Phonon spectroscopy of photochemical reactions in organic solids: Photodimerization of 2-benzyl-5-benzylidenecyclopentanone and photopolymerization of 2,5-distyrylpyrazine

The nature of the photoreactions in 2-benzyl-5-benzylidenecyclopentanone (abbreviated as BBCP) and 2,5-distyrylpyrazine (abbreviated as DSP) is investigated by a combination of Raman spectroscopy and electronic spectroscopy. The Raman phonon spectra, obtained as a function of reaction progress, reveal that photodimerization of BBCP in the solid state proceeds by a homogeneous mechanism. But a similar study of photopolymerization of the DSP crystal indicates a heterogeneous behavior for this reaction. The 4.2 K electronic spectra for both systems show a strong electron-phonon coupling in the reactive excited electronic state. The observation of a strong electron-phonon coupling in the reactive electronic state supports a model of phonon-assisted photochemical aggregation (dimerization, polymerization) reaction. In this model, a strong electron-phonon coupling localizes the excitation energy as well as provides a local lattice configuration, which is a precursor of the product lattice.