Viscosity-Dependent Internal Rotation in Polymethine Dyes Measured by Picosecond Fluorescence Spectroscopy
Polymethine dyes are known to undergo ultrafast internal conversion, the rate of which depends on solvent viscosity (1), and this is thought to be due to a torsional motion of the heterocyclic quinolyl rings which allows the excited state energy to be dissipated by intramolecular vibration and rotation. The radiationless relaxation rate has been found to obey an empirical law of the form τ= const.ηα, where τ is the relaxation lifetime, and τ, for a particular dye molecule, is strongly dependent on the type of solvent used. In ethanol/glycerol mixtures, it is found that the power dependence varies between 0.35 and 0.70 (2). A recent study (3), using both mixed solvents and a homologous series of unbranched alcohols, established that a approaches 0.5 in the former, but is close to unity in the latter.
KeywordsFluorescence Lifetime Homologous Series Excited State Energy Torsional Motion Polymethine Chain
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