Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

Abstract

The dynamics of the bimolecular quenching of triplet excited benzophenone by anisole was studied by nanosecond flash photolysis. We carried out a detailed study of the solvent dependence of the reaction rates and efficiencies in a number of protic and non-protic solvents. These studies were augmented by theoretical modelling and experimental investigation of solute/solvent interactions in the triplet excited and the ground state, respectively. The triplet quenching that follows Stern-Volmer kinetics in all cases is profoundly dependent on the nature of the solvent, with the highest reactivity being consistently found in protic solvents. The results in non-protic solvents are compatible with unproductive quenching via a charge-transfer state, whereas the generally fast quenching in protic solvents is accompanied by efficient formation of free-radical products. Analysis of the solvent dependence in terms of Marcus theory reveals the impact of specific solvation of benzophenone by protic solvents on the ET driving force and kinetics. Specific solvation is found to support efficient free radical ion formation in media of moderate and low polarity as well.

Unusual photobehavior of benzophenone triplets in hexafluoroisopropanol. Inversion of the triplet character of benzophenone

“…This finding is in agreement with our recent observation of enhanced photoactivity of BP in protic media toward quenching by anisole [18], and it parallels the results from a related study by Miranda et al [26]. In our system, the gain in the reactivity in HFIP was accompanied by a switch in the quenching mechanism itself.…”

“…This red shift of the → * band amounts to as much as 12 nm in HFIP with respect to ACN, corresponding to a stabilization of the singlet , * state relative to the ground state by 22.0 kJ mol −1 . These data are in accord with previous findings [16][17][18][19] where the most important contribution to the observed solvatochromism was attributed to the effect of specific solute-solvent interactions resulting from the formation of hydrogen bonding between the solute molecule and the protic solvent.…”

“…In our system, the gain in the reactivity in HFIP was accompanied by a switch in the quenching mechanism itself. Recently, we found some evidence that the overall appearance of the BP triplet quenching by AN changes from charge-transferinduced nonreactive quenching in nonprotic solvents to reactive ET-quenching in protic solvents, concomitant with efficient freeradical formation in the protic solvents [18]. This was interpreted as due to a decrease in the free energy of the ET and the free energy of activation, both as an effect of the specific H-bonding of the BP chromophore by protic solvents and additives.…”

“…These UV-vis spectra of the BP and AN system are the sum of the UV-vis spectra of BP and AN. Accordingly, the triplet quenching by anisole in ACN solution, which was studied in detail recently [18], was not connected with any formation of free radical-ion products that might signify that a full ET occurred. In contrast, the significant spectral evolution seen in HFIP signals that there was efficient formation of quenching-derived transients at 540 and 430 nm that were stable on the microsecond timescale ( Fig.…”

“…H-bonding interaction between the solvent molecule and BP was recently confirmed by density functional theory calculations for TFE as a solvent. The calculations showed that hydrogen bonding plays an important role both in the ground state and in the excited state of BP [18].…”