FORMATION AND STABILIZATION OF TWISTED INTRAMOLECULAR CHARGE-TRANSFERSTATES IN BINARY MIXED-SOLVENTS

Spectral and time resolved fluorescence studies were performed for 4-( N,N-dimethylamino)-benzonitrile (DMABN) and some derivatives which wer e designed to model twisted intramolecular charge transfer (TICT) stat es, in hexane binary solvent mixtures with ethanol, It is attempted to elucidate the mechanism of local interactions between solute and pola r 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 stron gly on the structural and photophysical features of the solute. Wherea s 2-MODMABN shows effective ground, state solvation because of its sho rt fluorescence lifetime, 3-MDMABN is the best example for the appeara nce of successive excited state association with single alcohol molecu les which leads to different solvate cluster sizes and finally to TICT emission, As excited DMABN reacts much slower with ethanol monomers t han the sterically hindered derivative, complexation with cyclic alcoh ol multimers which become the most important structural feature at hig her ethanol concentrations is the predominant mechanism in that case, It leads to efficient charge transfer in these solute clusters with se veral alcohol molecules, and bulk polarity effects on the resulting TI CT fluorescence describe the spectral shift in the high alcohol concen tration regime.