Unique solvent properties of straight-chain carboxylic acids for electron-transfer photoreactions

We have studied the electron-transfer fluorescence quenching of pyrene by diphenyliodonium cation in a series of straight-chain carboxylic acid solvents. Use of the Py-scale of solvent polarity suggests specific solvation of the pyrene by the polar head groups of the acid solvents. We hypothesize involvement of hydrogen bonding between the carboxyl functions and the pi-electron cloud of pyrene. Electron-transfer quenching of pyrene fluorescence by diphenyliodonium cation is, surprisingly, not diffusion controlled. From experimental data we have estimated values for the bimolecular reaction rate constant, k(r) in each solvent over the temperature range 20-60 degrees C, and calculated, therefrom, reorganization energies. We find that lambda is more-or-less independent of acid chain length. We, therefore, infer that the head groups or the carboxylic acids are principally involved in solvent relaxation, and propose that a relaxation mechanism involving rupture (or formation) of hydrogen bonds is most likely. (C) 2000 Elsevier Science S.A. All rights reserved.