Structure-Property Relationship of Thiophene-Isoindigo Co-Oligomers: A First-Principles Study

In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) are employed to systematically investigate the impact of the molecular structure of thiophene-isoindigo-based co-oligomers (PnTI, where n denotes the number of thiophene units) on their conformational, electrochemical, optical, and charge transfer properties. A twist in the backbone conformation of the co-oligomers was observed to increase as the number of thiophene units grew. The highest occupied molecular orbital (HOMO) level was found to shift upward, while the lowest unoccupied molecular orbital (LUMO) remained insensitive to the increasing number of thiophene rings. This was attributed to the enhanced electron-donating strength of the thiophene rings, as confirmed by the increase in electrostatic potential (ESP) of the donor units. In contrast to the electrochemical bandgap, the optical bandgap of the co-oligomers remained constant despite the increase in electron-donating properties, suggesting that the intramolecular charge transfer in the co-oligomers is primarily determined by the strong electron-withdrawing isoindigo moiety. Additionally, the exciton binding energies, hole mobilities, and charge transfer properties in the co-oligomers were found to be dictated by the number of thiophene units in the donors. This detailed exploration of structure-property relationships offers valuable insights for the rational design of high-performance organic semiconductor materials.