The synthesis and photophysical studies of cyclometalated iridium(III) complexes
In 1985, fac-Ir(ppy)3 was characterised as the first triply ortho-metalated iridium(III)
species. This bright yellow solid exhibits green phosphorescent emission. Since then,
numerous related iridium(III) complexes of the formula Ir(L)3 and Ir(L)2acac have been
synthesised utilising ligands such as 2-phenylpyridine (ppy), 4-(2-pyridyl)benzaldehyde
(fppy), benzo[h]-quinoline (bzq) and 2-(2-thienyl)pyridine (thpy), to name, but a few.
These complexes give rise to tuneable emission wavelengths via ligand
modification and also exhibit electroluminescence allowing them to be used as
phosphorescent dopants in Organic Light Emitting Devices (OLEDs), the next
generation of flat panel displays.
In this work, ortho-metalating ligands, especially substituted 2-phenylpyridines,
have been produced by a variety of synthetic pathways. The subsequent cyclometalating
reactions of these ligands with iridium(III) and rhodium(III) have afforded a series of
complexes whose photophysical properties can be related in part to the substituents
upon the ligands. In general, these complexes exhibit phosphorescent emission that is
derived from an excited metal to ligand charge transfer state (MLCT), which possesses
mostly triplet character. Emission from this triplet excited state to the ground state is
formally forbidden. However, strong spin-orbit coupling provides mixing of the 3MLCT
state with higher energy singlet states thus' providing this transition with intensity. Long
lifetimes and oxygen-quenched emission are therefore typical of these complexes.
Tuning the emission wavelength of these complexes is possible by altering the
relative energy of the emissive 3MLCT state and is achieved by altering the substituents
that reside upon the cyclometalating ligands. The reversible oxidation of these
complexes under anaerobic conditions has also been demonstrated. For these
iridium(III) complexes the position of the oxidative wave follows a pattern in which
more positive values are found for complexes of ligands bearing electron-withdrawing
substituents and less positive values result from ligands with electron-donating
Studies of polymer films doped with these iridium(Ill) complexes have shown
efficient energy transfer between the host and guest species. This is inferred by the lack
of host emission even at doping concentrations as low as 0.5 % wt. /wt. This is vital if
these compounds are to be used as electroluminescent dopants in OLEDs.