Electronic spectra of jet-cooled 1-phenylpyrrole: Large-amplitude torsional motion and twisted intramolecular charge-transfer phenomenon

The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra in the region of the S0⇆S1 transition were measured for jet-cooled 1-phenylpyrrole. The 0–0 band was observed at 35 493 cm−1. Long and low-frequency progressions with somewhat irregular intensity distributions appeared on both spectra, and were assigned to torsional motion. The torsional energy levels in the S0 and S1 states were obtained up to 25 and 16 quanta, respectively. The torsional potentials in both states could be determined from the sufficient number of energy levels observed. In the S0 state the most stable conformation was determined to be a twisted form with a dihedral angle of 38.7°, where the planar barrier height was calculated to be 457 cm−1, and the perpendicular to be 748 cm−1. On the other hand, it was discovered that 1-phenylpyrrole in the S1 state also had a twisted form with a somewhat smaller dihedral angle of 19.8°, and that the barrier to planarity was 105 cm−1 and to perpendicularity, 1526 cm−1. These facts indicated that the electronic excitation caused 1-phenylpyrrole to be rigid to twist. 1-Phenylpyrrole and its derivatives have been reported as a group of twisted intramolecular charge-transfer (TICT) molecules. No indication of TICT appeared on the shape of the S1-state torsional potential determined. The relation between torsional potential and TICT is discussed based on the results of this study.