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-yl)thieno-[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 (∼0.66 eV). Investigation of field-effect transistor properties indicates that both polymers show a wellbalanced ambipolar transport behavior, affording the hole/electron mobilities of 1.3 × 10 −3 and 2.0 × 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 π-conjugated systems with low band gap and high NIR activity.

No abstract

A hitherto unreported heterocyclic imide, thiadiazolo[3,4-g]quinoxaline-fused acenaphthenequinone imide acceptor (TQAI), is designed and synthesized by incorporating an auxiliary electron-deficient thiadiazolo[3,4-g]quinoxaline (TQ) annulated in the lateral direction of the acenaphthenequinone imide skeleton. This molecular design makes it possible to generate strong electron affinity by integrating the highly electron-deficient TQ unit with the dicarboxylic acid imide groups. In addition, a largely π-extended and S/N-enriched π-conjugated structure can be produced, which facilitates both intermolecular and intramolecular interactions. As a result, the alternating arrangement of TQAI acceptor and thiophenederivative donor segments, such as 2,5-bis(3-(2-decyltetradecyl)thiophen-2-yl)thieno-[3,2-b]thiophene or 3,3‴-bis(2-decyltetradecyl)-2,2′:5′,2″:5″, 2‴-quaterthiophene, in the polymer main chain, endows the target D−A conjugated polymers with desirable photophysical and electrochemical properties. These properties include deep lowest unoccupied molecular orbital levels (LUMO < −3.80 eV), narrow optical band gaps (as low as 0.84 eV), strong light-capturing ability in the second near-infrared window, and strong solid-state organization with a preferred face-on orientation. The organic field-effect transistors, which were made with our synthesized polymers, exhibit typical ambipolar charge transport in an inert atmosphere. PTQAIT-2T shows maximum hole and electron mobilities of 8.1 × 10 −3 and 3.6 × 10 −2 cm 2 V −1 s −1 , respectively. In particular, with a C 60 layer, which is added to facilitate efficient electron trapping and exciton dissociation, the bilayer C 60 /PTQAIT-2T phototransistors show a pronounced photosensitivity of 1780, which is 50 times higher than that of pristine PTQAIT-2T devices.

Despite the important advances in high-mobility electron-transporting polymers built from aromatic six- and seven-membered diimides, aromatic five-membered diimide (AFMDI)-containing polymers rarely access satisfactory n-type or ambipolar performance. Herein, a UV-photocyclization protocol is applied to create two novel AFMDI units, named as dibrominated naphtho­[1,2-b:4,3-b′]­dithiophene diimides (NDTI-2Br) and dibrominated naphtho­[1,2-b:4,3-b′]­diselenophene diimides (NDSI-2Br). Both NDTI- and NDSI-based small molecules are demonstrated to possess not only a highly π-extended conjugation backbone but also high electron deficiency and low-lying energies of lowest unoccupied molecular orbital (LUMO, as low as −3.74 eV), which is due to the incorporation of two electron-poor imide units into the fused-ring parent cores. With these attractive properties, we further disclose their applications in the construction of four novel conjugated copolymers, including NDTI and NDSI derivative acceptors coupled with a weak electron-donating vinyl unit (P1 and P2) or linked with the 4,7-di­(thiophen-2-yl)­benzo­[c]­[1,2,5]­thiadiazole unit (P3 and P4). It is found that the backbone structure, optical property, electronic structure (energy levels and band gap), and charge transport property of the resulting polymers are fine-tuned by regulating the copolymerization units or sulfur/selenium heteroatoms embedded in the acceptor units. All the polymers display a near-coplanar conjugation backbone, outstanding thermal stability (T d > 460 °C), and desirable reduction waves coupled with low-lying LUMO energies below −3.78 eV. Investigation of charge transport properties indicates that P1 and P2 show typical unipolar n-type characteristics with the highest electron mobility of 0.01 cm2 V–1 s–1, while P3 and P4 exhibit balanced ambipolar charge transport properties, with the maximum hole and electron mobilities of 2.0 × 10–4 and 0.005 cm2 V–1 s–1, respectively. The electron mobility (0.01 cm2 V–1 s–1) observed here can be compared to the classical n-type semiconductor PCBM (∼10–3 cm2 V–1 s–1), which is sufficient for electron transport to the electrode in the all-polymer solar cells.

Ladder-like aromatic diketones (LADK), which possess a coplanar π-extended geometry, a high electron deficiency as well as various attractive optoelectronic properties, are demonstrated as the promising candidates in building small-molecule organic electron-transporting materials, yet reports on direct integration of these structural motifs into n-type polymers are rarely accessed. Herein, it is demonstrated that a possibility of realizing unipolar n-type characteristics of such acceptor system by developing two novel donor-acceptor type polymers, in which the newly developed LADK unit, named as 3,8-bis(2-decyltetradecyl)-5,10-difluoro-s-indaceno[1,2-b:5,6-b′] dithiophene-4,9-dione (FIDTO-R), is adopted as the acceptor segments. The resulting polymers present deep-lying unoccupied molecular orbital levels (as low as −3.84 eV), compact π-π stacking (d-spacing, ≈3.57 Å) coupled with uniform nanofiber-like surface morphology. All these factors contribute to excellent unipolar n-type characteristics with high electron mobilities of 0.27 and 1.01 cm 2 V −1 s −1 , together with high inverter gain values of 141 and 80, respectively. The recorded values are among the best in n-type polymer field-effect transistors and associated inverter circuits. These findings unambiguously reveal that the as-prepared FIDTO-R and its analog LADK derivatives are another type of excellent building blocks for the construction of high-mobility n-type polymers.