The kinetics of the interaction of singlet excited state methyl-2-naphthoate with sulfuric acid; search for excited state acid catalysis

Abstract

Steady state and time-resolved fluorometry were used to investigate the kinetics of the interaction of singlet excited methyl-2-naphthoate (*2MN) with H 2SO 4 in CH 3CN and CH 3OH. The *2MN fluorescence intensity was quenched by H 2SO 4 through associative interaction giving an emissive species (*2MNH +) at [H 2SO 4] ⩽ 0.1 M, in the range too low for ground state protonation. Rate constants for the association and decay of *2MN and the decay of *2MNH + were moderately larger on going from CH 3CN to CH 3OH, where the *2MN association rate constant was less than that of a diffusion-controlled reaction in the same solvent. However, the rate constant for *2MNH + dissociation was 36 times larger on going from CH 3CN to CH 3OH, which parallels the proton affinity of the solvent, i.e. the association in CH 3OH is reversible. The *2MNH + species is quenched by H 2SO 4 in CH 3CN but not in CH 3OH. In addition, *2MNH + is quenched by acetylacetone (AA) in CH 3CN and CH 3OH; this process is shown to contribute only a small fraction to the catalysis of the photocycloaddition between 2MN and AA.