Heteroatom Structural Engineering Enables Ethenylene-Bridged Bisisoindigo-Based Copolymers to Exhibit Unique n-Type Transistor Performance

Herein, we report three novel electron-deficient aromatics, ethenylene-bridged bisisoindigos 3,3 '-((3E,3 ' E)- ((E)-ethene-1,2-diyl)bis(1-(4-decyltetradecyl)-2-oxoind-oline-6-yl-3-ylidene))bis(1-(4-decyltetradecyl)-1,3-dihydro-2H-pyrrolo[2,3-b]pyridin-2-one) (NCCN), 3,3 '-((3E,3 ' E)-((E)-ethene-1,2-diyl)bis(1-(4-decyltetradecyl)- 7-fluoro-2-oxoindoline-6-yl-3-ylidene))bis(1-(4-decyltetradecyl)-1,3-dihydro-2H-pyrrolo[2,3-b]pyridin-2-one) (NFFN), and (3E,3 '' E)-6,6 ''-((E)-ethene-1,2-diyl)bis(1,1 '-bis(4-decyltetradecyl)-[3,3 '-bipyrrolo[2,3-b] pyridinylidene]-2,2 '(1H,1 ' H)-dione) (NNNN), and their derived donor-acceptor (D-A) copolymers, namely poly[3,3 '-((3E,3 ' E)-((E)-ethene-1,2-diyl)bis(1-(4-decyltetradecyl)-2-oxoindoline-6-yl-3-ylidene))bis(1-(4- decyltetradecyl)-1,3-dihydro-2H-pyrrolo[2,3-b]pyridin-2-one-6-yl)]-alt-[5,6-difluoro-4,7-di[(thiophen-2-yl)-5-yl)]benzo[c][1,2,5]thiadiazole] (PNCCN-FBT), poly[3,3 '-((3E,3 ' E)-((E)-ethene-1,2-diyl)bis(1-(4-decyltetradecyl)-7-fluoro-2-oxoindoline-6-yl-3-ylidene))bis(1-(4-decyltetradecyl)-1,3-dihydro-2H-pyrrolo [2,3-b]pyridin-2-one-6-yl)]-alt-[5,6-difluoro-4,7-di[(thiophen-2-yl)-5-yl)]benzo[c][1,2,5]thiadiazole] (PNFFN-FBT), and poly[(3E,3 '' E)-6 ',6"'-((E)-ethene-1,2-diyl)bis(1,1 '-bis(4-decyltetradecyl)-[3,3 '-bipyrrolo[2,3-b]pyridi-nylidene]-2,2 '(1H,1 ' H)-dione-6-yl)]-alt-[5,6-difluoro-4,7-di[(thiophen-2-yl)-5-yl)]benzo[c][1,2,5]thiadiazole] (PNNNN-FBT), in which 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (FBT) acts as the electron-donating units. The ethenylene-bridging unit reduces the steric hindrance of the three bisisoindigos. Incorporation of heteroatoms, such as fluorine and sp2-nitrogen atoms, endows them with multiple CH center dot center dot center dot F, CH center dot center dot center dot N, and N center dot center dot center dot S intramolecular hydrogen bonds/nonbinding interactions, resulting in increasing backbone planarity from NCCN, NFFN, to NNNN, and thus from PNCCN-FBT, PNFFN-FBT, to PNNNN-FBT. We found that all copolymers formed an improved molecular packing in the 1-chloronaphthalene (CN)-processed thin film compared with the 1,2-dichlorobenzene-processed one. The CN-processed PNCCN-FBT-based polymer field-effect transistors showed ambipolar transport characteristics with the electron mobility (mu e) and hole mobility of 1.20 and 0.46 cm2 V-1 s-1, respectively, while the PNFFN-FBT-and PNNNN-FBT-based ones afforded unique n-type transport characteristics with impressively high mu e up to 3.28 cm2 V-1 s-1. The lower frontier molecular orbital energy levels of PNFFN-FBT are the key reason for its higher mu e. This study demonstrated that heteroatom structural engineering on ethenylene-bridged bisisoindigos is an effective