The synthesis and photophysical studies of cyclometalated iridium(III) complexes

• Main Author: Bettington, Sylvia
• Published: Durham University 2003
• Subjects: 535.357
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435656

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 substituents. 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.