9,9-Bis(4-diphenylaminophenyl)fluorene Modified Naphthothiadiazoles as Ambipolar Non-doped Near-Infrared Emitters for High-Efficiency Electroluminescent Devices

Herein, we report the synthesis and characterization of efficient ambipolar charge-carrier transporting deep-red to near-infrared (NIR) fluorophores, namely MNTFT, PNTFT, and BMNTFT, based on an asymmetric donor-acceptor-donor' (D-A-D') structure, as well as their application as non-doped emitters in NIR organic light-emitting diodes (OLEDs). The molecules comprise naphtho[2,3-c][1,2,5]thiadiazole (Nz) as a strong A, bis(4-diphenylaminophenyl)fluorene as D, and either diphenylamine or dimethylaminophenyl as D'. Both experimental and theoretical investigations confirm that they exhibit hybridized local and charge-transfer (HLCT) excited states, which enable the pathways for harvesting triplet hot exciton via high-lying reverse intersystem crossing (hRISC) with intense NIR emissions (∼708 nm). Their asymmetrical D-A-D' features benefit well-balanced hole and electron mobilities, resulting in a successful application as a non-doped emissive layer in the OLEDs with decent performance. Particularly, the MNTFT-based non-doped device shows an NIR emission with a peak at 710 nm, a high maximum external quantum efficiency (EQEmax) of 2.51%, and a low turn-on voltage of 3.2 V. These results represent a significant advancement in the development of NIR fluorescent materials and OLED devices, meeting key requirements for various applications.