Femtosecond dynamics of "TICT" state formation in small clusters:: The dimethylaminobenzomethyl ester acetonitrile system

The solvent effect on the appearance of a red-shifted twisted intramolecular charge-transfer (TICT) emission is studied in molecular clusters. Using a supersonic expansion, dimethylaminobenzomethyl ester-acetonitrile (DMABME-(CH3CN)(n)) clusters are studied by monitoring at the same time mass spectra and dispersed fluorescence spectra as well as by lifetime measurements. For DMABME-(CH3CN), clusters, a clear redshifted fluorescence is observed readily when the cluster contains one solvent molecule and the fluorescence decay becomes biexponential. Short-time evolution of the system has been monitored using a femtosecond pump/probe technique and picosecond photoelectron spectroscopy. The femtosecond dynamics is solvent dependent and is interpreted as a fast decay from the locally excited state to the TICT state reaching an equilibrium between these two states. The equilibrium shifts to the TICT states as the cluster size increases. The role of the triplet state in this process and in the biexponential character of the fluorescence is discussed.