Fluorene copolymers containing bithiophene/2,5-or 2,6-pyridine units: A study of their optical, electrochemical, and electroluminescence properties

Abstract

Two alternating copolymers, poly[(2,5-di(2-thienyl)-pyridine-5,5'-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)], PFO-TPy25T, and poly[(2,6-di(2-thienyl)-pyridine-5,5'diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)], PFO-TPy26T, were synthesized by the Pd-catalyzed Suzuki polymerization method. The pyridine units are present as trimeric monomers in these copolymers and have different connectivities to their two neighboring thiophenes, para- and meta-linkages. We investigated the variations in the optical and electrochemical properties of the copolymers that arise from these different connectivities. The two polymers exhibit 5% weight loss above 410 degrees C and high glass transition temperatures (T-g: 113 degrees C for PFO-TPy25T, 142 degrees C for PFO-TPy26T). The UV-vis absorption maximum peaks of PFO-TPy25T and PFO-TPy26T in the solid state were found to be 449 and 398 nm respectively, with photoluminescence maximum peaks in the solid state of 573 and 490 nm respectively. Using cyclic voltammetry, we determined their energy band gaps: 3.08 eV for PFO-TPy25T and 3.49 eV for PFO-TPy25T. The cyclic voltammetry study of these polymers revealed that there are some differences. The electroluminescence (EL) properties of the copolymers were measured for the device configuration of ITO/PEDOT/polymers/CaW. The device fabricated with the polymer containing 2,5-pyridine exhibits pale orange emission, whereas the device fabricated with the polymer containing 2,6-pyridine exhibits pale blue emission. The EL device fabricated with PFO-TPy25T has a higher brightness (2010 cd/m(2)) and external quantum efficiency (0.1%) than the PFO-TPy26T device (260 cd/m(2), 0.008%), because it has a smaller energy barrier to the injection of charges from PEDOT and Ca into the HOMO and LUMO levels. (c) 2006 Wiley Periodicals, Inc.