Two Isomeric Thienoacenes\nin Thin Films: Unveiling\nthe Influence of Molecular Structure and Intermolecular Packing on\nElectronic Properties

Isomerism of molecular\nstructures is often encountered\nin the field\nof organic semiconductors, but little is known about how it can impact\nelectronic and charge transport properties in thin films. This study\nreveals the molecular orientation, electronic structure, and intermolecular\ninteractions of two isomeric thienoacenes (DN4T and isoDN4T) in thin\nfilms, in relation to their charge transport properties. Utilizing\nscanning tunneling microscopy (STM), angle-resolved photoemission\nspectroscopy (ARUPS), and near-edge X-ray absorption fine structure\nmeasurements (NEXAFS), we systematically analyze the behavior of these\nisomers from submonolayer to multilayer coverage on highly ordered\npyrolytic graphite (HOPG) as substrates. We find that at submonolayer\ncoverage both DN4T and isoDN4T molecules predominantly adopt a nearly\nflat-lying orientation on the surface, minimizing intermolecular interactions.\nThe distinct emission features of the highest occupied molecular orbital\n(HOMO) level in ARUPS enables the determination of molecular reorganization\nenergies. These are found to be in good agreement with theoretical\npredictions, suggesting superior charge transport in DN4T compared\nto isoDN4T. Notably, thickness-dependent photoemission measurements\nreveal a significant splitting (approximately 450 meV) of the HOMO\nlevel of isoDN4T, attributed to polarization-induced effects rather\nthan wave function overlap, indicating a nuanced interplay between\nmolecular packing and electronic properties. Our results underscore\nthe importance of molecular packing and substrate interactions in\ndetermining the electronic structure and transport properties of organic\nsemiconductor thin films. Substrate-induced polymorphism and the crucial\nrole of polarization-induced effects influencing charge transport\nare highlighted. These insights are pivotal for future engineering\nof molecular and thin film structures, aiming to enhance the performance\nof organic semiconductor-based devices.