Charge Accumulation and Multi‐Electron Photoredox Chemistry with a Sensitizer–Catalyst–Sensitizer Triad

Engineering Soluble Diketopyrrolopyrrole Chromophore Stacks from a Series of Pd(II)-Based Ravels

A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd3 L6 three-ring, a heteroleptic Pd2 L2 L'2 ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd2 L3 triple and a Pd2 L4 quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd2 L2 L'2 , which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.

Photochemical charge accumulation in a heteroleptic copper(i)-anthraquinone molecular dyad via proton-coupled electron transfer

Developing efficient photocatalysts that perform multi electron redox reactions is critical to achieving solar energy conversion. One can reach this goal by developing systems which mimic natural photosynthesis and exploit strategies such as proton-coupled electron transfer (PCET) to achieve photochemical charge accumulation. We report herein a heteroleptic Cu(i)bis(phenanthroline) complex, Cu-AnQ, featuring a fused phenazine-anthraquinone moiety that photochemically accumulates two electrons in the anthraquinone unit via PCET. Full spectroscopic and electrochemical analyses allowed us to identify the reduced species and revealed that up to three electrons can be accumulated in the phenazine-anthraquinone ring system under electrochemical conditions. Continuous photolysis of Cu-AnQ in the presence of sacrificial electron donor produced doubly reduced monoprotonated photoproduct confirmed unambiguously by X-ray crystallography. Formation of this photoproduct indicates that a PCET process occurred during illumination and two electrons were accumulated in the system. The role of the heteroleptic Cu(i)bis(phenanthroline) moiety participating in the photochemical charge accumulation as a light absorber was evidenced by comparing the photolysis of Cu-AnQ and the free AnQ ligand with less reductive triethylamine as a sacrificial electron donor, in which photogenerated doubly reduced species was observed with Cu-AnQ, but not with the free ligand. The thermodynamic properties of Cu-AnQ were examined by DFT which mapped the probable reaction pathway for photochemical charge accumulation and the capacity for solar energy stored in the process. This study presents a unique system built on earth-abundant transition metal complex to store electrons, and tune the storage of solar energy by the degree of protonation of the electron acceptor.

Ruthenium polypyridine complexes containing prenyl groups as antibacterial agents against Staphylococcus aureus through a membrane-disruption mechanism

Four new ruthenium polypyridyl complexes with prenyl groups, [Ru(bpy)2 (MHIP)](PF6 )2 (Ru(II)-1), [Ru(dtb)2 (MHIP)](PF6 )2 (Ru(II)-2), [Ru(dmb)2 (MHIP)](PF6 )2 (Ru(II)-3), and [Ru(dmob)2 (MHIP)](PF6 )2 (Ru(II)-4) (bpy = 2,2'-bipyridine, dtb = 4,4'-di-tert-butyl-2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, dmob = 4,4'-dimethoxy-2,2'-bipyridine, and MHIP = 2-(2,6-dimethylhepta-1,5-dien-1-yl)-1H-imidazo[4,f][1,10]phenanthroline), were synthesized and characterized. Their antibacterial activities against Staphylococcus aureus were assessed, and the minimum inhibition concentration (MIC) value of Ru(II)-2 against S. aureus was only 0.5 µg/mL, showing the best antibacterial activity among them. S. aureus could be quickly killed by Ru(II)-2 in 30 min and Ru(II)-2 displayed an obvious inhibitive effect on the formation of a biofilm, which was essential to avoid the development of drug-resistance. Meanwhile, Ru(II)-2 exhibited a stable MIC value against antibiotic-resistant bacteria. The antibacterial mechanism of Ru(II)-2 was probably related to depolarization of the cell membrane, and a change of permeability was associated with the formation of reactive oxygen species, leading to leakage of nucleic acid and bacterial death. Furthermore, Ru(II)-2 hardly showed toxicity to mammalian cells and the Galleria mellonella worm. Finally, murine infection studies also illustrated that Ru(II)-2 was highly effective against S. aureus in vivo.

A Novel Pump-Pump-Probe Resonance Raman Approach Featuring Light-Induced Charge Accumulation on a Model Photosystem

Light-induced charge accumulation is at the heart of biomimetic systems aiming at solar fuel production in the realm of artificial photosynthesis. Understanding the mechanisms upon which these processes operate is a necessary condition to drive down the rational catalyst design road. We have built a nanosecond pump-pump-probe resonance Raman setup to witness the sequential charge accumulation process while probing vibrational features of different charge-separated states. By employing a reversible model system featuring methyl viologen (MV) as a dual electron acceptor, we have been able to watch the photosensitized production of its neutral form, MV⁰, resulting from two sequential electron transfer reactions. We have found that, upon double excitation, a fingerprint vibrational mode corresponding to the doubly reduced species appears at 992 cm^-1 and peaks at 30 μs after the second excitation. This has been further confirmed by simulated resonance Raman spectra which fully support our experimental findings in this unprecedented buildup of charge seen by a resonance Raman probe.

Accumulation of Four Electrons on a Terphenyl (Bis)disulfide

The activation of N₂ , CO₂ or H₂ O to energy-rich products relies on multi-electron transfer reactions, and consequently it seems desirable to understand the basics of light-driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four-electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible-light irradiation using the organic super-electron donor tetrakis(dimethylamino)ethylene, a cascade of light-induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage.

Synthesis of ruthenium polypyridine complexes with benzyloxyl groups and their antibacterial activities against Staphylococcus aureus

Four new ruthenium polypyridyl complexes, [Ru(bpy)₂(BPIP)](PF₆)₂ (Ru(II)-1), [Ru(dtb)₂(BPIP)](PF₆)₂ (Ru(II)-2), [Ru(dmb)₂(BPIP)](PF₆)₂ (Ru(II)-3) and [Ru(dmob)₂(BPIP)](PF₆)₂ (Ru(II)-4) (bpy = 2,2'-bipyridine, dtb = 4,4'-di-tert-butyl-2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, dmob = 4,4'-dimethoxy-2,2'-bipyridine and BPIP = 2-(3,5-bis(benzyloxyl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline) had been synthesized and characterized. Their antimicrobial activities were investigated against Staphylococcus aureus (S. aureus) and four complexes showed obvious antibacterial effect, especially the minimum inhibition concentration (MIC) value of Ru(II)-3 was only 4 μg/mL. In addition, Ru(II)-3 was able to kill bacteria quickly and inhibit the formation of biofilm. Meanwhile, the cooperative effect between Ru(II)-3 and general antibiotics were tested and the results showed that Ru(II)-3 could enhance the susceptibility of S. aureus to different types of antibiotics. Most importantly, Ru(II)-3 hardly showed cytotoxicity to mammalian erythrocytes both in homelysis experiment and G. mellonella model. After being injected with high doses of the Ru(II)-3in vivo, the G. mellonella worms still exhibited high survival rates. Finally, a mouse skin infection model and G. mellonella infection model was built to determine the antibacterial activity of Ru(II)-3in vivo. The antibacterial mechanism of Ru(II)-3 was probably related to the membrane-disruption. Taken together, ruthenium polypyridine complexes with benzyloxyl groups had the potential to develop an attractive and untraditional antibacterial agent with new mode of action.

Recent Advances and Perspectives in Photodriven Charge Accumulation in Molecular Compounds: A Mini Review

The formation of so-called solar fuels from abundant low-energetic compounds, such as carbon dioxide or water, relies on the chemical elementary steps of photoinduced electron transfer and accumulation of multiple redox equivalents. The majority of molecular systems explored to date require sacrificial electron donors to accumulate multiple electrons on a single acceptor unit, but the use of high-energetic sacrificial redox reagents is unsustainable. In recent years, an increasing number of molecular compounds for reversible light-driven accumulation of redox equivalents that do not need sacrificial electron donors has been reported. Those compounds are the focus of this mini review. Different concepts, such as redox potential compression (achieved by proton-coupled electron transfer, Lewis acid-base interactions, or structural rearrangements), hybrids with inorganic nanoparticles, and diffusion-controlled multi-component systems, will be discussed. Newly developed strategies to outcompete unproductive reaction pathways in favor of desired photoproduct formation will be compared, and the importance of identifying reaction intermediates in the course of multiphotonic excitation by different time-resolved spectroscopic techniques will be discussed. The mechanistic insights gained from molecular donor-photosensitizer-acceptor compounds inform the design of next-generation charge accumulation systems for solar energy conversion.

Efficient Convergent Energy Transfer in a Stereoisomerically Pure Heptanuclear Luminescent Terpyridine-Based Ru(II)-Os(II) Dendrimer

The stereoisomerically pure synthesis of a novel heptanuclear Ru(II)-Os(II) antenna bearing multitopic terpyridine ligands is reported. An unambiguous structural characterization was obtained by ¹H NMR spectroscopy and ion mobility spectrometry (IMS-MS). The heptanuclear complex exhibits large molar absorption coefficients (77900 M^-1 cm^-1 at 497 nm) and undergoes unitary, downhill, convergent energy transfer from the peripheral Ru(II) subunits to the central Os(II) that displays photoluminescence with a lifetime (τ = 161 ns) competent for diffusional excited-state electron transfer reactivity in solution.

Role of MLCT States in the Franck-Condon Region of Neutral, Heteroleptic Cu(I)-4H-imidazolate Complexes: A Spectroscopic and Theoretical Study

The impact of the electronic structure of a series of 4H-imidazolate ligands in neutral, heteroleptic Cu(I) complexes is investigated. Remarkable broad and strong ligand-dependent absorption in the visible range of the electromagnetic spectrum renders the studied complexes promising photosensitizers for photocatalytic applications. The electronic structure of the Cu(I) complexes and the localization of photoexcited states in the Franck-Condon region are unraveled by means of UV-vis absorption and resonance Raman (rR) spectroscopy supported by time-dependent density functional theory (TD-DFT) calculations. The visible absorption bands stem from a superposition of bright metal-to-ligand charge-transfer (MLCT) and π-π* as well as weakly absorbing MLCT states. Additionally, the analysis of involved molecular orbitals and rR spectra upon excitation of MLCT and π-π* states highlights the impact of the electronic structure of the 4H-imidazolate ligands on the properties of the corresponding Cu(I) complexes to avail a toolbox for predictive studies and efficient complex design.

Electrochemical and Photophysical Properties of Ruthenium(II) Complexes Equipped with Sulfurated Bipyridine Ligands

The development of new solar-to-fuel scenarios is of great importance, but the construction of molecular systems that convert sunlight into chemical energy represents a challenge. One specific issue is that the molecular systems have to be able to accumulate redox equivalents to mediate the photodriven transformation of relevant small molecules, which mostly involves the orchestrated transfer of multiple electrons and protons. Disulfide/dithiol interconversions are prominent 2e^-/2H⁺ couples and can play an important role for redox control and charge storage. With this background in mind, a new photosensitizer [Ru(^S-Sbpy)(bpy)₂]²⁺ (1²⁺) equipped with a disulfide functionalized bpy ligand (^S-Sbpy, bpy = 2,2'-bipyridine) was synthesized and has been comprehensively studied, including structural characterization by X-ray diffraction. In-depth electrochemical studies show that the ^S-Sbpy ligand in 1²⁺ can be reduced twice at moderate potentials (around -1.1 V vs Fc^+/0), and simulation of the cyclic voltammetry (CV) traces revealed potential inversion (E₂ > E₁) and allowed to derive kinetic parameters for the sequential electron-transfer processes. However, reduction at room temperature also triggers the ejection of one sulfur atom from 1²⁺, leading to the formation of [Ru(^Sbpy)(bpy)₂]²⁺(2²⁺). This chemical reaction can be suppressed by decreasing the temperature from 298 to 248 K. Compared to the archetypical photosensitizer [Ru(bpy)₃]²⁺, 1²⁺ features an additional low energy optical excitation in the MLCT region, originating from charge transfer from the metal center to the ^S-Sbpy ligand (aka MSCT) according to time-dependent density functional theory (TD-DFT) calculations. Analysis of the excited states of 1²⁺ on the basis of ground-state Wigner sampling and using charge-transfer descriptors has shown that bpy modification with a peripheral disulfide moiety leads to an energy splitting between charge-transfer excitations to the ^S-Sbpy and the bpy ligands, offering the possibility of selective charge transfer from the metal to either type of ligands. Compound 1²⁺ is photostable and shows an emission from a ³MLCT state in deoxygenated acetonitrile with a lifetime of 109 ns. This work demonstrates a rationally designed system that enables future studies of photoinduced multielectron, multiproton PCET chemistry.

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

Photoinduced PCET meets catalysis, and the accumulation of multiple redox equivalents is of key importance.

Recent advances in bioinspired proton-coupled electron transfer

Fundamental aspects of PCET continue to attract attention. Understanding this reaction type is desirable for small-molecule activation and solar energy conversion.