From monomers to pi-stacks. A comprehensive study of the structure and properties of monomeric, pi-dimerized, and pi-stacked forms of the cation radical of 3',4'-dibutyl-2,5''-diphenyl-2,2':5',2''-terthiophene

3',4'-Dibutyl-5,5 ''-diphenyl-2,2':5',2 ''-terthiophene (Bu(2)Ph(2)Tth) can be readily converted to the stable cation radical [Bu(2)Ph(2)Tth](+). The hexafluorophosphate salts of the cation radical ([Bu(2)Ph(2)Tth]PF6) are synthesized in millimolar quantities by either chemical or constant current electrochemical oxidation; the latter method gave X-ray quality single crystals. X-ray powder diffraction studies indicate the materials from both syntheses have the same structure. The crystal structure of [Bu(2)Ph(2)Tth]PF6 was determined and is temperature dependent. At 293 K, [Bu(2)Ph(2)Tth]PF6 crystallizes in the C2/c space group while at 106 K it belongs to the P2(1)/n space group and exhibits a superlattice structure where the c axis is approximately double that at 293 K. Both structures consist of columnar ''slipped pi-stacks'' of [Bu(2)Ph(2)Tth](+) cations and channels of PF6- anions. At 293 K, the cations stack with a regular interplanar contact distance of 3.47 Angstrom; at 106 K, the contact distances alternate regularly such that each cation has interplanar contact distances of 3.36(5) and 3.42(5) Angstrom with the two neighboring cations in the pi-stack. The crystal structure of Bu(2)Ph(2)Tth was also determined, and comparisons are made to the structure of [Bu(2)Ph(2)Tth]PF6. Several physical techniques (UV-vis-NIR-IR absorption, spectroelectrochemistry, ESR, conductivity) were used to investigate the electronic structure of the cation radical in solution, embedded in a thin film polymer network and as a pure solid sample. The measured physical properties correlate with the crystal structures of [Bu(2)Ph(2)Tth]PF6 and indicate that the material has a band-like electronic structure characteristic of a semiconductor.