Investigation of the N∧C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N∧C)2(N∧N)]+ Complexes

A series of bis-metalated phosphorescent [((NC)-C-boolean AND)(2)Ir(bipyridine)](+) complexes with systematic variations in the structure and electronic characteristics of the (NC)-C-boolean AND ligands were synthesized and characterized by using elemental analysis, mass spectrometry, NMR spectroscopy and X-ray crystallography. Investigation of the complexes' spectroscopic properties together with DFT and TD DFT calculations revealed that metal-to-ligand charge transfer (MLCT) and intraligand (LC) transition play key roles in the generation of emissive triplet states. According to the results of theoretical studies, the (LC)-L-3 excited state is more accurate to consider as an intraligand charge transfer process (ILCT) between N- and C-coordinated moieties of the (NC)-C-boolean AND chelate. This hypothesis is completely in line with the trends observed in the experimental absorption and emission spectra, which display systematic bathochromic shifts upon insertion of electron-withdrawing substituents into the N-coordinated fragment. An analogous shift is induced by expansion of the aromatic system of the C-coordinated fragment and insertion of polarizable sulfur atoms into the aromatic rings. These experimental and theoretical findings extend the knowledge of the nature of photophysical processes in complexes of this type and provide useful instruments for fine-tuning of their emissive characteristics.

Automated summary

A series of bis-metalated phosphorescent complexes were synthesized and characterized. The investigation of the complexes' spectroscopic properties and theoretical calculations revealed the importance of metal-to-ligand charge transfer and intraligand transition in the generation of emissive triplet states. The results showed systematic bathochromic shifts in the absorption and emission spectra upon insertion of electron-withdrawing substituents and expansion of the aromatic system. These findings provide valuable insights into the photophysical processes in complexes of this type and offer useful tools for fine-tuning their emissive characteristics.