Formation and stabilization of twisted intramolecular charge transfer states in binary mixed solvents

Abstract

Spectral and time resolved fluorescence studies were performed for 4-(N,N-dimethylamino)-benzonitrile (DMABNZ) and some derivatives which were designed to model twisted intramolecular charge transfer (TICT) states, in hexane binary solvent mixtures with ethanol. It is attempted to elucidate the mechanism of local interactions between solute and polar solvent molecules and their capacity to induce intramolecular charge transfer. It is shown that crossing to the TICT state occurs in these mixed solvents at low alcohol concentrations but the mechanism of the intermolecular interaction responsible for this effects depends strongly on the structural and photophysical features of the solute. Whereas 2-MODMABN shows effective ground state solvation because of its short fluorescence lifetime, 3-MDMABN is the best example for the appearance of successive excited state association with single alcohol molecules which leads to different solvate cluster sizes and finally to TICT emission. As excited DMABN reacts much slower with ethanol monomers than the sterically hindered derivative, complexation with cyclic alcohol multimers which become the most important structural feature at higher ethanol concentrations is the predominant mechanism in that case. It leads to efficient charge transfer in these solute clusters with several alcohol molecules, and bulk polarity effects on the resulting TICT fluorescence describe the spectral shift in the high alcohol concentration regime.