Steric Control in the π-Dimerization of Oligothiophene Radical Cations Annelated With Bicyclo[2.2.2]octene Units.

Significance Statement

“Cationic oligothiophenes play a key role in the study of conduction mechanism of p-doped polythiophene, and radical cations of oligothiophenes, which are made persistent with appropriate substituents, have been shown to undergo self-association, so-called p-dimer formation, in the condensed phase. A study of the p-dimer of terthiophenes radical cation reported two decades ago proposed that the p-dimer-like interpolymer chain interaction of the two polarons can be an alternative to bipolaron. However, no detailed studies based on both experiments and theoretical calculations for the quantitative estimation of the attractive and repulsive intermolecular interactions in the π-dimers of oligothiophene radical cations in solution have been performed. In the present study, series of oligothiophenes annelated with bicyclo[2.2.2]octene units were synthesized to investigate the driving forces of p-dimerization and the structure–property relationships of the p-dimers of oligothiophene radical cations. Combined with the results of DFT calculations, SOMO–SOMO interaction was found to be the most important factor affecting p-dimerization enthalpy. In recent years, there has been growing interest in π-dimer-based supramolecular chemistry, and the present study provides an important guidance for developing oligothiophene-π-dimer-based supramolecular chemistry and also functional p-conjugated polymers.”

 

 

Steric Control in the {Pi}-Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[octene Units

Chemistry. 02/2013; DOI:10.1002/chem.201204332.

Masaki Tateno, Masayoshi Takase, Masahiko Iyoda, Koichi Komatsu, Tohru Nishinaga.

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397 (Japan).

Abstract

 A Two series of oligothiophenes 2(nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a-c, annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π-dimerization and the structure-property relationships of the π-dimers of oligothiophene radical cations. Their radical-cation salts were prepared through chemical one-electron oxidation by using nitrosonium hexafluoroantimonate. From variable-temperature electron spin resonance and electronic absorption measurements, the π-dimerization capability was found to vary among the members of the 2(nT)+ . SbF6 – series and 3+ . SbF6 – series of compounds. To examine these results, density functional theory (DFT) calculations at the M06-2X/6-31G(d) level were conducted for the π-dimers. This level of theory was found to successfully reproduce the previously reported X-ray structure of (2(3T))2 2+ having a bent π-dimer structure with cis-cis conformations. The absorption bands obtained by time-dependent DFT calculations for the π-dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π-dimerization were divided into four factors: 1) SOMO-SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain-length dependence are discussed in detail.

 

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