Photoinduced Reactions Research Articles

Stereospecific Formation of the rctt Isomer of Bis-crown-Containing Cyclobutane upon [2 + 2] Photocycloaddition of an (18-Crown-6)stilbene Induced by Self-Assembly via Hydrogen Bonding.

The formation and the spectroscopic and structural properties of 1:1 and 2:1 (ligand-to-dication) complexes of an (18-crown-6)stilbene with ethane-1,2-diammonium diperchlorate in MeCN were studied by UV-vis and NMR spectroscopy and by density functional theory calculations. Prolonged UV irradiation of 2:1 mixtures of the crown stilbene and the diammonium salt led to the formation of two main photoproducts, namely, the single syn-"head-to-head" photodimer of the crown stilbene (rctt cyclobutane) due to supramolecular-assisted [2 + 2] photocycloaddition and a crown ether derivative of phenanthrene due to a photoinduced electrocyclization reaction. The rctt cyclobutane was isolated by preparative photolysis, followed by chromatography. The selectivity of the [2 + 2] photocycloaddition is explained by supramolecular pre-organization of crown stilbene molecules into the 2:1 complexes that have a pseudo-sandwich structure with stacking interactions between the stilbene moieties.

Aquatic photochemistry of Cu(II) in the presence of As(III): Mechanistic insights from Cu(III) production and As(III) oxidation under neutral pH conditions

Surface complexation between arsenite (As(III)) and colloidal metal hydroxides plays an important role not only in the immobilization and oxidation of As(III) but also in the cycle of the metal and the fate of their ligands. However, the photochemical processes between Cu(II) and As(III) are not sufficiently understood. In this work, the photooxidation of As(III) in the presence of Cu(II) under neutral pH conditions was investigated in water containing 200 μM Cu(II) and 5 μM As(III) under simulated solar irradiation consisting of UVB light. The results confirmed the complexation between As(III) and Cu(II) hydroxides, and the photooxidation of As(III) is attributed to the ligand-to-metal charge transfer (LMCT) process and Cu(III) oxidation. The light-induced LMCT process results in simultaneous As(III) oxidation and Cu(II) reduction, then produced Cu(I) undergoes autooxidation with O2 to produce O2•⁻ and H2O2, and further the Cu(I)-Fenton reaction produces Cu(III) that can oxidize As(III) efficiently (kCu(III)+As(III) = 1.02 × 109 M–1 s–1). The contributions from each pathway (ρrCu(II)-As(III)+hv = 0.62, ρrCu(III)+As(III) = 0.38) were obtained using kinetic analysis and simulation. Sunlight experiments showed that the pH range of As(III) oxidation could be extended to weak acidic conditions in downstream water from acid mine drainage (AMD). This work helps to understand the environmental chemistry of Cu(II) and As(III) regarding their interaction and photo-induced redox reactions.

The role of predissociation states in the UV photooxidation of acetylene

We study the photo-induced oxidation of acetylene in two different spectral areas, around 220 and 320 nm. In both spectral areas, the dominant reaction pathways involve the rupture of the acetylene C−H bond, thus primarily generating C2H and H fragments. The released fragments react in O2 presence resulting in glyoxal and other molecular species formation. We observe strong variations in the photooxidation rates, which point to the increased participation of excited, predissociation states, in the oxidation process. Such effects of increased excited state participation in photooxidation should be observed in a variety of photo-induced reactions, affecting the reaction spectral response and eventually the reaction cycles in terrestrial or planetary environments.

Palladium Hydride-Enabled Hydroalkenylation of Strained Molecules.

We report the first palladium hydride enabled hydroalkenylation of strained molecules. This new mild protocol proceeds via a regio- and chemoselective hydropalladation step, followed by a photoinduced radical alkyl Heck reaction. This methodology represents a new reactivity mode for strained molecules and opens new avenues for photoinduced palladium catalysis. The reaction is compatible with a wide range of functional groups and can be applied to complex structures, delivering a diverse array of highly valuable and modifiable alkenylated cyclobutanes and cyclopropanes. A hydroalkenylation/diastereoselective rearrangement cascade toward a cyclopentene scaffold has also been demonstrated.

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Understanding the Effect of an Amorphous Surface on the Ultrafast Dynamics of a Heterogeneous Photoinduced Reaction: CD3I Photoinduced Reaction on Amorphous Cerium Oxide Films.

In this work, to understand how an amorphous surface influences the dynamics of surface photoinduced reactions, pump-probe spectroscopy in conjunction with mass spectrometry is employed to track the ultrafast evolution of intermediates and final products with time, mass, and energy resolution. As a model system, the photoinduced reaction of CD3I adsorbed on amorphous cerium oxide films is investigated. A fraction of the first intermediates produced on a freshly prepared surface is trapped to passivate the surface. After the A-band excitation, the minimum dissociation time of CD3I indicates that CD3I adsorption geometries with either CD3 or I facing the gas phase exist; however, the transient data suggest that most molecules are adsorbed with the I atom facing the surface. CD3 and I are consumed to form I2 and reform CD3I, which are produced at a steady rate only after the intermediates have lost the excess translational energy released from photodissociation.

Decarboxylative Selective Phosphorylation of Aliphatic Acids: A Transition‐Metal‐ and Photocatalyst‐Free Avenue to Dialkyl and Trialkyl Phosphine Oxides from White Phosphorus

Developing practical and mild strategies for the direct functionalization of white phosphorus (P4 ) without chlorination is an appealing but formidable challenge. To this end, we report a breakthrough in the preparation of structurally diverse dialkylphosphines and trialkylphosphines that rely on the successive generation of carbon-centered radicals from N-hydroxyphthalimide (NHPI) esters and the controllable alkylation of the P4 molecule under transition-metal- and photocatalyst-free conditions. To facilitate separation and prevent product losses during purification, the corresponding oxidation products dialkylphosphine oxides (DAPOs) and trialkylphosphine oxides (TAPOs) were isolated. This photoinduced phosphorylation reaction features one-pot operation, high product selectivity, and tolerates a broad range of alkyl NHPI esters, including derivatives of complex natural products and pharmaceuticals. Further diversified transformation of DAPOs to construct P-F, P-C, P-N, and P-O bonds was also demonstrated.

Photoexcited Palladium-Initiated Remote Desaturation of N-Alkoxypyridinium Salts.

1,5-Hydrogen atom transfer (HAT) is an effective strategy to achieve remote desaturation of nonfunctionalized alkanes. Herein, we report a photoinduced remote desaturation reaction of N-alkoxypyridinium salts, which serve as alkoxyl radical precursors. Mechanistic studies show that a single electron transfer between the excited palladium complex and a N-alkoxypyridinium salt initiates a radical chain process leading to desaturation of N-alkoxypyridinium salts. This chain mechanism is supported by the measurement of the quantum yield of this reaction (Φ = 82). This reaction is applicable to a range of N-alkoxypyridinium salts, including some complex molecule-derived ones.

Decarboxylative Selective Phosphorylation of Aliphatic Acids: A Transition‐Metal‐ and Photocatalyst‐Free Avenue to Dialkyl and Trialkyl Phosphine Oxides from White Phosphorus

AbstractDeveloping practical and mild strategies for the direct functionalization of white phosphorus (P4) without chlorination is an appealing but formidable challenge. To this end, we report a breakthrough in the preparation of structurally diverse dialkylphosphines and trialkylphosphines that rely on the successive generation of carbon‐centered radicals from N‐hydroxyphthalimide (NHPI) esters and the controllable alkylation of the P4 molecule under transition‐metal‐ and photocatalyst‐free conditions. To facilitate separation and prevent product losses during purification, the corresponding oxidation products dialkylphosphine oxides (DAPOs) and trialkylphosphine oxides (TAPOs) were isolated. This photoinduced phosphorylation reaction features one‐pot operation, high product selectivity, and tolerates a broad range of alkyl NHPI esters, including derivatives of complex natural products and pharmaceuticals. Further diversified transformation of DAPOs to construct P−F, P−C, P−N, and P−O bonds was also demonstrated.

Visible Light-Driven Photoelectrochemical Platform Based Biosensor

The characterization of the photoinduced carriers' production, separation, and interfacial reaction processes is critical in developing effective photoelectrochemical (PEC) detection. The design and fabrication of a visible-light-driven PEC device based on CdSe-Co3O4@TiO2 nanoflower are presented in this report. In addition, an application for the ultrasensitive detection of viruses, including hepatitis E virus (HEV), HEV-like particles (HEV-LPs), and SARS-CoV-2 spike protein in complex lysate solution. The photocurrent response output of a CdSe-Co3O4@TiO2 based PEC device is improved compared to the individual components, TiO2 and CdSe-Co3O4. This can be ascribed to forming a heterojunction structure obtained by the hybridization of TiO2 and CdSe-Co3O4. CdSe QDs sensitization effect and strong visible light absorption can extend interfacial carrier lifetime and enhance energy conversion efficiency coupling with a robust oxygen-evolving catalyst (Co3O4) at the hole trapping site (CdSe) to improve overall system stability. The developed PEC-based biosensing device has been effectively used for quantitative virus detection through effective hybridization among antibody and viral specimens. The effective capture of the virus resulted in a linear relationship of change in photocurrent output versus the HEV concentration ranging from 10 fg mL–1 to 100 ng mL–1 with a detection limit of 3.5 fg mL–1. This CdSe-Co3O4@TiO2-based PEC device shows a significant ability to develop an improved device with affordable and portable biosensing capabilities.

Molecular-Modified Photocathodes for Applications in Artificial Photosynthesis and Solar-to-Fuel Technologies.

Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors.

Radical Mediated Rapid In Vitro Formation of c-Type Cytochrome.

A cytochrome c552 mutant from Thermus thermophilus HB8 (rC552 C14A) was reported, where the polypeptide with replaced Cys14 by alanine, overexpressed in the cytosol of E. coli. The apo-form of the C14A mutant (apo-C14A) without the original prosthetic group was obtained by simple chemical treatments that retained compact conformation amenable to reconstitution with heme b and zinc(II)-protoporphyrin(IX), gradually followed by spontaneous formation of a covalent bond between the polypeptide and porphyrin ring in the reconstituted apo-C14A. Further analysis suggested that the residual Cys11 and vinyl group of the porphyrin ring linked through the thiol-ene reaction promoted by light under ambient conditions. In this study, we describe the kinetic improvement of the covalent bond formation in accordance with the mechanism of the photoinduced thiol-ene reaction, which involves a thiyl radical as a reaction intermediate. Adding a radical generator to the reconstituted C14A mutant with either heme-b or zinc(II) porphyrin accelerated the bond-forming reaction, which supported the involvement of a radical species in the reaction. Partial observation of the reconstituted C14A in a dimer form and detection of sulfuryl radical by EPR spectroscopy indicated a thiyl radical on Cys11, a unique cysteinyl residue in rC552 C14A. The covalent bond forming mediated by the radical generator was also adaptable to the reconstituted apo-C14A with manganese(II)-protoporphyrin(IX), which also exhibits light-mediated covalent linkage formation. Therefore, the radical generator extends the versatility of producing c-type-like cytochrome starting from a metallo-protoporphyrin(IX) and the apo-C14A instantaneously.

Trajectory Surface Hopping for a Polarizable Embedding QM/MM Formulation.

We present the implementation of trajectory surface-hopping nonadiabatic dynamics for a polarizable embedding QM/MM formulation. Time-dependent density functional theory was used at the quantum mechanical level of theory, whereas the molecular mechanics description involved the polarizable AMOEBA force field. This implementation has been obtained by integrating the surface-hopping program Newton-X NS with an interface between the Gaussian 16 and the Tinker suites of codes to calculate QM/AMOEBA energies and forces. The implementation has been tested on a photoinduced electron-driven proton-transfer reaction involving pyrimidine and a hydrogen-bonded water surrounded by a small cluster of water molecules and within a large water droplet.

Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production.

Fe(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole-electron pairs, reactive radicals, and Fe(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Photoinduced Three‐Component Cyclization of Arylamines, Enaminones and Difluorobromoacetates to 2,3‐Difunctionalized Quinolines

AbstractA photoinduced multicomponent reaction of arylamines, enaminones and difluorobromoacetates for the synthesis of 2,3‐difunctionalized quinolines is reported. This strategy features broad functional groups tolerance and wide substrate scopes that enables further synthetic applications of the obtained products. Mechanistic studies reveal that intermolecular [3+3] cyclization between in‐situ generated 1,3‐vinylimine ions and arylamines is the key step in this transformation.magnified image

Visible-light-enabled multicomponent synthesis of trifluoromethylated 3-indolequinoxalin-2(1H)-ones without external photocatalysis

A novel and sustainable visible-light-enabled multicomponent reaction involving quinoxalin-2(1H)-ones, indoles, and CF3SO2Na that does not require an external photocatalyst is described. This photoinduced reaction employs air as the sole oxidant, thereby providing a green and highly step-efficient approach to a series of biologically important trifluoromethylated 3-indolequinoxalin-2(1H)-ones.

Ultrafast Charge Relocation Dynamics in Enol-Keto Tautomerization Monitored with a Local Soft-X-ray Probe.

Proton-coupled electron transfer (PCET) is the underlying mechanism governing important reactions ranging from water splitting in photosynthesis to oxygen reduction in hydrogen fuel cells. The interplay of proton and electronic charge distribution motions can vary from sequential to concerted schemes, with elementary steps occurring on ultrafast time scales. We demonstrate with a simulation study that femtosecond soft-X-ray spectroscopy provides key insights into the PCET mechanism of a photoinduced intramolecular enol* → keto* tautomerization reaction. A full quantum treatment of the electronic and nuclear dynamics of 2-(2′-hydroxyphenyl)benzothiazole upon electronic excitation reveals how spectral signatures of local excitations from core to frontier orbitals display the distinctly different stages of charge relocation for the H atom, donating, and accepting sites. Our findings indicate that ultraviolet/X-ray pump-probe spectroscopy provides a unique way to probe ultrafast electronic structure rearrangements in photoinduced chemical reactions essential to understanding the mechanism of PCET.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons.

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

Adsorption and Oxidation of NO2 on Anatase TiO2: Concerted Nitrate Interaction and Photon-Stimulated Reaction

The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination.

Spontaneous and photo-induced decay processes of WF5 - and HfF5 - molecular anions in a cryogenic storage ring.

Spontaneous and photo-induced decay processes of HfF5 - and WF5 - molecular anions were investigated in the Double ElectroStatic Ion Ring ExpEriment (DESIREE). The observation of these reactions over long time scales (several tens of ms) was possible due to the cryogenic temperatures (13K) and the extremely low residual gas pressure (∼10-14mbar) of DESIREE. For photo-induced reactions, laser wavelengths in the range 240 to 450nm were employed. Both anion species were found to undergo spontaneous decay via electron detachment or fragmentation. After some ms, radiative cooling processes were observed to lower the probability for further decay through these processes. Photo-induced reactions indicate the existence of an energy threshold for WF5 - anions at about 3.5eV, above which the neutralization yield increases strongly. By contrast, HfF5 - ions exhibit essentially no enhanced production of neutrals upon photon interaction, even for the highest photon energy used in this experiment (∼5.2eV). This suppression will be highly beneficial for the efficient detection, in accelerator mass spectrometry, of the extremely rare isotope 182Hf using the 182HfF5 - anion while effectively reducing the interfering stable isobar 182W in the analyte ion 182WF5 -. The radionuclide 182Hf is of great relevance in astrophysical environments as it constitutes a potential candidate to study the events of nucleosynthesis that may have taken place in the vicinity of the solar system several million years ago.