π-Bonded molecular wires: self-assembly of mixed-valence cation-radical stacks within the nanochannels formed by inert tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anions†
A series of mixed-valence cation-radical salts D(n+1) TFPBn, where D is a planar organic donor of about 0.7 to 0.9 nm wide and 1.1 to 1.3 nm long (e.g., tetramethyltetraselenafulvalene, octamethylanthracene, octamethylbiphenylene, perylene) and TFPB is a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion, were crystallized and characterized via single-crystal X-ray analyses and UV–vis-NIR-IR measurements. All these salts showed isolated one-dimensional arrays of π-stacked partially-oxidized donors (resulting from the self-assembly of planar organic cation-radicals with their parent molecules) located within the nanochannels formed by the bulky inert anions. The essentially parallel hydrocarbon moieties within the π-stacks are arranged at interplanar separations of ~3.4–3.6 Å. As a result of the intermolecular interactions within the mixed-valence arrays, all these salts show intense absorption bands in the NIR to IR range (8000–1500 cm−1) characteristic of conducting organic materials with a low HOMO/LUMO gap. Quantum mechanical computations afforded electronic coupling elements between adjacent species of 0.1 eV to 0.3 eV. These values are larger than half of the corresponding reorganization energy for electron transfer, which implies significant electron/hole delocalization between neighboring π-bonded species. In comparison, crystallization of smaller tetrathiafulvalene, tetramethyltetrathiafulvalene or N,N,N′,N′-tetramethyl-p-phenylenediamine cation-radicals with the same counter-ion afforded 1 : 1 salts which contained isolated cationic moieties and were transparent in the NIR range. On the whole, the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion appears as an efficient building block for creating supramolecular insulating networks sheathing π-bonded molecular wires with a cross-section of about 1 nm2.