Design, Synthesis and Characterization of Fused Thiophene Based Copolymers and Evaluation of their Optoelectro

Abstract

Conjugated polymers have grabbed significant attention in the last few years due to their growing use in optoelectronic devices. These polymers are flexible and lightweight with tunable electronic and optoelectronic properties which make them useful as semiconductors for the fabrication of organic light emitting diodes (OLEDs), field effect transistors (FETs), plastic lasers, photovoltaic cells, chemical sensors and indicators. Fused thiophenes, generated by the annulation of thiophene rings, have rigid structures with an extended n-conjugation compared to thiophene. While the simplest fused thiophene called thieno[3 ,2-b ]thiophene (IT) is formed by annulation of two thiophene rings, fusing another thiophene ring into TT gives dithieno[3,2-b:2',3'-d]thiophene (DTT). TTs and DTTs are electron rich compounds with flat and rigid delocalized systems, which make them appropriate candidates to be used in conjugated and low bandgap semiconductors. Since the electronic characteristics of conjugated polymers strongly depend on the electronic energy levels of the polymer, it is very important to tune the position of the energy levels and the extent ofthe bandgap in the design and synthesis of conjugated polymers. In this study novel donor-acceptor n-conjugated copolymers based upon thieno[3,2- b ]thiophene (TT) and dithieno[3,2-b:2',3'-d]thiophene (DTT) were synthesized to investigate their optoelectronic properties. TTs and DITs were employed as donor building blocks while benzothiadiazole (BT) was used as an acceptor unit. Donor and acceptor units were polymerized via Sonogashira cross-coupling polymerization under standard reaction conditions to afford new conjugated copolymers, PI-P14 incorporating differently substituted TTs, M2-M12 and DTTs, M13-M15 as the donor, acetylene as the n-bridge and benzothiadiazole as the acceptor units. The optical, electrochemical and thermal properties of these conjugated polymers were evaluated via UV -vis, fluorescence, cyclic voltarnmetry and thermogravimetric analysis. weight average molecular weight, number average molecular weight and polydispersity index (PDI) of the polymers were studied using gel permeation chromatography. Brunner-Emmett-Teller (BET) analysis was done to investigate porosity and surface area ofTT-based polymer. All the polymers were found to have narrow polydispersity indices (less than 1.9), low bandgaps (2.30-2.42 eV), high Stocks shift values (97-148), moderate to high char yields (20-71 %), high limiting oxygen