Electron Transfer Reaction from Triplet 1,4-Dimethoxybenzene to Hydronium Ion in Aqueous Solution

Abstract

The electron transfer (ET) reaction from the triplet state of 1,4-dimethoxybenzene (3DMB*) to the hydronium ion (Haq+) in aqueous solution at 295 K was studied by means of the laser photolysis method. 3DMB* was quenched by Haq+ and Daq+ with the quenching rate constants of 7.7 (±0.4) × 106 and 2.6 (±0.4) × 106 M-1 s-1, respectively. The occurrence of the ET reaction was substantiated by formation of a DMB cation radical (DMB+•) coinciding with disappearance of 3DMB*. From kinetic analyses, the rate constant (ketH) for the ET reaction from 3DMB* to Haq+ was determined to be 7.5 (±0.4) × 106 M-1 s-1. The observed ET rate exhibited a significantly smaller value compared to the diffusion-controlled rate in aqueous solution. The free energy changes for the ET reaction were estimated to be +28 and −175 kJ mol-1 for the reactions 3DMB* + Haq+ → DMB+• + H and 3DMB* + Haq+ → DMB+• + 1/2H2, respectively. The observed small ET rate constant (7.5 (±0.4) × 106 M-1 s-1) revealed that the ET corresponds to the former re...