Abstract
Time-resolved laser-induced optoacoustic spectroscopy was used for the determination of the enthalpy, DeltaTH, and structural volume changes, DeltaTV, concomitant with triplet state formation upon excitation of meso-tetrakis(4-sulfonatophenyl)porphyrin, TSPP(4-), as well as with the triplet state electron-transfer (ET) quenching by benzoquinone, BQ (DeltaRH and DeltaRV). The values of DeltaTH and DeltaTV for (3)TSPP(4-) formation in the presence of different cations (Li+, Na+, K+, NH4+, and Cs+) correlated with each other and afforded a value of DeltaTG = 140 +/- 20 kJ mol(-1), equal to the value of E+ at 77 K, but much larger than the DeltaTH values in solution at room temperature, due to the large entropic factor in solution. The influence of the cations on DeltaTH and DeltaTV (a contraction ranging from 5.4 to 3.8 cm3 mol(-1)) is attributed to changing chromophore-water interactions in the ground and triplet states. Upon quenching of 3TSPP(4-) by BQ, the quantum yield of free radical formation, PhiR = 0.66 +/- 0.04, is the same in the solutions of the five cations. The values of DeltaRH and DeltaRV are small and have a large error. The energy level of the free radicals formed is thus very similar to that of 3TSPP(4-). TDeltaRS and X = TDeltaRS/DeltaRV, i.e., the structural volume change-normalized entropy change for free radicals formation, were derived using average values of DeltaRH and DeltaRV together with the calculated DeltaRG degrees . The measured Marcus reorganization energy, lambda, and X fall into the lambda vs X linear dependence we previously found for the radical formation upon ET quenching of triplet flavins (3FMN and 3FAD) by amines and amino acids. Thus, X = TDeltaRS/DeltaRV in aqueous solutions is a property of the particular donor-acceptor pair linearly correlated to the corresponding Marcus reorganization energy. The value of X is much larger than the predicted value applying the electrostriction concepts in view of the noncontinuum nature of the aqueous solutions.
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