Understanding trends in conductivity in four isostructural multifunctional crystals of Se substituted bis-dithiazolyl radicals

Abstract

Materials based on stable organic radicals are very promising for the development of single-component organic conductors. However, the lack of studies addressing the quantitative calculation of the parameters defining their conductivity hampers progress. To contribute to this field, we computationally study four isostructural compounds with different Se-contents belonging to the key pyridine-bridged bisdithiazolyl family (namely, (S,S)-bisdithiazolyl, (S,Se) and (Se,S) mixed-thiaselenazolyl, and (Se,Se)-bisdiselenazolyl) with remarkable variation in the electrical conductivity (σ_SS < σ_SeS < σ_SSe < σ_SeSe) that cannot be explained on simple grounds. This trend here is explained by analyses of the local microscopic parameters playing the leading role in charge transport mediated by the molecular hopping mechanism: reorganization energy (λ), electronic couplings (H_DA), electron-transfer rate constants (k_DA), and charge-carrier density (ρ_c). Our results reveal the preference for hole conduction. The lowest conductivity of (S,S) arises from its largest λ, and smallest H_DA's and ρ_c, resulting in a 1D conductor along the π-stack. Instead, the largest conductivity of (Se,Se) originates in its smallest λ, largest ρ_c and a set of H_DA electronic couplings that not only are the largest but also define a 3D topology of conduction pathways along both lateral contacts and π-stacking. Comparison of (Se,S) and (S,Se) shows that although (Se,S) features the largest k_DA and the smallest λ values, (S,Se) exhibits the largest electrical conductivity since it shows a 3D conduction topology because of lateral contacts and has a larger ρ_c value. Our take-home message is that one needs to master a holistic view of the parameters governing the charge transport process (namely, λ, H_DA, topology of conduction paths, and ρ_c) to understand the trends in conductivity in radical-based molecular materials.