Thienoquinoidal System: Promising Molecular Architecture for Optoelectronic Applications

Abstract

Thienoquinoids are a class of compounds that consist of a quinoidal conjugation path in the thiophene-based π-extended structures. Owing to the quinoidal conjugation path, the characteristic features of thienoquinoidal compounds are their high-lying energy levels of the highest occupied molecular orbitals (HOMOs) and low-lying energy levels of the lowest unoccupied molecular orbitals (LUMOs) compared to their aromatic thiophene-based counterparts. The thienoquinoidal cores are often terminated with electron-withdrawing groups such as dicyanomethylene units, which further lowers the LUMOs making the resulting thienoquinoidal compounds attractive as electron-accepting molecules for charge-transfer complexes and n-type organic semiconductors. In fact, the thienoquinoidal molecules were initially focused as the electron accepter in 1980s. Since 2002 when the first report that a terthienoquinoidal derivative can act as a superior n-type organic semiconductor, this class of compounds have attracted researchers in the field of organic electronics and organic semiconductors, and a vast number of thienoquinoidal molecules have been synthesized in the last decade. The present review focuses on the methods for the synthesis of thienoquinoidal compounds with different terminal units and their electronic structures depending on the terminal units and thienoquinoidal cores. Then, several representative results on the application of thienoquinoidal molecules in the n-type field-effect transistors are mentioned, together with future prospects of this intriguing class of compounds.