Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence

We report on new anionic tridentate benzimidazole-pyridine-tetrazolate ligands that form neutral 3:1 complexes with trivalent lanthanides. The ligands are UV-absorbing chromophores that sensitize the red luminescence of europium with energy-transfer efficiency of 74–100%. The lifetime and quantum yield of the sensitized europium luminescence increase from 0.5 ms and 12–13% for the as-prepared solids to 2.8 ms and 41% for dichloromethane solution. From analysis of the data, the as-prepared solids can be described as aqua-complexes [Ln­(κ³-ligand)₂(κ¹-ligand)­(H₂O)_<i>x</i>] where the coordinated water molecules are responsible for the strong quenching of the europium luminescence. In solution, the coordinated water molecules are replaced by the nitrogen atoms of the κ¹-ligand to give anhydrous complexes [Ln­(κ³-ligand)₃] that exhibit efficient europium luminescence. X-ray structures of the anhydrous complexes confirm that the lanthanide ion (La^III, Eu^III) is nine-coordinate in a distorted tricapped trigonal prismatic environment and that coordination of the lanthanide ion by tetrazolate is weaker than by carboxylate

Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence

We report on new anionic tridentate benzimidazole-pyridine-tetrazolate ligands that form neutral 3:1 complexes with trivalent lanthanides. The ligands are UV-absorbing chromophores that sensitize the red luminescence of europium with energy-transfer efficiency of 74–100%. The lifetime and quantum yield of the sensitized europium luminescence increase from 0.5 ms and 12–13% for the as-prepared solids to 2.8 ms and 41% for dichloromethane solution. From analysis of the data, the as-prepared solids can be described as aqua-complexes [Ln­(κ³-ligand)₂(κ¹-ligand)­(H₂O)_<i>x</i>] where the coordinated water molecules are responsible for the strong quenching of the europium luminescence. In solution, the coordinated water molecules are replaced by the nitrogen atoms of the κ¹-ligand to give anhydrous complexes [Ln­(κ³-ligand)₃] that exhibit efficient europium luminescence. X-ray structures of the anhydrous complexes confirm that the lanthanide ion (La^III, Eu^III) is nine-coordinate in a distorted tricapped trigonal prismatic environment and that coordination of the lanthanide ion by tetrazolate is weaker than by carboxylate

Near-Infrared Optical Imaging of Necrotic Cells by Photostable Lanthanide-Based Metallacrowns

Sensitive detection of cell necrosis is crucial for the determination of cell viability. Because of its high resolution at the cellular level and sensitivity, optical imaging is highly attractive for identifying cell necrosis. However, challenges associated with this technique remain present such as the rapid photobleaching of several types of organic fluorophores and/or the interference generated by biological autofluorescence. Herein, we synthesized novel biologically compatible Zn²⁺/Ln³⁺ metallacrowns (MCs) that possess attractive near-infrared (NIR) emission and are highly photostable. In addition, these MCs have the ability to label differentially necrotic HeLa cells from living cells. This work is also the first demonstration of (i) the use of the NIR emission arising from a single lanthanide­(III) cation for optical biological imaging of cells under single photon excitation, (ii) the first example of a lanthanide­(III)-based NIR-emitting probe that can be targeted to a specific type of cell

Near-Infrared Optical Imaging of Necrotic Cells by Photostable Lanthanide-Based Metallacrowns

Sensitive detection of cell necrosis is crucial for the determination of cell viability. Because of its high resolution at the cellular level and sensitivity, optical imaging is highly attractive for identifying cell necrosis. However, challenges associated with this technique remain present such as the rapid photobleaching of several types of organic fluorophores and/or the interference generated by biological autofluorescence. Herein, we synthesized novel biologically compatible Zn²⁺/Ln³⁺ metallacrowns (MCs) that possess attractive near-infrared (NIR) emission and are highly photostable. In addition, these MCs have the ability to label differentially necrotic HeLa cells from living cells. This work is also the first demonstration of (i) the use of the NIR emission arising from a single lanthanide­(III) cation for optical biological imaging of cells under single photon excitation, (ii) the first example of a lanthanide­(III)-based NIR-emitting probe that can be targeted to a specific type of cell

Rare Earth pcu Metal–Organic Framework Platform Based on RE₄(μ₃‑OH)₄(COO)₆²⁺ Clusters: Rational Design, Directed Synthesis, and Deliberate Tuning of Excitation Wavelengths

The T_<i>d</i> point group symmetry of rare earth (RE³⁺) metal clusters RE₄(μ₃-OH)₄(COO)₆²⁺ makes them attractive building blocks for creating metal–organic frameworks (MOFs) with controllable topologies. Herein, we describe the design and synthesis of a series of isoreticular MOFs featuring <b>pcu</b> topology [<b>MOF-1114­(RE)</b> and <b>MOF-1115­(RE)</b>] with variable rare earth metal ions (RE³⁺ = Y³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺) and linear amino-functionalized dicarboxylate linkers of different lengths. In total, we report 22 MOFs that vary in both composition and structure yet share the same RE₄(μ₃-OH)₄ cluster motif. We demonstrate that these <b>pcu</b> MOFs are cationic and that anion exchange can be used to affect the MOF properties. We also investigate the luminescence properties of a representative member of this MOF series [<b>MOF-1114­(Yb)</b>] that exhibits near-infrared emission. We show that the excitation energy for Yb³⁺ sensitization can be carefully adjusted to lower energy via covalent postsynthetic modification at the amino group sites within the MOF

Rare Earth pcu Metal–Organic Framework Platform Based on RE₄(μ₃‑OH)₄(COO)₆²⁺ Clusters: Rational Design, Directed Synthesis, and Deliberate Tuning of Excitation Wavelengths

The T_<i>d</i> point group symmetry of rare earth (RE³⁺) metal clusters RE₄(μ₃-OH)₄(COO)₆²⁺ makes them attractive building blocks for creating metal–organic frameworks (MOFs) with controllable topologies. Herein, we describe the design and synthesis of a series of isoreticular MOFs featuring <b>pcu</b> topology [<b>MOF-1114­(RE)</b> and <b>MOF-1115­(RE)</b>] with variable rare earth metal ions (RE³⁺ = Y³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺) and linear amino-functionalized dicarboxylate linkers of different lengths. In total, we report 22 MOFs that vary in both composition and structure yet share the same RE₄(μ₃-OH)₄ cluster motif. We demonstrate that these <b>pcu</b> MOFs are cationic and that anion exchange can be used to affect the MOF properties. We also investigate the luminescence properties of a representative member of this MOF series [<b>MOF-1114­(Yb)</b>] that exhibits near-infrared emission. We show that the excitation energy for Yb³⁺ sensitization can be carefully adjusted to lower energy via covalent postsynthetic modification at the amino group sites within the MOF

A Dual-Mode Near-Infrared Optical and Photoacoustic Imaging Agent Based on a Low Energy Absorbing Ytterbium Complex

Near-infrared (NIR) luminescence and photoacoustic (PA) imaging have attracted increasing attention for the real-time monitoring of biological samples due to high sensitivity, resolution, and pronounced signal detection depth, respectively. For improved contrast, both techniques require imaging agents possessing high absorption in the red-NIR range. Herein, we took advantage of a ternary complex formed with the anionic ytterbium(III) tetrakis(2-thenoyltrifluoroacetonate) ([Yb(tta)4]−) and the cationic NIR-absorbing chromophore, 1,1′-diethyl-2,2′-dicarbocyanine (Cy+), to evaluate its potential to act as a dual-mode NIR luminescence and PA imaging agent. We demonstrated that, upon excitation with red-NIR light, Cy[Yb(tta)4] encapsulated into polystyrene nanoparticles is able to generate both NIR Yb3+ emission and a PA signal in an imaging experiment performed in a tissue-mimicking phantom

Ga³⁺/Ln³⁺ Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains

Luminescent lanthanide­(III)-based molecular scaffolds hold great promises for materials science and for biological applications. Their fascinating photophysical properties enable spectral discrimination of emission bands that range from the visible to the near-infrared (NIR) regions. In addition, their strong resistance to photobleaching makes them suitable for long duration or repeated biological experiments using a broad range of sources of excitation including intense and focalized systems such as lasers (e.g., confocal microscopy). A main challenge in the creation of luminescent lanthanide­(III) complexes lies in the design of a ligand framework that combines two main features: (i) it must include a chromophoric moiety that possesses a large molar absorptivity and is able to sensitize several different lanthanide­(III) ions emitting in the visible and/or in the near-infrared, and (ii) it must protect the Ln³⁺ cation by minimizing nonradiative deactivation pathways due to the presence of −OH, −NH and −CH vibrations. Herein, a new family of luminescent Ga³⁺/Ln³⁺ metallacrown (MC) complexes is reported. The MCs with the general composition [LnGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N) (CH₃OH)] (<b>Ln-1</b>, Ln = Sm³⁺–Yb³⁺) were synthesized in a one pot reaction using salicylhydroxamic acid (H₃shi) with Ga³⁺ and Ln³⁺ nitrates as reagents. The molecular structure of [DyGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N) (CH₃OH)] was obtained by X-ray analysis of single crystals and shows that the complex is formed as a [12-MC_Ga(III)shi-4] core with four benzoate molecules bridging the central Dy³⁺ ion to the Ga³⁺ ring metals. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The extended analysis of the luminescence properties of these complexes, excited by the electronic states of the chromophoric ligands, showed the presence of characteristic, sharp f–f transitions that can be generated not only in the NIR (Sm, Dy, Ho, Er, Yb) but also in the visible (Sm, Eu, Tb, Dy, Tm). All <b>Ln-1</b> complexes possess very high quantum yield values with respect to other literature compounds, indicating a good sensitization efficiency of the [12-MC_Ga(III)shi-4] scaffold. Especially, as of today, the <b>Yb-1</b> complex exhibits the highest NIR quantum yield reported for a lanthanide­(III) complex containing C–H bonds with a value of 5.88(2)% in the solid state. This work is a significant step forward toward versatile, easily prepared luminescent lanthanide­(III) complexes suitable for a variety of applications including highly in demand biological imaging, especially in the NIR domain

Near-Infrared to Visible Light-Upconversion in Molecules: From Dream to Reality

Light-upconversion via stepwise energy transfer from a sensitizer to an activator exploits linear optics for converting low-energy infrared or near-infrared incident photons to higher energy emission. This approach is restricted to activators possessing intermediate long-lived excited states such as those found for trivalent lanthanide cations dispersed in solid-state matrices. When the activator is embedded in a molecular complex, efficient nonradiative relaxation processes usually reduce excited state lifetimes to such an extent that upconversion becomes too inefficient to be detected under practical excitation intensities. Theoretical considerations presented here predict that the combination of at least two millisecond time scale sensitizers with a central lanthanide activator in supramolecular complexes circumvents this bottleneck by creating a novel upconversion pathway, in which successive excitations are stored on the sensitizers prior to inducing stepwise energy transfer processes. Application of this concept to the chromium/erbium pair demonstrates that strong-field trivalent chromium chromophores irradiated with near-infrared photons produce upconverted green erbium-centered emission in discrete dinuclear and trinuclear triple-stranded helicates

A Tripodal Ruthenium–Gadolinium Metallostar as a Potential α_vβ₃ Integrin Specific Bimodal Imaging Contrast Agent

Gd^III-containing <i>metallostar</i> contrast agents are gaining increased attention, because their architecture allows for a slower tumbling rate, which, in turn, results in larger relaxivities. So far, these <i>metallostars</i> find possible applications as blood pool contrast agents. In this work, the first example of a tissue-selective <i>metallostar</i> contrast agent is described. This RGD-peptide decorated Ru^II(Gd^III)₃ <i>metallostar</i> is synthesized as an α_vβ₃-integrin specific contrast agent, with possible applications in the detection of atherosclerotic plaques and tumor angiogenesis. The contrast agent showed a relaxivity of 9.65 s^–1 mM^–1, which represents an increase of 170%, compared to a low-molecular-weight analogue, because of a decreased tumbling rate (τ_R = 470 ps). The presence of the MLCT band (absorption 375–500 nm, emission 525–850 nm) of the central Ru^II(Ph-Phen)₃-based complex grants the <i>metallostar</i> attractive luminescent properties. The ³MLCT emission is characterized by a quantum yield of 4.69% and a lifetime of 804 ns, which makes it an interesting candidate for time-gated luminescence imaging. The potential application as a selective MRI contrast agent for α_vβ₃-integrin expressing tissues is shown by an <i>in vitro</i> relaxometric analysis, as well as an <i>in vitro</i> <i>T</i>₁-weighted MR image