Photoreduction of 2,3-dihydro-oxoisoaporphine dye (A) by amines occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the metastable N-hydrogen 2,3-dihydro-oxoisoaporphine anion (A-NH^-), eventually. During photoreduction that occurs from the triplet manifold of the oxoisoaporphine (³A), a radical anion (A^-·) and neutral-hydrogenated radical (A-NH^·) derived from the 2,3-dihydro-oxoisoaporphine are postulated as intermediate products. Quantum mechanical semi-empirical PM3 and ZINDO/S calculations for the transient species formed during the flash photolysis of 2,3-dihydro-oxoisoaporphine in the presence of amines reproduce adequately the experimental time-resolved absorption spectra of the triplet-triplet and that of neutral hydrogenated radical (A-NH^·) with maxima located at 450 and 390 nm, respectively. However, the ZINDO/S/PM3 spectral calculations performed for the isolated oxoisoaporphine radical anion A^-· predict the strongest transitions above 600 nm. Therefore, a transient absorption observed in flash photolysis experiments, with maximum located near 490 nm, for all of the systems studied, was assigned to a radical ion pair between the radical anion of the oxoisoaporphine, A^-· and the radical cation of the respective amine. This tentative assignment was confirmed further by Molecular Mechanics and ZINDO/S calculations. Calculated spectrum generates the strongest transition near the experimentally observed absorption maximum at 490 nm.

The validity of prediction of the strongest transitions for the isolated oxoisoaporphine radical anion A^-· was separately confirmed in complementary pulse radiolysis experiments. The oxoisoaporphine radical anion (A^-·) was generated in methanol and acetonitrile solutions by the reaction of e^-_solv with 2,3-dihydro-oxoisoaporphine dye. The product of this reaction is characterized by a spectral absorption band with l_max located at 605-610 nm. This observation further supports formation of a radical ion pair complex in the first step of the photoreduction.

We thank to FONDECYT, Project Nº1030963, 7060244 and to DI University of Chile

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1. Charge Transfer Complexes Formation Between Benzoanthracenones and Amines