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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 (k et H) 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 reaction. The large isotope effect (k et H/k et D = 3.0) was observed for the ET reaction between 3DMB* and Haq + (or Daq +). The fluorescence of DMB was also quenched by Haq + with a quenching rate constant of 6.2 (±0.4) × 107 M-1 s-1. The singlet quenching was similarly ascribable to the ET reaction from 1DMB* to the hydronium ion.

The loss of HF is one of the main fragmentation processes in the metastable molecular ions of fluorophenols and fluoroanilines. By investigation of D-labelled compounds, it could be established that the eliminated HF molecule contains the hydroxylic or amino hydrogen atom. The other ring hydrogen atoms are not involved in the fragmentation. In order to rationalize this process, a 'ring-walk' mechanism of the fluorine atom is proposed.