Monosubstituted naphthalenes as quenchers and generators of singlet molecular oxygen

The monosubstituted naphthalenes (MSN) 1-hydroxynaphthalene (1-OHN), 2-hydroxynaphthalene (2-OHN), 1-methoxynaphthalene, 2-methoxynaphthalene, 1-aminonaphthalene (1-AN), 2-aminonaphthalene (2-AN), 1-methylnaphthalene, 1-fluoronaphthalene and 1-cyanonaphthalene, and the parent compound naphthalene have been evaluated as physical and chemical quenchers and as generators of singlet molecular oxygen, [O-2((1) (g))], in water and in organic solvents. Rate constant values for the overall (k(t)) and chemical (k(r)) quenching of O-2((1) (g)), respectively, determined by time-resolved phosphorescence detection and by substrate or oxygen consumption- were from less than 10(4)-3.92 10(8) M-1 s(-1), depending on the solvent polarity and the pH of the solution. The derivatives 1-OHN, 2-OHN, 1-AN and 2-AN react with O-2((1) (g)), generated with external sensitisers, with quantum efficiencies in the range 0.03-0.67. The photooxidation mechanism must include the formation of an encounter complex with partial charge-transfer character of the type [O-2((1) (g))(-)...MSN+], in accordance with the observed variation of k(t) with the global free energy change. All the herein studied MSN generate O-2((1) (g)) upon direct irradiation, with fair to good quantum yields. The structure of the photooxidation products from 1-OHN and 2-OHN, two compounds with environmental interest, have been determined by combined GC/MS analysis. Benzoic acid, phthalic acid, 1,4-naphthoquinone and a dimer were identified in the reaction mixture from 1-OHN, while 2-OHN yielded phthalic acid and the dimer 2,2 -dihydroxy-1,1-binaphthyl. (C) 1999 Elsevier Science S.A. All rights reserved.