Low Band Gap Donor-Acceptor Conjugated Polymers with Indanone-Condensed Thiadiazolo[3,4-g]quinoxaline Acceptors

Donor-acceptor (D-A) conjugated polymers with the band gaps below 1.0 eV can exhibit unique near-infrared (NIR) activities and multiple functional applications. However, it is still a big challenge to develop such materials because of the scarcity of effective synthetic strategies and strong acceptor building blocks. Herein, we report the design, synthesis, and application of two novel indanone-condensed thiadiazolo[3,4-g]quinoxaline (TQ) acceptor units, which display high electron affinities and low-lying lowest unoccupied molecular orbital (LUMO) levels because of the incorporation of auxiliary electron-deficient carbonyl or cyano groups into the TQ core. Moreover, two low band gap D-A conjugated polymers are synthesized via Stille condensation reactions between the newly developed TQ acceptor units and 2,5-bis(3-(2-decyltetradecyl)thiophen-2-yothieno-[3,2-b]thiophene donor units. The effect of the substitute groups (carbonyl and cyano groups) on the geometry, optical property, electronic structure [highest occupied molecular orbital (HOMO)/LUMO levels and band gap], film organization, and charge transport of the polymers are discussed carefully. The resulting polymers exhibit very broad NIR absorptions extended to around 1880 nm, deep-lying LUMO energy levels (< -3.90 eV), and low optical band gaps (similar to 0.66 eV). Investigation of field-effect transistor properties indicates that both polymers show a well- balanced ambipolar transport behavior, affording the hole/electron mobilities of 1.3 X 10(-3) and 2.0 X 10(-3) cm(2 )V(-1) s(-1), respectively. These results indicate that both newly developed TQ acceptors are excellent building blocks for the development of novel pi-conjugated systems with low band gap and high NIR activity.