Synthesis, Characterization, and Theoretical Study of Sulfur-Containing Donor-Acceptor DCNQI Derivatives with Photoinduced Intramolecular Electron Transfer.

The donor-acceptor compounds N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine (9a) and N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respectively) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The (1)H-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical data suggest that the preferred configuration of the N-CN groups corresponds to a syn isomer for 9 and to an anti isomer for 10. The X-ray analysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moieties of adjacent molecules. In agreement with X-ray data, theoretical calculations predict that both for 9 and 10 the acceptor DCNQI moiety is folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy difference 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 broad absorption in the visible which corresponds to a photoinduced intramolecular electron transfer from the high-energy HOMO furnished by the donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two one-electron reduction waves to the anion and dianion. Theoretical calculations show the planarization of the acceptor/donor moiety induced by reduction/oxidation. The formation of stable radical anions is corroborated by the intense EPR signals recorded for reduced 9. The assignment of the hyperfine coupling constants of the EPR spectra is consistent with the existence of a preferred syn configuration.