Fluorescence quenching of the ketocyanine dyes in polar solvents: anti-TICT behavior

Spectral properties and fluorescence decay in a series of differently N-substituted ketocyanine dyes belonging to the dibenzalcyclopentanone type were studied in solvents of various polarity and hydrogen bonding ability. The significant solvent-induced fluorescence quenching was explained by the increase of internal conversion rate owing to the lowering of the energy gap between S1 and S0 states in polar solvents. The alternative model of non-luminescent TICT states formation was shown to be less probable in the discussed series.