A theoretical study of the molecular and electronic structure of benzoannulated tetracyanoquinodimethanes

Abstract

The molecular and electronic structure of a series of TCNQ derivatives with laterally extended π-systems is investigated using quantum-chemical methods. The steric hindrance introduced by lateral benzoannulation determines the loss of planarity of the molecular system which adopts a butterfly-type structure with the TCNQ ring folded in a boat conformation. PM3-optimized geometries are in good agreement with experimental X-ray data and show that the structural identity of the polyacenic units laterally fused to TCNQ is preserved. The evolution of the acceptor properties is rationalized with the help of VEH calculations. For the more extended TCNQs, these calculations show that the lateral naphthalene or anthracene units act as donors and determine the appearance of low-energy charge-transfer absorption bands. The geometries of negatively charged TCNQs up to the trianion are optimized at the PM3 level. The anions are calculated to be non-planar, thus explaining their low stability. For the dianions, the polycyclic skeleton is planar and the dicyanomethylene groups are rotated out of the molecular plane. While the first two extra electrons are mostly incorporated into the dicyanomethylene groups, the third electron added to form the trianion is shown to enter the central polycyclic unit. This result explains the formation of relatively stable trianions and tetra-anions for π-extended TCNQs.