Effect of binary solvent mixtures (DMSO/water) on the dipole moment and lifetime of coumarin dyes

Absorption and fluorescence spectra of four coumarin laser dyes, Coumarin 440 (C440), Coumarin 450 (C450), Coumarin 466 (C466) and Coumarin 151 (C151), have been recorded in binary mixtures of DMSO and water at room temperature (298 K) to study the solvent effect and to evaluate the ground and excited state dipole moments using solvatochromic shift method. The anomalous feature in the complex systems of these mixtures due to the structural heterogeneities is discussed. The effect of solvent polarity parameters of binary mixture on the ground and excited state dipole moments was examined. Experimental results indicate that the dipole moments in the excited state, for all the molecules investigated, are higher than the corresponding values in the ground state. The ground and excited state dipole moments of these probes have also been computed from ab initio calculations and compared with those obtained experimentally. Fluorescence lifetimes of all the probes measured using time domain fluorescence spectrometer employing time correlated single photon counting technique, show a nonlinear variation with the composition of binary mixture.