Direct C-H arylation as a tool for synthesis of conjugated polymers : from 1D linear polymers to 2D/3D porous networks

Abstract

Organic π-conjugated linear and network polymers, owing to their large synthetic variety of finely tunable structures and properties, are promising semiconducting materials for optoelectronic devices and light harvesting applications. A vast library of such π-conjugated systems has been synthesized through conventional tools of coupling (e.g. Suzuki coupling, Stille coupling) which involves tedious preactivation of C-H bonds using highly toxic and flammable reagents. In recent years an emerging synthetic technique called direct C–H arylation has been extensively studied as a facile, atom-efficient and environmentally benign pathway for the synthesis of conjugated polymers and small molecules. In this thesis, direct arylation has been applied to rationally chosen monomers to produce conjugated small molecules, linear polymers as well as 2D/ 3D porous polymers that exhibit performances comparable with those made from conventional reactions.

In this thesis, we use naphthodithiophenediimide (NDTI) – a new electron-accepting building block that has shown high performance in ambi-polar organic field-effect transistors to synthesize polymers and small molecules by direct arylation. To examine its regioselectivity, NDTI was used to synthesize small molecules by coupling with 2-bromo-9,9-dihexylfluorene. Two NDTI-based narrow bandgap polymers, one with benzothiadiazole and the other with 9,9-dioctylfluorene, were synthesized via polymerization (DAP). The chemical structures and optoelectronic properties of these molecules and polymers were characterized.

Benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDTD) is another electron-accepting building block which has demonstrated outstanding performance in organic bulk heterojunction (BHJ) solar cells. We use DAP to synthesize a series of wide bandgap D-A copolymers with a common acceptor building block of BDTD and study the structure-property relationship in these polymers. We also present the device performances of these polymers in both thin-film field-effect transistors and organic solar cells using BDTD-based polymers as the electron donors and fullerene derivatives as the electron acceptors.

In this thesis, we have also demonstrated direct arylation as an efficient synthetic tool for conjugated porous polymers (CPPs). A series of narrow bandgap conjugated porous polymers have been synthesized by facile direct arylation polymerization of thiophene-flanked thienothiadiazole (TTD) with multi-brominated monomers with different geometries. The polymer products show strong light absorption in the near infrared region, corresponding to narrow optical bandgaps below 1.3 eV. Under the same polymerization conditions, the morphologies, porosities and optoelectronic properties of the resulting polymers are determined by the chemical structures of the aryl bromides.

In order to extend the library of thiophene-containing CPPs synthesized by direct arylation, we chose three thiophene-flanked monomers – bithiophene (BT), 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) and 2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) with active C-H bonds and polymerized them with tri- and tetra-brominated monomers. This modular design has allowed us to study the effect of thiophene monomers on the morphology, porosity and optical properties of the CPPs.

In our final research work, we present the direct arylation synthesis of a new series of conjugated porous polymers (CPPs) containing triazine. We used 2,4,6-(tri-2-thienyl)-1,3,5-triazine (TTT) to synthesize a series of robust triazine-core polymers by with DAP without need for preactivating the C–H bonds in the arene monomers. The resulting triazine-core polymers were used to catalyze the photo-oxidation of benzylamines and the polymers containing the highest fraction of triazine units showed > 99% conversion with a relatively low loading of the catalyst.