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Immu- ABSTRACT A STUDY OF CHARGE TRANSFER CHARACTER IN THE PHOTOREDUCTION OF VARIOUS Q-TRIFLUOROACETOPHENONES BY Henry Man Hung Lam The photoreduction of a—trifluoroacetophenone by tolu— ene in acetonitrile has been examined. Since the ability of alkylbenzenes to quench triplet a-trifluoroacetophenone is linearly dependent on their ionization potentials, the presence of charge transfer process in the triplet quenching is affirmed. The slope from a plot of log quenching rate constant versus ionization potential indicates only ~'21% electron transfer in the triplet charge transfer complex. The photoreduction of several substituted g-trifluoro- acetOphenones having either nv* and WW* lowest triplet has been studied. The WF* lowest triplet ketones have lower reactivity than the nn* low-lying triplet ketones. The plot of log quenching rate constant versus the sum of triplet energy and reduction potential does not show a good linear relationship and thus obscures the interpretation of the data. Perhaps the different nature of the two lowest excited states may explain the difference in reactivities. However, the WW* lowest triplet ketones may be quenched by Henry Man Hung Lam an hydrogen abstraction process rather than a charge trans- fer process as observed in the quenching of nv* lowest triplet ketones. A STUDY OF CHARGE TRANSFER CHARACTER IN THE PHOTOREDUCTION OF VARIOUS a-TRIFLUOROACETOPHENONES BY Henry Man Hung Lam A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1975 ACKNOWLEDGMENT I wish to thank Professor Peter J. wagner, who has provided me with the opportunity to complete this work. His guidance, and inspiration are sincerely appreciated. I am particularly thankful to my fellow graduate stu- dents for their helpful assistance. I would also like to acknowledge financial assistance from the Department of Chemistry throughout the course of my study. ii TABLE OF CONTENTS INTRODUCTION . . . . . . . . . . . . General Mechanistic Process . . The Nature of the Excited States The Correlations between the Reactivities of Hydrogen Abstraction and the Nature of the Excited State . . . . . . . . Charge Transfer Mechanism . . . Previous Study of the Photoreduction of TrifluoroacetOphenone . . . . Objectives . . . . . . . . . . Kinetics Expressions . . . . . RESULTS 0 O O O O O O O O O O O O 0 Quantum Yield Determinations . Triplet Quenching Studies . . . Intersystem Crossing . . . . . Absorbance and Emission Spectra DISCUSSION . . . . . . . . . . . . . EXPERIMEM‘AL O O O O O O O O Q O O 0 PART I: PROCEDURE . . . . . . A. Preparation and Purification 1. Ketones . . . . . . 2. Solvent . . . . . . 3. Hydrogen Donors . . 4. Quenchers . . . . . 5. Internal Standard . iii 10 11 12 15 15 15 16 17 29 39 39 39 39 41 41 42 42 TABLE OF CONTENTS 1. Irradiation Procedure -- Continued. Photolysis 2. Analysis of Sample . C. D. PART II: A. B. C. LIST OF REFERENCES KINETIC DATA Quenching Studies Reduction Potential iv Absorption and Emission Spectra Quantum Yield of Bibenzyl Formation Determination of Intersystem Crossing Quantum Yield Page 43 43 43 46 46 47 47 49 56 57 LIST OF TABLES Table Page 1. Least squares SIOpe and intercept values from ¢ 1 versus [BH] 1 plot for the photoreduction 0 indicated ketones with toluene . . . . . . 17 2. Triplet quenching parameters for the reaction of indicated ketone with 1M toluene by using the quencher, naphthalene in acetonitrile . . 18 3. Triplet quenching parameters for trifluoroaceto- phenone . . . . . . . . . . . . . . . . . . . 18 4. Kinetics data for photoreduction of 0.05M indicated ketones with toluene in acetonitrile 19 5. Ultra violet absorption bands of indicated ke tones . O I O O O O O O O O O O O O O O C O 19 6. Phosphorescence data of indicated ketones . . 20 7. Substituent effects on triplet ketones . . . 37 8. Quantum yield of bibenzyl formation for 0.05M ketone in acetonitrile with toluene, indi- cated Cn and conditions . . . . . . . . . . . 47 Part A: a-Trifluoroacetophenone . . . . . . 47 Part B: pfiMethoxy-a-trifluoroacetophenone . 47 Part C: pfiMethyl-a-trifluoroacetophenone . . 48 Part D: mfchloro-a-trifluoroacetophenone . . 48 Part E: m-Trifluoromethyl-a-trifluoroaceto- phenone . . . . . . . . . . . . . . 48 Part F: EflTrifluoromethyl-a—trifluoroaceto- phenone . . . . . . . . . . . . . . 48 9. System with 0.05M a-trifluoroacetophenone and 0.1M toluene in acetonitrile quenching with specific quencher in indicated Cn and conditions . . . . . . . . . . . . . . . . . 49 Part A: Anisole . . . . . . . . . . . . . . 49 Part B: Benzene . . . . . . . . . . . . . . 49 Part C: Benzotrifluoride . . . . . . . . . . 50 Part D: Benzonitrile . . . . . . . . . . . . 50 Part E: Chlorobenzene . . . . . . . . . . . 50 V LIST OF TABLES -- Continued. Table 10. 11. 12. 13. 14. 15. 15. Part F: Fluorobenzene . . . . . . . . . . . Part G: EfDimethoxybenzene . . . . . . . . . Part H: lgfinichlorobenzene . . . . . . . . . System with 0.05M a-trifluoroacetOphenone, 0.001M C20 and indicated amount of toluene in acetonitrile quenching with naphthalene . . . Part A: 1M Toluene . . . . . . . . . . . . . Part B: 0.4M Toluene . . . . . . . . . . . . System with 0.05M pfmethoxy-a-trifluoroaceto- phenone, 0.001M C20 and indicated amount of toluene in acetonitrile, quenching with naphthalene . . . . . . . . . . . . . . . . . Part A: 1M Toluene . . . . . . . . . . . . . Part B: 2M Toluene . . . . . . . . . . . . . 0.05M merifluoromethyl-a-trifluoroaceto— phenone-0.1M toluene system in acetonitrile quenching with indicated concentration of naphthalene . . . . . . . . . . . . . . . . . 0.05M archloro-a-trifluoroacetophenone-1M toluene system in acetonitrile quenching with indicated concentration of naphthalene . . . 0.05M pfiMethyl-a-trifluoroacetophenone-lM toluene system in acetonitrile quenching with indicated concentration of naphthalene . . . 0.05M prrifluoromethyl-a-trifluoroaceto- phenone—0.1M toluene system in acetonitrile quenching with indicated concentration of naphthalene . . . . . . . . . . . . . . . . . Triplet sensitized cis-trans isomerization of piperylene . . . . . . . . . . . . . . . . vi 53 53 54 54 55 55 56 Figure 1. 2. 10. 11. 12. Jablon'ski's LIST OF FIGURES diagram . . . . . . . . . . . . Reciprocal quantum yield plot for indicated ketones with toluene . . . . . . . . . . . . Reciprocal quantum yield plot for indicated ketones with Stern-Volmer toluene . . . . . . . . . . . . plots for naphthalene quenching of photoreduction of indicated ketones by 1M toluene . Stern-Volmer plots for naphthalene quenching of photoreduction of indicated ketones by indicated concentration of toluene . . . . . Stern-Volmer plots for naphthalene quenching of photoreduction of indicated ketones by 0.1M toluene Stern-Volmer reduction of plots for the quenching of photo- a-trifluoroacetophenone with 0.1M toluene by indicated quenchers . . . . . . . Stern-Volmer reduction of 0.1M toluene Stern-Volmer reduction of 0.1M toluene plots for the quenching of photo- a-trifluoroacetophenone with by indicated quenchers . . . . . plots for the quenching of photo- a-trifluoroacetophenone with by anisole . . . . . . . . . . . Phosphorescence of pftrifluoromethyl-a-tri- fluoroacetOphenone at 770K . . . . . . . . . PhosPhorescence of mftrifluoromethyl—a-tri- fluoroacetophenone at 770K . . . . . . . . . Phosphorescence of m-chloro-a-trifluoro- acetophenone at7701‘c'............ vii Page 4 21 22 22 23 23 24 25 25 26 26 27 LIST OF FIGURES--Continued. Figure 13. 14. 15. 16. 17. 18. Phgsphorescence of a-trifluoroacetOphenone at 77 K O O O O O O O O O O O O O O O O O O O O Phosphorescence of pfmethyl-a-trifluoroaceto- phenone at 77°K . . . . . . . . . . . . . . . Phosphorescence of pfmethoxy-a-trifluoro- acetophenone at 770K . . . . . . . . . . . . Correlation of rate constant for quenching of triplet a-trifluoroacetophenone by aromatics with their ionization potentials in acetOP nitrile . . . . . . . . . . . . . . . . . . . Log kr for quenching of acceptor triplets by toluene gs triplet energy minus reduction mtential O O O O O O O O O O O O O O O O O 0 Fluorescence quenching rate constants k as a function of the free enthalpy AG . . . . . . viii Page 27 28 28 30 33 35 INTRODUCTION Photoreduction of ketones has become a common topic for investigation since the original discovery by Ciamician and Sil‘ber1 that the action of sunlight on benzophenone in 2-propanol yields benzopinacol and acetone. Studies of the mechanism of photochemical pinacolization were intensely 0 on o u I u 2C5H5CC5H5 + CH30HCH3 ——-> (c611,),c -C(C5H5)2 + casccn3 (1) 0 I 0H 0H conducted since 1900. Most of the work done in the 1950's emphasized synthetic application because the chemical yields in some of these photochemical reactions were good and the products were more readily prepared than by non-photochemi- cal methods. The advancement of technology during the past few years enabled more thorough investigation on the mechan- ism of photoreduction. The photoreduction of ketones, either intra— or inter- molecular, involving hydrogen abstraction could be divided into radical—like and charge-transfer processes. Radical— like intramolecular photoreduction,usua11y referred to as the Norrish type II reactions, involves the transfer of y hydrogen to the carbonyl oxygen in the excited state and the subsequent reaction of the 1,4-biradica1.2 1 H R R . ,_., + ( 2 ) I] ‘—+ OH’ R Intermolecular photoreduction represents the abstrac- tion of hydrogen from a substrate, which can be an alcohol,3 hydrogen bromide,4 toluene,5 alkanes,6 and amines,7 by the electronically excited carbonyl compound. :c=o* +I-IB -—-—> -c-0H +B° (3) The resulting radicals may proceed to form different pro- ducts depending on the reaction conditions.8 2 -(?.-OH > -c—c- (4) I I 0H 0H (A) —C:!-OH + B' > -C-B (5) I OH (B) B- + B- -—--> B-B (6) (c) -¢-0H 4- BH >- -C-OH +B' (7) 13 (D) The reaction of benzophenone with toluenéh9gives products A, B, and C which are shown in equations 4, 5, and 6, but the photoreduction of acetone by tributyl stannane10 gives only product D. Although aliphatic ketones usually undergo 3 radical-like photoreduction with alcohols, many aromatic ketones as fluorenone, aminobenzophenone, a and B-naphthyl carbonyl compounds are not efficiently photoreduced by alcohols. Recent studies reveal that these aromatic ketones could be photoreduced efficiently in a charge-transfer mechanism by amines which contain a-hydrogens.11 ;c==o* + RCH2NR5 l [-c;-0' RCHz'fiRh (8) 1 -¢—0H + RéHNR; General Mechanistic Process Before any further discussion on photoreduction of ketones, a brief descriptionof the fate of the electronic excitation is necessary to achieve a better understanding of the mechanism. Figure 1 is a modern elaboration of Jablonski's diagram, representing the energies of the various low-lying states of a typical molecule with respect to the ground state So. In solution, when an organic molecule absorbs light, it is elicited to an excited singlet state, and then decays rapidly to the lowest singlet state («:10" sec). The lowest singlet state either deactivates to ground state by fluorescence and radiationless decay or populates triplet states by inter- system crossing. Since the rate of intersystem crossing 53' . a T- S, 'r k1 : T3 c . T 2 ‘Ti k P02 -..----8’.-..-- 4’ IL Figure 1. Jablon'ski's diagram. Solid arrows represent radiative processes, corresponding to absorp- tion or emission of light. Dashed and wiggly lines denote non—radiative unimolecular and bimolecular processes respectively. for phenyl ketones is much faster than the rate of fluores- cence and radiationless decay, the triplet state is usually populated with unit efficiency.12 Subsequent emission from the lowest triplet state to ground state is known as phos- phorescence. The Nature of the Excited States The molecular electronic state, as mentioned in the last section, can be described by the appropriate electronic configuration and electron distribution, which are expressed in terms of one-electron wave functions by molecular-orbital 5 methods. Recent studies have concentrated on the chemical properties of the two lowest electronic states nv* and vw*. nv* transition is the promotion of an electron from the non-bonding orbital on oxygen to an antibonding v orbital. As a result, the excited state is electrophilic and radical- like in the proximity of the oxygen atom and nucleophilic (and possibly radical-like) above and below the carbonyl faces.13‘15 Alkoxy radicals have been found to be good analogues of nw* carbonyl triplets in relation to their behaviors in radical-like hydrogen abstraction reactionsQ'la‘18 The formation of a vv* triplet involves the promotion offlan'electron of the conjugated carbonyl group to an anti- bonding v* orbital. Although the nature of the 'pure' vw* triplet of the benzoyl group is not altogether clear, it can be alternatively viewed as having its excitation localized on the aromatic ring with carbonyl providing a strong eneru getic perturbation,19 or as a charge-transfer state with electron migration from the benzene ring to the carbonyl.20 It might be a mixture of both and therefore the carbonyl oxygen in a vw* triplet should always be electron rich as compared with nw* triplet.21 It is also known that the electronic transitions of benzene are strongly affected by substituents.22 Experi- mental results’3c3‘ show that the addition of an electron- donating group to the benzene ring stabilizes the VW* trip- let with respect to the nv* triplet. Electron—withdrawing substituents, on the other hand, tend to lower the nw* 6 triplet. Solvent effect on the electronic states has also been studied. Polar solvents usually stabilize vw* relative to nw* state. The Correlations between the Reactivities of Hydrogen Abstraction and the Nature of the Excited State Recent studies showed that those ketones which are mostly vw* in character display highly reduced reactivity in hydrogen abstraction reactions relative to those ketones with nw* low-lying triplet.35 But in aromatic carbonyl com- pounds, since the excited state associated with the aromatic ring is frequently mixed into the lowest ww* states of the carbonyl, the subsequent lowering of the WW* states which in some cases leads to the energetic proximity of nv* and ww* triplet, has caused the interpretation in ketone re- activities to become more challenging. In studying the correlation of the photochemical reactivity and the nature of excited states of acetOphenone and substituted aceto- 23 26 . . . . . ' observed a Significant decrease in trip- phenones, Yang let reactivity when the vw* triplet is lower than the nw* triplet. Since the vibronic coupling between two states ‘will increase as the energy gap decreases, the enhanced photochemical reactivity of the low-lying ww* state of 2: methyl and 3,4-dimethyl acetophenone with respect to those ketones possessing a much lower lying vw* triplet state could be explained by the vibronic coupling of the vw* trip- let with the nearby nw* triplet in methylated acetophenones. 7 Wagner,3"v28 on the other hand, suggested that a thermal equilibrium between two states would allow the re- active upper nw* triplet to be responsible for the observed reactivity. In studying the reactivity of various pfmethoxy phenyl ketones which have vv* lowest triplets, it was re- ported that the observed rates of triplet state y-hydrogen abstraction of p—methoxy phenyl ketones and the rate of simple phenyl ketones which have nw* lowest triplet vary identically with C-H bond strength and with the inductive effect of substituents. It was then concluded that the hydrogen abstraction comes from the low equilibrium popula- tions of the upper nw* triplet.39 Recently, the observation that interchromphore energy transfer in 1-benzoyl-4-anisoy1— butane is fast enough to allow almost complete equilibration between the nv* triplet of the benzoyl group and the ww* triplet of the anisoyl group supports the proposed theory of intrachromophore energy transfer between nw* triplet and vr* triplet.3° This interesting correlation of the nature of the ex- cited states with the reactivities of direct hydrogen abstrac— tion leads us to question whether the same correlation ‘would appear in charge transfer reactions. Charge Transfer Mechanism Quenching of fluorescence from aromatic compounds has been observed19 even when the lowest excited singlet of the quenchers have higher excitation energies than those of the 8 quenched molecules. In order to explain this observation, charge transfer mechanism has been proposed to be the pre- dominant quenching mechanism for the singlet states of aromatic compounds with amines,31 and quadricyclene,32 , Eh) ,,,Q(-+—) (9) F* + Q —:—> F* 0.00 < where F* is the excited molecules and Q is the quenchers. Charge transfer interactions have also been suggested in the quenching of triplet ketones and aldehydes. Reduc- tions of ketones by amines appear to occur by rapid inter- action at the non-bonding electons of the heteroatom leading to a charge transfer complex.11'33 The complex will either go through a transfer of a-hydrogen to form radicals or a charge destruction to regenerate ketone and amine. Although the rate of charge transfer is usually faster than expected for direct hydrogen abstraction, the ;c=o*3 + -N-C-H [-é-o’ -N+-E;- H] (10) V N -¢-oa + 131-9- =c=o + .i;i—<';-H subsequent self-quenching of the charge transfer complex prevents the attainment of a maximum quantum yield of 2, which has been observed in benzophenone-isopropanol reaction. In order to support the proposal of the charge transfer 34 9 mechanism, Davidson and Wilson35 confirmed the production of 9-hydroxy fluorenyl radical in the photoreduction of fluorenone with tertiary amines by e.s.r. study. To obtain a quantitative relationship between kinetics of charge transfer and thermodynamic properties of the re- actants, Cohen and Guttenplan36 modified the published equation for singlet quenching by electron transfer31 to equations 11 and 12, AGC a: “313.130,0 + IPD - E(A‘/A) + c (11) log kr ~8 K:AGc (12) where kr is the rate constant of a quenching interaction between an electron donor D and an acceptor A, AGc is the free energy change for charge transfer formation in triplet excited state, 3AE0,0 is the triplet energy of the excited species at the 0,0 band, IPD is the ionization potential of the donor, E(A-/A) is the reduction potential of the ac- ceptor, and C is a constant. For the reaction of an ac- ceptor with a series of donors, log kt ~'IPD + C' and a reasonably linear relationship was observed for interaction of nv* triplet benzophenone with 17 donors,36 and vw* trip- let fluorenone with a limited series of compounds.37 For a series of acceptors with a constant donor, the equation log kr -[-3AEo'o - E(A-/A)] + C is applicable and a linear relationship was reported for the interaction of triethyl- amine with a series of carbonyl acceptors with varying reduc- tion potential, triplet energy, and triplet configuration.37 10 Previous Study of the Photoreduction of Trifluoroacetophenone The involvement of charge transfer in the photoreduction of trifluoroacetophenone by alkylbenzenes has been suggested?“39 The general mechanistic scheme can be written for the photo- reduction of ketone A, by subStrate BB, in the presence of quencher, Q. A3* + BH —————9 [A' ---- BH+] 1 §———'T"' i A0 + BH < [AH- + B'] (13) f / ‘ l Free Radicals > AHB 1 \ BB AHAH A3* .+ Q ———> A0 + 03* (14) Charge transfer interaction was suggested to be in- volved in the photoreduction of trifluoroacetophenone by toluene, because the photoreactivities do not depend on the C-H bond strength of the donor, but depend on their ioniza- tion potential. A small solvent effect and a small slope in the log kr 33 IP plot indicates only partial charge transfer in the transitional state. Deuterium isotope ef- fects suggest that the reaction of trifluoroacetophenone with cumene involves both charge transfer and direct hydro- gen abstraction. Since the rate of hydrogen abstraction in the reaction of cyclohexane with trifluoroacetophenone is 11 faster than with acetophenone, the addition of the trifluoro- methyl group to the carbonyl not only increases the rate of charge transfer complexing, but also increases the rate of direct hydrogen abstraction. Although the v7* triplet in trifluoroacetophenone is lower in energy than the nw* trip- let, they are so close that it is difficult to determine the extent to which each state is taking part in the charge transfer photoreduction. Objectives In order to confirm the involvement of partial charge transfer in the transitional state of the photoreduction of trifluoroacetophenone, experiments were set up similar to what Cohen and Guttenplan36 did on the triplet quenching of benzophenone by different amines in benzene. The kinetics of the photoreduction for trifluoroacetophenone with several substituted benzenes were examined in acetonitrile solvent and a quantitative relationship between rate constants for charge transfer and thermodynamic preperties of the react- ants was drawn. As mentioned above, correlations have been shown be- tween relative disposition of nr* and ww* triplets with reactivity of triplets in direct hydrogen abstraction. Analysis of the reactivity of various pfmethoxy phenyl ketones suggests that hydrogen abstraction occurs by equi- librium concentrations of upper nw* triplet,29 but so far no data are available leading to the same conclusion on 12 charge transfer photoreduction. As proposed by Cohen,37 a linear relationship could be obtained by plotting log kr with -3AEo'o - E(A-/A) in the system of a series of ac- ceptors reacting with a constant donor. But in studying a series of substituted trifluoroacetophenones with nw* or ww* lowest triplet, if the ketones with nw* and VV* lowest triplet have different reactivities in charge trans- fer reactions, a linear relationship could not be found on the plot of log kr yg_-3AE0'0 - E(A-/A) because the dif- ferent reactivities of nw* and vw* triplets are not taken into account in that quantitative relationship. Kinetics Expressions Quantum yield is the only kinetic parameter associated with a photoreaction which is directly measurable under steady state conditions and is generally defined by the relationship between the number of molecules which react or are formed and the number of photons absorbed.“o no. of molecules or ions of X formed/cm3,sec No. of quanta absorbedmby reactant/cm3,sec (15) ¢u The quantum yield of a particular photoreaction can also be defined.‘1 i ' 438.54331 (16) Where ¢ ES light will produce the requisite excited state, 0R is the represents the probability that absorption of 13 probability that the excited state will undergo the primary photoreaction necessary for process i, and Pi is the prdbability that any metastable ground state intermediate will proceed to stable product. Since the probability, ¢R' that the excited state will react in the manner responsible for process i depends on the rates of all the competing processes of the reactive excited state, a bimolecular re- action with BB would give the following expression, ¢R = kr[BH]T (17) where kr is the rate constant for interaction of the ex- cited state with BH and the lifetime, I, is the reciprocal of the sum of the rate of all reactions undergone by the excited state. Applying equations 14 and 15 to the mechanistic scheme of the photoreduction of trifluoroacetophenone with toluene, an useful expression can be obtained = 1 “’33 YBBPBB (kr [BH] 1) ( 8) where Y B' the actual chemical yield of the product, is B [33} defined as 2[BB]+[AHB] is based on only one of the products containing B' radical. because the measured quantum yield Since 1 Rd + krlBH] (19) the reciprocal of equation 17 would give 14 k ...1 -1 -1 (1 ¢ = -———— BB YBB PBB (1 + krlgnl) (20) A plot of reciprocal quantum yield ¢;; 1 is linear and a lepe/inter- versus reciprocal substrate concentration [EH]- cept value gives kd/kr' When a triplet quencher is used, equation 17 becomes kr[BH] kr[BH] +'kd + kqlol ¢ = Y 9 ( ) (21) BB BB BB Dividing equation 18 by 21 gives the Stenn-Volmer equation ¢°/¢ = 1 + kq[Q]r (22) where ¢° and ¢ represent the quantum yield of BB formation in the absence and presence of quencher respec- tively. A plot of ¢°/¢ versus the concentration of the quencher gives a linear plot with a slope equal to qu. Using a quencher with known kq, we can calculate T,‘which equals [kr(BH) + kdl-l. Since the ratio of kd/kr is known, the rate constant of the excited state reaction can be obtained. RESUDTS Quantum Yield Determinations Absolute quantum yields of bibenzyl formation were determined for trifluoroacetophenone and substituted tri- fluoroacetophenone as a function of toluene concentration. Degassed acetonitrile solutions containing 0.05M ketone, 0.001M C17 and various concentrations of toluene were indi- cated at 313 nm and analyzed for bibenzyl concentration by VPC. ValeroPhenone actinometry, described in the experi- mental section, was used to calculate the light absorption. Reciprocal quantum yield 33 reciprocal toluene concentration plots were shown in Figures 2 and 3. SlOpe and intercept values, obtained from Figures 2 and 3 were used to calculate kd/k and ¢max r BB which were shown in Table 4. Triplet Quenching Studies The lifetimes of trifluoroacetophenone and substituted trifluoroacetophenones in acetonitrile were obtained by using the Stern-volmer relationship and an assumed constant, the kq value for naphthalene. Degassed acetonitrile solution containing 0.05M ketone, 0.001M C17 and various concentra- tions of naphthalene were irradiated at 366 nm. Bibenzyl and C17 peak areas were analyzed and linear Stern-volmer 15 16 plots were obtained as shown in Figures 4 - 6. With the lifetime as indicated in Table 2 and the kd/kr ratio ob- tained from the absolute quantum yield determination, kd and kr were calculated and shown in Table 4. On the other hand, the rate constants of different quenchers in quenching the reaction of trifluoroacetophenone with toluene were revealed. Degassed solutions containing 0.05M ketones, 0.001M C17, 0.1M or In toluene and different concentrations of quenchers were irradiated in parallel at 313 nm to less than 5 percent ketone conversion. Bibenzyl and C30 peak areas which were analyzed by VPC permitted the calculation of ¢gB/¢BB. The qu values obtained from the Stern-volmer plots in Figures 7 - 9 were shown in Table 3. Intersystem Crossing A comparison of the amount of Sigfpiperylene isomerized to Eggggfpiperylene by pfmethoxy-a-trifluoroacetOphenone and mytrifluoromethyl—a-trifluoroacetophenone with acetophenone would give the ¢. of the substituted trifluoroaceto- 18C phenones, since the ¢isc of acetophenone is unity.12 De— gassed acetonitrile solutions containing 0.1M ketone and 0.2 M gigfpiperylene were irradiated in parallel at 313 nm. The ¢isc of pfmethoxy-a-trifluoroacetophenone is considered to be unity within experimental error, DUt the ¢isc of m: trifluoromethyl-a-trifluoroacetophenone in 0.94. 17 Absorbance and Emission Spectra Table 5 showed the In,“ <—--1A19(1Lb <-——-la) and 1B1u <———-1a19(1La <——— AA) bands of different ketones. The 0-0 band and the lifetime of the phosphorescence of trifluoroacetophenone and substituted trifluoroacetophenone were given in Table 6. Figures 10 - 15 show the phosphor- escence spectra of the six ketones. -1 Table 1. Least squares lepe and intercept values from ¢BB versus [EH]-1 plot for the photoreduction of indicated ketones with toluene Ketone Slopeb Intercept TFAa 6.23 26.63 g-MeO-TFA ' 106.18 15.9 p-Me-TFA 13 .66 13 .5 m—Cl-TFA 1 .92 15.11 b aTrifluoroacetophenone. Single run. 18 Table 2. Triplet quenching parameters for the reaction of indicated ketone with 1M toluene by using the quencher, naphthalene, in acetonitrile. -1830 Ketone qu,M 1, sec. 1/1, sec-1 TFA 790 7.89 x 10'8 1.3 x 107 B-MeO-TFA 24300 2.43 x 10"3 4.1 x 105 g-Me-TFA 5400 5* .40 x 10'7 1.8 x 108 ngl-TPA 180 1.80 x 10"8 5.6 x 107 B-CFa-TFA 93 9.30 x 10'9 1.1 x 108 g-cra-JrFA 76 7.64 x 10‘9 1.3 x 108 akq = 1.0 x 1010 M-1 sec-l. bSingle run. Table 3. Triplet quenching parameters for trifluoroaceto- phenone with toluene. _1a,b __1 _1 c Quencher qu,M kq,M sec 1PD, e.v. Anisole 60 1.9 x 108 8.2 Benzene 0.14 4.5 x 105 9.25 Chlorobenzene 0.32 1.0 x 105 9.07 Benzonitrile 0.025 9.9 x 104 9.7 Benzotrifluoride 0.04 1.3 x 105 9.68 prichlorobenzene 0.42 1.3 x 106 8.94 Fluorobenzene 0.1 3.2 x 105 9.19 primethoxybenzene 3780 1.2 x 1010 -- Toluene -- 1.1 x 107 8.80 a1 - 3.1 x 10-5 sec. bSingle run. cReference 4 2. 19 Table 4. Kinetic data for photoreduction of 0.05M indicated ketones with toluene in acetonitrile. Ketone 0:;xa kd/krb kr.Mflsec-1c kd,sec-1c E(:.é§)d e TFA 0.0375 0.2342 1.07 x 107e 1.75 x 106 -1.37 gyneO-TFA 0.0627 6.665 5.13 x 104f 3.42 x 105 -1.72 gynedTFA 0.0740 1.0118 9.2 x 105f 9.3 x 105 -1.71 mel-TFA 0.0660 0.1276 4.9 x 107f 6.28 x 106 -1.20 g-CFs-TFA 0.1043 0.0499 7.17 x 1031: 3.57 x 10" -1.11 mam 0.0498 0.0304 1.00 x 109 3.04 x 10" -1.17 aExtrapolated quantum yield of bibenzyl at infinite toluene concentration. bSlope/intercept value from Table 1. cCalculated from r values in Table 2 and kd/k values in this table. r dReduction potential. eThe standard deviation of k for TFA and peneodTFA is 0.05 and 0.16 respectively. The§ are measured by triplet quench- ing of the two particular ketones with different concentra- tions of toluene by naphthalene. Table 5. Ultra violet absorption bands of indicated ketones. Ketone 1B,“ <- 1A19, 2 1131“ <- 1A19, R TFA 2940 2520 g-MeO-TFA -- 2870 pfifle-TFA -- 2640 ngl-TFA 3010 2580 gyCFa-TFA 2930 2510 g-CF, -TFA 2980 2445 20 Table 6. PhOSphorescence data of indicated ketones at 770K.a 0-0 Bands Ketone Lifetime, sec nm Kcal/mole TFA 57 40611 70.41 24160-er ~600 430 66 .48 p-Me-TFA 208 . 5 414 69 .05 EfCl-TFA 85.6 409 69.90 g-cr, -TFA 18 .2 406 70 .41 g-CF, -TFA 24 405 70 .59 aSolvent is 4:1 mixture of methyl cyclohexane and isopentane. 21 240 ErMeo-TFA 220 P 200F' 180 ’ 160 r —1 BB 140 ’ 120 ’ 100 ' p_-_-Me "TFA TFA EfC1“TFA 2 1 3 7r [Toluene] M-1 “b Figure 2. Reciprocal quantum yield plot for indi— cated ketones with toluene 22 50» 40 -1 BB 30)- g-CF, ~TFA 20 10 A A L 10 30 _ 5 [Toluene] 1,M‘9 Figure 3. Reciprocal quantum yield plot for indicated ketones with toluene. 2.0r’ mel-TFA l 1 0.001 0.002 [Naphthalene], M Figure 4. Stern-Volmer plots for naphthalene quenching of photoreduction of indicated ketones by 1M toluene. , 23 g-Meo -‘I'FA with 2M Toluene -Me -’I‘FA wi th M Toluene ..L— L .0001 .0002 [Toluene], M Figure 5. Stern-Volmer plots for naphthalene quenching of photoreduction of indicated ketone by indi- cated concentration of toluene. 7. 2 b ¢0 '3 [Cl-CFa “TFA 1 .51. ' o .005 .01 .015 [Naphthalene], M Figure 6. Sterndvolmer plots for naphthalene quenching of photoreduction of indicated ketones by 0.1M toluene. 24 1.9 » Benzene ¢0 '5' 1.7 r 1.5 P Fluorobenzene A» b 1.3 ) Benzotrifluoride A II :enzonitrile 1.1 b ‘ - .A 1 3 5 7 Quencher, M Figure 7. Stern-volmer plots for the quenching of photoreduc— tion of a-trifluoroacetOphenone with 0.1M toluene by indicated quenchers. 25 prichlorobenzene 1. (O 1. $0 Chlorobenzene Ti . 1. p /. J J 1 2 [Quencher] , [M] Figure 8. Stern-Volmer plots for the quenching of photoreduction of a-trifluoroacetOphenone with 0.1M toluene by indicated quenchers. Anisole I 1 J .01 .02 .03 [Anisole], M Figure 9. Stern-volmer plots for the quenching of photoreduction of a—trifluoroaceto henone with 0.1M toluene by anisole. P 26 L l l 350 450 x, nm 550 Figure 10. Phosphorescence of pftriflugromethyl-a-tri- fluoroacetophenone at 770K. I l l 350 450 A.nm 550 Figure 11. Phosphorescence of mftrifluoromethyl-a-tri- fluoroacetophenone at 770K.a 27 l I l 350 450 A, nm 550 Figure 12. Phosphorescence of mfchloro-a-trifluoroaceto- phenone at 770K.a ' I) I I L 350 450 A, nm 550 Figure 13. Pngsphorescence of a-trifluoroacetophenone at 77 K.3 28 I L 350 450 A,nm 550 Figure 14. Phosphorescence of pfmethyl-a-trifluoroaceto- phenone at 770K.a l l 350 450 1, nm 550 Figure 15. Phosphorescence of pfmethoxy-q-trifluoroaceto- phenone at 770K.a aThe phosphorescence intensities of all the six ketones are not exactly measured. DISCUSSION PART I A quantitative relationship33 between kinetics of charge transfer and thermodynamic properties of the reactants has been established. The details of equation 23 have log Jere-3660,, + 19D - E(A’/A) + c (23) already been thoroughly reviewed during the discussion of charge transfer reactions in the introduction. Since it has been shown that the photoreduction of a-trifluoroaceto- phenone by toluene in benzene involves charge transfer com- plexing with only a modest degree of electron transfer,38 the linear relationship between the logarithm of the rate constant, kr' for quenching a-trifluoroacetophenone in ben- zene and the ionization potential of aromatic electron donors, lPD, supports the same conclusion.39 To confirm the in- volvement of partial charge transfer reaction, the relation of log kr to lPD in quenching triplet a-trifluoroaceto- phenone by substituted aromatics in acetonitrile has been studied. The plot of these data, Figure 16, has a slope of -2.22//e.v. which is parallel to the slope of the same re— action in benzene. However, the value of the sloPe is much lower than the -17.06/e.v. observed in systems in which 29 30 log kr 8 9 1P 10 Figure 16. Correlation of rate constant for quenching of triplet c-trifluoroacetophenone by aromatics with their ionization potentials in acetonitrile. 31 quenching of fluorescence occurs by complete electron transfer.43 The value 17.06/e.v. is obtained from the plot of quenching rate versus halfdwave potential of the quenchers, whereas innquenching a-trifluoroacetOphenone, the quenching rate is plotted against ionization potential instead of the halfdwave potential of the monosubstituted aromatic quenchers because the unstable cations of the quenchers eliminate the possibility of achieving accurate halfdwave potential. The correlation between ionization and halfdwave potential has been established from the data of aromatic compounds with known ionization and reduction potentials. Although this relationship“ is deduced from stable cations generated from 61/: = 0.66 lP - 3.86 (24) aromatic molecules, the linear relationship could be extra- polated to those monosubstituted benzene cations. Conversion of the plot log kr yg.lP to log kr yg_halfdwave potential for triplet quenching of a-trifluoroacetophenone by mono- substituted benzene in acetonitrile increases the value of the lepe only by a factor of 1.5. The readjusted slope indicates that only 21% of the reaction is by electron transfer. A small solvent effect is observed. The rate constant for the a-trifluoroacetOphenone-toluene system in aceto- nitrile is only twice as large as in benzene, but the rate of fluorescence quenching of aniline, N,N-dimethyl- and N,N- diethyl-aniline involving full electron transfer in 32 acetonitrile is 13 times larger than in benzene.45w The available data seem to support our proposed theory that photoreduction of a-trifluoroacetOphenone with toluene in acetonitrile is by charge transfer. PART II In investigating the correlation of the nature of excited states and the reactivities of a charge transfer reaction, six ketones with FW* or nr* low lying triplet state have been studied. The plot of log kr versus -3AE0'0 — E(A-/A) should give a straight line according to Weller's equation31 if the photoreduction of the six ketones involves charge transfer. However, problems arise in re- lating the six points in the plot as shown in Figure 17. Two straight lines or a curve linking all the six points could be drawn, but a straight line through all six points could also be prOposed. Statistically speaking, the standard deviation and correlation coefficient of the straight line that is drawn through the six points and the four point line are 0.49; 0.96: and 0.51; 0.94, respectively. The values are so close to each other that it is difficult to distinguish which is the better line to interpret the data. The six-point line suggests that the nature of the lowest excited state does not have any effect on the charge transfer reaction. The low lying ww* triplet a-trifluoro- acetophenones are at least as reactive as the low lying nw* triplet ketones. The terms in equation 23, as Weller 33 \ B-MeO \ A 1 ‘ A 30 Ammo " 3(A /A) Figure 17. Log kr for quenching of acceptor triplets by toluene gs triplet energy minus reduction potential. originally pr0posed, have already accounted for all the factors affecting the charge-transfer reaction. But the standard deviation of the straight line is 49%, whereas the greatest experimental error in individual determinations is only 16%. The absence of large experimental error suggests that the straight line is improbable to interpret the cor- relation of the six ketones. An S-shaped curve can be proposed as the correlation among those points. The slope of the curve on the portion of nm* low lying triplet ketones is sharp as compared with 34 the zero slope when the nw* state of the ketone is close to the vv* state. When the curve reaches the pfmethyl-a- trifluoroacetophenone, the slope increases again, but to a smaller extent than the limiting slope of the lowest nv* ketone. The large slope between a-trifluoroacetophenone and the nv* low lying triplet pftrifluoromethyl-a-trifluoro- acetophenone, reveals that a difference of 6 kcal/mole in reduction potential causes a 70 fold difference in reactivity when the triplet energies of the ketones are about the same although their low lying triplet state is different. By comparing pfmethyl and unsubstituted a-trifluoroacetophenone, both with low lying ww* triplets, a difference of 7.8 kcal/ mole in reduction potential induces only a factor of 10 dif- ference in reactivity and their difference in triplet energy is only 0.5 kcal. This phenomenon explains the dis- continuity of the two lepes for nv* and vw* low lying trip- let ketone. However, the higher sensitivity of log kr to the reduction potential of nw* low lying triplet ketones indicates a higher degree of electron transfer. The lower sensitivity observed in pfmethyl- and pfmethoxy-a-trifluoro- acetophenone implies that a lower degree of electron transfer might be due to the difference in the nature of energy state. In an electron transfer process, the electron would first occupy the lowest n orbital if it transfers from the donor to the nv* low lying triplet ketones, but it would occupy the lowest empty 7 orbital if the low lying triplet is vw*. For a charge transfer reaction, the formation of 35 an exciplex by interaction of the molecular orbital of the donor with the lowest empty orbital of the excited ketone is rate determining for the photoreduction of a-trifluoro- acetophenone. It is assumed that the rate determining step for substituted and unsubstituted a-trifluoroacetophenone would be the same in the following discussion. For the vw* low lying triplet ketones, the v electron donor may inter- act with the aromatic ring or the localized aromatic ring with carbonyl, but for the nw* low lying triplet ketones, during exciplex formation it interacts with the oxygen of the carbonyl group. The difference in activation energy for the two processes could explain the reactivity differences between the nv* and vw* triplet ketones. Weller, in studying the kinetics of fluorescence quenching, plotted log kr versus AG as shown in Figure 18.46 1011} 109 f . £.CF3 k . q L M-lsec"1 107 - TFA p ' pfiMe 105 ’ ,,prMeo -20 -10 6 10 20 30 mole/kcal, AG Figure 18. Fluorescence quenching rate constants kq as a function of the free enthalpy AG. experimental data of electron transfer process done by Weller. 36 However, the difference of AS between the photoreduction of substituted and unsubstituted a-trifluoroacetophenone with the reaction system that Weller studied would only shift the curve along the x-axis rather than change the shape of the curve. 32527 and mgggfa-trifluoromethyl, pgggfchloro and the unsubstituted a-trifluoroacetophenone fall within Weller's curve, but the other two vv* low lying triplet ketones have a much higher reactivity than expected for an electron transfer reaction according to the extrapolation of‘Weller's curve. Although the differences in the nature of states might explain the higher reactivity of the ”7* low lying triplet ketone, without any experimental evidence, it cannot rule out the possibility that a different mechan- ism is involved. A comparison of reactivity among a-tri- fluoroacetophenone, acetophenone and valerophenone as shown in Table 7 suggests that the photoreduction of pfmethyl and pfmethoxy-a-trifluoroacet0phenone might possibly occur yia_ hydrogen abstraction, because 10-fold difference in reac- tivity between a-trifluoroacetophenone and pfmethylra-tri- fluoroacetophenone can be observed as in the case of aceto- phenone and pfmethyl-acetOphenone. A ZOO-fold difference in reactivity between pfmethyl and unsubstituted valerophenone is very close to the difference between pfmethoxy and un- substituted a-trifluoroacetophenone. On comparing the dif- ference in reactivity, it seems evident that the photoreduc- tion of the 77* low lying a—trifluoroacetophenone with toluene is a charge transfer reaction, but the photoreduction 37 Table 7. Substituent effects on triplet ketones. Ketone kr' MDJ-sec-1 A330.° kcal/mole TFA 1.1 x 107 70.41 pyMe-TFA 9.1 x 105 69.05 pfiMeO-TFA 5.1 x 104 66.48 Acetophenonea 8.6 x 105 E-Me-ACP 7.8 x 104 Valerophenoneb 1.2 x 108 sec-1 72 p-Me-VAL 1.8 x 107 sec".1 73 p-MeO-VAL 5 x 105 sec-1 71 aReference 26. bReference 29. of the nw* low lying acetophenone and valerOphenone is yia_ hydrogen abstraction. However, the coincidence of the re- activity difference among various substituted and unsubsti- tuted ketones suggests the possibility of the involvement of hydrogen abstraction which could explain the reactivities of the two nv* lowest triplet ketones. A definite conclusion cannot be drawn to explain the available data. Further experiments must be conducted before the reactions could be fully interpreted and explained. The six points on the plot, in fact, are not sufficient to show the exact shape of the curve. Since the 38 pfmethyl-a-trifluoroacetophenone is the inflection point of the curve, any experimental error on that point could cause large deviation on the shape of the curve. It would be desirable to obtain more points between the trifluoro- methyl-a-trifluoroacet0phenone and pfmethoxy-a-trifluoro- acetophenone in order to confirm the shape of the curve. If the nature of the energy state affects the reactiv- ity of ketones in a charge transfer reaction, the site of charge transfer on the ketone would be the determining factor on the reactivities. In order to judge whether an electron donor would interact with the aromatic ring or with the carbonyl of the vw* triplet ketones, substituting bulky groups on the benzene ring might decrease the reactivity of the #7* low lying triplet ketones to a greater extent than that of the nu* ketones. On the other hand, the relation of log kr y§.lP for the triplet quenching of pfigfbutyl-a- trifluoroacetophenone by a series of substituted tfbutyl benzenes would suggest the possibility of any interaction of localized w electrons with the donor. The above discussed shows that apart from charge trans- fer, hydrogen abstraction might possibly be another mechanism for the photoreduction of pfmethyl- and pfmethoxy-a-tri- fluoroacetophenone. For an hydrogen abstraction reaction, there would be a significant deuterium isotope effect on kr' More work on deuterium isotope effects is required in order to confirm the possible involvement of hydrogen abstraction on the low lying vv* triplet ketones. EXPERIMENTAL PART I . PROCEDURE A. Preparation and Purification 1. Ketones a) a-Trifluoroacetophenone (Columbia Organic Chemicals). Purification by spinning band distillation with 20 mm vacuum at 500 gave a center cut of >99.9% pure trifluoroacetophenone. b) ijethoxy-a-trifluoroacetopheno e was pre- pared by the method of Dishan and Levine.‘7 In a dry 1- liter 3-necked round bottom flask fitted with a mechanical stirrer, an addition funnel and an efficient reflux con- denser, 0.6 g-atom of magnesium turnings were placed. The reaction required 0.6 mole of pfbromoanisole, 10 ml of pr bromoanisole with 50 ml of ether and a small crystal of iodine were added to the magnesium turnings. Nitrogen was passed through during the reaction. When the color of the solution changed to yellow after mild heating with no stir- ring, the remaining pfbromoanisole/ether mixture was added dropwise. The reaction mixture was stirred vigorously for two hours at room temperature. The reaction mixture was 39 40 refluxed for fifteen minutes, then 0.2 mole of trifluoro- acetic acid in ether was added to the<3rignard solution slowly within two hours, and the solution was refluxed for another hour. The reaction mixture was cooled and poured into a 10% hydrochloric acid. After the extraction, the ether layer was washed with sodium bicarbonate solution and dried. The product was purified by spinning band distilla- tion with 0.9 mm vacuum at 75°. The purity of the product was 3. 99.9%. c) pyMethyl-a-trifluoroacetophenone was prepared by the Grignard procedure essentially as described for pr methoxy-a-trifluoroacetophenone, starting with pfbromo- toluene. d) mehloro-a-trifluoroacetOphenone was prepared by the Grignard procedure essentially as described for pr methoxy-a-trifluoroacetophenone, starting with mfbromo- Chlorobenzene. e) merifluoromethyl-a-trifluoroacet0phenone was prepared by the Griganrd procedure essentially as described for pfmethoxy-a-trifluoroacetophenone, starting with E7 bromotrifluoromethyl benzene. f) prrifluoromethyl-a-trifluoroacetophenone was prepared by the Grignard procedure essentially as described for pfmethoxy-a-trifluoroacetophenone, starting with 27 bromotrifluoromethyl benzene. 41 g) Valerophenone (Aldrich Chemical Company) was distilled under reduced pressure, passed through alumina, and redistilled. 2. Solvent a) Acetonitrile (Fisher Scientific Company) was distilled from KMnO, and Nazcoa according to the procedure of O'Donnell, Ayres and Mann.‘8 The final cut of the final fractional distillation was retained. b) Benzene (Mallinckrodt Chemical Works) was purified by stirring over concentrated H3804 several times until the acid layer no longer turned yellow. The benzene was washed with NaOH solution,distilled water and NaCl solution, followed by drying over anhydrous M9804 and frac- tional distillation from P205. The middle 80% was collected. 3. Hydrogen Donors a) Toluene (Fisher Scientific Company) was purified analogous to benzene. b) Anisole (Matheson.Coleman and Bell) was purified by successively washing with 1M NaOH, distilled water and saturated NaCl solution. After it was dried over anhydrous M9804, pure anisole was obtained by the center cut from fractional distillation. c) Chlorobenzene (Matheson, Coleman and Bell) was purified analogous to benzene. 42 d) Benzonitrile (Eastman Organic Chemicals) was purified analogous to anisole. e) Benzotrifluoride was purified analogous to anisole. f) prichlorobenzene (Matheson, Coleman and Bell) was fractionally distilled and subsequently sublimed at reduced pressure. 9) Fluorobenzene (Aldrich Chemical Co.) was purified analogous to benzene. h) primethoxybenzene was purified by Dr. Michael Thomas. 4. (Quenchers a) cis-Piperylene (Aldrich Chemical Co.) was passed through alumina followed by distillation. b) Naphthalene (Matheson, Coleman and Bell) was purified by three crystallizations from ethanol. 5. Internal Standard a) Hexadecane (Aldrich Chemical Co.) was purified analogous to benzene. b) Heptadecane (Chemical Sample Co.) was purified analogous to benzene. 43 c) Eicosane (Matheson, Coleman and Bell) was used without further purification. B . Photolysis 1. Irradiation Procedure After solutions of proper concentration were prepared, two to three 2.8 ml portions from each solution were with- drawn and injected into 13 x 100 mm culture tubes. The solutions were degassed four times at 3 x 10“ torr vacuum in freeze-thaw cycles and sealed off with a torch. The sample tubes were irradiated in parallel on a rotating ”merry-go-around” apparatus“9 immersed in a water bath to make sure that each sample absorbed the same amount of light. A 450 watt Hanovia medium pressure mercury lamp was used for photolysis. The quantum yield determinations and most of the quenching studies were done in 300-320 nm region which was isolated with a filter solution of 0.002M potas- sium chromate in a 1% aqueous solution of potassium carbonate. For naphthalene quenching experiments, the 366 nm region was isolated with a set of Corning No. 7083 filter combinations. 2. Analysis of Sample a) Instruments: Analysis for all photoproducts and standards were made on the following two vpc's which used flame ionization detectors. 44 VPC-1: Aerograph Hy-Fi model 600D gas chromatograph with 550 oven and 328 programmer connected to Leeds and Northrup Speedomax H recorder. This vpc was equipped with a 25' x 1/8" aluminum column containing 25% 1,2,3-tris- (2-cyanoethoxy)propane on 60/80 chromosorb P. VPC-2: Varian Aerograph 1200 gas chromatograph con- nected to Leeds and Northrup Speedomax W recorder. This vpc was equipped with a 8' x 1/8“ aluminum column contain- ing 4% QF-l, 1% carbowax 20M on 60/80 chromosorb G. Both instruments were connected to an Infrotronics Automatic Digital Integrator model CBS-208. b) Standardization: Known amounts of internal standard had to be used to determine the concentration of a photoproduct. The internal standard was selected not to overlap with any peaks in the Vpc traces, and the concentra- tion or the internal standard had to be adjusted so that the peak area of internal standard would be close to the area of the larger product peak. The standardization fac- tOr SF, which is different for different combinations of internal standards and photoproducts, could be determined by the following expression: _ [Product] Area of IS Peak [13] Area of Product Peak (25) SF where [IS] is the concentration of the internal standard. With the known SF, the concentration of the product could be determined: 45 _ Area of Product Peak [Product] SF x [IS] x Area of IS Peak (26) c) Actinometry and Quantum Yield Determination: The amount of’light absorbed by the parent ketone had to be determined in order to calculate the quantum yield of the reaction. Parallel irradiation of the actinometer tubes containing 0.1M valerophenone and 0.01M 015 internal standard in benzene provided a measurable amount of aceto- phenone produced yig_the type II elimination of valero- phenone. Since the quantum yield of acetophenone formation under these conditions was 0.33,25 the amount of light ab- sorbed by the reaction could be measured - [ACP] Ia " 6:33" (27) The quantum yield of the parent ketone reaction would then be measured from equation [Product] Ia (28) The intersystem quantum yield for erF3 and pfmethoxy- a-trifluoroacetophenone were measured by using gigfpiperyl- ene-acetophenone actinometry. Tubes containing 0.2M gig: piperylene and 0.1M ketones in acetonitrile were irradiated in parallel at 313 nm. Since the amount . I . . 55 . [9.1.9:PJ-Plo 1“ W = [PIP*3] (29) of excited triplets produced could be calculated for 46 acetOphenone (ACP), mftrifluoromethyl-a- trifluoroaceto- phenone (37CF3-TFA) and pfmethoxy-a-trifluoroacetophenone (pflMeO-TFA). The intersystem quantum yield could be derived from the following expression by knowing that the inter- system quantum yield of acetophenone is 1. [Pip*3] = g-MeO-TFA (¢isc)ACP [Pip* (3°) lACP (¢ ) isc preO-TFA C. Absorption and Emission Spectra Ultra violet spectra were taken on a Varian Cary 17 recording spectrophotometer. Phosphorescence spectra were taken on Aminco Bowman Spectrophotofluorometer equipped with a Houston Instrument 2000 Recorder. Lifetime of the ketones were measured on the apparatus of Professor Alfred Hang28 in the MSU-AEC Plant Research Laboratory. D. Reduction Potential Reduction potential of the ketones were measured at a dropping mercury electrode with a Sargent Model XV polaro- graph. A saturated potassium chloride bridge connected sample solutions (lo-SM ketone and 0.1M.TEAP as supporting electolyte in acetonitrile) to a freshly calibrated standard calomel electrode. 47 PART II: KINETIC DATA A. gpantum Yield of Bibenzyl Formation The quantum yields of bibenzyl formation were measured as a function of the concentrations of the hydrogen donors. BB refers to the bibenzyl formed from the toluene. Cn is the internal standard. Ia values were obtained from valerophenone actinometry and were averages of at least two tubes per run. Table 8. Quantum yield of bibenzyl formation for 0.05M ketone in acetonitrile with toluene, indicated cn and conditions. Part A: a-‘Trifluoroacetophenonea Toluene.M BB/Clg [BB].IO'°M Ia.El 433 2.00 1.1125 168 0.0491 0.0342 1.00 1.0198 154 0.0491 0.0313 0.75 1.8543 280 0.0977 0.0286 0.50 1.6225 245 0.0977 0.0251 0.25 1.2582 190 0.0977 0.0195 a0.00211 61.; SF - 1.5100; vpc -2. Part B: _p__-.Methoxy-cx-trifluoroacetophenonea Toluene.M BB/c20 [BB],10‘§M 151.21"1 ¢BB 4.00 1.0107 182 0.0853 0.0213 2.00 0.9996 180 0.1284 0.0140 1.00 0.6441 116 0.1284 0.0090 0.75 0.7497 135 0.2136 0.0063 0.50 0.5103 91.9 0.2136 0.0043 'ao.oo1u 630; SF . 1.8007; vpc — 2. Table 8 (Cont.) 48 Part C: fippMethyl-a~trifluoroacetophenonea Toluene,M BB/C17 [BB] .1041: 1am"1 ¢BB 4.00 1.0749 146 0.0244 0.0597 2.00 1.7972 244 0.0488 0.0500 1.00 1.3027 177 0.0488 0.0362 0.75 2.1722 295 0.0959 0.0307 0.50 1.8144 246 0.0959 0.0256 a0.00m C17: 89 - 1.3588; vpc-2. Part D: t_n_-Chloro-a-trifluoroacetophenonea 2.00 0.7636 103 0.0164 0.0626 1.00 0.7287 98.8 0.0164 0.0599 0.75 0.6868 93.4 0.0164 0.0567 0.50 1.2100 163 0.0323 0.0505 0.25 2.1237 143 0.0323 0.0443 a0.001M 01,; SE = 1.3514; vpc-2. Part E: m-Trifluoromethyl-a-trifluoroacet0phenonea 0.20‘ 1.2919 174 0.0423 0.0412 0.10 1.2463 168 0.0423 0.0397 0.07 1.5963 215 0.0624 0.0345 0.05 1.4588 197 0.0624 0.0315 0.02 0.9096 122 0.0624 0.0196 a0.001M c17; SF - 1.3514; vpc-2. Part F: jprTrifluoromethyl-a-trifluoroacetophenonea 0.10 1.8083b 250 0.0346 0.0721 0.07 3.0968: 214 0.0346 0.0617 0.05 3.7419 258 0.0510 0.0506 0.03 2.8816 199 0.0510 0.0390 asp - 1.3832; vpc-2. b c0.000511 C17. 0.001M C17. 49 B. ggenching Studies Two different sets of reactions were separately studied. In the reaction of trifluoroacetophenone with toluene, a series of substituted benzenes were used as quenchers to calculate the rate constants of the quenchers. On the other hand, when substituted trifluoroacetOphenones were reacted with toluene, naphthalene was used as the quencher in order to figure out the kd/kr ratio. Concentra- tions of bibenzyl formation were always kept in the 5 x 10-4- 2 x 10-3M region to ensure linearity in the Stern-Volmer plots. Table 9. System with 0.05M a-trifluoroacetophenone and 0.1M toluene in acetonitrile quenching with specific quencher in indicated Cn and conditions. Part A: Anisolea [Quencher],M BB/Clab ¢°/¢ 0.000 1.9591 0.005 1.5190 1.2897 0.007 1.4133 1.3861 0.01 1.2070 1.6231 0.03 0.6471 3.0272 a0.001M C13: VPC-2. bVPC area ratio. Part B: Benzene 0.00 2.2709 0.50 2.2293 1.0186 1.00 2.0878 1.0876 4.00 1.5018 1.5121 7.00 1.1563 1.9639 b a0.001M C15: VPC-2. VPC area ratio. Table 9. (Cont.) 50 Part C: Benzotrifluoridea [Quencher],M BB/C17 00/0 0.0 1.5911 0.5 1.5614 1.0190 1.0 1.5243 1.0438 5.0 1.3115 1.2131 a b . 0.001M C17; VPC-2. VPC area ratio. Part D: Benzonitrilea 0.0 1.7569 0.5 1.7224 1.0200 1.0 1.6801 1.0457 3.0 1.6131 1.0891 6.0 1.5000 1.1712 a b . 0.001M C17: VPC-2. VPC area ratio Part E: Chlorobenzenea [Quencher],M BB/Clsb ¢°/¢ 0.0 2.8914 0.2 2.6997 1.0710 0.3 2.6374 1.0963 0.5 2.5018 1.1557 1.0 2.1826 1.3247 2.0 1.7614 1.6415 a0.001M C15: vpc-2. b VPC area ratio. 51 Table 9. (Cont.) Part F: Fluorobenzenea b [Quencher],M BB/C17 90/0 0.0 2.0449 0.5 1.9057 1.0730 1.0 1.8402 1.1114 3.0 1.6337 1.2516 4.0 1.4900 1.3724 5.0 1.4473 1.4129 a b . 0.001M 017; VPC-2. VPC area ratio. Part G: gngimethoxybenzenea 0.00000 1.8745 0.00001 1.8097 1.0358 0.00004 1.6564 1.1316 0.00007 1.4850 1.2622 0.0001 1.3362 1.4028 a0.001M C17, VPC-2. bVPC area ratio. Part H:A_‘p_-Dichlorobenzenea 0.00 2.0186 0.25 1.8680 1.0806 1.00 1.4565 1.3859 1.50 1.2221 1.6517 a0.001M C17: VPC-2. bVPC area ratio 52 Table 10. System with 0.05M a~trifluoroacetophenone, 0.001M C20 and indicated amount of toluene in aceto- nitrile quenching with naphthalene. Part A: 1M Toluene aVPC area ratio: VPC-2. [Quencher],M BB/Cgoa ¢°/¢ 0.0000 2.0116 0.0003 1.6774 1.1992 0.0005 1.4088 1.4278 0.001 1.1631 1.7295 0.0015 0.9147 2.1991 aVPC area ratio; VPC-2. Part B: 0.4M Toluene 0.0000 1.9824 0.0001 1.7541 1.1301 0.0003 1.3110 1.5121 0.0005 1.1994 1.6528 0.001 0.6786 2.9213 53 Table 11. System with 0.05M pfmethoxy-a-trifluoroaceto— phenone, 0.001M C20 and indicated amount of toluene in acetonitrile, quenching with naphtha- lene. Part A: 1M Toluene [Quencher],M BB/Cgoa 00/4 0.000000 1.8159 0.000005 1.5836 1.1467 0.00001 1.4217 1.2773 0.00003 1.0814 1.6781 0.00005 0.8498 2.1369 aVPC area ratio: VPC-2. Part B: 2M Toluene 0.00000 2.6832 0.00001 2.0718 1.2951 0.00003 1.6005 1.6764 0.00005 1.2149 2.2085 0.0001 0.8088 3.3175 aVPC area ratio: VPC-2. Table 12. 0.05M m-Trifluoromethyl-a-trifluoroacetophenone- 54 0.1M t31uene system in acetonitrile quenching with indicated concentration of naphthalene. [Quencher],M BB/C17 00/0 0.000 1.3246 0.002 1.1843 1.1199 0.006 0.9082 1.4584 0.01 0.7581 1.7472 0.015 0.6045 2.1912 b VPC area ratio. Table 13. 0.05M archloro-aetrifluoroacetophenone- 1M toluene system in acetonitrile quenching with indicated concentration of naphthalene.a [Quencher],M BB/Clqb ¢°/¢ 0.0000 2.3209 0.0001 2.2766 1.0194 0.0005 2.1180 1.0957 0.001 1.9777 1.1735 0.002 1.6936 1.3703 a0.001M C17: vpc-2. b vpc area ratio. 55 Table 14. 0.05M pfiMethyl-a-trifluoroacetophenone- 1M toluene system in acetonitrile quenching with indicated concentration of naphthalene.a [Quencher],M BB/C17 00/0 0.00000 0.8750 0.00001 0.8242 1.0616 0.00003 0.7466 1.1719 0.0001 0.5682 1.5399 0.0002 0.4168 2.0993 a0.001M C17: VPC-2. bVPC area ratio. 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