Synthesis and characterization of a new light-emitting fluorene-thieno[3,2-b]thiophene-based conjugated copolymer

A new alternating polyfluorene copolymer, poly(9,9'-dioctylfluorene-alt-thieno[3,2-blthiophene) (PFTT), containing a thiophene-condensed thieno[3,2-b]thiophene moiety has been synthesized via a palladium-catalyzed Suzuki coupling reaction. The synthesized polymer was successfully characterized by H-1 NMR, C-13 NMR, and elemental analysis. It shows good thermal stability and displays unique phase transition behavior between the crystalline and liquid-crystalline states. The ionization potential and electron affinity of PFTT are -5.38 eV and -2.40 eV, respectively, as determined by cyclic voltammetry. Thus, PFTT has an electrochemical band gap of approximately 2.98 eV, which is smaller than that of common polyfluorene (PF) homopolymers. As a film, PFTT exhibits UV-vis and photoluminescence maxima at 471 and 511 nm, respectively. A light-emitting diode device fabricated with an ITO/PEDOT/PFTT/LiF/Al configuration exhibits pure green light emission with the full width at half-maximum (fwhm) of only 57 nm and a low turn-on voltage of 3.3 V. Especially, this emission has the CIE coordinates (0.29, 0.63), which are very close to the standard for green used by the National Television System Committee. In addition, PFTT exhibits better electroluminescence performance than other similar PF homopolymers and than fluorene- and thiophene-based copolymers.