Structure and intramolecular charge-transfer dynamics of p-(dimethylamino)benzonitrile in solvent clusters

Excited-state dynamics of p-(dimethylamino)benzonitrile (DMABN) complexed with water and acetonitrile, DMABN(H2O)(n) and DMABN(acetonitrile)(n), have been investigated as a function of cluster size by the spectroscopic techniques of resonant two-photon ionization, fluorescence excitation, dispersed fluorescence, and hole-burning. In water clusters, a distinct red shift of fluorescence is observed to occur for n = 3. A hole-burning measurement reveals that this behavior is strongly dependent on the excess vibrational energy deposited on the cluster, thereby suggesting the occurrence of intramolecular charge transfer (CT). However, the extent of the CT stabilization, which is manifested as the fluorescence red shift, is found to be much less than in bulk water. As the solvation number increases up to n approximate to 30, the red shift appears to saturate and does not converge to the bulk limit. Similar saturation behavior is observed for acetonitrile clusters. The absence of bulklike CT emissions in both solvent clusters is attributed to their low internal temperatures or cluster rigidity, which impede large structural changes of the DMABN solute leading to CT. This cluster rigidity scenario suggests that the fluorescence from larger clusters originates from an unrelaxed CT state trapped in a solvent cage and thus further stabilization can be activated upon increasing cluster internal energy.