A Mesoionic Carbene as Neutral Ligand for Phosphorescent Cationic Ir(III) Complexes

Two phosphorescent Ir­(III) complexes bearing a mesoionic carbene ligand based on 1,2,3-triazolylidene are obtained for the first time. A silver–iridium transmetalation of the in situ-generated mesoionic carbene affords the cationic dichloro complex [Ir­(trizpy)₂Cl₂]⁺ (<b>3</b>, trizpy = 1-benzyl-3-methyl-4-(pyridin-2-yl)-1<i>H</i>-1,2,3-triazolylidene) that reacts with a bis-tetrazolate (b-trz) dianionic ligand to give [Ir­(trizpy)₂(b-trz)]⁺ (<b>5</b>). The new compounds are fully characterized by NMR spectroscopy and mass spectrometry, and the X-ray structure of <b>3</b> is determined. The electrochemical behavior is somewhat different compared to most standard cationic iridium complexes. The first oxidation process is shifted to substantially higher potential in both <b>3</b> and <b>5</b>, due to peculiar and different ligand-induced effects in the two cases, which stabilize the highest occupied molecular orbital; reduction processes are centered on the mesoionic carbene ligands. Both compounds exhibit a mostly ligand-centered luminescence band in the blue-green spectral region, substantially stronger in the case of <b>5</b> versus <b>3</b>, both in CH₃CN solution and in poly­(methyl methacrylate) matrix at room temperature. Optimized geometries, orbital energies, spin densities, and electronic transitions are determined via density functional theory calculations, which support a full rationalization of the electrochemical and photophysical behavior. This work paves the way for the development of Ir-based emitters with neutral mesoionic carbene ligands and anionic ancillary ligands, a new concept in the area of cationic Ir­(III) complexes

Blue Phosphorescence of Trifluoromethyl- and Trifluoromethoxy-Substituted Cationic Iridium(III) Isocyanide Complexes

We report the first comprehensive comparative synthetic, structural, electrochemical, and spectroscopic study of an extended series of fluorocarbon-modified iridium­(III) complexes. We prepared seven new cationic Ir­(III) complexes with <i>tert</i>-butyl isocyanide and trifluoromethyl- or trifluoromethoxy-substituted cyclometalating 2-phenylpyridines, [(C^∧N)₂Ir­(CN<i>t</i>Bu)₂]­(CF₃SO₃), and characterized five of them by crystal structure analysis. The redox potentials and photophysical properties of Ir­(III) complexes are determined by the type, position, and number of fluorocarbon groups in the cyclometalating ligand. The complexes exhibit pale blue to yellow-green phosphorescence at room temperature with quantum yields and excited-state lifetimes up to 73% and 84 μs in solution (under argon) and 7.5% and 4.3 μs in neat solid (under air). The structured and solvent-independent phosphorescence spectra, with 0–0 emission transition at 445–467 nm, and the long calculated radiative lifetimes, 43–160 μs, indicate that the complexes emit from a cyclometalating-ligand-centered triplet excited state. Bulky fluorocarbon groups prevent intermolecular interaction (aggregation) of the complexes, thereby minimizing red-shift of phosphorescence color in going from solution to neat solid

Anilino-Substituted Multicyanobuta-1,3-diene Electron Acceptors: TICT Molecules with Accessible Conical Intersections

A theoretical investigation based on DFT, TD-DFT, and CASSCF/CASPT2 methods has been carried out to elucidate the photophysics of two anilino-substituted pentacyano- and tetracyanobuta-1,3-dienes (<b>PCBD</b> and <b>TCBD</b>, respectively). These molecules exhibit exceptional electron-accepting properties, but their effective use in multicomponent systems for photoinduced electron transfer is limited because they undergo ultrafast (∼1 ps) radiationless deactivation. We show that the lowest-energy excited states of these molecules have a twisted intramolecular charge-transfer character and deactivate to the ground state through energetically accessible conical intersections (CIs). The topology of the lowest-energy CI, analyzed with a linear interpolation of the two branching-space vectors (<b>g</b> and <b>h</b>), indicates it is a sloped CI, ultimately responsible for the ultrafast deactivation of this class of compounds

Extreme Tuning of Redox and Optical Properties of Cationic Cyclometalated Iridium(III) Isocyanide Complexes

We report seven heteroleptic cationic iridium­(III) complexes with cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(C^∧N)₂Ir­(CNR)₂]­(CF₃SO₃), and characterize two of them by crystal structure analysis. The complexes are air- and moisture-stable white solids that have electronic transitions at very high energy with absorption onset at 320–380 nm. The complexes are difficult to reduce and oxidize; they exhibit irreversible electrochemical processes with peak potentials (against ferrocene) at −2.74 to −2.37 V (reduction) and 0.99–1.56 V (oxidation) and have a large redox gap of 3.49–4.26 V. The reduction potential of the complex is determined by the azole heterocycle (pyrazole or indazole) and by the isocyanide (<i>tert</i>-butyl or 2,6-dimethylphenyl) and the oxidation potential by the Ir–aryl fragment [aryl = 2′,4′-R₂-phenyl (R = H/F), 9′,9′-dihexyl-2′-fluorenyl]. Three of the complexes exhibit phosphorescence in argon-saturated dichloromethane and acetonitrile solutions at room temperature with 0–0 transitions at 473–478 nm (green color; the emission spectra are solvent-independent), quantum yields of 3–25%, and long excited-state lifetimes of 62–350 μs. All of the complexes are phosphorescent at 77 K with 0–0 transitions at 387–474 nm (blue to green color). The extremely long calculated radiative lifetimes, 0.5–3.5 ms, confirm that the complexes emit from a cyclometalating-ligand-centered excited state

Blue-Emitting Dinuclear N-heterocyclic Dicarbene Gold(I) Complex Featuring a Nearly Unit Quantum Yield

Dinuclear N-heterocyclic dicarbene gold­(I) complexes of general formula [Au₂(RIm-Y-ImR)₂]­(PF₆)₂ (R = Me, Cy; Y = (CH₂)_1–4, <i>o</i>-xylylene, <i>m</i>-xylylene) have been synthesized and screened for their luminescence properties. All the complexes are weakly emissive in solution whereas in the solid state some of them show significant luminescence intensities. In particular, crystals or powders of the complex with R = Me, Y = (CH₂)₃ exhibit an intense blue emission (λ_max = 450 nm) with a high quantum yield (Φ_em = 0.96). The X-ray crystal structure of this complex is characterized by a rather short intramolecular Au···Au distance (3.272 Ǻ). Time dependent density functional theory (TDDFT) calculations have been used to calculate the UV/vis properties of the ground state as well as of the first excited state of the complex, the latter featuring a significantly shorter Au···Au distance

Extreme Tuning of Redox and Optical Properties of Cationic Cyclometalated Iridium(III) Isocyanide Complexes

We report seven heteroleptic cationic iridium­(III) complexes with cyclometalating N-arylazoles and alkyl/aryl isocyanides, [(C^∧N)₂Ir­(CNR)₂]­(CF₃SO₃), and characterize two of them by crystal structure analysis. The complexes are air- and moisture-stable white solids that have electronic transitions at very high energy with absorption onset at 320–380 nm. The complexes are difficult to reduce and oxidize; they exhibit irreversible electrochemical processes with peak potentials (against ferrocene) at −2.74 to −2.37 V (reduction) and 0.99–1.56 V (oxidation) and have a large redox gap of 3.49–4.26 V. The reduction potential of the complex is determined by the azole heterocycle (pyrazole or indazole) and by the isocyanide (<i>tert</i>-butyl or 2,6-dimethylphenyl) and the oxidation potential by the Ir–aryl fragment [aryl = 2′,4′-R₂-phenyl (R = H/F), 9′,9′-dihexyl-2′-fluorenyl]. Three of the complexes exhibit phosphorescence in argon-saturated dichloromethane and acetonitrile solutions at room temperature with 0–0 transitions at 473–478 nm (green color; the emission spectra are solvent-independent), quantum yields of 3–25%, and long excited-state lifetimes of 62–350 μs. All of the complexes are phosphorescent at 77 K with 0–0 transitions at 387–474 nm (blue to green color). The extremely long calculated radiative lifetimes, 0.5–3.5 ms, confirm that the complexes emit from a cyclometalating-ligand-centered excited state

Anionic Cyclometalated Iridium(III) Complexes with a Bis-Tetrazolate Ancillary Ligand for Light-Emitting Electrochemical Cells

A series of monoanionic Ir­(III) complexes (<b>2</b>–<b>4</b>) of general formula [Ir­(C^N)₂(b-trz)]­(TBA) are presented, where C^N indicates three different cyclometallating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)­pyridine; Hpqu = 2-methyl-3-phenylquinoxaline), b-trz is a bis-tetrazolate anionic N^N chelator (H₂b-trz = di­(1H-tetrazol-5-yl)­methane), and TBA = tetrabutylammonium. <b>2</b>–<b>4</b> are prepared in good yields by means of the reaction of the suitable b-trz bidentate ligand with the desired iridium­(III) precursor. The chelating nature of the ancillary ligand, thanks to an optimized structure and geometry, improves the stability of the complexes, which have been fully characterized by NMR spectroscopy and high-resolution MS, while X-ray structure determination confirmed the binding mode of the b-trz ligand. Density functional theory calculations show that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are mainly localized on the metal center and the cyclometalating ligands, while the bis-tetrazolate unit does not contribute to the frontier orbitals. By comparison with selected classes of previously published cationic and anionic complexes with high ligand field and even identical cyclometallating moieties, it is shown that the HOMO–LUMO gap is similar, but the absolute energy of the frontier orbitals is remarkably higher for anionic vs cationic compounds, due to electrostatic effects. <b>2</b>–<b>4</b> exhibit reversible oxidation and reduction processes, which make them interesting candidates as active materials for light emitting electrochemical cells, along with red, green, and blue emission, thanks to the design of the C^N ligands. Photoluminescence quantum yields range from 28% (<b>4</b>, C^N = pqu, red emitter) to 83% (<b>3</b>, C^N = dfppy, blue emitter) in acetonitrile, with the latter compound reaching 95% in poly­(methyl methacrylate) (PMMA) matrix. In thin films, the photoluminescence quantum yield decreases substantially probably due to the small intersite distance between the complexes and the presence of quenching sites. In spite of this, surprisingly stable electroluminescence was observed for devices employing complex <b>2</b>, demonstrating the robustness of the anionic compounds

Photocatalytic Radical Alkylation of Electrophilic Olefins by Benzylic and Alkylic Zinc-Sulfinates

Alkyl radicals are obtained by photocatalytic oxidation of readily prepared or commercially available zinc sulfinates. The convenient benzylation and alkylation of a variety of electron-poor olefins triggered by the iridium­(III) complex <b>6</b> Ir­[dF­(CF₃)­ppy]₂(dtbbpy)­PF₆ as photocatalyst is described. Moreover, it is shown that zinc sulfinates can be used for facile nonradical sulfonylation reactions with highly electrophilic Michael acceptors

Bright Blue Phosphorescence from Cationic Bis-Cyclometalated Iridium(III) Isocyanide Complexes

We report new bis-cyclometalated cationic iridium­(III) complexes [(C^∧N)₂Ir­(CN-<i>tert</i>-Bu)₂]­(CF₃SO₃) that have <i>tert</i>-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4′-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-<i>tert</i>-butyl, or 5-trifluoromethyl) as C^∧N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58–3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440–459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46–82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky <i>tert</i>-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium­(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid

Bright Blue Phosphorescence from Cationic Bis-Cyclometalated Iridium(III) Isocyanide Complexes

We report new bis-cyclometalated cationic iridium­(III) complexes [(C^∧N)₂Ir­(CN-<i>tert</i>-Bu)₂]­(CF₃SO₃) that have <i>tert</i>-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4′-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-<i>tert</i>-butyl, or 5-trifluoromethyl) as C^∧N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58–3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440–459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46–82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky <i>tert</i>-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium­(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid