Enhanced π-conjugation in hybridized local and charge transfer state by intramolecular hydrogen bonding to construct efficient red emitters for OLEDs and cellular imaging

Red emitting materials are the key to development of organic light-emitting diodes (OLEDs) and bioimaging, but limited by the energy gap law and aggregation-caused quenching (ACQ), impeding their applications. Herein, two red emitters TPA-QxPy and Cz-Ph-QxPy were designed and synthesized with aggregation-induced emission (AIE) characteristics. TPA-QxPy had a strong charge transfer (CT) state, while due to the different spatial configurations of the donor units, intramolecular hydrogen bonds were formed in the Cz-Ph-QxPy, enabling the locally excited (LE) state to be incorporated into CT emissive state to form hybridized local and charge-transfer (HLCT) state. The results demonstrated that the higher planarity of the Cz-Ph-QxPy enhanced the pi-conjugation and hybridization between the CT and LE, showing a red emission at 600 nm and a high fluorescence quantum yield of (phi PL) 30.8% in film. Cz-Ph-QxPy-based OLED achieved highly efficient red emission with external quantum efficiency (EQE) of 7.56% at 580 nm. Moreover, benefiting from AIE characteristics, the fabricated TQx NPs and CQx NPs with near-infrared (NIR)/red emission showed high photostability and biocompatibility and were successfully used for cellular imaging. This work provides new insights for promoting the luminescence performance of red emitting materials with HLCT state by spatial configuration changes.

Automated summary

In this study, two red emitters TPA-QxPy and Cz-Ph-QxPy with aggregation-induced emission characteristics were designed and synthesized. TPA-QxPy showed a strong charge transfer state, while Cz-Ph-QxPy had intramolecular hydrogen bonds to form hybridized local and charge-transfer state. Cz-Ph-QxPy-based organic light-emitting diode achieved highly efficient red emission with external quantum efficiency of 7.56% at 580 nm.