Photochromic activity of dihydropyridine derivatives: energetics and kinetics of photochemically driven reactions in polycrystalline 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine

Kinetics of thermally driven reactions in a crystalline photochromic solid 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine (DHP) has been studied. Th e processes have been monitored by measuring the isothermal bleaching of co loured species produced upon UV irradiation of DHP, and by measuring the he at flow in non-isothermal differential scanning calorimetry experiments. Si gnatures of two processes were detected; a combination of quantum-chemical calculations and spectroscopic measurements allowed us to attribute them to reactions occurring in the photochromic system. The model put forward in t he paper assumes that the primary photochemically driven reaction (intramol ecular phenyl shift) is irreversible; the photochromic cycle consists of tw o parallel processes: 2,4-ring closing and intramolecular hydrogen shift. ( C) 1999 Elsevier Science S.A. All rights reserved.