The molecular and electronic structures of TCNQ and its pi-extended derivatives benzo-TCNQ and TCAQ have been investigated using the semiempirical PM3 method and ab initio 6-31G* calculations. The steric hindrance introduced by lateral benzoannulation determines the loss of planarity of the TCNQ moiety for benzo-TCNQ and TCAQ. For both molecules, the most stable conformation corresponds to a butterfly-type structure, in which the TCNQ ring adopts a boat conformation and the lateral benzene rings remain planar. This structure lies, at the 6-31G* level, 33.45 kcal/mol below the fully planar conformation and 17.50 kcal/mol below a second local minimum for TCAQ, while it only lies 3.35 kcal/mol below the planar structure for benzo-TCNQ. The geometries of the anions and dianions have been also optimized at the semiempirical and ab initio levels. Although reduction induces the aromatization of the TCNQ moiety, the anions of benzo-TCNQ and TCAQ remain nonplanar. The stability of the anions with respect to the neutral systems is thus calculated to decrease along the series TCNQ > benzo-TCNQ > TCAQ, thus explaining the decrease of the electron-acceptor properties along this series. For the dianions, the polycyclic skeleton is fully planar and the C(CN)(2) groups are twisted out of the molecular plane. While the first two extra electrons are mostly incorporated into the C(CN)(2) groups, the third electron added to form the trianion is shown to enter the central polyacenic unit for TCAQ. This result explains the formation of relatively stable trianions and tetraanions for TCAQ. PM3 calculations have been especially useful in characterizing the stationary structures located for each compound and have been used as a guide for 6-31G* calculations. PM3-optimized geometries are in good agreement with experimental X-ray data and 6-31G* results.