Photoinduced Intramolecular Electron Transfer Mechanism for Photochemical Dissociation of Para-Substituted Benzyl 9,10-Dimethoxyanthracene-2-sulfonates

Article abstract of DOI:10.1021/j100180a046

The photochemical behavior of para-substituted benzyl 9,10-dimethoxyanthracene-2-sulfonates with varying electron-withdrawing ability of substituents has been investigated by both steady-state and laser flash photolysis techniques. p-Nitrobenzyl 9,10-dimethoxyanthracene-2-sulfonate is photochemically dissociated to give 9,10-dimethoxyanthracene-2-sulfonic acid, 9,10-dimethoxy-2-(p-nitrobenzyl)anthracene, and p,p'-dinitrobibenzyl which were assigned on the basis of mass spectroscopic data and 1H NMR spectroscopy.Quantum yields for the photodissociation of p-nitrobenzyl 9,10-dimethoxyanthracene-2-sulfonate and for the formation of 9,10-dimethoxyanthracene-2-sulfonic acid with excitation at 436 nm in degassed acetonitrile solution are 0.12 +/- 0.03 and 0.10 +/- 0.03, respectively.Photodissociation of p-nitrobenzyl 9,10-dimethoxyanthracene-2-sulfonate is considered to proceed via an intramolecular electron transfer from the excited singlet state of 9,10-dimethoxyanthracene moiety to p-nitrobenzyl moiety followed by the heterolytic bond cleavage at oxygen-carbon bond of the sulfonyl ester from the fact that the transient absorptions at 310 and 425 nm due to 9,10-dimethoxyanthracene-2-sulfonate radical cation and around 340 nm due to the p-nitrobenzyl radical anion are detected in laser spectroscopy.The rate constant for dissociation of p-nitrobenzyl 9,10-dimethoxyanthracene-2-sulfonate was determined to be (1.1 +/- 0.5) * 10⁵ s^-1 by kinetic analyses.Possible mechanisms for the photodissociation are discussed and it is concluded that the three factors which control the rate of photodissociation are intramolecular electron transfer, feasibility of oxygen-carbon bond scission, and stability of benzyl-type radical formed by the photodissociation.