Reactivity of the 3-nitroanisole triplet: Methanol inhibition of photohydroxylation

The reactivity of the triplet state of 3-nitroanisole (1) toward a series of quenchers has been investigated in water and water alcohol mixtures. The triplet is a relatively efficient hydrogen abstractor, a very efficient electron acceptor, but a poor electron donor, Hydroxide quenches the triplet state via nucleophillic aromatic substitution. Methanol inhibition of the S(N)2 Ar* reaction of hydroxide with triplet 1 in water at pH 12 was examined by steady-state and time-resolved methods. HPLC measurements show that a product in addition to 3-nitrophenolate is formed in trace amounts (less than or equal to 1% of total material) when methanol is present. 2-Propanol, tert-butyl alcohol, and trifluoroethanol do not quench the reaction nor does their presence lead to formation of additional products, Methanol quenching of the reaction follows Stern-Volmer behavior with K-SV = 0.36 +/- 0.03 M-1 leading to a rate constant of (2.3 +/- 0.2) x 10(6) M-1 s(-1) for methanol quenching of product formation. Transient absorption studies show that methanol quenches triplet 1 at pH 11.5 with a rate constant of (1.8 +/- 0.1) x 10(6) M-1 s(-1). In the absence of hydroxide, methanol does not quench the triplet state. The results are explained in terms of formation of low concentrations of methoxide in alkaline solution.