Sulfonyl-Substituted Heteroleptic Cyclometalated Iridium(III) Complexes as Blue Emitters for Solution-Processable Phosphorescent Organic Light-Emitting Diodes

The synthesis is reported of a series of blue-emitting heteroleptic
iridium complexes with phenylpyridine (ppy) ligands substituted with
sulfonyl, fluorine, and/or methoxy substituents on the phenyl ring
and a picolinate (pic) ancillary ligand. Some derivatives are additionally
substituted with a mesityl substituent on the pyridyl ring of ppy
to increase solubility. Analogues with two ppy and one 2-(2′-oxyphenyl)­pyridyl
(oppy) ancillary ligand were obtained by an unusual in situ nucleophilic
displacement of a fluorine substituent on one of the ppy ligands by
water followed by N^O chelation to iridium. The X-ray crystal structures
of seven of the complexes are reported. The photophysical and electrochemical
properties of the complexes are supported by density functional theory
(DFT) and time-dependent DFT calculations. Efficient blue phosphorescent
organic light-emitting devices (PhOLEDs) were fabricated using a selection
of the complexes in a simple device architecture using a solution-processed
single-emitting layer in the configuration ITO/PEDOT:PSS/PVK:OXD-7(35%):Ir
complex­(15%)/TPBi/LiF/Al. The addition of a sulfonyl substituent blue-shifts
the electroluminescence by ca. 12 nm to λmaxEL 463 nm with CIEx,y coordinates (0.19, 0.29), compared to the benchmark complex FIrpic
(λmaxEL 475 nm, 0.19, 0.38) in directly comparable devices, confirming the
potential of the new complexes to serve as effective blue dopants
in PhOLEDs. Replacing a fluorine by a methoxy group in these complexes
red shifts the PL and EL λmax by ca. 4–6 nm.
The efficiency of the blue PhOLEDs of the sulfonyl-substituted complexes
is, in most cases, significantly enhanced by the presence of a mesityl
substituent on the pyridyl ring of the ppy ligands.