Efficient Non-doped Near Infrared Organic Light-Emitting Devices Based on Fluorophores with Aggregation-Induced Emission Enhancement

A family of donor–acceptor–donor (D–A–D) type near-infrared (NIR) fluorophores containing rigid nonplanar conjugated tetraphenylethene (TPE) moieties was designed and synthesized through Stille coupling reactions with electron-deficient [1,2,5]­thiadiazolo­[3,4-g]­quinoxaline (QTD) or benzo­[1,2-c;4,5-c′]­bis­[1,2,5]­thiadiazole (BBTD) as acceptors. The absorption, fluorescence, and electrochemical properties were studied. These compounds exhibited good aggregation-induced emission enhancement (AIEE) property, as a result of the twisted TPE units, which restrict the intramolecular rotation and reduce the π–π stacking. Photoluminescence of these chromophores ranges from 600 to 1100 nm, and their HOMO–LUMO gaps are between 1.85 and 1.50 eV. Non-doped organic light-emitting diodes (OLEDs) based on these fluorophores were made and exhibited EL emission spectra peaking from 706 to 864 nm. The external quantum efficiency (EQE) of these devices ranged from 0.89% to 0.20% and remained fairly constant over a range of current density of 100–300 mA cm^–2. The device with the highest solid fluorescence efficiency emitter 1a shows the best performance with a maximum radiance of 2917 mW Sr^–1 m^–2 and EQE of 0.89%. A contrast between nondoped and doped OLEDs with these materials confirms that AIEE compounds are suitable for fabricate efficient nondoped NIR OLEDs.