TOPOCHEMICALLY CONTROLLED SOLID-STATE PHOTOREACTION IN 7-ACETOXY COUMARIN - A RAMAN PHONON SPECTROSCOPIC STUDY

The solid state photodimerization reaction mechanism in 7-acetoxy coum arin (7AC) crystal has been investigated. Raman phonon spectroscopic s tudies reveal that the reaction mechanism is homogeneous and the latti ce becomes disordered with the increase of product concentration. No m ode softening is observed. Electronic excitation and emission spectra show that exciton-phonon coupling is weak in the monomer crystal. Thus the reaction seems to be topochemically controlled and lattice phonon s do not take part in the reaction. Formation of cyclobutane rings thr ough the > C=C < of the pyrone moiety is established by infrared and R aman spectroscopy.