Luminescent Rhenium Complexes Research Articles

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 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.

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.

Unprecedented staining of polar lipids by a luminescent rhenium complex revealed by FTIR microspectroscopy in adipocytes.

Fourier transform infrared (FTIR) microspectroscopy and confocal imaging have been used to demonstrate that the neutral rhenium(i) tricarbonyl 1,10-phenanthroline complex bound to 4-cyanophenyltetrazolate as the ancillary ligand is able to localise in regions with high concentrations of polar lipids such as phosphatidylethanolamine (PE), sphingomyelin, sphingosphine and lysophosphatidic acid (LPA) in mammalian adipocytes.

Potential theranostic and multimodal iron oxide nanoparticles decorated with rhenium-bipyridine and -phenanthroline complexes.

Two structurally similar nanoparticles were designed for multimodal imaging and possible radiotherapy. The assembly consists of ultrasmall superparamagnetic iron oxide nanoparticles that act as contrast agents for MRI with a luminescent rhenium complex, for optical imaging, attached to the surface. Rhenium has the advantage of being luminescent and carries two radio-isotopes 186Re and 188Re making it possible to act as a contrast agent for SPECT (γ) and to be used for radiotherapy (β). The iron oxide nanoparticles were treated with a silane and further functionalized with picolyl. This picolyl was used to capture rhenium(i)(CO)3-1,10-phenanthroline (ReL1) or rhenium(i)(CO)3-2,2'-bipyridine (ReL2) and forms the final product Fe3O4-picolyl-rhenium(i)(CO)3-1,10-phenanthroline (IO-ReL1) or Fe3O4-silica-picolyl-rhenium(i)(CO)3-2,2'-bipyridine (IO-ReL2), respectively. All products were characterized properly (TEM, XRD, NMR, IR and TXRF) and a full investigation of the relaxometric and optical properties was conducted. Although iron oxide nanoparticles suffer from strong Rayleigh scattering, an efficient sensitized luminescence was observed and the orange emission wavelength was found to be 585 nm for IO-ReL1 and 592 nm for IO-ReL2 after irradiation at 395 nm. The relaxometric study of these ultrasmall nanoparticles showed very promising results. The r2 values measured at a magnetic field strength of 60 MHz of the nanoparticles being 92.9 mM-1 s-1 and 97.5 mM-1 s-1 for IO-ReL1 and IO-ReL2, respectively, were at least 1.5 times larger than Sinerem®.

Nanosized Micelles Formed by the Self-assembly of Amphiphilic Block Copolymers with Luminescent Rhenium Complexes

We have synthesized a series of polystyrene-block-poly(4-vinylpyridine)s (PS-b-PVP) to which luminescent tricarbonyl(2,2‘-bipyridyl)rhenium(I) complexes were attached. The rhenium complexes could induce the self-assembly of the copolymers into nanosized micelles with different shapes and dimensions, depending on the block size distribution of the copolymers and the solvent system being used. In general, spherical micelles were observed when methanol, a nonsolvent for the polystyrene block, was added to a copolymer solution in dichloromethane. Micellization was observed when the added methanol concentration was approximately 30%. When toluene, a nonsolvent for the poly(4-vinylpyridine) block, was added to the copolymer solution in dichloromethane, micelles with different interesting shapes were observed. For copolymers with larger PVP block size, spherical micelles were observed. When the relative block size of the PVP block was reduced, the micelles gradually changed to disk or vesicle structures and then to rodlike structures. The rhenium complex can act as a luminescent probe in the resulting nanosized micelles and provide sufficient contrast for electron microscopic studies. Significant changes in luminescence spectra were observed after the micellization.

Luminescence-Based Oxygen Sensors: ReL(CO)3Cl and ReL(CO)3CN Complexes on Copolymer Supports

A new class of luminescent rhenium complexes has been tested as oxygen sensors based on luminescent quenching. ReL(CO)3Cl and ReL(CO)3CN (L = 2,2′-bipyridine or 1,10-phenanthroline and substituted analogues) have several features that seem to indicate suitability as oxygen sensors. These include simple synthesis, long excited-state lifetimes, and high luminescence quantum yields. Intensity and lifetime oxygen quenching measurements were used to study the complexes in various polymer supports including homopolymers of PDMS (polydimethylsiloxane), a methacryloxy containing PDMS (Gp-163), and trimethylsilylmethylmethacrylate (T3642), and copolymers containing Gp-163 and T3642. In contrast to previous studies utilizing [Ru(4,7-diphenyl-1,10-phenanthroline)3]2+ as an oxygen sensor, quenching of the Re complexes proved much more sensitive to the polymer support. With suitable supports, the rhenium chloro complexes demonstrated significant quenching; but the cyano complexes, in spite of being robust in solution, exhibited severe photochemical instability in polymers. The potential of this class of complexes as oxygen sensors and as molecular probes as well as the ramifications in the design of new and different types of sensors is discussed.

Polarized emission from a rhenium metal-ligand complex

We report the first observation of polarized emission from a rhenium-phenanthroline complex, Re(CO)3(phen)Cl. Highly luminescent rhenium complexes are known, with quantum yields near 0.5 and lifetimes in excess of 10 μs. The detection of polarized emission suggests the use of rhenium complexes as probes of the hydrodynamics of large macromolecular complexes and for use in fluorescence polarization immunoassays with gated detection.