Substitution effect and charge transport properties in organic field-effect transistors (OFETs) of simply functionalized carbazole derivatives

Development of low cost, easy-preparation and good performance organic semiconductors is in high demand for the practical application of organic field-effect transistors (OFETs). Even though the π-extended carbazole derivatives have been intensively studied in OFETs, the application potential of simple carbazole derivatives, i.e., owning simple substituents directly at carbazole backbone, remains unclear. In this work, we designed and synthesized a series of carbazole derivatives with simple phenyl or thienyl functionalization directly at 2,7- or 2,6-positions, respectively. The photophysical and electrochemical properties, together with their theoretical frontier molecular orbitals and energy levels were investigated. The molecular conformations and packing structures in the solid state of these molecules were understood by single-crystal structure analysis. In addition, single-crystal based OFETs were fabricated to explore their intrinsic charge transporting properties, in which the 2,7-diphenyl modified carbazole derivative showed hole-transporting mobilities up to 0.16 cm² V⁻¹ s⁻¹. Our study not only proves that simply functionalized carbazoles have great potentials to act as easy-preparation high-performance p-type semiconducting materials, but also illustrates that 2,7-positon functionalization is more advantageous for balancing the molecular conformations and packing structures of carbazole derivatives to achieve good charge transport performance.