Cationic Heteroleptic Cyclometalated Iridium Complexes with 1‐Pyridylimidazo[1,5‐α]pyridine Ligands: Exploitation of an Efficient Intersystem Crossing

• Published in: Chemistry : a European journal 2009-06, Vol.15 (26), p.6415-6427
• Main Authors: Volpi, Giorgio, Garino, Claudio, Salassa, Luca, Fiedler, Jan, Hardcastle, Kenneth I., Gobetto, Roberto, Nervi, Carlo
• Format: Article
• Language: English
• Subjects: Cations - chemistry; Crystallography, X-Ray - methods; density functional calculations; Electrochemistry; fluorescence; Imidazoles - chemical synthesis; Imidazoles - chemistry; intersystem crossing; iridium; Iridium - chemistry; Ligands; Luminescence; Luminescent Measurements; Magnetic Resonance Spectroscopy; Molecular Structure; Organometallic Compounds - chemistry; phosphorescence; Photochemistry; Pyridines - chemical synthesis; Pyridines - chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200801474

Luminescent ligands in IrIII cyclometalated complexes. The photophysical and photochemical properties of Ir‐cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir‐cyclometalated complexes containing N(amido) groups directly bound to Ir. A series of phosphorescent cyclometalated heteroleptic iridi um(III) phenylpyridinato (ppy) complexes containing luminescent 1‐pyridylimidazo[1,5‐α]pyridine (pip) ligands, namely [Ir(ppy)2(pip)]+, have been synthesised, characterised and their electrochemical, photophysical and electronic properties studied. Seven X‐ray structures have been resolved. Excitation of [Ir(ppy)2(pip)]+ in acetonitrile at room temperature results in a dual luminescence, strongly quenched by O2. Four complexes show, in absence of O2, a high‐energy emission (assigned to a 3MLLCT transition) with two maxima in the blue region of the visible spectra, and a second structured emission (assigned largely to a 3LC π–π* transition) centred around λ=555 nm. Lifetimes of high‐energy emissions are between 0.6 and 1.3 μs. Time‐dependent density functional calculations combined with the conductor‐like polarisable continuum model method, with acetonitrile as solvent, have been used to calculate a series of ground and excited states of the derivatives under investigation, and the transitions compared with the experimental UV/Vis absorption spectra. A quick and efficient photochemical reaction has been observed for these iridium derivatives that leads to the formation of a new class of cyclometalated iridium complexes containing a stable deprotonated amide unusually coordinated to the metal through a nitrogen bond. The synthesis of a 15N enriched selected ligand has been performed to investigate, by means of NMR, the particular facile route to these new set of derivatives. The electrochemical behaviour of all complexes is also reported. Luminescent ligands in IrIII cyclometalated complexes. The photophysical and photochemical properties of Ir‐cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir‐cyclometalated complexes containing N(amido) groups directly bound to Ir.