Covalently linked acceptor-donor systems based on isoquinoline N-oxide acceptor:: Photoinduced electron transfer produces dual-channel luminescent systems that evolve chemically to photohydroxylation of the aromatic donor

Acceptor-donor compounds containing the isoquinoline N-oxide acceptor and (methoxy) benzene (n = 0, 1, 2, 3) electron donors were studied. The two chromophores are connected by a CH2 bridging unit. All acceptor-donor compounds exhibit photoinduced electron transfer in acid medium that results in the formation of a charge-transfer (CT) state. Measurements of the corresponding electronic emission spectra revealed that these bichromophoric systems exhibit a dual fluorescence that is strongly dependent on the protonation of the N-oxide function and the donor ability. The CT state responsible for the red-shifted luminescence in the studied compounds is directly connected with the initial excited state S-1. On the basis of the spectroscopic and photochemical evidence, N-O bond scission is the dominant primary photochemical process involving the CT state, the subsequent radical coupling resulting in efficient aromatic hydroxylation. The outcome of both quenching and sensitization experiments confirms this assertion. The results strongly suggest that the ensuing photohydroxylation reaction is not a concerted process, but rather a two-step N-O bond scission followed by C-O bond formation, which is regioselectively guided by the electronic distribution of the resulting donor cation-radical.