Enhanced π-conjugation in hybridized local and charge transfer emitters for efficient red OLEDs and bioimaging.

Red-emitting materials have significantly advanced the development of materials for OLEDs and bioimaging. However, traditional red dyes often suffer from the 5 % external quantum efficiency (EQE) limit and aggregation-caused quenching (ACQ), which impede their practical applications. Herein, two red-emitting materials, TPA-QP and Cz-Ph-QP were designed and synthesized with donor-acceptor (D-A) structures. The ratio of localized π-conjugation to intramolecular charge transfer components was rationally adjusted due to the different spatial configurations of the donor units. Both emitters exhibited hybridized local and charge transfer (HLCT) characteristics and the higher planarity of the TPA-QP molecular structure enhanced π-conjugation and facilitated effective hybridization between the locally excited (LE) and charge transfer (CT), resulting in deep-red (DR) emission at 680 nm and high photoluminescence quantum yields of 15.6 %. Furthermore, TPA-QP-based OLED demonstrated emission closer to the standard saturated red (λ = 657 nm, CIE coordinates of (0.65, 0.33)) with an excellent maximum EQE of 6.47 % and low efficiency roll-off. Additionally, the fabricated nanoparticles (TQ NPs) showed deep-red emission (λ = 682 nm) and were successfully applied in cellular imaging. The results provide deeper insights into the effects of spatial configuration changes on promoting the luminescence performance of the long-wavelength HLCT molecules.