Crystalline state photoreaction in 4-methylcinnamic acid: a Raman phonon spectroscopic study

The solid-state photodimerization reaction mechanism in 4-methylcinnamic acid crystal was investigated. Raman and infrared spectroscopy were used to study the intramolecular vibrations of the reactant and the product in order to characterize them. The absence of any large Stokes shift between the excitation and emission bands of the monomer crystal shows that the exciton–phonon coupling is weak. Raman phonon spectroscopic studies reveal that the reaction proceeds by a homogeneous mechanism in the initial stage and subsequently by a heterogeneous mechanism. The appearance of new sharp phonon bands suggests that the product lattice is ordered. No mode softening is observed. Hence the reaction seems to be topochemically controlled and lattice phonons do not take part in the reaction.