Kinetics of photochromic processes in dihydropyridine derivatives

摘要

substitued dihydropyridines belong to a group of photoactive compounds whose properties have been relatively little known (cf. the accompanying paper [1] reviewing the structure and properties of dihydropyridines, pyrans, thiopyrans and selenopyrans). An interesting feature of the photochemical activity of many representatives of this group is that reactions following an electronic excitation are reversible (i.e., the compounds exhibit the photochromic effect), and that the effect can be observed both in liquid solutions, in solid matrices (e.g. in polymer foils), and in crystallites. Moreover, the effective rate constants deduced from the kinetics of colouring and bleaching have been shown to differ by several orders of magnitude [2-8], thus opening (at least in principle) an interesting possibility of tailoring the response kinetics of the systems to particular needs. Several molecules belonging to this group have ben synthesized to date (cf. [1,9] and references therein), complex studies, however, have been performed only on few systems. Thus it comes as no surprise that the molecular mechanism of the activity of these moleculaes has not been unequivocally established. it has been postulated that the initial step in the photochromic cycle involves an intramolecular phenyl shift [9-11] and/or a 3,5-bridge formation [12-16]. Some further steps in the sequence of elementary processes have also been proposed, e.g., a 2,4-bridge formation [2,3,9,14] or a hydrogen shift [6, 17]. It has also been believed that initial stages of the photochromic process are identical, irrespective of the nature of the heteroatom in the central ring.