Effect of solvents on the barrierless reaction in the excited state of cyanine dye
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UV-Vis, fluorescence spectroscopy and time resolved spectroscopic technique on picosecond and femtosecond 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 analyzingwavelength 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.
Keywordspicosecond time-resolved fluorescence spectroscopy femtosecond transient absorption technique barrierless isomerization
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