Covalently linked benzothiadiazole-fullerene adducts for organic optoelectronic devices: synthesis and characterization

Abstract

Fullerene adducts have attracted attention in a variety of applications including organic optoelectronic devices. In this regard, we have designed a covalently linked donor–acceptor dyad comprising a fluorobenzothiadiazole-thiophene (BTF₂-Th) unit with the electron acceptor fullerene in an Acceptor–Donor–Acceptor (A–D–A) molecular arrangement. We synthesized and characterized two new covalently bonded benzothiadiazole-based fullerene molecules, mono-adduct, 7 (benzothiadiazole : PC₆₁BM = 1 : 1, anchored terminally via esterification reaction) and multi-adduct, 10-I (benzothiadiazole : PC₆₁BM = n : 1, where n ≥ 1, attached directly to the fullerene core via the Prato reaction) using different synthetic strategies. A broadening of the UV-visible spectra of the modified fullerene derivative with strong absorption from 350 to 500 nm and at low wavelengths is observed as compared to PC₆₁BM. A suitable bandgap, good electronic conductivity, and appreciable solubility in solvents suggest their utility in optoelectronic devices. The mono-adduct 7 showed two-order higher electron mobility as compared to bis-adduct 10-I due to retention of extended conjugation in fullerene, as in the case of PC₆₁BM. Experimentally determined optical properties and energy levels of the fullerene adducts were found to be in good agreement and supported by theoretical calculations. The presence of BTF₂ affects the ground state dipole moments as well as the absorption strengths, most noticeable in the case of two attached BTF₂ moieties. The HOMO and LUMO levels are found to be localized on the fullerene cage with the extension of the HOMO to the BTF₂ unit more and the same is noticed in ground state dipole moment in the side-chain functionalized structure. Such structural arrangement provides easy charge transfer between acceptor and donor units to allow a concomitant effect of favorable optoelectronic properties, energy levels of the frontier orbitals, effective exciton dissociation, and charge transport which may reduce processing complexity to advance single material-based future optoelectronic devices.