New photochromic diarylethenes with a pyrimidine moiety

A novel class of photochromic diarylethene derivatives based on the hybrid skeleton of six-membered pyrimidine and five-membered thiophene moieties has been firstly synthesized. The substituent effects on their properties, including photochromism, fatigue resistance, and fluorescence, have been systematically investigated. All these diarylethenes showed significant photochromism and notable fluorescence switching properties in both solution and poly(methylmethacrylate) films. The electron-donating substituent enhanced their cyclization quantum yield, fatigue resistance, and fluorescence quantum yield, whereas electron-withdrawing group exhibited contrary effects.     

The effects of heteroaryl ring on the photochromism of diarylethenes with a naphthalene moiety

Three new asymmetrical naphthalene-containing diarylethenes with different heteroaryl groups have been synthesized to investigate the heteroaryl effects on their properties. The three diarylethenes exhibited distinctive photochromism with good thermal stability, which may be attributed to the different heteroaryl effects. Their cycloreversion quantum yields increased in the order of 2-methylbenzofuran < 2-methylbenzothiophene < 1,2-dimethylindole, while the cyclization quantum yields exhibited a reverse trend. Compared to indole and benzothiophene, the benzofuran moiety could effectively shift the absorption maximum to a shorter wavelength and notably enhance the cyclization quantum yield and fluorescence quantum yield of the diarylethene. The results indicated that the category of heteroaryl groups played a vital role during the process of photoisomerization of naphthalene-containing diarylethene derivatives.     

Substituent position effect on the properties of new unsymmetrical isomeric diarylethenes having a chlorine atom

Three new unsymmetrical photochromic diarylethenes bearing a chlorine atom at para-, meta- and ortho-position of one terminal benzene ring, namely {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(4-chlorophenyl)-3-thienyl]}perfluorocyclopentene (para 1o), {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-chlorophenyl)-3-thienyl]}perfluorocyclopentene (meta 2o), and {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(2-chlorophenyl) -3-thienyl]}perfluorocyclopentene (ortho 3o), have been synthesized. The substituent position effect on their properties, including photochromism, fluorescence both in solution and in PMMA amorphous film and their electrochemical properties were investigated in detail. The results elucidated that the chlorine atom and its substituent position had a remarkable effect on the absorption characteristics, photochromic reactivity, fluorescence, as well as the electrochemical performances of these diarylethene compounds. For diarylethenes 1, 2 and 3, the cycloreversion quantum yields were gradually increased when the chlorine atom was attached to the para-, meta- and ortho-positions of the one terminal benzene rings; but their molar absorption coefficients both of their open-ring and closed-ring isomers were remarkably decreased. The fluorescent properties of para-substituted diarylethene embedded in poly(methyl methacylate) (PMMA) amorphous film showed good fluorescent switches (quenched to 28%). Furthermore, the cyclic voltammograms experiments elucidated that the electrochemical properties of these diarylethene derivatives were also remarkably dependent on the chlorine atom position effect, which may be attributed to the different electron-inducing ability and steric effect when the chlorine atom was substituted on the different position of the terminal benzene ring.     

Photochromism of new diarylethenes bearing both thiazole and benzene moieties

A new class of photochromic diarylethenes bearing both thiazole and benzene moieties has been developed, and the effects of substitution on their properties, including photochromism, fatigue resistance, and fluorescence properties have been investigated. They exhibited good photochromism and functioned as a fluorescence switch upon alternating irradiation with UV and visible light both in solution and in PMMA film. The electron-donating substituents could significantly enhance the cyclization quantum yield and depress the cycloreversion quantum yield whereas the electron-withdrawing groups functionalized an inverse action for these diarylethene derivatives. Relatively big differences exist among the properties of these diarylethenes which may be attributed to the different substituent effects.     

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.     

Substituent position effect on the optoelectronic properties of photochromic diarylethenes

Photochromic diarylethenes bearing fluorine atoms at the ortho-, meta-, or para-position of both terminal phenyl rings have been synthesized. Effect of the substituent positions on their optoelectronic properties, including photochromism in solution, fluorescence, and electrochemical properties were investigated for the first time. The results indicated that the position of the fluorine substituent had remarkable impacts on molar absorption coefficient of the closed-ring forms of diarylethenes, quantum yields of the cyclization and cycloreversion reactions, fluorescence intensity, as well as the oxidation potential.     

Photochromism of new 3,5-position hybrid diarylethene derivatives bearing both thiophene and thiazole moieties

Five new diarylethenes based on a hybrid structure of bis(5-thiazolyl)ethene and bis(3-thienyl)ethene were synthesized, and the structures of the four compounds were determined by single-crystal X-ray diffraction analysis. The properties of these diarylethenes, such as photochromism, fluorescence, and electrochemical properties were investigated in detail. All of these compounds showed good photochromism and fluorescence both in solution and in PMMA films. The electron-donating substituents could effectively increase the cyclization and cycloreversion quantum yields, and the fluorescence emission peaks, whereas the electron-withdrawing groups functionalized an inverse action for these diarylethene derivatives. Cyclic voltammetry revealed that great differences existed amongst the electrochemical behaviors of these compounds. The oxidation potentials and the band gaps of these diarylethenes increased remarkably with the increase in electron-withdrawing ability. All results suggested that the effects of substitution have a significant effect on the photochemical and electrochemical behaviors of these diarylethene derivatives.     

New photochromic diarylethenes bearing a pyridine moiety

A new class of diarylethenes bearing a pyridine unit has been firstly developed and their properties such as photochromism, fatigue resistance, and fluorescence have been discussed. The pyridine unit was connected directly to the central cyclopentene ring as one aryl moiety and availably participated in the photoinduced cyclization reaction even in the crystalline phase. All of these diarylethenes exhibited excellent photochromism, remarkable fatigue resistance, and notable fluorescence photoswitches both in solution and in poly(methylmethacrylate) films. Moreover, the different substituents had a significant effect on their properties. The results indicated that the electron-donating substituent could significantly enhance the cyclization quantum yield and depress the cycloreversion quantum yield while the electron-withdrawing group had a notable contribution to the cycloreversion quantum yield and fluorescence quantum yield for these diarylethenes.     

Minimal modification approach to red-shifted absorption and fluorescence in 1,8-naphthalimides

As a minimum modification approach toward longer wavelength chromophores, a series of (4-diethylamino-1,8-naphthaloyl)-aminopyridines were prepared with the pyridine nitrogen located at the ortho, meta, and para positions. Comparison between these isomeric neutral dyes and their corresponding pyridinium-1,3-propanesulfonate salts reveals a red-shift in both absorption (up to an emission of 1682 cm⁻¹ in ethyl acetate) and emission. These observed shifts along with increased fluorescence quantum yield are attributed to polarization induced by the quaternary nitrogen of the pyridinium cation.     

Control of the photoreactivity of diarylethene derivatives by quaternarization of the pyridylethynyl group

Photochromic behavior of diarylethene derivatives with (4-pyridyl)ethynyl group directly attached to the 6-pi hexatriene moieties of the diarylethenes was investigated. Upon quaternarization of the pyridine moieties, the photoreactivity was strongly suppressed. On the other hand, diarylethene derivatives with nonconjugated (4-pyridyl)ethyl group exhibited the photochromic reactivity, regardless of whether pyridyl rings are quaternarized or not. In the case of the (4-pyridyl)ethynyl-substituted compounds, the photochromic reactivity was suppressed by the addition of trifluoroacetic acid and was restored by diethylamine.     

The effects of phenyl-bridged glucosyltriazolyl groups on the properties of water-soluble diarylethene derivatives

Three water-soluble diarylethenes with glycosyltriazolyl groups and variable phenyl units have been synthesized, and their properties, such as water solubility, photochromism, and fluorescence have been discussed. Upon alternating irradiation with UV and visible light, each of the diarylethenes exhibited favorable photochromism and functioned as fluorescent switches in water. Experimental results showed that their water solubility as well as absorption features, cyclization quantum yields, photoconversion ratios, thermal stability, and fatigue resistance exhibited a strong correlation with the number of phenyl-bridged glucosyltriazolyl groups in diarylethene systems. The absorption maximum, thermal stability, and fatigue resistance were significantly improved, but the water solubility and fluorescent modulation efficiency were suppressed with the increase of the number of phenyl-bridged glucosyltriazolyl groups for these diarylethene derivatives.     

Synthesis and the effects of substitution upon photochromic diarylethenes bearing an isoxazole moiety

A new class of unsymmetrical photochromic diarylethenes bearing an isoxazole moiety was synthesized and the effects of substitution on their optical and electrochemical properties were investigated systematically. Each of the compounds exhibited remarkable photochromism and functioned as a fluorescent photoswitch both in solution and in poly(methyl methacrylate) films. The electron-donating substituents effectively shifted the absorption maximum and the emission peak to a longer wavelength direction, while the electron-withdrawing substituents notably enhanced the fluorescent quantum yields and oxidation onsets of these diarylethene derivatives. As compared to the unsubstituted parent diarylethene, introduction of the electron-donating/withdrawing substituents could efficiently modulate the optical and electrochemical properties of the diarylethenes bearing an isoxazole moiety. All results indicated that the isoxazole moiety and the substitution effects played a very important role during the process of photochromic reaction for these diarylethene derivatives.     

Large geometrical structure changes of photochromic diarylethenes upon photoirradiation

Photochromic diarylethene derivatives having phenylethynyl and pent-1-ynyl groups at the reactive carbons have been synthesized. These ethynyl groups enhance the cycloreversion quantum yields of diarylethenes without affecting the absorption maxima and the absorption coefficients of the closed-ring isomers. The derivatives exhibited large geometrical structural changes in the photoisomerization process.     

Synthesis and optical properties of C3-ethynylated chlorin and π-extended chlorophyll dyads

A C3-ethynylated chlorophyll derivative was prepared from methyl pyropheophorbide-d possessing a 3-formyl group by treatment of Bestmann-Ohira reagent. The mono-substituted acetylene was subjected to coupling reactions at the terminal acetylenic carbon atom to form a series of π-extended chlorophyll derivatives. Replacement of the terminal hydrogen to phenyl, phenylethynyl and C3-chlorin-ethynyl caused red-shifts of their Q_y (0,0) maxima from 675 to 679, 686, and 696 nm, respectively. Optical properties of C3²-substituted 3-ethynyl-chlorophyll derivatives including chlorophyll dyads were investigated in comparison with those of their related compounds. Partial quenching of the fluorescence emission (Φ_flu=0.14) was observed for ortho-substituted dyad, compared to meta- (Φ=0.27) and para-dyads (Φ=0.29), suggesting a through-space interaction between the two chlorin macrocycles in a molecule.     

New photochromic 1,2-diarylperfluorocyclopentenes bearing unsymmetrical six-membered aryl units

A new class of unsymmetrical photochromic diarylethenes based on the six-membered naphthalene-benzene and naphthalene-pyridine backbones has been firstly developed and their properties have been discussed. The two different six-membered aryl moieties were connected directly to the central cyclopentene ring and available to participate in the photoinduced cyclization reaction. The three diarylethenes exhibit distinctly different photochromism by photoirradiation in solution: they turn red, yellow, and orange upon photocyclization, which may be resulted from the different substituent and six-membered aryl moiety effects. Compared with the analog bearing a benzene unit, diarylethene bearing a pyridine moiety has a shorter absorption maximum and a smaller fluorescent quantum yield, and it shows a dual-addressable photo-switch by photoirradiation and acid/base stimulation.     

Synthesis and Cytotoxic and Antiviral Activity Profiling of All-Four Isomeric Series of Pyrido-Fused 7-Deazapurine Ribonucleosides

All four isomeric series of novel 4-substituted pyrido-fused 7-deazapurine ribonucleosides possessing the pyridine nitrogen atom at different positions were designed and synthesized. The total synthesis of each isomeric fused heterocycle through multistep heterocyclization was followed by glycosylation and derivatization at position 4 by cross-coupling reactions or nucleophilic substitutions. All compounds were tested for cytostatic and antiviral activity. The most active were pyrido[4′,3′:4,5]pyrimidine nucleosides bearing MeO, NH₂, MeS, or CH₃ groups at position 4, which showed submicromolar cytotoxic effects and good selectivity for cancer cells. The mechanism involved activation by phosphorylation and incorporation to DNA where the presence of the modified ribonucleosides causes double-strand breaks and apoptosis.     

Efficient One‐step Synthesis and Properties of Photochromic Diarylethenes Having an Indene Bridging Unit

Two novel isomeric photochromic diarylethenes with an indene bridging unit have been prepared by a simple and efficient one‐step synthesis method. Their properties, including photochromic behavior, fluorescent properties and fatigue resistance, have been investigated. These two isomeric compounds showed photochromic back‐and‐forth reactions with ultraviolet and visible light both in solution and in PMMA film. Their ring‐open forms exhibited appreciable fluorescence, which was quenched by the ring‐closed forms. All results indicated that diarylethenes derivatives with indene‐aryl bridges exhibited rather high fatigue resistance and good thermally irreversible photochromic properties.     

A new class of flavonoids bearing macrocyclic polyethers by stereoselective photochemical cycloaddition reaction

A new class of flavonoids bearing cyclic polyethers involving a phenyl ring was conveniently provided by the intramolecular photochemical dimerization of 2-chromonecarboxylic esters. Irradiation of 2-chromonecarboxylate with a polyether tethered at both ends promoted intramolecular [2 + 2] cyclobutane formation leading to 14- to 27-membered cyclic polyethers. The efficiency depended on the substituted position of the phenyl ring, with ortho- and meta-substituted derivatives giving cycloadducts in good chemical yields and quantum efficiencies, whereas the para-derivatives were inert toward photolysis. X-ray crystallographic analysis revealed that the stereochemistry of the macrocyclic cycloadducts exhibited C₂-symmetry.     

Synthesis and Properties of Photochromic Diarylethenes Bearing Thiazole Moiety

Four new diarylethene derivatives based on thiophene or thiazole moieties were designed and synthesized, and the structures of compounds 4a and 5a were determined by single‐X‐ray diffraction analysis. All of these compounds showed reversible photochromic reactions and notable fluorescence photo‐switches in solution. Furthermore, diarylethenes 4a and 5a also showed good photochromism in single crystal phase. The electron‐donating and electron‐withdrawing substituents play the same role in the photochromic process: red‐shifted absorption and fluorescence ( 1a and 2a compared with 3a ). The fluorescent modulation efficiencies of asymmetrical diarylethenes 4 and 5 were significantly enhanced compared with the similar dithienylethenes, and the fatigue resistance of 5 was much better than 4 , which showed that the diarylethene bearing electron‐withdrawing group could improve its fatigue resistance.     

Synthesis and properties of quinazoline derivatives containing cymantrenyl group

The synthesis of new quinazoline derivatives with 2,3- and 4-positioned cymantrenyl fragment is described. Alkylation of 2-substituted quinazolin-4-ones with cymantrenylalkyl bromides on using K₂CO₃, potassium tert-butoxide, and sodium hydride as bases has been studied. It is established for the first time that condensation of 2-methylquinazolin-4-ones with aldehydes can cause elimination of the substituent at the nitrogen atom in position 3. The new cymantrene derivatives possess fluorescence properties.