Preferential Solvation of Coumarin 153The Role of Hydrogen Bonding

Preferential solvation of solvatochromic probe coumarin 153 has been studied in toluene−acetonitrile and toluene−methanol solvent mixtures. Solvatochromic shifts of absorption and fluorescence spectra of coumarin 153 in these nonideal systems are analyzed as a function of solvent polarity. There is no apparent preferential solvation of coumarin 153 in the ground state. For the excited-state molecule preferential solvation is observed in toluene−acetonitrile mixtures and it is confirmed by a theoretical model. For the excited C153 in toluene−methanol mixtures strong nonlinearity in the fluorescence solvatochromic shifts is attributed mainly to formation of a hydrogen bond between methanol and C153. IR spectra of C153 in solvent mixtures containing methanol confirm formation of a hydrogen bond between methanol and C153. The presence of a hydrogen-bonded C153 in toluene−methanol mixtures causes an unusual dependence of the C153 fluorescence lifetime on the mole fraction of methanol in solution.