Intramolecular Charge-Transfer Dynamics in p-Dimethylaminobenzonitrile·Acetonitrile Clusters. A New Twist

Excited-state dynamics of p -dimethylaminobenzonitrile (DMABN) complexed with acetonitrile solvent molecules, DMABN(CH 3 CN) n, have been investigated as a function of the cluster size using dispersed fluorescence spectroscopy implemented with time-of-flight mass analysis. No significant red shift in the fluorescence spectra is found to occur irrespective of the cluster size, when the clusters are excited in the vicinity of the S 1 electronic origin of the DMABN solute. As the excitation energy is increased, strongly red-shifted fluorescence which resembles the charge-transfer (CT) fluorescence in solution is detected for clusters of n ≥ 5, while those of smaller sizes exhibit fragment fluorescence upon solvent evaporation. The size- and excitation-energy-dependent behavior suggests that the CT dynamics and subsequent relaxation in the jet-cooled clusters are controlled by microscopic hindrance of intramolecular structural changes due to solvent cage effects. Thus, the CT stabilization can be activated by increasing cluster internal temperatures and facilitating solvent reorganization.