Charge-transfer complexes based on C_2v-symmetric benzo[ghi]perylene: Comparison of their dynamic and electronic properties with those of D_6h-symmetric coronene

Single crystals of three neutral charge-transfer complexes and a cation radical salt based on a C_2v-symmetric polycyclic aromatic hydrocarbon, benzo[ghi]perylene (bper), were obtained. The 1: 1 complex with 7,7,8,8-tetracyanoquinodimethane (TCNQ) involves DA-type alternating p-columns, whereas the alternating p-columns in the 2: 1 TCNQ complex are flanked by another bper molecule. The in-plane rotation of bper in the 1: 1 complex was significantly suppressed compared with that of coronene in (coronene) (TCNQ), which is associated with the lower molecular symmetry of bper. Whereas the 3: 1 TCNQ complex involves DDA-type alternating p-columns flanked by another bper molecule, bper molecules in the 3: 1 cation radical salt with Mo₆O₁₉ ² have a columnar structure with a [101]-like charge-ordered pattern associated with intermolecular interactions in the bay region of bper. The dimerisation of charge-rich bper molecules results in an increased energy gap at the Fermi level, and consequently, semiconducting behaviour of the salt has a larger activation energy than that of the isostructural coronene salt in the partially charged state. The lower molecular symmetry of bper also affects the degeneracy of the frontier-orbitals; the energy difference between the HOMO and the HOMO1 of bper is significantly larger than that of coronene, which is comparable to the intermolecular transfer integrals.