Polar dyes in solution: A joint experimental and theoretical study of absorption and emission band shapes

Absorption and steady-state emission spectra of two fluorescent dyes are measured in a series of aprotic solvents with similar refractive index and different polarity. The spectra are interpreted in terms of a two-state electronic model accounting for the coupling to internal vibrations and to an effective solvation coordinate. The proposed approach naturally accounts not only for solvatochromic shifts of absorption and emission bands but also for the evolution of band shapes with solvent polarity and for the observation of nonspecular absorption and fluorescence bands. The good agreement between experimental and calculated spectra confirms the validity of a two-state picture for the low-energy spectral properties of these donor-acceptor molecules, provided that the molecular polarizability is fully accounted for. The role of conformational degrees of freedom in flexible chromophores is also addressed.