New Copolymers with Thieno[3,2-b]thiophene or Dithieno[3,2 b:2 ',3 '-d]thiophene Units Possessing Electron-Withdrawing 4-Cyanophenyl Groups: Synthesis and Photophysical, Electrochemical, and Electroluminescent Properties

New monomers containing 4-cyanophenyl (-PhCN) groups attached to a thieno[3,2-b] thiophene (TT) or dithieno [3,2-b: 2',3'-d] thiophene (DTT) structure were synthesized and characterized as 4-(2,5-dibromothieno[3,2-b] thiophen-3-yl) benzonitrile (Br-TT-PhCN) or 4,4'-(2,6-dibromodithieno[3,2-b: 2',3'-d] thiophene3,5-diyl) dibenzonitrile (Br-DTT-PhCN). The Suzuki coupling of 9,9dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester and the Br-TT-PhCN or Br-DTT-PhCN monomer was utilized for the syntheses of novel copolymers poly{9,9-dioctylfluorene-2,7-diylalt-3-(4'-cyanophenyl) thieno[3,2-b] thiophene-2,5-diyl} (PFTT-PhCN) and poly{9,9-dioctylfluorene-2,7-diyl-alt-3,5-bis(4'-cyanophenyl) dithieno[3,2-b: 2',3'-d] thiophene-2,6-diyl} (PFDTT-PhCN), respectively. The photophysical, electrochemical, and electroluminescent (EL) properties of these novel copolymers were studied. Their photoluminescence (PL) exhibited the same emission maximum for both copolymers in solution. Red-shifted PL emissions were observed in the thin films. The PL emission maximum of PFTT-PhCN was more significantly redshifted than that of PFDTT-PhCN, indicating more pronounced excimer or aggregate formation in PFTT-PhCN. The ionization potential (HOMO level) and electron affinity (LUMO level) values were 5.54 and 2.81 eV, respectively, for PFTT-PhCN and were 5.57 and 2.92 eV, respectively, for PFDTT-PhCN. Polymer light-emitting diodes (LEDs) with copolymer active layers were fabricated and studied. Anomalous behavior and memory effects were observed from the current-voltage characteristics of the LEDs for both copolymers. (C) 2017 Wiley Periodicals, Inc.