Photoinduced electron transfer in quinoid systems

We have recently reported the synthesis, together with a detailed study of the structural, optical, and electrochemical properties, of benzo[Z ]phenoxazine-6,11 -dione derivatives 84 as novel nitrogen containing 

The electronic absorption spectra of compounds 84 show the presence of a very broad, low-energy band in the visible region, centred around 650 nm. Based on the results reported previously [26], this band is expected to arise from intramolecular electron-transfer from the donor benzoxazine moiety to the acceptor naphthoqui- 

The redox potentials of compounds 83-85 provide evidence of the presence of both reduction and oxidation waves to form stable anion and cation radicals in the same organic molecule. The redox potentials thus obtained are collected in Table 1.19. For compounds 84 and 85 the two reduction waves are reversible, while for the A -tosyl-substituted compounds 83 the first reduction potential is irreversible and the second reduction potential to the dianion is reversible. 

The presence of the tosyl substituent on the nitrogen atom on compounds 83 is responsible for the striking decrease in the oxidation potential values that now appear as a single quasi-reversible oxidation wave at more positive voltages. 

VEH calculations predict that the HOMO LUMO transition corresponds to an electronic transfer from the phenoxazine moiety, acting as a donor, to the acceptor naphthoquinone moiety. This result supports the intramolecular charge-transfer nature of the lowest energy absorption band observed experimentally for these quinones. The second absorption band of this compound corresponds to the lowest energy absorption band of 1,4-naphthoquinone which is hypsochromically shifted due to the destabilization of the LUMO in 83 and 84. 

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