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]thiophene-3,5-diyl)dibenzonitrile (Br–DTT–PhCN). The Suzuki coupling of 9,9-dioctylfluorene-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-diyl-alt-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.