Electron Spin Polarization and Time-Resolved Electron Paramagnetic Resonance: Applications to the Paradigms of Molecular and Supramolecular Photochemistry

Most molecular and supramolecular organic photochemical reactions involve paramagnetic reactive intermediates (such as molecular triplet states, triplet radical pairs, and free radicals). In a number of cases these species are created with "anomalous" spin populations which are far from thermal equilibrium. Such paramagnetic species are said to be "spin polarized" and may be observed directly by time-resolved electron paramagnetic resonance (TREPR). The TREPR technique can be applied to exploit spin polarization, which, in addition to providing an enormous signal to noise enhancement, also reveals the mechanisms involved in photochemical reactions. TREPR spectroscopy provides a means of tracking the reaction of radicals with molecules and the nonreactive interactions of radicals with other radicals in real time. The latter interactions provide a systematic investigation of supramolecular interactions of geminate radicals in micelles.     

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.     

Rydberg states in chemistry

The excited states of molecules can be divided into three categories: valence-shell, Rydberg, and intermediate. Reasons are given why this classification is important for photochemistry. The photolytic primary steps should be different for excited states falling into these different categories. Rydberg states should be taken into account when the photochemical reaction path is interpreted in terms of orbital symmetry conservation or diradical intermediates. They might play a role in certain photobiological events.     

Physical insights into the photoactivated Ullmann coupling process producing highly conjugated oligothiophene films on a copper substrate

This paper describes the details of surface reactions producing >100-nm-thick conjugated polymer films. When 2,5-diiodothiophene films deposited on copper are irradiated with UV at room temperature in Ar environments, oligothiophene films are synthesized. The average conjugation length of the produced film varies from approximately 7 to 3-4 as the film thickness increases from approximately 100 to approximately 500 nm. The X-ray photoelectron spectroscopy analysis of the produced films reveals evidence for the formation of organo-copper intermediate species at the copper-monomer film interface and their diffusion from the copper surface into the monomer film during the photochemical process. A one-dimensional diffusion-reaction model is presented to explain the formation, diffusion, and reaction of organo-copper intermediates in the multilayer film during the photochemical reaction. The model simulation results qualitatively explain the decrease of the Ullmann coupling contribution in the photochemical reaction with the film thickness.     

Applications of time-resolved step-scan Fourier-transform infrared spectroscopy in revealing the light-initiated reactions in condensed phases

Step-scan Fourier-transform infrared spectroscopy (ssFTIR) simultaneously provides the spectroscopic and kinetic information of a given reaction. ssFTIR has been extensively employed to acquire the transient absorption and emission spectra in gas phase for identifying unstable species, for example, various Criegee intermediates, and elucidating the dynamics and kinetics of the reaction, such as the molecular elimination dynamics of haloalkenes and the bimolecular reactions involving chlorine atoms and singlet oxygen atoms. In addition to gaseous studies, ssFTIR has been also utilized to record the time-resolved difference spectra of the photochemical reactions in condensed phases, such as the photolysis of metal–ligand complexes, photocycles of the retinal proteins, coordination capability of solvents to unstable transient species, chemical reactions of atmosphere-related molecules in aqua, and the exciplex dynamics of organic light emitting materials. Moreover, my group has pioneered the recording of the transient thermal infrared emission of gold nanostructures upon photoexcitation. The experimental setups and the working principles for probing the time-resolved infrared absorption and emission in condensed phases will be revealed and a number of studies on chemical, biological, and materials systems will be described. These reported results demonstrate that ssFTIR is a versatile tool for exploring the properties of novel materials and photoreactions in condensed phases.     

Mechanistic Organic Photochemistry

Organic photochemical reactions can be understood as transformations of the electronically excited states of the reactant molecules. By considering Lewis structure or molecular orbital representations of these excited states it is possible to outline the several possible reactions available in the case of a given reactant. A number of different types of photochemical transformations are now reasonably well understood. In these cases one finds the same common controlling feature, namely the tendency for an excited state species to follow mechanistic pathways of minimum energy and the requirement for continuous electron redistribution in following these pathways. These preferred transformations can often be selected by inspection of relative bond orders for different types of bonding, by comparison of the potential energy surfaces available to the excited state molecules, and by use of correlation diagrams. The reactions derive from both singlet and triplet states, and one of the more reliable methods now available for identifying excited states reacting is termed the “fingerprint method”. Examples of the author's mechanistic approach are given both for ketone and for hydrocarbon photochemistry.     

Spectral Characteristics and Transformations of Intermediates in Irradiated Freon 11, Freon 113, and Freon 113a

Optical absorption bands observable in Freon 11, Freon 113, and Freon 113a irradiated at 77 K were assigned to various intermediates (radical cations, radical ion pairs, and complexes of radicals with ions). The transformations of these species in thermal and photochemical reactions occurring at 77 K were studied. On the basis of experimental results, it was suggested that the radical anions of Freon 11 and Freon 113 are unstable at 77 K and the spatial distribution of the intermediates produced is inhomogeneous.     

Photoinduced bimolecular cyclization of diarylamines with polyhalomethanes into acridines

It is shown on the basis of measurements of the activation parameters for the reaction of nucleophilic substitution, an intermediate stage of photochemical reactions, that the entropy factor makes the main contribution to the decrease in the reactivities of the intermediates that occurs on going from CHBr₃ to CBr₄ The low efficiency ofN-alkyl-substituted diarylamines in these photochemical transformations is explained by taking into consideration another intermediate stage (cyclization) and by quantum chemical modeling of the preceding isomerization.For Part 3, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 663–667, April, 1995.     

Regioselective Arene C−H Alkylation Enabled by Organic Photoredox Catalysis

Expanding the toolbox of C−H functionalization reactions applicable to the late-stage modification of complex molecules is of interest in medicinal chemistry, wherein the preparation of structural variants of known pharmacophores is a key strategy for drug development. One manifold for the functionalization of aromatic molecules utilizes diazo compounds and a transition-metal catalyst to generate a metallocarbene species, which is capable of direct insertion into an aromatic C−H bond. However, these high-energy intermediates can often require directing groups or a large excess of substrate to achieve efficient and selective reactivity. Herein, we report that arene cation radicals generated by organic photoredox catalysis engage in formal C−H functionalization reactions with diazoacetate derivatives, furnishing sp²–sp³ coupled products with moderate-to-good regioselectivity. In contrast to previous methods utilizing metallocarbene intermediates, this transformation does not proceed via a carbene intermediate, nor does it require the presence of a transition-metal catalyst.     

Time-resolved thermodynamics: heat capacity change of transient species during photoreaction of PYP

Heat capacity changes of short-lived transient species in different time ranges were measured for the first time by using the thermal component of the transient grating and transient lens signals at various temperatures. This method was applied to the transient intermediates of Photoactive Yellow Protein (PYP). The temperature dependence of the enthalpy change shows that the heat capacity of the short-lived intermediate pR2 (also called I1 or PYP(L)) species is the same as that of the ground state (pG) species within our experimental accuracy, whereas that of the long-lived intermediate pB (I2 or PYP(M)) is much larger (2.7 +/- 0.4 kJ/mol K) than that of pG. The larger heat capacity is interpreted in terms of the conformational change of the pB species such as melted conformation and/or exposure of the nonpolar residues to the aqueous phase. This technique can be used for photochemical reactions in general to investigate the conformational change and the hydrophobic interaction in a time domain.     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Regioselective Arene C−H Alkylation Enabled by Organic Photoredox Catalysis

Expanding the toolbox of C?H functionalization reactions applicable to the late‐stage modification of complex molecules is of interest in medicinal chemistry, wherein the preparation of structural variants of known pharmacophores is a key strategy for drug development. One manifold for the functionalization of aromatic molecules utilizes diazo compounds and a transition‐metal catalyst to generate a metallocarbene species, which is capable of direct insertion into an aromatic C?H bond. However, these high‐energy intermediates can often require directing groups or a large excess of substrate to achieve efficient and selective reactivity. Herein, we report that arene cation radicals generated by organic photoredox catalysis engage in formal C?H functionalization reactions with diazoacetate derivatives, furnishing sp²–sp³ coupled products with moderate‐to‐good regioselectivity. In contrast to previous methods utilizing metallocarbene intermediates, this transformation does not proceed via a carbene intermediate, nor does it require the presence of a transition‐metal catalyst.     

HNCO与CX（X=F，Cl，Br）自由基反应机理的密度泛函理论研究

用量子化学密度泛函理论的B3LYP方法,在6-31+G~*水平上按BERNY能量梯度解 析全参数优化了HNCO与CX（X=F,Cl,Br）反应势能面上各驻点的几何构型,通过 振动频率分析确认了中间体和过渡态,内禀反应坐标（IRC）对反应物、中间体、 过渡态和产物的相关性予以证实,对各驻点进行了零点能校正（ZPE）在此基础上 计算了反应能垒。研究结果表明,与HNCO和其它小分子自由基反应不同,HNCO与 CX自由基反应首先发生分子间H原子迁移,随后N与CX的C（1）原子相互靠近成键并 生成较稳定的中间体,再发生N-C（2）键的断裂,完成N向C（1）上的迁移并进一 步解离为产物。反应按反应物→TS1→IM→TS2→产物通道进行。反应为放热反应。     

Blue‐Light‐Induced Carbene‐Transfer Reactions of Diazoalkanes

Carbenes are very important reactive intermediates to access a variety of complex molecules and are applied widely in organic synthesis and drug discovery. Typically, their chemistry is accessed by the use of transition metal catalysts. Herein, we describe the application of low‐energy blue light for the photochemical generation of carbenes from donor–acceptor diazoalkanes. This catalyst‐free and operationally simple approach enables highly efficient cyclopropenation reactions with alkynes and the rearrangement of sulfides under mild reaction conditions, which can be utilized for both batch and continuous‐flow processes.     

Efficient photochemical electron transfer sensitization of homogeneous organic reactions

Photochemical electron transfer induced reactions have become an interesting tool in organic synthesis since transformations can be easily performed which are difficult or impossible with more conventional organic reactions. In this context, electron transfer sensitized reactions are frequently used since the sensitizer can be considered as a catalyst. Various intermediates such as radical ions with a variety of reaction possibilities are involved. Nevertheless, the reactions have been performed with high yields and high selectivities. Particular attention is paid to the stability of the sensitizer. Reaction steps regenerating the sensitizer from different intermediates are discussed. In photochemical electron donor and acceptor sensitized transformations, these steps are often part of the main reaction course. In other cases, co-sensitizers or mediators significantly enhance the efficiency of the transformations although the number of reactive intermediates is increased.     

1,3-Difunctionalizations of [1.1.1]Propellane via 1,2-Metallate Rearrangements of Boronate Complexes

1,3-Disubstituted bicyclo[1.1.1]pentanes (BCPs) are valuable bioisosteres of para-substituted aromatic rings. The most direct route to these structures is via multicomponent ring-opening reactions of [1.1.1]propellane. However, challenges associated with these transformations mean that difunctionalized BCPs are more commonly prepared by multistep reaction sequences with BCP-halide intermediates. Herein, we report three- and four-component 1,3-difunctionalizations of [1.1.1]propellane with organometallic reagents, organoboronic esters, and a variety of electrophiles. This process is achieved by trapping intermediate BCP-metal species with boronic esters to form boronate complexes, which are versatile intermediates whose electrophile-induced 1,2-metallate rearrangement chemistry enables a broad range of C−C bond-forming reactions.     

High-performance liquid chromatographic/tandem mass spectrometric identification of the phototransformation products of tebuconazole on titanium dioxide

Tebuconazole is a widely used fungicide. The formation of by-products on irradiated titanium dioxide as a photocatalyst was evaluated. Several species derived from tebuconazole degradation were identified and characterized by HPLC/MS(n). A pattern of reactions accounting for the observed intermediates is proposed. Different parallel pathways are operating (and through these pathways the transformation of the molecule proceeds), leading to a wide range of intermediate compounds. All these molecules are more hydrophylic than tebuconazole. The main steps involved are (1) the hydroxylation of the molecule with the formation of three species having [M + H](+) 324; the hydroxylation occurs on the C-1 carbon and on the aromatic ring in the two ortho-positions; (2) the cleavage of a C--C bond with the release of the tert-butyl moiety and the formation of a species having m/z 250; analogously to step 1, also on this species a further hydroxylation reaction occurs; (3) through the loss of the triazole moiety with the formation of a structure with m/z 257.     

Application of cycloaddition reactions to the syntheses of novel boron compounds

This review covers the application of cycloaddition reactions in forming the boron-containing compounds such as symmetric star-shaped boron-enriched dendritic molecules, nano-structured boron materials and aromatic boronic esters. The resulting boron compounds are potentially important reagents for both materials science and medical applications such as in boron neutron capture therapy (BNCT) in cancer treatment and as drug delivery agents and synthetic intermediates for carbon-carbon cross-coupling reactions. In addition, the use of boron cage compounds in a number of cycloaddition reactions to synthesize unique aromatic species will be reviewed briefly.     

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.     

Room Temperature Coupling of Aryldiazoacetates with Boronic Acids Enhanced by Blue Light Irradiation

A visible-light-promoted photochemical protocol is reported for the coupling of aryldiazoacetates with boronic acids. This photochemical reaction shows great enhancement compared to the same protocol performed in the absence of light. Except for a few cases, the room temperature coupling in the dark (thermal process) generally does not work. When it does, it is likely to also involve free carbenes as key intermediates. Alternatively, photochemical reactions show a broad scope, can be performed under air and tolerate a wide variety of functional groups. Reaction-evolution monitoring, DFT calculations and control experiments have been used to evaluate the main aspects of this intricate mechanistic scenario. Biologically active molecules Adiphenine, Benactyzine and Aprophen have been prepared as examples of synthetic applications.