Excited state acid catalysis of the 2 + 2 photocycloaddition of methyl 2-naphthoate and acetylacetone; the interaction of a nonfluorescent exciplex with H2SO4

In this paper, the striking acid catalysis of the photocycloaddition of methyl 2-naphthoate (2MN) to acetylacetone (AA) to give 1 and 2 was traced to the enhanced conversion of the nonemissive *(2MN–AA) exciplex to 1 and 2 by H2SO4. The partial reversibility of the exciplex in CH3CN and CH3OH was established by extended fluorescence-quenching analysis using oxygen to perturb the system, whereas time-resolved fluorescence kinetics failed to reveal the presence of this exciplex. A similar analytical method demonstrated the interaction of the nonfluorescent *(2MN–AA) exciplex and H2SO4 with the rate constants of kap = (2 ± 1) × 109 M−1 s−1 in CH3CN and (4 ± 1) × 109 M−1 s−1 in CH3OH. The quantum yield of photocycloadducts was significantly enhanced at high 2MN conversions owing to the retardation of the competing excimer *(2MN)2 formation. In the range of low conversion (< 15%) and [H2SO4] ≤ 1 mM, the quantum yield was shown to be proportional to H2SO4 concentration; the slope from such plots was analyzed to give kap values similar to those obtained from the extended fluorescence-quenching analysis. This agreement, along with the elimination of other potential acid catalytic routes, unambiguously proves that the acid catalysis originates from the interaction of the *(2MN–AA) exciplex with H2SO4.