Microscopic solvation of acetonitrile to 9,9′-bianthryl studied in gas phase clusters:: Polar excited state formation

Dynamics of the electronically excited state of 9,9'-bianthryl(BA) was studied in gas-phase clusters with acetonitrile (ACN), BA-(ACN)(n) (n = 1-5). The gas-phase clusters were formed by supersonic expansion of BA/acetonitrile/He mixtures. In the laser-induced fluorescence spectra, new absorption bands ascribed to the BA-(ACN)(n) clusters were observed at longer wavelength side of the S-1-S-0 transition of BA. The mass-selected two-color resonance-enhanced multiphoton ionization (REMPI) spectra were measured to determine the absorption band for the cluster of BA with the specific number of the acetonitrile molecules. The dispersed fluorescence (DF) spectra and lifetimes were measured for these absorption bands to study the dynamics of the excited state of BA in the clusters, BA-(ACN)(n). For all clusters studied, the DF spectra of BA were broad and largely red shifted compared with that of unclustered BA. The fluorescence lifetimes of BA in the clusters were considerably longer than that of unclustered BA. These results suggest that the photoexcited state of BA in the clusters change to the polar excited state which is analogous to the twisted intramolecular charge transfer (TICT) state of BA in the acetonitrile solution. The Stokes shift and lifetime of the BA-(ACN)(1) suggested that the charge-transfer reaction becomes possible by adding one acetonitrile. However, these two properties of the larger clusters do not show monotonic change toward those of BA in acetonitrile solution. These results implied that the structure and dynamics of the clusters play some important roles in the charge-transfer reaction.