Cholesteric Aggregation at the Quinoidal-to-Diradical Border Enabled Stable n-Doped Conductor

Abstract

Molecular Kekulé diradicals have been demonstrated to show unique optoelectronic properties as a function of their diradical character. A series of thienoquinoidal oligothiophenes from dimer to pentamer and substituted with an odd and even number of pyrrolo-dione groups have been prepared and proven to be n-dopable materials showing outstanding ambient stability and excellent electrical and thermoelectric behavior. Going from dimer to pentamer, a progressive change in the diradical character and aggregation mode is observed, with the tetramer showing an optimal diradical character that allows favorable intermolecular contacts with π−π multi-bonding features, while the presence of the two dione groups promotes a cholesteric-like π−π stacking. Both features synergistically contribute to form a material with exceptional ambient stability for an n-doped system exhibiting high electrical conductivities and thermoelectric performance. Doping is a vital strategy in achieving high-performance organic electronic devices. Compared to p-doped materials, there are only a few complementary n-doped conductors reported and all of these are unstable in ambient conditions, a limitation for the consolidation of organic materials in the electronic market. The well-known instability of the electrically active radical anions in the n-doped substrates, which partially relies on their high-lying LUMO energy levels, becomes one of the most critical challenges to solve. Our findings demonstrate that diradical character and deep LUMO energy levels, lower than −4.6 eV, such as in the limit of quinoidal-to-aromatic diradical conversion, are conditions for achieving stable n-type doped conducting materials. We believe these findings offer a new design concept to the organic electronic community for the realization of new and improved applications, such as n-doped conductors and in organic thermoelectrics. Highly stable n-doped conductors based on quinoidal oligothiophenes are achieved. The suitable synergy between intra- and inter-molecular effects dictates the exceptional properties of 2DQQT. Uniquely, its incipient diradical character and cholesteric-like aggregation both enhance electrical conductivity (i.e., 14.0 S cm −1 ) and unprecedented air stability. At the molecular level, our findings demonstrate that small diradical character and deep LUMO energy levels, lower than −4.6 eV, are conditions suitable for achieving stable n-type doping.