Photophysics of some styryl thiazolo quinoxaline dyes in organic media

The photophysics of a new class of styryl dyes, 2-styryl thiazolo quinoxali ne (STQ) based structures was investigated in organic solvents and organize d molecular assemblies. The absorption, steady state and time-resolved fluo rescence characteristics of the STQ dyes in low-viscosity organic solvents are consistent with a single species in the ground and excited state. The o ne electron electrochemical oxidation and reduction potentials of the dyes are within +/-1 V vs. NHE, The spectral shifts of the dyes in organic solve nts are linearly correlated with the variation of solvent polarity paramete rs. The dipole moments in the ground and excited state of the dyes were cal culated without assuming a value for the cavity radius. The temperature dep endence of the nonradiative rate of STQ dye in DMSO indicated an activation barrier (DeltaE = 10.7 kJ/mol) which is comparable to the activation energ y (E-a = 13.7 kJ/mol) of viscous friction in DMSO. In dichloromethane, the activation barrier is 34.0 kJ/mol which is very high compared to E-a = 6.64 kJ/mol. Formation of a dye-solvent complex is suggested in dichloromethane . The fluorescence decay of STQ dye is multiexponential in a viscous solvent (2-octanol) or when bound to a protein (Lysozyme), micelle or lipid membran e. In 2-octanol, the decay parameters are wavelength dependent and the resu lts are consistent with the mechanism of excited state kinetics of solvent relaxation. In other systems, the multiexponential decay is due to multiple sites of solubilization of the dye in the organized molecular assembly. (C ) 2000 Elsevier Science B.V. All rights reserved.