Selenium-Substituted β-Methylthiobenzo[1,2- b:4,5- b′]dithiophenes: Synthesis, Packing Structure, and Transport Properties

Systematic investigation on selenium modification of 3,7-bis(methylthio)benzo[1,2-b:4,5-b′]dithiophene (1) that affords the rubrene-like pitched π-stack in the solid state was carried out to elucidate the effects of selenium modification on their electronic properties, packing structures in the solid state, and transport properties. Straightforward access to the target compounds, 3,7-bis(methylseleno)benzo[1,2-b:4,5-b′]dithiophene (2), 3,7-bis(methylthio)benzo[1,2-b:4,5-b′]diselenophene (3), and 3,7-bis(methylseleno)benzo[1,2-b:4,5-b′]diselenophene (4), was accomplished by employing methyl-thiohypochlorite and -selenohypochlorite as the electrophilic reagent in the Larock cyclization reactions on 1,4-bis(methylthio)- and 1,4-bis(methylseleno)-2,5-bis(trimethylsilylethynyl)benzene. The packing structures of 2-4 were found to be classified into two classes, i.e., the rubrene-like pitched π-stack (3 and one of the polymorphs of 4) and the brickwork packing (2 and the other polymorph of 4). With the quantum chemical calculations using the symmetry-adapted perturbation theory (SAPT) and the Hirshfeld surface analysis, we elucidated how the packing structure of 1-4 can be controlled by the pitched π-stack or brickwork packing from the herringbone packing of the parent benzo[1,2-b:4,5-b′]-dithiophene and -diselenophene. The transport properties were evaluated by the single-crystal field-effect transistors, and the mobility extracted from the linear regime of 1, 3, and 4 with the pitched π-stack increases with the number of incorporated selenium atoms. On the other hand, fiberlike crystals of 2 obtained by the microspacing in-air sublimation method showed much higher mobility of up to 0.87 cm²/(V s). Although the exact packing structure of the fiberlike crystals of 2 could not be elucidated, the methylseleno groups that largely contribute to the highest occupied molecular orbital (HOMO) are speculated to enhance the intermolecular electronic coupling of HOMO in the solid state. We can thus conclude that the selenium modification not only in the π-core but also in the substituent is beneficial to enhance the transport properties in the benzodichalcogenophene system.