Emission-wavelength-dependent decay of the fluorescent probe N-phenyl-1-naphthylamine

We have measured the fluorescence decay of N-phenyl-1-naphthylamine using the phase-modulation method, in several solvent systems and egg phosphatidylcholine vesicles. The decay is monoexponential in pure solvents (both polar and non-polar) of low viscosity. In polar viscous solvents or in non-polar solvents containing an added polar solute, the decay is heterogeneous and emission wavelength dependent. In such cases, dielectric relaxation and/or excited-rate complexing give rise to a shift of the emission spectrum on the nanosecond time scale. Emission-wavelength-dependent decay was also observed when N-phenyl-1-naphthylamine was bound to egg phosphatidylcholine vesicles. From these results as well as the position of the emission spectral maximum, we conclude that N-phenyl-1-naphthylamine probes the ester-carbonyl region of the phospholipid acyl chains, where it undergoes an excited-state reaction. This result contradicts the often made assumption that N-phenyl-1-naphthylamine probes the deeper hydrocarbon region of the bilayer.