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 o r constant current electrochemical oxidation; the latter method gave X -ray quality single crystals. X-ray powder diffraction studies indicat e the materials from both syntheses have the same structure. The cryst al structure of [Bu(2)Ph(2)Tth]PF6 was determined and is temperature d ependent. 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 exhibit s a superlattice structure where the c axis is approximately double th at 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 tha t 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 crysta l 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 techniqu es (UV-vis-NIR-IR absorption, spectroelectrochemistry, ESR, conductivi ty) were used to investigate the electronic structure of the cation ra dical in solution, embedded in a thin film polymer network and as a pu re solid sample. The measured physical properties correlate with the c rystal structures of [Bu(2)Ph(2)Tth]PF6 and indicate that the material has a band-like electronic structure characteristic of a semiconducto r.