Fluorescence excitation and excited state intramolecular relaxation dynamics of jet-cooled methyl-2-hydroxy-3-naphthoate

Two distinct S₀ → S₁ fluorescence excitation spectra of methyl-2-hydroxy-3-napthoate (MHN23) have been obtained by monitoring fluorescence separately in the short (∼410 nm) and long (∼650 nm) wavelength emission bands. The short wavelength fluorescence is assigned to two MHN23 conformers which do not undergo excited state intramolecular proton transfer (ESIPT). Analysis of the 'long wavelength' fluorescence excitation spectrum, which arises from the proton transfer tautomer of MHN23 indicates an average lifetime of τ ≥ 18 ± 2 fs for the initially excited states. Invoking the results of Catalan et al. [J. Phys. Chem. A, 1999, 103, 10921], who determined the N tautomer to decay predominantly via a fast non-radiative process, the limit of the rate of intramolecular excited proton transfer in MHN23 is calculated as, k_pt ≤ 1 × 10 ¹² s^-1.