SOLUTE SOLVENT RELAXATION OF ELECTRONICALLY EXCITED XANTHENE DYES

Time-resolved fluorescence measurements were carried out on rhodamine 3B, pyronine B and rhodamine 101 ethylester in protic solvents of different viscosity but of similar dielectric properties by dual-beam multiple frequency phase fluorometry. Emission wavelength dependent biexponential fluorescence decay is reported for all combinations. The electronic nature of the xanthene skeleton with different N-substitution patterns was investigated as a function of the molecular conformation by semi-empirical calculations according to the extended Hückel molecular orbitals and the Pariser-Parr-Pople methods. No evidence in support of the twisted intramolecular charge transfer state model for the S1 state is found in the correlation of experimental and computational results, but a qualitative interpretation for the viscosity dependence of the fluorescence quantum yields of dialkylamino-substituted xanthene dyes is given. The key factor determining the quantum yield was found to be the rigidity of the amino groups and their substitution pattern. The variation of the decay law with emission wavelength is discussed in terms of structural relaxation of the solute, dielectric relaxation of the solvent and specific solute-solvent interactions. © 1990.