SYNTHESIS, CHARACTERIZATION, AND THEORETICAL-STUDY OF SULFUR-CONTAINING DONOR-ACCEPTOR DCNQI DERIVATIVES WITH PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER

The donor-acceptor compounds o[b]naphtho[2,3-e][1,4]dithiin-6,11-quino nediimine (9a) and [b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respecti vely) have been prepared, and their structural and electronic properti es have been characterized by both experimental techniques and quantum -chemical calculations. The H-1-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical da ta suggest that the preferred configuration of the N-CN groups corresp onds to a syn isomer for 9 and to an anti isomer for 10. The X-ray ana lysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moiet ies of adjacent molecules. In agreement with X-ray data, theoretical c alculations predict that both for 9 and 10 the acceptor DCNQI moiety i s folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy diffe rence between planar and butterfly structures is calculated to be < 3 kcal/mol at the ab initio 6-31G level. The UV-vis spectra present a b road absorption in the visible which corresponds to a photoinduced int ramolecular electron transfer from the high-energy HOMO furnished by t he donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two o ne-electron reduction waves to the anion and dianion. Theoretical calc ulations show the planarization of the acceptor/donor moiety induced b y reduction/oxidation. The formation of stable radical anions is corro borated by the intense EPR signals recorded for reduced 9. The assignm ent of the hyperfine coupling constants of the EPR spectra is consiste nt with the existence of a preferred syn configuration.