Recent progress toward the exploitation of organic radical compounds with photo-responsive magnetic properties

Recent studies on the exploitation of novel organic radical compounds that exhibit photo-responsive properties by illumination are described in this tutorial review. Those so far reported consist of a variety of photo-responsive pi-electron moieties such as azobenzene, diarylethene, biindenylidene, terphenoquinone, anthracene, naphthopyran, arylimine, or hexaarylbiimidazoles, bearing stable organic radicals or generating organic radicals as spin centres for magnetic properties. The switching behaviours of their magnetic properties evoked by the stimuli of irradiation are discussed by taking the associated structural changes into consideration.     

Kinetics of the reaction of the TEMPO radical with alkylarenes

The kinetics of the reaction of the stable radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with a series of alkylarenes containing primary and secondary benzyl C—H bonds was studied by ESR, and the reaction rate constants were determined. The scheme of the process under study was examined, and the applicability boundaries of the simplification during analysis were shown. The selectivities of TEMPO and the more reactive cumylperoxyl radical were compared.     

Photochromism of diarylethene derivatives having an indene unit

Photochromic indene derivatives, 1-(2-methyl-1-inden-3-yl)-2-(2-methyl-1-benzothien-3-yl)perfluorocyclopentene and 1-(2-methyl-1-inden-3-yl)-2-(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene, were synthesized and their photochromic performance was examined.     

Synthesis and photochromism of novel unsymmetrical diarylethenes with an azaindole unit

A new class of unsymmetrical photochromic diarylethenes with an azaindole moiety has been firstly synthesized. Their properties, including photochromism, crystal structure, as well as fluorescence, were investigated systematically. The azaindole was connected directly to the central cyclopentene ring as a heteroaryl moiety and available to participate in the photoisomerization reaction. Each of the diarylethenes exhibited favorable photochromism, good thermal stability, remarkable fatigue resistance, and notable fluorescence switches in both solution and solid media. The substituents at the para-position of the terminal benzene ring affected evidently their properties: the electron-donating methoxy could be effective to enhance the cyclization quantum yield, while the electron-withdrawing cyano could shift the absorption maximum to a longer wavelength in both hexane and solid film. The results revealed that the introduction of azaindole moieties and different substituents played an important role in the photoisomerization process of these diarylethenes.     

A Dendritic Macrocyclic Organic Polyradical with a Very High Spin of S=10

Designed as a ferromagnetically coupled spin pentamer , the macrocyclic polyradical 1 possesses an average spin of S=10 in the ground state. This is the highest spin quantum number that has yet been measured for an organic molecule. Ar=tBuC₆H₄.     

Intermolecular radical carboaminohydroxylation of olefins with aryl diazonium salts and TEMPO

Highly reactive aryl radicals can selectively be reacted with a broad variety of activated and nonactivated olefinic substrates in the presence of nitroxyl radicals. Direct recombination of the aryl radical and the nitroxide as well as telomerization of the olefin is successfully suppressed by the reaction conditions.     

A Spin-Frustrated System Composed of Organic Radicals and Magnetic Metal Ions

Despite antiferromagnetic coupling between any two neighboring spins, parallel alignment of the 3d spins of the Mn^II ions and the 2p spins of the organic ligand is observed for the (degenerate) ground state of [{Mn(hfac)₂}₂(bnn)] (shown on the right). The “butterfly”-shaped arrangement of the spins contributes to this noteworthy spin configuration. hfac=hexafluoroacetylacetonate, bnn=2,2′-bis(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolinyl).     

Direct aminoalkylation of cycloalkanes through dimethylzinc-initiated radical process

Cycloalkyl radicals were directly generated from cycloalkanes via C-H bond cleavage by the action of dimethylzinc-air and underwent an addition reaction with N-tosylimines, giving aminoalkylation products in fair to high yields.     

Synthesis and catalytic activity of a fluorous-tagged TEMPO radical

A fluorous-tagged TEMPO radical has been prepared and its catalytic activity in the chemoselective oxidation of alcohols to carbonyl compounds has been investigated. The new fluorous radical proved to be an efficient, selective and easily recoverable catalyst, which can be conveniently used in standard organic solvents and then isolated and recycled by fluorous liquid-liquid extraction.     

Synthesis,Structure and Magnetic Properties of dimeric Nickel(II) Benzoate with Pyridyl‐substituted Nitronyl Nitroxides

The novel heterospin complex [Ni₂(PhCOO)₄(NITpPy)₂]·2CH₃CN ( 1 ) was synthesized by the reaction of nickel benzoate and 2‐(4‐pyridyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (NITpPy) in acetonitrile and dichloromethane solutions. The X‐ray structure determination shows that complex 1 consists of a symmetrical dimeric Ni^II benzoate paddle‐wheel core and pyridyl nitrogen atoms of radical ligands at the apical position, in addition, the temperature (2–300K) dependent magnetic susceptibility measurements indicate that 1 has antiferromagnetic behavior.     

Average Octet Radical Polymer: A Stable Polyphenoxyl with Star-Shaped π Conjugation

An average spin quantum number of 7/2 is displayed by the star-shaped, π-conjugated polyradical 1 , even with a spin concentration of 0.8 per monomer unit. Polymer 1 is stable at room temperature, and it is a powerful candidate for the synthesis of a feasible polymer with very high spin.     

New phosphaalkenes with small rings as substituents

Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1324–1325, July, 1993.     

A fluorescent sensor for Al3+ based on a photochromic diarylethene with a hydrazinobenzothiazole Schiff base unit

A new fluorescent turn-on chemosensor for Al³⁺ based on a diarylethene unit was designed and synthesized. Photochromism, fluorescence switch, and metal ion recognition behaviors of this diarylethene derivative were investigated by absorption and fluorescence emission spectra. It shows an outstanding fluorometric sensing ability toward Al³⁺ ion, and the detection limit was measured to be 9.3 × 10^?8 mol L^?1 via fluorescence methods. Based on these interesting properties, a combinational logic circuit was constructed successfully.     

The effect of substituents on the reactivity of aniline with phosphate radical

The reaction between aromatic amines and phosphate radical has been investigated. The reaction is accelerated by electron-releasing substituents and retarded by electron-withdrawing substituents pointing to an electrophilic attack by the PO₄^2xxx radical. σ⁺para values correlate the effect of substituents well. The rho value for PO₄^2xxx is more positive than that for CO₃^xxx indicating higher reactivity of phosphate radical towards aromatic amines than the carbonate radical.     

Tris(trimethylsilyl)methane is not an effective mediator of radical reactions

The reductive dehalogenation of organohalides by tris(trimethylsilyl)methane has been re-investigated. Contrary to claims made in a recent publication (Tetrahedron Lett.2006, 47, 5163-5165), (TMS)₃CH does not reduce organohalides. In competition experiments between (TMS)₃CH and the poor chain mediator Et₃SiH, the latter performed the reduction. Computational investigations support these experimental findings and indicate that the C-H bond of (TMS)₃CH is too strong for this compound to serve as an effective mediator of radical reactions.     

Light-fluorous TEMPO: reagent, spin trap and stable free radical

The synthesis of two light-fluorous TEMPO derivatives is reported, along with their fluorous-organic solvent partition coefficients and their ESR spectra. Applications of the fluorous-TEMPO reagents in oxidation reactions and as radical traps are discussed.     

A proton and optic dual-control molecular switch based on photochromic diarylethene bearing a rhodamine unit

A novel fluorescent switch based on rhodamine B and photochromic diarylethene, 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4-rhodamine B hydrazine-Schiff base-phenyl)-3-thienyl]perfluorocyclopentene (1), has been successfully synthesized through the condensation of rhodamine B hydrazine and 1-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]-2-[2-methyl-5-(4-formylphenyl)-3-thienyl]perfluorocyclopentene. UV and FL measurements reveal that the compound exhibits good photochromic properties responsive to proton and optic dual inputs. Upon irradiation with 297 nm light, the colorless solution of compound 1 turns blue, while the blue solution becomes colorless after irradiated with visible light (λ>450 nm). Furthermore, upon an addition of H⁺, the fluorescence resonance energy transfers from the rhodamine unit (FRET donor) to the closed-ring diarylethene unit (FRET acceptor), although no energy transfer occurs when the diarylethene is in the open-ring form. The emission intensity of the rhodamine can be modulated with proton and UV/vis light and molecular-level signal communication has been constructed, indicating high potentials of the compound in molecular switches or naked eye recognition systems.     

Phosphonyl radical addition to enol ethers. The stereoselective synthesis of cyclic ethers

Addition of diethyl phosphite or diethyl thiophosphite to enol ethers, in the presence of a radical initiator, results in the regioselective synthesis of organophosphonate or phosphonothioate derivatives, respectively, under mild conditions. This method can be applied to the stereoselective formation of substituted tetrahydrofurans and tetrahydropyrans, on cyclisation of vinyl ethers bearing unsaturated side chains.