THE KINETICS OF THE INTERACTION OF SINGLET EXCITED-STATE METHYL-2-NAPHTHOATE WITH SULFURIC-ACID - SEARCH FOR EXCITED-STATE ACID CATALYSIS

Steady state and time-resolved fluorometry were used to investigate the kinetics of the interaction of singlet excited methyl-2-naphthoate (*2MN) with H2SO4 in CH3CN and CH3OH. The *2MN fluorescence intensity was quenched by H2SO4 through associative interaction giving an emissive species (*2MNH(+)) at [H2SO4]less than or equal to O.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 CH3CN to CH3OH, 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,CN to CH3OH, which parallels the proton affinity of the solvent, i.e. the association in CH3OH is reversible. The *2MNH(+) species is quenched by H2SO4 in CH3CN but not in CH3OH. In addition, *2MNH(+) is quenched by acetylacetone (AA) in CH,CN and CH3OH; this process is shown to contribute only a small fraction to the catalysis of the photocycloaddition between 2MN and AA.