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Abstract: Fluorene decorated with multiple imidazole units in a non-conjugated fashion and featuring polyaromatic hydrocarbons such as anthracene or pyrene were synthesized and characterized as blue emitters suitable for application in organic light-emitting diodes. The optical absorption and emission peak profiles remained unaltered on increasing the imidazole loading on fluorene. But the molar extinction coefficient progressively increased on introduction of additional imidazole unit attributable to the increment in the chromophore density. Replacement of phenyl group at the C-2 of imidazole nucleus with polyaromatic hydrocarbons such as anthracene and pyrene led to a red-shift in the absorption and emission spectra due to their characteristic absorption features. All the compounds exhibited high thermal decomposition temperature in the range 395–509 °C showing significant thermal robustness. The anthracene and pyrene derivatives were demonstrated as efficient blue emitters in 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl host for multilayered organic light-emitting diodes. [Copyright &y& Elsevier]

New organic luminescent materials containing carbazole and benzimidazole chromophores were synthesized by tethering them together using thiophene or phenyl spacer. Both 3,6- and 2,7-disubstitutions were explored. The functional properties of the new materials were analyzed by photophysical, electrochemical, thermal and electroluminescence properties. The thiophene containing dyes showed red-shifted absorption and emission profiles than that of the phenyl analogs due to effective electronic delocalization in the former. Also, the thiophene-based dyes exhibited low oxidation potentials than that of the phenyl analogs attributable to electron richness. Electroluminescence devices were fabricated using these dyes as emitting layer or dopant in a multi-layered configuration. The devices containing thiophene-based dyes showed relatively low turn-on voltage in the neat devices due to the favorable alignment of energy levels, which facilitated balanced charge transport. A thiophene-based dye exhibited better performance in the series with external quantum efficiency as high as 1.5%. [ABSTRACT FROM AUTHOR]

Abstract: Low color temperature (CT) lighting provides a warm and comfortable atmosphere and shows mild effect on melatonin suppression. A high-efficiency low CT organic light emitting diode can be easily fabricated by spin coating a single white emission layer. The resultant white device shows an external quantum efficiency (EQE) of 22.8% (34.9lm/W) with CT 2860K at 100cd/m2, while is shown 18.8% (24.5lm/W) at 1000cd/m2. The high efficiency may be attributed to the use of electroluminescence efficient materials and the ambipolar-transport host. Besides, proper device architecture design enables excitons to form on the host and allows effective energy transfer from host to guest or from high triplet guest to low counterparts. By decreasing the doping concentration of blue dye in the white emission layer, the device exhibited an orange emission with a CT of 2280K. An EQE improvement was observed for the device, whose EQE was 27.4% (38.8lm/W) at 100cd/m2 and 20.4% (24.6lm/W) at 1000cd/m2. [Copyright &y& Elsevier]