Learning from Nature's Example: Repair Strategies in Light-Driven Catalysis

The continuous repair of subunits of the photosynthetic apparatus is a key factor determining the overall efficiency of biological photosynthesis. Recent concepts for repairing artificial photocatalysts and catalytically active materials within the realm of solar fuel formation show great potential in reshaping the research directions within this field. This perspective describes the latest advances, concepts, and mechanisms in the field of catalyst repair and catalyst self-healing and provides an outlook on which additional steps need to be taken to bring artificial photosynthetic systems closer to real-life applications.

Self-Healing of Polymers and Polymer Composites

This review is devoted to the description of methods for the self-healing of polymers, polymer composites, and coatings. The self-healing of damages that occur during the operation of the corresponding structures makes it possible to extend the service life of the latter, and in this case, the problem of saving non-renewable resources is simultaneously solved. Two strategies are considered: (a) creating reversible crosslinks in the thermoplastic and (b) introducing a healing agent into cracks. Bond exchange reactions in network polymers (a) proceed as a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration, or as an associative process, the limiting stage of which is the interaction of the reactive end group and the crosslink. The latter process is implemented in vitrimers. Strategy (b) is associated with the use of containers (hollow glass fibers, capsules, microvessels) that burst under the action of a crack. Particular attention is paid to self-healing processes in metallopolymer systems.

Metal-Terpyridine Assembled Functional Materials for Electrochromic, Catalytic and Environmental Applications

Molecular assembly induced by metal-terpyridine-based coordinative interactions has become an emergent research topic due to its ease of synthesis and diverse applications. This article highlights recent significant developments in the metal-terpyridine-based supramolecular architectures. At first, the design aspect of the molecular building blocks has been described, followed by elaboration on how the ligand backbone plays an important role for achieving different dimensionalities of the resulting assemblies which exhibit a wide range of potential applications. After that, we discussed different synthetic approaches for constructing these assemblies, and finally, we focused on their significant developments in three specific areas, viz., electrochromic materials, catalysis and a new application in wastewater treatment. In the field of electrochromic materials, these assemblies made important advancements in various aspects like sub-second switching time (<1 s), low switching voltage (<1 V), increased switching stability (>10000 cycles), tuning of multiple colors by using multimetallic systems, fabrication of charge storing electrochromic devices, utilizing and storing solar energy etc. Similarly, the catalysis field witnessed application of the metal-terpyridine assemblies in C-H monohalogenation, heterogeneous Suzuki-Miyaura coupling, photocatalysis, reduction of carbon dioxide, etc. Finally, the environmental application of these coordination assemblies includes capturing Cr(VI) from waste water efficiently with high capture capacity, good recyclability, wide pH independency etc.

Heterogeneous photocatalysis in flow chemical reactors

The synergy between photocatalysis and continuous flow chemical reactors has shifted the paradigms of photochemistry, opening new avenues of research with safer and scalable processes that can be readily implemented in academia and industry. Current state-of-the-art photocatalysts are homogeneous transition metal complexes that have favourable photophysical properties, wide electrochemical redox potentials, and photostability. However, these photocatalysts present serious drawbacks, such as toxicity, limited availability, and the overall cost of rare transition metal elements. This reduces their long-term viability, especially at an industrial scale. Heterogeneous photocatalysts (HPCats) are an attractive alternative, as the requirement for the separation and purification is largely removed, but typically at the cost of efficiency. Flow chemical reactors can, to a large extent, mitigate the loss in efficiency through reactor designs that enhance mass transport and irradiation. Herein, we review some important developments of heterogeneous photocatalytic materials and their application in flow reactors for sustainable organic synthesis. Further, the application of continuous flow heterogeneous photocatalysis in environmental remediation is briefly discussed to present some interesting reactor designs that could be implemented to enhance organic synthesis.

Self-healing and shape memory metallopolymers: state-of-the-art and future perspectives

Recent achievements and problems associated with the use of metallopolymers as self-healing and shape memory materials are presented and evaluated.

[Cr(ttpy)2]3+ as a multi-electron reservoir for photoinduced charge accumulation

[Cr(ttpy)2]3+ (ttpy = 4'-(4-methylphenyl)-2,2':6,2''-terpyridine) exhibits rich electrochemical and photophysical properties. Cyclic voltammetry performed in CH3CN shows in the cathodic part the presence of three one-electron reversible systems at -0.47, -0.85 and -1.35 V vs. Ag/AgNO3 10-2 M. These systems are attributed to the reduction of the terpyridine ligands with a partial delocalization of the charge on the tolyl for the last reduction event. The three different reduced species were generated by exhaustive electrolysis and characterized by EPR and UV-visible spectroscopy; DFT calculations were performed to locate the spin density of the electrons added during the reduction. Visible light irradiation of [Cr(ttpy)2]3+ induces the population of a luminescent metal-centered excited state with a lifetime of 270 ns in deoxygenated CH3CN. This excited state can be quenched by an electron transfer process with triphenylphosphine (PPh3) or triethanolamine (TEOA). Using TEOA as a sacrificial electron donor, the doubly reduced species (i.e.[Cr(ttpy)2] +) can be generated under continuous irradiation. In the presence of [Ru(bpy)3]2+ as an additional photosensitizer, the photoreduction of [Cr(ttpy)2]3+ towards [Cr(ttpy)2]+ is accelerated. The trinuclear [{RuII(bpy)2(bpy-O-tpy)}2CrIII]7+ complex ([Ru2Cr]7+) in which a CrIII-bis-terpyridine centre is covalently linked to two RuII-tris-bipyridine moieties by oxo bridges has been synthesised. Its electrochemical, photophysical and photochemical properties were investigated in deoxygenated CH3CN. Cyclic voltammetry indicates only a poor electronic communication between the different subunits, whereas luminescence experiments show a strong quenching of the RuII* excited state by an intramolecular process. Continuous irradiation of [Ru2Cr]7+ under visible conditions in the presence of TEOA leads to [Ru2Cr]4+ where three electrons are stored on the [Cr(ttpy)] subunit.

Reversible Assembly of Terpyridine Incorporated Norbornene-Based Polymer via a Ring-Opening Metathesis Polymerization and Its Self-Healing Property

We induced a terpyridine moiety into a norbornene-based polymer to demonstrate its self-healing property, without an external stimulus, such as light, heat, or healing agent, using metal⁻ligand interactions. We synthesized terpyridine incorporated norbornene-based polymers using a ring-opening metathesis polymerization. The sol state of diluted polymer solutions was converted into supramolecular assembled gels, through the addition of transition metal ions (Ni2+, Co2+, Fe2+, and Zn2+). In particular, a supramolecular complex gel with Zn2+, which is a metal with a lower binding affinity, demonstrated fast self-healing properties, without any additional external stimuli, and its mechanical properties were completely recovered.

Photoactive Molecular Dyads [Ru(bpy)3-M(ttpy)2] n+ on Gold (M = Co(III), Zn(II)): Characterization, Intrawire Electron Transfer, and Photoelectric Conversion

We propose in this work a stepwise approach to construct photoelectrodes. This takes advantage of the self-assembly interactions between thiol with a gold surface and terpyridine ligands with first-row transition metals. Here, a [Ru(bpy)₃]²⁺ photosensitive center bearing a free terpyridine group has been used to construct two linear dyads on gold (Au/[Zn^II-Ru^II]⁴⁺ and Au/[Co^III-Ru^II]⁵⁺). The stepwise construction was characterized by electrochemistry, quartz crystal microbalance, and atomic force microscopy imaging. The results show that the dyads behave as rigid layers and are inhomogeneously distributed on the surface. The surface coverages are estimated to be in the order of 10^-11 mol cm^-2. The kinetics of the heterogeneous electron transfer is determined on modified gold ball microelectrodes using Laviron's formula. The oxidation rates of the terminal Ru(II) subunits are estimated to be 700 and 2300 s^-1 for Au/[Zn^II-Ru^II]⁴⁺ and Au/[Co^III-Ru^II]⁵⁺, respectively. In the latter case, the rate is limited by the kinetics of electron transfer between an intermediate Co(II) center and the gold surface. For Au/[Zn^II-Ru^II]⁴⁺, the Zn-bis-terpyridine center is not involved in the electron-transfer process and the oxidation of the Ru(II) subunit occurs through a superexchange process. In the presence of a tertiary amine in solution, the electrodes at a bias of 0.12 V behave as photoanodes when subjected to visible light irradiation. The magnitude of the photocurrent is around 10 μA cm^-2 for Au/[Co^III-Ru^II]⁵⁺ and 5 μA cm^-2 for Au/[Zn^II-Ru^II]⁴⁺, proving the importance of an electron relay on the photon-to-current conversion. The results suggest an efficient conversion for Au/[Co^III-Ru^II]⁵⁺, since each bound dyad, once excited, injects an electron around 10 times per second.

Terpyridine-functionalized stimuli-responsive microgels and their assembly through metal–ligand interactions

We developed a terpyridine-functionalized microgel (tpy-mG) for its supramolecular assembly.