Decay time distribution analysis of Y_t-base in benzene-methanol mixtures

Frequency-domain fluorometry was used to measure intensity decays of synthetic Y_t-base in mixtures of benzene-methanol at 20 °C. Multiexponential analysis shows that the decay of Y_t-base fluorescence in benzene and methanol can be well fitted to a single-exponential model with τ = 9.67 ns and 6.25 ns respectively. In mixtures of benzene-methanol the decays became heterogeneous, and the maximum of heterogeneity observed was in a mixture containing 6% methanol. Since we expected a distribution of Y_t-base solvation states in the solvent mixtures, and because the decay times of Y_t-base are sensitive to solvent, we analyzed the data in terms of decay time distributions. The goodness-of-fit for the unimodal distribution model which has two floating parameters was equivalent to that found using the double exponential model with three floating parameters. The Lorentzian distribution model appears to provide a slightly superior fit relative to the Gaussian distribution model. These results suggest that the intensity decays of solvent-sensitive fluorophores in mixed solvents are described by a distribution of decay times.