2-Phenylbenzothiazolyl iridium complexes as inhibitors and probes of amyloid β aggregation

The aggregation of amyloid β (Aβ) peptides is a significant hallmark of Alzheimer's disease (AD), and the detection of Aβ aggregates and the inhibition of their formation are important for the diagnosis and treatment of AD, respectively. Herein, we report a series of benzothiazole-based Ir(III) complexes HN-1 to HN-8 that exhibit appreciable inhibition of Aβ aggregation in vitro and in living cells. These Ir(III) complexes can induce a significant fluorescence increase when binding to Aβ fibrils and Aβ oligomers, while their measured log D values suggest these compounds could have enhanced blood-brain barrier (BBB) permeability. In vivo studies show that HN-1, HN-2, HN-3, and HN-8 successfully penetrate the BBB and stain the amyloid plaques in AD mouse brains after a 10-day treatment, suggesting that these Ir(III) complexes could act as lead compounds for AD therapeutic and diagnostic agent development.

Substituent-Dependent Azide Addition to Isocyanides Generates Strongly Luminescent Iridium Complexes

Ligand-centered functionalization reactions offer diverse strategies to prepare luminescent organometallic compounds. These compounds can have unique structures that are not accessible via traditional coordination chemistry and can possess enhanced or unusual photophysical properties. Here we show that bis-cyclometalated iridium bis-isocyanide complexes (1) react with azide (N₃^-) to form novel luminescent structures. The fate of the reaction with azide is determined primarily by the substituent on the aryl isocyanide. Those with electron-withdrawing substituents (CF₃ or NO₂) react with 1 equiv of azide followed by N₂ extrusion, forming aryl cyanamido products (2). With electron-donating groups on the aryl isocyanide the reactivity is more diverse, and three outcomes are possible. In two cases, the isocyanide and azide undergo a [3 + 2] cycloaddition to form a C-bound tetrazolato structure (3). In three other cases, 2 equiv of azide are involved in the formation of a previously unobserved structure, where a tetrazolato and aryl cyanamido couple and rearrange to form a chelating ligand comprised of an N-bound tetrazolato and an acyclic diaminocarbene (4). Finally, a bimetallic aryl cyanamido complex (5) is isolated in one case. All compounds are luminescent, some with exceptional photoluminescence quantum yields as high as 0.81 in solution for sky-blue emission, and 0.87 for yellow emission and 0.65 for orange-red emission in polymer films.

Substituent and redox effects on the second-order NLO response of Ru(ii) complexes with polypyridine ligands: a theoretical study

Modifying ligands in four studied Ru(II) complexes have slight influence on their static first hyperpolarizabilities (βtot) values. However, the βtot values of complexes 1+/−–4+/− are all enhanced and the largest βtot value is observed for complex 4−.

Theoretical investigation of electronic structures, second-order NLO responses of cyclometalated Ir(iii) and Rh(iii) counterpart complexes: effect of metal centers

Tuning center metal is an effective manner to modulate second-order NLO responses. When the big size Ir is introduced in complexes, the superior NLO responses can be found over its Rh counterparts.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

The diverse functions of isocyanides in phosphorescent metal complexes

In this Perspective, we highlight many examples of photoluminescent metal complexes supported by isocyanides, with an emphasis on recent developments including several from our own group. Work in this field has shown that the isocyanide can play important structural roles, both as a terminal ligand and as a bridging ligand for polynuclear structures, and can influence the excited-state character and excited-state dynamics. In addition, there are many examples of isocyanide-supported complexes where the isocyanide serves as a chromophoric ligand, meaning the low-energy excited states that are important in the photochemistry are partially or completely localized on the isocyanide. Finally, an emerging trend in the design of luminescent compounds is to use the isocyanide as an electrophilic precursor, converted to an acyclic carbene by nucleophilic addition which imparts certain photophysical advantages. This Perspective aims to show the diverse roles played by isocyanides in the design of luminescent compounds, showcasing the recent developments that have led to a substantial growth in fundamental knowledge, function, and applications related to photoluminescence.

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

In this paper, we report a series of six neutral, blue-phosphorescent 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 σ-donating 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.

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.

Ligand-triplet migration in iridium(iii) cyclometalates featuring π-conjugated isocyanide ligands

The manipulation of the triplet excited state manifold leads to large differences in the photophysical properties within a given class of metal-organic chromophores. By the appropriate choice of ancillary ligand, large changes can be made both to the order and nature of the lowest excited states and therefore to the resulting photophysical properties. Herein, a series of four bis-2-phenylpyridine (ppy) cyclometalated Ir(iii) compounds bearing two arylisocyanide ligands were synthesized and photophysically characterized to understand the effects of using ancillary ligands featuring systematic changes in π-conjugation. By varying the arylisocyanide ligands, the photoluminescence quantum yield ranged from 5% to 49% and the excited state lifetime ranged between 24 μs and 2 ms. These variations in photophysical response are consistent with lowering the triplet ligand-centered (³LC) state of the arylisocyanide ligand as the π system was extended, confirmed by 77 K photoluminescence emission spectra and ultrafast transient absorption experiments. The latter analysis gleaned detailed insight into the importance of the interplay of the ³LC state of the phenylpyridine and arylisocyanide ligands in these polychromophic Ir(iii) molecules.

Cyclometalated iridium-BODIPY ratiometric O2 sensors

Here we introduce a new class of ratiometric O2 sensors for hypoxic environments. Two-component structures composed of phosphorescent cyclometalated Ir(iii) complexes and the well-known organic fluorophore BODIPY have been prepared by the 1 : 1 reaction of bis-cyclometalated iridium synthons with pyridyl-substituted BODIPY compounds. Two different cyclometalating ligands are used, which determine the relative energies of the iridium-centered and BODIPY-centered excited states, and the nature of the linker between iridium and BODIPY also has a small influence on the photoluminescence. Some of the conjugates exhibit dual emission, with significant phosphorescence from the iridium site and fluorescence from the BODIPY, and thus function as ratiometric oxygen sensors. Oxygen quenching experiments demonstrate that as O2 is added the phosphorescence is quenched while the fluorescence is unaffected, with dynamic ranges that are well suited for hypoxic sensing (pO2 < 160 mmHg).

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

Transmetallation with tris(pentafluorophenyl)borane produces bis-cyclometallated iridium complexes with a perfluorophenyl ancillary ligand.