This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Photocatalysis in organic and polymer synthesis

It is commonly accepted that it is almost not possible to realize the large-scale practical application of fuel cells if the expensive noble metal-based electrocatalysts for oxygen reduction reactions (ORR) cannot be replaced by other low-cost, efficient, and stable ones. Herein, our studies demonstrate that iron phthalocyanine (FePc) supported on chemically reduced graphene through π–π interaction can act as a noble metal-free electrocatalyst with a comparable activity, long-term operation stability, and better tolerance to methanol crossover and CO poisoning compared with commercial Pt/C for ORR in alkaline media. The improved electrochemical activity and stability of FePc by graphene is mainly attributed to the inherent properties of graphene and the π-stacking interaction between FePc and planar aromatic structure of graphene. The as-prepared graphene–iron phthalocyanine (g-FePc) composite exhibits an efficient 4-electron pathway and can be used as a promising Pt-free ORR electrocatalyst.

Enhanced Catalytic Performance of Pt-Free Iron Phthalocyanine by Graphene Support for Efficient Oxygen Reduction Reaction

This review provides an overview of the literature regarding heterogeneous molecular catalysts for electrochemical CO2 reduction (ECR). Fundamental aspects of the science, including aggregation, electrochemical rate laws, and electrode-catalyst electronic coupling, are discussed to provide a solid foundation on which to design experiments and interpret results. Mechanistic aspects of ECR are presented based on electrokinetic and spectroscopic measurements as well as density functional theory (DFT) calculations. Consensus is improving for electrokinetic measurements, but the redox state of the metal center under reaction conditions and DFT reaction pathways lack agreement in the literature. Concerning the tunable aspects of the molecular catalyst, the impacts of the metal center, ligand substituents, and electrode support on the activity and selectivity toward ECR are presented with an emphasis on those studies that controlled for aggregation and minimized mass-transport limitations. Extended three-dimensional (3D) structures such as polymers, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) are discussed as highly tunable architectures that begin to mimic the catalytic pockets of enzyme active sites. To achieve the full potential of these catalysts, design principles must emerge based on a combination of deconvoluting measurements to extract intrinsic catalyst properties and more reliable theoretical calculations to predict reaction pathways.

https://link.springer.com/content/pdf/10.1007/s12274-019-2403-y.pdf

Heterogeneous molecular catalysts for electrocatalytic CO 2 reduction

The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg−1. By the use of heterogeneous catalyst cobalt phthalocyanine on a carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 °C (83.2 °C measured temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations.

/pdf/macroscopic-heat-release-in-a-molecular-solar-thermal-energy-5scgs6rwgm.pdf

Macroscopic Heat Release in a Molecular Solar Thermal Energy Storage System

We report for the first time the use of phthalocyanine compounds, in conjunction with the common solvent N-methyl-2-pyrrolidone (NMP), as a photoinitiation system for radical polymerization under various visible and near-infrared wavelengths. A library of redox-inactive metallic phthalocyanine compounds were investigated for their ability to act as photosensitizers in these new two-component photoinitiation systems, for the radical polymerization of (meth)acrylate and (meth)acrylamide monomers. Conventional radical polymerization under these conditions was successful, and moreover, the addition of reversible addition–fragmentation chain transfer (RAFT) agents to these systems led to the production of polymers with molecular weights in agreement with theoretical predictions and molecular weight distributions typically lower than 1.15. In contrast to our previous systems described as photoinduced electron/energy transfer–reversible addition–fragmentation chain transfer (PET-RAFT) where RAFT agent is activat...

A Photoinitiation System for Conventional and Controlled Radical Polymerization at Visible and NIR Wavelengths

We investigated quantitatively the solubility properties of a number of unsubstituted metal phthalocyanines with various metal atoms such as Co, Cu, Fe, Mg, Mn, Ni, Sn, and Zn. We studied three categories of solvents: conventional solvents, ionic liquids, and acids. The conventional liquids comprised 20 typical representatives from acetone to toluene. We tested two ionic liquids and five acids. For each solution we measured the molar absorption coefficient and the saturation concentration (or estimated these values if the solubility was too low). The absorption coefficients and saturation concentrations were derived from UV–vis absorption measurements of the Q-band. The solution spectra are presented and discussed as well as the chemical properties (stability) of the solutions.

Juliane Kristen

Papers

Solubility Properties of Unsubstituted Metal Phthalocyanines in Different Types of Solvents