Cationic Bis(cyclometalated) Ir(III) Complexes with Pyridine–Carbene Ligands. Photophysical Properties and Photocatalytic Hydrogen Production from Water

Abstract

Precursors of chelate pyridine-N-heterocyclic carbene (N^C:) ligands with methyl- or benzyl-substituted imidazolylidene fragments were synthesized. They were used to obtain 12 iridium bis-cyclometalated complexes of the type [Ir(C^N)(N^C:)] (C^N = 2-(phenyl)pyridinato-C,N, ppy, 2-(4,6-difluorophenyl)pyridinato-C,N, dfppy). The ancillary N^C: ligands contain different structural modifications. The aim of the work was to analyze the effect that changes in the two types of ligands have on the photophysical and electrochemical properties and also on the behavior of these materials as photosensitizers. The X-ray crystal structures of five complexes were determined. The complexes emitted in the blue-green region. It was expected that the frontier orbitals and thus the photophysical and electrochemical properties would be controlled by the main C^N ligands, and it was demonstrated that the effect of the modifications in the N^C: ligand, especially the presence of a nitro group in the pyridine ring or the interruption of conjugation between the two rings, also affected these properties. The quenching with O and photostability studies were also performed. Density functional theory calculations were used to explain the behavior of some derivatives. The complexes and other previously reported compounds were employed as photosensitizers (PS) in preliminary studies on the production of H from water using [Co(bpy)]Cl (bpy = 2,2'-bipyridine) as catalyst and triethanolamine (TEOA) as the sacrificial reductant. The absence of quenching of the PS with TEOA allowed us to propose an oxidative quenching mechanism.

Ancillary Ligand Effects on Red‐Emitting Cyclometalated Iridium Complexes

“…Therefore, tuning the emission color and excited‐state dynamics by altering the ancillary ligand structure in heteroleptic iridium complexes with known cyclometalating ligands could be an easier, more efficient, and more expeditious approach. There are several well‐known strategies that use ancillary ligands to tune the emission and/or photochemical properties of cyclometalated iridium complexes over a wide range, including ancillary ligands that are chromophoric and completely change the nature of the excited state or those that have subtler effects on the frontier orbitals and excited states . Our group has recently reported the synthesis and characterization of red‐emitting bis‐cyclometalated Ir III complexes with high efficiency, good device performance, and excellent color purity by using β‐ketoiminate (acNac), β‐diketiminate (NacNac), and smaller bite‐angle N′N ‐diisopropylbenzamidinate (dipba) as the ancillary ligands .…”

“…There are several well-known strategies that use ancillary ligandst o tune the emission and/orp hotochemical properties of cyclometalated iridium complexes over aw ide range, including ancillary ligandst hat are chromophoric and completely change the nature of the excited state or those that have subtler effects on the frontier orbitalsa nd excited states. [20,21,[33][34][35][36] Our group has recently reported the synthesis and characterization of red-emittingb is-cyclometalated Ir III complexes with high efficiency,g ood device performance, and excellent color purity by using b-ketoiminate (acNac), b-diketiminate (NacNac), and smaller bite-angle N'N-diisopropylbenzamidinate (dipba) as the ancillary ligands. [37] We were able to show that the different L^X ligands influence the electronic structures of bis-cyclometalated iridium complexes, in some cases inducing significant redshiftso ft he emission maxima, and in other cases enhancing phosphorescence quantum yields relative to the corresponding acetylacetonate (acac) analogues or the homoleptic tris-cyclometalated complexes featuring the same cyclometalating ligands.…”

Non‐emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

“…In the cathodic region, all the new complexes feature two common reversible peaks: the first one between −1.83 and −1.92 V is imputed to the reduction of one bipyridine ligand ([Ru II (bpy) 2 (N^N − )] + →[Ru II (bpy − )(bpy)(N^N − )]) and the second one, between −2.07 and −2.24 V, is attributed to the reduction of the other bipyridine ligand ([Ru II (bpy − )(bpy)(N^N − )]→[Ru II (bpy − )(bpy − )(N^N − )] − ) in agreement with the topology of the LUMO level that is based on bpy(π*) orbitals (theoretical calculations below). In the voltammogram of [RuL4]Cl , there are extra peaks assigned to the typical reduction of the −NO 2 group . The electrochemical band gaps (Δ E 1/2 ) calculated from the first reversible oxidation potential and the first reversible reduction potential for the new PC s (2.07–2.22 V) are lower than for [1] 2+ (2.61 V) and anticipate a lower S 0 ←T 1 energy for the new Ru complexes relative to [1] 2+ (Table ).…”

Strong Influence of the Ancillary Ligand over the Photodynamic Anticancer Properties of Neutral Biscyclometalated IrIII Complexes Bearing 2‐Benzoazole‐Phenolates

“…The iridium centerd isplays as lightly distorted octahedral coordination geometry with the expected cis-C,C and trans-N,N mutual disposition for the dfppy ligands. [40] The IrÀC dfppy and IrÀN dfppy bond distances lie in the expected range typical of phenylpyridinate type ligands( very Chem. Eur.J.…”