Kinetics of photochromic processes in substituted dihydropyridines in the solid state and in solutions

Results are presented of studies of the kinetics of thermally driven reacti ons in substituted dihydropyridines: 1-methyl-2,4,4,6-tetraphenyl-1,4-dihyd ropyridine (DHP), and 4,4-(diphenyl-2,2 ' -diyl)-2,6-diphenyl- 1-methyl-1,4 -dihydro-pyridine (BDH). The processes have been monitored by measuring the isothermal bleaching of coloured species produced upon UV irradiation of t he samples, and by measuring the heat flow in non-isothermal differential s canning calorimetry experiments. A comparison of quantum-chemical calculati ons with results of spectroscopic and kinetic measurements seems to indicat e that the initial step following an electronic excitation of DHP may be a radical, the absorption at 520 nm being probably associated with S-0 and T- 1 states of one of intermediate products. The primary photochemically drive n reaction (intramolecular phenyl shift) is irreversible: the photochromic cycle consists of two parallel processes: 2,4-ring closing and intramolecul ar hydrogen shift, The results described above are compared with those obta ined on BDH - an analogue of DHP in which the potentially mobile phenyl gro ups are linked with a chemical bond.