Shpol'skii spectroscopy and vibrational analysis of [N]phenylenes

Vibrationally resolved fluorescence spectra of four angular [N]phenylenes were recorded with laser excited Shpol'skii spectroscopy (LESS) in an n-octane matrix at 10 K. In general, the same vibrational frequencies were observed in the fluorescence excitation and emission spectra, indicating that the geometries of ground and electronically excited state are very similar. Because of intensity borrowing from the S₂ state, vibrations of two different symmetries were observed in the fluorescence excitation spectra of angular [3]phenylene and zig-zag [5]phenylene. This finding allowed the location of the S₂ state for these compounds. DFT calculations (RB3LYP/6-31G*) of the ground state vibrational frequencies were made. The calculated vibrational modes were in reasonably good agreement with the experimental data. A new very low-frequency vibration of approximately 100 cm^-1 was predicted and experimentally confirmed for all [N] phenylenes investigated. This vibration seems to be unique for [N]phenylenes and is attributed to an in-plane movement of the carbon backbone.