Supramolecular Photocatalyst Research Articles

The asymmetrical-structure of supramolecular precursor to improve internal electric field for simultaneously enhancing contaminant degradation and H2O2 production performance

Asymmetric-structure perylene diimide (PDI) supramolecular photocatalysts were successfully constructed to induce robust internal electric field for efficient photocatalytic activity. Benefitting from the large dipole moment of the asymmetric-structure PDI, the internal electric field has been significantly enhanced by 8.4 times. And then the charge separation efficiency of the asymmetric-structure PDI has been dramatically improved by 2.4 times from 4.60% up to 11.22%. Thus, the asymmetric-structure PDI supramolecular presents evidently enhanced photocatalytic degradation performance, up to 4.8 times, even surpassing many reported high-activity organic photocatalysts. Meanwhile, they also exhibited a maximum 9.5-fold increase in photocatalytic hydrogen peroxide production activity. Briefly, this work throws light on enhancing the internal electric field via molecular structure modification to dramatically improve the photocatalytic performance.

Create a strong internal electric-field on PDI photocatalysts for boosting phenols degradation via preferentially exposing π-conjugated planes up to 100%

Nearly 100% exposure of π-conjugated planes, whose structure inherently exhibits large electron delocalization and fast charge transfer, has been achieved in perylene diimide (PDI) supramolecular photocatalysts by a solvent-induced self-assembly method. The high exposure ratio of π-conjugated planes is found to cause a larger surface potential and higher surface charge density by experimental data, and higher electron distribution by DFT calculations, relative to π-stacked planes exposed on PDI nanorods or (020) planes exposed on PDI particles, resulting in a strong internal electric field. This gives π-conjugated PDI ca. 8–17 times higher activity on phenols photodegradation than reported PDI, and 4–6 times higher activity than well-known photocatalysts like Bi2WO6 or g-C3N4. The successful control of PDI to preferentially expose π-conjugated planes may not only boost the photocatalytic activity in this system, but also give some guidelines in the design and development of more efficient organic photocatalysts with wide spectrum response.

Design and fabrication of a perylene dimiide functionalized g-C3N4@UiO-66 supramolecular photocatalyst: Insight into enhancing the photocatalytic performance

Photocatalytic degradation of organic pollutants (dyes, drugs, pesticides...) is of high interest. In this study, a unique supramolecular architecture of perylene diimide functionalized graphitic carbon nitride (PCN) was prepared and used as a support for UiO-66 nanoparticles. The organic-inorganic heterojunction (PCN@UiO-66) was employed for the photodegradation of rhodamine B (RhB). The synthesized materials were characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Under visible light illumination, PCN@UiO-66 achieved a superior degradation efficiency of ∽99% ± 1%. In comparison, pristine g-C3N4, PCN and UiO-66 exhibited degradation efficiencies of 14% ± 0.5%, 44% ± 1.5% and 62% ± 1%, respectively. The pH was found to affect the photodegradation process and a pH value of 7 resulted in maximum photodegradation efficiency. As confirmed by UV-vis diffuse reflectance spectroscopy and photoluminescence measurements, the enhanced efficiency can be attributed to the improved visible light absorption, as well as reduction of the charge recombination rate brought by perylene dimiide (PDI) photofunctional groups. Moreover, the π-conjugated structure of g-C3N4 along with proper matching between energy levels of components bring synergistic effect in this photocatalytic system. In addition, the photocatalytic mechanism was investigated using radical scavengers, which suggested superoxide radical (O2−•) to be the main species involved in the RhB degradation. Repeated experimental studies and structural analysis of photocatalyst before and after degradation revealed that the photocatalyst exhibits good stability and reusability. It is noteworthy that after four consecutive runs, 91% ± 1% of the photocatalytic efficiency was still preserved. The findings of this study can be used to design high-performance photocatalysts for environmental remediation.

A Full-Spectrum Porphyrin-Fullerene D-A Supramolecular Photocatalyst with Giant Built-In Electric Field for Efficient Hydrogen Production.

A full-spectrum (300-850nm) responsive donor-acceptor (D-A) supramolecular photocatalyst tetraphenylporphinesulfonate/fullerene (TPPS/C60 ) is successfully constructed. The theoretical spectral efficiency of TPPS/C60 is as high as 70%, offering the possibility of full-solar-spectrum light harvesting. The TPPS/C60 performs a highly efficient photocatalytic H2 evolution rate of 276.55µmol h-1 (34.57mmol g-1 h-1 ), surpassing many reported organic photocatalysts. The D-A structure effectively promotes electron transfer from TPPS to C60 , which is beneficial to the photocatalytic reaction. Specifically, a giant internal electric field in the D-A structure is built via the enhanced molecular dipole, which dramatically promotes the charge separation (CS) efficiency by 2.35 times. Transient absorption spectra results show a long-lived CS state TPPS•+ -C60 •- in the D-A structure, which effectively promotes participation of photogenerated electrons in the reduction reaction. Briefly, this work provides a novel approach for designing high-performance photocatalytic materials via enhancing the interfacial electric field.

Supramolecular Zinc Porphyrin Photocatalyst with Strong Reduction Ability and Robust Built‐In Electric Field for Highly Efficient Hydrogen Production

AbstractThe ultrathin 2D self‐assembled tetrakis (4‐carboxyphenyl) zinc porphyrin (SA‐ZnTCPP) is successfully fabricated for photocatalytic hydrogen production. The H2 evolution rate is 3487.3 µmol g–1 h–1 under wide‐spectrum (300–698 nm), which is increased by ≈85 times compared with metal‐free self‐assembled tetrakis (4‐carboxyphenyl) porphyrin (SA‐TCPP). The significant promotion in photocatalytic activity of SA‐ZnTCPP comes from the deep conduction band (−1.01 V) contributed by the elevation of orbital energy level of porphyrin after Zn2+ coordination. In addition, the strong built‐in electric field boosts the rapid separation and mobility of photogenerated carriers. Consequently, the strong reduction driving force and faster electron transfer kinetics result in highly efficient photocatalytic hydrogen production of SA‐ZnTCPP. This work improves the understanding of the structure‐property relationship of metalloporphyrin‐based supramolecular photocatalysts and provides a general strategy at the molecular level for performance optimization of photocatalysts.

Molecule/Semiconductor Hybrid Materials for Visible-Light CO2 Reduction: Design Principles and Interfacial Engineering

ConspectusBecause of increasing concerns over the depletion of energy sources and the concomitant increase in CO2 emissions, much attention has been devoted to carbon capture and utilization technologies. Among the various methods and schemes proposed, visible-light-driven CO2 reduction in combination with water oxidation, one of the representative models of artificial photosynthesis, is an attractive solution because it enables abundant water and inexhaustible solar energy to be used to produce value-added chemicals.Molecular metal complexes and semiconductors are promising candidates for photocatalysts that can reduce CO2 to CO, formate, formaldehyde, or other hydrocarbons. Although both molecular metal complexes and semiconductors have strengths and weaknesses, their weaknesses (low oxidation ability and low selectivity for reduction reactions) can be overcome via the construction of a suitable molecule/semiconductor hybrid material. However, facilitating electron transfer at the molecule/semiconductor junction while suppressing unfavorable back electron transfer events is challenging. Consequently, the number of molecule/semiconductor hybrid systems that show a reasonable level of visible-light photocatalytic activity is limited, despite the development of a large number of visible-light-driven semiconductors and molecular photocatalysts (or catalysts).In this Account, we describe our approaches to developing hybrid photocatalysts and photoelectrodes for CO2 reduction. We have been developing both molecular (photo)catalysts and semiconductor photocatalysts individually, the latter of which are also designed for visible-light water splitting. For example, supramolecular photocatalysts that possess both photosensitizer and catalyst units in a single molecule can reduce CO2 to formate or CO in a homogeneous system, with high selectivity toward the desired product and high quantum yields of several tens of percent. However, nonoxide semiconductors such as C/N-based polymers and mixed-anion compounds exhibit a strong photooxidation ability under visible light. Carefully designed molecule/semiconductor hybrid materials achieve CO2 reduction under visible light with high product selectivity and stability even in an aqueous environment, where the concentration of CO2 is low but that of protons is high. Visible-light CO2 reduction combined with H2O oxidation is possible via the construction of a photoelectrochemical cell that comprises a molecular photocathode and an n-type semiconductor photoanode.Although our photosystems can be regarded as model systems for artificial photosynthesis, their light-energy conversion efficiencies are still unsatisfactory. To improve the efficiency, materials design, including interfacial engineering at the molecule/semiconductor junction, is important and is the general theme of the results highlighted in this Account.

Uncovering the origin of full-spectrum visible-light-responsive polypyrrole supramolecular photocatalysts

Environmentally friendly polypyrrole (PPy) catalysts with conjugated nanostructures for superior photophysical and photoelectric applications are developed. The high coplanar structure via α-α connection is benefit for the transferring of photo-induced electron and improving the life of the light generated carrier. Importantly, the energy level structure of conjugated polymers was obtained via theoretical calculation and the photocatalytic mechanism of PPy photocatalyst was illuminated in detail. O2− are the main radical responsible for the degradation of MO. Moreover, conjugated PPy photocatalysts are stable with cycling. Compared to P25 and g-C3N4, PPy showed excellent photocatalytic performance for degradation of organic pollutants in full spectrum and visible light, the reaction rate constants of PPy are 4.27 and 5.31 times as much as the P25, respectively. Furthermore, PPy also exhibited effective photocatalytic performance towards different organic pollutants. These results obviously open interesting perspectives of using conjugated polymer and provide theoretical guidance for designing new organic photocatalysts to solve the energy and environment issues.

Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)-Re(i) supramolecular photocatalyst.

Supramolecular photocatalysts comprising [Ru(diimine)3]2+ photosensitiser and fac-[Re(diimine)(CO)3{OC(O)OC2H4NR2}] catalyst units can be used to reduce CO2 to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO2 to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO2, e.g., 10% CO2, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO2 was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s−1 at 298 K and is a unimolecular reaction, independent of the concentrations of both CO2 and RuC2(Re)−.

A supramolecular single-site photocatalyst based on multi-to-one Förster resonance energy transfer.

A multi-to-one supramolecular photocatalyst is fabricated by aqueous electrostatic self-assembly of multiple graphene quantum dot (GQD) antennas onto a single-Pt-site porphyrin unimolecular micelle (PtTHPD-UM) catalytic center. Light energy is transferred from GQDs to PtTHPD-UM to catalyze water splitting into hydrogen up to 57 190 μmol g(Pt)-1 h-1 under visible light.

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation.

The development of systems for photocatalytic CO2 reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO2 reduction in water featuring a polypyrrole layer, [Ru(diimine)3]2+ as a redox photosensitizer unit, and Ru(diimine)(CO)2Cl2 as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO2 to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoOx/BiVO4 and RhOx/TaON) to construct full cells that photocatalytically reduce CO2 using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10−2% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10−2% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm−2).

Photocatalysis of a Dinuclear Ru(II)-Re(I) Complex for CO2 Reduction on a Solid Surface.

The development of CO2-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO2-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating Al2O3 particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction.

SummaryPhotoelectrochemical CO2 reduction into syngas (a mixture of CO and H2) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H2-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H2 ratios (0–10) under one-sun illumination. Moreover, unassisted solar CO2 reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

Photogenerated-hole-induced rapid elimination of solid tumors by the supramolecular porphyrin photocatalyst.

The rapid, complete, targeted and safe treatment for tumors remains a key issue in cancer therapy. A novel treatment of solid tumors by supramolecular photocatalyst Nano-SA-TCPP with the irradiation of 600–700 nm wavelength is established. Solid tumors (100 mm3) can be eliminated within 10 min. The 50-day mouse survival rate was increased from 0% to 100% after the photocatalytic therapy. The breakthrough was owing to the cell membrane rupture and the cytoplasmic loss caused by photogenerated holes inside cancer cells. The porphyrin-based photocatalysts can be internalized in a targeted manner by cancer cells due to the size selection effect, without entering the normal cells. The therapy has no toxicity or side effects for normal cells and organisms. Moreover, the photocatalytic therapy is effective for a variety of cancer cell lines. Because of its high efficiency, safety and universality, the photocatalytic therapy provides us with a new lancet to conquer the tumor.

A Highly Crystalline Perylene Imide Polymer with the Robust Built-In Electric Field for Efficient Photocatalytic Water Oxidation.

A highly crystalline perylene imide polymer (Urea-PDI) photocatalyst is successfully constructed. The Urea-PDI presents a wide spectrum response owing to its large conjugated system. The Urea-PDI performs so far highest oxygen evolution rate (3223.9 µmol g-1 h-1 ) without cocatalysts under visible light. The performance is over 107.5 times higher than that of the conventional PDI supramolecular photocatalysts. The strong oxidizing ability comes from the deep valence band (+1.52 eV) which is contributed by the covalent-bonded conjugated molecules. Besides, the high crystallinity and the large molecular dipoles of the Urea-PDI contribute to a robust built-in electric field promoting the separation and transportation of photogenerated carriers. Moreover, the Urea-PDI is very stable and has no performance attenuation after 100 h continuous irradiation. The Urea-PDI polymer photocatalyst provides with a new platform for the use of photocatalytic water oxidation, which is expected to contribute to clean energy production.

Photophysical Activity and Host-Guest Behavior of Ruthenium Polypyridyl Catalysts Encapsulated in Cucurbit[10]uril.

This work outlines a strategy to combine the use of visible light and confined spaces to form a supramolecular photocatalyst system. Polypyridyl ruthenium(II) complexes [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine), [Ru(bpy)2(bpm)]2+ (bpm = 2,2'-bipyrimidine), and [Ru(bpy)2(bpz)]2+ (bpz = 2,2'-bipyrazine) are encapsulated in cucurbit[10]uril to form host-guest systems in aqueous solution. The photophysical properties of the complexes are altered by encapsulation, with improved emissive behavior for the heteroleptic complexes. Oxidative quenching of the photocatalyst's excited state via intermolecular charge transfer to methyl viologen can occur within the internal cavity, which acts to preorganize the reagents. The host-guest system containing [Ru(bpy)3]2+ can bind suitable substrates, and essential criteria for its use as a supramolecular photocatalyst are investigated.

Photocatalytic degradation of tetracycline antibiotics using three-dimensional network structure perylene diimide supramolecular organic photocatalyst under visible-light irradiation

The occurrence of antibiotics in the ambient environment has raised serious concerns. In this work, the kinetics and mechanism of photocatalytic degradation tetracycline (TC) was investigated using three-dimensional network structure perylene diimide supramolecular organic photocatalyst (3D-PDI). Under visible-light irradiation, 3D-PDI exhibited excellent degradation performance and stability for several tetracycline-based antibiotics (e.g., tetracycline; chlortetracycline; oxytetracycline.). The adsorption and degradation rate of TC by 3D-PDI were 8.21 and 12.7 times higher than that of bulk-PDI. The enhanced adsorption and degradation performance of TC by 3D-PDI were mainly due to the larger specific surface area and π-electron conjugation of 3D network supramolecular system. Superoxide radical (O2−), hydrogen peroxide (H2O2) and hole (h+) the main reactive species (RSs) for TC degradation. Under the attack of photocatalytic RSs, TC undergoes hydroxylation, demethylation, aromatization, and ring-opening processes, and finally complete mineralization into CO2 and H2O. These results revealed that perylene diimide supramolecular photocatalyst may be efficiently applied for the remediation of tetracycline contaminated natural waters.

(Invited) New Directions to Development of Photocatalytic CO2 Reduction

The artificial systems which can convert CO2 to useful and energy-rich compounds by using renewable energy especially solar energy should be developed because the human beings have produced a huge amount of CO2 mainly from fossil resources for making energy. We have been tackling this target by using transition-metal complexes as main players. In this presentation, I mainly talk about three projects, i.e., (1) highly efficient supramolecular photocatalysts with both functions as photosensitizer and catalysts, (2) reduction of low concentration of CO2 using metal-complex catalysts, and (3) photocatalytic CO2 coupled to water oxidation.1. Highly efficient supramolecular photocatalysts for CO2 reduction1 Since CO2 is the most oxidized state of carbon, one-electron reduction of CO2 is an extraordinarily high endergonic reaction. From the viewpoints of low-energy light application, the multi-electron reduction of CO2 via chemical reactions promises to be a more valuable process. This is a main reason why both a photosensitizer and a catalyst are required to promote efficient photocatalytic CO2 reduction.We have developed so-called supramolecular photocatalyts where the photosensitizer and the catalyst are combined to each other with a bridging ligand. This bonding in the supramolecular photocatalysts can accelerate electron transfer between the two components, which improves the performance of the photocatalytic system. In particular, this advantage should be heightened on the surface of photofunctional solid materials as described below.2. Reduction of low concentration of CO2 using metal-complex photocatalysts2 Exhaust gases from such as thermal power plants and iron manufactures contained several % - 20% of CO2. Although several methods such as adsorption and desorption methods using amines and membrane filtration have been already developed for enrichment of CO2, these procedures require high energy consumption. If low concentration of CO2 can be directly reduced by artificial methods, it should give a new direction of researches for artificial photosynthesis.In this presentation, I report such systems using CO2-capturing properties of metal complexes. It has been reported that fac-[ReI(diimine)(CO)3X]n+ can work as both photocatalyst and electrocatalysts for CO2 reduction. We found that the Re complexes also have CO2-capturing properties with the assistance of triethanolamine (TEOA) as shown in the eq.1. Although this reaction is in equilibrium, the equilibrium constant is very large (K = 1.7 x 103 M-1). This means that the Re complex can efficiently capture CO2 from gases containing low concentration of CO2 such as 1% CO2. This type of CO2adducts work as a catalyst for photochemical reduction of CO2. A Ru(II)-Re(I) supramolecular photocatalyst consisting of a [Ru(diimine)3]+ type photosensitizer unit and the Re complex as a catalyst unit efficiently photocatalyzed reduction of low concentration of CO2 (even 1% CO2).3. Photocatalytic CO2 coupled to water oxidation3 The molecular photocatalytic systems have another weakness, i.e., weak oxidation power in the excited state. For practical application, water should be used as an electron donor. For overcoming this problem, we have developed a hybrid system consisting of the supramolecular photocatalyst and semiconductor electrodes. The hybrid photoelectrochemical cell consisting of a NiO-RuRe hybrid photocathode and a CoOx/TaON photoanode showed the activity for visible-light-driven CO2 reduction with water as a reductant to generate CO and O2 (Figure).

A Ru(II)–Mn(I) Supramolecular Photocatalyst for CO2 Reduction

Supramolecular photocatalysts for CO2 reduction, constituted of redox photosensitizer, catalyst, and bridging ligand, play crucial roles in constructing hybrid systems with solid materials and phot...

Efficient trinuclear Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction based on a new tris-chelating bridging ligand built around a central aromatic ring.

We have designed and synthesized a new tris-chelating polypyridine ligand (bpy3Ph) suitable to be used as a bridging ligand (BL) for constructing various supramolecular photocatalysts. This BL is a phenylene ring with three ethylene chains at 1, 3, and 5 positions, of which the other terminals are connected to 2,2'-bipyridine moieties. The ligand bpy3Ph has been used to prepare, according to a multi-step synthetic protocol, trinuclear supramolecular photocatalysts containing different metal subunits. In particular, the compounds Ru2Re and RuRe2 have been prepared, containing different ratios of components based on Ru(dmb)3 2+-type and Re(dmb)(CO)3Cl-type units (dmb = 4,4'-dimethyl-2,2'-bipyridine), which can play the roles of photosensitizers and catalyst units for photocatalytic CO2 reduction, respectively. The trinuclear model Ru3 and mononuclear and dinuclear Ru and Ru2 precursor metal complexes, containing free chelating sites, have also been synthesized using the same bridging ligand. The absorption spectra, redox behaviour and photophysical properties of the new species indicate that there is no strong electronic interaction among the Ru and Re units. The trinuclear complexes Ru2Re and RuRe2 could photocatalyze CO2 reduction to CO with high selectivity (up to 97%), high efficiency (Φ COs of 28% and 25%, respectively: BIH as a reductant), and high durability (TONCOs of 5232 and 6038, respectively: BIH as a reductant) which are the largest TONs for CO2 reduction using supramolecular photocatalysts in homogeneous solutions. The absence of negligible accumulation of the mono-reduced form of the photosensitizer indicates fast electron transfer to the catalyst unit(s) through the relatively large bridging ligand and is proposed to contribute to the outstanding photocatalytic properties of the new species, including their durability. The relevant photocatalytic behaviour of the new systems indicates new avenues for the design of extended bridging ligands capable of efficiently and functionally integrating photosensitizers and catalysts towards the preparation of new, larger supramolecular photocatalysts for selective CO2 reduction.

Synthesis of a Novel Re(I)-Ru(II)-Re(I) Trinuclear Complex as an Effective Photocatalyst for CO2 Reduction

Abstract Supramolecular photocatalysts, which consist of redox photosensitizer and catalyst units, have recently attracted attention in the field of artificial photosynthesis. Aiming to construct a durable supramolecular photocatalyst for CO2 reduction, a new Re–Ru–Re trinuclear complex was successfully developed, where one Ru unit, acting as photosensitizer, was linked to two Re units, acting as catalysts, through ethylene chains. The novel complex was synthesized by creating two chelate moieties on the ligand of a Ru complex using the Negishi-coupling reaction, which were then coordinated to Re complexes. This trinuclear complex selectively photocatalyzed the reduction of CO2 to CO under visible light irradiation, with high durability compared with the corresponding Ru–Re binuclear complex and a mixed system of model mononuclear complexes. Absorption spectra and mass spectroscopic data of the reaction solutions revealed a rapid consumption of the electrons on the one-electron reduced species of the Ru unit, which led to suppression of the Ru unit decomposition, thereby maintaining its photosensitizing ability and product selectivity.