Abstract
Octithiophene sterically segregated by annelation with bicyclo[2.2.2]octene (BCO) units was synthesized to study the unimolecular properties of longer oligothiophene dications. For the preparation of such longer oligomers, a new route for the synthesis of the monomer annelated with BCO unit at the 3,4-positions of thiophene ring was developed. Attempted synthesis of octithiophene 1(8T) fully annelated with BCO units was hampered by low solubility of the product, and octithiophene 2(8T) having octyl groups instead of the BCO units at the second rings from the both ends of 1(8T) was synthesized to solve the solubility problem. Neutral 2(8T) has a lower planarity due to the steric repulsion between the octyl substituents and the neighboring thiophene units. However, dication 2(8T)2+, which was obtained as a stable salt by chemical two-electron oxidation with NO+SbF6–, has a planar quinoid structure, as judged from a linear correlation between the inverse chain length and the absorption energy for 1(nT)2+ (n = 3,4,6) and 2(8T)2+. Based on the comparison with the calculated absorption spectra and the result of ESR inactive properties, 2(8T)2+ appears to have a singlet ground state with open-shell biradical character rather than a closed-shell singlet structure.
Highlights
Polythiophenes are one of the most useful conducting polymers, and their conduction mechanism is a subject of intense theoretical and experimental studies [1,2,3,4,5]
Measurements and semi-empirical theoretical calculations, they concluded that the absorption spectrum of the dication of 12T was assigned as an intra-chain polaron pair
We designed and synthesized 2(8T), in which two BCO units were replaced by octyl groups, to solve the solubility problem, and conducted Cyclic voltammetry (CV), UV-vis-NIR, and ESR measurements of the neutral and cationic species as a part of a series of studies on sterically segregated oligothiophenes by the BCO-annelation
Summary
Polythiophenes are one of the most useful conducting polymers, and their conduction mechanism is a subject of intense theoretical and experimental studies [1,2,3,4,5]. We succeeded in the first systematic X-ray structural analysis of cationic oligothiophenes, i.e., radical cation and dication salts of 1(nT) (n = 2, 3, 4, 6) [17] These results unambiguously demonstrated that the cationic oligothiophenes are sterically segregated and serves as good models of single polythiophene chain at p-doped states. We designed and synthesized 2(8T), in which two BCO units were replaced by octyl groups, to solve the solubility problem, and conducted CV, UV-vis-NIR, and ESR measurements of the neutral and cationic species as a part of a series of studies on sterically segregated oligothiophenes by the BCO-annelation We report these results and compare them with the previous results [16,17] for shorter chain homologs 1(nT) (n = 2, 3, 4, 6)
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