PHONON PARTICIPATION IN SOLID-STATE PHOTOREACTION IN 6-BROMOCOUMARIN - LASER RAMAN-SPECTROSCOPIC STUDY

The photodimerization reaction in 6-bromocoumarin in the crystalline s tate is shown to be mediated by lattice phonon. Significant overlap be tween the electronic absorption and emission spectra suggests that the exciton-phonon coupling is not very strong in the monomer crystal. Ra man phonon spectra show softening of a specific phonon mode with photo reaction progress. This suggests that phonon participation in the reac tion is through a mode softening in the excited state of the crystal o n photoirradiation, not via strong exciton-phonon coupling. Raman phon on spectroscopic study also reveals that the reaction mechanism is hom ogeneous in the initial stage of reaction when the reactant and the pr oduct form solid solution and is heterogeneous in the later stage when the product segregates out. IR and Raman spectroscopy are used to cha racterize the reactant and the product, and the results confirm that r eaction occurs by cyclobutane ring formation through the > C = C < bon d of the pyrone ring. (C) 1998 Elsevier Science Ltd. All rights reserv ed.