BENZOANNULATED QUINONE-TYPE ELECTRON-ACCEPTORS - EVOLUTION OF THE MOLECULAR AND ELECTRONIC-STRUCTURES UPON REDUCTION

The molecular and electronic structures of DCNQI and its pi-extended d erivatives benzo-DCNQI and DCAQI have been investigated using ab initi o 6-31G calculations. The steric hindrance introduced by lateral benz oannulation determines the loss of planarity of the DCNQI moiety for D CAQI. The most stable conformation of DCAQI corresponds to a butterfly -type structure, in which the DCNQI ring adopts a boat conformation an d the lateral benzene rings remain planar. This structure lies 2.75 kc al mol(-1) below the fully planar conformation. MP2/6-31G calculation s including full geometry optimization confirm the greater stability o f the butterfly structure. The geometries of the anions and dianions h ave also been optimized. The reduction process leads to the aromatizat ion of the DCNQI moiety and both the anion and the dianion of DCAQI ar e fully planar. The stability of the anions with respect to the neutra l systems is calculated to decrease along the series DCNQI > benzo-DCN QI > DCAQI, thus explaining the decrease of the electron acceptor prop erties along this series. For the dianions, the polycyclic skeleton is aromatic and forms a polyacenic unit substituted with two negatively charged NCN groups. (C) 1998 Elsevier Science B.V.