Direct observation of the change in transient molecular structure of 9,9′-bianthryl using a 10 fs pulse UV laser

Photoexcitation of 9,9′-bianthryl has been considered to produce a locally excited (LE) state and then a charge transfer (CT) state by intramolecular charge transfer (ICT) in polar solvents. In contrast, the CT state is considered to be unstable in non-polar solvents and triggers a dipole flip-flop between the two anthracene rings. The central C–C bond between the two anthracene rings is considered to twist during relaxation in the LE state and CT state, and this was visualized in the present work using an ultrashort ultraviolet (UV) pulse. The C–C stretching frequency was recovered in a period of ca. 500 fs, which implies that the C–C bond undergoes repeated torsion back and forth in the electronic excited state. Investigation of samples in a polar solvent and a non-polar solvent indicated that ICT does not affect the torsion around the central C–C bond.