Three Structural Isomers of Dinaphthothieno[3,2-b]thiophenes: Elucidation of Physicochemical Properties, Crystal Structures, and Field-Effect Transistor Characteristics

In order to gain an insight into the relationship between the molecular structure and the semiconductor characteristics of highly pi-extended heteroarene-based organic semiconductors, three structural isomers of dinaphthothieno[3,2-b]thiophenes with C-2h symmetry were investigated. Of these, two isomers, dinaphtho[2,1-b:2',1'-f]thieno[3,2-b]thiophene (2) and dinaphtho[1,2-b:1',2'-f]thieno[3,2-b]thiophene (3), were newly synthesized, characterized, and utilized as active semiconducting layers in organic field-effect transistors (FETs). Detailed investigation of the physicochemical properties of 2 and 3, together with another isomer, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (1), indicated that the electronic structures of the three isomers are fairly different from each other despite having the same molecular formula and the same aromatic constituents. Comparison of the molecular arrangements in the crystals elucidated by X-ray structural analysis implied that the molecular shape and the thus-induced favorable intermolecular interactions play important roles in determining the entire molecular arrangement. The characteristics of 2- and 3-based FETs with maximum field-effect mobilities (mu(FET)s) of 10(-3)-10(-2)cm(2)V(-1)s(-1) were inferior to those of 1-based FETs with mu(FET)s up to 3.0cm(2)V(-1)s(-1). The inferior characteristics of 2- and 3-based devices were due to film morphology as elucidated by atomic force microscopy (AFM) and supported by theoretical calculations of electronic structure in the solid state. Together, the results indicate that the molecular structure and shape, even for similar heteroarenes with the same molecular formula and symmetry, are important parameters to determine the solid-state properties of organic semiconductors.