Cyano-Isocyanide Iridium(III) Complexes with Pure Blue Phosphorescence

Abstract

In this paper, we report a series of six neutral, bluephosphorescent cyclometalated iridium complexes of the type Ir(C^Y) 2 (CNAr)(CN). The cyclometalating ligands in these compounds (C^Y) are either aryl-substituted 1,2,4-triazole or NHC ligands, known to produce complexes with blue phosphorescence. These cyclometalating ligands are paired with π-acidic, strongly σdonating cyano and aryl isocyanide (CNAr) ancillary ligands, the hypothesis being that these ancillary ligands would destabilize the higher-lying ligand-field (d−d) excited states, allowing efficient blue photoluminescence. The compounds are prepared by substituting the cyanide ancillary ligand onto a chloride precursor and are characterized by NMR, mass spectrometry, infrared spectroscopy, and, for five of the compounds, by X-ray crystallography. Cyclic voltammetry establishes that these compounds have large HOMO− LUMO gaps. The mixed cyano-isocyanide compounds are weakly luminescent in solution, but they phosphoresce with moderate to good efficiency when doped into poly(methyl methacrylate) films, with Commission Internationale de L'Eclairage coordinates that indicate deep blue emission for five of the six compounds. The photophysical studies show that the photoluminescence quantum yields are greatly enhanced in the cyano complexes relative to the chloride precursors, affirming the benefit of strong-field ancillary ligands in the design of blue-phosphorescent complexes. Density functional theory calculations confirm that this enhancement arises from a significant destabilization of the higher-energy ligand-field states in the cyanide complexes relative to the chloride precursors.

Trimetallic Iridium–Nickel–Iridium Bis(formazanate) Assemblies

“…These reports include a structural investigation and applications in oxygen sensitization, metal-ion sensing, and electrophosphorescence, but a detailed and systematic account of their photophysics is not available from these previous works. In these complexes, photoluminescence in the blue-to-green regions ( 6 and 7 ; C^N = F 2 ppy and ppy) is weak, as we have observed for a number of other blue-phosphorescent chloride-terminated cyclometalated iridium complexes . The photoluminescence in the yellow-orange-to-red region, in complexes 8 and 9 (C^N = bt and piq, respectively), is much stronger (Table ).…”

High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

“…Prominent examples of TTET catalyzed organic reactions include E / Z isomerizations, − cycloadditions, − the disulfide-ene reaction, C-(sp 3 )–H methylations, and the photocatalyzed Paternò–Büchi reaction. , The scope of substrates is typically dictated by the triplet energy of the available photosensitizers, and the development of photocatalysts with higher triplet energies is desirable to enable more challenging substrates or even new reactions. Transition metal complexes are widely used as TTET and photoredox catalysts, due to their high intersystem crossing efficiencies that usually lead to quantitative population of triplet excited states, as well as the tunability of their photophysical properties and their photostability. − Though earth-abundant first- and second-row transition metal complexes become increasingly popular, − so far mostly precious Ru­(II)- and Ir­(III)-based photocatalysts have been employed, with cyclometalated Ir­(III) complexes representing a particularly popular choice for reactions requiring comparatively high triplet energies. , However, most commonly used Ir­(III) photosensitizers do not exceed triplet energies of 2.7 eV, and the metal-based TTET catalyst with the highest triplet energy that is typically employed is [Ir­(dFppy) 3 ] (dFppy = 2-(2,4-difluoro­phenyl)­pyridine), with a triplet energy of 2.75 eV (Figure a). ,,− Other metal complexes also exhibit high triplet energies (up to 3.26 eV), − but it seems that their performance in TTET catalysis has not yet been explored. Very recently, a tris-cyclometalated Ir­(III) complex with a triplet energy of 3.18 eV and its application in a De Mayo type reaction was reported .…”

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

“…Metal-to-ligand charge-transfer (MLCT) excited states play a key role in many coordination complexes and organometallic compounds because they enable a range of different applications in photophysics and photochemistry. Precious and rare elements such as ruthenium­(II), − osmium­(II), − rhenium­(I), − or iridium­(III) ,− in polypyridine or cyclometalating coordination environments often feature a long-lived MLCT excited state, , whereas among first-row d 6 transition metal elements, this is yet a very rare occurrence. Iron­(II) is by far most investigated in this regard, − yet only a handful of iron­(II) complexes with MLCT lifetimes in the nanosecond time regime are known. ,, Building on early reports of hexakis­(arylisocyanide) manganese­(I) complexes with a focus on UV–vis absorption and electrochemical properties, − we recently discovered that manganese­(I) complexes with chelating bi- and tridentate ligands have luminescent and photoredox active MLCT states .…”