Multi-orbital hybridization in a one-dimensional monolayer of DPh-BTBT

[1]Benzothieno[3,2-b][1]benzothiophene (BTBT)-based molecules exhibit remarkably high hole mobility, sparking interest in their charge transport mechanisms. However, for thin films, the theoretically proposed mixed-orbital charge transport (MOCT) mechanism, which involves the hybridization of different frontier orbitals between neighboring molecules in the bulk, remains unexplored both experimentally and theoretically. In this study, we prepared a monolayer of 2,7-diphenyl-BTBT (DPh-BTBT) with a unique one-dimensional structure and investigated its molecular-level structure and electronic state. The formation of this monolayer led to significant energetic broadening of both highest occupied molecular orbital (HOMO) and second-highest occupied molecular orbital (HOMO-1) levels, observed via photoemission spectroscopy. Density functional theory calculations indicate that electrostatic effects cause a shift in the on-site energies of the two molecules within the monolayer unit cell. However, this on-site energy in turn facilitates hybridization between the HOMO and HOMO-1 of adjacent molecules. Thus, the multi-orbital hybridization effect can still be observed in the one-dimensional monolayer, which is a unique and important property of the thin films of this class of molecules.