Porphocyanines: Expanded Aromatic Tetrapyrrolic Macrocycles

Design of Boron and Transition Metal Embedded Two-Dimensional Porous Carbon Nitride for Electrocatalytic Synthesis of Urea

Electrocatalytic coupling of CO and N2 to synthesize urea under ambient conditions is considered a promising strategy to replace traditional industrial technology. It is crucial to find efficient electrocatalysts that can adsorb and activate N2 and promote the C-N coupling reaction. Herein, a new two-dimensional porous carbon nitride material with multiactive sites is designed, in which boron and transition metal are embedded. Through a series of screening, B2Cr2, B2Mn2, and B2Os2 are predicted to be potential electrocatalysts for urea synthesis. Mechanistic studies are performed on bidentate metal-metal and metal-boron sites, and both NCON and CO mechanisms are explored. The electronic structure analysis shows that there is a strong N2 chemical adsorption within the bidentate site and that the N≡N bond is significantly activated. A new mechanism where free CO is inserted for C-N coupling within the two-dimensional porous structure is proposed.

Design of a Four-Atom Cluster Embedded in Carbon Nitride for Electrocatalytic Generation of Multi-Carbon Products

The development of efficient and stable catalysts for the electrocatalytic CO₂ and CO reduction reactions (CORR) is under active investigation, but the problems of poor selectivity and low efficiency for C₂ products still exist. We design a two-dimensional carbon nitride material (C₅N₂H₂) that contains an eight N-atom structure capable of coordinating four-metal atom clusters and supporting simultaneously two carbon oxide molecules needed for the C₂ coupling. The designed material has excellent electrical conductivity and stability. After high-throughput screening of catalytic performance of multiple four-metal clusters embedded into the framework, we systematically investigate the CORR process of 11 candidates. We find that Cu₄-C₅N₂H₂ has superior selectivity and low limiting potential for generating ethylene, while Cu₂Zn₂-C₅N₂H₂ is selective and efficient to synthesize ethanol. Further, we discover a novel type of descriptor related to 2D material flexibility to evaluate the potential-determining step for generating ethylene. Our report both broadens the possibilities for few-atom CO reduction and demonstrates a novel substrate flexibility-related descriptor to predict the catalytic performance of materials.

5,20-Diheterohexaphyrins: metal-template-free synthesis and aromaticity switching

As the first example of expanded heteroporphyrins with heteroatoms at meso-positions, 5,20-dithiahexaphyrins and 5,20-diazahexaphyrins were synthesized via nucleophilic substitution reactions of α,α'-dibromotripyrrin as the key step. While 5,20-dithiahexaphyrins are practically nonaromatic, 5,20-[28]diazahexaphyrins show weakly antiaromatic characters and can be cleanly converted into aromatic 5,20-[26]diazahexaphyrins upon oxidation with PbO2, demonstrating distinct aromaticity switching with maintenance of dumbbell-like conformations.

Schiff-base porphyrin and expanded porphyrin analogs

In recent years, the field of porphyrin chemistry has expanded to include several new analogs of the original four pyrrole, carbon-bridged systems. This review traces the development of one such class of new macrocycles, namely Schiff-base porphyrin analogs. The review's focus is on the synthesis and properties of these new Schiff-base macrocycles that for ease of division have been divided into four classes: systems with two pyrrole rings, systems with three pyrrole or heterocyclic rings, systems with four pyrrole or heterocyclic rings, and systems with five or more pyrrole or heterocyclic rings. In addition to the role of metal ions as templating agents in the synthesis of these complexes, the rich coordination chemistry of several of the macrocycles is also discussed. X-ray crystallography has played an important role in determining the structures of many of these Schiff-base porphyrin analogs and many of the available structures have been incorporated into this review. Aromaticity of the macrocycles is discussed and has been evaluated from available electronic and NMR spectra. Finally, several potential applications of these molecules are discussed, briefly.

A versatile building block for pyrazole–pyrrole hybrid macrocycles

This contribution describes the synthesis of a novel pyrazole-pyrrole building block and its use in the formation of a non-aromatic, Schiff base-type macrocycle incorporating a chromophore and H-bonding donor and acceptor functionalities inside and outside of the macrocycle, which makes it predestined for molecular recognition systems.

Effect of Aza Substitution on the Photophysical and Electrochemical Properties of Porphycenes: Characterization of the Near-IR-Absorbing Photosensitizers 2,7,12,17-Tetrakis(p-substituted phenyl)-3,6,13,16-tetraazaporphycenes

The need for new photodynamic-therapy photosensitizers has stimulated the search of new families of compounds absorbing strongly in the 700-900 nm range, the region where tissue is most transparent to radiation capable to induce the photodynamic effect. Using computational chemistry techniques, 3,6,13,16-tetraazaporphycenes were previously identified as interesting target candidates. This work reports on the photophysical and electrochemical properties of selected members of this new family of macrocycles. Compared to porphycenes, the tetra-aza counterparts show stronger absorption in the near-infrared, lower-lying singlet and triplet excited states, and substantially larger internal conversion quantum yield (Phi(IC) = 0.93). Energy transfer to oxygen is observed, which results in the formation of the cytotoxic species singlet oxygen. The process is found to be reversible, consistent with a triplet-energy value close to that of singlet oxygen.