Blue-emitting Ir(III) complexes using fluorinated bipyridyl as main ligand and 1,2,4-triazol as ancillary ligand: syntheses, photophysical properties and performances in devices

Polysubstituted Ligand Framework for Color Tuning Phosphorescent Iridium(III) Complexes

A series of ligands have been synthesized based upon a polysubstituted 2-phenylquinoxaline core structure. These ligands introduce different combinations of fluorine and methyl substituents on both the phenyl and quinoxaline constituent rings. The resultant investigation of these species as cyclometalating agents for Ir(III) gave cationic complexes of the form [Ir(C^N)₂(bipy)]PF₆ (where C^N = cyclometalating ligand; bipy = 2,2'-bipyridine). X-ray crystallographic studies were conducted on four complexes and each revealed the expected distorted octahedral geometry based upon a cis-C,C and trans-N,N ligand arrangement at Ir(III). Supporting computational studies predict that each of the complexes share the same general descriptions for the frontier orbitals. TD-DFT calculations suggest MLCT contributions to the lowest energy absorption and a likely MLCT/ILCT/LLCT nature to the emitting state. Experimentally, the complexes display tunable luminescence across the yellow-orange-red part of the visible spectrum (λ_em = 579-655 nm).

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

The photocatalytic reduction of water to form hydrogen gas (H₂) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.