Excited-state kinetics of the hydrophobic probe Nile red in membranes and micelles

Nile red is a widely used hydrophobic dye for probing the structure, dynamics, and environment in many biological and microheterogeneous systems. This paper reports emission-wavelength-dependent fluorescence intensity decay of Nile red in membranes and micelles. Global analysis of these multiple fluorescence decays reveals a double-exponential decay with negative amplitudes for the short-lifetime component at longer emission wavelengths. This indicates an excited-state kinetics leading to the formation of a new species in the excited state from the initially excited state. In both the cases, the short lifetime corresponds to that of the initially excited species. This excited-state kinetics is also observed in the case of viscous organic solvents such as l-octanol and glycerol and is attributed to that of an excited-state solvent relaxation.