One-Step Assembly of Visible and Near-Infrared Emitting Metallacrown Dimers Using a Bifunctional Linker
Résumé
A family of dimeric Ln III [12-MCGa(III)N(shi)-4] metallacrowns (MCs) (Ln III = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) was synthesized using the isophthalate group (ip 2-) as a linker. The [LnGa4]2 complexes exhibit remarkable photophysical properties, with large molar absorptivities of ~ 4×10 4 M-1 cm-1 , high quantum yields and long luminescence lifetimes with values of (i) 31.2(2)% and 1.410(1) ms respectively for the visible-emitting [TbGa4]2 complex and (ii) 2.43(6)% and 30.5(1) µs for the near-infrared (NIR) emitting [YbGa4]2 in the solid state. The NIR emission was obtained not only from Yb, Nd, and Er complexes but also from the less frequently observed emitters such as Pr and Ho. In addition, emission in both visible and NIR domains could be detected for Dy and Sm MCs. ESI-MS and UV-Vis data revealed that the complexes are highly stable in dimethylsulfoxide (DMSO) solution with the 1 Hand COSY-NMR spectra of the diamagnetic [YGa4]2 analogue providing evidence for long-term solution stability. This new approach allows one to construct a basis for highly luminescent MCs that may be further modified to be adapted for applications such as optical imaging. Metallacrowns (MCs) are a unique class of inorganic macrocycles forming repeating [Metal-N-O] subunits with a large variety of structures of different sizes and topologies being synthesized in the last two decades. [1] These MC complexes typically use tetradentate hydroximate types of ligands such as salicylhydroximate (shi 3-) (Figure 1) with oxygen atoms oriented toward the center of the ring allowing an efficient pre-organization for guest encapsulation in a manner similar to classical organic crown ethers. Accordingly, MCs coordinate a central metal cation through these oxygen binding sites and provide an appealing route toward the a priori design of multimetallic complexes. Recently, lanthanide (Ln III)-encapsulated MCs have drawn strong interest due to their remarkable luminescent properties in the near-infrared (NIR) range, with the highest quantum yield values reported to date for Nd III and Yb III complexes containing C-H bonds. [2] In addition, the MC approach has led to families of complexes displaying luminescent sensitization of multiple lanthanide ions in both visible and NIR regions, [2c] a feature that has been described only for a few systems. [3] Very recently, we have reported the first direct applications of NIR-emitting MCs for the selective imaging of necrotic cells versus apoptotic or healthy cells, or as simultaneous cell fixation and counter-staining agents. [4] For many applications such as optical imaging, the total number of emitted photons is crucial for detection sensitivity. Since most of the f-f transitions are forbidden, free lanthanide cations have very low absorption cross sections and need to be sensitized by highly absorbing chromophoric groups that will transfer the resulting energy to the accepting electronic levels of the Ln III. This process is named "antenna effect" [5] and has a high impact on the total number of emitted photons. A major advantage of Ln III-based MC imaging agents is that they can be constituted with chromophoric aromatic ligands connected to several diamagnetic metal ions creating a rigid tridimensional structure with large absorptivity. This assembly allows to establish a controlled, constant and relatively long distance between the Ln III and the O-H, N-H or C-H oscillators located on these chromophoric ligands. Such organization is advantageous as overtones of these vibrations can quench significantly Ln III emission if the distance between the two partners is very short. [6] In addition, MCs incorporate a large number of chromophoric ligands, increasing the absorption of each discrete MC molecule, and in turn the number of emitted photons. All mentioned features of the MC structure should contribute to the increase of the overall luminescence intensity and brightness of the complexes. Figure 1. Structures of salicylhydroxamic acid (H3shi) (left) and isophthalic acid (H2ip) (right). One particularly promising class of MCs previously reported by our group, [2c] Ln III (benzoate)4[12-MCGa(III)N(shi)-4] (hereafter LnGa4), are assembled by the reaction between salicylhydroxamic acid (H3shi) with sodium benzoate, pyridine, gallium (Ga) and lanthanide nitrates to form a series of LnGa4 complexes. These MCs are attractive precursors for higher nuclearity MCs because of the presence of four benzoate ligands on the same side of the Ga4 plane (Figure 2, top-left), which could favor the formation of [a] Dr.
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