Electron n-doping of a highly electron-deficient chlorinated benzodifurandione-based oligophenylene vinylene polymer using benzyl viologen radical cations

Abstract

Benzodifurandione-based oligophenylene vinylene (BDOPV) is a very strong electron-accepting moiety that has a very low lowest unoccupied molecular orbital (LUMO) value of −4.24 eV. Consequently, benzodifurandione-based polyphenylene vinylene (BDPPV) polymers also possess low LUMO values of less than −4.0 eV and have been reported to show good thermoelectric performances. However, all n-doping of BDPPV polymers have been insofar restricted to hydride-donors. Using tetrakis(dimethylamino)ethylene (TDAE) as a control, we found out that the lack of suitable electron-donors for BDPPV polymers is due to the strong tendency of over-doping, as a result of their very low LUMO values that promotes very rapid electron transfer and near room-temperature alkyl side chain melting that facilitates dopant infiltration. Here, we demonstrate well-controlled n-doping of a highly electron-deficient chlorinated BDPPV polymer (Cl-BDPPV, LUMO = −4.3 eV) using an electron-donor benzyl viologen radical cation (BV˙⁺). Unlike other electron-donors like N,N′-dimethylbenzimidazoline dimers, BV˙⁺ can be easily prepared using easily accessible reagents and shows excellent stability under a N₂ atmosphere. The BV˙⁺-doped films were fully characterized and their thermoelectric performances were evaluated. A maximum σ and PF of 0.16 ± 0.01 S cm⁻¹ and 0.43 ± 0.01 μW m⁻¹ K⁻² were obtained, respectively. Our results indicate that BV˙⁺ is a suitable electron-donor n-dopant candidate for highly electron-deficient n-type conjugated polymers.