Overlapping of Frontier Orbitals in Well-Defined Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene and Picene Monolayers

Well-defined monolayers with single-crystalline-like molecular arrangements of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT) and picene, which are a new class of organic semiconductors with enhanced intermolecular interactions, were fabricated and characterized. Although both molecules initially form a loosely packed monolayer with a flat-lying molecule, they undergo phase transition into a densely packed monolayer with single-crystalline-like molecular arrangements with increasing molecular density. Upon the phase transition of the monolayer, the highest occupied molecular orbital (HOMO) level of these molecules splits into two features, as suggested from both the ultraviolet photoelectron spectroscopy and density functional theory calculations. The splitting of the HOMO level was observed to be similar to that expected for the molecular arrangement in the single crystal. This splitting, which has not been observed in the polycrystalline film, suggests a substantial overlap of the HOMO in the well-ordered monolayers.