Homoleptic Tris(Pyridyl Pyrazolate) Ir^III Complexes: En Route to Highly Efficient Phosphorescent OLEDs

Abstract

Treatment of the metal reagent IrCl(3)nH(2)O with two equivalents of 2-pyridyl pyrazole (N;N)H (3-tert-butyl-5-(2-pyridyl) pyrazole, (bppz)H and 3-trifluoromethyl-5-(2-pyridyl) pyrazole, (fppz)H), afforded the isomeric Ir(III) metal complexes with a general formula cis-[Ir(bppz)(2)Cl(2)]H (2 a), trans-[Ir(bppz)(2)Cl(2)]H (3 a), cis-[Ir(fppz)(2)Cl(2)]H (2 b), and trans-[Ir(fppz)(2)Cl(2)]H (3 b). Single-crystal X-ray diffraction studies on 2 b and 3 a revealed the coexistence of two pyrazolate chelates and two terminal chloride ligands on the coordination sphere. Subsequent reactivity studies confirmed their intermediacy to the preparation of homoleptic mer-[Ir(bppz)(3)] (1 a) and mer-[Ir(fppz)(3)] (1 b) that showed dual intraligand and ligand-to-ligand charge-transfer phosphorescence at room temperature. To attain bright, room-temperature phosphorescence further, we then synthesized two isoquinolinyl pyrazolate complexes, mer-[Ir(bipz)(3)] (4 a) and mer-[Ir(fipz)(3)] (4 b) ((bipz)H=3-tert-butyl-5-(1-isoquinolyl) pyrazole and (fipz)H=3-trifluoromethyl-5-(1-isoquinolyl) pyrazole). Their orange luminescence is mainly attributed to the mixed MLCT/pipi* transition, and the quantum yields were as high as 86 (4 a) and 50 % (4 b) in degassed CH(2)Cl(2) solution at RT. The organic light-emitting diodes (OLEDs) were then fabricated by using 4 a as a dopant, giving orange luminescence with CIE(x,y)=0.55, 0.45 (CIE(x,y)=the 1931 Commission Internationale de L'Eclairage (x,y) coordinates) and peak efficiencies of 14.6 % photon/electron, 34.8 cd A(-1), 26.1 lm W(-1). The device data were then compared with the previously reported heteroleptic complex [Ir(dfpz)(2)(bipz)] (5) ((dfpz)H=1-(2,4-difluorophenyl) pyrazole), revealing the possible effect of the bipz chelate and phosphor design on the overall electrophosphorescent performance, which can be understood by the differences in the carrier-transport properties.

Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary

“…This is in sharp contrast to the trisbidentate analogue mer-[Ir(fppz) 3 ] that showed the meridional geometry and poor emission efficiency observed at room temperature (RT). 30,31 This photophysical character of mer-[Ir(fppz) 3 ] is also in agreement with the structure−photo- physical behaviors of fac-and mer-[Ir(46dfppy) 3 ], 46dfppyH = 4,6-difluorophenylpyridine, for which the fac-derivative is known to exhibit better photoluminescence at RT versus the mer isomer. 32 We expected that the destabilized metal d π orbitals within the fac isomer might relax, in part, the radiationless deactivation and hence afford better emission efficiency than that of mer counterpart.…”

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

“…The accompanied long tailing band, signifying the dual emission, possesses the inherent MLCT character that is mixed slimly with respect to ligand-centered ππ* transition. This takes place once the energy gap between the lower-lying MLCT and ligand-centered ππ* states are small and thermally accessible as well as the two states are distinct in electronic configuration and hence the corresponding structures . In order to analyze the two-state relation between MLCT and ππ* states, a series of time-resolved emission spectra were acquired across 380–600 nm (Figure S5).…”

“…This takes place once the energy gap between the lower-lying MLCT and ligandcentered ππ* states are small and thermally accessible as well as the two states are distinct in electronic configuration and hence the corresponding structures. 45 In order to analyze the two-state relation between MLCT and ππ* states, a series of time-resolved emission spectra were acquired across 380−600 nm (Figure S5). The results led us to propose a mechanism depicted in Figure S6, where there exist two close lying triplet states, the LE (ππ*) and CT (MLCT) states, both of which possess slightly different geometry and are populated by the fast rate of adiabatic crossing (beyond our instrument response time of 100 ps).…”

Metallo‐organic Conjugated Systems for Organic Electronics

“…Chen et al. reported a series of homoleptic tris(pyridyl pyrazolate) Ir(III) complexes ( 108 – 113 ) 86. The OLED study of 112 (which had the highest quantum efficiency amongst those reported) as the dopant showed a maximum external quantum efficiency as high as 14.6 %.…”