Photo-Catalyzed α-Arylation of Enol Acetate Using Recyclable Silica-Supported Heteroleptic and Homoleptic Copper(I) Photosensitizers

Earth-abundant photosensitizers are highly sought after for light-mediated applications, such as photoredox catalysis, depollution and energy conversion schemes. Homoleptic and heteroleptic copper(I) complexes are promising candidates in this field, as copper is abundant and the corresponding complexes are easily obtained in smooth conditions. However, some heteroleptic copper(I) complexes suffer from low (photo)stability that leads to the gradual formation of the corresponding homoleptic complex. Such degradation pathways are detrimental, especially when recyclability is desired. This study reports a novel approach for the heterogenization of homoleptic and heteroleptic Cu complexes on silica nanoparticles. In both cases, the photophysical properties upon surface immobilization were only slightly affected. Excited-state quenching with aryl diazonium derivatives occurred efficiently (108 -1010  M-1  s-1 ) with heterogeneous and homogeneous photosensitizers. Moderate but almost identical yields were obtained for the α-arylation of enol acetate using the homoleptic complex in homogeneous or heterogeneous conditions. Importantly, the silica-supported photocatalysts were recycled with moderate loss in photoactivity over multiple experiments. Transient absorption spectroscopy confirmed that excited-state electron transfer occurred from the homogeneous and heterogeneous homoleptic copper(I) complexes to aryl diazonium derivatives, generating the corresponding copper(II) center that persisted for several hundreds of microseconds, compatible with photoredox catalysis applications.

Photophysics of Anionic Bis(4H-imidazolato)CuI Complexes

In this paper, the photophysical behavior of four panchromatically absorbing, homoleptic bis(4H-imidazolato)Cu^I complexes, with a systematic variation in the electron-withdrawing properties of the imidazolate ligand, were studied by wavelength-dependent time-resolved femtosecond transient absorption spectroscopy. Excitation at 400, 480, and 630 nm populates metal-to-ligand charge transfer, intraligand charge transfer, and mixed-character singlet states. The pump wavelength-dependent transient absorption data were analyzed by a recently established 2D correlation approach. Data analysis revealed that all excitation conditions yield similar excited-state dynamics. Key to the excited-state relaxation is fast, sub-picosecond pseudo-Jahn-Teller distortion, which is accompanied by the relocalization of electron density onto a single ligand from the initially delocalized state at Franck-Condon geometry. Subsequent intersystem crossing to the triplet manifold is followed by a sub-100 ps decay to the ground state. The fast, nonradiative decay is rationalized by the low triplet-state energy as found by DFT calculations, which suggest perspective treatment at the strong coupling limit of the energy gap law.

Photoactive Copper Complexes: Properties and Applications

Photocatalyzed and photosensitized chemical processes have seen growing interest recently and have become among the most active areas of chemical research, notably due to their applications in fields such as medicine, chemical synthesis, material science or environmental chemistry. Among all homogeneous catalytic systems reported to date, photoactive copper(I) complexes have been shown to be especially attractive, not only as alternative to noble metal complexes, and have been extensively studied and utilized recently. They are at the core of this review article which is divided into two main sections. The first one focuses on an exhaustive and comprehensive overview of the structural, photophysical and electrochemical properties of mononuclear copper(I) complexes, typical examples highlighting the most critical structural parameters and their impact on the properties being presented to enlighten future design of photoactive copper(I) complexes. The second section is devoted to their main areas of application (photoredox catalysis of organic reactions and polymerization, hydrogen production, photoreduction of carbon dioxide and dye-sensitized solar cells), illustrating their progression from early systems to the current state-of-the-art and showcasing how some limitations of photoactive copper(I) complexes can be overcome with their high versatility.

Modulating the Excited-State Decay Pathways of Cu(I) 4H-Imidazolate Complexes by Excitation Wavelength and Ligand Backbone

Cu(I) 4H-imidazolato complexes are excellent photosensitizers with broad and intense light absorption properties, based on an earth-abundant metal, and hold great promise as photosensitizers in artificial photosynthesis and for accumulation of redox equivalents. In this study, the excited-state relaxation dynamics of three novel heteroleptic Cu(I) 4H-imidazolato complexes with phenyl, tolyl, and mesityl side groups are systematically investigated by femtosecond and nanosecond time-resolved transient absorption spectroscopy and theoretical methods, complemented by steady-state absorption spectroscopy and (spectro)electrochemistry. After photoexcitation into the metal-to-ligand charge transfer (MLCT) and intraligand charge transfer absorption band, fast (0.6-1 ps) intersystem crossing occurs into the triplet MLCT manifold. The triplet-state population relaxes via the geometrical planarization of the N-aryl rings on the Cu(I) 4H-imidazolato complexes. Depending on the initial Franck-Condon state, the remaining small singlet state population relaxes into two geometrically distinct minima geometries with similar energy, S_1/2,relax and S_3/4,relax. Subsequent ground-state recovery from S_1/2,relax and internal conversion from S_3/4,relax to S_1/2,relax take place on a 100 ps time scale. The internal conversion can be understood as hole transfer from a d_yz-orbital to a d_xz-orbital, which is accompanied with the structural reorganization of the coordination environment. Generally, the photophysical processes are determined by the steric hindrance of the side groups on the ligands. And the excited singlet-state pathways are dependent on the excitation wavelength.

Light-Induced Migration of Spin Defects in TiO2 Nanosystems and their Contribution to the H2 Evolution Catalysis from Water

The photocatalytic activity for H₂ production from water, without presence of hole scavengers, of thermally reduced TiO₂ nanoparticles (H-500, H-700) and neat anatase were followed by in-situ continuous-wave light-induced electron paramagnetic resonance technique (CW-LEPR), in order to correlate the H₂ evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e^- +h⁺ ) recombination and were the crucial electronic factor affecting catalysis. In the best-performant system (H-500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti³⁺ sites, including interstitial Ti³⁺ , and singly ionized electrons trapped in oxygen vacancies (V_O ^. ). The interplay of these species fed successfully surface exposed Ti⁴⁺ sites, which became a catalytically active, fast reacting Ti⁴⁺ ⇄Ti³⁺ state that was key for the H₂ evolution process.

A facile route to transfer Cu nanoparticles to organic medium for better stabilization and improved photocatalytic activity towards N-formylation reaction

Cu nanoparticles were prepared in an aqueous phase by means of a simple reduction-route using sodium borohydride as the reducing agent in the presence of ascorbic acid and polyvinylpyrrolidone (PVP). The hydrosol of the Cu nanoparticles deteriorated within a day. It compelled to initiate a scheme to stabilize the nanoparticles for a long period of time. Phase transfer to organic solvents using Benzyldimethylstearylammonium chloride (BDSAC) as a phase transfer agent was found to be an effective path in this respect. BDSAC performed the dual role of dragging the Cu nanoparticles from water to organic solvent and also acted as a capping agent along with PVP for better stabilization of Cu nanoparticles. The organosol of the Cu nanoparticles exhibited excellent stability and promising catalytic activity towards N-formylation reactions on a number of amine substrates in presence of visible green LED light. The yield and reusability of the catalyst were promising. All the samples were thoroughly characterized by UV-visible spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, energy dispersive analysis of x-rays, x-ray photoelectron spectroscopy and thermo gravimetric analysis.

The Coordination Behaviour of CuI Photosensitizers Bearing Multidentate Ligands Investigated by X-ray Absorption Spectroscopy

A systematic series of four novel homo‐ and heteroleptic Cu^I photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.

Ligand electronic fine-tuning and its repercussion on the photocatalytic activity and mechanistic pathways of the copper-photocatalysed aza-Henry reaction

Subtle electronic ligand effects have a strong impact on the mechanistic pathway of a photocatalytic coupling reaction.

Excited-state dynamics of heteroleptic copper(i) photosensitizers and their electrochemically reduced forms containing a dipyridophenazine moiety – a spectroelectrochemical transient absorption study

Heteroleptic copper(i) dipyridophenazine complexes were investigated by transient absorption spectroelectrochemistry to examine their multi-electron photoaccumulation properties.

Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production

Advances in transition-metal (Ru, Co, Cu, and Fe) complex-based catalysts since 2000 are briefly summarized in terms of catalyst selection and application for photocatalytic H₂ evolution.

Morphology, Optical Properties and Photocatalytic Activity of Photo- and Plasma-Deposited Au and Au/Ag Core/Shell Nanoparticles on Titania Layers

Titania is a promising material for numerous photocatalytic reactions such as water splitting and the degradation of organic compounds (e.g., methanol, phenol). Its catalytic performance can be significantly increased by the addition of co-catalysts. In this study, Au and Au/Ag nanoparticles were deposited onto mesoporous titania thin films using photo-deposition (Au) and magnetron-sputtering (Au and Au/Ag). All samples underwent comprehensive structural characterization by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Nanoparticle distributions and nanoparticle size distributions were correlated to the deposition methods. Light absorption measurements showed features related to diffuse scattering, the band gap of titania and the local surface plasmon resonance of the noble metal nanoparticles. Further, the photocatalytic activities were measured using methanol as a hole scavenger. All nanoparticle-decorated thin films showed significant performance increases in hydrogen evolution under UV illumination compared to pure titania, with an evolution rate of up to 372 μL H₂ h−1 cm−2 representing a promising approximately 12-fold increase compared to pure titania.

Metal-complex chromophores for solar hydrogen generation

Solar H₂ generation from water has been intensively investigated as a clean method to convert solar energy into hydrogen fuel. During the past few decades, many studies have demonstrated that metal complexes can act as efficient photoactive materials for photocatalytic H₂ production. Here, we review the recent progress in the application of metal-complex chromophores to solar-to-H₂ conversion, including metal-complex photosensitizers and supramolecular photocatalysts. A brief overview of the fundamental principles of photocatalytic H₂ production is given. Then, different metal-complex photosensitizers and supramolecular photocatalysts are introduced in detail, and the most important factors that strictly determine their photocatalytic performance are also discussed. Finally, we illustrate some challenges and opportunities for future research in this promising area.

Stabilisation effects of phosphane ligands in the homogeneous approach of sunlight induced hydrogen production

Most of the systems for photochemical hydrogen production are not stable and suffer from decomposition. With bis(bidentate) tetraphosphane ligands the stability increases enormously, up to more than 1000 h. This stability was achieved with a system containing osmium(ii) as a light harvesting antenna and palladium(ii) as a water reduction catalyst connected with a bis(bidentate) phosphane ligand in one molecule with the chemical formula [Os(bpy)₂(dppcb)Pd(dppm)](PF₆)₄. With the help of electrochemical measurements as well as photophysical data and its single crystal X-ray structure, the electron transfer between the two active metal centres (light harvesting antenna, water reduction catalyst) was analysed. The distance between the two active metal centres was determined to be 7.396(1) Å. In a noble metal free combination of a copper based photosensitiser and a cobalt diimine-dioxime complex as water reduction catalyst a further stabilisation effect by the phosphane ligands is observed. With the help of triethylamine as a sacrificial donor in the presence of different monophosphane ligands it was possible to produce hydrogen with a turnover number of 1176. This completely novel combination is also able to produce hydrogen in a wide pH-range from pH = 7.0 to 12.5 with the maximum production at pH = 11.0. The influence of monophosphane ligands with different Tolman cone angles was investigated. Monophosphane ligands with a large Tolman cone angle (>160°) could not stabilise the intermediate of the cobalt based water reduction catalyst and so the turnover number is lower than for systems with an addition of monophosphane ligands with a Tolman cone angle smaller than 160°. The role of the monophosphane ligand during sunlight-induced hydrogen production was analysed and these results were confirmed with DFT calculations. Furthermore the crystal structures of two important Co(i) intermediates, which are the catalytic active species during the catalytic pathway, were obtained. The exchange of PPh₃ with other tertiary phosphane ligands can have a major impact on the activity, depending on the coordination properties. By an exchange of monophosphane ligands with functionalised phosphane ligands (hybrid ligands) the hydrogen production was raised 2.17 times.

Ultrafast excited state dynamics of iridium(III) complexes and their changes upon immobilisation onto titanium dioxide layers

Time-resolved spectroscopy was applied to investigate the excited state dynamics of two heteroleptic Ir(III) complexes with the general formula [Ir(C^N)2(N^N)](+), where C^N and N^N represent different cyclometalating and diimine ligands, respectively. The excited state relaxation is influenced by the ligand substitution as well as the light polarisation. Vibrational relaxation occurs in the sub-ps timescale and interligand charge transfer results in polarisation dependent signal dynamics with a time constant of about 30 ps. Electron injection from the iridium dye to TiO2 is analysed with respect to potential applications in solar energy conversion.

General Scheme for Oxidative Quenching of a Copper Bis-Phenanthroline Photosensitizer for Light-Driven Hydrogen Production

A new, general reaction scheme for photocatalytic hydrogen production is presented based on oxidative quenching of a homoleptic copper(I) bis-1,10-phenanthroline photosensitizer (PS) by 1-methyl-4-phenyl-pyridinium (MPP(+) ) as the electron relay and subsequent regeneration of the so formed copper(II) complex by a sacrificial electron donor. Electron transfer from the relay to various cobalt based water reduction catalysts and subsequent H2 production was shown to close the catalytic cycle. Transient absorption experiments unambiguously confirmed the proposed pathway, both the oxidative quenching and subsequent regeneration of oxidized PS. Photocatalytic test runs further confirmed the role of MPP(+) and up to 10 turnovers were achieved in the relay. The performance limiting factor of the system was shown to be the decomplexation of the copper PS. Quantum yields of the system were 0.03 for H2 production, but 0.6 for MPP(.) formation, clearly indicating that unproductive pathways still prevail.

Determination of side products in the photocatalytic generation of hydrogen with copper photosensitizers by resonance Raman spectroelectrochemistry

A combination of UV-Vis, resonance Raman spectroscopy and electrochemistry is employed to reveal the nature of a side product when using heteroleptic Cu(i)-photosensitizers for photocatalytic hydrogen production.

Neutral, heteroleptic copper(i)-4H-imidazolate complexes: synthesis and characterization of their structural, spectral and redox properties

Facile synthetic access to four novel, neutral, heteroleptic copper(i)-complexes, incorporating 4H-imidazolates as well as the phosphane ligands XantPhos and DPEPhos is reported.

Photocatalytic H2 Production Using Semiconductor Nanomaterials via Water Splitting – An Overview

Heterogeneous semiconductor based photocatalytic hydrogen (H2) production by water splitting is one of the widely recognized promising sustainable technologies to deliver clean energy for future energy demands. The present review article mainly focus on the overview of principle of water splitting, different semiconductor nanomaterials used for photocatalytic water splitting in the presence of UV and solar light irradiation, role of sacrificial reagents, simultaneous degradation of pollutants and H2 production reaction, strategy for development of efficient photocatalyst for H2 production. Further the flaws associated with present photocatalytic system like recombination rate of electron–hole pairs, low visible-light response, use of hazardous irradiation sources and surface area of photocatalyst etc. has also been discussed. Recently the use of energy efficient light emitting diodes (LEDs) as an irradiation source for H2 production is highly attracted due to its unique characteristics. Recent literature on LED source based photocatalytic system for H2 production has also been summarized and highlighted. At last, the future prospects and challenges towards the designing of better photocatalytic system for H2 production have also been discussed. From the literature survey, it is concluded that construction of efficient photocatalytic system for simultaneous degradation of pollutants and H2 production under energy efficient irradiation source offer clean and simple system for solving the futuristic environmental concerns and energy crisis.

Long-lived excited states of zwitterionic copper(I) complexes for photoinduced cross-dehydrogenative coupling reactions

Four heteroleptic copper(I) complexes containing phenanthroline and monoanionic nido-carborane-diphosphine ligands have been prepared and structurally characterized by various spectroscopic techniques and X-ray diffraction. These complexes exhibit intense absorptions in the visible range and excited-state lifetimes on the microsecond scale. Their application in visible-light-induced cross-dehydrogenative coupling reactions was investigated. Preliminary studies showed that one of the four copper(I) complexes is an efficient catalyst for photoinduced oxidative C-H functionalization using oxygen as oxidant. Furthermore, α-functionalized tertiary amines were obtained in good-to-excellent yields by light irradiation (λ>420 nm) of a mixture of our Cu(I) complex, tertiary amines, and a variety of nucleophiles (nitroalkane, acetone, or indoles) under aerobic conditions. Electron paramagnetic resonance measurements provided evidence for the formation of superoxide radical anions (O2(-⋅)) rather than singlet oxygen ((1)O2) during these photocatalytic reactions.

Substitution-controlled excited state processes in heteroleptic copper(I) photosensitizers used in hydrogen evolving systems

Four different heteroleptic [Cu(N^N)(P^P)]PF6 complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time-resolved spectroscopy and quantum chemical calculations. The light-induced excited state processes, accompanied by a structural change, are discussed with respect to the application of these complexes as a new class of noble-metal-free photosensitizers in proton reducing systems. In particular, the influence of different substituents in the ligand backbone on the photophysical properties is highlighted.