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Keywords:

  • conjugated polymers;
  • light-emitting diodes (LED);
  • luminescence;
  • photophysics;
  • thin films

Abstract

New donor–acceptor conjugated copolymers called poly}2,7-(9,9′-dihexylfluorene)-co-5,10-[pyrazino(2,3-g)quinoxaline]{s or PFPQs [where F represents the 2,7-(9,9′-dihexylfluorene) moiety and PQ represents the 5,10-(pyrazino[2,3-g]quinoxaline) moiety], synthesized by the palladium-catalyzed Suzuki coupling reaction, are reported. The PQ contents in the PFPQ copolymers were 0.3, 1, 5, and 50 mol %, and the resulting copolymers were named PFPQ0.3, PFPQ01, PFPQ05, and PFPQ50, respectively. Absorption spectra showed a progressive redshift as the PQ acceptor content increased. The relatively small optical band gap of 2.08 eV for PFPQ50 suggested strong intramolecular charge transfer (ICT) between the F and PQ moieties. The photoluminescence emission peaks of the PFPQ copolymer films also exhibited a large redshift with enhanced PQ contents, ranging from 551 nm for PFPQ0.3 to 592 nm for PFPQ50. However, the PFPQ copolymer based electroluminescence (EL) devices showed poor device performances probably due to the strong confinement of the electrons in the PQ moiety or significant ICT. This problem was resolved with a binary blend of poly[2,7-(9,9-dihexylfluorene)] (PF) and PFPQ with a volume ratio of 95/5 (BPQ05). Multiple emission peaks were observed at 421, 444, 480, 516, and 567 nm in the BPQ05-based EL devices because the low PQ content led to incomplete energy transfer. The Commission Internationale de L'Eclairage 1931 coordinates of the BPQ05-based EL device were (0.31, 0.32), which were very close to the standard white emission of (0.33, 0.33). Furthermore, the maximum luminescence intensity and luminescence yield were 524 cd/m2 and 0.33 cd/A, respectively. This study suggested that a pure white light emission was achieved with the PFPQ copolymers or PF/PFPQ blends through the control of the energy transfer between F and PQ. Such PFPQ copolymers or PF/PFPQ blends would be interesting for electronic and optoelectronic devices. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009