Search

Abstract Two new supramolecular photocatalysts containing Ru(II) polypyridine units as light-harvesting photosensitizers and Re(I) polypyridine subunits as catalytic centers have been prepared. The new species, RuRe2A and Ru2ReA, contain catalytic Re(I) subunits coordinated by the preformed CO2TEOA adduct (known to be the effective catalytic subunits; TEOA is triethanolamine) and exhibit quite efficient and selective photoreduction of CO2 to CO, with outstanding TONs of 2368 and 2695 and a selectivity of 99.9% and 98.9%, respectively. Such photocatalytic properties are significantly improved with respect to those of previously studied RuRe2 and Ru2Re parent compounds, containing chloride ligands instead of the CO2TEOA adduct. Comparison between photocatalytic performance of the new species and their parent compounds allows to investigate the effect of the CO2TEOA insertion process as well as the eventual effect of the presence of chloride ions in solution on the photocatalytic processes. The improved photocatalytic properties of RuRe2A and Ru2ReA compared with their parent species are attributed to a combined effect of different distribution of the one-electron reduced form of the supramolecular photocatalysts on the Ru-subunit(s) (leading to decreased CO formation due to a poisoning ligand loss process) and on the Re-subunit(s) and to the presence of chloride ions in solution for RuRe2 and Ru2Re, which could interfere with the CO2TEOA adduct formation, a needed requisite for CO forming catalysis. These results strongly indicate the utility of preparing supramolecular photocatalysts containing preformed adducts.

Graphitic carbon nitride (g-C3N4) was synthesized by heating urea at different temperatures (773–923 K) in air, and was examined as a photocatalyst for CO2 reduction. With increasing synthesis temperature, the conversion of urea into g-C3N4 was facilitated, as confirmed by X-ray diffraction, FTIR spectroscopy and elemental analysis. The as-synthesized g-C3N4 samples, further modified with Ag nanoparticles, were capable of reducing CO2 into formate under visible light (λ > 400 nm) in the presence of triethanolamine as an electron donor, with the aid of a molecular Ru(II) cocatalyst (RuP). The CO2 reduction activity was improved by increasing the synthesis temperature of g-C3N4, with the maximum activity obtained at 873–923 K. This trend was also consistent with that observed in photocatalytic H2 evolution using Pt-loaded g-C3N4. The photocatalytic activities of RuP/g-C3N4 for CO2 reduction and H2 evolution were thus shown to be strongly associated with the generation of the crystallized g-C3N4 phase.

The visible-light responsive Cu(I)-complex photosensitizers were developed by introducing various aromatic substituents at the 4,7-positions of a 2,9-dimethyl-1,10-phenanthroline (dmp) ligand in a heteroleptic CuI(dmp)(DPEphos)+-type complexes (DPEphos = [2-(diphenylphosphino)phenyl]ether) for photocatalytic CO2 reduction. Introducing biphenyl groups (Bp-) on the dmp ligand enhanced the molar extinction coefficient (ε) of the metal-to-ligand charge transfer (MLCT) band in the visible region (ε = 7,500 M−1cm−1) compared to that of the phenyl (Ph-)-containing analog (ε = 5,700 M−1cm−1 at λmax = 388 nm). However, introducing 4-R-Ph- groups (R = the electron-withdrawing groups NC-, or NO2-) led to a red shift in the band to λmax = 390, 400, and 401 nm, respectively. Single-crystal X-ray analysis showed the Ph- groups were twisted because of the steric repulsion between the 2,6-protons of the Ph- groups and 5,6-protons of the dmp ligand. The result strongly indicated that the π-conjugation effect of the 4-R-Ph- groups is so weak that the lowering of the energy of the dmp π* orbitals is small. However, when 4-R-ph- was substituted by a 5-membered heterorings, there was a larger red shift, leading to an increase in the ε value of the MLCT absorption band. Thus, the substitution to 2-benzofuranyl- groups resulted in visible-light absorption up to 500 nm and a shoulder peak at around 420 nm (ε = 12,300 M−1cm−1) due to the expansion of π-conjugation over the substituted dmp ligand. The photocatalytic reaction for CO2 reduction was tested using the obtained CuI complexes as photosensitizers in the presence of a Fe(dmp)2(NCS)2 catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as a sacrificial reductant, which showed improved CO generation.

A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands [Ir(pyr)] has a strong absorption band in the visible region (ε444nm = 67,000 M−1 cm−1) but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as an electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO2) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)2Cl2 (dmb = 4,4′-dimethyl-2,2′-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ruvia an ethylene bridge and investigated its photochemical CO2 reduction activity in the presence of BI(OH)H.

Ruthenium(II) picolinate complex, [Ru(dmb)2(pic)]+ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N∧N)3]2+ (N∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λabs < 670 nm) and stronger reduction ability in a one-electron reduced state (E1/2red = −1.86 V vs. Ag/AgNO3), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO2 to CO under red-light irradiation (λex > 600 nm). TONCO reached 235 and ΦCO was 8.0%. Under these conditions, [Ru(dmb)3]2+ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOFCO = 6.7 min−1) and larger TONCO (2347) than Ru(dmb) (TOFCO = 3.6 min−1; TONCO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.

In the field of hydride transfer reactions, overcoming the slowness of the transfer is an important challenge. In this study, we investigated the reaction mechanism of the transfer from a (2,2¤:6¤,2¤¤-terpyridine)(2,2¤-bipyridine)ruthenium(II) hydrido complex to a NAD(P)+ model compound, 3-carbamoyl-1- methylpyridinium by means of density functional theory. One of the calculated transition states is in good agreement with our kinetics observations (activation Gibbs energy, activation volume by means of high pressure apparatus, and kinetic isotope effect) about the transition state in the rate-determining step, indicating that the carbamoyl group (especially carbonyl group) in the NAD(P)+ model helps to complete the transfer by modulating the kinetic hydricity on an ad hoc basis. [ABSTRACT FROM AUTHOR]

The photophysical and photochemical properties of transition metal complexes have attracted considerable attention because of their recent applications as photocatalysts in artificial photosynthesis and organic synthesis, as light emitters in electroluminescent (EL) devices, and as dyes in solar cells. The general control methods cannot be always used to obtain transition metal complexes with photochemical properties that are suitable for the above-mentioned applications. In the fields of solar energy conversion, strong metal-to-ligand charge-transfer (MLCT) absorption of redox photosensitizers and/or photocatalysts in the visible region with long wavelength is essential. However, the usual methods, i.e., introduction of electron-withdrawing groups into the electron-accepting ligand and/or weak-field ligands into the central metal, have several drawbacks, including shorter excited-state lifetime, lower emission efficiency, and lower oxidation and reduction power. Herein we describe a new method to control the photophysical, photochemical, and electrochemical properties of Re(I) diimine carbonyl complexes that have been widely used in various fields such as photocatalysts for CO2 reduction and emitters in EL devices and sensors. This method involves the introduction of interligand interactions (π-π and CH-π interactions) into the Re(I) complexes; the aromatic diimine ligand coordinating to the Re center approaches the aryl groups on the phosphine ligand or ligands at the cis position, which "compulsorily" induces a weak interaction between these aromatic groups. As a result of this interligand interaction, the Re complexes with the aromatic diimine ligand and the arylphosphine ligand(s) exhibit red-shifted 1MLCT absorption but afford blue-shifted emission from the triplet metal-to-ligand charge-transfer (³MLCT) excited state. This increases the oxidation power and lifetime of the ³MLCT excited state. These unique property changes are favorable, particularly for redox photosensitizers. The interligand interaction is strongly expressed by the ring-shaped multinuclear Re(I) complexes (Re-rings). In the case of Re-rings with high steric hindrance due to a small inner cavity, the lifetime of the ³MLCT excited state is up to 8 µs and the emission quantum yield is up to 70%. These properties cannot be obtained by the corresponding mononuclear Re(I) complexes, which generally exhibit shorter lifetimes (<1 µs) and lower emission quantum yields (<10%). Some of the Re-rings could be successfully applied as efficient photosensitizers in photocatalytic systems for CO2 reduction; the highest quantum yields for CO2 reduction were achieved by using photocatalytic systems composed of Re-rings as the photosensitizers and Re(I) (82%), Ru(II) (58%), and Mn(I) (48%) complexes as catalysts. This interligand interaction potentially provides unique and useful methods for controlling the photophysical, photochemical, and electrochemical functions of various metal complexes, paving the way to create new functions for metal complexes. [ABSTRACT FROM AUTHOR]

A series of highly luminescent trinuclear and tetranuclear ring-shaped Re(I) complexes wherein the Re units are linked with rigid bidentate phosphine ligands, namely, bis(diphenylphosphino)-p-phenylene, -trans-vinylene, and -ethynylene, were synthesized and fully characterized. Their strong emissive properties and the long lifetimes of their triplet metal-to-ligand charge transfer excited states originate primarily from enhanced, rigidity-induced interligand interactions between the 2,2′-bipyridine (bpy) ligand and the phenyl groups of the phosphine ligands. In addition, another type of interligand interaction was also observed between the bpy ligand and the phosphine-bridging group; this interaction also strongly affected the photophysical and redox properties of the Re-rings. [ABSTRACT FROM AUTHOR]

Three types of photocatalytic systems for CO2 reduction, which were recently developed in our group, are reviewed. First, two-component systems containing different rhenium(I) complexes having different roles; i.e., redox photosensitizer and catalyst in the reaction solution are described. The mixed system of a ring-shaped rhenium(I) trinuclear complex and fac-[Re(bpy)(CO)3(MeCN)]+ is currently the most efficient photocatalytic system for CO2 reduction (φCO = 0.82 at λex = 436 nm). The second is a series of supramolecular photocatalysts, which have units with different functions in one molecule, i.e., redox photosensitizer, catalyst, and bridging ligand. The highest durability and speed of photocatalysis were achieved by using this system (φCO = 0.45, TONCO = 3029, and TOFCO = 35.7 min-1). The third is a novel type of artificial Z-Scheme photocatalyst for CO2 reduction, of which photocatalysis is revealed by stepwise excitation of both a semiconductor photocatalyst unit and the supramolecular photocatalyst unit. This system has both strong oxidation and reduction powers. [ABSTRACT FROM AUTHOR]

A novel tripodal ligand, tris[(4′-methyl-2,2′-bipyridin-4-yl)methyl]carbinol (tb-carbinol) and its homonuclear and heteronuclear Ru(II)–Re(I) complexes have been synthesized and characterized by NMR spectroscopy, elemental analysis, and mass spectroscopy. The spectroscopic, electrochemical and photocatalytic properties of the Ru(II)–Re(I) complexes have been investigated. In these supramolecular complexes with tb-carbinol as a bridging ligand, the intramolecular interaction among the terminal metal centers is very weak. In the cases of Ru(II) and Re(I) heteronuclear systems, when the Re(I) moieties are excited, the emission from the Re(I) moiety is efficiently quenched and the intensity of the emission from the Ru(II) moiety increases. The rate constant of energy transfer from Re(I) moieties to Ru(II) unit in RuRe2is 1.7 × 108s−1. From the point of view of the free energy change, the intramolecular electron transfer from the Ru(II) moiety to the Re(I) moiety could proceed smoothly in the ground state. Both of Ru2Re and RuRe2show excellent photocatalytic activities to the CO2reduction. RuRe2exhibits a turnover number of 190 for CO formation compared with 89 from the model complexes system after 16 h of irradiation (TNCOcalculated based on Ru(II) moiety concentration). Ru2Re shows a higher turnover number than the model complexes system, 110 compared with 55 from the model system (TNCOcalculated based on Re(I) moiety concentration). The bridging ligand of Ru(II)–Re(I) heteronuclear tripodal systems, tb-carbinol, plays an important role in converting radiant energy to chemical energy in the form of CO from CO2. Enhancement of the photocatalytic response to light in the visible region has been achieved by fabricating supramolecular systems featuring covalently linked Ru(II) and Re(I) moieties. [ABSTRACT FROM AUTHOR]

We report the most efficient homogeneous photocatalyst yet for CO2 reduction using a wide range of visible-light wavelength. We synthesized new Ru(ii)–Re(i) binuclear complexes with 1,3-bis(4′-methyl-[2,2′]bipyridinyl-4-yl)-propan-2-ol (bpyC3bpy) as a bridge ligand, specifically [Ru-ReP(OEt)3]3+ and [Ru-Repy]3+ where a P(OEt)3 or pyridine ligand coordinates on the Re site. Their photocatalytic activities were compared with [Ru-ReCl]2+, which has a Cl− ligand on the Re site and has recently been reported as a much better photocatalyst (Φ = 0.12, TNCO = 160) than a 1 : 1 mixed system of the corresponding Ru(ii) and Re(i) mononuclear complexes. The best photocatalyst was [Ru-ReP(OEt)3]3+, for which Φ = 0.21 and TNCO = 232. A mechanistic study clearly showed that [Ru-ReP(OEt)3]3+ is rapidly converted into the solvento complex [Ru-ReSol]3+, (Sol = DMF or TEOA) which is the actual photocatalyst. Although similar rapid ligand substitution occurs with other supramolecules, the pyridine and Cl− anions accelerate the decomposition of the supramolecular photocatalysts. [ABSTRACT FROM AUTHOR]

Semiconducting C3N4 powder modified with a catalytic Ru(II) complex having carboxylic acid groups as anchors showed higher activity for CO2 reduction into HCOOH under visible light (λ > 400 nm) at the initial stage of the reaction, whereas an analogue anchored with phosphonic acid groups exhibited higher stability. [ABSTRACT FROM AUTHOR]

We successfully performed the reduction of CO2 to CO by selective illumination of red visible light to a chlorophyll derivative covalently linked with a rhenium bipyridine complex. This is the first example of the photochemical reduction of CO2 using a sensitizer that has strong absorption at a wavelength over 650 nm. [ABSTRACT FROM AUTHOR]

A photocatalytic system with a Mn() complex as a catalyst and a RU(II) complex as a photosensitizer efficiently reduced CO2 to formic acid. [ABSTRACT FROM AUTHOR]

A novel molecular photocatalytic system with not only high reduction ability of CO2 but also high capture ability of CO2 has been developed using a Ru(II)–Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO2 of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO2 atmosphere (TONCO > 1000, ΦCO > 0.4). Even 0.5% concentration of CO2 was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO2 (TONCO > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO2, which proceeds CO2 insertion to the Re–O bond, and then reduce the captured CO2 by using an electron supplied from the photochemically reduced Ru photosensitizer unit. [ABSTRACT FROM AUTHOR]