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

A series of phosphorescent cyclometalated heteroleptic iridium(III) phenylpyridinato (ppy) complexes containing luminescent 1-pyridyl-imidazo[1,5-alpha]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 pi-pi* transition) centred around lambda = 555 nm. Lifetimes of high-energy emissions are between 0.6 and 1.3 us. 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 N-15 enriched selected ligand hits 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.