Intramolecular electron transfer in the photochemistry of substituted 1-naphthylmethyl esters of benzoic acids

Abstract

Direct excitation of the esters 5 in methanol solvent leads to rapid intramolecular exciplex formation (kex = 1010 s−1 for X = CH3O, Y = CN) with electron transfer from the naphthalene to the benzoate ring. This process dominates the usual fluorescence and reaction of the excited singlet state. The rate of this process can be varied over 103 by suitable change in the substituents X and Y. The electron-transfer rates can be correlated with the two-parameter Hammett equation: log kex = 8.48 − 1.5σ+ + 0.77σ. For cases where the rate of exciplex formation is slow, the usual homolytic carbon–oxygen bond cleavage occurs from the excited singlet state. The eventual products result from the ion pair since the rate of electron transfer in the radical pair to form the ion pair is considerably faster than the rate of decarboxylation of the benzoyloxy radical.