SYNTHESIS AND CHARACTERIZATION OF NOVEL DONOR-ACCEPTOR NAPHTHOQUINONE DERIVATIVES WITH PHOTOINDUCED CHARGE-TRANSFER PROPERTIES - A JOINT EXPERIMENTAL AND THEORETICAL-STUDY

The synthesis of substituted benzo[b]phenoxazine-6,11-diones (7a-d) and their precursor N-tosyl derivatives (6a-d) is reported. The N-methylbenzo[b]phenoxazine-6,11-dione (8) and the analogous benzo[b]phenothiazine-6,11-dione (9) are also prepared. The UV-vis spectra of compounds 6, 7, and 9 show the existence of an intramolecular electronic transfer from the respective benzoxazine or benzothiazine donor fragments to the acceptor p-benzoquinone moiety. In agreement with this donor-acceptor character, the cyclic voltammograms of 7-9 exhibit two one-electron reduction waves to the corresponding radical-anion and dianion and two quasireversible oxidation peaks; however, the tosyl group in compounds 6 results in only one quasireversible oxidation wave at higher positive values. This causes a strong hypsochromic shift of the low-energy charge-transfer band. The molecular and electronic structures of compounds 6-8 are investigated using the semiempirical PM3 method and the nonempirical VEH approach, respectively. PM3 calculations predict that both the neutral and the reduced/oxidized compounds are nonplanar, the largest distortions from planarity being found for the most hindered N-tosylated derivatives 6. VEH calculations are used to rationalize the redox potentials and justify the low oxidation potentials measured for compounds 7 and 8. They also show the charge-transfer nature of the lowest-energy HOMO --> LUMO electronic transition, thus supporting the experimental UV-vis results. The higher oxidation potentials and blue-shifted absorption bands observed for compounds 6 are due to their highly distorted geometries which lead to a stabilization of the HOMO and a destabilization of the resulting cation-radical.