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The development of fluorescent materials capable of harvesting triplet excitons efficiently is of great importance in achieving high-performance low-cost organic light-emitting diodes (OLEDs). Among the three mechanisms converting triplet to singlet excitons, triplet fusion delayed fluorescence (TFDF) plays a key role in the demonstration of highly efficient and reliable OLEDs, especially blue devices, for practice applications. This review focuses on the recent development of TFDF materials and their applications in OLEDs. Fundamental TFDF mechanism, molecular design principles, and the structure-property relationship of TFDF materials with a particular emphasis on their different excited state characters, are presented and discussed. Moreover, the future perspectives and ongoing challenges of TFDF materials are also highlighted.

Very recent investigations are described, which have shown how basic and unprotected nitrogen functionalities can be included, problem-free, in the furan photooxidation step of singlet oxygen-initiated cascade reaction sequences. The amine groups do not react with singlet oxygen, but, instead, participate later on in the sequences that ultimately yield a diverse range of important alkaloid motifs. To illustrate the versatility of this chemistry, six natural products were synthesised very rapidly and efficiently. Furthermore, all the new technologies operated under green conditions and without the use of a single protecting group.

The application of heavy-metal complexes in bulk-heterojunction (BHJ) solar cells is a promising new research field which has attracted increasing attention, due to their strong spin–orbit coupling for efficient singlet to triplet intersystem crossing. This review article focuses on recent advances of heavy metal complex containing organic and polymer materials as photovoltaic donors in BHJ solar cells. Platinum–acetylide containing oligomersor and polymers have been firstly illustrated due to the good solubility, square planar structure, as well as the fairly strong Pt–Pt interaction. Then the cyclometalated Pt or Ir complex containing conjugated oligomers and polymers are presented in which the triplet organometallic compounds are embedded into the organic/polymer backbone either through cyclometalated main ligand or the auxiliary ligand. Pure triplet small molecular cyclometalated Ir complex are also briefly introduced. Besides the chemical modification, physical doping of cyclometalated heavy metal complexes as additives into the photovoltaic active layers is finally demonstrated.

Thermally activated delayed fluorescence (TADF) emitters are primarily comprised of intramolecular charge-transfer (ICT) molecules with small energy difference between the lowest singlet and triplet excited states. They lend extremely favorable electroluminescent performance to organic light-emitting diodes (OLEDs). This paper summarizes relevant issues and research efforts in the theoretical prediction of singlet- and triplet-transition energies of ICT molecules via time-dependent density functional theory (TDDFT). The successful application of the descriptor-based optimal Hartree–Fock percentage method and the optimally tuned range-separated functional to many TADF systems represent an interesting approach to the exact prediction of the complex excited-state molecular dynamics within TDDFT.

A new series of spiropyrans based on 1,3-benzoxazine-2-one were synthesized using acid catalysis. Contrary to expectations, 6′-bromo-6-chloro-3-methyl-8′-nitro-spiro[1,3-benzoxazine-4,2′-chromene]-2-one and 6,6′-dichloro-3-methyl-8′-nitro-spiro[1,3-benzoxazine-4,2′-chromene]-2-one did not exhibit photochromic behavior, which was explained by TD DFT calculations to be a consequence of the existence of an effective non-radiative decay pathway allowing the structural relaxation of the spiropyrans in the first singlet excited state.

Deactivation pathways of the triplet metal-to-ligand charge transfer (3MLCT) excited state of cyclometalated polypyridine ruthenium complexes with [RuN5C]+ coordination are discussed on the basis of the available experimental data and a series of density functional theory calculations. Three different complex classes are considered, namely with [Ru(N^N)2(N^C)]+, [Ru(N^N^N)(N^C^N)]+ and [Ru(N^N^N)(N^N^C)]+ coordination modes. Excited state deactivation in these complex types proceeds via five distinct decay channels. Vibronic coupling of the 3MLCT state to high-energy oscillators of the singlet ground state (1GS) allows tunneling to the ground state followed by vibrational relaxation (path A). A ligand field excited state (3MC) is thermally accessible via a 3MLCT → 3MC transition state with the 3MC state being strongly coupled to the 1GS surface via a low-energy minimum energy crossing point (path B). Furthermore, a 3MLCT → 1GS surface crossing point directly couples the triplet and singlet potential energy surfaces (path C). Charge transfer states either with higher singlet character or with different orbital parentage and intrinsic symmetry restrictions are thermally populated which promote non-radiative decay via tunneling to the 1GS state (path D). Finally, the excited state can decay via phosphorescence (path E). The dominant deactivation pathways differ for the three individual complex classes. The implications of these findings for isoelectronic iridium(III) or iron(II) complexes are discussed. Ultimately, strategies for optimizing the emission efficiencies of cyclometalated polypyridine complexes of d6-metal ions, especially RuII, are suggested.

In this review, we address recent studies of porphyrin-based molecular systems for efficient singlet oxygen (1O2) generation. Porphyrins have a highly conjugated, delocalized 18-π electron system (for the shortest cyclic path) that is responsible for the strong absorption and emission characteristics in the visible region. Singlet oxygen is useful for applications in photodynamic cancer therapy (PDT), photooxidation of toxic molecules, and photoproduction of important intermediates for various chemicals owing to its high oxidation ability. Porphyrin and its analogues have been investigated as photosensitizers for 1O2 generation. The production of 1O2 is successfully regulated by photophysical parameters, such as intersystem crossing, triplet state (T1) lifetime, and photosensitizer to 3O2 energy transfer in molecular systems. Introduction of substituents with heavy atoms (halogen, metal) and radicals onto the photosensitizer (PS) exerts a strongly positive impact on intersystem crossing of the singlet excited state of the PS, promoting generation of the triplet excited state. Environmental conditions, such as solvent and pH, may also influence 1O2 generation. As a means to limit cellular photodamage, two-photon absorption and DNA switches for 1O2 generation have been proposed and developed. Achieving control of singlet oxygen generation is pertinent to many arenas at the cutting edge of modern science, ranging from the chemical industry to biomedical applications.

Carbonyl compounds studied are confined to acetyl halide (CH3COCl), acetyl cyanide (CH3COCN), acetyl sulfide (CH3COSH), acetaldehyde (CH3CHO), and methyl formate (HCOOCH3). They are asymmetrically substituted, but do not follow the well-known Norrish type I reactions. Each compound ejected in an effusive beam at about 300 K is commonly excited to the 1(n, π*)CO lower state; that is, a nonbonding electron on O of the CO group is promoted to the antibonding orbital of π*CO. The photolysis experiments are conducted in the presence of Ar gas and the corresponding fragments are detected using time-resolved Fourier-transform Infrared (FTIR) emission spectroscopy. The enhancement of the collision-induced internal conversion or intersystem crossing facilitates the dissociation channels via highly vibrational states of the ground singlet (So) or triplet (T1) potential energy surfaces. In this manner, an alternative nonadiabatic channel is likely to open yielding different products, even if the diabatic coupling strength is strong between the excited state and the neighboring state. For instance, the photodissociation of CH3COCl at 248 nm produces HCl, CO, and CH2 fragments, in contrast to the supersonic jet experiments showing dominance of the Cl fragment eliminated from the excited state. If the diabatic coupling strength is weak, dissociation proceeds mainly through internal conversion, such as the cases of CH3COCN and CH3COSH. The photodissociation of CH3COCN at 308 nm has never been reported before, while for CH3COSH matrix-isolated photodissociation was conducted that shows a distinct spectral feature from the current FTIR method. The CH3CHO and HCOOCH3 molecules belong to the same type of carbonyl compounds, in which the molecular products, CO + CH4 and CO + CH3OH, are produced through both transition state and roaming pathways. Their products are characterized differently between molecular beam and current FTIR experiments. For instance, the photodissociation of HCOOCH3 at 248 nm yields CO with the vibrational state v ≥ 4, in contrast to the molecular beam experiments producing CO at v = 1. The photodissociation of CH3CHO at 308 nm intensifies a low energy component in the CH4 vibrational distribution, thus verifying the transition state pathway for the first time.

Carbenoids are a class of highly reactive reagents that play an important role in modern organic synthesis. These species are very similar to singlet state carbenes since they have an ambiphilic character and react by a concerted mechanism allowing stereospecific transformations. Herein, we discuss these and other chemical aspects of zinc, lithium and magnesium carbenoids as well as relevant applications of these intermediates in organic synthesis.

A honeycomb layer of transition metal-centered bismuth cubes has attracted attention as part of the weak topological insulator (TI) Bi14Rh3I9. The intermetallic layer itself is a two-dimensional TI and has been found in Bi13Pt3I7 and Bi12Pt3I5, as well as in the new material Bi38Pt9I14, yet with different stacking sequences and types of insulating spacer layers in all cases. The arrangements strongly influence the electronic situation, especially the coupling between the TI layers. X-ray diffraction and electron microscopy show that in Bi38Pt9I14 the intermetallic layers are exclusively separated by single layers of iodide ions. The eclipsed stacking of the honeycomb layers results in hexagonal channels. These host hitherto unknown linear [BiI2]− anions with bismuth atoms in the uncommon +I oxidation state. Quantum chemical calculations as well as magnetic susceptibility data indicate a singlet state stabilized by strong spin–orbit coupling. Bi38Pt9I14 is a semiconductor with a very narrow energy gap, as ...

Radicals are an important class of species which act as intermediates in numerous chemical and biological processes. Most of the radicals have short lifetimes. However, radicals with longer lifetimes can be isolated and stored in a pure form. They are called stable radicals. Over the last five decades, the syntheses of several stable radicals have been reported. Recently, highly unstable radicals have been successfully stabilized via strong σ-donation of singlet carbenes. Cyclic aklyl(amino) carbene (cAAC) is regarded as a stronger σ-donor and a better π-acceptor when compared with that of an N-heterocyclic carbene (NHC). In this article we review preferentially the results of our group to generate stable radical centers on the carbene carbon atoms by employing the so far hidden and unique ability of the cAACs. We focus on the development of new synthetic routes to stable and isolable radicals containing silicon atoms. All the compounds have been well characterized by single crystal X-ray analysis; the mono-radicals have been distinguished by EPR spectroscpy and the ground state of the diradicals has been studied by magnetic susceptibility measurements and theoretical calculations. Many of these compounds are studied by cyclic voltammetry and are often converted to their corresponding radical cations or radical anions via electron abstraction or addition processes. Some of them are stable, having long lifetimes and hence are isolated and characterized thoroughly. Not much information has been obtained on the short lived persistent radical species. Herein, we discuss some of the examples of such a type of species and focus on what kind of chemical reactions are initiated by these short-lived radical species in solution. We also briefly mention the syntheses and charaterization of the so far reported stable silicon centered radicals.

Boron dipyrromethene (Bodipy) is one of the most extensively investigated organic chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochemistry study, photodynamic therapy (PDT), photocatalysis and triplet–triplet annihilation (TTA) upconversion. The recent developments in the study of the production, modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Forster-resonance-energy-transfer (FRET), or the intermolecular charge transfer (ICT). Controlling the triplet excited state will give functional molecules such as activatable PDT reagents or molecular devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) production, photoredox catalytic organic reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.

Elusive nature in spin state is a feature of singlet biradical molecules and arises from the weak coupling of unpaired electrons. This highlight review focuses on recent advances in experimental ap...

Carbene intermediates are very useful species for a range of reactions including C-H insertions and cycloadditions. They are most commonly generated by metal-catalyzed release of nitrogen gas from diazo precursors. Herein, we present a novel C-H insertion of simple β-ketoamide substrates, through reaction with (diacetoxyiodo)benzene (DIB) in the presence of a base. This unprecedented transformation bypasses the use of either diazo precursors or metal catalysts and directly delivers β-lactam products by an iodonium ylide, in a single step under mild conditions. Mechanistic studies support the intermediacy of a free singlet carbene of unique reactivity and selectivity.

ConspectusCarbenes are compounds that feature a divalent carbon atom with only six electrons in its valence shell. In the singlet state, they possess a lone pair of electrons and a vacant orbital and therefore exhibit Lewis acidic and Lewis basic properties, which explains their very high reactivity. Following the preparation by our group in 1988 of the first representative, a variety of stable carbenes are now available, the most popular being the cyclic diaminocarbenes.In this Account, we discuss another class of stable cyclic carbenes, namely, cyclic (alkyl)(amino)carbenes (CAACs), in which one of the electronegative and π-donor amino substituents of diaminocarbenes is replaced by a σ-donating but not π-donating alkyl group. As a consequence, CAACs are more nucleophilic (σ-donating) but also more electrophilic (π-accepting) than diaminocarbenes. Additionally, the presence of a quaternary carbon in the position α to the carbene center provides steric environments that differentiate CAACs dramatically fr...

The phenalenyl radical is a polycyclic aromatic hydrocarbon (PAH) radical. Owing to its widely distributed spin structure, phenalenyl is relatively stable compared to other hydrocarbon radicals and has been studied from the viewpoint of its application to electroconductive and magnetic materials. In addition, a strong intermolecular spin-spin coupling nature is another feature of phenalenyl. This account summarizes my studies so far into PAH radicals containing the phenalenyl scaffold in terms of their amphoteric redox properties and singlet biradical character, which strongly rely on the characteristic electronic structure, that is, non-bonding character and sixfold symmetry of a singly occupied molecular orbital of the phenalenyl radical.

coupled aromatic protons H-5′, H-2′ and H-6′, respectively. The aromatic protons H-10 resonated as a one-proton singlet at δ 7.58. The oxygenated methylene protons appeared at δ 6.08 as two-proton broad singlets where as methoxy protons resonated as two three-proton broad singlets at δ 4.05 and 3.99. A one-proton doublet at δ 3.85 (

In this paper, we offer a brief review of the ground-state properties of the cubic oxides AnO2, where An = U , Np, Pu and Am, as revealed mainly by NMR studies of the 17O2− ligand. For PuO2, where the ground state is a nonmagnetic singlet eigenstate of the cubic crystal field, only the 239Pu has been studied, in a recent breakthrough observation of this elusive isotope [1] . For UO2 and NpO2, the former has an exotic four-sublattice antiferromagnetic (AFM) ground state, while the latter has the first multipolar ground state to be identified among actinide compounds, namely a mixture of octupolar and rank 5 (triakontadipolar) order. On the other hand AmO2, even with the longest-lived isotope 243Am, becomes disordered so quickly from radiation self-damage that its ground state in a recent study was simply a spin glass, while the actual ground state of the cubic crystalline compound remains obscured by this experimental problem. The emphasis throughout is on how 17O NMR studies complement other experimental data to confirm and verify the known exotic magnetic ground states of AnO2 systems.

A new photoinduced three-component reaction between a cyanoarene, an alkene and an N-protected pyrrole has been developed. This reaction extended the scope of the photo-NOCAS reaction by introducing pyrrole as a neutral carbon-centered nucleophile. The cyanoarenes used include tetracyanobenzene (TCB), 2,3,5,6-tetrafluoro-1,4-dicyanobenzene (TFDCB) and 1,4-dicyanobenzene (DCB). N-Methyl, N-phenyl and N-Boc pyrroles are suitable nucleophiles in the reaction. Taking advantage of the strong electron acceptor ability of the singlet excited TCB, a wide range of alkenes, including the highly electron deficient 4-fluoro-, 4-chloro-, 2,3,4,5,6-pentafluorostyrenes and N-methylmaleimide take part in this reaction, leading to the simultaneous 1,2-diarylation of the alkene and the regioselective 2-alkylation of the pyrrole ring via sequential formation of two new C–C bonds between the three reactants.

This paper reviews results from study of mononuclear ruthenium complexes capable of catalyzing the oxidation of water to molecular oxygen. These catalysts may be classified into three groups, with different rate laws associated with O2 evolution. In one class, O2 evolution proceeds via radical coupling of the oxygen atom of an RuV=O species with a hydroxocerium(IV) ion. O2 evolution catalyzed by the second class occurs via acid–base reaction of the oxygen atom of an RuV=O species with a water molecule. In the third group, the dominant mechanism is oxo–oxo radical coupling between two RuV=O species. Several significant properties of the oxidant Ce(IV) are also discussed, including the singlet biradical character of the hydroxocerium(IV) ion.

Conversion of a simple furan into the ABCD-ring skeleton of the azaspiracids via a singlet oxygen-initiated one-pot process is described.

Owing to the electron spin-orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure-property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.

Polycrystalline samples of AgxVP2O7 (0.7 ≤ x ≤ 0.9) are prepared by solid state reaction of mixtures of composition 2xAg3PO4, 4xVPO4, 3(1-x)VPO5, and (3-x)V(PO3)3 (sealed quartz tubes, 1073 K, 12 h).

ConspectusResearchers have studied polycyclic aromatic hydrocarbons (PAHs) for more than 100 years, and most PAHs in the neutral state reported so far have a closed-shell electronic configuration in the ground state. However, recent studies have revealed that specific types of polycyclic hydrocarbons (PHs) could have a singlet biradical ground state and exhibit unique electronic, optical, and magnetic activities. With the appropriate stabilization, these new compounds could prove useful as molecular materials for organic electronics, nonlinear optics, organic spintronics, organic photovoltaics, and energy storage devices. However, before researchers can use these materials to design new devices, they need better methods to synthesize these molecules and a better understanding of the fundamental relationship between the structure and biradical character of these compounds and their physical properties. Their biradical character makes these compounds difficult to synthesize. These compounds are also challen...

The properties of singlet and triplet excited states are strongly medium-dependent. Hence, these species constitute valuable tools as reporters to probe compartmentalised microenvironments, including drug@protein supramolecular systems. In the present review, the attention is focused on the photophysical properties of the probe drugs (rather than those of the protein chromophores) using transport proteins (serum albumins and α1-acid glycoproteins) as hosts. Specifically, fluorescence measurements allow investigation of the structural and dynamic properties of biomolecules or their complexes. Thus, the emission quantum yields and the decay kinetics of the drug singlet excited states provide key information to determine important parameters such as the stoichiometry of the complex, the binding constant, the relative degrees of occupancy of the different compartments, etc. Application of the FRET concept allows determination of donor–acceptor interchromophoric distances. In addition, anisotropy measurements can be related to the orientation of the drug within the binding sites, where the degrees of freedom for conformational relaxation are restricted. Transient absorption spectroscopy is also a potentially powerful tool to investigate the binding of drugs to proteins, where formation of encapsulated triplet excited states is favoured over other possible processes leading to ionic species (i.e. radical ions), and their photophysical properties are markedly sensitive to the microenvironment experienced within the protein binding sites. Even under aerobic conditions, the triplet lifetimes of protein-complexed drugs are remarkably long, which provides a broad dynamic range for identification of distinct triplet populations or for chiral discrimination. Specific applications of the laser flash photolysis technique include the determination of drug distribution among the bulk solution and the protein binding sites, competition of two types of proteins to bind a drug, occurrence of drug–drug interactions within protein binding sites, enzymatic-like activity of the protein or determination of enantiomeric compositions. The use of proteins as supramolecular hosts modifies the photoreactivity of encapsulated substrates by providing protection against oxygen or other external reagents, by imposing conformational restrictions in the binding pockets, or by influencing the stereochemical outcome. In this review, a selected group of examples is presented including decarboxylation, dehalogenation, nucleophilic addition, dimerisation, oxidation, Norrish type II reaction, photo-Fries rearrangement and 6π electrocyclisation.

The potential energy surfaces of the singlet and triplet states of the title clusters are computed by DFT methods.

In modern research on organic light-emitting diodes (OLEDs), cyclometalated iridium(III) complexes represent one of the most studied class of compounds. The high emission efficiency caused by the strong spin-orbit coupling in the presence of heavy metals leads to the mixing of singlet and triplet manifolds so that both the singlet and triplet excitons can be harvested. For OLEDs to be useful in displays application, true red, green, and blue emissions of sufficient luminous efficiencies and proper chromaticity are required. In recent years, the development of materials for phosphorescent red OLEDs has indeed gone through several important evolutional stages. However, the luminescent quantum yields of red-emitting iridium(III) phosphors tend to be intrinsically low which are governed by the energy gap law for triplet states in which the luminescence quantum yields tend to decrease with an increase in the emission wavelength. Many red organic dyes currently in use do not show a good compromise between device efficiency and color purity. In general, a dilemma facing red OLEDs was realized in which efficient and bright dopants are not red enough, and red-enough dopants are not efficient and bright. In this review article, we highlight the recent progress and current challenges of efficient OLEDs based on cyclometalated iridium(III) dyes which exhibit saturated red and near-infrared electroluminescence. Optimization of the phosphorescent red OLED efficiency/color purity trade-off and extension of the work to other organometallic phosphors are also presented and discussed.

Various reactions for a stable dialkylsilylene, 2,2,5,5-tetrakis(trimethylsilyl)silacyclopentane-1,1-diyl (1), are summarized and their mechanisms are discussed. Silylene 1 isomerizes to the corresponding silaethene via the 1,2-trimethylsilyl migration. Reduction of 1 with alkali metals affords the corresponding radical anion 1 . − with a relatively small 29Si hfs constant (2.99 mT) and a large g-factor (g = 2.0077) compared with those for trivalent silyl radicals. Photo-excitation of 1 generates the corresponding singlet excited state (1 1 ∗ ) with the lifetime of 80.5 ns. The excited state reacts with C=C double bond compounds including benzene, naphthalene, and (E)- and (Z)-2-butenes. Although the thermal reactions of 1 with haloalkanes occur via radical mechanisms, the insertion into O–H, Si–H and Si–Cl bonds proceeds concertedly via the three-membered cyclic transition states. The reaction of 1 with H2SiCl2 gives the Si–Cl insertion product exclusively, while the quantitative insertion to Si–H bond occurs when Me2SiHCl is used as a substrate. The origin of the rather unusual Si–H/Si–Cl selectivity is elucidated using DFT calculations. Silylene 1 adds to C=C, C≡C, and C=O π bonds to afford the corresponding silacycles as stable compounds. The importance of the carbonyl silaylides during the reactions of silylenes with aldehydes and ketones is emphasized.

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

The dimerization of anthracene by a [4π + 4π] cycloaddition is one of the oldest and best known reactions in photochemistry. In the series of tethered bichromophoric arenes, this reaction type could be extended to anthracene–naphthalene, naphthalene–naphthalene and recently even to anthracene–benzene and naphthalene–benzene systems. Cyclophanes, which can be regarded as twofold or multiple tethered systems, are not discussed here. The cycloisomerizations are performed by irradiation at the long-wavelength absorption (λ > 270 nm), whereas shorter wavelengths (λ < 270 nm) lead to cycloreversions, which can be also achieved by a thermal route. The systems represent therefore a P- and T-type photochromism, which can be used for optical or chiroptical switches. An acceleration of the switch is possible by a singlet energy transfer (light harvesting antenna effect) in dendritic compounds. In the past 5 to 10 years many applications of these switches were studied in the context of photonic devices, sensor techniques, lithographic processes, imaging techniques, data processing and data storage.

Hetero-doped nitrogen-rich compounds are of great interest for potential use in high energy density materials (HEDM) fields. In this paper, we report in detail the structures and stabilities of a tetra-atomic cluster CN3−, which is isoelectronic to the well-known N4. A series of higher-level energetic calculations were carried out at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d), CASPT2(12,12)/6-311+G(d)//CASSCF (12,12)/6-311+G(d) and CASPT2(12,12)/aug-cc-pVTZ//CASSCF(12,12)/aug-cc-pVTZ levels. The kinetic stability of CN3− was evaluated for the first time by studying the dissociation, isomerization and intersystem crossing barriers, as well as the Born–Oppenheimer molecular dynamic (BOMD) simulations. We found that the ground state isomer, i.e., chain-like triplet NCNN−(Cs), and a higher-energy isomer, i.e., tetrahedral-like singlet CN3−(C3v), are both kinetically stable, even when the intersystem crossing (ISC) is included. Therefore, the two isomers, 3NCNN− and 1CN3− (C3v), could be observable in future laboratory studies. By comparison with the isoelectronic N4 system, we conclude that the carbon-doping for XN3 (X = N to C−) greatly enhances the kinetic stability of the chain-like triplet and the tetrahedral-like singlet structures. Finally, for more practical use, our studies on the related salts M+[CN3]− (M = Li, Na, K) showed that both the chain-like 3NCNN− and tetrahedral-like 1CN3− could act as promising building blocks for novel HEDMs.

Photoreactions of oximes have been known for 120 years. The earliest recognized reaction was geometric isomerization, which is still the most prominent oxime photoreaction; it proceeds via a ππ* triplet state. More than sixty years later the intriguing photo-Beckmann rearrangement came to light, a C–C=N–O to O=C–N–C conversion, generating, for example, ɛ-caprolactam from cyclohexanone ξιμɛ. The rearrangement proceeds via a ππ* singlet state forming an oxazirane intermediate; the oxazirane ring usually opens in stereospecific fashion. Detailed insight into the stereo- and regiochemistry of the rearrangement was gained from steroidal oximes, the most thoroughly investigated class of oximes. Additional photoreactions of oximes or of their ethers and esters include: net loss of hydroxylamine generating carbonyl compounds; loss of water from aldoximes forming nitriles; conversion of ketoximes to nitriles by more complex mechanisms; photochemical O–H cleavage generating iminoxyl radicals, an interesting family of free radicals; N–O cleavage giving rise to iminyl radials, which undergo radical cyclizations. β,γ-Unsaturated oximes undergo aza-di-π-methane rearrangements, cyclize to oxazolines, and undergo further reorganizations. The palette of oxime reactions has been further enriched by triplet energy transfer or electron transfer sensitization; oximes serve as electron donors or acceptors. Keywords: Light-induced chemistry of oximes and derivatives; HEINZ D. ROTH; E, Z isomerization; photo-Beckmann rearrangement; oxazirane formation; O–H cleavage; N–O cleavage; aza-di-π-methane rearrangement; isoxazoline formation; iminoxyl radical; iminyl radial; steroidal oximes; oxo-oximes

This microreview describes the tremendous progress that has been made during the past decade in the area of ferrocene-based singlet carbenes, the vast majority of which belong to the family of N-heterocyclic carbenes (NHCs). The ferrocene motif is present in such NHCs either as a ferrocenyl (Fc) substituent or a 1,1′-ferrocenediyl (fc) backbone. In terms of applications, the modification of popular ferrocene-based chiral P ligands by following the phosphane-NHC analogy has been an important guideline for much of the work directed towards catalysis. A particularly exciting development has occurred with NHCs that contain an fc backbone, since these compounds have proved to be capable of small-molecule activation. The recent development of singlet carbenes that are not based on ferrocene but rather on related ruthenium sandwich complexes demonstrates the wide scope and highly dynamic nature of the field.

Nuclear singlet states may have lifetimes that are much longer than the conventional relaxation time of nuclear spin magnetization. This review covers how these states may be generated, observed, and exploited in solution nuclear magnetic resonance (NMR). Potential applications include the study of slow molecular processes, the elucidation of molecular geometry, and the transport of hyperpolarized nuclear spin order.

The title reaction proceeds via singlet N-nitrene intermediates to give a series of allylic diazene products.

The recent and rapid enhancement in power conversion efficiencies of organic-based, bulk-heterojunction solar cells has been a consequence of both improved materials design and better understanding of the underlying physical processes involved in photocurrent generation. In this Perspective, we first present an overview of the application of quantum-chemical techniques to study the intrinsic material properties and molecular- and nano-scale processes involved in device operation. In the second part, these quantum-chemical tools are applied to an oligomer-based study on a collection of donor–acceptor copolymers that have been used in the highest-efficiency solar cell devices reported to date. The quantum-chemical results are found to be in good agreement with the empirical data related to the electronic and optical properties. In particular, they provide insight into the natures of the electronic excitations responsible for the near-infrared/visible absorption profiles, as well as into the energetics of the low-lying singlet and triplet states. These results lead to a better understanding of the inherent differences among the materials, and highlight the usefulness of quantum chemistry as an instrument for material design. Importantly, the results also point to the need to continue the development of integrated, multiscale modeling approaches to provide a thorough understanding of the materials properties.

[Ru(terpy)(bpy)(OH2)]2+ and its analogues were found to be highly active as catalysts toward water oxidation in the presence of Ce4+ as an oxidizing reagent in acidic aqueous media. These findings were considered as a significant breakthrough in this field because there had been a long belief that the four-electron process (i.e., 2H2O→O2+4H++4e−) is much more effectively accelerated by dinuclear or tetranuclear metal complexes. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration, revealing that these catalysts can be classified into two groups exhibiting different rate laws for O2 evolution. Moreover, the singlet biradical character of the hydroxocerium(IV) ion is realized, indicating that the radical coupling of the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis. Several important insights into the mechanism of oxygen evolution from water catalyzed by the mononuclear aquaruthenium complexes will also be discussed.

The utility of 1,2,3,4-tetrahydropyrimidine-5-carboxylate (1a,b), and 1,2-dihydropyrimidine-5-carboxylate (2) in the synthesis of some new functionalized pyrimidine derivatives such as 6-(2-(dimethylamino)vinyl)-1,2,3,4tetrahydropyrimidine, 6-(1-cyano-2-(phenylamino)-2-thioxoethyl)-1,2,3,4-tetrahydropyrimidine and 4,6-distyryl-1,2-dihydropyrimidine derivatives is reported. Antimicrobial evaluation of some selected examples from the synthesized products was carried out and showed promising results. Pyrimidine derivatives are reported to possess antibacterial, antifungal anticancer, antiinflammatory, and cardioprotective effects. In addition, certain pyrimidine derivatives are known to display antimalarial, antifilarial and antileshmainal activities. Also, pyrimidine nucleus is considered as one of the most important classes of chemotherapeutic drugs. Many pyrimidine derivatives are used for thyroid drugs and leukemia. In view of the these results and as an extension of our recent work concerned with the synthesis of heterocycles of interested biological activity we decided to synthesize some new pyrimidine derivatives starting from the readily obtainable 1,2,3,4-tetrahydropyrimidine-5-carboxylate derivatives 1a, 1b, and dihydropyrimidine 2 as highly versatile and useful building blocks for the synthesis of a wide variety of pyrimidine derivatives and to test their antimicrobial activity. 1,2,3,4-Tetrahydropyrimidine 1a was found to react readily with dimethylformamide dimethyl acetal (DMF-DMA) to produce ethyl 6-(2-(dimethylamino)vinyl)-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine5-carboxylate (3) (Scheme 1). The IR spectrum of the product 3 showed absorption bands at, 1624, 1701, 3117, 3244 cm due to two carbonyl groups, and two imino functions, respectively. Its H NMR spectrum showed a triplet signal at  1.11 (J = 6.9 Hz) due to CH3 protons, singlet signal at  2.85, characteristics for two methyl protons, a quartet signal at  3.96 (J = 6.9 Hz) due to CH2 protons, a doublet signal at  5.17 (J = 3.6 Hz) due to CH proton, two doublets at  6.18 and  7.48 (J = 14.1 Hz) due to two olefinic protons, and two D2O-exchangeable signals at  7.52 and 8.61 corresponding to two NH protons, in addition to an aromatic multiplet in the region  7.22-7.29. The value of the coupling constant for the ethylenic protons indicates that the pyrimidine 3 exists exclusively in the E-configuration.

Over the last three decades the rational design of diradicals has been a challenging issue because of their special features and activities in organic reactions and biological processes. The orbital phase theory has been developed for understanding the properties of diradicals and designing new candidates for synthesis. The orbital phase is an important factor in promoting the cyclic orbital interaction. When all of the conditions: (1) the electron-donating orbitals are out of phase; (2) the accepting orbitals are in phase; and (3) the donating and accepting orbitals are in phase, are simultaneously satisfied, the system is stabilized by the effective delocalization and polarization. Otherwise, the system is less stable. According to the orbital phase continuity requirement, we can predict the spin preference of π-conjugated diradicals and relative stabilities of constitutional isomers. Effects of the intramolecular interaction of bonds and unpaired electrons on the spin preference, thermodynamic and kinetic stabilities of the singlet and triplet states of localized 1,3-diradicals were also investigated by orbital phase theory. Taking advantage of the ring strains, several monocyclic and bicyclic systems were designed with appreciable singlet preference and kinetic stabilities. Substitution effects on the ground state spin and relative stabilities of diradicals were rationalized by orbital interactions without loss of generality. Orbital phase predictions were supported by available experimental observations and sophisticated calculation results. In comparison with other topological models, the orbital phase theory has some advantages. Orbital phase theory can provide a general model for both π-conjugated and localized diradicals. The relative stabilities and spin preference of all kinds of diradicals can be uniformly rationalized by the orbital phase property. The orbital phase theory is applied to the conformations of diradicals and the geometry-dependent behaviors. The insights gained from the orbital phase theory are useful in a rational design of stable 1,3-diradicals.

2,4,6-Tri-tert-butyl-1,3,5-triphosphinine (7) was combined with 1,5-COD or 2,4-di-tert-butyl-1,3-diphosphete to form iron π-complexes. The ligands of highly reactive [(2,4-di-tert-butyl-1,3-diphosphete)(2,4,6-tri-tert-butyl-1,3,5-triphosphinine)Fe] (8) are formed by cyclooligomerization of tert-butylphosphaalkyne (3) under the influence of the iron atom. 8 rearranges spontaneously to yield penta-tert-butyl-pentaphosphaferrocene (9) as the isolable product. An intramolecular electron transfer product [(1,5-COD)(η6-1,3,5-triphosphacyclohexa-2,5-dine-1,4-diyl)Fe(2+)] (12) is obtained with 1,5-COD. Addition of [(CO)5Cr(THF)] initiates an interligand hydrogen transfer to form [(η5-trihydropentalenyl)Fe(μ,1-3-η-4,5,6-trihydro-1,3,5-triphosphinine)Cr(CO)5] (13) Extensive DFT calculations of isolated and reactive species, and several possible intermediates agree with the experimental observations and revealed for the first time a possible reaction sequence, which allows a low-activated exchange of ring elements between the ligands of sandwich complex 8 to form 9. The process is based on the specific combination of the metal and its heterocyclic π-ligands. Both, singlet and triplet spin states play an important role in the process.

The first intramolecular thermal reactions of cyclopropenone ketals are reported and the work examined substrates tethered to an electron-deficient olefin bearing a single electron-withdrawing substituent. Whereas the intermolecular variants of the reactions provide only the products of an endo-selective [1 + 2] cycloaddition or a carbonyl addition reaction of a thermally generated π-delocalized singlet vinylcarbene, the intramolecular variants provide either [1 + 2] or [3 + 2] cycloadducts in reactions that depend on the reaction conditions, the alkene-activating substituent, and the nature of the tethering. In addition to providing key mechanistic insights into the thermal [3 + 2] cycloaddition reaction for such substrates, they were also found to proceed under conditions that reflect the ease and regioselectivity of the cyclopropenone ketal cleavage for π-delocalized singlet vinylcarbene generation. The most effective combination of structural features that impact the reactivity was observed with subst...

Photolysis of matrix-isolated diazoethane (1a) at 8 K produces mainly ethene (4a), along with a small amount of 3-methyldiazirine (2a). ESR experiments employing a variety of irradiation conditions and matrix media (Ar, N 2 , Xe) show that triplet ethylidene (3T) is not an observable photoproduct. Similar experiments fail to reveal direct evidence for either singlet or triplet ethylidene by IR or UV-vis spectroscopy. Photolysis of nascent 3-methyldiazirine (2a) also fails to yield ethylidene. Deuterium substitution produces no detectable change in the chemistry

"True sensitized" photo-SET reactions from a variety of tertiary amines and singlet excited cyanoarenes [mainly 1,4-dicyanonaphthalene and 9,10-dicyanoanthracene] have been observed. The free amine radical cation, formed via a solvent separated ion pair, undergoes efficient deprotonation-desilylation at the carbon adjacent to nitrogen. The synthetic usefulness of this step is demonstrated in generating reactive intermediates such as nitrones, iminium cations, α -amino radicals and azomethine ylides depending on the substrates and reaction medium. 1. Introduction 2. Generation of Cyclic Nitrones: A Versatile 1,3-Dipole 3. Generation of Regiospecific Iminium Cation 3.1. Synthesis of Oxaazabicyclo[m.n.o]alkanes and Related Compounds 3.2. Stereoselective Synthesis and Utilization of Tetrahydro-1,3-oxazines 3.3. SET Promoted Photo N-Debenzylation and N-Dealkylation 4. Generation and Reactivity of α-Amino Radical (or Equivalent) 5. Generation of Non-Stabilized Azomethine Ylides 6. Conclusion.

Ab initio GVB-CI (generalized valence bond theory with configuration interaction) and MCSCF (multiconfiguration self-consistent-field) calculations have been carried out on methylene and ethylidene (CH3CH) to investigate the effects of methyl substitution on the singlet-triplet splitting of methylene. We predict that ethylidene has a singlet-triplet splitting of 3 f 2 kcal/mol, about 6 kcal/mol less than the value for methylene. Furthermore, our investigations show that neither steric effects nor hyperconjugative singlet stabilization effects are large enough to account for this decrease in the singlet-triplet splitting. Rather, the dominant effects of the CH3 group are to (i) decreasep character in the nonbonding u orbital of the triplet state, (ii) reduce electron donation to the methylenic carbon, and (iii) induce repulsions in the triplet state between the CH3 and the singly occupied methylenic C PA orbital. All of these effects destabilize the triplet ground state and thus decrease the singlet-triplet splitting. I. Introduction The geometric and electronic structure of substituted carbenes is of importance in synthetic chemistry. The diverse reactivity of such molecules can be understood in terms of their electronic structure. The singlet state (u2) has both of its nonbonding electrons in a u orbital localized on the carbenoid carbon while the triplet state (uW) has one electron in that u orbital and one in a a orbital also localized on the carbenoid carbon. As a result, the chemistry in which they participate is very different: singlet carbenes undergo one-step stereospecific reactions while triplet carbenes participate in two-step, nonspecific reactions.’ The trends in the reactivity of carbenes can thus be better understood upon determination of their singlet-triplet splittings. The singlet-triplet gap in a substituted carbene depends strongly on the hybridization of the u and a orbitals of the central carbon. For both the triplet and the singlet states, the A orbital is essentially a pure p-like orbital while the u orbital is a hybrid of s- and p-like atomicorbitals. Thus, u orbitals with increased p character favor