Charge-Neutral Ir(III) Metal Phosphors Bearing Functional Pyrazole–Pyridine–Pyrazole Chelates

Homoleptic charge-neutral Ir(III) metal complexes bearing
tridentate
chelates composed of two orthogonally aligned pyrazole–pyridine–pyrazole
fragments were designed and synthesized. It is notable that the substituents
play an essential role in tuning both the photophysical and electrochemical
properties. Upon the introduction of CF3 substituents at
the central pyridyl fragment of one tridentate chelate and peripheral
pyrazolate fragments of alternative tridentate chelate, phosphorescence
can be fine-tuned from green to orange, together with an improved
emission quantum yield, as recorded in degassed tetrahydrofuran (THF)
solution at room temperature (RT). Further replacement of the N–H
fragment of monoanionic chelate with a methyl substituent gives Ir(III)
complexes [Ir(Bpz2pyMe)(Fpz2py)] (2Me) and [Ir(Bpz2FpyMe)(Fpz2py)] (3Me), showing decomposition temperatures (Td) greater than 300 °C. Therefore, this work paves
a new avenue for designing and synthesizing emissive Ir(III) complexes
without the traditional cyclometalating aromatics, albeit with a lowered
emission efficiency. As a proof of concept, Ir(III) complexes 2Me and 3Me were employed as the dopant emitters
for organic light-emitting diode (OLED) fabrication, giving a maximum
external quantum efficiency (EQE) of 7.1 and 15.9% and a current efficiency
(CE) of 22.5 and 50 cd·A–1, respectively.