Thieno[3,4‑b]thiophene-Based Novel Small-Molecule Optoelectronic Materials

Because of the tailorable photoelectric properties derived from judicious molecular design and large-area and low-temperature processability especially on flexible substrates, design and synthesis of new organic π-functional materials is always a central topic in the field of organic optoelectronics, which siginificantly contributed to the development of high-performance optoelectronic devices such as organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Compared with polymers, small molecules with well-defined molecular structures benefit the establishment of structure–property relationships, which may provide valuable guidelines for the design of new optoelectronic materials to further promote the device performance. New building blocks are essential for the construction of optoelectronic materials. As is well recognized, thiophene-based functional materials have played an indispensable role in the development of organic optoelectronics. Compared with six-membered benzene, five-membered thiophene shows weaker aromaticity and lower steric hindrance and may provide extra sulfur–sulfur interactions in solid state. Among various thiophene building blocks, thieno­[3,4-b]­thiophene (TbT) is an asymmetric fused bithiophene containing four functionalization positions, in which the proaromatic thiophene can effectively stabilize the quinoidal resonance of the aromatic thiophene. Thus, TbT exhibits a unique characteristic of quinoid-resonance effect that is powerful to modulate electronic structures. Although the application of TbT in polymer donor materials represented by PTB-7 has achieved a great success, its application in small-molecule optoelectronic materials is almost an untouched field. In this Account, we summerize the rational design of a series of TbT-based small-molecule optoelectronic materials designed and optimized by quinoid-resonance effect, regiochemistry, and side-chain engineering and demonstrate the crucial effect of TbT building blocks on the electronic structures, photophysical and charge transport properties, and photovoltaic performance. With well-defined regioregular oligothieno­[3,4-b]­thiopenes, we revealed the quinoid-resonance effect of the TbT moiety and its geometric origin. TbT-based small molecules exhibit full-color tunable emissions in the visible to near-infrared regions and excellent performance in OFETs and OPVs. For instance, TbT-based quinoidal molecules with nea