A THEORETICAL-STUDY OF THE MOLECULAR AND ELECTRONIC-STRUCTURE OF BENZOANNULATED TETRACYANOQUINODIMETHANES

The molecular and electronic structure of a series of TCNQ derivatives with laterally extended pi-systems is investigated using quantum-chem ical methods. The steric hindrance introduced by lateral benzoannulati on determines the loss of planarity of the molecular system which adop ts a butterfly-type structure with the TCNQ ring folded in a boat conf ormation. PM3-optimized geometries are in good agreement with experime ntal X-ray data and show that the structural identity of the polyaceni c units laterally fused to TCNQ is preserved. The evolution of the acc eptor properties is rationalized with the help of VEH calculations. Fo r the more extended TCNQs, these calculations show that the lateral na phthalene or anthracene units act as donors and determine the appearan ce of low-energy charge-transfer absorption bands. The geometries of n egatively charged TCNQs up to the trianion are optimized at the PM3 le vel. The anions are calculated to be non-planar, thus explaining their low stability. For the dianions, the polycyclic skeleton is planar an d the dicyanomethylene groups are rotated out of the molecular plane. While the first two extra electrons are mostly incorporated into the d icyanomethylene groups, the third electron added to form the trianion is shown to enter the central polycyclic unit. This result explains th e formation of relatively stable trianions and tetra-anions for pi-ext ended TCNQs.