Bithiophene-based molecular systems have emerged as a cornerstone in the field of organic electronics owing to their remarkable structural versatility, strong π conjugation, and favorable optoelectronic properties. Their relatively simple chemical framework makes them ideal building blocks for more complex thiophene-based oligomers and polymers, widely used in organic semiconductors. In this work, we specifically investigate a dimer composed of two parallel bithiophene chains in order to evaluate the impact of intermolecular coupling on the electronic structure and optical response of the system. The parallel configuration is of particular interest because it mimics the π–π stacking interactions commonly observed in thin films and crystalline domains of organic materials.

Our analysis reveals that the interaction between the π orbitals of the two chains significantly modifies the energy levels of the dimer. The effective coupling reduces the electronic band gap, leading to a pronounced red shift in the absorption spectrum and enhancing the oscillator strength of the main electronic transitions. This results in improved light-harvesting ability, an essential requirement for photovoltaic and photodetector applications. Moreover, the parallel stacking promotes stronger charge delocalization along the conjugated framework, thereby lowering the reorganization energy and enhancing charge transfer efficiency. As a consequence, both electron and hole mobilities are improved, which is crucial for charge transport in optoelectronic devices.

These findings underline the central role of interchain interactions in dictating the performance of bithiophene-based materials. By controlling stacking geometry and intermolecular distances, it becomes possible to tune their electronic and optical properties, paving the way toward high-performance organic field-effect transistors (OFETs), organic photovoltaic cells (OPVs), and organic light-emitting diodes (OLEDs).