SYNTHESIS AND CHARACTERIZATION OF NOVEL DONOR-ACCEPTOR NAPHTHOQUINONEDERIVATIVES WITH PHOTOINDUCED CHARGE-TRANSFER PROPERTIES - A JOINT EXPERIMENTAL AND THEORETICAL-STUDY
B. Illescas et al., SYNTHESIS AND CHARACTERIZATION OF NOVEL DONOR-ACCEPTOR NAPHTHOQUINONEDERIVATIVES WITH PHOTOINDUCED CHARGE-TRANSFER PROPERTIES - A JOINT EXPERIMENTAL AND THEORETICAL-STUDY, Journal of organic chemistry, 60(17), 1995, pp. 5643-5650
The synthesis of substituted benzo[b]phenoxazine-6,11-diones (7a-d) an
d their precursor N-tosyl derivatives (6a-d) is reported. The N-methyl
benzo[b]phenoxazine-6,11-dione (8) and the analogous benzo[b]phenothia
zine-6,11-dione (9) are also prepared. The UV-vis spectra of compounds
6, 7, and 9 show the existence of an intramolecular electronic transf
er from the respective benzoxazine or benzothiazine donor fragments to
the acceptor p-benzoquinone moiety. In agreement with this donor-acce
ptor character, the cyclic voltammograms of 7-9 exhibit two one-electr
on reduction waves to the corresponding radical-anion and dianion and
two quasireversible oxidation peaks; however, the tosyl group in compo
unds 6 results in only one quasireversible oxidation wave at higher po
sitive values. This causes a strong hypsochromic shift of the low-ener
gy charge-transfer band. The molecular and electronic structures of co
mpounds 6-8 are investigated using the semiempirical PM3 method and th
e nonempirical VEH approach, respectively. PM3 calculations predict th
at both the neutral and the reduced/oxidized compounds are nonplanar,
the largest distortions from planarity being found for the most hinder
ed N-tosylated derivatives 6. VEH calculations are used to rationalize
the redox potentials and justify the low oxidation potentials measure
d 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 thei
r highly distorted geometries which lead to a stabilization of the HOM
O and a destabilization of the resulting cation-radical.