Polythieno[3,4-b]pyrazine: pathways to metallic charge transport

Abstract

For the first time, an alkyl-chain free variant of polythieno[3,4-b]pyrazine (PTP) was created in a conductive state via oxidative chemical vapour deposition (oCVD). The charge transport properties of this new PTP are superior to those of the state-of-the-art ones by ten-fold with a conductivity of 0.5 S cm⁻¹ and the material is found in the metallic regime of the metal–insulator-transition as the conductivity is vastly independent on temperature. Herein, we investigate the chemical and physical composition of the material in order to explain this improvement and discuss the results in light of the polythiophenes produced using the same method. Using GIWAXS, we discover large crystalline features combined with smaller, highly organized assemblies and gain insight into the chain length. The chemical composition of the material is determined via XPS and FTIR and further supplemented by computational methods. We employ DFT calculations (ωB97X-V/def2-QZVP) to understand the differences between unsubstituted and alkylated PTP. The former shows stronger proclivity to form planar structures, and the conjugation is broken upon distortion. Alternatively, in the latter case, such disorder does not considerably affect the conjugation. Furthermore, we simulate the effects of doping based on XPS results and reveal the possibility of protonation, despite the low pK_a of the materials. Furthermore, this protonation allows the materials to achieve a cis conformer despite the typical expectation suggesting the prevalence of the trans form. Molecular dynamics simulations (NPT MD) of 24 dodecamers confirm the prevalence of a trans conformer. However, at multiple occasions, trimers of the cis motif form. The simulations predict stronger charge transport in the trans conformer. However, the cis isomer exhibits relevant levels and an extremely low HOMO–LUMO difference (<0.1 eV). This indicates the possibility of high conductivities and explains the hallmarks of metallic charge transport we observe experimentally.