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.     

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.     

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.     

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 two novel protoncontrollable isomeric diarylethenes with a pyridine moiety: substituent effects on their properties

Two novel hybrid isomeric diarylethenes with pyridine and pyrrole units have been synthesized by a parallel reaction, and their structures were determined by single-crystal X-ray diffraction analysis. They exhibited distinctly different photochromic behavior and fluorescent properties upon photoirradiation due to the different substituent effects. In the crystalline phase, one is photoactive with an anti-parallel conformation, while the other is non-photoactive with a parallel conformation. Moreover, the closed-ring isomers showed different acidichromism in dichloromethane. The absorption maxima of one showed extraordinarily large red shifts of 120 nm, whereas that of the other exhibited extraordinarily large blue shifts of 194 nm.     

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.     

Unique photochromism of two solid-emissive boron difluoride-based diarylethenes with isomeric structures

Two solid-emissive boron difluoride-based diarylethenes with isomeric structures are synthesized and the boron-difluoride chromophores are one part of the photoisomerization units in both diarylethenes. These two diarylethenes show similar fluorescent properties in the solid state as in dilute chloroform solutions because intermolecular planar π-π interactions are significantly alleviated as verified by their X-ray single crystal structures. However, only one of these two diarylethenes exhibits typical reversible absorption and fluorescence changes upon UV or visible light irradiation, and their diverse response to light irradiation is investigated by X-ray single crystal analysis and also DFT calculation. The investigation presented here provides valuable insight into the designing and development of diarylethene-based fluorescent switches in the solid state.     

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.     

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.     

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.     

A sensitive sensor for Cu(II) based on a novel diarylethene with a bipyridyl moiety

A novel diarylethene with a bipyridyl unit has been designed and synthesized for the first time. Its photochromic behaviors could be modulated by protonation and coordination with Cu(II). The absorption maximum of the closed-ring isomer shifted from 569 to 666 nm when trifluoroacetic acid was added. Furthermore, the closed-ring isomer behaved as a sensitive colorimetric sensor, exhibiting an open-ring reaction upon exposure to Cu(II). Its high selectivity toward Cu(II) over other competitive species makes the ‘naked-eye’ detection of Cu(II) possible.     

Molecular design for a write-by-light/erase-by-heat recording system using photochromic diarylethenes with thermal cycloreversion

We designed and synthesized photochromic diarylethenes for a write-by-light/erase-by-heat recording system. The introduction of polar substituents at both sides of the diarylethene maintained the photocyclization and photocycloreversion reactivities, but significantly affected the thermal cycloreversion reactivity. The introduction of electron-withdrawing substituents accelerated the thermal reaction and the introduction of electron-donating substituents suppressed the thermal reaction. The rate constants of the thermal reaction were well correlated with Brown-Okamoto's substituent constant σ⁺ that is a modified value of Hammett's substituent constant σ. The large rate constants are ascribed to the lower activation energy for the thermal reaction. These results provide new knowledge for the molecular design of diarylethenes for a write-by-light/erase-by-heat recording system.     

Solvent effect on photochromism of a dithienylperfluorocyclopentene having diethylamino group

Some photochromic diarylethenes having polar substituents were synthesized, and their photochromic behavior was examined. They exhibited photochromism with the similar photoreactivity in a non-polar solvent. However, the photocyclization quantum yield of dithienylperfluorocyclopentene having diethylamino group was found to decrease in polar solvents. The yield in acetonitrile was estimated to be 60 times smaller than that in hexane. The yields determined in various solvents were found to be correlated with the solvent polarity parameter E_T(30). Such an effect was not observed in dithienylfluorocyclopentenes bearing methoxy or acetoxy group and in non-fluorinated dithienylcyclopentene bearing diethylamino group.     

Characterization and photophysical properties of sulfur-oxidized diarylethenes

Controlled oxidation of sulfur atoms in benzothiophene rings of a photochromic diarylethene, 1,2-bis(2-methyl-1-benzothiophene-3-yl)perfluorocyclopentene (BTF6) by 3-chloroperbenzoic acid afforded various oxidation products such as BTFO1, BTFO2, and BTFO4. Upon irradiation with UV light, colorless o-BTFO1 and o-BTFO2 turned to red color, whereas colorless o-BTFO4 turned to yellow color. Theoretical calculation was performed to understand the absorption spectra of closed-ring isomers. All of those compounds returned back to their open-ring isomers upon irradiation with visible light. The cyclization conversions of BTFOn (n=1, 2, and 4) at the photostationary state are higher than that of BTF6. Interestingly, in the case of BTFO1, because of the unsymmetrical environment around the sulfoxide subunit in the molecule, the diastereoselective photocyclization was observed in 25% de. In addition, c-BTFO2 and c-BTFO4 also exhibited a marked fatigue resistance and strong fluorescence, respectively. Oxidation state of sulfur atom in a diarylethene can play an important role in determining its photophysical and photochromic properties.     

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.     

Photoswitching of an alcohol-sensitive photochromic diarylethene

The closed-ring isomer of diarylethene 1a, 1-(2-methyl-5-(4-N,N-diethylaminophenyl)thien-3-yl)-2-(2-methyl-5-phenylthien-3-yl)perfluorocyclopentene was found to cause the substitution reaction with primary alcohols at room temperature. The open-ring isomer 1a was stable in the alcohols. The product obtained in methanol was isolated by HPLC, and the structure was identified by ¹H NMR, mass spectrometry, and X-ray crystallographic analysis. It was revealed that two fluorine atoms were replaced with methoxy groups. The substitution reaction was also caused with ethylene glycol to form the five-membered ring. Both the products also showed photochromism, and had absorption maxima and photocycloreversion quantum yields different from those of 1a.     

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.     

A new DMAP-catalyzed and microwave-assisted approach for introducing heteroarylamino substituents at position-4 of the quinazoline ring

We report herein a new methodology for synthesizing quinazoline derivatives bearing a heteroarylamino moiety at position-4 of the quinazoline ring. As an alternative to the Buchwald–Hartwig cross-coupling reaction, which appears, until now, as the only efficient way to react 4-chloroquinazolines with numerous amino nitrogen-containing heterocycles displaying poor nucleophilicity, we developed a DMAP-catalyzed reaction involving microwave irradiation. Optimization of the reaction conditions led to the use of 30 mol % of DMAP in toluene, using a monomode microwave reactor and sealed vials. Moreover, the S_NAr reaction intermediate salt was isolated and fully characterized. Finally, the procedure was extended to two different 2-substituted-quinazoline series and also to various anilines, demonstrating that this approach was a general efficient way to access to such 4-substituted quinazoline scaffolds of high pharmaceutical interest.     

Characterization of the photochromism of dihydropyrenes with photophysical techniques

Dihydropyrenes are negative photochromes and their photophysical characterization, achieved in the past decade, is reviewed. Special emphasis is given to the theoretical background and to the experimental details which are relevant for the characterization of bi-stable systems, such as photochromic ones. In contrast to most photochromic compounds, dihydropyrenes have long lifetimes for their photoreactive excited states making it possible to tune their reactivity with the introduction of substituents. This knowledge is applicable for the design of more efficient photochromic molecules and for multichromophoric systems.     

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.