Improving the optoelectronic properties of blue hybridized local and charge-transfer emitters via rational utilization of intramolecular hydrogen bonds

Abstract

Efficient blue emitters are indispensable for manufacturing full-color organic light-emitting diodes. However, the performance of blue emitters still lags far behind that of green and red ones, due to their intrinsically wide energy gap which could result in a large barrier for carrier injection and low efficiency. Herein, we designed and synthesized two blue emitters – mP9PCZ and mPmPCZ – having a donor–π-bridge–acceptor (D–π–A) architecture and hybridized local and charge-transfer characteristics. In these two emitters, pyridinyl was introduced as a bridging unit between phenanthro[9,10-d]imidazole (PI) and 9-phenyl-9H-carbazole (PCZ). Intramolecular hydrogen bond (IHB) interactions between the pyridine N atom and the H atoms at the adjacent phenyl of PCZ were observed using single-crystal analysis. Such interactions help suppress molecular vibrations and dramatically increase molecular rigidity. Theoretical calculations verified that the IHB can induce small torsion angles in the central part of the molecules, resulting in increased overlap of frontier molecular orbitals and enhanced oscillator strength. Owing to the IHB interactions, both emitters exhibited enhanced fluorescence quantum efficiencies (Φ_PLs) and balanced bipolar carrier mobilities. Moreover, the effect of various substitution positions of PCZ on the structure–property relationship was also analyzed. Non-doped devices based on mP9PCZ and mPmPCZ achieved good performance, with high maximum luminance (>21 000 cd m⁻²) and maximum external quantum efficiencies ≥4.88% with negligible efficiency roll-off. Furthermore, the doped devices displayed deep-blue emissions at 418 and 413 nm with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of (0.160, 0.067) and (0.160, 0.056) for mP9PCZ and mPmPCZ, respectively. These results demonstrate that the introduction of appropriate IHB interactions is a promising strategy for the design of efficient blue emitters.