Environmental effects on the photophysics of pyrromethene 556

Visible absorption and fluorescence (steady-state and time-correlated) spectroscopies were applied to study the photophysics of the dye pyrromethene 556 in liquid solutions. Environmental factors, such as dye concentration, pH and nature of the solvent, were considered. The shape of the absorption spectrum does not change with the dye concentration, suggesting that the dye does not aggregate up to the solubility limit. The changes in the fluorescence signal by increasing the dye concentration (shift to lower energies in the fluorescence band and decrease/increase in the fluorescence quantum yield/lifetime) are attributed to re-absorption and re-emission effects. The solvent effects are analysed in terms of multicomponent linear regressions in which several solvent parameters (dipolarity/polarizability, H-bond donor, electron donor capacities, etc.) are individually and simultaneously treated. The influence of the solvent on the maximum of the absorption and fluorescence bands and on the rate constant of the radiative deactivation is explained on the basis of the stabilization of several resonance structures of the chromophore by general solvent effects and by specific solute-solvent interactions.