Photoredox Properties Research Articles

A Covalent Triazine Framework for Photocatalytic Anti‐Markovnikov Hydrofunctionalizations

AbstractPorous materials‐based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)‐based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C−N and C−O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron‐donating and ‐withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late‐stage functionalization of bio‐active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

A Covalent Triazine Framework for Photocatalytic Anti-Markovnikov Hydrofunctionalizations.

Porous materials-based heterogeneous photocatalysts, performing selective organic transformations, are increasing the applicability of photocatalytic reactions due to their ability to merge traditional photocatalysis with structured pores densely decorated with catalytic moiety for efficient mass and charge transfer, as well as added recyclability. We herein disclose a porous crystalline covalent triazine framework (CTF)-based heterogeneous photocatalyst that exhibits excellent photoredox properties for different hydrofunctionalization reactions such as hydrocarboxylations, hydroamination and hydroazidations. The high oxidizing property of this CTF enables the activation of styrenes, followed by regioselective C-N and C-O bond formation at ambient conditions. A change in the physicochemical and optoelectronic properties of the CTF, upon protonation during catalysis, lies at the basis of its photocatalytic properties. This allows us to obtain hydrocarboxylations, hydroamination, and hydroazidations from a myriad of electron-donating and -withdrawing aromatic and aliphatic substrates. This catalytic approach is further extended to late-stage functionalization of bio-active molecules. Finally, detailed characterizations of the CTF and further mechanistic investigations provide mechanistic insights into these reactions.

Synthesis, crystal structures and properties of three sodium coordination polymers based on bridging polyoxometalate ligands

The syntheses, crystal structures, spectral characterization, electrochemical, photoredox properties and biological studies of three sodium coordination polymers viz. (ImH)4[(µ2-HMo7O24)Na(H2O)8]∙7H2O 1 (Im = Imidazole), [(TEAH)Na5(H2O)12(µ5-Mo7O24)] 2 (TEA=triethanolamine) and [Na2(PyrH)2(µ2-Mo8O26)(H2O)7]∙8H2O 3 (Pyr = pyrazole) are reported. The unique Na atom in 1 exhibits ten coordination unlike hexa coordination observed in 2 and 3. The µ2-bridging heptamolybdate (or octamolybdate) anion in 1 (or 3) links the sodium cations into an infinite chain resulting in a one-dimensional coordination polymer. In 2, the heptamolybdate anion exhibits an unprecedented µ5-nonadentate binding resulting in a three- dimensional (3D) structure. Compounds 1–3 exhibit characteristic Raman bands. Solar irradiation of 2 results in photodimerization, which is further supported by electrochemical studies. Compounds 1–3 exhibit antibacterial activity, with 3 being the most effective against S. Pyogenes, followed by P. aeruginosa, S.typhi and S. aureus. Compound 3 exhibits excellent antidiabetic and anti-inflammatory properties. A comparative study of the binding modes of several heptamolybdates and octamolybdates based compounds is presented.

Manganese Complexes with Consecutive Mn(IV) → Mn(III) Excitation for Versatile Photoredox Catalysis.

Manganese complexes stand out as promising candidates for photocatalyst design, attributed to their eco- and biocompatibility, versatile valence states, and capability for facilitating multiple electronic excitations. However, several intrinsic constraints, such as inadequate visible light response and short excited-state lifetimes, hinder effective photoinduced electron transfer and impede photoredox activation of substrates. To overcome this obstacle, we have developed a class of manganese complexes featuring boron-incorporated N-heterocyclic carbene ligands. These complexes enable prolonged excited-state durations encapsulating both Mn(IV) and Mn(III) oxidation stages, with lifetimes reaching microseconds for Mn(IV) and nanoseconds for Mn(III), concurrently exhibiting robust redox capabilities. They efficiently catalyze direct, site-selective cross-couplings between diverse arenes and aryl bromides, at a low catalyst loading of 0.5 mol %. Their proficiency spans an extensive array of substrates including both highly electron-rich and electron-deficient molecules, which underscore the superior performance of these manganese complexes in tackling intricate transformations. Furthermore, the versatility of these complexes is further highlighted by their successful applications in various photochemical transformations, encompassing reductive cross-couplings for the formation of C-P, C-B, C-S and C-Se bonds, alongside oxidative couplings for creating C-N bonds. This study sheds light on the distinctive photoredox properties and the remarkable catalytic flexibility of manganese complexes, highlighting their immense potential to drive progress in photochemical synthesis and green chemistry applications.

Highly Efficient, Noble-Metal-Free, Fully Aqueous CO2 Photoreduction Sensitized by a Robust Organic Dye.

The development of efficient, selective, and durable CO2 photoreduction systems presents a long-standing challenge in full aqueous solutions owing to the presence of scarce CO2 and the fierce competition against H2 evolution, which is even more challenging when noble metals are not utilized. Herein, we present the facile decorations of four phosphonic acid groups on a donor-acceptor-type organic dye to obtain a water-soluble photosensitizer (4P-DPAIPN), which succeeds the excellent photophysical and photoredox properties of its prototype, exhibiting long-lived delayed fluorescence (>10 μs) in aqueous solutions. Combining 4P-DPAIPN with a cationic cobalt porphyrin catalyst has accomplished record-high apparent quantum yields of 9.4-17.4% at 450 nm for CO2-to-CO photoconversion among the precedented systems (maximum 13%) in fully aqueous solutions. Remarkable selectivity of 82-93% and turnover number of 2700 for CO production can also be achieved with this noble-metal-free system, outperforming a benchmarking ruthenium photosensitizer and a commercial organic dye under parallel conditions. Such high performances of 4P-DPAIPN can be well maintained under real sunlight. More impressively, no significant decomposition of 4P-DPAIPN was detected during the long-term photocatalysis. Eventually, the photoinduced electron transfer pathways were proposed.

Covalent Organic Frameworks Based Photoenzymatic Nano-reactor for Asymmetric Dynamic Kinetic Resolution of Secondary Amines.

Bio-catalysis represents a highly efficient and stereoselective method for the synthesis of valuable chiral compounds, however, the poor stability and limited reaction types of free enzymes restrict their wide application in industrial production. In this work, to overcome these problems, a multifunctional photoenzymatic nanoreactor CALB@COF-Ir was developed through the encapsulation of Candida antarctica lipase B (CALB) in a photosensitive covalent organic framework COF-Ir. This bio-nanocluster serves as efficient catalysts in asymmetric dynamic kinetic resolution (DKR) of secondary amines to give a series of chiral amines in high yields (up to 99 %) and enantioselectivities (up to 99 % ee). The well-designed COF-Ir not only acts as safety cover to prevent CALB from deactivation but promotes racemization of secondary amines via photo-induced hydrogen atom transfer (HAT) process. Photoelectric characterization and TDDFT calculation revealed that (ppy)2Ir units in COF-Ir play crucial role in this photocatalytic system which enhance its photo-redox properties through facilitating the separation between photoelectrons (e-) and holes (h+). Furthermore, the heterogeneous photoenzymatic nanoreactor could be recycled for five rounds with slight decline of catalytic reactivity.

Photoredox matching of earth-abundant photosensitizers with hydrogen evolving catalysts by first-principles predictions.

Photoredox properties of several earth-abundant light-harvesting transition metal complexes in combination with cobalt-based proton reduction catalysts have been investigated computationally to assess the fundamental viability of different photocatalytic systems of current experimental interest. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations using several GGA (BP86, BLYP), hybrid-GGA (B3LYP, B3LYP*), hybrid meta-GGA (M06, TPSSh), and range-separated hybrid (ωB97X, CAM-B3LYP) functionals were used to calculate relevant ground and excited state reduction potentials for photosensitizers, catalysts, and sacrificial electron donors. Linear energy correction factors for the DFT/TD-DFT results that provide the best agreement with available experimental reference results were determined in order to provide more accurate predictions. Among the selection of functionals, the B3LYP* and TPSSh sets of correction parameters were determined to give the best redox potentials and excited states energies, ΔEexc, with errors of ∼0.2eV. Linear corrections for both reduction and oxidation processes significantly improve the predictions for all the redox pairs. In particular, for TPSSh and B3LYP*, the calculated errors decrease by more than 0.5V against experimental values for catalyst reduction potentials, photosensitizer oxidation potentials, and electron donor oxidation potentials. Energy-corrected TPSSh results were finally used to predict the energetics of complete photocatalytic cycles for the light-driven activation of selected proton reduction cobalt catalysts. These predictions demonstrate the broader usefulness of the adopted approach to systematically predict full photocycle behavior for first-row transition metal photosensitizer-catalyst combinations more broadly.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

Introducing Iodide Ligands on IrAu12 Cluster Enhances Phosphorescence Efficiency and Photoredox Activity

AbstractThe effects of halide ligands on the photoluminescence (PL) and photoredox properties were examined on [IrAu12(dppe)5X2]+ (1X; X=Cl, Br, I; dppe=1,2‐bis(diphenylphosphino)ethane). The PL quantum yield was enhanced in the order of 1Cl (67 %)<1Br (75 %)<1I (77 %). The lifetime of the photoexcited state was shortened in the order of 1Cl>1Br>1I due to the acceleration of radiative relaxation by the heavy atom effect in 1I. Despite the shortest PL lifetime, cluster 1I exhibited the highest conversion in the photocatalytic [2+2] cycloaddition of bisenone. The PL from 1Cl and 1Br was quenched by adding Lewis acid Li+, required for activation of bisenone, whereas that from 1I remained unaffected. Theoretical calculations showed that the Au−Cl and Au−Br bonds of 1Cl and 1Br were weakened by interaction with Li+, while the Au−I bond of 1I was not. Clusters 1Cl and 1Br underwent halide exchange with isocyanide in the presence of Li+, whereas 1I did not. These results suggested that the photoredox activity of 1Cl and 1Br was suppressed by the Li+‐induced nonradiative quenching of the photoexcited state. This study highlights the unexpected importance of halide ligands in the design of efficient cluster photocatalysts.

Luminescent and Photoredox-Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines.

Ruthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero-valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal-to-ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited-state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet-triplet annihilation, to convert low-energy input radiation into higher-energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

Luminescent and Photoredox‐Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines

AbstractRuthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero‐valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal‐to‐ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited‐state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet‐triplet annihilation, to convert low‐energy input radiation into higher‐energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

Reductive Photocycloreversion of Cyclobutane Dimers Triggered by Guanines.

The quest for simple systems achieving the photoreductive splitting of four-membered ring compounds is a matter of interest not only in organic chemistry but also in biochemistry to mimic the activity of DNA photorepair enzymes. In this context, 8-oxoguanine, the main oxidatively generated lesion of guanine, has been shown to act as an intrinsic photoreductant by transferring an electron to bipyrimidine lesions and provoking their cycloreversion. But, in spite of appropriate photoredox properties, the capacity of guanine to repair cyclobutane pyrimidine dimer is not clearly established. Here, dyads containing the cyclobutane thymine dimer and guanine or 8-oxoguanine are synthesized, and their photoreactivities are compared. In both cases, the splitting of the ring takes place, leading to the formation of thymine, with a quantum yield 3.5 times lower than that for the guanine derivative. This result is in agreement with the more favored thermodynamics determined for the oxidized lesion. In addition, quantum chemistry calculations and molecular dynamics simulations are carried out to rationalize the crucial aspects of the overall cyclobutane thymine dimer photoreductive repair triggered by the nucleobase and its main lesion.

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers.

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein, we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes that can be used as sensitizers to promote the light-driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (εvis > 103 M-1 cm-1), maintain long excited-state lifetimes (τ ∼ 0.2-1 μs), and perform stable photoinduced charge transfer to a target substrate with high photoreducing potential (E+/* up to -2.33 V vs Fc+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that the two-coordinate complexes herein can perform photodriven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this "catalyst-free" system, the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuel photosensitizers that offer exceptional tunability and photoredox properties.

Partial Oxidation of Methane Enabled by Decatungstate Photocatalysis Coupled to Free Radical Chemistry

The decatungstate anion, [W10O32]4– or DT, is a useful photocatalyst for organic transformations involving C–H functionalization. Herein, we leverage the unique photoredox properties of DT to generate a chlorine radical from chloride ion for the photochemical partial oxidation of methane. Under optimized conditions, the DT–chloride–iodine ensemble achieves methane to methyl trifluoroacetate conversion with >350 photocatalyst turnovers at ∼60% yield based on methane in trifluoroacetic acid solvent. Methyl trifluoroacetate exhibits excellent stability under reaction conditions with minimal amounts of degradation (<6%) detected after 41 h. Based on density functional theory calculations, we propose a mechanism that involves synergistic relationships among the DT, chloride, and iodine species with the following key features: (1) photoredox electron transfer reaction of DT with Cl– to generate Cl•, (2) reaction of photoexcited DT with methane to generate methyl radicals via net hydrogen atom abstraction, (3) a Cl/I radical-based pathway in which methane is converted to MeTFA, and (4) reoxidation of reduced DT species by dioxygen. This mechanism takes advantage of the unique redox potential of DT and the ability of DT to mediate both electron transfer and hydrogen atom transfer reactions, ultimately generating an efficient pathway for aerobic methane partial oxidation.

Neutral, Heteroleptic [Cu(I)(PPh3 )2 (4H-imidazolato)] Complexes: Ligand Exchange Reactivity, Redox Properties, Excited-State Dynamics.

Cu(I) 4H-imidazolate complexes are rare examples of Cu(I) complexes with chelating anionic ligands and are potent photosensitizers with unique absorption and photoredox properties. In this contribution, five novel heteroleptic Cu(I) complexes with monodentate triphenylphosphine co-ligands are investigated. As a consequence of the anionic 4H-imidazolate ligand and in contrast to comparable complexes with neutral ligands, these complexes are more stable than their homoleptic bis(4H-imidazolato)Cu(I) congeners. Here, the ligand exchange reactivity was studied by 31 P-,19 F-, and variable temperature NMR and the ground state structural and electronic properties by X-ray diffraction, absorption spectroscopy, and cyclic voltammetry. The excited-state dynamics were investigated by femto- and nanosecond transient absorption spectroscopy. The observed differences, with respect to chelating bisphosphine bearing congeners, are often due to the increased geometric flexibility of the triphenylphosphines. These observations render the investigated complexes interesting candidates for photo(redox)reactions not accessible with chelating bisphosphine ligands.

Interfacial Chemical Bond Engineering in a Direct Z-Scheme g-C3N4/MoS2 Heterojunction.

The Z-scheme heterojunction shows great potential in photocatalysis due to its superior carrier separation efficiency and strong photoredox properties. However, how to regulate the charge separation at the nanometric interface of heterostructures still remains a challenge. Here, we take g-C3N4 and MoS2 as models and design the Mo-N chemical bond, which connects exactly the CB of MoS2 and VB of g-C3N4. Thus, the Mo-N bond could act as an atomic-level interfacial "bridge" that provides a direct migration path of charge carriers between g-C3N4 and MoS2. Experiments confirmed that the Mo-N bond and the internal electric field promote greatly the photogenerated carrier separation. The optimized photocatalyst exhibits a high hydrogen evolution rate that is about 19.6 times that of the pristine bulk C3N4. This study demonstrates the key role of an atomic-level interfacial chemical bond design in heterojunctions and provides a new idea for the design of efficient catalytic heterojunctions.

Band-structure tunability via the modulation of excitons in semiconductor nanostructures: manifestation in photocatalytic fuel generation.

Understanding the energetics of electron transfer at the semiconductor interface is crucial for the development of solar harvesting technologies, including photovoltaics, photocatalysis, and solar fuel systems. However, modern artificial photosynthetic materials are not efficient and limited by their fast charge recombination with high binding energy of excitons. Hence, reducing the exciton binding energy can increase the generation of charge carriers, which improve the photocatalytic activities. Extensive research has been dedicated to improving the exciton dissociation efficiency through rational semiconductor design via heteroatom doping, vacancy engineering, the construction of heterostructures, and donor-π-acceptor (D-π-A) interfaces to extend the charge carrier migration, promoting the dissociation of excitons. Consequently, functionalized photocatalysts have demonstrated remarkable photocatalytic performances for solar fuel production under visible light irradiation. This review provides the fundamental aspects of excitons in semiconductor nanostructures, having a high binding energy and ultrafast exciton formation together with promising photo-redox properties for solar to fuel conversion application. In particular, this review highlights the significant role of the excitonic effect in the photocatalytic activity of newly developed functional materials and the underlying mechanistic insight for tuning the performance of nanostructured semiconductor photocatalysts for water splitting, CO2 reduction, and N2 fixation reactions.

Keggin type heteropolyacid-mediated novel green protocol for the synthesis of porphyrins

Phosphovanadotungstic acid mediated novel strategy employs mild reaction conditions without use of halogenated solvents/oxidising agents and affords a commendable spectroscopic yield of 12–39%.

Extending the π-conjugation system of covalent organic frameworks for more efficient photocatalytic H2O2production

Three highly crystalline and porous COFs with promising photoredox properties have been prepared.Viaa post-sulfurization process, the imine-linked COFs were converted into thiazole-linked COFs, generating a highly conjugated and ultrastable photocatalyst for H2O2 production.

Chalcogenide‐Doped Anthracenes as Organophotocatalysts for Metal‐Free Atom Transfer Radical Polymerization

AbstractOrganocatalytic atom transfer radical polymerization (O‐ATRP) is emerging as a powerful tool for the synthesis of well‐defined, metal‐free polymers, with additional spatiotemporal control by light. Herein, based on the design logic of heteroatom‐doping of polycyclic arenes for photocatalyst development, a series of chalcogenide atom (e.g., O, S, Se, Te)‐doped anthracenes as potential photocatalysts for O‐ATRP is explored. It is found that these heteroatoms have a significant effect on the photophysical and photoredox properties, leading to different performance in the O‐ATRP of methyl methacrylate (MMA). Upon further optimization, the selenium analogues of N‐aryl phenothiazines are successfully identified as effective photocatalysts for O‐ATRP, which can promote a controlled radical polymerization of MMA.