Columnar benzoperylene-hexa- and tetracarboxylic imides and esters: synthesis, mesophase stabilisation and observation of charge-transfer interactions between electron-donating esters and electron-accepting imides

Benzoghi]perylene 1,2,4,5,10,11-hexacarboxylic trialkylimide and dialkylimido-dialkyl ester derivatives, displaying a thermodynamically stable hexagonal columnar liquid-crystalline phase at room temperature, have been obtained by the use of previously unexplored chiral racemic α-branched alkylimide functions. One of the trialkylimides described here is the first room temperature columnar solely oligo-alkylimide-substituted arene, and thus constitutes a prototype case of self-assembling organic acceptor materials. As the related hexacarboxylic hexaesters are found to exhibit only a weak tendency to form columnar mesophases, benzoghi]perylene 1,2,5,10-tetracarboxylic tetraalkyl esters have been synthesized by regioselective oxidative Diels-Alder addition of maleic anhydride to 3,10-dicyanoperylene, and a room temperature hexagonal columnar mesophase was obtained with branched alkyl chains. The acceptor-type electronic properties of the tri- and diimides have been found to be considerably more pronounced than those of the hexa- and tetracarboxylic esters, and to approach those of the prototype acceptor material C(60). The formation of bathochromically absorbing donor-acceptor complexes was observed with a di- or triimide as acceptor and a tetraester as donor, but not with a hexaester as donor. Exploiting the non-negligible differences in reduction and oxidation potentials between all four types of materials, the minimum HOMO energy difference necessary for charge-transfer-complex formation has been determined to lie between 0.29 and 0.35 eV.