Effect of solvents on the barrierless reaction in the excited state of cyanine dye

UV-Vis, fluorescence spectroscopy and time resolved spectroscopic technique on picosecond and femto- second scales have been utilized to investigate the effect of steric hindrance and multiple hydroxyl groups of solvents on the barrierless isomerization in excited state of 3, 3′- di(3-sulfopropyl)thiacyanine triethylaminium salt. The results show that the factors mentioned above may be responsible for a different "short-time behavior" of the bleaching signal on the ground state and nonlinear viscosity dependence of the excited-state lifetime in isopropanol and glycol. These phenomena are distinguished from the situation reported in the long-chained monohydroxyl alcohol solvents. However, the dye molecule still exhibits the analyzing- wavelength dependence of the observed fluorescence lifetime, that is, the fluorescence lifetime increases with the increasing of analyzing-wavelength. The conclusions are in agreement with those drawn from femtosecond upconversion fluorescence experiments and further exemplify the barrierless isomerization model in the excited state.