Luminescent Iridium-Terpyridine Complexes with Various Bis-Cyclometalated Ligands

A series of luminescent bis-cyclometalated iridium complexes with 2,2':6',2″-terpyridine (tpy), [Ir(C^N)₂(tpy)]PF₆ (C^N = 2-phenylpyridinate (ppy) for 1; benzo[h]quinolinate (bzq) for 2; 1-phenylisoquinolinate (piq) for 3; and 2-phenylbenzothiazolate (pbt) for 4), have been synthesized and structurally characterized. Single-crystal X-ray diffraction analyses reveal that the tpy ligands of 1-4 are coordinated to the iridium center in a bidentate fashion, and the uncoordinated pendant pyridine rings in the tpy ligands of 1-4 form intramolecular π-π stacking interactions with a phenyl moiety of C^N ligands. In addition, the pendant pyridine ring in the tpy ligand of 1 forms an intramolecular hydrogen bonding interaction, unlike in 2-4. Of interest, the photophysical properties of 1-4 are strongly influenced by the C^N ligands; 1 shows a luminescence band at 572 nm, with a short lifetime (τ) value of 80 nsec and a lower absolute luminescence quantum yield (Φ) of 3.72%, whereas 3 exhibits an intense luminescence band at 588 nm with a long τ value of 1965 nsec and a moderate Φ value of 9.57%. The density functional theory calculations revealed that the luminescence originates from the triplet metal-ligand to ligand charge transfer (³MLL'CT) excited state.