Influencing the spectral stability and the electroluminescence behavior of new blue-emitting bifluorene-based materials by the 7,7 '-functionalization of the core

This study concerns the preparation of novel monodispersed organic materials based on a tetrahexyl-bifluorene core linked to end-groups by double or triple bonds. The molecules were designed aiming at tuning their HOMO-LUMO levels without remarkably changing their optical properties. The syntheses were carried out through a convergent-divergent approach by reaction of a 7,7'-diiodobifluorene building block with 2-fluoren-7-yl-ethynyl (BF1), 2-carbazol-3-yl-ethynyl (BF2), 2-fluoren-7-yl-ethenyl (BF3), and 2-carbazol-3-yl-ethenyl (BF4) moieties. The analysis of the optical properties evidenced for BF1-4 an efficient blue emission in solution. Their emission profile is preserved also in the solid state, except for that of BF4, the PL spectrum of which showed the appearance of a green band deriving from aerobic degradation. The HOMO value of -5.00 eV, measured for BF4 by cyclic voltammetry, suggested a correlation of the spectral instability with its low ionization potential. This hypothesis was confirmed by the photoluminescence behavior of BF1-3 upon UV photoirradiation showing a correlation between spectral stability and ionization potential. OLED devices of ITO/PEDOT-PSS/BF1-4/Ca/Al configuration exhibited a blue electroluminescence. Fair performances were obtained for the carbazole-containing BF2 (1000 cd/m(2) with a current efficiency of 0.039 cd/A at 10 V) and BF4 (1431 cd/m(2) and 0.027 cd/A at 14 V) presumably due to a more balanced transport of charge carriers in the active layer. The results obtained in devices embodying 7.5 nm of a hole-blocking layer (BCP) between the calcium cathode and the emitting material showed a remarkable enhancement of current efficiencies for all devices and confirmed a more balanced transport of charge carriers in the carbazole-containing molecules. The figures of merit obtained for the carbazole-containing BF2 (1088 cd/m(2) and 0.070 cd/A at 10 V) represent one of the best results for blue-emitting materials embodying ethynyl groups.