Luminescent Rhenium Research Articles

Luminescent rhenium(I) complex with cyanopentafluorophosphate ligand: Synthesis, characterization, and photophysics

A tricarbonyl Re(I) phenanthroline complex (1) with anionic cyanopentafluorophosphate [NCPF5]– ligand has been synthesized and characterized. Although the structure of the complex has also been determined by X-ray crystallography, the identity of the [NCPF5]– needs to be confirmed by the vibrational spectroscopic study. The cyanopentafluorophosphate Re(I) phenanthroline complex 1 also exhibits phosphorescence derived from a 3MLCT excited state. Importantly, as the MLCT absorption and emission energies of 1 are comparable to those of the structurally related complexes with the strongest π-accepting anionic isocyanoborate ligands, it reveals the exceptionally strong π-accepting ability of [NCPF5]–. This result suggests that [NCPF5]– is a promising alternative to isocyanoborate ligands for designing phosphorescent materials, catalysts and photocatalysts.

Aryl, bi-functionalised imidazo[4,5-f]-1,10-phenanthroline ligands and their luminescent rhenium(I) complexes

Five new imidazo[4,5-f]-1,10-phenanthroline based ligands (1–5) have been synthesised and characterised. The facile synthesis of 1–5 allows two regiochemical points of structural variety allowing highly conjugated and bulky aryl groups of varying functionalities, including azobenzene, trityl and terpyridine constituents, to be attached to the ligand core. 1–5 are fluorescent (λem = 410–415 nm), and react readily with [ReBr(CO)5] in toluene to give neutral coordination complexes of the form fac-[ReBr(CO)3(1–5)]. The series of complexes was characterised using a variety of spectroscopic and analytical techniques. Two examples of this series were characterised in the solid state using single crystal X-ray diffraction which confirmed the octahedral geometry and formulation. Photophysical studies showed that fac-[ReBr(CO)3(1–5)] are phosphorescent in solution under ambient conditions, revealing visible emission (558–585 nm) in aerated solution with corresponding lifetimes in the range 149–267 ns. These attributes are consistent with a triplet metal to ligand charge transfer (3MLCT) emitting state.

Strategic Design of Luminescent Rhenium(I), Ruthenium(II), and Iridium(III) Complexes as Activity-Based Probes for Bioimaging and Biosensing.

The rapid development of responsive fluorescent probes has advanced optical imaging for biological research and biomedical applications. Among different sensing strategies, activity-based sensing, which exploits the unique reactivity of the target chemical species to achieve high chemoselectivity, has emerged as a promising paradigm for the development of responsive probes for selective molecular imaging. Luminescent transition metal complexes have received considerable attention for bioimaging and biosensing applications over the last decade due to their remarkable photophysical behavior including intense emission with large Stokes' shifts, long emission lifetimes, strong two-photon absorption, and high photostability. In this Review, we summarize the design strategies and applications of luminescent complexes of rhenium(I), ruthenium(II), and iridium(III) polypyridines as activity-based probes for the detection of various chemical species and bioactive molecules in live cells and organisms. The current challenges and future prospects of these complexes as activatable reagents for disease diagnosis and treatment are also discussed.

Hydrogen-bonded organic frameworks based on luminescent rhenium octahedral cluster complexes

Hydrogen-bonded organic frameworks based on luminescent rhenium octahedral cluster complexes

Rhenium(I) conjugates as tools for tracking cholesterol in cells.

Cholesterol is vital to control membrane integrity and fluidity, but is also a precursor to produce steroid hormones, bile acids, and vitamin D. Consequently, altered cholesterol biology has been linked to many diseases, including metabolic syndromes and cancer. Defining the intracellular pools of cholesterol and its trafficking within cells is essential to understand both normal cell physiology and mechanisms of pathogenesis. We have synthesized a new cholesterol mimic (ReTEGCholestanol), comprising a luminescent rhenium metal complex and a cholestanol targeting unit, linked using a tetraethylene glycol (TEG) spacer. ReTEGCholestanol demonstrated favourable imaging properties and improved water solubility when compared to a cholesterol derivative, and structurally related probes lacking the TEG linker. A non-malignant and three malignant prostate cell lines were used to characterize the uptake and intracellular distribution of ReTEGCholestanol. The ReTEGCholestanol complex was effectively internalized and mainly localized to late endosomes/lysosomes in non-malignant PNT1a cells, while in prostate cancer cells it also accumulated in early endosomes and multivesicular bodies, suggesting disturbed cholesterol biology in the malignant cells. The ReTEGCholestanol is a novel imaging agent for visualizing endosomal uptake and trafficking, which may be used to define cholesterol related biology including membrane integration and altered lipid trafficking/processing.

Photochemistry of Rhenium(i) Diimine Tricarbonyl Complexes in Biological Applications.

Luminescent rhenium complexes continue to be the focus of growing scientific interest for catalytic, diagnostic and therapeutic applications, with emphasis on the development of their photophysical and photochemical properties. In this short review, we explore such properties with a focus on the biological applications of the molecules. We discuss the importance of the ligand choice to the contribution and their involvement towards the most significant electronic transitions of the metal species and what strategies are used to exploit the potential of the molecules in medicinal applications. We begin by detailing the photophysics of the molecules; we then describe the three most common photoreactions of rhenium complexes as photosensitizers in H₂ production, photocatalysts in CO₂ reduction and photochemical ligand substitution. In the last part, we describe their applications as luminescent cellular probes and how photochemical ligand substitution is utilized in the development of photoactive carbon monoxide-releasing molecules as anticancer and antimicrobial agents.

Luminescent rhenium(I) perfluorobiphenyl complexes as site-specific labels for peptides to afford photofunctional bioconjugates.

We report herein new luminescent rhenium(I) perfluorobiphenyl complexes that reacted specifically with the cysteine residue of the π-clamp sequence (FCPF) to afford novel peptide-based imaging reagents, photosensitisers for singlet oxygen and enzyme sensors.

The role of zero-field splitting and π-stacking interaction of different nitrogen-donor ligands on the optical properties of luminescent rhenium tricarbonyl complexes

In this work, a systematic evaluation of the role of zero-field splitting (ZFS), and the geometric arrangement of different nitrogen-donor ligands, including π-stacking interactions, in five selected rhenium luminescent complexes was performed.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts.

Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)3(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na2CO3 and NaOH gave neutral fac-[Re(CO)3(pz*H)(pypz)] and anionic Na{fac-[Re(CO)3(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)3(pz*H)(pypz)], and fac-[Re(CO)3(pypz)2Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently 3MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to ReI → ReII occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO2 saturated media is consistent with CO2 electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/icat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)3Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Bioorthogonal Phosphorogenic Rhenium(I) Polypyridine Sydnone Complexes for Specific Lysosome Labeling.

Invited for this month's cover is the group of Prof. Kenneth Kam-Wing Lo at City University of Hong Kong, Hong Kong, P. R. China. The cover picture shows the selective landing of a bioorthogonal spacecraft on a lysosomal planet modified with a strained cyclooctyne moiety in an intracellular environment with other organelles and a plethora of biomolecules. A sydnone moiety is appended to a luminescent rhenium(I) diimine unit as both an emission quencher and a bioorthogonal handle. Selective strain-promoted sydnone-alkyne cycloaddition (SPSAC) of the complex with a strained alkyne leads to impressive emission turn-on, which can be exploited in bioimaging and phototherapeutic applications. Read the full text of the article at 10.1002/cplu.202000029.

Rhenium(I)-tetrazolato functional luminescent polymers: Organic-inorganic hybrids via RAFT and post-polymerization modification

A series of well-defined tetrazole-functionalized (co)polymers based on the styrenic structural motif that are able to serve as macromolecular ligands for the coordination of a luminescent rhenium(I) complex are presented. Copolymers of styrene with 4-cyanostyrene containing different ratios of cyano appended groups have been synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Post-polymerization modification of the cyano groups via reaction with NaN3 yielded the corresponding tetrazole functionalized copolymers. These tetrazole functional materials are able to serve as macromolecular ligands for the coordination of metal complexes: a rhenium(I) species has been incorporated into the polymeric structure by coordination to the tetrazole groups on the copolymers. The parent (co)polymers have been characterised by size exclusion chromatography (SEC), while inverse-gated proton decoupled 13C NMR spectroscopy and infrared spectroscopy confirmed the formation of the tetrazole functional group and subsequent metal coordination. The photophysical properties of the copolymer-rhenium(I) hybrid luminescent materials have been fully investigated. Absorption and emission profiles confirmed coordination between the tetrazole functionality appended to the polymer material and the rhenium(I) precursor complex. Lifetime measurements and quantum yields confirmed that the presence of the polymeric scaffold does not interfere with the luminescent properties of the rhenium(I) complex.

Luminescent Rhenium(I)-Polypyridine Complexes Appended with a Perylene Diimide or Benzoperylene Monoimide Moiety: Photophysics, Intracellular Sensing, and Photocytotoxic Activity.

This communication reports novel luminescent rhenium(I)-polypyridine complexes appended with a perylene diimide (PDI) or benzoperylene monoimide (BPMI) moiety through a non-conjugated linker. The photophysical and photochemical properties originating from the interactions of the metal polypyridine and perylene units were exploited to afford new cellular reagents with thiol-sensing capability and excellent photocytotoxic activity.

Luminescent dinuclear rhenium(I)[sbnd]PNA conjugates for microRNA-21 targeting: Synthesis, chemico-physical and biological characterization

Two different luminescent rhenium complexes, having the general formula [Re2(μ-Cl)2(CO)6(μ-1,2-diazine)] and containing two different diazine ligands, namely 4-(pyridazin-4-yl)-butanoic acid (Re-1) and 8-(5-methylpyridazin-4-yl)-octanoic acid (Re-2), were prepared. Exploiting the presence of the carboxylic acid moieties, both complexes were further covalently linked to a 20-mer oligomer (PNA1), bearing a nucleobase sequence complementary to that of mature miR-21–5p (5′-TCAACATCAGTCTGATAAGCTA-3′), as well as to a 20-mer oligomer (PNA2) used as a negative control bearing the sequence 5′-GTGTAACACGTCCTATACGCC-3’. The resulting four Re-PNA conjugates were characterized and used to target miR-21 in the DU145 prostate cancer cell line. The conjugation with PNA did not perturb the photoluminescence behavior of the organometallic fragments: emission from 3MLCT excited states, centered in a range between 580 and 610 nm, was observed, with satisfactory photoluminescence quantum yields (Φ = ca. 0.03–0.01, in aerated water/acetonitrile solution 1/1). Experiments of cell uptake, performed with living prostate cancer cells showed that the length of the aliphatic linker between the rhenium complex and the PNA sequence, strongly affects the cellular localization.

Luminescent rhenium(I) carbonyl complex with redox noninnocent ONS donor azo-phenol ligand: Synthesis, X-ray structure, photophysical properties and live cell imaging

Herein, we have synthesized a new fluorescent rhenium(I) carbonyl complex 1, bearing {Re(CO)3}+ core with ONS donor thioether containing azo-phenol redox noninnocent ligand. The distorted octahedral geometry of the complex is confirmed by single crystal X-ray diffraction method. Cyclic voltammogram in acetonitrile exhibits irreversible oxidation peak (Epa = 1.36 V) along with quasi-reversible reduction peak at E1/2 = −0.92 V (ΔE = 210 mV). The complex exhibits low energy emission band at 525 nm with high emission quantum yield (ϕ = 0.115). Cytotoxicity of the complex is studied by MTT method with human breast cancer cell lines (MCF-7) and IC50 value is found to be 23.6 μM. In presence of the complex (10 μM) a bright green fluorescence image of MCF-7 cell lines is observed under fluorescence microscope.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes.

Two new neutral fac-[Re(CO)3(phen)L] compounds (1,2), with phen = 1,10-phenanthroline and L = O2C(CH2)5CH3 or O2C(CH2)4C≡CH, were synthetized in one-pot procedures from fac-[Re(CO)3(phen)Cl] and the corresponding carboxylic acids, and were fully characterized by IR and UV-Vis absorption spectroscopy, 1H- and 13C-NMR, mass spectrometry and X-ray crystallography. The compounds, which display orange luminescence, were used as probes for living cancer HeLa cell staining. Confocal microscopy revealed accumulation of both dyes in mitochondria. To investigate the mechanism of mitochondrial staining, a new non-emissive compound, fac-[Re(CO)3(phen)L], with L = O2C(CH2)3((C5H5)Fe(C5H4), i.e., containing a ferrocenyl moiety, was synthetized and characterized (3). 3 shows the same mitochondrial accumulation pattern as 1 and 2. Emission of 3 can only be possible when ferrocene-containing ligand dissociates from the metal center to produce a species containing the luminescent fac­[Re(CO)3(phen)]+ core. The release of ligands from the Re center was verified in vitro through the conjugation with model proteins. These findings suggest that the mitochondria accumulation of compounds 1–3 is due to the formation of luminescent fac-[Re(CO)3(phen)]+ products, which react with cellular matrix molecules giving secondary products and are uptaken into the negatively charged mitochondrial membranes. Thus, reported compounds feature a rare dissociation-driven mechanism of action with great potential for biological applications.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

Synthesis, characterization and computational studies of luminescent rhenium(I) tricarbonyl diimine complexes with 8-hydroxyquinoline-containing alkynyl ligands

A series of 8-hydroxyquinoline-containing alkynylrhenium(I) tricarbonyl diimine complexes has been designed and synthesized. Their UV-vis absorption, emission as well as electrochemical properties have been studied. Computational studies have also been performed to provide insights into the electronic transitions, excited state origins and electrochemical properties of the complexes.

Synthesis, Structures, and CO Release Capacity of a Family of Water-Soluble PhotoCORMs: Assessment of the Biocompatibility and Their Phototoxicity toward Human Breast Cancer Cells.

Two manganese(I) carbonyl complexes derived from 2-(pyridyl)benzothiazole (pbt) and 1,10-phenanthroline (phen) release carbon monoxide (CO) under low-power broad-band visible-light illumination. CO photorelease from [Mn(CO)3(pbt)(PTA)]CF3SO3 (1, where PTA = 1,3,5-triaza-7-phosphaadamantane) is accompanied by an emergence of a strong fluorescence around 400 nm from almost nonfluorescent preirradiated 1. However, [Mn(CO)3(phen)(PTA)]CF3SO3 (2) showed no such phenomenon upon prolonged illumination under similar experimental conditions. The two analogous rhenium(I) complexes, namely, [Re(CO)3(pbt)(PTA)]CF3SO3 (3) and [Re(CO)3(phen)(PTA)]CF3SO3 (4), have also been synthesized and characterized to compare their photo properties with the manganese congeners. Complexes 3 and 4 exhibit moderate CO release upon irradiation with low-power UV light. All four complexes are highly soluble in anaerobic/aerobic aqueous media and are also considerably more stable when kept under dark conditions. The inherently luminescent rhenium complex 3 was utilized to demonstrate cellular internalization of these types of compounds by MDA-MB-231 (human breast cancer) cells, while the two biocompatible manganese(I) complexes (1 and 2) have been applied to assess the cell viability of these malignant cells upon CO delivery.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents.

There has been fast-growing interest in the exploitation of the photophysical and photochemical properties of luminescent transition metal complexes in biological applications, with a focus on both diagnostic and therapeutic aspects. In particular, the design of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents and anticancer drugs has received considerable attention for a number of reasons. First, most rhenium(i) tricarbonyl polypyridine complexes possess diverse photophysical and photochemical properties through the coordination of functionalized ligands. The typical photophysical properties of the complexes such as large Stokes shifts, long emission lifetimes, and high photostability allow them to serve as attractive candidates for optical imaging. Also, the cellular uptake of the complexes can be readily quantified by atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry. Additionally, owing to the characteristic infrared absorption bands and the isostructural relationship between rhenium and technetium-99m, rhenium(i) tricarbonyl complexes have been exploited as multimodal imaging reagents for vibrational and radio-imaging, respectively. Furthermore, the facile photosensitizing properties and the three carbon monoxide (CO) ligands render rhenium(i) tricarbonyl complexes promising candidates as photodynamic therapy reagents and photoactivatable CO-releasing molecules, respectively, for cancer treatment. In this Perspective, we describe the recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents.

Intracellular distribution and stability of a luminescent rhenium(i) tricarbonyl tetrazolato complex using epifluorescence microscopy in conjunction with X-ray fluorescence imaging.

Optical epifluorescence microscopy was used in conjunction with X-ray fluorescence imaging to monitor the stability and intracellular distribution of the luminescent rhenium(i) complex fac-[Re(CO)3(phen)L], where phen = 1,10-phenathroline and L = 5-(4-iodophenyl)tetrazolato, in 22Rv1 cells. The rhenium complex showed no signs of ancillary ligand dissociation, a conclusion based on data obtained via X-ray fluorescence imaging aligning iodine and rhenium distributions. A diffuse reticular localisation was detected for the complex in the nuclear/perinuclear region of cells, by either optical or X-ray fluorescence imaging techniques. X-ray fluorescence also showed that the rhenium complex disrupted the homeostasis of some biologically relevant elements, such as chlorine, potassium and zinc.