CONTROLLING STRUCTURE AND FLUIDITY OF OCTADECYL PHOPHONIC ACID (ODPA) MONOLAYERS FORMED BY LANGMUIR BLODGETT (LB) DEPOSITION By Homa Sadeghzadeh A DISSERTATION Submitted to Michigan State University in partial fulfillment for the requirements for the degree of Chemistry – Doctor of Philosophy 2023 ABSTRACT Langmuir-Blodgett (LB) deposition methodology has applications ranging from chemical sensing to synthesis of complex multilayer structures. The interactions of metal ions with mono- and multilayer amphiphiles have been investigated extensively. Copper, Cadmium, and Ruthenium have received relatively little attention as a subphase constituent. These metal ions are of interest because of the potential to control their oxidation states reversibly, once they are incorporated into a monolayer structure that is deposited on an electrode surface. In the first part of the project, we report on the formation and organization of a Cu2+-complexed octadecylphosphonic acid (ODPA) monolayer formed by LB deposition. The formation of the Cu-complexed monolayer is seen to depend sensitively on subphase pH and Cu2+ concentration and it is possible to form a monolayer containing regions of complexed and free ODPA. From pressure-area isotherm data for these monolayers we can determine the equilibrium constant and free energy of formation for the Cu2+-ODPA complex. For the second part of this project, we modified the surface of Indium Tin Oxide (ITO). ITO has been used extensively as a transparent conductor. The surface chemistry of ITO is amenable to reactions similar to those used to modify silica, but a long-standing issue has been understanding the density and robustness of the ITO surface-modification. We report on the formation of chemically bound Cd2+-complexed ODPA monolayer formed on a Langmuir trough and deposited using LB methodology onto an ITO surface, either in its native form or functionalized with phosphonate (RPO32-). The organization of the Langmuir monolayer depends on the pH and [Cd2+] in the aqueous subphase on which it is formed and on the functionalization of the ITO surface. We probe the permeability of the resulting LB-support interface electrochemically and the motional freedom characteristic of chromophores contained within the monolayer using fluorescence recovery after photobleaching (FRAP). Our data demonstrate that, without modification of the ITO surface, the monolayer is significantly permeable by the electrophores used (ferrocene and Ru3+), and surface modification to produce covalently bound phosphonate functionality results in a monolayer that is impermeable to the electrophores. FRAP studies reveal a relatively rigid monolayer aliphatic chain region for deposition on either native or modified ITO, suggesting direct Cd2+-ITO interactions. We have also used Ru3+ as a metal ion for the fabrication of ODPA monolayers. The motivation for the use of Ru3+ is that it can be converted reversibly between several oxidation states, resulting in changes in ODPA monolayer properties. We show that Ru3+ can be changed to Ru2+ without loss of the monolayer. The CV data for this monolayer points to complex electrode morphology, with consequent complexity in the order and permeability of the adsorbed monolayer. Copyright by HOMA SADEGHZADEH 2023 I dedicate this thesis to my parents, sister and brother for their constant support and unconditional love. I love you all dearly v ACKNOWLEDGEMENTS I would like to thank to my PhD advisors, Professors Gary Blanchard, for supporting me during these past four years. Gary is someone you will instantly love and never forget once you meet him. He is always available to give you a hand for anything. He’s one of the smartest people I know. I hope that I could be as lively, enthusiastic, and energetic as Gary and to someday be able to command an audience as well as he can. I also have to thank the members of my PhD committee, Professors Greg Swain, Liangliang Sun, and Marcos Dantus for their helpful career advice and suggestions in general. I am thankful for funding from the Army Research Office with grant W911-NF-14-10063. Also I want to show my gratitude to Michigan State University for all support during this project. Thank you to the Blanchard group members, both past and present, who have supported me. I would like to thank Dr. Stephen Baumler, Dr. Briana Carpitsan, Dr. Corbin Livingston, Dr. Yufeng Wang, Iqbal Hossain, Andrew Wendel, Emily Simonis, Neelajana Mukherjee, Diana Nazario, and Shannon Cartwright. Special thanks to current member of Dr. Swain’s lab, Aaron Jacob, for his help and kindness. His support and collaboration allowed this thesis to be completed! I’m so grateful, to have Dr. Qiu’s students in our office, Einar Jacobson, Jasmin Reza, Eidan Reynolds, because of their everlasting friendship. I want to say special thanks to my lovely friends, peers of hard and soft time, Yufeng Wang, Andrew Wendel, Emily Simonis. Lastly, to my family who have always supported me. To my parents, thank you for your never-ending support and belief in me. I am forever grateful for you both. To my sister, Negar, for vi being angel in my life, she is always standing beside me. To my brother, Amirreza, for all the time that we spend together over the phone, and he gave me strength to continue my research. vii TABLE OF CONTENTS CHAPTER I: Introduction. ....................................................................................................... 1 1.1 Background and Motivation ....................................................................................... 2 1.2 Monolayer formation .................................................................................................. 3 1.3 Monolayer deposition and characterization ................................................................ 7 1.4 Summary ................................................................................................................... 14 REFERENCES.................................................................................................................. 16 CHAPTER II: Quantitating the Binding Energy of Metal Ions to Langmuir-Blodgett Monolayers. The Copper (II)-Octadecylphosphonic Acid System. ....................................... 19 2.1 Introduction ............................................................................................................... 20 2.2 Methods..................................................................................................................... 21 2.2.1 Materials: .......................................................................................................... 21 2.2.2 LB Film Formation: .......................................................................................... 22 2.2.2 XPS Measurements: .......................................................................................... 22 2.3 Result and discussion ................................................................................................ 23 2.4 Conclusions ............................................................................................................... 38 REFERENCES.................................................................................................................. 39 CHAPTER III: Permeability and Dynamics of a Monolayer are Mediated by ITO Support Surface-Modification (under review- submission date: April 2023). ..................................... 43 3.1 Introduction ............................................................................................................... 44 3.2 Methods..................................................................................................................... 45 3.2.1 Materials: .......................................................................................................... 45 3.2.2 Monolayer deposition: ...................................................................................... 46 3.2.3 Phosphonation reaction: .................................................................................... 46 3.2.4 Langmuir Film Formation: ............................................................................... 47 3.2.5 Cyclic voltammetry studies: ............................................................................. 48 3.2.6 Fluorescence Recovery after Photobleaching (FRAP): .................................... 48 3.3 Results and Discussion ............................................................................................. 48 3.3.1 Relevant equilibria: ........................................................................................... 49 3.3.2 Langmuir monolayer formation and LB deposition: ........................................ 53 3.3.3 Electrochemical characterization: ..................................................................... 57 3.3.4 Chromophore dynamics within the monolayers: .............................................. 62 3.4 Conclusions ............................................................................................................... 68 REFERENCES.................................................................................................................. 69 CHAPTER IV: A Switchable Monolayer using a Ru (III)-ODPA Complex (currently under review). ......................................................................................................................................... 73 4.1 Introduction ............................................................................................................... 74 4.2 Methods..................................................................................................................... 75 4.2.1 Materials: .......................................................................................................... 75 4.2.2 Monolayer deposition: ...................................................................................... 75 4.2.3 Phosphonation reaction: .................................................................................... 76 4.2.4 Langmuir Film Formation: ............................................................................... 76 viii 4.2.5 Cyclic voltammetry studies: ............................................................................. 77 4.2.6 Fluorescence Recovery after Photobleaching (FRAP): .................................... 77 4.3 Results and Discussion ............................................................................................. 77 4.3.1 Isotherm studies, BAM images and monolayer stoichiometry: ........................ 78 4.3.2 Electrochemical characterization: ..................................................................... 80 4.3.3 FRAP measurements:........................................................................................ 84 REFERENCES.................................................................................................................. 87 CHAPTER V: Conclusion and Future Work. ....................................................................... 88 5.1 Conclusions and Future Work .................................................................................. 89 5.1.1 Cu2+-Complexed Langmuir-Blodgett (LB) Films: ........................................... 89 5.1.2 Cd2+-Complexed Langmuir-Blodgett (LB) Films: ........................................... 89 5.1.3 Ru3+-Complexed Langmuir-Blodgett (LB) Films: ........................................... 90 ix CHAPTER I: Introduction. 1 1.1 Background and Motivation The chemical modification of surfaces has found wide use in science and technology, with goals ranging from chemical separations and sensing to tribology and heterogeneous catalysis, among others.1-3 Macroscopic surface modifiers (ca. µm length scale and thicker) have found widespread use, including paint and other protective coatings, but this dissertation is concerned with molecular-scale surface chemical modification, with thickness being in the nm range. There are a variety of means for the deposition and bonding of single layers of molecules to surfaces, with the nature of the surface-monolayer bonding ranging from physisorption (ca. 5 kJ/mol or less) through ionic coordination (20 – 200 kJ/mol) and covalent bond formation (80 – 400 kJ/mol).4 The organization of molecules within the monolayer can be controlled by the method of monolayer formation and the structure or order of the support surface. For all these monolayer interfaces, the organization of the monolayer is established during growth or deposition, and once formed, the organization cannot be altered reversibly. One of the goals of the work presented in this dissertation is to explore the feasibility of forming monolayers on support surfaces where the organization of the monolayer can be switched reversibly between two states. The ability to achieve reversible switching of monolayer organization could find application in chemical separations, sensing, and information storage, for example. The ability to reversibly change the organization of a single molecular layer places limits on how the layer can be bonded to the support surface. Physisorption will not allow reproducible, reversible changes in organization because of the low energy of interaction between the monolayer and the support surface. Covalent bonding of the monolayer to the support surface will not be appropriate for reversible change because of the energy of the covalent bond and the typically limited efficiency of geminate recombination. The use of metal ion coordination chemistry as a 2 means of bonding a monolayer to a support surface is attractive for several reasons. The first is that metal ion coordination to functionalities such as carboxylate, sulfonate or phosphonate is reversible and is characterized by an equilibrium constant. In addition, the energy of the bond to the support surface can be controlled through the identity and oxidation state of the metal ion used. Controlling the oxidation state of the coordinating metal ion in situ will provide the driving force for reversible change between monolayer structures. There are several issues that must be addressed for the formation of structurally switchable monolayers to be realized. In the discussion that follows, we consider the reasons for the choice of monolayer formation and the metal ions used. 1.2 Monolayer formation As noted above, monolayers of amphiphilic molecules can be formed in a number of ways, and the underlying organization of the support on which the monolayer is formed can have a strong influence on order within the formed monolayer.4 For example, alkanethiols will spontaneously form a monolayer on a gold or silver surface, with the resulting Au-S or Ag-S bond being on the order of DG = -20kJ/mol. The structure of the alkanethiol monolayer is determined by the metal lattice structure, with Au(111) surfaces producing a hexagonal close-packed geometry of the monolayer.5-6 Likewise, monolayers deposited on non-crystalline support surfaces, such as silica, exhibit disordered monolayer and multilayer structures, where the organization of the monolayer is determined by the spatial arrangement of the surface silanol functionalities. For both types of monolayer, the reason the support surface imposes organization on the monolayer lies in the energy of the monolayer-support bond. In order to overcome this limitation, it is of advantage to form a well-ordered monolayer prior to depositing it onto a support. 3 Irving Langmuir and Katharine Blodgett pioneered the formation of single layers of amphiphiles at the air-water interface, and the subsequent deposition of the formed monolayers on solid supports.7-10 The formation of Langmuir monolayers is schematized in Figure 1.1. Subphase Figure 1.1. Schematic of amphiphile monolayer formed using a Langmuir-Blodgett trough. A key advantage of Langmuir monolayers is that their organization at the air-water interface is determined by the intermolecular interactions of the amphiphiles, and the pressure imposed on the monolayer determines the average distance between amphiphile molecules and consequently the characteristic order of the amphiphile non-polar aliphatic chains. If highly ordered monolayers are desired, the length of the aliphatic chains must be sufficient for the attractive interaction energy between aliphatic chains exceeds the thermal energy of the bath. For this reason, amphiphile aliphatic chains of C16 or longer are used most commonly. In this work, we have used C18 aliphatic chains. For the amphiphiles used in the formation of Langmuir monolayers, the identity of the polar head group is an important consideration. Typically, the polar head group is either a carboxylate or a phosphonate, though other functionalities, including sulfate and ammonium, have been reported. For the carboxylate and phosphonate headgroups, the pH of the water subphase plays a critical role in determining the extent of headgroup protonation and thus the order of the resulting monolayer.11-12 4 O P OH OH Hydrophobic Aliphatic Tail Hydrophilic Head Figure 1.2. Structure of octadecylphosphonic acid (ODPA). The organization of the Langmuir monolayer can be inferred by measuring the relationship between surface pressure of the monolayer and the extent of compression. In the formation of a Langmuir monolayer, the monolayer compression arms are separated and an aliquot of the amphiphile in n-hexane is deposited on the water surface between the arms. After evaporation of the n-hexane, the density of amphiphiles at the water surface is substantially less than a monolayer, and the average distance between amphiphile molecules is large, analogous to a two-dimensional gas, and the surface pressure, as measured by a Wilhelmy plate balance, is low. As the compression arms are brought closer together, the average distance between amphiphile molecules diminishes, and intermolecular interactions can be observed in the form of an increase of surface collapsed solid surface pressure liquid 2 barrier liquid 1 subphase liquid 1, gas gas area per molecule Figure 1.3. Schematic of Langmuir pressure-area isotherm and its relationship to monolayer density and order. 5 pressure. This region is termed the liquid region. As the compression arms are brought closer together, the amphiphile molecules reach an average distance commensurate with the corresponding two-dimensional solid, and the surface pressure is seen to increase rapidly with monolayer compression. This is termed the solid phase region, and extrapolation of the slope the pressure vs. area response to zero pressure yields the average surface area per amphiphile molecule. Further compression of the monolayer results in a rapid decrease in measured surface pressure, corresponding to monolayer buckling or collapse to form irregular multilayer regions. This progression is schematized in Figure 1.3. The above description and Figure is appropriate when the amphiphile is spread on an aqueous subphase that does not contain any species that can interact with the amphiphile headgroup. When certain metal ions are introduced to the subphase and the amphiphile headgroups are carboxylate of phosphonate, the interactions between headgroup and metal ion can have a significant effect on the organization of the resulting monolayer. The Talham group has reported on the formation of ODPA Langmuir monolayers for several divalent and trivalent metal ions. By sequential layer deposition they formed metal bisphosphonate multilayers and found that the structure of the metal bisphosphonate regions of the multilayers were the same as those of the corresponding solid state metal bisphosphonate. This is an important finding because it provides a facile means of predicting the structure imposed on the Langmuir monolayer by the metal ion introduced to the aqueous subphase during monolayer formation.13-23 The Blanchard group has recently examined ODPA Langmuir monolayer formation with Cu2+, Cd2+ and Ru3+ in the aqueous subphase during monolayer formation. While these metal ions have received relatively little attention in the Langmuir monolayer literature, they do participate in ODPA monolayer formation and exhibit characteristic pH-dependence, analogous to that reported 6 by the Talham group. The motivation for this choice of metal ions is that all are known to exhibit more than one non-zero oxidation state, and the reduction potentials for the relevant redox chemistry lies in a window that is not overlapped by either ODPA or water. An ultimate goal of this work is to control the oxidation state of the metal ion contained in a monolayer deposited onto a conductive support using Langmuir-Blodgett methodology. 1.3 Monolayer deposition and characterization A critical step in this project is the formation of the Langmuir monolayer with selected metal ions in the aqueous subphase, and the deposition of the monolayer onto conductive supports. The formation of the monolayers requires knowledge of multiple competing equilibria, experimental verification of the pH- and metal ion-concentration dependence of the monolayer formation, and control over the surface functionalities present on the conductive support. Characterizing the ODPA monolayers, both during formation, and following their deposition on the conductive support requires measurement of pressure-area isotherms (Figure 1.3), characterization of monolayer during formation using Brewster angle microscopy, electrochemical characterization and demonstration of the ability to control metal ion oxidation state in the formed and deposited monolayer, and characterization of the structural properties of the monolayer following deposition. We use fluorescence recovery after photobleaching (FRAP) measurements on chromophores embedded in the monolayers to infer the extent of organization and constituent mobility as a function of experimental conditions. Each of these characterization techniques is discussed briefly in the following paragraphs, with full reports on the specific monolayer systems in Chapters 2-4 of this dissertation. The relationship between surface pressure and average area per amphiphile molecule is seen in the pressure-area (P-A) isotherm for a Langmuir monolayer. The surface pressure is measured 7 using a Wilhelmy plate balance, and the functional form of the isotherm is related to the dominant chemical interactions that mediate monolayer formation. For ODPA monolayers formed on an aqueous subphase containing metal ions, it is expected that the metal ions will interact with the amphiphile headgroups even when the surface pressure is low, where the average distance between amphiphiles is comparatively large. Even with a large average distance between amphiphiles, the presence of metal ions can lead to the formation of a heterogeneous surface, where island-like structures form. The ability to detect such islands can be challenging because of the small absolute number of amphiphile molecules present. Brewster angle microscopy is an established technique laser polarization Imaging detector Figure 1.4. Schematic of the Brewster angle microscope for the characterization of surface structures at the sub-monolayer level. Such high sensitivity is the result of the fundamental properties of reflection of light from an interface. Brewster angle microscopy can be understood by examining the Fresnel equations. While the laws of reflection (qi = qr) and refraction (Snell’s law, n1sinq1 = n2sinq2) describe the directions of the reflected and transmitted components of an incident electric field at an interface, these equations do not provide information on the fraction of light reflected and transmitted. The Fresnel 8 equations describe the incident angle-dependence and electric field polarization-dependence of specular reflection and transmission at an interface. The Fresnel equations describes reflection and transmission at an interface as a function of angle of incidence (qi) and polarization of the incident electric field for two limiting cases; where the electric field is transverse to the plane of incidence (TE polarization) and where the magnetic field is transverse to the plane of incidence (TM polarization). For TM polarization (i.e. the electric field lies in the plane of incidence), there is an angle of incidence for which the reflection coefficient is identically zero. This angle is called Brewster’s angle, and it depends on the refractive indices of the materials on either side of the interface. (1.1) n = 1.00 qB qB n = 1.00 X air n = 1.45 air n = 1.33 water n = 1.33 water Figure 1.5. Reflection and refraction (transmission) of incident light in different condition, with n0, n1 and n2 being the refractive indices of the three media. For the air-water interface, n1 = 1.00 and n2 = 1.33, with a corresponding qB = 53.06°. For an aqueous subphase, the Brewster angle microscope (Figure 1.4) is set to qB = 53.06°, resulting in a null signal. The refractive index of bulk ODPA is ca. 1.45, and while the refractive index of a monolayer may not be identical to that of the bulk material, n2 will be different from 1.33, leading to a change in qB, and the surface containing monolayer or island structures, reflection will occur. 9 Acquiring a spatial image of the reflected light provides an image of the ODPA structures present on the aqueous subphase.24-26 Deposition of the formed Langmuir monolayer onto a solid support is accomplished using the Langmuir-Blodgett technique. In this technique, a solid support is immersed into the aqueous subphase prior to formation of the Langmuir monolayer, and once the monolayer is formed, the support is drawn vertically at a fixed rate out of the aqueous subphase, allowing transfer of the Langmuir monolayer onto the solid support, with the polar amphiphilic headgroup in closest proximity to the solid support. The polarity of and chemical functionality on the solid support plays a critical role in determining the properties of the supported monolayer. Interactions varying in energy from physisorption and hydrogen bonding (£ 20 kJ/mol) to ionic complexation (~60 – 200 kJ/mol) characterize the formation of the supported monolayer. Such a range of interaction energies is expected to be associated with a wide range of dynamics that characterize the monolayer constituents as well as chromophores and electrophores embedded in the monolayer. Electrochemical characterization of the resulting interfaces is an information-rich means of understanding the properties of interest, such as monolayer permeability, and whether the metal ions involved in complexation with the amphiphiles can undergo reversible redox chemistry in situ. In this work, cyclic voltammetry (CV) has been of use because of its ability to evaluate the reversibility of a redox reaction and its concentration and scan rate dependence. (1.2) Where ip is the peak current, n is the number of electrons involved in the reaction, S is the surface area of the working electrode, DA is the diffusion constant of the analyte, V is the potential scan rate, and CA is the bulk concentration of the analyte. It is the energy of the analyte lowest unoccupied molecular orbital (LUMO) relative to the energy of the electrode, and electron transfer 10 takes place at the point where these energies are equal. In a CV experiment, there are both Faradaic (electron-transfer) and non-Faradaic (capacitive) contributions to the experimental data, and both components can provide insight into the system under examination. We use CV measurements in the experiments reported in this dissertation to evaluate the permeability of deposited monolayers, the presence of metal ions in the supported monolayer structure, and their ability to undergo reversible reduction and oxidation in situ.27 The electrochemical data provide information on the integrity and permeability of the monolayer as a function of experimental conditions. While this information is of critical importance, it does not address issues such as mobility of either monolayer constituents or species Mn+ Mn+ Mn+ Mn+ Mn+ Mn+ Mn+ O- O- OH O- OH OH O- OH Figure 1.6. Schematic of two deposited monolayers with different support functionality, and with perylene incorporated into the monolayer for FRAP measurements. 11 of interest contained in the monolayer. This complementary information is available from fluorescence recovery after photobleaching (FRAP) experiments. For FRAP measurements, a fluorescent chromophore must be incorporated into the sample, and the goal of the measurement is to extract the translational diffusion constant, DT, from the experimental data. In a FRAP measurement, a pre-selected spot of known diameter is irradiated with high intensity light to photobleach the chromophores within the irradiated region. After the photobleaching light is removed, the resulting image is monitored as a function of time to determine the rate and functional form of photobleached chromophore diffusion out of the region of observation and active chromophores to diffuse into the same volume. The instrumentation used for this measurement is schematized in Figure 1.7. Diode Laser (405, 488, 561, 640 nm) sample Pinhole Aperture Galvo- Dichromic scanner Mirror Bandpass Filter Confocal Pinhole 3 PMT Detector Figure 1.7. Schematic of confocal inverted microscope used to perform FRAP measurements. The functional form of the experimental signal is schematized in Figure 1.8, For the FRAP measurement, there are three distinct facets of the experimental data that are relevant. A region of interest (ROI) is identified in the sample, with a pre-defined radius, and upon 12 Prebleaching Bleaching Postbleaching intensity time Figure 1.8. Schematic of FRAP data, indicating pre- bleached region of interest (ROI, top left), photobleached ROI (top center) and recovered ROI (top right). photobleaching a fraction of the fluorescence signal in that region is depleted by the photobleaching process. It is the intensity of the recovery signal in the ROI (Ifrap(t)) that recovers subsequent to photobleaching. The background or reference signal (Ibkg(t)) is observed in an area that is well isolated from the ROI, and time-dependent changes in the intensity of this region determine the background signal. Recovery of the signal will, of necessity, be lower in intensity after full recovery due to the destruction of some fraction of the total number of fluorophores in the sample. Normalization of the signal is described in Soumpasis treatment of FRAP data (Eq. 1.3) acquisition (1.3) Where Ifrap-pre is the intensity of the signal prior to photobleaching the ROI. The curve schematized in Fig. 1.8 is normalized to span the range of 1 and 0 to compensate for variations in absolute intensities between individual measurements. Equation 1.4 is used to calculate the translational diffusion constant (DT) of a fluorescent probe, 13 (1.4) where tD is the diffusion time, or the time at which the fluorescence intensity recovers to half of the maximum intensity, in seconds, and w is the bleaching radius, or spot size, in µm. Extraction of tD from the experimental data is accomplished using software written in-house (MATLAB) which fits the data to Eq. 1.5, derived originally by Soumpasis, where I0 and I1 are modified Bessel functions. (1.6) There are appropriate limiting cases in the Soumpasis model to account for free diffusion or diffusion that competes with complexation in the sample. The quantity of interest is DT, and how it varies with the identity of the monolayer amphiphile, metal ion and solid support surface chemical functionality. DT is related to the properties of the monolayer through the Stokes- Einstein equation, (1.7) Where kBT is the thermal energy of the system, h is the bulk viscosity of the medium and r is the radius of the diffusing moiety. FRAP measurements thus provide a facile means of evaluating the extent of ordering and packing, and how those properties change, as a function of monolayer identity, metal ion and chemical functionalization of the support surface.28-30 1.4 Summary In this dissertation, we describe in Chapter 2 quantitating binding energy of metal ions to Langmuir-Blodgett monolayers, in Copper (II)-Octadecylphosphonic Acid system. In Chapter 3 we report the permeability and dynamics of a monolayer that mediated by ITO support surface- 14 modification, and in Chapter 4 we report a switchable monolayer in a Ru(III)-ODPA complex. The work contained in this dissertation describes the foundational work in characterizing that will enable future efforts aimed directly at demonstrating reversible structural modulation of monolayers in situ. 15 REFERENCES 1. Malik, S.; Tripathi, C. C., Thin Film Deposition by Langmuir Blodgett Technique for Gas Sensing Applications. Journal of Surface Engineered Materials and Advanced Technology 2013, 03 (03), 235-241. 2. Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H. Friction and Wear of Langmuir-Blodgett Films Observed by Friction Force Microscopy; J. Phys. Rev. Lett. 1992, 69, 1777-1783. 3. Tieke, B., Langmuir–Blodgett membranes for separation and sensing. Advanced Materials 1991, 3 (11), 532-541. 4. Mavukkandy, M. O.; McBride, S. A.; Warsinger, D. M.; Dizge, N.; Hasan, S. W.; Arafat, H. A., Thin film deposition techniques for polymeric membranes– A review. Journal of Membrane Science 2020, 610. 5. Ghosh, M.; Yang, D. S., Structures of self-assembled: N -alkanethiols on gold by reflection high-energy electron diffraction. Physical Chemistry Chemical Physics 2020, 22 (30), 17325- 17335. 6. Luo, X.; Ma, K.; Jiao, T.; Xing, R.; Zhang, L.; Zhou, J.; Li, B., Graphene Oxide-Polymer Composite Langmuir Films Constructed by Interfacial Thiol-Ene Photopolymerization. Nanoscale Research Letters 2017, 12 (1). 7. Blodgett, K. B., Films built by depositing successive monomolecular layers on a solid surface. J. Am. Chem. Soc. 1935, 57, 1007-1022. 8. Langmuir, I.; Schaefer Vol, V. J.; Irving Langmuir, B.; Schaefer Katharine Blodgett, V. J. Composition of Fatty Acid Films on Water Containing Calcium or Barium Salts; J. Am. Chem. Soc.1936, 58, 284-287. 9. Langmuir, I. Surface Chemistry, Chem. Rev.,1932, 13, 147-191. 10. Hann, R. M.; Hudson, C. S. A Calcium Chloride Compound of α-d-Galactose, J. Am. Chem. Soc. 1937, 59, 2075. 11. Ulman, A., An Introduction to Ultrathin Organic Films: From Langmuir--Blodgett to Self- -Assembly. Academic Press: 1991. 12. Petty, M. C., Langmuir-Blodgett Films: An Introduction. Cambridge University Press: 1996. 13. Byrd, H.; Pike, J. K.; Talham, D. R., Extended-Lattice Langmuir-Blodgett-Films - Manganese Octadecylphosphonate Langmuir-Blodgett-Films Are Structural and Magnetic Analogs of Solid-State Manganese Phosphonates. J. Am. Chem. Soc. 1994, 116, 7903-7904. 16 14. Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. Langmuir-Blodgett Films of Known Layered Solids: Preparation and Structural Properties of Octadecylphosphonate Bilayers with Divalent Metals and Characterization of a Magnetic Langmuir-Blodgett Film; J. Am. Chem. Soc. 1997, 119, 7084-7094. 15. Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Stephen; Nagler, E.; Talham, D. R. Role of the Template Layer in Organizing Self-Assembled Films: Zirconium Phosphonate Monolayers and Multilayers at a Langmuir-Blodgett Template; J. Am. Chem. Soc. 1994, 116, 295-301. 16. Seip, C. T.; Byrd, H.; Talham, D. R. Electron Paramagnetic Resonance Study of a Langmuir-Blodgett Film of Manganese Octadecylphosphonate and Comparison of the Magnetic Properties to Those of Solid-State Manganese Alkylphosphonates; Inorg. Chem. 1996, 35, 3479- 3483. 17. Byrd, H.; Pike, J. K.; Talham, D. R., Inorganic Monolayers Formed At an Organic Template - a Langmuir-Blodgett Route to Monolayer and Multilayer Films of Zirconium Octadecylphosphonate. Chem. Mat. 1993, 5, 709-715. 18. Byrd, H.; Pike, J. K.; Talham, D. R., Langmuir-Blodgett-Films As Single-Layer Analogs of Known Organic-Inorganic Solid-State Materials. Synth. Met. 1995, 71, 1977-1980. 19. Byrd, H.; Pike, J. Κ.; Showalter, M. L.; Whipps, S.; Talham, D. R. Langmuir-Blodgett Monolayers as Templates for the Self-Assembly of Zirconium Organophosphonate Films; 1994. 20. Seip, C. T.; Talham, D. R. Organic/inorganic Langmuir-Blodgett films based on known layered solids: characterization and reaction of cobalt octadecylphosphonate, Mat. Resear. Bulletin, 1999, 34, 437-445. 21. Fanucci, G. E.; Talham, D. R., Langmuir-Blodgett films based on known layered solids: Lanthanide(III) octadecylphosphonate LB films. Langmuir 1999, 15, 3289-3295. 22. Byrd, H.; Pike, J. K.; Talham, D. R. Inorganic Monolayers Formed at an Organic Template: A Langmuir-Blodgett Route to Monolayer and Multilayer Films of Zirconium Octadecylphosphonate; Chem. Mater. 1993, 5, 709-715. 23. Bujoli, B.; Roussière, H.; Montavon, G.; Laïb, S.; Janvier, P.; Alonso, B.; Fayon, F.; Petit, M.; Massiot, D.; Bouler, J. M.; Guicheux, J.; Gauthier, O.; Lane, S. M.; Nonglaton, G.; Pipelier, M.; Léger, J.; Talham, D. R.; Tellier, C., Novel phosphate-phosphonate hybrid nanomaterials applied to biology. Progress in Solid State Chemistry 2006, 34 (2-4), 257-266. 24. Daear, W.; Mahadeo, M.; Prenner, E. J., Applications of Brewster angle microscopy from biological materials to biological systems. Biochimica et Biophysica Acta - Biomembranes, 2017, 1859, 1749-1766. 25. Winsel, K.; Hönig, D.; Lunkenheimer, K.; Geggel, K.; Witt, C., Quantitative Brewster angle microscopy of the surface film of human broncho-alveolar lavage fluid. European Biophysics Journal 2003, 32 (6), 544-552. 17 26. The Fresnel Equations and Brewster's Law. 27. Elgrishi, N.; Rountree, K. J.; McCarthy, B. D.; Rountree, E. S.; Eisenhart, T. T.; Dempsey, J. L., A Practical Beginner's Guide to Cyclic Voltammetry. Journal of Chemical Education 2018, 95 (2), 197-206. 28. Soumpasis, D. M., Theoretical analysis of fluorescence photobleaching recovery experiments. Biophys. J. 1983, 41, 95-97. 29. Sprague, B. L.; Pego, R. L.; Stavreva, D. A.; McNally, J. G., Analysis of binding reactions by fluorescence recovery after photobleaching. Biophys. J. 2004, 86, 3473-3495. 30. Lorén, N.; Hagman, J.; Jonasson, J. K.; Deschout, H.; Bernin, D.; Cella-Zanacchi, F.; Diaspro, A.; McNally, J. G.; Ameloot, M.; Smisdom, N.; Nydén, M.; Hermansson, A. M.; Rudemo, M.; Braeckmans, K., Fluorescence recovery after photobleaching in material and life sciences: Putting theory into practice. Quarterly Reviews of Biophysics 2015, 48 (3), 323-387. 18 CHAPTER II: Quantitating the Binding Energy of Metal Ions to Langmuir-Blodgett Monolayers. The Copper (II)-Octadecylphosphonic Acid System. Adapted with permission from: Sadeghzadeh, H.; Blanchard, G.; Quantitating the Binding Energy of Metal Ions to Langmuir– Blodgett Monolayers: The Copper(II)–Octadecylphosphonic Acid System; The Journal of Physical Chemistry B 2022 126 (17), 3366-3373. 19 2.1 Introduction Langmuir-Blodgett (LB) deposition methodology1-3 has found use in applications ranging from chemical sensing to control over the synthesis of complex multilayer structures. Indeed, there is an extensive literature on the formation of LB mono- and multilayers when metal ions are present in the aqueous subphase on which the monolayers are formed.4-18 The Talham group has pioneered the effort to understand and characterize the interactions between amphiphiles and metal ions in planar structural formats such as LB mono- and multilayer structures.19-41 Among the key findings of that work, some of which focused on the magnetic properties of certain metal ions, was the correspondence between metal-phosphonate structure in the solid state and in LB layers.19-26 While a variety of divalent metal ions have been studied, Cu has received comparatively little attention.15-16, 42-43 We are interested in the formation of Cu2+-ODPA monolayers because of the ability, in principle, to reduce Cu2+ to Cu+ reversibly, in situ, in an effort to control the organization of the ODPA monolayer. A first step in this effort is understanding the conditions required to form a Cu2+-ODPA LB monolayer and how the formation process can be used to control monolayer morphology. In the course of this work we have developed a means of evaluating the Cu2+-ODPA equilibrium constant based on the functional form(s) of the pressure-area isotherms acquired during LB monolayer formation. The characterization and control of ODPA monolayer growth is important in areas where monolayer morphology and fluidity are relevant to their application, including tribology and corrosion inhibition, where the growth of ODPA mono- and multilayers on Cu and oxidized Cu surfaces has been reported.42-43 In that body of work, the various surface species of Cu were Cu0, Cu2O, Cu(OH)2 and CuO could all play a role in determining interface properties. The ability to controllably alter the morphology of a monolayer on a metallic or oxide surface has implications 20 not only for tribology and corrosion inhibition, but also for chemical sensing and molecular-scale electronics. In the work we present here, we find evidence for the ability to control the morphology of the LB monolayer through the pH and [Cu2+] in the aqueous subphase during monolayer formation, and from p-A isotherm data for the monolayer we can extract the equilibrium constant for Cu2+ complexation with the ODPA monolayer, and from the equilibrium constant we can determine DG for the reaction. We find DG = -22.5 kJ/mol for the formation of (Cu2+(C18H37PO3H-))+, similar in magnitude to a hydrogen bond in an aqueous medium, suggesting the labile nature of the metal- amphiphile complex. The fluid nature of the resulting monolayer allows for the possibility of facile structural change. 2.2 Methods 2.2.1 Materials: ODPA (Sigma-Aldrich, ≥99.0%) was used as received. All the ODPA solutions used for monolayer deposition were prepared at a concentration of 1 mg/mL in tetrahydrofuran (THF). For the growth of Cu2+−ODPA monolayers, copper(II) chloride (CuCl2 , Jade Scientific, 99.0%) was dissolved in Milli-Q water at the desired concentration to constitute the aqueous subphase. Filtered water (18 MΩ) from a Milli-Q filtration system was used for the subphase in all the experiments. The subphase pH was controlled with hydrochloric acid (HCl, 1 M, CCI, Inc.). Films were deposited on glass cover slides (#1, 22 mm × 22 mm, Alkali Scientific, Inc.). All the substrates were cleaned by immersion into the piranha solution (1H2O2:3H2SO4; caution: strong oxidizer!) for 10−15 min, followed by rinsing with Milli- Q water until the pH of the rinse was ca. 7. Substrates were stored in Milli-Q water to minimize adventitious contami- nation by airborne organic compounds. 21 2.2.2 LB Film Formation: All the LB monolayers were formed using an LB trough (KN 2003, KSV Nima, Biolin Scientific, Gothenburg, Sweden) equipped with a Brewster angle microscope attachment (UltraBAM, Accurion, Göttingen, Germany) and a platinum Wilhelmy plate balance for measuring the surface pressure. The aqueous subphase of the trough is temperature controlled at 20.0 ± 0.5 °C. The glass substrate is immersed in the trough well prior to the application of the monolayer deposition solution. Approximately 1 h was allotted for subphase equilibration, then 70 μL of the ODPA in the THF spreading solution was deposited beneath the subphase surface by using a syringe, with care taken to ensure the initial surface pressure did not exceed 0.5 mN/m. Following a 20 min solvent evaporation period, monolayer compression was initiated at a barrier speed of 5 mm/min. For monolayer deposition, barrier compression stopped when the surface pressure reached the desired pressure (30 or 63 mN/m depending on the experiment). The barrier position was placed at the desired surface pressure for ca. 10−15 min to allow for monolayer equilibration. Film deposition was performed by the vertical removal of the immersed substrate from the subphase at a rate of 2 mm/min. Following complete removal from the subphase, the substrate was maintained above the trough for ca. 30 min to allow any residual water to evaporate. All the experiments were controlled by using KSV NIMA LB software (v2.2, Biolin Scientific). The Brewster angle microscope was controlled using UltraBAM 1.1.2 software (Accurion) and the angle of incidence was set to 53.2° (θB for H2O). Film formation images were acquired using Accurion Image 1.1.3 software (Accurion). 2.2.2 XPS Measurements: X-ray photoelectron spectroscopy (XPS) measurements were performed using a PerkinElmer Phi 5600 ESCA instrument equipped with an Al Kα X-ray source to illuminate the sample at a 22 takeoff angle of 45°. Samples were analyzed at pressures between 10−9 and 10−8 Torr with a spot size of ca. 250 μm2 and a pass energy of 187.9 and 29.4 eV for the survey and detailed scans, respectively. Survey scans of 0− 1100 eV binding energy and detailed scans for Cu2+, C 1s, O 1s, P 2p, and Si 2p were measured for all the samples. All the peaks were referenced to the C 1s peak at 284.8 eV. 2.3 Result and discussion This work is the first step in an effort to control the structure of molecular monolayers in situ and reversibly by electrochemical means. The formation of the monolayer using LB deposition and the dependence of monolayer properties on the extent of amphiphile headgroup protonation and subphase metal ion concentration is the focus of this work. As noted above, we use Cu2+ because it can be reduced to Cu+ reversibly in a potential window that is not overlapped with the redox reactions of water or the phosphonate amphiphile. We first consider the formation of ODPA monolayers in the absence of Cu2+ in the aqueous subphase, then examine the changes in monolayer formation associated with the presence of Cu2+. From these two bodies of information, we extract the equilibrium constant and free energy for Cu2+−ODPA formation. The formation of phosphonate monolayers by LB deposition has been reported before by several groups. In that work, it was found that the formation of metal bisphosphonate structures by LB deposition yielded the same structures found in the corresponding bulk metal phospho- nates. An important difference between that work and this report is that they deposited LB monolayers on nonpolar supports to produce Y-type LB mono- and multilayer structures. Our aim is to deposit one LB monolayer by emersion (Z-type deposition) onto polar, hydrophilic supports. Despite this important difference in monolayer orientation, the π−A isotherms and formation of the ODPA monolayers on the aqueous subphase are fully comparable to those reported by the Talham group. 23 The characteristic form of the π−A isotherm for ODPA is shown in Figure 2.1. Significantly, the functional form of the isotherm is pH independent, and based on well-established assignments of isotherm regions, the monolayer that forms is consistent with solid phase packing for pressures above ca. 20 mN/m. Interestingly, with increasing monolayer compression, the pressure plateaus at ca. 70 mN/m and does not exhibit an explicit collapse. It is also of use to examine the Brewster angle microscopy (BAM) images for ODPA over the range of subphase pH values at different pressures. For ODPA, there is little evidence for the formation of nonuniform monolayer structures. The isotherms appear to be the same over a range of pH values spanning pKa1 and approaching pKa2, consistent with the dominant role of van der Waals interactions between the amphiphile aliphatic chains in determining the intermolecular spacing of ODPA. pH 7.76 pH 4.30 pressure (mN/m) pH 3.85 2 mean molecular area (Å ) Figure 2.1. Pressure−area (π−A) isotherms of ODPA Z-type LB monolayers for three aqueous subphase pH values. It is instructive to consider the monolayer morphology, which can be viewed as a function of subphase pH and pressure using BAM. These BAM images are shown in Figure 2.2. The primary observation is that the morphology of the monolayer depends little on the pH, indicating that van der Waals interactions in the aliphatic chain region of the monolayer are significant and that H-bonding is extensive in the headgroup region of the ODPA monolayer. The uniformity of 24 the images arises from the facile nature of amphiphile mobility and intermolecular interactions. It is possible, at submonolayer densities, that island formation could occur, but increasing compression force produces a uniform ODPA monolayer. 0 mN/m 1 mN/m 30 mN/m 68 mN/m pH 7.76 pH 4.30 pH 3.85 Figure 2.2. BAM images of the ODPA monolayers corresponding to four different pressures and at three pH values. The corresponding π− A isotherms for these images are shown in Figure 2.1. The addition of Cu2+ to the aqueous subphase gives rise to a pronounced subphase pH dependence in the functional form of the adsorption isotherms (Figure 2.3). The isotherm data also depend on [Cu2+]. The complexation of Cu2+ by ODPA is operative in these monolayers. In the pH region where the ODPA headgroup is fully protonated (C18H37PO3H2), that is, well less than pKa1 (2.60), the π−A isotherm data resemble that seen for ODPA in the absence of Cu2+. As the pH of the subphase is increased above pKa1, ODPA exhibits an increasing fraction of monodeprotonated ODPA (C18H37PO3H−) is present in the monolayer. The α fraction plot for 25 ODPA is shown in Figure 2.4 with the inclusion of the Cu2+ complexation equilibria. Because of the pH range in which we operate, there is essentially no contribution (α2) from C18H37PO32− (pKa2 = 8.20). 26 2+ -3 [Cu ] = 1x10 M pressure (mN/m) pH 4.98 pH 3.75 pH 3.55 pH 3.25 2 mean molecular area (Å ) 2+ -4 [Cu ] = 5x10 M pH 4.80 pressure (mN/m) pH 4.05 pH 3.75 pH 3.65 pH 3.55 pH 3.25 2 mean molecular area (Å ) 2+ -4 [Cu ] = 1x10 M pressure (mN/m) pH 5.49 pH 4.15 pH 4.07 pH 3.83 2 mean molecular area (Å ) Figure 2.3. Pressure−area (π−A) isotherms of ODPA monolayers formed on an aqueous subphase containing Cu2+ at concentrations of (a) 1 × 10−3 M, (b) 5 × 10−4 M, and (c) 1 × 10−4 M at a series of pH values, as indicated in the panels. 27 Figure 2.4. Plots of the α fractions of ODPA and the complexed Cu2+ species calculated using Eqs 2.1−2.7, the solution phase values of Ka1 and Ka2 for ODPA, the determined value of K1 and the estimated value of K2. The pH used in the calculation is the local pH based on the PB theory with φ(0) = kBT. The plots show the fractional composition of the ODPA monolayers as a function of pH. It is important to consider that the pKa values indicated above are for the bulk solution. The pKas for monolayers of amphiphiles will be shifted to lower values relative to bulk values because of the comparatively higher charge density in the headgroup region of the monolayer, which will serve to alter the local pH of the medium. This effect is well established and has been treated in the context of Poisson−Boltzmann (PB) theory. The PB theory relates the pH in the vicinity of the headgroups to the bulk pH of the solution through [H+]local = [H+]bulk exp(−eφ(0)) where φ(0) is the contact potential. We show in Figure 2.5 the change in α fractions for ODPA resulting from changes in local pH for φ(0) values ranging from 0 (bulk solution) to 3kBT. The effective shift is ΔpH ∼ −0.43/kBT.47−50 28 local vs. bulk pH via Poisson-Boltzmann theory 1.0 f(0) = 0kBT a0 a1 0.8 a2 f(0) = 1kBT a0 0.6 a1 a fraction a2 DpK ~ -0.434/kBT f(0) = 2kBT a0 0.4 a1 a2 f(0) = 3kBT 0.2 a0 a1 a2 0.0 0 2 4 6 8 10 12 14 bulk pH Figure 2.5. Calculation of a-fractions for ODPA accounting for local pH at the planar phosphonate headgroup interface. For these calculations, [H+]local = [H+]bulk*exp(-ef(0)), where f(0) is the contact potential. The calculation shows that the local pKa decreases by 0.434 per kBT. It has been reported that ODPA LB monolayers formed on aqueous subphases containing several different metal ions produce π−A isotherms that resemble the formation of liquid phase monolayers. The reason for this effect is that, for monolayers formed in the presence of metal ions, at low pressures the submonolayer system contains islands of complexed ODPA, and at higher pressures, metal ion- phosphonate complexation determines the intermolecular spacing within the monolayer. Because the intermolecular spacing is greater when metal ion complexation proceeds, it is not possible for the aliphatic chains to interact to produce a quasi-crystalline monolayer environment. The result is that a liquid-like monolayer forms rather than a solid-like monolayer. 29 Thus, the π−A isotherm for ODPA monolayers is a diagnostic for the presence or absence of metal ions under conditions where the ODPA headgroup is not fully protonated. We have found, consistent with the Talham group’s earlier work, that over a relatively narrow pH range for a given metal ion concentration, π−A isotherms appear to exhibit liquid-like behavior up to a certain pressure, and above that pressure, the isotherm is consistent with solid- like behavior. Empirically, we have determined that the (narrow) pH range where this mixed isotherm is seen depends on [Cu2+]. We can understand this behavior in the context of a parameter space (pH, [Cu2+]) where there are regions of complexed and uncomplexed ODPA present in the monolayer and the relative fractions of each component can be estimated from the isotherm. Indeed, the BAM images for monolayer formation under conditions where Cu2+ is present in the aqueous subphase reveal significant spatial heterogeneity, especially under conditions of low compression force, and there is a qualitative difference between monolayer morphology for pH values below and above pKa1 for ODPA (Figure 2.6). We recognize that there are several possible explanations for this spatial heterogeneity, but, unfortunately, BAM does not provide the information needed to resolve the chemical and physical properties of these heterogeneities. 30 Figure 2.6. BAM images of the ODPA monolayers formed in the presence of 1 × 10−3 M Cu2+ in the aqueous subphase at selected pressures and subphase pH values. The corresponding π−A isotherms for these images are shown in Figure 3a. Consideration of the several connected equilibria that are operative in this system allows for the extraction of the equilibrium constant for (Cu2+( C18H37PO3H−))+ from the π− A isotherm data, subphase [Cu2+], and pH. The relevant equilibria for the ODPA−Cu2+ LB monolayer system are given in Scheme 2.1. 31 Scheme 2.1. ODPA acid-base reactions and Cu2+ complexation reactions for LB monolayers. As noted above, Ka2 plays essentially no role in the observed monolayer formation process because of the pH range in which we operate. The equilibrium expressions for the ODPA dissociation reactions in scheme 1 are given in Eq. 2.1 and Cu2+ association in Eq. 2.2, [C18 H 37 PO3 H - ][ H + ] [C18 H 37 PO3= ][ H + ] K a1 = Ka 2 = [C18 H 37 PO3 H 2 ] [C18 H 37 PO3 H - ] [2.1] [Cu 2+ (C18 H 37 PO3 H - ) + ] [Cu 2+ (C18 H 37 PO32- )] K1 = K2 = [Cu 2+ ][C18 H 37 PO3 H - ] [Cu 2+ ][C18 H 37 PO32- ] [2.2] And the corresponding a-values for each species are given in Eqs. 2.3-2.7 (Fig. 2.5), [C18 H 37 PO3 H 2 ] [ H + ]2 a0 = = + 2 CT [ H ] + K a1[ H ] + K a1K a 2 + K a1K1[Cu 2+ ][ H + ] + K a1K a 2 K 2 [Cu 2+ ] + [2.3] [C18 H 37 PO3 H - ] K a1[ H + ] a1 = = + 2 CT [ H ] + K a1[ H ] + K a1K a 2 + K a1K1[Cu 2+ ][ H + ] + K a1K a 2 K 2 [Cu 2+ ] + [2.4] [C18 H 37 PO3= ] K a1K a 2 a2 = = + 2 CT [ H ] + K a1[ H ] + K a1K a 2 + K a1K1[Cu 2+ ][ H + ] + K a1K a 2 K 2 [Cu 2+ ] + [2.5] [Cu 2+ (C18 H 37 PO3 H - ) + ] a Cu 2+ - + ( C18 H 37 PO3 H ) = CT K a1 K1[Cu 2+ ][ H + ] = [ H ] + K a1[ H ] + K a1 K a 2 + K a1K1[Cu 2+ ][ H + ] + K a1K a 2 K 2 [Cu 2+ ] + 2 + [2.6] 32 [Cu 2+ (C18 H 37 PO3= ) 2 ] a Cu 2+ = ( C18 H 37 PO3 )2 = CT K a1 K a 2 K 2 [Cu 2+ ] = [ H + ]2 + K a1[ H + ] + K a1 K a 2 + K a1 K1[Cu 2+ ][ H + ] + K a1K a 2 K 2 [Cu 2+ ] [2.7] where CT=[C18H37PO3H2]+[C18H37PO3H-]+[C18H37PO3=]+[Cu2+(C18H37PO3H-)+]+[Cu2+C18H37PO32- ]. The (Cu2+(C18H37PO3H-))+ association equilibrium expression is given in Eq. 2.-2 for K1. In this system it is likely that the species (Cu2+C18H37PO3H-)+ is the primary complexed species, rather than Cu2+(C18H37PO3H-)2, based on headgroup spacing (MMA from the p-A isotherm is ca. 37 Å2 for the complex (Fig. 2.3). The experimental p-A isotherm data at the pH and [Cu2+] values that exhibit both liquid and solid phase behavior reflect the relative fraction of the free (C18H37PO3H2) and complexed (Cu(C18H37PO3H-))+ components of the monolayer and from this information we can determine K1. We have performed XPS measurements on the deposited monolayers and find that the Cu:P stoichiometry varies in the expected manner, from ca. 1:4 at high pH to ca. 1:10 at low pH (Table 2.1). The XPS data are shown in Figs. 2.7-2.11. Figure 2.7. XPS spectrum of Sample 1. 33 Figure 2.8. XPS spectrum of Sample 2. Figure 2.9. XPS spectrum of Sample 3. 34 Figure 2.10. XPS spectrum of Sample 4. Figure 2.11. XPS spectrum of Sample 5. 35 Table 2.1. Relative Intensities of the Copper and Phosphorus XPS Signals for LB monolayers deposited on silica at the pressure and subphase pH values indicated. For all measurements [Cu2+] in the subphase was 10-4 M. The signal intensities shown are corrected and reflect relative abundances. Sample Pressure pH Cu2p P2p (mN/m) 1 30 5.43 0.52 1.97 2 30 4.07 0.62 1.90 3 30 3.75 0.26 2.77 4 63 4.07 0.57 2.42 5 63 3.75 0.41 2.60 From the adsorption isotherm data we estimate the fraction of monolayer that is C18H37PO3H2 and the fraction that is (Cu2+(C18H37PO3H-))+. We also know the pH and [Cu2+] in the subphase. With these data we can estimate the association constant for (Cu2+(C18H37PO3H-))+. For the formed monolayer, XA is the mole fraction of the monolayer composed of C18H37PO3H2 and XC is the mole fraction of the monolayer composed of and (Cu2+(C18H37PO3H-))+, and XA + XC =1. Cast in these terms, the isotherm data provide an estimate of XA/XC. For a given pH the fractional amount of each form of ODPA is given by the a-fractions, and for all the equilibria relevant to the monolayer formation, a2 ~ 0 and a0 + a1 ~ 1. With these definitions, XA = a0, and [(Cu 2+ (C18 H 37 PO3 H - ))+ ] XC K1 = » [2.8] 2+ [Cu ][C18 H 37 PO3 H ] - [Cu 2+ ]a1 The a-term in Eq. 2.8 contains terms with K1 and K2, both of which are not known. While the estimation of either term involves substantial uncertainty, it is known that the ratio of K1/K2 is ca. 50 based on Bjerrum theory and electrostatic considerations for a planar charged surface.49-51 With 36 this information, the term containing K2 in a1 is sufficiently small that it does not contribute and can be ignored. With this approximation, we can estimate K1 (Table 2.2). Table 2.2. Values of K1 extracted from p-A isotherm data. The [Cu2+] is controlled experimentally, the [H+] indicated in the second column is that at which the mixed p-A isotherm is seen for the given [Cu2+] and XA/XC is determined by from the p-A isotherm data. [Cu2+] (M) [H+] (M) XA/XC KCu (M-1) 1x10-3 2.82x10-4 0.17 6.65x103 5x10-4 1.78x10-4 0.40 5.36x103 1x10-4 8.51x10-5 0.56 1.85x104 The average value of K1 is (1.02 ± 0.72)x104 M-1. We note that these values of K1 were determined with bulk solution phase values of Ka1 and Ka2 for ODPA (2.5x10-3 and 6.3x10-9, respectively), and these values are expected to be slightly different for a nominally planar monolayer of ODPA.52-53 The value of K1 determined from the p-A isotherm data is an association equilibrium constant. We can calculate the free energy of the formation reaction through DG = -RTlnKeq. Based on the values in Table 1, DG = -22.5 ± 2.1 kJ/mol for the formation of the Cu-ODPA monolayer complex. We note that this is a relatively modest driving force for the reaction but its value is consistent with the observed pH dependence. When the ODPA is fully protonated, the Cu2+ complexation process is not sufficiently favorable from an energetic standpoint to displace a proton from each of two ODPA headgroups. The literature on the formation of Cu2+-phosphonates is relatively sparse, but one report of the complexation of Cu2+ with ethane-1-hydroxy-1,1-diphosphonic acid (EDPA) does report formation constants in aqueous solution.54 Their value of for the formation of the complex 37 analogous to the one we report is 6.3x104, corresponding to DG = -26.9 kJ/mol. This value is in good agreement with the result we report here. 2.4 Conclusions Our work with the Cu2+-ODPA LB system has shown that the coordination of Cu2+ is similar in functional form to that reported for other metal ions. We have reported a relatively simple means of evaluating the equilibrium constant for the metal ion complexation in LB monolayers. The equilibrium constant information is useful for making more comparisons to other LB monolayers and it provides a means of evaluating the free energy of complexation. As noted above, this work is a first step in the exploration of the relationship between metal ion oxidation state and LB monolayer organization. In particular, understanding the energetics of metal ion complexation for metal ions in different oxidation states will serve as a guide in determining the feasibility of controlling monolayer morphology in situ. 38 REFERENCES 1. Blodgett, K. B., Films built by depositing successive monomolecular layers on a solid surface. J. Am. Chem. Soc. 1935, 57, 1007-1022. 2. Blodgett, K. B.; Langmuir, I., Built-up films of barium stearate and their optical properties. Phys. Rev. 1937, 51, 964-982. 3. Langmuir, I., The constitution and fundamental properties of solids and liquids. II. Liquids. J. Am. Chem. Soc. 1917, 39 (9), 1848-1906. 4. Das, N. M.; Roy, D.; Gupta, M.; Gupta, P. S., Structural and surface morphological studies of long chain fatty acid thin films deposited by Langmuir-Blodgett technique. Physica A 2012, 407, 4777-4782. 5. Kurnaz, M. L.; Schwartz, D. K., Morphology and microphase separation in arachidic acid/cadmium arachidate Langmuir-Blodgett multilayers. J. Phys. Chem. 1996, 100, 11113- 11119. 6. Roy, D.; Das, N. M.; Gupta, P. S., Structural morphology study of Cd2+ induced Langmuir Blodgett multilayer films of arachidic acid. Appl. Surf. Sci. 2013, 271, 394-401. 7. Dynarowicz-Latka, P.; Dhanabalan, A.; Oliviera, O. N. J., A Study on Two-Dimensional Phase Transitions in Langmuir Monolayers of a Carboxylic Acid with a Symmetrical Triphenylbenzene Ring System. J. Phys. Chem. B 1999, 103, 5992-6000. 8. Ganguly, P.; Paranjape, D. V.; Sastry, M.; Chaudhari, S. K.; Patil, K. R., Deposition of Yttrium Ions in Langmuir-Blodgett Films Using Arachidic Acid. Langmuir 1993, 9, 487-490. 9. Huang, X.; Jiang, S.; Liu, M., Metal Ion Modulated ORganization and Function of the Langmuir-Blodgett Films of Amphiphilic Diacetylene: Photopolymerization, Thermochromism and Supramolecular Chirality. J. Phys. Chem. B 2005, 109, 114-119. 10. Linden, D. J. M.; Peltonen, J. P. K.; Rosenholm, J. B., Adsorption of Some Multivalent Transition-Metal Ions to a Stearic Acid Monolayer. Langmuir 1994, 10, 1592-1595. 11. Linden, M.; Rosenholm, J. B., Influence of Multivalent Metal Ions on the Monolayer and Multilayer Properties of Some Unsaturated Fatty Acids. Langmuir 1995, 11, 4499-4504. 12. Zasadinski, J. A.; Viswanthan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K., Langmuir- Blodgett Films. Science 1994, 263, 1726-1733. 13. Baumler, S. M.; Blanchard, G. J., The Influence of Metal Ions on the Dynamics of Supported Phospholipid Langmuir Films. Langmuir 2017, 33, 2986-2992. 14. Baumler, S. M.; McHale, A. M.; Blanchard, G. J., Surface Charge and Overlayer pH Influence the Dynamics of Supported Phospholipid Films. J. Electroanal. Chem. 2018, 812, 159- 165. 39 15. Capistran, B. A.; Blanchard, G. J., Effects of Cu(II) on the Formation and Orientation of an Arachidic Acid Langmuir-Blodgett Film. Langmuir 2019, 35, 3346-3353. 16. Capistran, B. A.; Blanchard, G. J., Spectroscopic Analysis of Cu(II)-Complexed Thin Films to Characterize Molecular-Level Interactions and Film Behavior. Langmuir 2021, 37, 5089-5097. 17. Livingston, C.; Blanchard, G. J., Metal Ion Dependent Interfacial Organization and Dynamics of Metal Phosphonate Monolayers. Langmuir 2021, 37, 4658-4665. 18. Livingston, C.; Blanchard, G. J., Translational Diffusion Dynamics in Divalent Metal Phosphonate Monolayers. Langmuir 2021, 37, 7573-7581. 19. Byrd, H.; Pike, J. K.; Showalter, M. L.; Whipps, S.; Talham, D. R., Langmuir-Blodgett Monolayers As Templates For the Self-Assembly of Zirconium Organophosphonate Films. In Interfacial Design and Chemical Sensing, 1994; Vol. 561, pp 49-59. 20. Byrd, H.; Pike, J. K.; Talham, D. R., Inorganic Monolayers Formed At an Organic Template - a Langmuir-Blodgett Route to Monolayer and Multilayer Films of Zirconium Octadecylphosphonate. Chem. Mat. 1993, 5 (5), 709-715. 21. Byrd, H.; Pike, J. K.; Talham, D. R., Extended-Lattice Langmuir-Blodgett-Films - Manganese Octadecylphosphonate Langmuir-Blodgett-Films Are Structural and Magnetic Analogs of Solid-State Manganese Phosphonates. Journal of the American Chemical Society 1994, 116 (17), 7903-7904. 22. Byrd, H.; Pike, J. K.; Talham, D. R., Single Layers of Inorganic Extended Lattices Formed At Langmuir-Blodgett Templates. Thin Solid Films 1994, 242 (1-2), 100-105. 23. Byrd, H.; Pike, J. K.; Talham, D. R., Langmuir-Blodgett-Films As Single-Layer Analogs of Known Organic-Inorganic Solid-State Materials. Synth. Met. 1995, 71 (1-3), 1977-1980. 24. Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J. F.; Nagler, S. E.; Talham, D. R., Role of the Template Layer in Organizing Self-Assembled Films - Zirconium Phosphonate Monolayers and Multilayers At a Langmuir- Blodgett Template. Journal of the American Chemical Society 1994, 116 (1), 295-301. 25. Byrd, H.; Whipps, S.; Pike, J. K.; Talham, D. R., Molecular Self-Assembly At a Preformed Langmuir-Blodgett Template. Thin Solid Films 1994, 244 (1-2), 768-771. 26. Fanucci, G. E.; Bowers, C. R.; Talham, D. R., Application of solid-state P-31 NMR to the study of Langmuir- Blodgett films. Journal of the American Chemical Society 1999, 121 (5), 1088-1089. 27. Fanucci, G. E.; Seip, C. T.; Petruska, M. A.; Nixon, C. M.; Ravaine, S.; Talham, D. R., Organic/inorganic Langmuir-Blodgett films based on known layered solids: divalent and trivalent metal phosphonates. Thin Solid Films 1998, 329, 331-335. 40 28. Fanucci, G. E.; Talham, D. R., Langmuir-Blodgett films based on known layered solids: Lanthanide(III) octadecylphosphonate LB films. Langmuir 1999, 15 (9), 3289-3295. 29. Nixon, C. N.; Le Claire, K.; Odobel, F.; Bujoli, B.; Talham, D. R., Palladium porphyrin containing zirconium phosphonate Langmuir- Blodgett films. Chem. Mat. 1999, 11 (4), 965-976. 30. Petruska, M. A.; Fanucci, G. E.; Talham, D. R., Organic/inorganic Langmuir-Blodgett films based on metal phosphonates 2: zirconium phosphonate-based alternating layer films. Thin Solid Films 1998, 329, 131-135. 31. Petruska, M. A.; Fanucci, G. E.; Talham, D. R., Organic/inorganic Langmuir-Blodgett films based on metal phosphonates: Preparation and characterization of phenoxy- and biphenoxy-substituted zirconium phosphonate films. Chem. Mat. 1998, 10 (1), 177-189. 32. Petruska, M. A.; Talham, D. R., Organic/inorganic Langmuir-Blodgett films based on metal phosphonates. 3. An azobenzene-derivatized phosphonic acid forms continuous lattice layers with divalent, trivalent, and tetravalent metal ions. Chem. Mat. 1998, 10 (11), 3672-3682. 33. Pike, J. K.; Byrd, H.; Morrone, A. A.; Talham, D. R., Template-Directed Synthesis - Oriented Cdi2 Prepared in a Langmuir-Blodgett-Film. Journal of the American Chemical Society 1993, 115 (18), 8497-8498. 34. Pike, J. K.; Byrd, H.; Morrone, A. A.; Talham, D. R., Oriented Cadmium Dihalide Particles Prepared in Langmuir- Blodgett-Films. Chem. Mat. 1994, 6 (10), 1757-1765. 35. Pike, J. K.; Byrd, H.; Talham, D. R., X-Ray Photoelectron-Spectroscopy, Attenuated Total Reflectance Fourier-Transform Ir Spectroscopy and Transmission Electron- Diffraction Studies of Oriented Cadmium Iodide Prepared in a Langmuir-Blodgett Template. Thin Solid Films 1994, 243 (1-2), 510-514. 36. Ravaine, S.; Fanucci, G. E.; Seip, C. T.; Adair, J. H.; Talham, D. R., Photochemical generation of gold nanoparticles in Langmuir- Blodgett films. Langmuir 1998, 14 (3), 708-713. 37. Seip, C. T.; Byrd, H.; Talham, D. R., Electron paramagnetic resonance study of a Langmuir-Blodgett film of manganese octadecylphosphonate and comparison of the magnetic properties to those of solid-state manganese alkylphosphonates. Inorg. Chem. 1996, 35 (12), 3479-3483. 38. Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R., Langmuir-Blodgett films of known layered solids: Preparation and structural properties of octadecylphosphonate bilayers with divalent metals and characterization of a magnetic Langmuir- Blodgett film. Journal of the American Chemical Society 1997, 119 (30), 7084-7094. 39. Seip, C. T.; Talham, D. R., Organic inorganic Langmuir-Blodgett films based on known layered solids: Characterization and reaction of cobalt octadecylphosphonate. Mater. Res. Bull. 1999, 34 (3), 437-445. 41 40. Talham, D. R.; Seip, C. T.; Whipps, S.; Fanucci, G. E.; Petruska, M. A.; Byrd, H., Incorporating inorganic extended lattice structures into Langmuir-Blodgett films: Comparing metal phosphonate LB films to their solid-state analogs. Comments Inorganic Chem. 1997, 19 (3), 133-151. 41. Whipps, S.; Khan, S. R.; O'Palko, F. J.; Backov, R.; Talham, D. R., Growth of calcium oxalate monohydrate at phospholipid Langmuir monolayers. J. Cryst. Growth 1998, 192 (1-2), 243-249. 42. Zhao, W.; Chang, T.; Leygraf, C.; Johnson, C. M., Corrosion Inhibition of Copper with Octadecyl Phosphonic Acid (ODPA) in a Simulated Indoor Atmospheric Environment. Corr. Sci. 2021, 192, 109777 (1-10). 43. Zhao, W.; Gothelid, M.; Hosseinpour, S.; Johansson, M. B.; Li, G.; Leygraf, C.; Johnson, C. M., The Nature of Self-Assembled Octadecylphosphonic Acid (ODPA) Layers on Copper Substrates. J. Coll. Surf. Int. Sci. 2021, 581, 816-825. 44. Hussain, S. A.; Dey, B.; Bhattacharjee, D.; Mehta, N., Unique supramolecular assembly through Langmuir – Blodgett (LB) technique. Heliyon 2018, 4 (12), e01038. 45. Ulman, A., An Introduction to Ultrathin Organic Films: From Langmuir--Blodgett to Self--Assembly. Academic press: 2013. 46. Fanucci, G. E.; Petruska, M. A.; Meisel, M. W.; Talham, D. R., Structural Characterization and Magnetic Order in Phenoxy-Substituted Divalent Metal Phosphonate Langmuir-Blodgett Films. J. Sol. St. Chem. 1999, 145, 443-451. 47. Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W., Stability of insoluble monolayers and ionization of Langmuir-Blodgett multilayers of octadecanoic acid. Thin Solid Films 1990, 188 (2), 361-373. 48. Davies, J. T.; Rideal, E. K., Interfacial Phenomena, 2nd Ed. Academic Press: New York, 1963. 49. Wada, H.; Fernando, Q., Determination of Formation Constants of Copper(II) Complexes of Ethane-1-Hydroxy-1,1,-Diphosphonic Acid with a Solid State Cupric Ion-Selective Electrode. Anal. Chem. 1971, 43, 751-755. 42 CHAPTER III: Permeability and Dynamics of a Monolayer are Mediated by ITO Support Surface-Modification (under review- submission date: April 2023). 43 3.1 Introduction Indium tin oxide (ITO) is a widely used semiconductor because of its combined properties of optical transparency in the visible region of the spectrum and its relatively high conductivity. We have used ITO as a support for interfacial monolayer growth and for control of surface charge in work with room temperature ionic liquids. In some instances, we have applied surface modification chemistry to facilitate either covalent or ionic bonding of molecules to the ITO surface. Indeed, there is a rich literature on the surface modification of ITO.1-17 Empirically, we have found that the chemical reactions that have been used to modify silica surfaces can also be used to modify ITO surfaces, but gaining a detailed understanding of the ITO surface remains to be achieved. Silica and ITO are distinctly different surfaces in terms of chemical functionality. Silica surfaces are known to possess silanol groups with a density of ca. 5 µmol/m2, and with a pKa for one subset of those groups of ca. 4.5. At neutral pH, the silica surface carries a net negative charge. ITO, in contrast, is known to exhibit a net positive surface charge for pH values below 6,18 and surface hydroxyl functionality on ITO surfaces is seen only after oxygen plasma treatment.1 Despite these clear differences, ITO that has not undergone oxygen plasma treatment reacts with POCl3 in the presence of a Lewis base to produce a surface containing phosphonate functionalities.19-20 Even though the surface reaction chemistry is similar for these two different surfaces, the question remains as to how the properties of the resulting interfaces differ. Making a direct electrochemical comparison between ITO and silica is not possible because ITO is conductive while silica is a dielectric material. Much is known, however, about the silica surface, including that its morphology is extremely complex and the distribution of surface silanol groups is not homogeneous. Rather, the surface silica functionality is seen to exist in “islands” of ca. 15 44 nm diameter with narrow regions of lower silanol density between the islands.21 Similar morphology has not been observed, to our knowledge, for ITO. The aim of this work is to evaluate the properties of surface monolayers deposited onto ITO, prior to and following its modification with POCl3 in the presence of a Lewis base to produce surface phosphonate functionality. The amphiphile used to form the monolayers is octadecyl phosphonic acid (ODPA), performed on a Langmuir trough and deposited using the Langmuir-Blodgett (LB) technique.22-24 We construct a series of monolayer structures deposited onto native and modified silica and ITO, and in the presence and absence of Cd2+ in the aqueous subphase of the Langmuir trough. The interfaces have been evaluated optically (ITO and silica) and electrochemically (ITO only). To gauge the permeability of the interfaces, we used ferrocene and Ru3+ to determine the extent to which the monolayers on ITO were penetrable by the electrophores. Our findings revealed that when a Cd2+- bisphosphonate linkage was formed to bind the ODPA monolayer to the modified surface, Faradaic current was not observed for either electrophore. Further investigation of the extent of organization within the resulting monolayer interface was performed by incorporating perylene into the Langmuir monolayer during formation and evaluating the diffusional properties of the chromophore within the interface following LB deposition onto the modified ITO interface. Our data demonstrate that the aliphatic region of the interface exists as a comparatively viscous fluid, with chromophore diffusional motion being mediated by the inter-chain interactions, organization within the monolayer, and the mobility of the amphiphile monolayer constituents. 3.2 Methods 3.2.1 Materials: Octadecyl phosphonic acid (ODPA), CdCl2, RuCl3, ferrocene (Fc), tetrabutylammonium hexafluorophosphate (TBAPF6), acetonitrile, 2,4,6-collidine, POCl3 and perylene, were obtained 45 from Sigma-Aldrich in their highest purity forms and used without further purification. All ODPA solutions used for monolayer deposition were prepared at a concentration of 1 mg/mL in tetrahydrofuran (THF). For the growth of Cd2+–ODPA monolayers, CdCl2 was dissolved in Milli- Q water at the desired concentration to constitute the aqueous subphase. Water (18 MΩ) from a Milli-Q filtration system was used for the subphase in all Langmuir-Blodgett monolayer growth and deposition cycles. Subphase pH was controlled with HCl (1 M, CCI, Inc.). 3.2.2 Monolayer deposition: Films were deposited on glass microscope cover slides that were coated with indium tin oxide (ITO) (#1, 22 mm × 22 mm, Alkali Scientific, Inc.). The resistance of the ITO films is 10 W/square. All glass substrates were cleaned by immersion in piranha solution (1H2O2:3H2SO4; caution: strong oxidizer!) for 10–15 min followed by rinsing with Milli-Q water until the pH of the rinse was ca. 7. Substrates were stored in Milli-Q water to minimize adventitious contamination by airborne organic compounds. For cleaning the ITO-coated supports, a beaker is filled with Milli Q water and detergent (Fisher Sparklin 1®), then the support is added and sonicated for 10 minutes. The beaker is then rinsed with Milli Q water until the detergent is removed, and then filled with only Milli Q water and sonicated for 10 minutes. Water is then decanted, and the beaker is filled with 2- propanol and sonicated for 15 minutes. The ITO-coated cover slide is then dried under a flowing stream of N2(g) to remove 2-propanol. The cleaned cover slide is then submersed (vertically) into the Langmuir trough. 3.2.3 Phosphonation reaction: ITO was modified by using a procedure reported previously.20 The clean surface was immediately phosphonated by immersion in 100 mM POCl3 and 100 mM 2,4,6-collidine in dry 46 acetonitrile for 1 h and then rinsed with anhydrous acetonitrile. Then the modified surface was stored under Milli-Q water. 3.2.4 Langmuir Film Formation: All Langmuir monolayers were formed using a Langmuir trough (KN 2003, KSV Nima, Biolin Scientific, Gothenburg, Sweden) equipped with a Brewster angle microscope (BAM) attachment (UltraBAM, Accurion, Göttingen, Germany) and a platinum Wilhelmy plate balance for measuring surface pressure. The aqueous subphase of the trough is temperature controlled, 20.0 ± 0.1 °C. The pH of the subphase was measured using a pH meter, with the pH being controlled through the addition of HCl or NaOH. The ITO/glass substrate is immersed in the trough prior to application of the monolayer deposition solution. Approximately 1 h was allotted for subphase equilibration, then 70 μL of the ODPA in THF spreading solution was deposited beneath the subphase surface by using a syringe, with care taken to ensure the initial surface pressure did not exceed 0.5 mN/m. Following a 20 min solvent evaporation period monolayer compression was initiated at a barrier speed of 5 mm/min. For monolayer deposition barrier compression stopped when the surface pressure reached the desired pressure 35 mN/m depending on the experiment. The barrier position was placed at the desired surface pressure for ca. 10–15 min to allow for monolayer equilibration. Film deposition was performed by vertical removal of the immersed substrate from the subphase at a rate of 2 mm/min. Following complete removal from the subphase, the substrate was maintained above the trough for ca. 30 min to allow any residual water to evaporate. All experiments were controlled by using the KSV NIMA LB software (v2.2, Biolin Scientific). The Brewster angle microscope was controlled with the UltraBAM 1.1.2 software (Accurion) and angle of incidence was set to 53.2° (qB for H2O). Film formation images were acquired using Accurion Image 1.1.3 software (Accurion). 47 3.2.5 Cyclic voltammetry studies: A 25mL solution of TBAPF6 (0.1 M) in acetonitrile was prepared and that solution was used to prepare a 1 mM solution of ferrocene or 1 mM of Ru3+. All the measurement were done using TBAPF6 as an electrolyte. The working electrode is 1.5 cm ITO, the reference electrode is Ag/AgCl 1M from CH Instruments, and the counter electrode is platinum wire. The scan rate was 100 mV/sec. and the windows varies based on the solution of studies. 3.2.6 Fluorescence Recovery after Photobleaching (FRAP): FRAP measurements are made using a Nikon Eclipse Ti-E inverted microscope is equipped with a confocal scanning system (Nikon Ti-S-CON). A 20× objective lens was used for all experiments. For perylene, the 405 nm diode laser was used for excitation, (Nikon C2-DU3, 400−700 nm). For all measurements, the initial image intensity was recorded for 1 min, bleached for 30 s, and the recovery was monitored for a minimum of 3 min. At least five spots across each plate were measured in this manner. 3.3 Results and Discussion Our long-term interest in complexed monolayer systems lies in the ability to modify the organization and/or fluid behavior of the monolayer reversibly through control over the oxidation state of the metal ion used in the formation of the monolayer. In the work reported here, we focus on the formation of metal-phosphonate and metal-bisphosphonate monolayers with Cd2+ as the metal ion. A precursor to being able to control monolayer morphology reversibly is that the organization, complexation chemistry and consequent properties of the monolayers are well understood. The use of Cd2+ offers an opportunity to evaluate the role of ITO surface modification on the properties of the resulting monolayers. We consider first the details of monolayer formation on the Langmuir trough as a function of aqueous subphase pH. We then probe the permeability of LB monolayers deposited on unreacted ITO and on phosphonated ITO using two 48 electrochemical probes, Ru3+ and ferrocene (Fc). With this information in place, we then consider the fluid properties of the monolayer on both native and modified ITO supports. 3.3.1 Relevant equilibria: The amphiphile used in the formation of the Langmuir monolayers is octadecylphosphonic acid, ODPA, and we use Cd2+ to form complexes with the monolayer at the interface. Modeling the complexation requires consideration of the extent to which the phosphonic acid head groups are dissociated, and the extent to which the Cd2+ forms complexes with partially and fully dissociated phosphonic acid headgroups. While there is limited direct information on the formation constants for this specific system, there is a model compound that can be used to approximate the complexation(s) of interest. The model system is hydroxyethyldiphosphonic acid (HEDP, Fig. 3.1). Figure 3.1. Structures of HEDP as a function of protonation (left) and complexation with Cd2+ (right). There exist data for the complexation of HEDP in its several deprotonated states. While the structure of HEDP is not the same as that expected for the complexed monolayer, it does possess 49 a structure that is closer to that of a plane of phosphonates than bulk solution phase complexation of Cd2+. With that caveat in mind, we consider the extent of protonation of HEDP as a function of pH as well as the extent of complexation of the various forms of HEDP with Cd2+. For the purposes of this discussion, we use designate the various forms of HEDP as H4L0, H3L-, H2L-2, HL-3, and L-4 (Eq. 3.1), #!" #!# #!$ #!% 𝐻! 𝐿" $% 𝐻$ 𝐿% + 𝐻& $% 𝐻' 𝐿'% + 𝐻& $% 𝐻𝐿$% + 𝐻& $% 𝐿!% + 𝐻& [3.1] and Cd2+ can form complexes with each form of HEDP (Eq. 3.2). #& #" #!" #!" 𝐶𝑑 '& + 𝐿%! ↔ 𝐶𝑑𝐿%' + 𝐻& ↔ 𝐶𝑑𝐻𝐿% + 𝐻& $% 𝐶𝑑𝐻' 𝐿" + 𝐻& $% 𝐶𝑑𝐻$ 𝐿& [3.2] Based on these pH-dependent reactions, we can calculate the a-fractions for each form of HEDP (Eqs. 3.3-3.8), [3.3] [3.4] [3.5] [3.6] [3.7] [3.8] [3.9] and for each Cd-HEDP complex (Eqs. 3.10-3.15). [3.10] [3.11] 50 [3.12] [3.13] [3.14] [3.15] [3.16] We have not included a term for CdH4L2+ because it is not seen in the LB and BAM data (vide infra) to contribute, and due to the absence of information available on the formation constant for this species. We present the constants ki used in Eqs. 3.3-3.16 in Table 3.1. Table 3.1. acid dissociation and Cd-complexation constants used in Eqs. 3.3-3.16. ka1 1.59x10-2 k0 9.6x105 ka2 1.61x10-3 k1 5.0x108 ka3 1.0x10-7 k2 3.2x107 ka4 6.3x10-12 k3 3.2x104 The a-values for HEDP and the Cd-complexes is shown as a function of pH in Fig. 3.2a and 3.2b, respectively. With this information in mind, we consider the pH-dependence of the LB monolayer formation. 51 Figure 3.2. (a) acid a-fractions for HEDP as a function of pH, and (b) complexation a-fractions for Cd-HEDP complexation as a function of pH. 52 3.3.2 Langmuir monolayer formation and LB deposition: We have formed ODPA Langmuir monolayers and have examined their organization through their pressure-area (P-A) isotherms and Brewster angle microscope (BAM) images as a function of aqueous subphase pH and [Cd2+]. We present these data in Fig. 3.3. The data reveal a clear pH-dependent trend, which can be understood in the context of the a-fraction plots shown in Fig. 3.2 and the corresponding schematics in Fig. 3.3. These results are consistent with other reports on ODPA mono- and multilayer formation using Langmuir films.25-27 pH 5.9 Cd2+ Cd2+ Cd2+ pH 4 Cd2+ Cd2+ Cd2+ Cd2+ 2+ Figure 3.3. Schematic of ODPA Langmuir monolayer coordination of Cd in the vicinity of pH 5.9 (top) and 4.0 (bottom). The schematic is intended for illustrative purposes only and is not a calculated result. 53 Figure 3.4. Pressure-area (p-A) isotherms of Cd-ODPA Langmuir-Blodgett monolayers for three pH values of 0.1 mM of CdCl2 subphase. pH Values for each isotherm are as indicated in the legend. The isotherm data (Fig. 3.4) exhibit a progression in functional form that varies in a regular manner with pH. At pH 5.9 the isotherm exhibits a rigid island-like isotherm and in the pH range of 4.3 to 4.0 there is a transition toward progressively greater liquid to solid phase contribution with decreasing pH. We attribute this trend to pH-dependent changes in the extent of complexation between ODPA and Cd2+. The a-fraction plot (Fig. 3.2) shows that pH 5.9, the dominant form of the complex is between two mono-deprotonated ODPA molecules and one Cd2+ ion. In the region of pH 4, there are approximately equal amounts of complexation between two mono-deprotonated ODPA molecules and Cd2+, and one mono-deprotonated ODPA molecule and Cd2+. As the 54 ODPA:Cd2+ stoichiometry decreases based on headgroup charge, there is more opportunity for amphiphile interactions in the aliphatic chain region to contribute to monolayer structure, and at high pH values, such as 5.9, the Cd2+ interaction with two phosphonate functionalities dominates the spacing of the amphiphiles within the monolayer. The Brewster angle microscopy data (Fig.3.5) is consistent with this interpretation of the isotherm data. At pH 5.9, for all surface pressures a relatively amorphous image is seen, with progressively more order appearing with decreasing pH. For the monolayers formed in the pH range where the aliphatic chains contribute strongly to film organization, the order appears to be initiated even at low surface pressures, consistent with attractive aliphatic chain interactions. 55 Figure 3.5. Brewster angle microscopy images of the Cd-ODPA monolayers corresponding to three different pressures and at two pH values. The corresponding p-A isotherms for these images are shown in Figure 3.1. With the Langmuir monolayer formation established, the next stage in formation of the interfaces of interest is to deposit the monolayers onto solid supports by vertical withdrawal of the support from the Langmuir trough. We consider next the properties of the ODPA monolayers 56 formed under several different conditions, where the ITO-coated support is either in its native state or reacted with POCl3 and H2O to add surface phosphonate functionalities. Comparing the properties of ODPA monolayers complexed with Cd2+ present in the aqueous subphase deposited on native and phosphonated surfaces will provide insight into the role the Cd2+-bisphosphonate linkage plays in determining monolayer properties. We expect that the nature of the interfacial bond will be substantially different for the two surface modifications and have interrogated the differences in these monolayer systems both electrochemically and optically. 3.3.3 Electrochemical characterization: The issue of extent of coverage and defect density in monolayers can be examined electrochemically. We have used two electrochemical probes, Ru3+ and ferrocene (Fc), to evaluate their access to the ITO electrode surface. These two electrochemical probes were chosen because of their well-known electrochemical properties. All electrochemical experiments were performed in acetonitrile and used 0.1 M TBAPF6 as the electrolyte. Before considering the cyclic voltammetry (CV) of Ru3+ or Fc, we performed CV measurements on 0.1 M TBAPF6 in ACN (Fig. 3.6). The CVs exhibited Faradaic current only for solvent reduction, for all samples studied. The native ITO electrodes produced the highest currents, with the Cd2+-ODPA monolayer on native ITO showing an attenuated solvent reduction wave. The Cd2+-ODPA on phosphonated ITO exhibited very low non-Faradaic current, consistent with effective blockage of the electrode surface by the bound monolayer. No monolayer or electrode constituent produced a Faradaic response in this potential window. We consider next the CV data for Ru3+ and Fc. 57 Figure 3.6. bare ITO (black), phosphonated ITO (red), Cd-ODPA monolayer on ITO (blue), Cd-ODPA on phosphonated ITO (magenta) in 0.1 M TBAPF6 solution at scan rate= 500 mV/s. The CV data for Ru3+ are shown in Fig. 3.6 for the same electrode surfaces used for the TBAPF6 data. The CV of Ru2+/3+ has been examined extensively and is known to exhibit a number of redox waves associated with different Ru-oxide species.28-29 The data shown in Fig. 6 are characteristic of reduction and oxidation processes, and we assign the reduction feature at +0.40 V to reduction of Ru2+ and the reduction wave at +0.71 V to reduction of Ru3+. The data show that the largest currents are seen for the native ITO and phosphonated ITO electrodes. The Cd2+- ODPA monolayer on native ITO produce some that lower current, demonstrating that presence of the unbound monolayer does not prevent access of Ru3+ to the ITO electrode surface. The phosphonated ITO electrode with a Cd2+-ODPA monolayer produces lower current, suggesting significant coverage of the ITO surface by the surface modification. The deposition of a Cd2+- ODPA monolayer on phosphonated ITO reduces the current to the point where a Ru3+ reduction wave is barely discernible at a 100 mV/s scan rate but is seen clearly for a 500 mV/s scan rate. This result is due either to depletion of Ru3+ at the lower scan rate or disruption of the monolayer organization by the higher scan rates. The scan rate dependence of these data (Fig. 3.7) suggest the latter explanation is operative. 58 50 Ru3+ 100 mV/s current (µA) 0 -50 native ITO -100 phosphonated ITO Cd2+/ODPA on native ITO Cd2+/ODPA on phosphonated ITO -150 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 potential vs. Ag/AgCl (V) 100 Ru3+ 500 mV/s 50 current (µA) 0 -50 -100 -150 native ITO phosphonated ITO -200 Cd2+/ODPA on native ITO Cd2+/ODPA on phosphonated ITO -250 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 potential vs. Ag/AgCl (V) Figure 3.7. Cyclic voltammogram of the bare ITO (black), phosphonated bare ITO (red), Cd- ODPA monolayer on ITO (blue), Cd-ODPA on phosphonated ITO (magenta) in 1 mM RuCl3 (electrolyte: 0.1 M TBAPF6 solution.) Top: scan rate = 100mV/s, Bottom: scan rate = 500 mV/s. The CV data for Fc are shown in Fig. 3.8 Fc is known to be an ideal reversible redox couple under most conditions, but the change in oxidation and reduction potentials seen as a function of 59 ITO surface modification point to either a difference between the oxidized (charged) form and the reduced (neutral) form in terms of access to the electrode surface. Irreversibility has been seen before for Fc, and it has been understood in terms of the reaction of Fc with Cl-,30-32 and that is one possible contribution to the observed data. The presence of Cl- in our monolayers could be explained by the way the pH is controlled in the subphase with HCl or the phosphonation reaction of ITO involves the elimination of Cl- following initial reaction. For the Cd2+-ODPA monolayers, Cd can associate with Cl- until complexation, leaving displaced Cl- present in the monolayer, presumably in the region of the Cd-bisphosphonate functionality. Despite these possibilities, we have not observed Cl- in the XPS data on any of our support surfaces (data not shown), and the variation seen in the data are most likely due to surface modification of the ITO surface. The Fc CV data indicate that it is capable of penetrating the monolayer to an extent. For a scan rate of 100 mV/s, Fc Faradaic current is negligible, but it is seen clearly for a 500 mV/s scan rate. As for the Ru3+ data (Fig. 3.7), these results could be accounted for either by Fc depletion at low scan rates or monolayer disruption at higher scan rates (Fig. 3.8). The data appear to be more consistent with the latter explanation. 60 400 Fc 100 mV/s 200 current (µA) 0 native ITO phosphonated ITO Cd2+/ODPA on native ITO -200 Cd2+/ODPA on phosphonated ITO -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 potential vs. Ag/AgCl (V) 800 600 Fc 500 mV/s 400 current (µA) 200 0 -200 native ITO -400 phosphonated ITO Cd2+/ODPA on native ITO -600 Cd2+/ODPA on phosphonated ITO -800 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 potential vs. Ag/AgCl (V) Figure 3.8. Cyclic voltammogram of the bare ITO (black), phosphonated bare ITO (red), Cd- ODPA monolayer on ITO (blue), Cd-ODPA on phosphonated ITO (magenta) in 1 mM Ferrocene (electrolyte: 0.1 M TBAPF6 solution.) Top: scan rate = 100mV/s, Bottom: scan rate = 500 mV/s. 61 Of particular relevance to this work is the effective blockage of the ITO electrode for the Cd2+- ODPA monolayer deposited on the phosphonated ITO surface. This finding is consistent with strong monolayer bonding through the Cd-bisphosphonate functionality. The electrochemical data, taken collectively, point to the deposition of a Cd2+-ODPA monolayer on native ITO not being held in place by any substantial forces, and this is not surprising. The deposition of the Cd2+- ODPA monolayer on the phosphonated ITO surface results in relatively complete and robust coverage of the surface. Divalent metal ions are thought to form bisphosphonate complexes that are substantially more labile than those seen for Zr4+ or Hf4+,34-36 and in most cases M2+- bisphosphonates have been seen to be unstable in aqueous environments.37-40 Our findings, especially the effective blocking of access to the ITO electrode by the Cd2+-ODPA monolayer on phosphonated ITO, suggest that the monolayer is sufficiently fluid to “heal” defects in the monolayer. 3.3.4 Chromophore dynamics within the monolayers: In an effort to gauge the fluid nature of the Cd2+-ODPA monolayer, we have performed fluorescence recovery after photobleaching (FRAP) experiments, using perylene as a chromophore incorporated into the monolayer during its formation. FRAP measurements are used to characterize the diffusion constant of the perylene chromophore. The interpretation of FRAP data depends on the nature of the interactions between the chromophore and its environment, and for the system we consider here, there is no opportunity for the chromophore to bond to either the monolayer constituents or to the support surface. Using the free-diffusion model, we fit the FRAP recovery curves to established models.35-36 The translational diffusion constant is related to the thermal energy in the system (kBT), the size of the diffusing species (r), and the viscosity of the 62 monolayer medium (h). We extract DT from the FRAP data and from that information we determine h using Eq. 3.17, [3.17] where T = 293 K and the hydrodynamic volume of perylene is 225 Å3, yielding a value of r = 3.8 Å. We summarize the values for DT and h in Table 2 as a function of support surface chemistry and Langmuir trough subphase pH (subphase [Cd2+] = 0.1 mM). There are several pieces of useful information contained in these data. Considering the values of DT extracted from the experimental FRAP data (Figs. 3.9-3.11), there is a clear difference in the values for the LB monolayer deposited on native glass, phosphonated glass, and ITO. Specifically, the value of DT for perylene, which is contained in the aliphatic chain region of the monolayer, are relatively high and pH-independent for the monolayer deposited on a glass support. When the LB monolayer is deposited on phosphonated glass, DT for perylene is seen to be the same as that on glass at pH 4.1, but for monolayers deposited from subphases with pH 4.3 and 5.9, the measured value of DT for perylene is smaller by a factor of ca. 3. We understand these values as indicating that the measured diffusion of the embedded perylene chromophore is mediated both by the ability of the chromophore to move within the non-polar region of the monolayer, and also on the strength of interactions between the amphiphile monolayer and the support. In other words, it is the mobility of both the chromophore and amphiphile that contribute to the observed DT for the native glass support and on the phosphonated support at pH 4.1. For pH values of 4.3 and 5.9, DT for perylene becomes smaller because the mobility of the amphiphiles that comprise the monolayer has been greatly diminished by the formation of a Cd2+-bisphosphonate linkage. Interestingly, the values for perylene DT on native ITO and phosphonated ITO are the same to within the experimental uncertainty and are reflective of the amphiphiles exhibiting very limited mobility. These data 63 imply that the Cd2+ is coordinating strongly to surface functionality on both the native and modified ITO supports. For the native ITO support it is not clear what the nature of the coordination is, and this question remains to be investigated more thoroughly. Figure 3.9. Plot of time (s) vs relative intensity at three different conditions. a) monolayer formed on native glass at pH:4.1 on subphase containing 0.1 mM of CdCl2, b) same condition but pH:4.3, c) pH:5. These data were obtained from a single FRAP experiment with perylene in a monolayer. 64 Figure 3.10. Plot of time (s) vs relative intensity at three different conditions. a) monolayer formed on phosphonated glass at pH:4.1 on subphase containing 0.1 mM of CdCl2, b) same condition but pH:4.3, c) pH:5.9. These data were obtained from a single FRAP experiment with perylene in a monolayer. 65 Figure 3.11. Plot of time (s) vs relative intensity at three different conditions. a) monolayer formed at pH:4.2 on subphase containing 0.1 mM of CdCl2, deposited on phosphonated ITO. b) same condition but pH:4.2, deposited on native ITO. These data were obtained from a single FRAP experiment with perylene in a monolayer. It is instructive to view these results from the perspective of the extracted viscosity values (Table 3.2). On native glass at all pH values examined and for phosphonated glass at pH 4.1, the apparent viscosity of the LB monolayer is seen to be ca. 750 cP. For the phosphonated glass supports at pH values of 4.3 and 4.9, the monolayer viscosity is ca. 2200 cP, and for both the native and phosphonated ITO surfaces we recover a monolayer viscosity of ca. 3100 cP. By way of comparison, the viscosity of glycerol is ca. 1400 cP at room temperature. The measured perylene diffusion constants (and thus the calculated viscosities) depend on the mobility of both the chromophore and the amphiphiles that form the monolayer. 66 Table 3.2. Values of DT and h for the LB-monolayer derived from experimental FRAP data as function of support surface. For all measurements reported in this Table, [Cd2+] = 0.1 mM in the Langmuir trough aqueous subphase. Support Surface Diffusion constant DT (μm2/s ) Viscosity η (cP) Native glass pH:4.1 0.65±0.03 870±40 Native glass pH:4.3 0.86±0.29 710±240 Native glass pH:5 0.76±0.05 750±50 Phosphonated glass pH:4.1 0.78±0.25 770±260 Phosphonated glass pH:4.3 0.28±0.09 2120±690 Phosphonated glass pH:5.9 0.25±0.04 2310±390 Native ITO pH:4.2 0.19±0.01 3080±10 Phosphonated ITO pH:4.2 0.18±0.02 3160±260 It is important to compare the results of the electrochemical characterization to those of the FRAP characterization. For the data shown in Figs. 3.5-3.7, the Cd2+-ODPA monolayer on native ITO produces a higher Faradaic current than for the Cd2+-ODPA monolayer on the phosphonated ITO support. These data demonstrate more ready access to the ITO support (electrode) for the native ITO support, but the FRAP data produce identical FRAP results, with small DT for perylene in the amphiphile aliphatic chain region. These two findings, taken collectively, demonstrate that there is a bonding interaction between the native ITO surface functionalities and Cd2+ that renders the amphiphile immobile to an extent similar to that seen for the same monolayer bonded to the phosphonated ITO support. The difference between the two monolayers must lie with the density of the monolayer coverage. For the native ITO support there appears to be more access to the ITO electrode than for the phosphonated ITO support. Perylene diffusion senses the deposited monolayer regions but not the extent of monolayer coverage on the support while the electrochemical data sense the extent of monolayer coverage. 67 3.4 Conclusions We have demonstrated control over the pressure-area isotherms for ODPA Langmuir monolayers through control over the pH and [Cd2+] of the aqueous subphase, and we understand the distribution of species contained in the resulting Cd2+-complexed monolayer through the a- fractions of the multiple species present in this system. BAM data of these monolayers support the importance of Cd2+ complexation on the formation of the Langmuir monolayer. When these monolayers are deposited on silica and ITO supports, the extent and nature of the surface coverage is seen to depend on whether the monolayers are deposited on native of phosphonated supports, and whether Cd2+ is absent or present in the deposited monolayer assemblies. Interestingly, comparison of the electrochemical characterization of the monolayers deposited on the ITO supports show that the phosphonated ITO support produces monolayers that provide very limited access to the ITO electrode, while deposition of the same monolayer on the native ITO support leads to greater access to the ITO electrode. FRAP data for perylene contained in the aliphatic chain regions of these same monolayers shows the aliphatic chain regions of the monolayers to be essentially identical, suggesting a strong bonding of the amphiphiles to the ITO surface through Cd2+. The specific functionalities to which the Cd2+ binds on the native ITO surface remains to be identified, but these data demonstrate chemical control over monolayer properties. It is thus likely that we will be able to establish electrochemical control over monolayer morphology through the oxidation state of the metal ion used. 68 REFERENCES 1. Wu, C. C.; Wu, C. I.; Sturm, J. C.; Kahn, A., Surface modification of indium tin oxide by plasma treatment: An effective method to improve the efficiency, brightness, and reliability of organic light emitting devices. Appl. Phys. Lett. 1997, 70, 1348-1350. 2. Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P., Controlling charge injection in organic electronic devices using self-assembled monolayers. Appl. Phys. Lett. 1997, 71, 3528-3530. 3. Morgado, J.; Charas, A.; Barbagallo, N., Reduction of the light-onset voltage of light- emitting diodes based on a soluble poly(p-phenylene vinylene) by grafting polar molecules onto indium–tin oxide. Appl. Phys. Lett. 2002, 81, 933-935. 4. Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D.-Y.; Cho, K., Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics. Appl. Phys. Lett. 2007, 91, 112111. 5. Hanson, E. L.; Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L., Advanced Surface Modification of Indium Tin Oxide for Improved Charge Injection in Organic Devices. J. Am. Chem. Soc. 2005, 127, 10058-10062. 6. Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L., Direct Measurement of Surface Complex Loading and Surface Dipole and Their Effect on Simple Device Behavior. J. Phys. Chem. B 2005, 109, 3966-3970. 7. Yu, S.-Y.; Chang, J.-H.; Wang, P.-S.; Wu, C.-I.; Tao, Y.-T., Effect of ITO Surface Modification on the OLED Device Lifetime. Langmuir 2014, 30, 7369-7376. 8. Ho, P. K. H.; Granström, M.; Friend, R. H.; Greenham, N. C., Ultrathin Self-Assembled Layers at the ITO Interface to Control Charge Injection and Electroluminescence Efficiency in Polymer Light-Emitting Diodes. Adv. Mater. 1998, 10, 769-774. 9. Cui, J.; Huang, Q.; Veinot, J. G. C.; Yan, H.; Marks, T. J., Interfacial Microstructure Function in Organic Light-Emitting Diodes: Assembled Tetraaryldiamine and Copper Phthalocyanine Interlayers. Adv. Mater. 2002, 14, 565-569. 10. Yan, H.; Huang, Q.; Cui, J.; Veinot, J. G. C.; Kern, M. M.; Marks, T. J., High-Brightness Blue Light-Emitting Polymer Diodes via Anode Modification Using a Self-Assembled Monolayer. Adv. Mater. 2003, 15, 835-838. 11. Appleyard, S. F. J.; Day, S. R.; Pickford, R. D.; Willis, M. R., Organic electroluminescent devices: enhanced carrier injection using SAM derivatized ITO electrodes. J. Mater. Chem. 2000, 10, 169-173. 12. Hatton, R. A.; Willis, M. R.; Chesters, M. A.; Rutten, F. J. M.; Briggs, D., Enhanced hole injection in organic light-emitting diodes using a SAM-derivatised ultra-thin gold anode supported on ITO glass. J. Mater. Chem. 2003, 13, 38-43. 69 13. Nüesch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L., Chemical potential shifts at organic device electrodes induced by grafted monolayers. Chem. Phys. Lett. 1998, 288, 861-867. 14. Appleyard, S. F. J.; Willis, M. R., Electroluminescence: enhanced injection using ITO electrodes coated with a self assembled monolayer. Opt. Mater. 1998, 9, 120-124. 15. Susarova, D. K.; Akkuratov, A. V.; Kukharenko, A. I.; Cholakh, S. O.; Kurmaev, E. Z.; Troshin, P. A., ITO Modification for Efficient Inverted Organic Solar Cells. Langmuir 2017, 33, 10118-10124. 16. Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R., Phosphonic Acid Modification of Indium−Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies. J. Phys. Chem. C 2008, 112, 7809- 7817. 17. Kim, D.; Lee, A. W. H.; Eastcott, J. I.; Gates, B. D., Modifying the Surface Properties of Indium Tin Oxide with Alcohol-Based Monolayers for Use in Organic Electronics. ACS Appl. Nano Mat. 2018, 1, 2237-2248. 18. Peiris, T. A. N.; Senthilarasu, S.; Wijayantha, K. G. U., Enhanced Performance of Flexible Dye-Sensitized Solar Cells: Electrodeposition of Mg(OH)2 on a Nanocrystalline TiO2 Electrode. J. Phys. Chem. C 2012, 116, 1211-1218. 19. Mazur, M.; Blanchard, G. J., Surface Immobilized Optical Probes: Pyrene Molecules Covalently Attached to Silica and Indium-Doped Tin Oxide. Bioelectrochem 2005, 66, 89-94. 20. Mazur, M.; Krysinski, P.; Michota-Kaminska, A.; Bukowska, J.; Rogalski, J.; Blanchard, G. J., Immobilization of laccase on gold, silver and indium tin oxide by zirconium-phosphonate- carboxylate (ZPC) coordination chemistry. Bioelectrochem 2007, 71, 15-22. 21. Horne, J. C.; Huang, Y.; Liu, G.-Y.; Blanchard, G. J., The Correspondence Between Layer Morphology and Intralayer Excitation Transport in Zirconium-Phosphonate Monolayers. J. Am. Chem. Soc. 1999, 121, 4419-4426. 22. Blodgett, K. B., Films built by depositing successive monomolecular layers on a solid surface. J. Am. Chem. Soc. 1935, 57, 1007-1022. 23. Blodgett, K. B.; Langmuir, I., Built-up films of barium stearate and their optical properties. Phys. Rev. 1937, 51, 964-982. 24. Langmuir, I., The constitution and fundamental properties of solids and liquids. II. Liquids. J. Am. Chem. Soc. 1917, 39, 1848-1906. 25. Fanucci, G. E.; Seip, C. T.; Petruska, M. A.; Nixon, C. M.; Ravaine, S.; Talham, D. R., Organic/inorganic Langmuir-Blodgett films based on known layered solids: Divalent and trivalent metal phosphonates. Thin Solid Films 1998, 327-329, 331-335. 70 26. Fanucci, G. E.; Talham, D. R., Langmuir-Blodgett films based on known layered solids: Lanthanide(III) octadecylphosphonate LB films. Langmuir 1999, 15, 3289-3295. 27. Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R., Langmuir-Blodgett films of known layered solids: Preparation and structural properties of octadecylphosphonate bilayers with divalent metals and characterization of a magnetic Langmuir-Blodgett film. J. Am. Chem. Soc. 1997, 119, 7084-7094. 28. Herath, H. N. K.; MacRae, A. L.; Ugrinov, A.; Morello, G. R.; Parent, A. R., Electrochemical properties of Ru polypyridyl phosphonates. Eur. J. Inorg. Chem. 2023, 26, e202200747. 29. van der Westhuizen, D.; von Eschwege, K. G.; Conradie, J., Electrochemical data of polypyridine complexes of Ru(II). Data in Brief 2019, 27, 104759. 30. Cuartero, M.; Acres, R. G.; Bradley, J.; Jarolimova, Z.; Wang, L.; Bakker, E.; Crespo, G. A.; De Marco, R., Electrochemical Mechanism of Ferrocene-Based Redox Molecules in Thin Film Membrane Electrodes. Electrochim. Acta 2017, 238, 357-367. 31. Chen, J.; Ikeda, O.; Aoki, K., Electrode reaction of ferrocene in a nitrobenzene+water emulsion. J. Electroanal. Chem. 2001, 496, 88-94. 32. Sohail, M.; De Marco, R.; Jarolímová, Z.; Pawlak, M.; Bakker, E.; He, N.; Latonen, R.- M.; Lindfors, T.; Bobacka, J., Transportation and Accumulation of Redox Active Species at the Buried Interfaces of Plasticized Membrane Electrodes. Langmuir 2015, 31, 10599-10609. 33. Toma, S.; Sebesta, R., Applications of Ferrocenium Salts in Organic Synthesis. Synthesis 2015, 47, 1683-1695. 34. Obrien, J. T.; Zeppenfeld, A. C.; Richmond, G. L.; Page, C. J., Fourier-Transform Infrared-Spectroscopy Studies of Hafnium Alkylbis(Phosphonate) Multilayers On Gold - Effects of Alkylbis(Phosphonate) Chain-Length, Substrate Roughness, and Surface Functionalization On Film Structure and Order. Langmuir 1994, 10, 4657-4663. 35. Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J., Variation of Layer Spacing in Self-Assembled Hafnium-1,10- Decanediylbis(Phosphonate) Multilayers As Determined By Ellipsometry and Grazing Angle X-Ray-Diffraction. J. Am. Chem. Soc. 1994, 116, 9158-9165. 36. Kohli, P.; Rini, M. C.; Major, J. S.; Blanchard, G. J., Elucidating the Balance between Metal Ion Complexation and Polymer Conformation in Maleimide-Vinyl Ether Polymer Multilayer Structures. J. Mater. Chem. 2001, 11, 2996-3001. 37. Feldheim, D. L.; Mallouk, T. E., Layer-by-layer assembly and intercalation reactions of iron(III) and iron(II) alkanebisphosphonates on gold surfaces. Chem. Comm. 1996, (22), 2591- 2592. 71 38. Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E., Synthesis and Structural Characterization of a Homologous Series of Divalent-Metal Phosphonates, Mii(O3pr).H2o and Mii(Ho3pr)2. Inorg. Chem. 1988, 27, 2781-2785. 39. Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E., Structural Studies of Some New Lamellar Magnesium, Manganese and Calcium Phosphonates. Solid St. Ion. 1988, 26, 63-69. 40. Cao, G.; Mallouk, T. E., Shape-Selective Intercalation Reactions of Layered Zinc and Cobalt Phosphonates. Inorg. Chem. 1991, 30, 1434-1438. 41. Soumpasis, D. M., Theoretical analysis of fluorescence photobleaching recovery experiments. Biophys. J. 1983, 41, 95-97. 42. Sprague, B. L.; Pego, R. L.; Stavreva, D. A.; McNally, J. G., Analysis of binding reactions by fluorescence recovery after photobleaching. Biophys. J. 2004, 86, 3473-3495. 72 CHAPTER IV: A Switchable Monolayer using a Ru (III)-ODPA Complex (currently under review). 73 4.1 Introduction As noted in previous chapters, there are many practical and fundamental reasons to study monolayers, including chemical separations, sensing, lithography and tribology,1 and the means by which the monolayers are formed on support surfaces can determine their organization and properties. In this dissertation, the focus is on Langmuir monolayers, with deposition of the monolayers being accomplished using Langmuir-Blodgett methodology.2-5 The structural motif used in this dissertation is the formation of amphiphile monolayers onto oxide support surfaces, where the amphiphile-support interactions are mediated by the presence of a metal ion. The motivation for using this type of monolayer is that, in principle, the organization of the monolayer can be controlled in situ through the oxidation state of the metal ion, allowing for a level of reversible structural control that has not been achieved to date. In Chapter 3, we used two electrochemical probes, Ru3+ and ferrocene (Fc), to evaluate how the permeability of formed monolayers changes with the interactions of the binding metal ion with the support surface. The redox chemistry of Ru3+ appeared to be well suited for use as a monolayer bonding constituent, and we have explored the use of Ru3+ in that capacity. Because Ru3+ is trivalent, its interactions with phosphonates is expected to be more energetically favorable than it is for divalent metal ions.6- 9 For the divalent metal ions reported in Chapters 2 and 3, we found that the extent of deprotonation of the octadecylphosphonic acid (ODPA) Langmuir monolayer was critically important in the formation of metal ion-containing monolayers because of the competition between metal-phosphonate complexation and phosphonate protonation. The formation of Ru3+-ODPA Langmuir monolayers does not exhibit the same pH-dependence that is seen for the divalent metal ions reported in Chapters 2 and 3. The pH-range over which Ru3+ can exist in solution without hydroxide formation that overlaps with the range over which phosphonate complexation can 74 proceed is narrow. In the vicinity of pH 3, the formation of the Ru3+-ODPA monolayers is facile, and we explore the permeability and fluidity of these monolayers in this Chapter. The ability to reversibly control the oxidation state or Ru3+ in situ relies on the formation of the Langmuir- Blodgett monolayer on a conductive support. We use Indium Tin Oxide (ITO) for this purpose and explore how the properties of the deposited monolayer change with Ru oxidation state. 4.2 Methods 4.2.1 Materials: Octadecylphosphonic acid (ODPA), RuCl3, tetrabutylammonium hexafluorophosphate (TBAPF6), acetonitrile, 2,4,6-collidine, POCl3 and perylene, were obtained from Sigma-Aldrich in their highest purity forms and used without further purification. All ODPA solutions used for monolayer deposition were prepared at a concentration of 1 mg/mL in tetrahydrofuran (THF). For the growth of Ru3+–ODPA monolayers, RuCl3 was dissolved in Milli-Q water at the desired concentration to constitute the aqueous subphase. Water (18 MΩ) from a Milli-Q filtration system was used for the subphase in all Langmuir-Blodgett monolayer growth and deposition cycles. 4.2.2 Monolayer deposition: Langmuir monolayers were deposited on glass microscope cover slides that were coated with indium tin oxide (ITO) (#1, 22 mm × 22 mm, Alkali Scientific, Inc.). The resistance of the ITO films is 10 W/square. For cleaning the ITO-coated supports, a beaker is filled with Milli Q water and detergent (Fisher Sparkleen 1®), then the support is added and sonicated for ten minutes. The beaker is rinsed with Milli Q water until the detergent is removed, and then filled with only Milli Q water and sonicated for ten minutes. Water is then decanted, and the beaker is filled with 2- propanol and sonicated for fifteen minutes. The ITO-coated cover slide is then dried under a 75 flowing stream of N2(g) to remove 2-propanol. After the phosphonation reaction, the cleaned cover slide is then immersed (vertically) into the Langmuir trough. 4.2.3 Phosphonation reaction: ITO was modified by using a procedure reported previously.10 The clean surface was immediately phosphonated by immersion in 100 mM POCl3 and 100 mM 2,4,6-collidine in dry acetonitrile for 1 h and then rinsed with anhydrous acetonitrile. The modified surface was stored under Milli-Q water. 4.2.4 Langmuir Film Formation: All Langmuir monolayers were formed using a Langmuir trough (KN 2003, KSV Nima, Biolin Scientific, Gothenburg, Sweden) equipped with a Brewster angle microscope (BAM) attachment (UltraBAM, Accurion, Göttingen, Germany) and a platinum Wilhelmy plate balance for measuring surface pressure. The aqueous subphase of the trough is temperature controlled, 20.0 ± 0.1 °C. The pH of the subphase was measured using a pH meter, and adjusted by the addition of HCl or NaOH. The ITO/glass substrate is immersed in the trough prior to application of the monolayer deposition solution. Approximately 1 h was allotted for subphase equilibration, then 70 μL of the ODPA in THF spreading solution was deposited beneath the subphase surface using a syringe, with care taken to ensure the initial surface pressure did not exceed 0.5 mN/m. Following a 20 min solvent evaporation period monolayer compression was initiated at a barrier speed of 5 mm/min. For monolayer deposition barrier compression stopped when the surface pressure reached the desired pressure (ca. 35 mN/m, depending on the experiment). The barrier position was located to achieve the desired surface pressure for ca. 10–15 min to allow for monolayer equilibration. Film deposition was performed by vertical removal of the immersed substrate from the subphase at a rate of 2 mm/min. Following complete removal from the 76 subphase, the substrate was maintained above the trough for ca. 30 min to allow any residual water to evaporate. All experiments were controlled with KSV NIMA LB software (v2.2, Biolin Scientific). The Brewster angle microscope was controlled with UltraBAM 1.1.2 software (Accurion) and the angle of incidence was set to 53.2° (qB for H2O). Film formation images were acquired using Accurion Image 1.1.3 software (Accurion). 4.2.5 Cyclic voltammetry studies: The instrument used for cyclic voltammetry (CV) experiments was a CH Instruments 650E electrochemical bench. A solution of TBAPF6 (0.1 M) in acetonitrile was used as the electrolyte solution for CV experiments. The working electrode was 1.5 cm x 1.5 cm ITO, either surface modified or not. The reference electrode was Ag/AgCl 1M from CH Instruments, and the counter electrode is platinum wire. The scan rates used were in the range of 100 to 500 mV/sec. and the potential range was from 0 to 1.5V vs. Ag/AgCl. 4.2.6 Fluorescence Recovery after Photobleaching (FRAP): FRAP measurements were made using a Nikon Eclipse Ti-E inverted microscope equipped with a confocal scanning system (Nikon Ti-S-CON). A 20× objective lens was used for all experiments. For perylene, the 405 nm diode laser was used for excitation, (Nikon C2-DU3, 400−700 nm). For all measurements, the initial image intensity was recorded for 1 min, bleached for 30 s, and the recovery was monitored for a minimum of 3 min. At least five spots across each plate were measured in this manner. 4.3 Results and Discussion In this Chapter, we focus on the formation of metal-phosphonate and metal-bisphosphonate monolayers with Ru3+ as the metal ion. This work is distinct from that in Chapter 3 because the complexation chemistry of Ru3+ with phosphonate(s) is significantly different than that reported 77 for Cd2+. Among the consequences of the different metal ions is that the morphology and fluidity of the monolayers formed using the two metal ions differs markedly. We first consider the properties of the ODPA monolayers formed using Ru3+ in the aqueous subphase, and then the dynamics of perylene contained in the formed monolayers. Ultimately, the difference in monolayer properties points to the ability to tailor monolayers to specific applications through the use of complexation chemistry. 4.3.1 Isotherm studies, BAM images and monolayer stoichiometry: In contrast to the pH-dependence seen for Cd2+-ODPA monolayer formation (Chapter 3), the formation of Ru3+-ODPA monolayers does not exhibit the same pH window over which the process can proceed. This is because of the comparatively narrow balance between Ru3+- phosphonate complexation and Ru3+ hydrous oxide formation. ODPA in the absence of Ru3+ in the aqueous subphase forms monolayers, and the resulting monolayers depend on the pH of the aqueous subphase. Ru3+ is capable of complexing with OH-, with the solubility of the resulting hydrous oxide(s) being low. The complex between Ru3+ and phosphonates is empirically observed to be favored in the vicinity of pH 3. BAM images show the formation of island-like structures at the ODPA interface immediately after injection and before monolayer compression. Monolayer 78 32 Figure 4.1. Adsorption isotherm plot of ODPA on subphase containing 0.1 mM RuCl3, and BAM images of the monolayer at surface pressures of ~0 mN/m and 30 mN/m. compression gives rise to the expected monolayer structure, with the P-A isotherm shown in Fig. 4.1. The data contained in the P-A provide information that can be used in concert with electrochemical data to estimate the stoichiometry of the resulting Ru3+-ODPA monolayer. As shown in Fig. 4.1, the mean area per molecule is extrapolated to be 32 Å2/molecule. From this information, the surface density is [4.1] 79 And cyclic voltammetry data on the electrode surface can be used to determine the metal ion surface density. For this electrochemical measurement, we use a phosphonated ITO working electrode that has had a Ru3+-ODPA monolayer deposited by LB methodology (Fig. 4.2). Figure 4.2. Cyclic voltammograms of phosphonated ITO (yellow) and Ru3+-ODPA monolayer deposited by LB methodology onto the phosphonated ITO electrode (purple). Electrolyte solution is 0.1M TBAPF6 in acetonitrile, 500 mV/s scan rate. From these data, we can estimate the density of Ru3+, [4.2] From these results, the ODPA:Ru stoichiometry is CODPA/CRu = 3.93 ~ 4. While it is tempting to assign a two-dimensional lattice-like structure based on this stoichiometry, we do not know spatial distribution of surface phosphonate functionality on the ITO surface, or the characteristic domain sizes in the monolayer, and coordination along grain boundaries could be significantly distorted. 4.3.2 Electrochemical characterization: With the monolayer stoichiometry understood, we next consider how changes to the monolayer affect its permeability. Cyclic voltammetry is well suited to evaluating monolayer permeability. 80 1.40E-04 1.20E-04 1.00E-04 8.00E-05 ) 6.00E-05 A ( t n er 4.00E-05 r u 2.00E-05 C 0.00E+00 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -2.00E-05 -4.00E-05 -6.00E-05 Potential (V) vs. Ag/AgCl 1M Figure 4.3. Cyclic voltammograms of 0.1 M TBAPF6 electrolyte solution in acetonitrile for native ITO (orange), phosphonated ITO (yellow), ODPA monolayer on phosphonated ITO (red), and Ru3+-ODPA monolayer on phosphonated ITO (green). All data were acquired at a scan rate of 500mV/s. To evaluate the system, the measurements are for a series of systems, where the data are recorded in a 0.1 M TBAPF6 solution in acetonitrile. The CVs shown in Fig. 4.3 are for the native electrode (orange), where the expected charging current is observed and there is no identifiable Faradaic current. The phosphonated ITO surface (yellow) likewise shows no Faradaic current and exhibits a smaller charging current than the native ITO electrode, consistent with the phosphonate modification limits the ability of solution phase ions to achieve closest approach to the electrode. The ODPA monolayer deposited on a phosphonated ITO surface (red) also produces a small charging (non-Faradaic) current and no detectable Faradaic current. The Ru3+-ODPA monolayer deposited on the phosphonated ITO support exhibits a significant Faradaic current for the Ru2+/Ru3+ redox couple. The fact that we observe a Faradaic signal for the Ru-bisphosphonate monolayer is important because it demonstrates at the Ru-ODPA monolayer on the phosphonated 81 ITO surface is permeable to the electrolyte, allowing Faradaic processes to proceed. In contrast, the ODPA monolayer on the phosphonated ITO support does not show Faradaic current, indicating that monolayer is not having any active species in the structure to do electrochemistry. The comparison between monolayer without metal ion, and monolayer containing metal ions, is indicative that the Ru(III) is only active species in CV. To verify that the Faradaic current shown in Fig. 4.3 is associated with Ru redox couple(s), we compare the results in Fig. 4.3 to the Faradaic current associated with solution phase Ru3+ in TBAPF6 (Fig. 4.4). Aside from a small shift of the Ru3+/Ru2+ to higher potentials, due to coordination of Ru3+ within the monolayer, the data demonstrate the Faradaic signal observed for 5.00E-04 4.00E-04 3.00E-04 ) 2.00E-04 A ( t n er 1.00E-04 r u C 0.00E+00 0 0.2 0.4 0.6 0.8 1 1.2 -1.00E-04 -2.00E-04 -3.00E-04 Potential (V) vs. Ag/AgCl 1M Figure 4.4. Cyclic voltammograms of 0.1 M TBAPF6 in acetonitrile at native ITO (orange), phosphonated ITO (yellow), a Ru3+-ODPA monolayer phosphonated ITO (green), 1 mM Ru3+ in 0.1 M TBAPF6 acetonitrile at native ITO (blue), and at phosphonated ITO (purple). All data were acquired at a 500 mV/s scan rate. the Ru3+-ODPA monolayer is associated with the Ru3+/Ru2+ redox couple. 82 The scan rate dependence of the CV data for Ru3+-ODPA on phosphonated ITO in 0.1 M TBAPF6 in acetonitrile provide insight into the system (Fig. 4.5). As expected, the Ru3+/Ru2+ redox couple can be seen at higher scan rates but is not resolvable at lower scan rates. The stability of the CV scans through five cycles at each scan rate indicates that the monolayer is stable to Ru3+/Ru2+ cycling, and, significantly, no shift is seen in the redox wave with scan rate, indicating facile electron transfer kinetics at the ITO electrode. 8.00E-04 6.00E-04 4.00E-04 ) (A t n er 2.00E-04 r u C 0.00E+00 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 -2.00E-04 -4.00E-04 Potential (V) vs. Ag/AgCl 1M Figure 4.5. Cyclic voltammograms of a Ru3+-ODPA monolayer on phosphonated ITO in 0.1 M TBAPF6, acquired at a series of scan rates; 100 mV/s, 200 mV/s, 300 mV/s, 400 mV/s, and 500 mV/s, with signal magnitude increasing with scan rate. Examining the dependence of the peak (cathodic) current (ic) on scan rate (n) reveals a dependence that is not in agreement with the theoretical prediction of the Randles-Sevcik equation (ic µ n1/2). The dependence is shown in Fig. 4.6, and it is clear that a fit of either ic to n or n1/2 83 produce a poor fit. The discrepancy between the theoretical prediction and the experimental result points to a non-ideality in the electron transfer process. 50 40 ) 30 A µ ( t 20 n er r u C 10 0 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 -10 n1/2 Figure 4.6. Dependence of ic on scan rate (n) (points). The dotted blue line is included to underscore that ic does not scale with the square root of the scan rate. For systems where delivery of the redox-active species to the electrode surface is diffusion- mediated, ic µ n1/2 is the expected result. For a system where the electroactive species is adsorbed to the electrode surface, ic µ n is expected.11 Neither of these dependencies obtain, and it is possible that the system under investigation is behaving as a quasi-reversible process, or that the electron transfer process is mediated by another surface-adsorbed species. While we cannot resolve which of these processes is dominant, the structural complexity of the modified electrode surface almost certainly plays a significant role. Further investigation will be required to understand the details of electron transfer at these complex interfaces. 4.3.3 FRAP measurements: Perylene was used as a probe in an attempt to measure translational diffusion within the Ru3+- ODPA monolayer. The concentration of perylene was 3 mol% in the solutions used to form the monolayers. In contrast to the results we have reported in Chapter 3, perylene was observed to not 84 incorporate into the formed monolayers for Ru3+-ODPA (Fig. 4.7). This unexpected finding may be thought to indicate sufficiently tight monolayer packing that perylene cannot incorporate. Comparison of the images in Fig. 4.7 indicates a more uniform distribution of perylene in the Cd2+- ODPA monolayer, and apparently a higher extent of chromophore aggregation in the Ru3+-ODPA a b Figure 4.7. Confocal fluorescence images of (a) perylene in a Cd2+-ODPA monolayer and (b) perylene in a Ru3+-ODPA monolayer. The concentration of perylene used in forming the ODPA Langmuir monolayer was the same for both images. monolayer. The difference between these two images must reside with either the details of ODPA complexation with the two metal ions during formation or is the result of the rate of chromophore incorporation relative to complexation. In terms of the monolayers formed, a comparison of the P-A isotherms for Cd2+-ODPA and Ru3+-ODPA (Fig. 4.8), the average area per ODPA molecule is larger for Ru3+-ODPA than for Cd2+-ODPA, a finding that would suggest more facile incorporation of the chromophore into the Ru3+-ODPA monolayer. However, precisely the opposite result obtains, and this is a matter that points to our incomplete understanding of organization within the Ru3+-ODPA monolayer. 85 Cd monolayer Ru monolayer Figure 4.8. P-A isotherms for the formation of Cd2+-ODPA (blue) and Ru3+-ODPA (green) Langmuir monolayers. Future experiments to address whether this is an organization- or formation-mediated process may require the use of different chromophores, or the addition of metal ions to the LB trough following the initial formation of ODPA-chromophore monolayers in the absence of metal ions. 86 REFERENCES 1. Malik, S.; Tripathi, C. C., Thin Film Deposition by Langmuir Blodgett Technique for Gas Sensing Applications. Journal of Surface Engineered Materials and Advanced Technology 2013, 03 (03), 235-241. 2. Ghosh, M.; Yang, D. S., Structures of self-assembled: N -alkanethiols on gold by reflection high-energy electron diffraction. Physical Chemistry Chemical Physics 2020, 22 (30), 17325- 17335. 3. Mavukkandy, M. O.; McBride, S. A.; Warsinger, D. M.; Dizge, N.; Hasan, S. W.; Arafat, H. A., Thin film deposition techniques for polymeric membranes– A review. Journal of Membrane Science 2020, 610. 4. Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H. J. PHYSICAL REVIEW LETTERS Friction and Wear of Langmuir-Blodgett Films Observed by Friction Force Microscopy; 1992. 5. Tieke, B., Langmuir–Blodgett membranes for separation and sensing. Advanced Materials 1991, 3 (11), 532-541. 6. Fukuda, N.; Mitsuishi, M.; Aoki, A.; Miyashita, T., Photocurrent Enhancement for Polymer Langmuir−Blodgett Monolayers Containing Ruthenium Complex by Surface Plasmon Resonance. The Journal of Physical Chemistry B 2002, 106 (28), 7048-7052. 7. Grubb, M.; Wackerbarth, H.; Wengel, J.; Ulstrup, J., Direct imaging of hexaamine- ruthenium(III) in domain boundaries in monolayers of single-stranded DNA. Langmuir 2007, 23 (3), 1410-1413. 8. Herath, H. N. K.; MacRae, A. L.; Ugrinov, A.; Morello, G. R.; Parent, A. R., Electrochemical properties of Ru polypyridyl phosphonates. Eur. J. Inorg. Chem. 2023, 26, e202200747. 9. McCord, P.; Bard, A. J.; Miller, C. J., Study of Langmuir Monolayers of Ruthenium Complexes and Their Aggregation by Electrogenerated Chemiluminescence. Langmuir 1991, 7 (11), 2781-2787. 10. Livingston, C.; Blanchard, G. J., Metal Ion-Dependent Interfacial Organization and Dynamics of Metal-Phosphonate Monolayers. Langmuir 2021, 37 (15), 4658-4665. 11. Elgrishi, N.; Rountree, K. J.; McCarthy, B. D.; Rountree, E. S.; Eisenhart, T. T.; Dempsey, J. L., A Practical Beginner's Guide to Cyclic Voltammetry. Journal of Chemical Education 2018, 95 (2), 197-206. 87 CHAPTER V: Conclusion and Future Work. 88 5.1 Conclusions and Future Work 5.1.1 Cu2+-Complexed Langmuir-Blodgett (LB) Films: As noted, the longer-term goal of this work is to control the organization of an adsorbed monolayer in situ and reversibly. To this point, control over monolayer organization subsequent to its formation has not been feasible, and gaining this ability would not only represent an advance in interface science, but would also facilitate advances in applications ranging from chemical separations and sensing to surface passivation. We have used metal phosphonate complexation chemistry as a versatile means of linking amphiphilic monolayers to support surfaces, with Cu2+ being the initial metal ion because of its two electrochemically accessible oxidation states (Cu+ and Cu2+). Amphiphile Langmuir monolayers were formed on an aqueous subphase containing Cu2+ and transfer to solid supports was accomplished using Langmuir-Blodgett deposition methodology. It is interesting to find out how we can switch the structure of monolayer in solid from. Since it has wide application as sensing, coating, and optical enhancements. Trying to change the structure of monolayer in solid phase is a big progress in surface chemistry. The first part of this work was to try to form an interfaces with the addition of the metal ion in (Cu2+) to the ODPA complexed Langmuir-Blodgett (LB) films. Characterization of the Langmuir monolayer using Brewster angle microscopy (BAM) in conjunction with measurement of the P-A isotherm provided limited insight into monolayer morphology. Ultimately, the limitation to the use of Cu was the ability to isolate Cu+ in a stable form, leading to irreversibility in electrochemical cycling. 5.1.2 Cd2+-Complexed Langmuir-Blodgett (LB) Films: A subsequent study was aimed at modifying the surface of an ITO transparent conductive support to control the strength of interaction between the metal phosphonate monolayer and the support. Using Cd2+ as the metal ion in this work, we predicted the extent of protonation of the 89 amphiphile and the complexation equilibria based on metal phosphonate and hydroxide formation constants and the pKas of the ODPA amphiphile. These calculations provided useful guidance in the pH and metal ion concentration range appropriate for the formation of an organized Cd2+- ODPA Langmuir monolayer. Deposition of the Cd2+-ODPA monolayer onto native and modified ITO supports resulted in monolayers with distinct properties, as measured using electrochemical probes Ru3+ and Fc. For this system, we found that the stability of the resulting monolayers was limited and susceptible to irreversible changes to monolayer order and integrity for sufficiently high cyclic voltametric scan rates. 5.1.3 Ru3+-Complexed Langmuir-Blodgett (LB) Films: In an effort to produce a more robust amphiphile monolayer using the same structural motif, we used Ru3+ as the metal ion for complexation. Ru3+ is attractive for several reasons, ranging from stronger bonding associated with a trivalent metal ion to the ability of Ru3+ to be reversibly converted between multiple stable oxidation states. We demonstrated the formation of stable monolayers with Ru3+ as well as the ability to perform quasi-reversible conversion between Ru2+ and Ru3+ in situ. One significant result of this work was that the permeability of the aliphatic region of the monolayer was altered substantially from what was seen for ODPA monolayers formed with Cd2+. The results suggest that the Ru3+-ODPA monolayers were less able to incorporate perylene than Cd2+-ODPA monolayers despite the larger mean molecular area for the Ru3+-ODPA monolayer. This apparently contradictory finding remains under active investigation. Ru3+ shows promise for achieving the goal of reversible control over monolayer organization but a more complete understanding of the organization in the Ru3+-ODPA monolayer remains to be achieved. Given that Ru3+-bisphosphonate complexation appear to show promise, future directions in this work will likely focus on identifying useful structures for the nonpolar 90 regions of the amphiphilic monolayer, including the addition of reactive functionalities that could be used to impart chemical selectivity for the application of interest. 91