Light-driven Reactions Research Articles

Photoelectrochemical Kinetics: Hydrogen Evolution on p-Type Semiconductors

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants kt nd kr for charge transfer and recombination respectively. kt and kr are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.

A Bpp-based dinuclear ruthenium photocatalyst for visible light-driven oxidation reactions

The photocatalytic oxidation of organic substrates in water using a diruthenium chromophore-catalyst dyad molecule can be tuned by the nature of the bridging ligand.

Ultrasound-assisted two-step water-bath synthesis of g-C3N4/BiOBr composites: visible light-driven photocatalysis, sterilization, and reaction mechanism

g-C3N4/BiOBr photocatalysts with different weight ratios were prepared by a two-step ultrasound-assisted water-bath deposition method.

Design and application of diimine-based copper(i) complexes in photoredox catalysis.

Structurally different bis(imino)copper(i) complexes were prepared in a highly modular manner and utilized as copper-based photocatalysts in the ATRA reactions of styrenes and alkyl halides. The new photocatalysts showed good catalytic activity and ensured efficient chemical transformations.

Expanding the Spectrum of Light-Driven Peroxygenase Reactions.

Peroxygenases require a controlled supply of H2O2 to operate efficiently. Here, we propose a photocatalytic system for the reductive activation of ambient O2 to produce H2O2 which uses the energy provided by visible light more efficiently based on the combination of wavelength-complementary photosensitizers. This approach was coupled to an enzymatic system to make formate available as a sacrificial electron donor. The scope and current limitations of this approach are reported and discussed.

Computational design of metal-supported molecular switches: transient ion formation during light- and electron-induced isomerisation of azobenzene

In molecular nanotechnology, a single molecule is envisioned to act as the basic building block of electronic devices. Such devices may be of special interest for organic photovoltaics, data storage, and smart materials. However, more often than not the molecular function is quenched upon contact with a conducting support. Trial-and-error-based decoupling strategies via molecular functionalisation and change of substrate have in many instances proven to yield unpredictable results. The adsorbate-substrate interactions that govern the function can be understood with the help of first-principles simulation. Employing dispersion-corrected density-functional theory (DFT) and linear expansion delta-self-consistent-field DFT, the electronic structure of a prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed to (1 1 1) single crystal facets of copper, silver and gold, is investigated and the main reasons for the loss or survival of the switching function upon adsorption are identified. The light-induced switching ability of a functionalised derivative of azobenzene on Au(1 1 1) and azobenzene on Ag(1 1 1) and Au(1 1 1) is assessed based on the excited-state potential energy landscapes of their transient molecular ions, which are believed to be the main intermediates of the experimentally observed isomerisation reaction. We provide a rationalisation of the experimentally observed function or lack thereof that connects to the underlying chemistry of the metal-surface interaction and provides insights into general design strategies for complex light-driven reactions at metal surfaces.

Integrating the plasmonic effect and p-n heterojunction into a novel Ag/Ag2O/PbBiO2Br photocatalyst: Broadened light absorption and accelerated charge separation co-mediated highly efficient visible/NIR light photocatalysis

Among several factors that influence the effectiveness of a semiconductor mediated photocatalysis, efficient charge separation and broadened light absorption are regarded as two of the most important parameters. Herein, a series of plasmonic p-n heterojunction Ag/Ag2O/PbBiO2Br photocatalysts with wide spectrum responses were successfully fabricated through a facile precipitation-calcination method. The obtained samples display exceptional photocatalytic performance for the degradation of tetracycline (TC), and the results reveal that the 10 wt% Ag/Ag2O/PbBiO2Br exhibits the optimal activity, which can efficiently decompose 84.4% of TC after 90 min of visible light irradiation. While under NIR light irradiation, the TC removal ratio can still reach 50.9% with 240 min. Besides, no significant deterioration in the degradation performance is observed even after four cycling runs. The photoactivity enhancement of Ag/Ag2O/PbBiO2Br can be credited to the synergistic effect between the local surface plasmon resonance (LSPR) effect of metallic Ag and the p-n heterojunction of Ag2O/PbBiO2Br, which not only greatly broadens the light utilization, but also remarkably accelerates the charge separation. The radical trapping experiments and the ESR measurements ascertains that O2−, h+ and OH play a critical role in the degradation of TC under visible light irradiation, while O2− and h+ become the major reactive species in NIR light-driven reaction. According to the experimental results, the plausible reaction mechanisms for TC degradation over Ag/Ag2O/PbBiO2Br composite are proposed under both visible and NIR light irradiation. This work provides some guidance for rational construction of novel plasmonic p-n heterojunction photocatalysts to meet ever-increasing environmental requirements.

Wavelength‐Controlled Dynamic Metathesis: A Light‐Driven Exchange Reaction between Disulfide and Diselenide Bonds

AbstractWavelength‐controlled dynamic processes are mostly based on light‐triggered isomerization or the cleavage/formation of molecular connections. Control over dynamic metathesis reactions by different light wavelengths, which would be useful in controllable dynamic chemistry, has rarely been studied. Taking advantage of the different bond energies of disulfide and diselenide bonds, we have developed a wavelength‐driven exchange reaction between disulfides and diselenides, which underwent metathesis under UV light to produce Se−S bonds. When irradiated with visible light, the Se−S bonds were reversed back to those of the original reactants. The conversion of the exchange depends on the wavelength of the incident light. This light‐driven metathesis chemistry was also applied to tune the mechanical properties of polymer materials. The visible‐light‐induced reverse reaction was compatible with reductant‐catalyzed disulfide/diselenide metathesis, and could be utilized to develop a dissipative system with light as the energy input.

Oxygenic Photoreactivity in Photosystem II Studied by Rotating Ring Disk Electrochemistry.

Protein film photoelectrochemistry has previously been used to monitor the activity of photosystem II, the water-plastoquinone photooxidoreductase, but the mechanistic information attainable from a three-electrode setup has remained limited. Here we introduce the four-electrode rotating ring disk electrode technique for quantifying light-driven reaction kinetics and mechanistic pathways in real time at the enzyme–electrode interface. This setup allows us to study photochemical H2O oxidation in photosystem II and to gain an in-depth understanding of pathways that generate reactive oxygen species. The results show that photosystem II reacts with O2 through two main pathways that both involve a superoxide intermediate to produce H2O2. The first pathway involves the established chlorophyll triplet-mediated formation of singlet oxygen, which is followed by its reduction to superoxide at the electrode surface. The second pathway is specific for the enzyme/electrode interface: an exposed antenna chlorophyll is sufficiently close to the electrode for rapid injection of an electron to form a highly reducing chlorophyll anion, which reacts with O2 in solution to produce O2•–. Incomplete H2O oxidation does not significantly contribute to reactive oxygen formation in our conditions. The rotating ring disk electrode technique allows the chemical reactivity of photosystem II to be studied electrochemically and opens several avenues for future investigation.

Catalysts Based on Earth-Abundant Metals for Visible Light-Driven Water Oxidation Reaction.

Exploration of water oxidation catalyst (WOC) with excellent performance is the key for the overall water splitting reaction, which is a feasible strategy to convert solar energy to chemical energy. Although some compounds composed of noble metals, mainly Ru and Ir, have been reported to catalyze water oxidation with high efficiency, catalysts based on low-cost and earth-abundant transition metals are essential for realizing economical and large-scale light-driven water splitting. Various WOCs containing earth-abundant metals (mainly Mn, Fe, Co, Ni, Cu) have been utilized for visible light-driven water oxidation in recent years. In this Personal Account, we summarize our recent developments in WOCs based on earth-abundant transition metals including polyoxometalates (POMs), metal oxides or bimetal oxides, and metal complexes containing multidentate ligand scaffolds for visible light-driven water oxidation reaction.

Facile photo-induced growth of polymeric nanostructures onto cellulose: The poly(ethylene glycol) methacrylate (PEGMA)@cellulose case study

Photo-polymerization is a polymerization method widely exploited in many technological fields, due to its high versatility and fast reaction rates. In this study, poly(ethylene glycol) methacrylate (PEGMA) nanostructures are grown onto cellulose following a simple and promising light-driven reaction mechanism involving benzophenone (as photo-initiator) and UV-curing. The effectiveness of the surface functionalization has been determined by means of morphological and physicochemical characterizations, showing the formation of very sharp PEGMA nanometric architectures.

Photocatalytic anion oxidation achieves direct aerobic difunctionalization of alkenes leading to β-thiocyanato alcohols

A novel visible light-driven oxidative cascade reaction for the synthesis of β-thiocyanato alcohols via difunctionalization of alkenes is described for the first time. In this protocol inorganic ion thiocyanate was successfully converted into radical through photocatalysis by employing Rose Bengal as a photocatalyst to mediate the single-electron transfer. This hydroxylation process did not need extra reducing agent and the new CS and CO bonds could be constructed in one pot. Molecular oxygen not only was used as an excellent terminal oxidant, but also served as a green oxygen source, which is one of the most ideal processes to realize CH bond oxidation functionalization. Moreover, the reaction also proceeded very well under irradiation of sunlight.

Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light-Driven Hydrogen Evolution and Oxidative Dehydrogenation.

Dehydrogenation of the tetrahydroisoquinoline derivatives coupled with hydrogen production is important for hydrogen storage applications. Herein, we formulated a new system that embedded Dawson-type polyoxometalates as efficient photosensitizers into the pores of redox-active coordination polymers for the light-driven photocatalytic oxidative Mannich reaction and hydrogen evolution. In the designed Co-POM polymer, UV light excitation gives the excited state of the Dawson-type polyoxometalate first to oxidize electron donors or substrates; the reduced form (i.e., heteropolyblue) adsorbs visible light to achieve a new excited state, which reduced the cobalt redox sites and facilitates hydrogen evolution reaction. The photosensitizer recovered to the ground state, completing the catalytic cycle. Under the optimized conditions, Co-POM enabled the hydrogen evolution and dehydrogenation of tetrahydroisoquinoline without the presence of any other additives. The high catalytic efficiency and robustness indicated the advantages of the combining functional polyoxometalate-based catalysts and porous characters of the coordination polymers for the development of highly active heterogeneous catalysts.

Water oxidation catalytic ability of polypyridine complex containing a μ-OH, μ-O2 dicobalt(iii) core

Two polypyridine complexes containing μ-OH, μ-O2 dicobalt(III) cores, [(TPA)CoIII(μ-OH)(μ-O2) CoIII(TPA)](ClO4)3 and [(BPMEN)CoIII(μ-OH)(μ-O2)CoIII(BPMEN)](ClO4)3 (TPA = tris(2-pyridylmethyl)amine, BPMEN = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine), have previously been reported as inactive in the light-driven water oxidation reaction (ACS Catal., 2016, 6, 5062−5068). Herein, another dicobalt(III) compound, μ-OH, μ-O2-[{(enN4)2Co2}](ClO4)3 (enN4 = 1,6-bis(2-pyridyl-2,5-diazaocta-2,6-diene), with a similar core structure was synthesized, characterized, and applied to the light-driven water oxidation reaction. Collective experiments showed that the complex itself was also inactive in the light-driven water oxidation, and that the activity observed originated from Co(II) impurities. This research establishes that complexes possessing a μ-OH, μ-O2 dicobalt(III) core structure are not appropriate choices for true molecular catalysts of water oxidation.

Composition-Dependent Functionality of Copper Vanadate Photoanodes.

To understand functional roles of constituent elements in ternary metal oxide photoanodes, essential photoelectrochemical (PEC) properties are systematically analyzed on a series of copper vanadate compounds with different Cu:V elemental ratios. Homogeneous and highly continuous thin films of β-Cu2V2O7, γ-Cu3V2O8, Cu11V6O26, and Cu5V2O10 are grown via reactive co-sputtering and their performance characteristics for the light-driven oxygen evolution reaction are evaluated. All four compounds have similar bandgaps in the range of 1.83-2.03 eV, though Cu-rich phases exhibit stronger optical absorption and higher charge separation efficiencies. Transient photocurrent analysis reveals a reduction of surface catalytic activity with increasing Cu:V elemental ratio due to competitive charge recombination at Cu-related surface states. This comprehensive analysis of PEC functionalities-including photon absorption, carrier separation, and heterogeneous charge transfer-informs strategies for improving PEC activity in the copper vanadate materials system and provides insights that may aid discovery, design, and engineering of new photoelectrode materials.

A microfluidic photoreactor enables 2-methylbenzophenone light-driven reactions with superior performance.

Light-driven reactions of 2-methylbenzophenones (2-MBPs) occur with improved yields (up to >98%) and reaction rates (up to 0.240 mmol h-1) by using a tailored microfluidic photoreactor (MFP). For the first time, coumarins were converted into 4-benzylated chromanones in high yields (50-93%) and diastereoselectivity (up to >20 : 1 dr), thus by-passing their photo-dimerisation end-reaction.

Integration of Enzymes in Polyaniline-Sensitized 3D Inverse Opal TiO2 Architectures for Light-Driven Biocatalysis and Light-to-Current Conversion.

Inspired by natural photosynthesis, coupling of artificial light-sensitive entities with biocatalysts in a biohybrid format can result in advanced photobioelectronic systems. Herein, we report on the integration of sulfonated polyanilines (PMSA1) and PQQ-dependent glucose dehydrogenase (PQQ-GDH) into inverse opal TiO2 (IO-TiO2) electrodes. While PMSA1 introduces sensitivity for visible light into the biohybrid architecture and ensures the efficient wiring between the IO-TiO2 electrode and the biocatalytic entity, PQQ-GDH provides the catalytic activity for the glucose oxidation and therefore feeds the light-driven reaction with electrons for an enhanced light-to-current conversion. Here, the IO-TiO2 electrodes with pores of around 650 nm provide a suitable interface and morphology needed for the stable and functional assembly of polymer and enzyme. The IO-TiO2 electrodes have been prepared by a template approach applying spin coating, allowing an easy scalability of the electrode height and surface area. The successful integration of the polymer and the enzyme is confirmed by the generation of an anodic photocurrent, showing an enhanced magnitude with increasing glucose concentrations. Compared to flat and nanostructured TiO2 electrodes, the three-layered IO-TiO2 electrodes give access to a 24-fold and 29-fold higher glucose-dependent photocurrent due to the higher polymer and enzyme loading in IO films. The three-dimensional IO-TiO2|PMSA1|PQQ-GDH architecture reaches maximum photocurrent densities of 44.7 ± 6.5 μA cm-2 at low potentials in the presence of glucose (for a three TiO2 layer arrangement). The onset potential for the light-driven substrate oxidation is found to be at -0.315 V vs Ag/AgCl (1 M KCl) under illumination with 100 mW cm-2, which is more negative than the redox potential of the enzyme. The results demonstrate the advantageous properties of IO-TiO2|PMSA1|PQQ-GDH biohybrid architectures for the light-driven glucose conversion with improved performance.

Direct Carboxylation of C(sp3)-H and C(sp2)-H Bonds with CO2 by Transition-Metal-Catalyzed and Base-Mediated Reactions

This review focuses on recent advances in the field of direct carboxylation reactions of C(sp3)-H and C(sp2)-H bonds using CO2 encompassing both transition-metal-catalysis and base-mediated approach. The review is not intended to be comprehensive, but aims to analyze representative examples from the literature, including transition-metal catalyzed carboxylation of benzylic and allylic C(sp3)-H functionalities using CO2 which is at a “nascent stage”. Examples of light-driven carboxylation reactions of unactivated C(sp3)-H bonds are also considered. Concerning C(sp3)-H and C(sp2)-H deprotonation reactions mediated by bases with subsequent carboxylation of the carbon nucleophile, few examples of catalytic processes are reported in the literature. In spite of this, several examples of base-promoted reactions integrating “base recycling” or “base regeneration (through electrosynthesis)” steps have been reported. Representative examples of synthetically efficient, base-promoted processes are included in the review.

Engineering High-Potential Photo-oxidants with Panchromatic Absorption.

Challenging photochemistry demands high-potential visible-light-absorbing photo-oxidants. We report (i) a highly electron-deficient Ru(II) complex (eDef-Rutpy) bearing an E1/20/+ potential more than 300 mV more positive than that of any established Ru(II) bis(terpyridyl) derivative, and (ii) an ethyne-bridged eDef-Rutpy-(porphinato)Zn(II) (eDef-RuPZn) supermolecule that affords both panchromatic UV-vis spectral domain absorptivity and a high E1/20/+ potential, comparable to that of Ce(NH4)2(NO3)6 [E1/2(Ce3+/4+) = 1.61 V vs NHE], a strong and versatile ground-state oxidant commonly used in organic functional group transformations. eDef-RuPZn exhibits ∼8-fold greater absorptive oscillator strength over the 380-700 nm range relative to conventional Ru(II) polypyridyl complexes, and impressive excited-state reduction potentials (1E-/* = 1.59 V; 3E-/* = 1.26 V). eDef-RuPZn manifests electronically excited singlet and triplet charge-transfer state lifetimes more than 2 orders of magnitude longer than those typical of conventional Ru(II) bis(terpyridyl) chromophores, suggesting new opportunities in light-driven oxidation reactions for energy conversion and photocatalysis.

A small-scale reactor for light-driven chemistry

Seeking a way to standardize light-driven chemistry, researchers at Princeton University, led by David W. C. MacMillan, teamed up with scientists and engineers at Merck & Co., led by Ian W. Davies, to create a small-scale reactor to use for photochemical transformations (ACS Cent. Sci. 2017, DOI: 10.1021/acscentsci.7b00159). When a new reaction fails or performs poorly, there can be some ambiguity as to why that is, Davies explains. “We wanted to remove that ambiguity so that when we set up reactions and report them, they are set up consistently and are reproducible,” Davies says. The resulting reactor turns out to speed up many known light-driven reactions and boosts yields to boot. The researchers attribute this ability to the reaction vessel’s increased and highly uniform exposure to photons in the reactor. The system works with vials ranging from 2 to 40 mL. Different light sources can be swapped out, so photochemistry