Lanthanide-tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

Tetrapyrrole derivatives such as porphyrins, phthalocyanines, naphthalocyanines, and porpholactones, are highly stable macrocyclic compounds that play important roles in many phenomena linked to the development of life. Their complexes with lanthanides are known for more than 60 years and present breath-taking properties such as a range of easily accessible redox states leading to photo- and electro-chromism, paramagnetism, large non-linear optical parameters, and remarkable light emission in the visible and near-infrared (NIR) ranges. They are at the centre of many applications with an increasing focus on their ability to generate singlet oxygen for photodynamic therapy coupled with bioimaging and biosensing properties. This review first describes the synthetic paths leading to lanthanide-tetrapyrrole complexes together with their structures. The initial synthetic protocols were plagued by low yields and long reaction times; they have now been replaced with much more efficient and faster routes, thanks to the stunning advances in synthetic organic chemistry, so that quite complex multinuclear edifices are presently routinely obtained. Aspects such as redox properties, sensitization of NIR-emitting lanthanide ions, and non-linear optical properties are then presented. The spectacular improvements in the quantum yield and brightness of Yb^III-containing tetrapyrrole complexes achieved in the past five years are representative of the vitality of the field and open welcome opportunities for the bio-applications described in the last section. Perspectives for the field are vast and exciting as new derivatizations of the macrocycles may lead to sensitization of other Ln^III NIR-emitting ions with luminescence in the NIR-II and NIR-III biological windows, while conjugation with peptides and aptamers opens the way for lanthanide-tetrapyrrole theranostics.

Carbon Dioxide (CO2) Fixation: Linearly Bridged Zn2 Paddlewheel Nodes by CO2 in a Metal-Organic Framework

When the reaction of zinc nitrate with 4',4‴,4‴″,4‴‴'-(ethene-1,1,2,2-tetrayl)tetrakis[(1,1'-biphenyl-3-carboxylic acid)] (H₄tmpe) in dimethylformamide (DMF) under hydrothermal condition is performed in air or carbon dioxide (CO₂), [Zn₄(tmpe)₂(H₂O)₂(μ₂-CO₂)]·8DMF·18H₂O (1) crystallizes out. However, if it is in dioxygen, argon, or carbon monoxide, [Zn₂(tmpe)(DMF)]·2DMF·8H₂O (2) is the product. Both compounds are chemically stable coordination polymers. 1 contains zinc carboxylate paddlewheels as nodes linearly bridged by CO₂ into two interpenetrating lattices, and 2 has an infinite single framework formed by a tetranuclear node. 1 is the second example containing the linear CO₂-bridged paddlewheel node. Interestingly, CO₂ fixation in a μ₂-η²_O,O bridging mode is observed in 1, which is rarely characterized structurally and has been confirmed using IR and gas chromatography analysis. The stability of 1 is further verified by density functional theory calculations, which found an energy minimum with a Zn-O═C angle of 180°. Both compounds display strong emission around 490 nm and excited-state lifetimes around 2.4 ns.

Location controlled symmetry reduction: paradigm of an open metalloporphyrin framework based on the tetracarboxy porphyrin linker

The change in position of coordinating groups on symmetrical tetracarboxy porphyrin leads to novel class of reduced symmetry linker, which lay down pathway to obtain versatile coordination architectures to trap geometrically variant guest molecules.

Temperature dependent CO2 behavior in microporous 1-D channels of a metal-organic framework with multiple interaction sites

The MOF with the encapsulated CO₂ molecule shows that the CO₂ molecule is ligated to the unsaturated Cu(II) sites in the cage using its Lewis basic oxygen atom via an angular η¹-(O_A) coordination mode and also interacts with Lewis basic nitrogen atoms of the tetrazole ligands using its Lewis acidic carbon atom. Temperature dependent structure analyses indicate the simultaneous weakening of both interactions as temperature increases. Infrared spectroscopy of the MOF confirmed that the CO₂ interaction with the framework is temperature dependent. The strength of the interaction is correlated to the separation of the two bending peaks of the bound CO₂ rather than the frequency shift of the asymmetric stretching peak from that of free CO₂. The encapsulated CO₂ in the cage is weakly interacting with the framework at around ambient temperatures and can have proper orientation for wiggling out of the cage through the narrow portals so that the reversible uptake can take place. On the other hand, the CO₂ in the cage is restrained at a specific orientation at 195 K since it interacts with the framework strong enough using the multiple interaction sites so that adsorption process is slightly restricted and desorption process is almost clogged.

Ameliorated synthetic methodology for crystalline lanthanoid–metalloporphyrin open frameworks based on a multitopic octacarboxy-porphyrin scaffold: structural, gas sorption and photophysical properties

A novel synthetic methodology has been applied to obtain sizeable single crystals of wide-pore porphyrin-based MOFs.

Linearly bridging CO2 in a metal–organic framework

A very rare CO₂-coordinated metal–organic framework was structurally confirmed by single-crystal X-ray diffraction. The CO₂ ligand links two open Zn metal centers in an absolutely linear μ(O,O′) coordination mode with a CO distance of 1.107(4) Å. The new complex reported here is stable under ambient conditions and may provide a new strategy for CO₂ fixation.