Role of Photoionization on the Dynamics and Mechanism of Photoinduced Electron Transfer Reaction of Coumarin 307 in Micelles

Abstract

The dynamics and mechanism of the photoinduced electron transfer (PET) reaction between coumarin 307 (C307) and aromatic amines in micelles have been studied by using steady-state (S-S) and time-resolved (T-R) absorption and fluorescence spectroscopy. Based on the fluorescence quenching time scale, PET in micelles is grouped into two types: (i) ultrafast electron transfer (ET) due to the close contact of the donor and acceptor in micelles and (ii) diffusion averaged dynamic electron transfer (DADET) which is controlled by the diffusion of the reactants in micellar Stern layer and diffusion of the micelles. The DADET does not affect the photoionization and solvation processes whereas ultrafast ET competes with the photoionization and faster than the solvation process. Both ultrafast and DADET shows Marcus inversion in the ET rates at the similar exergonicity and indicates that the role of diffusion and solvent reorganization is negligible toward the activation barrier for the ET reaction in micelles. The activation barrier for the ET reactions in micelles is mainly due to intramolecular reorganization energy. The intramolecular reorganization energy must be higher in CTAB due to the photoionization and subsequent recombination and also involvement of triplet state in the PET. The ET reaction between coumarin radical cation and amine is reported for the first time in the C307-amine systems in micelles which are confirmed by the effect on amine concentration of the decay of coumarin radical cation and the dynamics of the ground-state recovery of C307. A mechanism for the PET reaction between C307-amine systems is proposed in micelles including photoionization, ultrafast and dynamic ET, and solvation dynamics.