Rotational coherence spectroscopy of 9,9′-bianthyrl and its van der Waals complexes with Ar and H2O

The excited state structure and dynamics of 9,9′-bianthryl and its Ar/H2O complexes have been studied using rotational coherence spectroscopy (RCS). The intensity and the decay of the RCS signal were found to be dependent on the torsional level of the bianthryl, reflecting the change of rotational constants with torsional levels. For the Ar and H2O complexes, no RCS signals were observed in a time-correlated single photon counting (TCSPC) experiment, though small signals were detected by a pump-probe time-resolved fluorescence depletion measurement. The smallness of the signal for the Ar complex is mostly attributed to structural reasons — deviation from a symmetric top and a hybrid-type transition unfavorable to RCS signals. For the H2O complex, the loss of coherence due to the rapid conversion of the electronic state is responsible for the absence of the signal in the TCSPC measurements.