Lanthanide Texaphyrins as Photocatalysts

Mechanism of the Visible-Light-Promoted C(sp3)-H Oxidation via Uranyl Photocatalysis

Uranyl cation, as an emerging photocatalyst, has been successfully applied to synthetic chemistry in recent years and displayed remarkable catalytic ability under visible light. However, the molecular-level reaction mechanisms of uranyl photocatalysis are unclear. Here, we explore the mechanism of the stepwise benzylic C-H oxygenation of typical alkyl-substituted aromatics (i.e., toluene, ethylbenzene, and cumene) via uranyl photocatalysis using theoretical and experimental methods. Theoretical calculation results show that the most favorable reaction path for uranyl photocatalytic oxidation is as follows: first, hydrogen atom transfer (HAT) from the benzyl position to form a carbon radical ([R•]), then oxygen addition ([R•] + O2 → [ROO•]), then radical-radical combination ([ROO•] + [R•] → [ROOR] → 2[RO•]), and eventually [RO•] reduction to produce alcohols, of which 2° alcohol would further be oxidized to ketones and 1° would be stepwise-oxygenated to acids. The results of the designed verification experiments and the capture of reactive intermediates were consistent with those of theoretical calculations and the previously reported research that the active benzylic C-H would be stepwise-oxygenated in the presence of uranyl. This work deepens our understanding of the HAT mechanism of uranyl photocatalysis and provides important theoretical support for the relevant application of uranyl photocatalysts in organic transformation.

Texaphyrin-Based Calcium Sensor for Multimodal Imaging

The ability to monitor intracellular calcium concentrations using fluorescent probes has led to important insights into biological signaling processes at the cellular level. An important challenge is to relate such measurements to broader patterns of signaling across fields of view that are inaccessible to optical techniques. To meet this need, we synthesized molecular probes that couple calcium-binding moieties to lanthanide texaphyrins, resulting in complexes endowed with a diverse complement of magnetic and photophysical properties. We show that the probes permit intracellular calcium levels to be assessed by fluorescence, photoacoustic, and magnetic resonance imaging modalities and that they are detectable by multimodal imaging in brain tissue. This work thus establishes a route for monitoring signaling processes over a range of spatial and temporal scales.

A pillar[5]arene noncovalent assembly boosts a full-color lanthanide supramolecular light switch

A photo-responsive full-color lanthanide supramolecular switch was constructed from a synthetic 2,6-pyridine dicarboxylic acid (DPA)-modified pillar[5]arene (H) complexing with lanthanide ion (Ln3+ = Tb3+ and Eu3+) and dicationic diarylethene derivative (G1) through a noncovalent supramolecular assembly. Benefiting from the strong complexation between DPA and Ln3+ with a 3 : 1 stoichiometric ratio, the supramolecular complex H/Ln3+ presented an emerging lanthanide emission in the aqueous and organic phase. Subsequently, a network supramolecular polymer was formed by H/Ln3+ further encapsulating dicationic G1via the hydrophobic cavity of pillar[5]arene, which greatly contributed to the increased emission intensity and lifetime, and also resulted in the formation of a lanthanide supramolecular light switch. Moreover, full-color luminescence, especially white light emission, was achieved in aqueous (CIE: 0.31, 0.32) and dichloromethane (CIE: 0.31, 0.33) solutions by the adjustment of different ratios of Tb3+ and Eu3+. Notably, the photo-reversible luminescence properties of the assembly were tuned via alternant UV/vis light irradiation due to the conformation-dependent photochromic energy transfer between the lanthanide and the open/closed-ring of diarylethene. Ultimately, the prepared lanthanide supramolecular switch was successfully applied to anti-counterfeiting through the use of intelligent multicolored writing inks, and presents new opportunities for the design of advanced stimuli-responsive on-demand color tuning with lanthanide luminescent materials.

Properties of Amine-Containing Ligands That Are Necessary for Visible-Light-Promoted Catalysis with Divalent Europium

We describe a study of the influence of amine-containing ligands on the photoredox-relevant properties of Eu^II toward the rational design of Eu^II-containing catalysts for visible-light-promoted photoredox reactions. We report our observations of the effects of the degree of functionalization of amines, denticity, and macrocylic ligands on the absorbance of Eu^II. Ligands that contain secondary amines bathochromically shift the absorbance of EuCl₂ relative to ligands that contain primary or tertiary amines. Similarly, ligands of larger denticity have a larger bathochromic shift of the absorbance than ligands of smaller denticity. We observed that macrocyclic ligands have a larger effect on the absorbance of EuCl₂ than nonmacrocyclic ligands. Also, we report the photoredox reactivity of four new Eu^II-containing complexes. These observations are potentially influential in understanding the ligand properties that promote the use of Eu^II in visible-light-promoted photoredox catalysis.

Lanthanide Luminescence in Visible-Light-Promoted Photochemical Reactions

The excitation of lanthanides with visible light to promote photochemical reactions has garnered interest in recent years. Lanthanides serve as initiators for photochemical reactions because they exhibit visible-light-promoted 4f→5d transitions that lead to emissive states with electrochemical potentials that are more negative than the corresponding ground states. The lanthanides that have shown the most promising characteristics for visible-light promoted photoredox are Sm^II, Eu^II, and Ce^III. By understanding the effects that ligands have on the 5d orbitals of Sm^II, Eu^II, and Ce^III, luminescence and reactivity can be rationally modulated using coordination chemistry. This review briefly overviews the photochemical reactivity of Sm^II, Eu^II, and Ce^III with visible light; the properties that influence the reactivity of these ions; and the research that has been reported towards modulating their photochemical-relevant properties using visible light and coordination chemistry.