Laser Raman phonon spectroscopy of solid-state photoreactions: photodimerization of O-methoxy trans cinnamic acid

Solid-state photodimerization reaction of O-methoxy trans cinnamic acid has been studied by laser Raman phonon spectroscopy. The progress of reaction has been estimated by monitoring the monomer 315-nm electronic absorption band which decreases in intensity with reaction progress and disappears in the dimer spectrum. The disappearance of the aliphatic C = C stretching monomer mode 1622 cm−1 in the infrared spectrum of the dimer suggests cyclobutane ring formation. Other infrared features also suggest four center type dimerization. In the monomer crystal, Raman phonon bands are observed at 32.5,57.5,65,89,95.5, and 160 cm−1. With reaction progress the sharp 32.5-cm−1 phonon shifts to lower frequency and finally disappears. This suggests softening of this mode with reaction progress and that the reaction is mediated by this phonon. This is the first example of phonon mode softening in solid state photoreaction. In the partially dimerized crystal, the monomer phonon bands lose their intensity and new phonon bands appear. When the reaction is almost complete, a new phonon spectrum is observed. The appearance of the segregated phonon spectra in the partially dimerized crystal suggests that the reaction is heterogeneous and the reactant and the product form separate lattices.