Steric Control in the π‐Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[2.2.2]octene Units

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.

Radical Cation π‐Dimers of Conjugated Oligomers as Molecular Wires: An Analysis Based on Nitronyl Nitroxide Spin Labels

“…showed almost quantitative π‐dimer formation. As shown in Figure , the diagnostic spectral change for the formation of oligomer radical cation π‐dimer, comprising three absorption bands due to a HOMO–LUMO transition in the π‐dimer with blueshifted original bands for the radical cation moiety, was observed in DCM at low temperatures. The longest absorption maxima were assigned to HOMO–LUMO transitions of the π‐dimer and found at 1040 nm for (DTP‐P‐NN . )…”

“…The B3LYP method is frequently used to estimate J values . However, in general, the method cannot be applied to the full optimization of ion radical π‐dimers without dispersion or long‐range correction . Therefore, B97D3(BJ) (recommended for the geometry optimization of ion radical dimers) and M06‐2X (used for π‐dimers in previous studies) and B3LYP with the 6‐31G(d) basis set were used to estimate J m intra .…”

“…From the diamagnetic character of the terthiophene radical cation π‐dimer, it was proposed that the π‐dimeric interchain interaction can act as an alternative charge carrier to bipolaron (an intrachain closed‐shell dication) in macroscopic conduction . Since then, various oligothiophene‐based π‐dimers have been prepared to understand the electronic structure of p ‐doped polythiophenes . Nevertheless, the microscopic conductive properties of long‐bonded π‐dimers as molecular wire have not been verified because of the absence of a suitable model system.…”

π‐Dimerization ability of conjugated oligomer dication diradicaloids composed of dithienylpyrrole and benzodithiophene units

“…As shown in Figure , all spectra of 1 2+ , 2 2+ , and 3 · + at room temperature consisted mainly of two absorption bands. These are typical absorption pattern of linear conjugated oligomer radical cation, and the longer and shorter wavelength bands are assigned as HOMO→SOMO and SOMO→LUMO transitions, respectively . In fact, the absorption maxima of 3 · + (950, 650 nm) were resemble to those of 9 · + (878, 570 nm).…”

“…Next, variable‐temperature (VT) measurements of these solutions were conducted. As shown in Figure C, the diagnostic spectral change due to the formation of π‐dimer ( 3 · + ) 2 , that is, new absorption band assigned as HOMO→LUMO transition of the π‐dimer in the longest wavelength region with a hypsochromic shift of the original two radical cation bands, was observed for 3 · + in dichloromethane at low temperatures. The absorption bands of ( 3 · + ) 2 were observed at ~1060, 850, and 550 nm, which were also ca.…”