Two Isomeric Thienoacenes in Thin Films: Unveiling the Influence of Molecular Structure and Intermolecular Packing on Electronic Properties

Isomerism of molecular structures is often encountered in the field of organic semiconductors, but little is known about how it can impact electronic and charge transport properties in thin films. This study reveals the molecular orientation, electronic structure, and intermolecular interactions of two isomeric thienoacenes (DN4T and isoDN4T) in thin films, in relation to their charge transport properties. Utilizing scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARUPS), and near-edge X-ray absorption fine structure measurements (NEXAFS), we systematically analyze the behavior of these isomers from submonolayer to multilayer coverage on highly ordered pyrolytic graphite (HOPG) as substrates. We find that at submonolayer coverage both DN4T and isoDN4T molecules predominantly adopt a nearly flat-lying orientation on the surface, minimizing intermolecular interactions. The distinct emission features of the highest occupied molecular orbital (HOMO) level in ARUPS enables the determination of molecular reorganization energies. These are found to be in good agreement with theoretical predictions, suggesting superior charge transport in DN4T compared to isoDN4T. Notably, thickness-dependent photoemission measurements reveal a significant splitting (approximately 450 meV) of the HOMO level of isoDN4T, attributed to polarization-induced effects rather than wave function overlap, indicating a nuanced interplay between molecular packing and electronic properties. Our results underscore the importance of molecular packing and substrate interactions in determining the electronic structure and transport properties of organic semiconductor thin films. Substrate-induced polymorphism and the crucial role of polarization-induced effects influencing charge transport are highlighted. These insights are pivotal for future engineering of molecular and thin film structures, aiming to enhance the performance of organic semiconductor-based devices.