Bis-Cyclometalated Iridium(III) Complexes Bearing Ancillary Guanidinate Ligands. Synthesis, Structure, and Highly Efficient Electroluminescence

We report the synthesis, structure, and photophysical
and electroluminescent
(EL) properties of a series of heteroleptic bis­(pyridylphenyl)­iridium­(III)
complexes with various ancillary guanidinate ligands. The reaction
of the bis­(pyridylphenyl)­iridium­(III) chloride [(ppy)2Ir­(μ-Cl)]2 with the lithium salt of various guanidine ligands Li­{(NiPr)2C­(NR1R2)} at 80 °C gave in 60–80% yield the corresponding heteroleptic
bis­(pyridylphenyl)/guanidinate iridium­(III) complexes having a general
formula of [(ppy)2Ir­{(NiPr)2C­(NR1R2)}], where NR1R2 = NPh2 (1), N­(C6H4tBu-4)2 (2), carbazolyl (3), 3,6-bis­(tert-butyl)­carbazolyl
(4), N­(C6H4)2S (5), N­(C6H4)2O (6), indolyl (7), NEt2 (8), NiPr2 (9), NiBu2 (10), and N­(SiMe3)2 (11). These heteroleptic cyclometalated
(C^N) iridium­(III) complexes showed intense absorption bands in the
UV region assignable to π–π* transitions and weaker
metal-to-ligand charge-transfer transitions extending to the visible
region. These complexes also showed intense emissions at room temperature.
Their photoluminescence spectra were influenced to some extent by
the ancillary guanidinate ligands, giving λmax values
in the range of 528–560 nm with quantum yields (Φ) of
0.16–0.37 and lifetimes of 0.61–1.43 μs. Organic
light-emitting diodes were fabricated by the use of these complexes
as dopants in various concentrations (5–100%) in a N,N′-dicarbazolylbiphenyl host.
High current efficiency (ηc; up to 137.4 cd/A) and
power efficiency (ηp; up to 45.7 lm/W) were observed
under appropriate conditions. Their high EL efficiency may result
from efficient trapping and radiative relaxation of the excitons formed
in the EL process. Because of the steric hindrance of the guanidinate
ligands, no significant intermolecular interaction was observed in
these complexes, thus leading to the reduction of self-quenching and
triplet–triplet annihilation at high currents. The EL emission
color could be changed in the range of green to yellow by choosing
appropriate guanidinate ligands.