Steady-state and time-resolved fluorescence study of excited-state proton transfer in 1-aminoalkyl-2-naphthols

Abstract

Intramolecular excited-state proton transfer in 1-morpholinylmethyl- and 1-piperidinylmethyl-2-naphthol in various solvents and binary solvent mixtures is demonstrated by steady-state and time-resolved fluorescence spectroscopy. The excited-state proton-transfer equilibrium constant was calculated from a fluorescence bandshape analysis, and its large solvent dependence arises primarily from local solute–solvent interactions. Only monoexponential fluorescence decay, giving the same lifetime irrespective of the emission wavenumber, was resolved (time resolution ca. 200 ps), and this proved proton transfer to be considerably faster than fluorescence decay. Therefore, equilibrium solvation is also established very fast at low concentrations of weakly associating solvents (e.g. tetrahydrofuran and butyl chloride), and local solvation is characterized by multiple encounter and re-encounter reactions. A good correlation between the spectral shift of the zwitterionic form, its non-radiative deactivation rate and the excited-state equilibrium constant was established.