Mechanism of photochemical conversion of 1,2-naphthoquinonediazides in solution

The photochemical conversion of 1,2-naphthoquinonediazide-5-sulfonic acid (NQDS) in solution was studied by nanosecond time-resolved spectroscopy, and the observed transient absorption was assigned to the intermediate produced by the photodecomposition of NQDS. The intermediate is formed by the hydration of photochemically generated ketene. The formation rate constant of ketene hydrate in the mixed solvent of 1,4-dioxane/water increases with the water concentration. The conversion process from the ketene hydrate to 3-indenecarboxylic acid in aqueous solution is the acid-catalyzed reaction with a rate constant of 5.2 × 10² s^-1 + (2.8 × 10⁷ M^-1 s^-1)[H⁺], [H⁺] being hydrogen ion concentration (in M^-1 unit).