Photochemical dissociation of p-nitrobenzyl sulfonate esters via an intramolecular electron transfer

Abstract

The photochemistry of p-nitrobenzyl sulfonate esters, p-nitrobenzyl 5-dimethylaminonaphthalene-1-sulfonate (NDS) and p-nitrobenzyl pyrene-1-sulfonate (NPS) has been investigated by both steady-state and laser flash photolysis techniques. These sulfonates are photochemically dissociated to give the parent sulfonic acids. Quantum yields for photodissociation of NDS and NPS with excitation at 365 nm in degassed acetonitrile solution are 0.20 ± 0.05 and 0.16 ± 0.04, respectively. Photodissociation of these sulfonates is considered to proceed via an intramolecular electron transfer from the excited singlet state of the 5-dimethylaminonaphthalene or pyrene moiety to the p-nitrobenzyl moiety from the fact that the transient absorption bands at 390 and 550 nm due to the radical cation of the 5-dimethylaminonaphthalene moiety, 420 and 450 nm due to the radical cation of the pyrene moiety, and at 340 nm due to the radical anion of the p-nitrobenzyl moiety are detected in laser spectroscopy. Intramolecular-electron-transfer-induced bond cleavage is observed at the oxygen–carbon bond of these sulfonate esters, producing the organic strong acids. Although photodissociation of o-nitrobenzyl esters of carboxylic and sulfonic acids is known, that of p-nitrobenzyl esters has not been previously reported. From the mechanistic point of view, the photochemical behaviour of these p-nitrobenzyl sulfonate esters is quite different from that of o-nitrobenzyl esters. While the latter involves a photoinduced intramolecular rearrangement, the former involves a photoinduced intramolecular electron transfer.