Syntheses and examining the electrochemical and spectroscopic properties of fused-thiophene based materials with benzonitrile functional group

Conjugated polymers with low band gaps have been taking growing attention due to their intrinsic electrical conductivities. These polymers are lightweight, flexible and have interesting electronic and optoelectronic properties which make them use in 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 π-conjugation compared to thiophene. While the simplest fused thiophene called thienothiophene (TT) is formed by annulation of two thiophene rings, fusing another thiophene ring into TT gives dithienothiophene (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 band gap semiconductors. Since the electronic characteristics of conjugated polymers are 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 of the band gap in the design and the synthesis of conjugated polymers. Use of strong donor (D) and acceptor (A) units is one of the most effective ways in low band gap materials architecture. In DA systems, both electron-withdrawing and donating groups diminish Eg. While electron withdrawing unit lowers LUMO (Lowest Unoccupied Molecular Orbital) energy, electron donating group increases HOMO (Highest Unoccupied Molecular Orbital) level. Fused thiophenes are commonly used as electron-donating groups, and cyano substituted units are the most widely employed as electron withdrawing units in design of low band gap conjugated polymers. In this study, novel π-conjugated monomers 4-(2,5-di(thiophen-2-yl)thieno[3,2-b]thiophen-6-yl)benzonitrile (69), 4-(2,5-bis(4-hexylthiophen-2-yl)thieno[3,2-b]-thiophen-3-yl)benzonitrile (70), 4,4'-(2,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)dithieno[3,2-b:2',3'-d]thiophene-3,5-diyl)dibenzonitrile (71), 4,4'-(dithieno[3,2-b:2',3'-d]thiophene-3,5-diyl)dibenzonitrile (74), 4,4'-(2,6-di(thiophen-2-yl)dithieno-[3,2-b:2',3'-d]thiophene-3,5-diyl)dibenzonitrile (76), 4,4'-(2,6-bis(4-hexylthiophen-2-yl)dithieno[3,2-b:2',3'-d]thiophene-3,5-diyl)dibenzonitrile (77), 4'-(2,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)dithieno[3,2-b:2',3'-d]thiophene-3,5-diyl)-dibenzonitrile (78) with electron-donating TT and DTT units and electron-withdrawing cyano group were designed to investigate the electronic and steric effects of substiuents on electrochemical and optoelectronic features of the final polymers. Syntheses of the TTs and DTTs were achieved through the ring closure reactions of mono and diketones using polyphosphoric acid (PPA) in refluxing chlorobenzene. Thiophene, 3-hexylthiophene and EDOT were linked to the TT and DTT units to rationalize the influence of electron density enhancement along monomer backbone. Monomers were synthesized via Suzuki or Stille cross-coupling methods. Polymers were prepared by electrochemical potentiodynamic method in acetonitrile/dichloromethane NaClO4/LiClO4 (0.1:0.1 M) solvent-electrolyte mixture. Chemical polymers were synthesized via Suzuki and Stille cross-oupling reactions or oxidative chemical polymerization through FeCl3. The electrochemical and spectroscopic properties of polymers were investigated via cyclic voltammetry (CV), and UV-Vis spectroscopy, respectively. The color changes emerging from the redox switching of polymers synthesized through electrochemical method were examined by CIE 1931 Yxy color space. Fine tuning of the band gaps was achieved by making structural variations to adjust optical band gap values between 1.79 and 2.18 eV for TT polymers, 1.71 and 2.34 eV for DTT polymers synthesized through electrochemical method, 1.69 and 2.51 eV for TT polymers, and 1.60 and 2.46 eV for DTT polymers synthesized through chemical method. In addition, the copolymer of 4-(thieno[3,2-b]thiophen-3-yl)benzonitrile (67) and 4,4'-(dithieno[3,2-b:2',3'-d]thiophene-3,5-diyl)dibenzonitrile (74) with 9,9-dioctyl-9H-fluorene were synthesized¬ to produce poly(9,9-dioctylfluorene¬-2,7-diyl-alt-¬¬3-(4'-cyanophenyl)-thieno[3,2-b]thiophene-2,5-diyl) (86) and poly(9,9-dioctylfluorene-2,7-diyl-alt-3,5-bis(4'-cyanophenyl)dithieno[3,2-b:2':3'-d]thiophene-2,5-diyl) (87), respectively. Their photophysical, electrochemical and electroluminescent (EL) were examined. Synthesis and investigation of properties of the polymers were performed by Polymer Photonics group in Institute of Macromolecular Chemistry, The Czech Academy of Sciences.