Abstract
Absorption and fluorescence data of a wide variety of substituted flavylium salts and their 2-naphthyl substituted derivatives in two solvents of different polarity are studied with respect to the influence of the flexibility of the chromophoric system both in their ground and excited states. The experimental results are in accordance with the assumption that for typical non-bridged 2-aryl substituted benzopyrylium salts electronic transitions occur from a distribution of non-planar conformations of the ground state conformers to the excited state ones, which in solution tend to relax to a more planar thermically equilibrated geometry, although fluorescence rate constant data indicate that also fluorescence may originate from somewhat twisted conformations. A polar solvent supports the tendency of certain substituted molecules to occur in the excited state in a non-fluorescent twisted configuration (TICT state). As the consequence, many flexible donor substituted benzopyrylium salts exhibit a strong decrease of their fluorescence quantum yield in polar solvents.
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