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Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence
Author(s) -
Mondal Ejabul,
Hung WenYi,
Chen YangHuei,
Cheng MingHung,
Wong KenTsung
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201300738
Subject(s) - phosphorescence , carbazole , fluorene , phosphor , common emitter , materials science , optoelectronics , fluorenone , photochemistry , chemistry , fluorescence , physics , polymer , optics , composite material
Two new molecules, CzFCBI and CzFNBI , have been tailor‐made to serve as bipolar host materials to realize high‐efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole‐transporting block and N ‐phenylbenzimidazole as the electron‐transporting block hybridized through the saturated bridge center (C9) and meta ‐conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N ‐phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy‐level alignment allows CzFCBI to outperform its N‐connected counterpart CzFNBI while employing other long‐wavelength‐emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue‐to‐red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low‐roll‐off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish‐green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish‐green with a sky‐blue emitter and a red emitter, a CzFNBI ‐hosted single‐emitting‐layer all‐phosphor three‐color‐based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A −1 , 28.3 lm W −1 ) and highly stable chromaticity (CIE x =0.43–0.46 and CIE y =0.43) at an applied voltage of 8 to 12 V, and a high color‐rendering index of 91.6.