Spectroscopic study of photoreaction dynamics in p-bromo cinnamic acid

摘要

The photodimerization reaction in p-bromocinnamic acid (p-BCA) in the crystalline state has been probed by Raman phonon spectroscopy. Monotonic redshift of the phonon bands and nonappearance of any new phonon bands on reaction progress suggest that the reaction is homogeneous and the reactant and the product form a solid solution. It is further observed that a low frequency phonon at 18.6 cm~(-1) softens on photoexcitation and progress of reaction. The photoreaction in p-BCA crystal therefore seems to be mediated by the lattice phonon. In the IR spectrum disappearance of the aliphatic > C=C < stretching frequency and appearance of cyclobutane ring deformation and ring stretching modes on reaction confirm photodimerization by cyclobutane ring formation. Considerable overlap between the absorption and emission bands shows that the exciton-phonon coupling is not very strong in the monomer crystal.