Copolyfluorenes containing phenothiazine or thiophene derivatives: Synthesis, characterization, and application in white-light-emitting diodes

Four copolyfluorenes chemically doped with 0.1 and 1 mol % 3,7-bis[2-thiophene-2-yl)-2-cyanovinyl]phenothiazine (PFPhT) or 2,5-bis[2-(thiophene-2-yl)-2-cyanovinyl]thiophene chromophores (PFThT) were synthesized using the Suzuki coupling reaction and applied in white-light-emitting devices. They were characterized by GPC, elemental analysis, DSC, TGA, optical spectra, and cyclic voltammetry. They exhibited good thermal stability (T_d > 420°C) and moderate glass transition temperatures (>95°C). The PhT-Br and ThT-Br showed PL peaks at 586 and 522 nm (with a shoulder at 550 nm). In film state, PL spectra of the copolymers comprised emissions from the fluorene segments and the chromophores due to incomplete energy transfer. Both monomers exhibited low LUMO levels around -3.50 to -3.59 eV, whereas the PhT-Br owned the higher HOMO level (-5.16 eV) due to its electron-donating phenothiazine core. Light-emitting diodes with a structure of ITO/ PEDOT:PSS/copolymer/Ca(50 nm)/Al(100 nm) showed broad emission depending on the chromophore contents. The maximum brightness and maximum current efficiency of PFPhT2 (PFThT1) device were 8690 cd/m² and 1.43 cd/A (7060 cd/m² and 0.98 cd/A), respectively. White-light emission was realized by further blending PFPhT2 with poly(9,9-dihexylfluorene) (w/w = 10/1), with the maximum brightness and maximum current efficiency being 10,600 cd/m² and 1.85 cd/A.