Remarkable accelerating effects of ammonium cations on electron-transfer reactions of quinones by hydrogen bonding with semiquinone radical anions

Remarkable accelerating effects of the ammonium cation (NH ₄ ⁺) have been observed on photoinduced electron-transfer reactions from the triplet excited state of tetraphenylporphyin (H ₂P) to quinones [p-benzoquinone (Q) and naphthoquinone (NQ)] in dimethyl sulfoxide (DMSO). The tetrabutylammonium cation (NBu ₄ ⁺) is also effective to accelerate the electron-transfer reduction of Q and NQ in dichloromethane (CH ₂Cl ₂). The hydrogen bonding interaction between the semiquinone radical anion (Q ^•- or NQ ^•-) and NH ₄ ⁺ was confirmed by the ESR spectra of the Q ^•-/NH ₄ ⁺ and Q ^•-/(NH ₄ ⁺) ₂ [NQ ^•/NH ₄ ⁺ and NQ ^•/(NH ₄ ⁺) ₂] complexes in DMSO. Accelerating effects of NH ₄ ⁺ in DMSO and NBu ₄ ⁺ in CH ₂Cl ₂ on the rates of photoinduced electron-transfer reduction of Q by H ₂P result from the positive shift of the E _red value of quinones together with a constant E _ox value of H ₂P determined from cyclic voltammetry measurements. The driving force dependence of the rate constants for photoinduced electron-transfer reduction of quinones in the presence of various concentrations of ammonium cations was evaluated in light of the Marcus theory of electron transfer.