On the mechanism of bleaching reactions in a photochromic dihydropyridine derivative

Kinetics of thermally driven reactions in solutions of photochromic 1-methy l-2,4,4,6-tetraphenyl-1,4-dihydropyridine (DHP) has been studied. The proce sses have been monitored by measuring the isothermal bleaching of species a bsorbing at ca. 520 nm, produced upon UV irradiation of solutions of DHP in chloroform and in chloroform/PMMA. The decay curves can be interpreted ass uming a contribution from two coloured species produced in a sequence of el ementary processes. A comparison of quantum-chemical calculations with resu lts of spectroscopic and kinetic measurements seems to indicate that the in itial step following an electronic excitation of DHP involves radical forma tion, the absorption around 520 nm being probably associated with S-0 --> S -1 and T-1 --> T-2 transitions in one of intermediate products. (C) 1999 El sevier Science S.A. All rights reserved.