Synthesis, properties and device applications of thienothiophene containing polymers with different functional groups

Abstract

Conjugated polymers have received increasing attention for use in various optical and electronic applications, such as organic light emitting diodes (OLEDs), organic solar cells, luminescent sensors, electrochromic devices, capacitors, organic field effect transistors (OFETs), and biological imagery. Many π-conjugated aromatic units, such as benzene, thiophene, pyridine, fluorene and carbazole, have been introduced in the conjugated polymer backbone. Amongst them, thiophene exhibits outstanding properties, due to its high π-electron density, band gap tunability, more reactivity and stability. Extension of the π system increases the conjugated length of the molecule, leading to a decreased band gap and often increased charge carrier mobility. Thienothiophenes (TTs) are the simplest fused and π-extended structures of the thiophene family. Especially, thieno[3,2-b]thiophene, is the most stable and highly conjugated isomer of TTs. Due to their electron rich, flat, rigid, and good electron delocalized skeleton, thienothiophene (TT) is one of the most impressive organic cores, and has gained significant attention in the organic polymeric materials in recent years. In this thesis, possessing cyano (CN), carboxylic acid (COOH) and dimethylamine (N(CH3)2) substituted thieno[3,4-b]thiophene (TT), π-extended conjugated three novel polymers (P1-P3) with 3- hexylthiophene were successfully synthesized by Suzuki coupling. All the polymers were found to be well soluble owing to the side chains. Effects of the functional groups (CN, COOH and N(CH3)2) on the polymers were compared and examined by optical, electronic, thermal, sensor study and memory applications as well as time-dependent DFT calculations for both monomers and dimers. Moreover, the photophysical characterization of the novel polymers demonstrated a significant mega Stokes shift, reaching 138 nm with a bathochromic shift, and a changing electronic band gap between 1.91 and 2.33 eV as well as good thermal stability of degradation temperature around 320 oC. Also, their F− anion-recognition abilities have been investigated with selectivity of different fluoride concentration for UV and emission titration spectra. Memristive switching properties were examined by using synthesized polymers as an active layer in memory devices. The highest ON/OFF ratio of the memory devices were recorded to be 106 with good stability, which makes them suitable for electronic applications.