Light-driven Reactions Research Articles

Long-Lived, Strongly Emissive, and Highly Reducing Excited States in Mo(0) Complexes with Chelating Isocyanides.

Newly discovered tris(diisocyanide)molybdenum(0) complexes are Earth-abundant isoelectronic analogues of the well-known class of [Ru(α-diimine)3]2+ compounds with long-lived 3MLCT (metal-to-ligand charge transfer) excited states that lead to rich photophysics and photochemistry. Depending on ligand design, luminescence quantum yields up to 0.20 and microsecond excited state lifetimes are achieved in solution at room temperature, both significantly better than those for [Ru(2,2'-bipyridine)3]2+. The excited Mo(0) complexes can induce chemical reactions that are thermodynamically too demanding for common precious metal-based photosensitizers, including the widely employed fac-[Ir(2-phenylpyridine)3] complex, as demonstrated on a series of light-driven aryl-aryl coupling reactions. The most robust Mo(0) complex exhibits stable photoluminescence and remains photoactive after continuous irradiation exceeding 2 months. Our comprehensive optical spectroscopic and photochemical study shows that Mo(0) complexes with diisocyanide chelate ligands constitute a new family of luminophores and photosensitizers, which is complementary to precious metal-based 4d6 and 5d6 complexes and represents an alternative to nonemissive Fe(II) compounds. This is relevant in the greater context of sustainable photophysics and photochemistry, as well as for possible applications in lighting, sensing, and catalysis.

The upper bound function of nonadiabatic dynamics in parametric driving quantum systems**Project supported by the National Natural Science Foundation of China (Emergency Project, Grant Nos. 11447025 and 11847308).

The adiabatic control is a powerful technique for many practical applications in quantum state engineering, light-driven chemical reactions and geometrical quantum computations. This paper reveals a speed limit of nonadiabatic transition in a general time-dependent parametric quantum system that leads to an upper bound function which lays down an optimal criteria for the adiabatic controls. The upper bound function of transition rate between instantaneous eigenstates of a time-dependent system is determined by the power fluctuations of the system relative to the minimum gap between the instantaneous levels. In a parametric Hilbert space, the driving power corresponds to the quantum work done by the parametric force multiplying the parametric velocity along the parametric driving path. The general two-state time-dependent models are investigated as examples to calculate the bound functions in some general driving schemes with one and two driving parameters. The calculations show that the upper bound function provides a tighter real-time estimation of nonadiabatic transition and is closely dependent on the driving frequencies and the energy gap of the system. The deviations of the real phase from Berry phase on different closed paths are induced by the nonadiabatic transitions and can be efficiently controlled by the upper bound functions. When the upper bound is adiabatically controlled, the Berry phases of the electronic spin exhibit nonlinear step-like behaviors and it is closely related to topological structures of the complicated parametric paths on Bloch sphere.

Coupling natural systems with synthetic chemistry for light-driven enzymatic biocatalysis

Visible light-driven redox reactions have been widely adopted for the production of chemicals to combat energy shortage and global warming. Key elements of such a reaction system include a photosensitizer, a catalyst, and an electron source. In this review, we introduce the small molecules and nanoparticles that are widely used as photosensitizers, as well as the development of a photosensitizer protein that is based on the expansion of genetic code, with a fluorescent protein that is used as a scaffold. Visible light-driven enzymes using proteins as photosensitizers or as catalysts such as carbon monoxide dehydrogenase (CODH), formic acid dehydrogenase (FDH), hydrogenase, nitrogenase, cytochrome P450 BM3, and alkane synthase are then described. CODH can be coupled with photosensitizing nanoparticles to reduce CO2 to CO, and hydrogenase can produce H2 using high-energy electrons produced from dye-sensitized nanoparticles. When water-soluble zinc porphyrin is coupled with FDH, visible light drives CO2 to produce formic acid. Nitrogenase can reduce N2 to NH3 using CdS nanoparticle as photosensitizer. Cytochrome P450 BM3 can be enhanced by a visible light-driven redox system and thus by hydroxylate lauric acid or fatty acids. CvFAP, an alkane synthase, can decarboxylate palmitic acid to pentadecane under blue light excitation. Moreover, we describe a genetically encoded photosensitive protein, which mimics the function of natural photosynthesis and catalyzes the conversion of CO2 to CO when covalently attached with a Ni-terpyridine complex.

Role of the Photosystem II as an Environment in the Oxidation Free Energy of the Mn Cluster from S1 to S2.

The manganese cluster (CaMn4O5) in the photosystem II (PSII) is the reaction center of the light-driven oxidation reaction, which generates the molecular oxygen. In this paper, we address the issue of the effect of the environment on the free energy associated with the oxidation of the Mn cluster in S1 state by conducting the large-scale quantum mechanical/molecular mechanical simulations, which involve the whole of the PSII monomer. It was found by the simulations at the level of the B3LYP functional that the environment surrounding the Mn cluster reduces the vertical oxidation free energy Δμvrt by 64.8 kcal/mol. A decomposition analysis of the free energy Δμvrt revealed that the system composed of peptide chains, ligands, lipids, and potassium ions contributes to lowering of Δμvrt by -98.0 kcal/mol, whereas the solvent water makes an opposite contribution of 38.9 kcal/mol. Reduction of the vertical oxidation free energy directly leads to the lowering of the activation free energy ΔGac for the electron transfer reaction from the Mn cluster in S1 state to the neighboring Tyrz+. Consequently, the electron transfer rate was found to be enhanced by a factor of 1012 by virtue of the influence of the environment.

Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State.

Propelling a ground state reaction by mode-specific vibrational excitation via infrared (IR) light offers a novel route to carry out ground state chemistry. Here, we describe the acceleration of a bimolecular alcoholysis reaction as a paradigm for IR light-driven ground state reactions. Instead of resorting to coherent control, IR light is used for direct or indirect vibrational excitation of the reaction coordinate (RC) overcoming the activation energy and promoting the ground state reaction with negligible heating of the sample. Thus, knowledge of the RC is crucial to pick the reaction accelerating vibrations. Alternatively, upon mapping the reaction accelerating vibrations an image of the RC can be reconstructed. We discuss the concept of RCs and examine strategies to use vibrational energy relaxation pathways to single out vibrations belonging to the RC. The influence of the solvent interaction and limitations due to conformational heterogeneity are considered. We provide an application example generating microstructures of polymers and address the use for chemical synthesis in general.

Light-Dependent and Aeration-Independent Gram-Scale Hydroxylation of Cyclohexane to Cyclohexanol by CYP450 Harboring Synechocystis sp. PCC 6803.

Oxygenase-containing cyanobacteria constitute promising whole-cell biocatalysts for oxyfunctionalization reactions. Photosynthetic water oxidation thereby delivers the required cosubstrates, that is activated reduction equivalents and O2 , sustainably. A recombinant Synechocystis sp. PCC 6803 strain showing unprecedentedly high photosynthesis-driven oxyfunctionalization activities is developed, and its technical applicability is evaluated. The cells functionally synthesize a heterologous cytochrome P450 monooxygenase enabling cyclohexane hydroxylation. The biocatalyst-specific reaction rate is found to be light-dependent, reaching 26.3 ± 0.6 U gCDW -1 (U = μmol min-1 and cell dry weight [CDW]) at a light intensity of 150 µmolphotons m-2 s-1 . In situ substrate supply via a two-liquid phase system increases the initial specific activity to 39.2 ± 0.7 U gCDW -1 and stabilizes the biotransformation by preventing cell toxification. This results in a tenfold increased specific product yield of 4.5 gcyclohexanol gCDW -1 as compared to the single aqueous phase system. Subsequently, the biotransformation is scaled from a shake flask to a 3 L stirred-tank photobioreactor setup. In situ O2 generation via photosynthetic water oxidation allows a nonaerated process operation, thus circumventing substrate evaporation as the most critical factor limiting the process performance and stability. This study for the first time exemplifies the technical applicability of cyanobacteria for aeration-independent light-driven oxyfunctionalization reactions involving highly toxic and volatile substrates.

Energy band edge alignment of anisotropic BiVO4 to drive photoelectrochemical hydrogen evolution

Bismuth vanadate (BiVO4) has been highlighted as the most efficient photoanode light absorber for the solar light-driven water oxidation reaction despite the lack of fundamental understanding of its poor performance. Though the BiVO4 performance has been significantly improved, the underlying reasons for this performance are not well understood. Here, anisotropically grown BiVO4 is designed to achieve band edge engineering induced by coupled crystal facets for hydrogen evolution reaction (HER), which is not possible with isotropic BiVO4 powder suspension. HER performances depending on the exposure area of the crystal facets in the anisotropic BiVO4 are investigated using electronic conduction and in situ X-ray absorption spectroscopy during HER. Our results reveal that the nature of the exposed area of the optimized coupled crystal facets in a BiVO4 photoelectrode critically dominates the surface charge density and conductivity, efficiently driving the HER. Sequentially, coupling between different crystal facets of BiVO4 photoelectrode led energy band edge to be upward. The results show that the energy band edge alignment tuned by coupled crystal facet can be applicable to other inorganic photocatalyst/photoelectrodes to promote efficient artificial photosynthetic reactions.

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor ("one-way" ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a "U-turn" ET-where the excited electron density of the donor is initially pushed away from the acceptor-to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

FeO x Derived from an Iron-Containing Polyoxometalate Boosting the Photocatalytic Water Oxidation Activity of Ti3+-Doped TiO2.

The development of efficient and stable catalyst systems using low-cost, abundant, and nontoxic materials is the primary demand for photocatalytic water oxidation. Distinguishing the true active species in a heterogeneous catalytic system is important for construction of efficient catalytic systems. Herein, hydrothermally synthesized Ti3+ self-doped TiO2, labeled as Ti3+/TiO2, was first used as a light absorber in a powder visible light-driven photocatalytic water oxidation reaction. When an iron-containing polyoxometalate Na27[Fe11(H2O)14(OH)2(W3O10)2(α-SbW9O33)6] (Fe11) was used as a cocatalyst, an amorphous layer of active species was wrapped outside the initial Ti3+/TiO2 nanorod and the in situ formed composite was labeled as F/Ti3+/TiO2. When the composite F/Ti3+/TiO2 was tested as a photocatalytic water oxidation catalyst, dramatically improved oxygen evolution performance was achieved. The composite F/Ti3+/TiO2 showed an oxygen evolution rate of 410 μmol/g/h, which was about 11-fold higher than that of prism Ti3+/TiO2. After 24 h of illumination, an O2 yield of 36.4% was achieved. The contrast experiments, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy characterization demonstrated that FeO x is the true cocatalyst that enhanced the oxygen evolution activity of TiO2. A recycling experiment proved that the composite F/Ti3+/TiO2 has favorable stability in the oxygen production process.

Hyperbranched Conjugated Polymer Dots: The Enhanced Photocatalytic Activity for Visible Light-Driven Hydrogen Production

Conjugated polymer (CP) aggregates with linear/cross-linked structures are widely reported as photocatalysts for hydrogen evolution reaction. For full disclosure of the relationship between the photocatalytic performance and structural features of the CP photocatalysts, homogeneous dispersion of these CPs in aqueous medium is necessary, which however is difficult to be achieved. Herein, we report several coassembled polymeric dots (Pdots) consisting of PEG45-b-PMMA103 and CPs with various structural features. We found that the Pdots of hyperbranched soluble CP nanoparticles (SCPNs) exhibit a high H2 evolution rate up to 840 μmol h–1 g–1 with no platinum or rhodium as a cocatalyst, superior to their analogues with the linear or cross-linked structure. A possible charge-transfer mechanism suggests that the photoelectrons directly mobilize to the surface of these single-particulate Pdots over three-dimensional skeleton and successfully avoid the ineffective intermolecular charge transfer, leading to the shortened diffusion path of photoelectrons and enhanced photolysis efficacy. We believe that the high dispersion stability (2 months), solution processability, and structural tunability of these Pdots with hyperbranched SCPNs would inspire further research on designing multicomponent photocatalysts for highly efficient visible light-driven hydrogen evolution reaction.

Visible-Light-Mediated Synthesis of β-Chloro Ketones from Aryl Cyclopropanes.

We report the visible-light-mediated synthesis of β-chloro ketones from aryl cyclopropanes, oxygen, hydrochloric acid, and nitric acid. The operationally simple and catalyst-free method uses cheap standard laboratory reagents and displays broad functional-group tolerance. Moreover, scale up of the reaction and late-stage functionalization of bioactive compounds is possible, providing the opportunity to utilize the cyclopropane ring as a masked β-chloro ketone in a reaction sequence. We propose a light-driven radical chain reaction initiated by the reaction of diluted hydrochloric and nitric acid to produce small quantities of molecular chlorine. The mechanistic hypothesis is supported by 18 O labelling and UV/Vis experiments, cyclovoltammetry, and several control reactions.

Quantum Dot Assembly for Light-Driven Multielectron Redox Reactions, such as Hydrogen Evolution and CO2 Reduction.

Light-driven multielectron redox reactions (e.g., hydrogen (H2 ) evolution, CO2 reduction) have recently appeared at the front of solar-to-fuel conversion. In this Minireview, we focus on the recent advances in establishing semiconductor quantum dot (QD) assemblies to enhance the efficiencies of these light-driven multielectron reduction reactions. Four models of QD assembly are established to promote the sluggish kinetics of multielectron transfer from QDs to cocatalysts, thus leading to an enhanced activity of solar H2 evolution or CO2 reduction. We also forecast the potential applications of QD assemblies in other multielectron redox reactions, such as nitrogen (N2 ) fixation and oxygen (O2 ) evolution from H2 O.

Defining optimal electron transfer partners for light-driven cytochrome P450 reactions

Plants and cyanobacteria are promising heterologous hosts for metabolic engineering, and particularly suited for expression of cytochrome P450 (P450s), enzymes that catalyse key steps in biosynthetic pathways leading to valuable natural products such as alkaloids, terpenoids and phenylpropanoids. P450s are often difficult to express and require a membrane-bound NADPH-dependent reductase, complicating their use in metabolic engineering and bio-production. We previously demonstrated targeting of heterologous P450s to thylakoid membranes both in N. benthamiana chloroplasts and cyanobacteria, and functional substitution of their native reductases with the photosynthetic apparatus via the endogenous soluble electron carrier ferredoxin. However, because ferredoxin acts as a sorting hub for photosynthetic reducing power, there is fierce competition for reducing equivalents, which limits photosynthesis-driven P450 output. This study compares the ability of four electron carriers to increase photosynthesis-driven P450 activity. These carriers, three plant ferredoxins and a flavodoxin-like engineered protein derived from cytochrome P450 reductase, show only modest differences in their electron transfer to our model P450, CYP79A1 in vitro. However, only the flavodoxin-like carrier supplies appreciable reducing power in the presence of competition for reduced ferredoxin, because it possesses a redox potential that renders delivery of reducing equivalents to endogenous processes inefficient. We further investigate the efficacy of these electron carrier proteins in vivo by expressing them transiently in N. benthamiana fused to CYP79A1. All but one of the fusion enzymes show improved sequestration of photosynthetic reducing power. Fusion with the flavodoxin-like carrier offers the greatest improvement in this comparison - nearly 25-fold on a per protein basis. Thus, this study demonstrates that synthetic electron transfer pathways with optimal redox potentials can alleviate the problem of endogenous competition for reduced ferredoxin and sets out a new metabolic engineering strategy useful for producing valuable natural products.

Phase-selective modulation of TiO2 for visible light-driven C[sbnd]H arylation: Tuning of absorption and adsorptivity

To understand and modify TiO2 for organic photoreaction, two points are important: improving light absorption and retaining adsorption sites for organic reagents. Herein, we tuning the absorption and adsorption of TiO2 by introducing the defects on each phase of TiO2, called phase-mixed structures of ordered anatase (OA) and disordered rutile (DR) (OA/DR P25) and disordered anatase (DA) and ordered rutile (OR) (DA/OR P25). Disordered structure of TiO2 broadened the absorption wavelength range including visible light, but changed surface structure reduced the adsorptivity of organic reactant. Additionally, anatase and rutile phase of TiO2 has different surface properties and energy band structure, so keeping the crystalline anatase surface for adsorption was important while introducing the TiO2. Thus, it is assumed that the phase combination of OA and DR can be the best photocatalytic structure of TiO2 even without any supporting materials and/or co-catalysts. In fact, the arylation yield of OA/DR P25 is the highest (63.4%), compared to those of untreated P25 (40.0%) and DA/OR (20.1%), highlighting the potential of OA/DR P25 in various visible light-driven photocatalytic organic reactions.

Visible Light-Driven Cross Coupling Reactions Catalyzed By B12 Complex with Photosensitizer

Organometallic B12 compounds are unique metal complexes having a cobalt-carbon σ-bond present in biological systems [1]. Cleavage of the metal-organic σ-bond initiates many B12-dependent enzymatic reactions. The alkylated cobalt complex is generally formed by the reaction of the Co(I) state of the complex with electrophiles such as an organic halide in vivo. Therefore, the reductive formation of the Co(I) species followed by formal oxidative addition of the organic halide to form the alkylated cobalt complex are key steps for the development of the B12 catalyzed enzymatic reactions. Our group has been reported the photosensitizing formation of the Co(I) species of the B12 derivative, heptamethyl cobyrinate perchlorate, using various photosensitizers (PSs) such as, [Ru(bpy)3]Cl2 or cyclometalated iridium(III) complexes and performed various B12 model reactions such as 1,2-migration of functional groups, dechlorination and so on [2,3]. Here, a visible light driven Heck-type coupling reaction of alkyl halides with styrene and its derivatives catalyzed by the cobalamin derivative (B12) with the [Ru(bpy)3]Cl2 photosensitizer at room temperature is reported. The catalytic efficiencies of the B12 catalyst were compared to that of other cobalt complexes such as cobaloxime. Various control experiments supported a radical-based mechanism similar to those for typical B12 model reactions. An unique coupling reaction combined with 1,2-migration of the functional group is also reported. Mild reaction conditions using an environmentally benign cobalt catalyst derived from the natural B12 provided a practical protocol for the synthetic organic chemistry of the B12 catalyzed reaction system. [1] Reviews: a) H. Shimakoshi, Y. Hisaeda, Current Opinion in Electrochem. 2018, 8, 24; b) H. Shimakoshi, Y. Hisaeda, ChemPlusChem. 2017, 82, 17 (Back cover). [2] H. Tian, H. Shimakoshi, T. Ono, Y. Hisaeda, ChemPlusChem 2019, in press (Front cover). [3] H. Shimakoshi, Y. You, Y. Hisaeda et al, Dalton Trans. 2018, 47, 675 (Front cover).

3-Amino-fluorene-2,4-dicarbonitriles (AFDCs) as Photocatalysts for the Decarboxylative Arylation of α-Amino Acids and α-Oxy Acids with Arylnitriles.

1-(4-(9 H-Carbazol-9-yl)phenyl)-3-amino-9 H-fluorene-2,4-dicarbonitrile as a new photocatalyst for the decarboxylative cross-coupling reaction of α-amino acids or α-oxy carboxylic acids with arylnitriles is described. This light-driven reaction enables a variety of benzylic amines and ethers to be prepared from readily available starting materials under mild conditions.

Redox active ionic liquid as efficient mediator and solvent for visible light-driven B12 catalytic reactions

The redox active ionic liquid, 1-ethyl-4-(methoxycarbonyl)pyridinium bis(trifluoromethanesulfonyl)amide (RIL), was synthesized from its iodide form by an anion exchange reaction of Li(NTf2) with viscos liquid (η = 122 cP at 25 °C) and characterized by NMR, IR, and elemental analysis. The compound showed reversible redox couples at −0.65 V and −1.48 V vs. Ag/AgCl and worked as an electron mediator in the B12 complex/[Ru(bpy)3]Cl2 photosensitizer catalytic system under visible light irradiation. The catalytic efficiency in the RIL was higher than those in DMF, MeOH, and the redox inactive ionic liquid, 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide.

Dynamic analysis of light‐driven DNA strand displacement

The photosensitive azobenzene-tethered DNA material provides an ideal force for DNA nanomachines. This work demonstrates the process of light-driven DNA strand displacement by physical and mathematical models. By data fitting, the strand displacement reaction rate constants under different sequence designs are discussed in detail. The results showed that the Gibbs free energy induced by azobenzene is around 2.52 kJ mol−1, which would hinder the light-driven strand displacement reaction. Increasing the length of the sticky ends can increase the displacement rate exponentially, but the displacement rate will not increase when it exceeds 12 bases.

N-hydroxyphthalimide-TiO2 complex visible light photocatalysis

TiO2 is the most established semiconductor photocatalyst. The prominence of TiO2 is becoming increasingly obvious because its interfacial redox reactions have implication on a wide range processes such as energy conversion and environmental remediation. Herein, we exploited the surface complex created by the interaction between organic molecules with binding sites and accommodating surface of TiO2 for visible light-driven selective aerobic oxidation reactions. A novel surface complex formed between N-hydroxyphthalimide (NHPI) and TiO2 was discovered. The NHPI-TiO2 complex turned out to be an outstanding visible light photocatalyst and was successfully used in the selective oxidation of amines into imines with atmosphere O2 under blue LED irradiation. The stability of the NHPI-TiO2 complex was preserved by 3 mol% of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) acting as a cooperative catalyst. Moreover, selectivities for the imine products were also promoted by TEMPO. Superoxide anion radical (O2−) were evidenced to be the primary reactive oxygen species (ROS) to execute the oxidative conversions. This work suggests that TiO2 surface complexes can be robust photocatalysts for visible light-driven selective aerobic reactions, provided that an appropriate cooperative redox catalyst exists to channel the photocatalytic electron transfer.

Visible light-driven cross-coupling reactions of alkyl halides with phenylacetylene derivatives for C(sp3)-C(sp) bond formation catalyzed by a B12 complex.

Visible light-driven cross-coupling reactions of alkyl halides with phenylacetylene and its derivatives catalyzed by the cobalamin derivative (B12) with the [Ir(dtbbpy)(ppy)2]PF6 photocatalyst at room temperature are reported. The robust B12 catalyst and Ir photocatalyst provided high turnover numbers of over 33 000 for the reactions.