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.     

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.     

New photochromic diarylethenes bearing a condensed aromatics moiety

A new class of diarylethenes based on a hybrid structure of naphthalene and thiophene have been developed and their properties including photochromism, fatigue resistance, and fluorescence have been investigated. The condensed aromatic naphthalene was connected directly to the central cyclopentene ring as an aryl moiety and available to participate in the photoinduced cyclization reaction. All of these compounds have exhibited good photochromism, notable fatigue resistance, and obvious fluorescence photo-switches in both solution and polymer matrix. Nevertheless, significantly different properties have been observed among these diarylethenes, which may be attributed to the effect of different substituent groups. Compared with the analogs bearing other six-membered aryl unit, the naphthalene moiety is more conducive, which leads to higher molar absorption coefficient and blue shift of the absorption maximum.     

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.     

Photochromism of new unsymmetrical isomeric diarylethenes bearing a pyridine group

A new class of unsymmetrical isomeric diarylethenes bearing a terminal pyridine group was synthesized and their structures were determined by single-crystal X-ray diffraction analysis. The effects of the nitrogen atom position in the pyridine group on their properties including photochromism, acidchromism, and fluorescence were investigated systematically. Among the three isomeric derivatives, the cyclization quantum yield and the absorption maxima of the ortho-substituted diarylethene were the biggest, while the fluorescence quantum yield of the para-substituted diarylethene was the biggest. Furthermore, the three isomeric diarylethenes exhibited multi-addressable switching behaviors by the stimulation of acid/base and light. Addition of trifluoroacetic acid to solutions of these diarylethenes produced the protonated derivatives, which also showed excellent photochromic behaviors, but only the para-substituted diarylethene exhibited a notable color change of solution from crimson to violet. The results revealed that the effects of the nitrogen atom position in the terminal pyridine group played a very important role during the process of photoisomerization for these isomeric diarylethene derivatives.     

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.     

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.     

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 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.     

Syntheses and properties of new photochromic diarylethene derivatives having a pyrazole unit

New photochromic diarylethenes 1a, 2a and 3a bearing a pyrazole unit have been synthesized. Their properties, including photochromic reactivity, fluorescence and electrochemical properties were investigated. These compounds showed good photochromic properties, high cycloreversion quantum yield and relatively strong fluorescence. The cycloreversion quantum yields of 1a, 2a and 3a are 0.46, 0.53 and 0.57, respectively, which are larger than those of their cyclization quantum yields (0.43, 0.45 and 0.47, respectively). The oxidations of diarylethenes 1a, 2a and 3a were initiated at 0.73, 1.11 and 0.79 V, respectively. Moreover, the position of the methoxyl substituent had remarkable impacts on the above optical and electrochemical properties.     

Effects of Aromatic Stabilization Energies on Photochromism of New Asymmetrical Azaindole‐Containing Diarylethenes

Three new asymmetrical diarylethenes containing an azaindole moiety and a variable heteroaryl ring have been synthesized. Their properties, such as photochromism, fatigue resistance, thermal stability, acidichromism, and fluorescence, were systematically investigated to elucidate the effects of aromatic stabilization energies (ASE) of the heteroaryl moieties. The results indicated that thermal stability decreased with the increment of the aromatic stabilization energies of the variable heteroaryl rings in the order of thiazyl<thienyl<pyrrolyl. Moreover, the dual switching behaviors of these azaindole‐containing diarylethenes were also studied by the stimulation of acid/base and light. Addition of trifluoroacetic acid to the solutions of these diarylethenes resulted in obvious hypochromic shifts, and their N‐protonated forms also exhibited favorable photochromism.     

New photochromic diarylethenes with a six-membered aryl unit

A new class of photochromic diarylethenes bearing a six-membered aryl unit has been developed. They underwent reversible cyclization and cycloreversion reactions upon alternating irradiation with UV and visible light both in solution and in PMMA film. However, diarylethenes 1, 2, and 4 showed no photochromism in the crystalline phase although they packed with an anti-parallel conformation and the distances between the two reactive carbon atoms were both less than 4.2 Å.     

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 the effect of alkyl chain length on optoelectronic properties of diarylethene derivatives

Photochromic symmetrical diarylethene derivatives 1a-6a bearing different long alkyl chains at 2-position of thiophene rings have been synthesized and their structures have been determined by single-crystal X-ray diffraction analysis. The effect of alkyl chain length on their optoelectronic properties, such as photochromism in solution as well as in the crystalline phase and electrochemical performance was investigated in detail. These diarylethenes have showed good photochromic behavior both in solution and in the single crystalline phase. Introduction of the long alkyl chains at 2-position of bis(5-formyl-3-thienyl)perfluorocyclopentene increased the absorption coefficients of both open- and closed-ring isomers and induced bathochromic shifts of the maximal wavelength absorption of the closed-ring isomers. The long alkyl chains can also decrease the cyclization/cycloreversion quantum yields and the oxidation potentials. The cyclic voltammetry indicated that the band gap of these diarylethene derivatives was significantly affected by the alkyl chain length.     

Substituent effects on the properties of photochromic diarylethenes

Photochromic symmetrical diarylethenes 1o-5o bearing different electron-donating or electron-withdrawing substitutents have been synthesized, and the structures of 1o, 2o, 4o, and 5o were determined by single-crystal X-ray diffraction analysis. Substitutent effects on their optoelectronic properties, including photochromism, fluorescence, and electrochemical properties were investigated in detail. The electron-withdrawing substituents can shift significantly the absorption maxima of the diarylethenes to a longer wavelength and increase their cyclization quantum yield, while the molar absorption coefficients increased with an increasing electron-donating ability. Diarylethenes 1, 2, and 4 show good photochromism both in solution and in the single crystalline phase; however, diarylethenes 5 show no photochromism in the crystalline phase because the distance between the reactive carbons become larger than 4.2 Å. Diarylethenes 1-3 exhibited good fluorescent switching upon alternating irradiation with UV and visible light, and their fluorescent conversions in the photostationary state were all larger than 80% in hexane. In addition, cyclic voltammetry tests showed that different electron-donating and electron-withdrawing substituents had a remarkable effect on the electrochemical behaviors of these diarylethenes.     

New thermally stable photochromic di(het)arylethenes of the cyclopentenone series

Oxidation of 2,3-diarylcyclopent-2-en-1-ones with m-chloroperoxybenzoic acid gave new thermally stable sulfone derivatives of photochromic diarylethenes. The spectral properties of the compounds obtained (the wavelengths of the maxima of the absorption bands of their initial and cyclic forms, the quantum yields of photocyclization and photobleaching reactions) as well as their thermal stability and fatigue resistance were examined. The relationship between the structures of the synthesized compounds and their photochromic properties was determined. The energy differences between the ground-state molecules of the starting and photoinduced isomers of 2,3-diarylcyclopent-2-en-1-ones were calculated by the DFT/B3LYP1 method with the 6-31G(d) basis set. The calculated energy differences can be used to predict and explain such spectral characteristics of photochromic diarylethenes as the thermal stability of photoinduced isomers and the quantum yields of cycloreversion reactions.     

A novel strategy for developing yellow diarylethenes based on acid stimulus

A novel strategy for the preparation of yellow diarylethenes was established based on the acidichromism of two new diarylethenes with a bromopyridine moiety. The bromopyridine was connected directly to the central perfluorocyclopentene ring to participate in photoisomerization reaction. The two diarylethenes exhibited favorable photochromism and function as notable fluorescence switches in solution. The absorption maxima of their closed-ring isomers shifted dramatically to shorter wavelengths with notable color change from red to yellow upon the stimulation of trifluoroacetic acid.     

New thermally irreversible and fluorescent photochromic diarylethenes

A photokinetic investigation is here carried out on four newly synthesized diarylethenes with the aim to test their performance as photoreversible chromogenic and light emitting materials. The pentatomic ring, which fixes these diarylethenes in the cis conformation, contains a Si atom or a PO group. The 1,2 positions at the ethenic bond are symmetrically substituted with thienyl or benzothienyl groups. The results are compared with those for the structurally related and widely studied 1,2-bis(2-methyl-benzo[b]thiophen-3-yl)perfluorocyclopentene (BTF6), investigated here under the same experimental conditions. Spectra of the colorless and colored forms and photoreaction quantum yields were determined; temperature, excitation wavelength and viscosity effects were explored. Compounds containing benzothienyl substituents were found to be good bistable photochromes, with high photochemical yields of both the cyclization and cycloreversion reactions, and to display appreciable fluorescence emission from the colored forms, which is a rare and desirable property for photochromes. In contrast, the molecules not bearing the benzene condensed rings were found unsuitable as photochromes because of side degradation processes occurring in competition with cyclization.     

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.