Coherent Phonons in a 1,3,5-Tri-Phenylbenzene Crystal

We studied low frequency vibrational modes in a light-harvesting (LH) antenna component molecule (1, 3, 5-tri-phenylbenzene: tri-ph) of a LH dendrimer by measuring a coherent phonon (CP) signal and molecular vibration analyses with semi-empirical molecular orbital calculations. In a Fourier transform spectrum obtained from the transient CP signal, we found three vibrating components with the frequencies of 35 ± 13, 60 ± 10, and 84 ± 14 cm−1. In the lower frequency range than 100 cm−1, the molecular vibration analyses give the following three kinds of vibrational modes: torsional vibrations, a butterfly vibration and bending vibrations, in which peripheral aromatic rings librate against the central ring in different manners. We identified the lowest frequency vibrating component (35 ± 13 cm−1) as the torsional vibrational mode changing dihedral angles and co-planarities between the central and the peripheral aromatic rings. In addition, it was also found that the vibrational frequencies obtained by our measurements are higher than those of the vibrational mode analyses. These hardenings are considered to account for steric hindrance between the neighbor molecules after the crystallization.