From monomers to π-stacks. A comprehensive study of the structure and properties of monomeric, π-dimerized, and π-stacked forms of the cation radical of 3', 4 '-dibuty 1- 2,5 _-diphenyl- 2,2': 5',2'-terthiophene

3',4'-Dibutyl-5,5_-diphenyl-2,2':5',2_-terthiophene (Bu₂Ph₂Tth) can be readily converted to the stable cation radical [Bu₂Ph₂Tth]⁺. The hexafluorophosphate salts of the cation radical ([Bu₂Ph₂Tth]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 [Bu₂Ph₂Tth]PF₆ was determined and is temperature dependent. At 293 K, [Bu₂Ph₂Tth]PF₆ crystallizes in the C2/c space group while at 106 K it belongs to the P2₁/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 π-stacks' of [Bu₂Ph₂Tth]⁺ cations and channels of PF₆^- anions. At 293 K, the cations stack with a regular interplanar contact distance of 3.47 Å; at 106 K, the contact distances alternate regularly such that each cation has interplanar contact distances of 3.36(5) and 3.42(5) Å with the two neighboring cations in the π-stack. The crystal structure of Bu₂Ph₂Tth was also determined, and comparisons are made to the structure of [Bu₂Ph₂Tth]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 [Bu₂Ph₂Tth]PF₆ and indicate that the material has a band-like electronic structure characteristic of a semiconductor.