American Chemical Society
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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‘-Dibutyl-2,5‘‘-diphenyl-2,2‘:5‘,2‘‘-terthiophene

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journal contribution
posted on 1997-06-25, 00:00 authored by David D. Graf, Robert G. Duan, John P. Campbell, Larry L. Miller, Kent R. Mann
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.