A microcapsule-based reusable self-reporting system using a donor-acceptor Stenhouse adduct.

Self-reporting systems automatically indicate damaged or corroded surfaces via color changes or fluorescence. In this study, a novel reusable self-reporting system is developed by exploiting the reversibility of a donor-acceptor Stenhouse adduct (DASA). The synthesized DASA precursor exhibits a color change when damaged upon reaction with diethylamine, and returns to its colorless form upon irradiation with visible light. Microcapsules are synthesized with a core comprising styrene and the DASA precursor, along with a shell formed of urea and formaldehyde. The optimal particle size and shell thickness of the microcapsules are 225 µm and 0.17 µm, respectively. The DASA precursor-containing microcapsules are embedded in a PEG gel matrix with secondary amine groups. This coating system, initially colorless, exhibits a color change, becoming pink after being damaged by scratching due to the reaction between the DASA precursor released from ruptured microcapsules with the secondary amine groups of the PEG gel, thus demonstrating self-reporting characteristics. Furthermore, the colored surface is restored to its initial colorless state by irradiation with visible light for 1.5 hours, demonstrating the reusability of the self-reporting system.

Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst.

Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s-1 and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.

Deciphering the mechanism of the Ni-photocatalyzed C‒O cross-coupling reaction using a tridentate pyridinophane ligand.

Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions. Although the involvement of paramagnetic Ni(I)/Ni(III) species as active intermediates in the catalytic cycle has been proposed, a thorough spectroscopic investigation of these species is lacking. Herein, we report the tridentate pyridinophane ligands RN3 that allow for detailed mechanistic studies of the photocatalytic C-O coupling reaction. The derived (RN3)Ni complexes are active catalysts under mild conditions and without an additional photocatalyst. We also provide direct evidence for the key steps involving paramagnetic Ni species in the proposed catalytic cycle: the oxidative addition of an aryl halide to a Ni(I) species, the ligand exchange/transmetalation at a Ni(III) center, and the C-O reductive elimination from a Ni(III) species. Overall, the present work suggests the RN3 ligands are a practical platform for mechanistic studies of Ni-catalyzed reactions and for the development of new catalytic applications.

Electrocatalytic H2 evolution promoted by a bioinspired (N2S2)Ni(II) complex.

A bioinspired (N2S2)Ni(II) electrocatalyst is reported that produces H2 from CF3CO2H with a turnover frequency (TOF) of ∼1250 s-1 at low acid concentration (<0.043 M) in MeCN. A mechanism for the H2 production by this electrocatalyst is proposed and its activity is benchmarked against those of other reported molecular Ni H2 evolution electrocatalysts. The involvement of a hemilabile pyridyl group of the N2S2 ligand is proposed to mimic the role of a cysteine residue involved in the biological proton reduction performed by [NiFe] hydrogenases.

Photoreductive chlorine elimination from a Ni(iii)Cl2 complex supported by a tetradentate pyridinophane ligand.

Herein we report the isolation, characterization, and photoreactivity of a stable NiIII dichloride complex supported by a tetradentate pyridinophane N-donor ligand. Upon irradiation, this complex undergoes an efficient photoreductive chlorine elimination reaction, both in solution and the solid-state. Subsequently, the NiIIICl2 species can be regenerated via a reaction with PhICl2.

Coordination-Driven Self-Assembly of Cyclometalated Iridium Squares Using Linear Aromatic Diisocyanides.

Here, we demonstrate facile [4 + 4] coordination-driven self-assembly of cyclometalated iridium(III) using linear aryldiisocyanide bridging ligands (BLs). A family of nine new [Ir(C^N)2(µ-BL)]44+ coordination cages is described, where C^N is the cyclometalating ligand-2-phenylpyridine (ppy), 2-phenylbenzothiazole (bt), or 1-phenylisoquinoline (piq)-and BL is the diisocyanide BL, with varying spacer lengths between the isocyanide binding sites. These supramolecular coordination compounds are prepared via a one-pot synthesis, with isolated yields of 40-83%. 1H NMR spectroscopy confirms the selective isolation of a single product, which is affirmed to be the M4L4 square by high-resolution mass spectrometry. Detailed photophysical studies were carried out to reveal the nature of the luminescent triplet states in these complexes. In most cases, phosphorescence arises from the [Ir(C^N)2]+ nodes, with the emission color determined by the cyclometalating ligand. However, in two cases, the lowest-energy triplet state resides on the aromatic core of the BL, and weak phosphorescence from that state is observed. This work shows that aromatic diisocyanide ligands enable coordination-driven assembly of inert iridium(III) nodes under mild conditions, producing supramolecular coordination complexes with desirable photophysical properties.

Mixed-carbene cyclometalated iridium complexes with saturated blue luminescence.

Cyclometalated iridium complexes have emerged as top-performing emitters in organic light-emitting diodes (OLEDs) and other optoelectronic devices. A persistent challenge has been the development of cyclometalated iridium complexes with deep blue luminescence that have the requisite color purity, efficiency, and stability to function in color displays. In this work we report a new class of cyclometalated iridium complexes with saturated blue luminescence. These complexes have the general structure Ir(C^C:NHC)2(C^C:ADC), where C^C:NHC is an N-heterocyclic carbene (NHC) derived cyclometalating ligand and C^C:ADC is a different type of cyclometalating ligand featuring an acyclic diaminocarbene (ADC). The complexes are prepared by a cascade reaction that involves nucleophilic addition of propylamine to an isocyanide precursor followed by base-assisted cyclometalation of the ADC intermediate. All three emit deep blue light with good quantum efficiencies (Φ PL = 0.13-0.48) and color profiles very close to the ideal primary blue standards for color displays.

Facile Synthesis of Luminescent Ir-Pt-Ir Trimetallic Complexes.

A new class of luminescent, heterotrimetallic supramolecular constructs partnering two bis-cyclometalated iridium centers with a diimine platinum acetylide center is introduced. Whereas most supramolecular constructs featuring cyclometalated iridium involve elaborate bridging ligands and are prepared under forcing conditions with low to moderate yields, the three Ir-Pt-Ir complexes described here are prepared at room temperature from simple precursors and isolated in near-quantitative yields. ESI-MS, NMR spectroscopy, and diffusion ordered spectroscopy confirm the identity and homogeneity of the trimetallic products. In comparison with monometallic model complexes, analysis of UV/Vis absorption, steady-state photoluminescence and time-resolved emission reveals the impacts of supramolecular assembly on the photophysical properties. UV/Vis absorption and cyclic voltammetry suggest perturbation of some frontier orbital energies as a result of assembly, and the emission spectra and lifetimes reveal efficient excited-state energy transfer via a Dexter mechanism, and show that the site of luminescence (platinum or iridium) depends on the identity of the cyclometalating ligand bound to iridium.

Highly Luminescent Cyclometalated Iridium Complexes Generated by Nucleophilic Addition to Coordinated Isocyanides.

In this work, we report a new class of blue-emitting cyclometalated iridium complexes supported by acyclic diaminocarbene (ADC) ancillary ligands. These neutral, tris-chelated complexes are not obtainable via traditional synthesis routes and instead are generated through metal-mediated nucleophilic addition to a metal-bound isocyanide, which is followed by orthometalation of the ADC under mild conditions. Importantly, four of the variants exhibit efficient phosphorescence when immobilized in PMMA matrix, achieving quantum yields of 79% for blue emitters with a 2-(2,4-difluorophenyl)pyridine (F2ppy) C^N ligand and 30-37% for orange emitters with a 2-phenylbenzothiazole (bt) C^N ligand. Electrochemical studies demonstrate significantly higher-lying HOMO levels in the ADC complexes relative to the NHC analogues, a phenomenon that results in enhanced charge-transfer character in the excited states of the ADC complexes. This study demonstrates that ADC ancillary ligands not only give rise to new structures for Ir(III)-based phosphorescent emitters but also are promising targets for use in light-emitting devices and other thin-film optical applications.

Room temperature transmetallation from tris(pentafluorophenyl)borane to cyclometallated iridium(iii).

Transmetallation reactions involving organoboron reagents and transition metals are legion in synthetic organometallic chemistry and homogeneous catalysis. Triarylboranes (BAr3) have been observed to participate in transmetallation reactions with many transition metals, typically following abstraction of an alkyl (R-) or hydride ligand by the Lewis acidic borane. Here, an unusual transmetallation strategy is described where an aryl group from a borane replaces a weakly coordinated PF6- ligand. The precursors Ir(C^N)2(CNAr)(FPF5) (C^N = cyclometallating ligand, CNAr = aryl isocyanide) react smoothly with B(C6F5)3 to give complexes of the type Ir(C^N)2(CNAr)(C6F5), a previously unobserved structure type featuring an unchelated aryl ligand. The reaction tolerates a variety of C^N ligands and a range of electronically and sterically varied aryl isocyanide ancillary ligands. A total of six complexes of this type are described, two of which are characterized by single-crystal X-ray diffraction. All but one of the complexes luminesces at room temperature, with the emission wavelength dependent on the C^N ligand.

Bis-cyclometalated iridium complexes with electronically modified aryl isocyanide ancillary ligands.

In this work we report a study on the effect of systematic ancillary ligand modifications on electrochemical and photophysical properties of cationic biscyclometalated bis(arylisocyanide)iridium(iii) complexes. Nine new Ir(iii) complexes were synthesized using three different cyclometalating (C^N) ligands (2,4-difluorophenylpyridine (F2ppy), 2-benzothienylpyridine (btp), and 2-phenylbenzothiazole (bt)) with three aryl isocyanide ancillary ligands (2,4-dimethoxyphenyl isocyanide (CNArOMe), 3,5-bis(trifluoromethyl)phenyl isocyanide (CNArCF3) and 4-nitrophenyl isocyanide (CNArNO2)). Systematic modifications of ancillary ligands with electron-donating or electron-withdrawing groups have a very minor influence on the positions of the absorption and emission bands, suggesting that aryl isocyanide ancillary ligands minimally perturb the primarily ligand-centered emissive states, but still can control the dynamics of the excited state. Replacing electron-donating groups with electron-withdrawing group influences kr and/or knr, resulting in changes in the lifetimes and quantum yields. In addition, we reveal that electronic structures can be substantially altered by incorporating electron-donating or electron-withdrawing groups on the aryl isocyanide ancillary ligand, with different magnitudes of the perturbation depending on the cyclometalating C^N ligand. Particularly, the formally IrIV/IrIII oxidation couple can be perturbed by over 200 mV when electron-donating substituents are replaced with electron-withdrawing groups.