Rational design of phenanthroimidazole derivatives with hybridized local and charge-transfer characteristics to achieve efficient blue emission in non-doped OLEDs

Abstract

In this work, two new blue emitters 4-(2-(3,5-di(9H-carbazol-9-yl) phenyl)-1H-phenanthro[9,10-d] imidazol-1-yl) benzonitrile (DCCPPI) and 4-(2-(3′,5′-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d] imidazol-1-yl) benzonitrile (DCBCPPI) were designed and synthesized by regulating the distance between phenanthroimidazole and 1,3-di(9H-carbazol-9-yl)benzene. Both DCCPPI and DCBCPPI exhibit relatively narrow full widths at half-maximum (FWHM) in favor of blue purity and high thermal decomposition temperatures (T_d) in excess of 440 °C. Compared with DCCPPI, a molecule where 1,3-bis(9H-carbazole-9-yl) benzene is directly linked to phenanthroimidazole with charge-transfer (CT), DCBCPPI with the introduction of a benzene bridge not only shows interesting hybridized local and charge-transfer (HLCT) characteristics, but also significantly improves the performance of an organic light-emitting diode (OLED) prepared based on it. The non-doped OLED prepared based on DCBCPPI displays an emission peak at 451 nm with CIE coordinates of (0.18, 0.14), a full width at half-maximum of 67 nm, the maximum brightness of 3767 cd m⁻², the maximum external quantum efficiency of 5.92% and an exciton utilizing efficiency (EUE) as high as 73%. The present work may provide a reliable strategy for the development of new HLCT-based optoelectronic materials by fine-tuning the molecular structure.