Near-infrared (NIR) luminescent lanthanide complexes hold great promise for practical applications, as their optical properties have several complementary advantages over organic fluorophores and semiconductor nanoparticles. The fundamental challenge for lanthanide luminescence is their sensitization through suitable chromophores. The use of the metallacrown (MC) motif is an innovative strategy to arrange several organic sensitizers at a well-controlled distance from a lanthanide cation. Herein we report a series of lanthanide “encapsulated sandwich” MC complexes of the form Ln3+[12-MCZn(II),quinHA-4]2[24-MCZn(II),quinHA-8] (Ln3+[Zn(II)MCquinHA]) in which the MC framework is formed by the self-assembly of Zn2+ ions and tetradentate chromophoric ligands based on quinaldichydroxamic acid (quinHA). A first-generation of luminescent MCs was presented previously but was limited due to excitation wavelengths in the UV. We report here that through the design of the chromophore of the MC assembly, we have significantly shifted the absorption wavelength toward lower energy (450 nm). In addition to this near-visible inter- and/or intraligand charge transfer absorption, Ln3+[Zn(II)MCquinHA] exhibits remarkably high quantum yields, long luminescence lifetimes (CD3OD; Yb3+, QLnL = 2.88(2)%, τobs = 150.7(2) μs; Nd3+, QLnL = 1.35(1)%, τobs = 4.11(3) μs; Er3+, QLnL = 3.60(6)·10–2%, τobs = 11.40(3) μs), and excellent photostability. Quantum yields of Nd3+ and Er3+ MCs in the solid state and in deuterated solvents, upon excitation at low energy, are the highest values among NIR-emitting lanthanide complexes containing C–H bonds. The versatility of the MC strategy allows modifications in the excitation wavelength and absorptivity through the appropriate design of the ligand sensitizer, providing a highly efficient platform with tunable properties.

Oscillators omitted: A self‐assembled inclusion complex between a LnIII[12‐metallacrown‐4]23+ sandwich motif (green and bronze) and a [24‐metallacrown‐8] (purple) is stable in methanol. The YbIII complex has a large quantum yield (0.89 %) and luminescent lifetime (14 μs) in methanol, which are attributed to the exclusion of high‐energy oscillators from within 6.7 Å of the emitting YbIII ion by the metallacrown topology.

We demonstrate adsorption and depolymerization of long-chain β-glu strands derived from cellulose, within the microporous confines of a zeolite-templated carbon (ZTC) material. The ZTC adsorbs β-glu strands that have a radius of gyration several-fold larger than the ZTC micropore diameter and do so rapidly (less than 2 min) and in adsorbed β-glu coverages of up to 80% of the ZTC mass. Principles of supramolecular chemistry predict that such adsorption occurs inside of the ZTC based on its micropore size as host being nearly ideal for glucan guest. A comparative study of partially etched materials and nitrogen physisorption at −196 °C indeed demonstrates β-glu adsorption to occur within internal ZTC micropores rather than on the external surface. Such adsorption under micropore confinement is expected to place significant mechanical strain on the β-glu strand, and this strain can be in principle relieved by depolymerization via hydrolysis. This hypothesis motivated us to investigate depolymerization of adsorbed β-glu strands in ZTC, where the ZTC serves as a catalyst for adsorbed β-glu hydrolysis. After a 3 h treatment in water at 180 °C, adsorbed β-glu was converted to soluble glucose in 73% yield. This represents the highest glucose yield observed to date for a carbon catalyst without postsynthetic surface functionalization and speaks to the effectiveness of weak-acid sites for β-glu hydrolysis within a constrained micropore environment.

Two intermediates in the assembly of lanthanide metallacrowns (MCs) of divalent transition metals and ligands in the picoline hydroxamic acid (picHA)/α-amino hydroxamic acid family were synthesized and crystallographically characterized. Structures of the elusive M(II)[12-MC(M(II),L)-4](2+) were obtained with M = Ni, Zn and L = picHA, quinaldic hydroxamic acid. Consistent with previous calculations, the complex is highly concave, particularly with Zn(II). ESI-MS and (1)H NMR reveal that the complexes retain their structure in solution. The Zn(II) analogue reacts with Ln(III) ions to form Ln(III)[15-MC(Zn(II),picHA)-5](3+) in pyridine. The greater stability of Zn(II)[12-MC(Zn(II),picHA)-4](2+) relative to the Cu(II) and Ni(II) analogues is inferred and attributed to the square-pyramidal Zn(II) ions being complementary with the concave MC topology. A Zn4(picHA)2(OAc)4(DMF)2 species bearing a tetranuclear [6-MC(Zn(II),picHA)-2] motif was also isolated. A mechanism for M(II)[12-MC(M(II),L)-4](2+) formation is proposed on the basis of structural analysis of tetranuclear [6-MC(M(II),L)-2] complexes. These results contribute to the goal of controlling the reactivity of intermediates in the assembly of lanthanide MCs, and coordination driven macrocycles in general, to prepare complexes with greater stability or enhanced physical properties.

Dimeric Ln(3+)[15-metallacrown-5] compartments selectively recognize carboxylates through guest binding to host metal ions and intermolecular interactions with the phenyl side chains. A systematic study is presented on how the size, selectivity, and number of encapsulated guests in the dimeric containers is influenced by the Ln(3+)[15-metallacrown(Cu(II))-5] ligand side chain and central metal. Compartments of varying heights were assembled from metallacrowns with S-phenylglycine hydroxamic acid (pgHA), S-phenylalanine hydroxamic acid (pheHA), and S-homophenylalanine hydroxamic acid (hpheHA) ligands. Guests that were examined include the fully deprotonated forms of terephthalic acid, isonicotinic acid, and bithiophene dicarboxylic acid (btDC). X-ray crystallography reveals that the side-chain length constrains the maximum and minimum length guest that can be encapsulated in the compartment. Compartments with heights ranging from 9.7 to 15.2 Å are formed with different phenyl side chains that complex 4.3-9.2 Å long guests. Up to five guests are accommodated in Ln(3+)[15-metallacrown(Cu(II))-5] compartments depending on steric effects from the host side chains. The nine-coordinate La(3+) central metal promotes the encapsulation of multiple guests, while the eight-coordinate Gd(3+) typically binds only one dicarboxylate. Electrospray ionization mass spectrometry reveals that the dimerization phenomenon occurs beyond the solid state, suggesting that these containers can be utilized in solid-state and solution applications.