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A series of a donor−acceptor−donor type of near-infrared (NIR) fluorescent chromophores based on 1,2,5thiadiazolo3,4-gquinoxaline (TQ) as an electron acceptor and triphenylamine as an electron donor are synthesized and characterized. By introducing pendent phenyl groups or changing the π-conjugation length in the TQ core, we tuned the energy levels of these chromophores, resulting in the NIR emission in a range from 784 to 868 nm. High thermal stability and glass transition temperatures allow these chromophores to be used as dopant emitters, which can be processed by vapor deposition for the fabrication of organic light-emitting diodes (OLEDs) having the multilayered structure of ITO/MoO3/NPB/Alq3:dopant emitter/BCP/Alq3/LiF/Al. The electroluminescence spectra of the devices based on these new chromophores cover a range from 748 to 870 nm. With 2 wt % of dopant 1, the LED device shows an exclusive NIR emission at 752 nm with the external quantum efficiency (EQE) as high as 1.12% over a wide range of current density (e.g., around 200 mA cm−2). The radiance reaches the largest value of 2880 mW Sr−1 m−2 at an operating voltage of 15 V.
Qian et al. (Wed,) studied this question.