Lattice phonon mediation of solid state photoreaction in 7-methoxycoumarin crystal: laser Raman spectroscopic study

The photodimerization reaction in 7-methoxycoumarin (7MC) in the crystalline state is shown to be mediated by a lattice phonon. The phonon participation is through a mode softening in the excited state of the crystal on photoirradiation and not via strong exciton—phonon coupling as in generally observed. IR and Raman spectroscopy were used to characterize the reactant and product, and it is confirmed that the reaction occurs by cyclobutane ring formation through the bond of the pyrone ring. Electronic spectroscopy suggests that the exciton—phonon coupling is weak in the monomer crystal.