An intermolecular hydrogen transfer process inducing triplet biradical photochromism of 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine in the solid state

An intermolecular hydrogen transport process, as a source of photochromism in a crystal, was identified in 1-methyl-2,4,4,6-tetraphenyl-1,4-dihydropyridine (DHP). The photochromic form of that compound, dark violet in color, according to EPR measurements is triplet biradical in nature. According to photoluminescence measurements, the lowest excited triplet state (T1) as well as the excited singlet state (S1) of the DHP molecule is of π,π* type. Two active non-symmetrical biradical forms, absorbing at 400 nm and 550 nm, are proposed connected with a hydrogen transfer process created by transformation of the forms on mutual terms (‘hydrogen cascade’). Optical generation of the photochromic form was followed by steady-state UV–vis spectroscopy. The rate constants are equal to k400 = 4.36 ± 0.17 × 10−3 s−1 and k550 = 5.87 ± 0.56 × 10−4 s−1, respectively. The values of energy of activation of these two parallel processes obtained by analysis of non-isothermal kinetics of the bleaching process are equal to 10.96 kJ/mol and 31.2 kJ/mol, respectively. The IR spectra together with EPR measurements as well as X-ray structural analysis and calculations on the semi-empirical AM1 level method seem to support the intermolecular hydrogen transfer chain process.