Rational Tuning of Bis-Tridentate Ir(III) Phosphors to Deep-Blue with High Efficiency and Sub-microsecond Lifetime

A new
class of bis-tridentate Ir­(III) complexes (Dap-1–4) was synthesized using carbene pincer pro-chelates PC1·H3(PF6)2 or PC2·H3(PF6)2 with either imidazolylidene or imidazo­[4,5-b]­pyridin-2-ylidene appendages, together with a second cyclometalating
2,6-diaryoxypyridine chelate, L1H2 and L2H2,
differed by a NMe2 donor at the central pyridinyl fragment.
The respective emission tuning between the ultraviolet and blue region
was rationalized using time-dependent density functional theory (TD-DFT)
approaches. Next, a highly efficient blue emitter (Dap-5) was synthesized by concomitant addition of two methyl groups and
a single CF3 substituent at the central phenyl and peripheral
imidazo­[4,5-b]­pyridin-2-ylidene entities of the carbene
pincer chelate, respectively. The organic light-emitting diode (OLED)
device with 15 wt % Dap-5 in DPEPO shows electroluminescence
at 468 nm and with CIE (0.14, 0.15) and a max external quantum efficiency
(max EQE) of 16.8% with low efficiency roll-off (EQE of 14.4% at 1000
cd m–2); the latter is attributed to the relatively
shortened triplet excited-state radiative lifetime. These results
highlight the adequateness of bis-tridentate Ir­(III) phosphors in
fabrication of practical blue-emitting OLEDs.