ACHIEVING A LONG-LIVED CHARGE-SEPARATED FE(II) CHROMOPHORE : INSIGHTS INTO THE ROLE OF REORGANIZATION ENERGY ON THE ULTRAFAST PHOTOPHYSICAL PROCESSES OF D6 POLYPYRIDYL COMPLEXES

Summary

Photoredox catalysis reactions are ubiquitous in nature. These processes require a long-lived charge-separated state that is ideally suited for redox-based chemistry and photovoltaic applications. Many common chromophores used in these systems are ruthenium(II)-based, but the low earth abundance of this metal makes it non-viable for large-scale applications in the long-term. The first row congener of Ru(II) is iron(II), but its decreased ligand field strength relative to the second row... Show morePhotoredox catalysis reactions are ubiquitous in nature. These processes require a long-lived charge-separated state that is ideally suited for redox-based chemistry and photovoltaic applications. Many common chromophores used in these systems are ruthenium(II)-based, but the low earth abundance of this metal makes it non-viable for large-scale applications in the long-term. The first row congener of Ru(II) is iron(II), but its decreased ligand field strength relative to the second row transition metal causes the metal-to-ligand charge transfer (MLCT) excited state to be depopulated on an ultrafast timescale, deactivating into metal-centered ligand field (LF) excited states that are inefficient for photovoltaic applications. The aim of this work is to understand the fundamental differences in the photophysical processes of Ru(II) and Fe(II) analogues. Three strategies can be envisioned for prolonging the MLCT lifetime in Fe(II) complexes: (1) prohibiting the vibrational modes associated with the MLCT→LF transition with synthetic modifications to the ligand, (2) increasing the ligand field strength to tune the LF and MLCT states such that the potential energy surface diagram for Fe(II) resembles that of Ru(II), or (3) extending conjugation within the ligand away from the metal center, thereby decoupling the MLCT and LF excited states. Any one of these approaches will inherently affect the reorganization energy, or the amount of energy required for the reactants to undergo vibrational and nuclear motions in order to achieve the geometry of the products without any electron transfer or electronic state crossing occurring. Variable-temperature transient absorption (VT-TA) spectroscopy is a methodology that has been developed to initially study the ground state recovery (GSR) processes of some low-spin Fe(II) polypyridyl complexes. Arrhenius parameters for this class of compounds are found experimentally for the first time and from these data, semi-classical Marcus theory analysis is performed, allowing for inner-sphere (i.e., complex-only) reorganization energies to be found for each. The Hab4/λ ratio is determined to be different between bis-tridentate and tris-bidentate species, which is postulated to imply a difference in nuclear coordinate for the relaxation process. The VT-TA methodology is also applied to a bis-tridentate compound for which GSR is both nearly barrierless and nearly at the crossing-point of the 5T2/3T1 as the lowest-energy excited state. The outer-sphere reorganization energy is adjusted through the use of counteranions and solvents in an attempt to tune the barrierless nature of the complex. The identity of the solvent did appear to affect the reorganization energy and the inverted region may have been accessed. The solvation dynamics of the vibrational cooling process in a Ru(II) chromophore were studied as a function of excitation wavelength in a series of alcohol and nitrile solvents. A dual solvation mechanism was observed depending on the amount of excess energy given to the system. Through the use of a sterically-encumbered analogue, the large aryl rotation in the MLCT excited state was determined to be the relevant nuclear coordinate in the vibrational cooling process as it related to the solvation. The Fe(II) analogue of this complex has also been prepared and studied in order to draw direct comparisons of the photophysical processes of these two related systems. These analogues are based on ligands with extended conjugation. In order to study the effects of delocalization on the excited state lifetime, other compounds of this type have been prepared and preliminary measurements of the MLCT lifetimes indicate that increasing delocalization away from the Fe(II) center lengthens the charge-separated lifetime, which is an important first step in achieving long-lived charge transfer states for this class of compounds Show less