Constructing deep-blue bis-tridentate Ir(III) phosphors with fluorene-based dianionic chelates

High efficiency and stable blue phosphors have been a persistent pursuit in the field of organic light-emitting diodes (OLEDs). Bis-tridentate Ir(III) complexes are considered as promising alternatives because of their excellent emission efficiency and good photostability. However, the emission colors of photostable bis-tridentate Ir(III) emitters reported so far do not reach the deep blue region. Herein, fluorene is introduced in the design of blue bis-tridentate Ir(III) complexes. By modifying a fluorene-based dianionic chelate with tert-butyl, methoxy, pyrrolidinyl and triazolyl appendages, five blue emitting bis-tridentate Ir(III) phosphors were successfully designed and prepared. Emissions of the new phosphors exhibited a gradual blue shift from the sky blue to the deep blue region according to the electron donating abilities of the added substituents. A deep blue OLED was successfully fabricated based on Flu-4 and presented an excellent maximum external quantum efficiency of 22.3% with CIE coordinates of (0.17, 0.19), which are comparable to the state-of-the-art deep blue phosphors.