Influence of solvent polarity on the photoreactivity of 2-4-ring aromatic o-quinones

The solvent polarity dependence of the photoreactivity of naphthalene-1,2-dione (1), acenaphthylene-1,2-dione (2), phenanthrene-9,10-dione (3), and aceanthrylene-1,2-dione (4) was investigated by means of nanosecond laser-flash photolysis techniques. The reactivity of the triplets of 1, 2, and 3 towards hydrogen abstraction increases as the polarity of the solvent decreases owing to an inversion between n,pi* and pi,pi* triplet states relatively close in energy. The relative increase in triplet reactivity follows the order 2 < 3 < 1. In the case of 4 no inversion of triplet levels is detected; the lowest excited triplet state of 4 is pi,pi* in character in both polar and nonpolar solvents.