Torsional band assignment and intramolecular twist potential of 9,9′-bianthryl and its 10-cyano derivative in a free jet

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Abstract
Using a pulsed supersonic beam and laser‐induced fluorescence spectroscopy the torsional structure of the S 0 and S 1 states of 9,9’‐bianthryl (BA) and its derivative 10‐(cyano)‐9,9’‐bianthryl (CBA) was investigated. Taking into account the very different equilibrium positions of the S 0 and S 1 potentials which result in a nonobservable 0–0 transition within the jet spectra, a new and straightforward procedure of torsional band assignment is carried out. This is based on a characteristic pattern of Franck–Condon factors within the dispersed fluorescence spectra. The torsional potentials were determined by a fit procedure of a one‐dimensional model to the experimental data. The results show that the S 1 double minimum potential for BA is shallower than for CBA indicating a stronger interaction between the molecular halfs of the latter compound. The observed rotational contours of torsional bands recorded for CBA reflect the change from a symmetric top molecule (for states above the S 1 torsional barrier) to an asymmetric top (for states below the barrier) and manifest the tunneling splitting of the level just below the barrier. The dispersed fluorescence spectra of CBA are discussed in terms of intramolecular vibrational redistribution (IVR) processes. The measuredfluorescence decay rates as a function of excess vibrational energy of CBA reflect a saturation behavior already within the origin region in contrast to BA (saturation near 380 cm−1). This is tentatively ascribed to a low lying dark background state possibly of charge transfer character.