THE INFLUENCE OF THE NITRO GROUP ON THE DISSOCIATIONS OF SOME AROMATIC ACIDS AND BASES IN THEIR LOWEST EXCITED TRIPLET STATES*

Abstract

Abstract— –Estimation of lowest excited triplet and singlet state dissociation constants of some nitro‐aromatic acids and bases, from shifts in their phosphorescence and absorption spectra, respectively, indicate that intramolecular charge transfer to the nitro group is much more important in the lowest excited singlet state than in the ground or lowest excited triplet states. As a result, the effect of a nitro group on the acidity of the lowest excited singlet state of an acid or base is more exaggerated than that on the ground or lowest excited triplet state of the same compound. Furthermore, the basicity of the nitro group is greatly enhanced in the lowest excited singlet state. On this basis the increased rate of photoreduction of nitrobenzene in acidic solutions is found to be thermodynamically unfeasible in the lowest excited triplet state. Although the reaction is thermodynamically feasible in the lowest excited singlet state, the short lifetime of that state may make the reaction kinetically unfeasible. Rate‐Hammett acidity profiles are therefore inadequate to alone establish the mechanism of photoreduction of nitrobenzene.