Self-assembly of photochromic diarylethenes with amphiphilic side chains: reversible thermal and photochemical control

Diarylethene derivatives with hexaethylene glycol side chains were synthesized and their self-assembling and photochromic reactivity were investigated. The diarylethenes showed photochromism in organic solvents and even in water. The aqueous solution of the compounds turned turbid quickly upon heating. The clouding behavior was investigated using 1H NMR spectroscopy, dynamic light scattering, and absorption spectroscopy. It was revealed that, in the aqueous solution, the compounds self-assembled into aggregates, and the aggregates were loosened by raising the temperature. The cloud-point temperature of the closed-ring isomer was 5-7 degrees C lower than that of the open-ring isomer. When asymmetric methyl groups were introduced in the amphiphilic side chains, induced circular dichroism (ICD) was observed upon irradiation with UV light in water. This ICD was explained by the difference in the self-assembling behavior between the open- and the closed-ring isomers. It was suggested that the closed-ring isomers assembled into a chiral nanostructure.     

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.     

Photo-, solvent-, and ion-controlled multichromism of imidazolium-substituted diarylethenes

Cationic diarylethenes with an imidazolium ring are synthesized for the first time. The imidazolium cationic moiety is connected directly to the ethene unit as one of the aryl units that take part in the photoinduced pericyclization reaction. The imidazolium-substituted diarylethenes undergo reversible photochromic reactions in a variety of organic media, including ionic liquids, even though they have a delocalized cationic charge in one of the five-membered aromatic rings. The closed-ring isomer shows solvatochromism depending on the solvent donor numbers. Addition of some tetraalkylammonium salts, such as tetrabutyl ammonium nitrate, into the colored organic solution of diarylethene also causes a color change, indicating its ionochromic property. These solvato- and ionochromic properties are considered in connection with the shift of chemical equilibrium between the closed-ring isomers, one with an extended pi-conjugation system and one with limited pi-conjugation due to the strong interaction with solvent molecules and anions with high donor number.     

New photoswitching unit for magnetic interaction: diarylethene with 2,5-bis(arylethynyl)-3-thienyl group

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene, and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have an extended pi-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity. Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum originating from the change in the magnetic interaction between two unpaired electrons. The open-ring isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic interaction between two radicals via the pi-conjugated chain was altered by photocyclization due to the change of the hybrid orbital at the 2-position of the thiophene ring from sp(2) to sp(3).     

Effective photoswitching of intramolecular magnetic interaction by diarylethene: Backgrounds and applications

Diarylethene is a photochromic molecule, which is potentially applicable to various optoelectronic devices. In diarylethenes the π-system of the two aryl rings is separated in the open-ring isomer, while the π-system is delocalized throughout the molecule in the closed-ring isomer. Based on this idea the exchange interaction between two nitronyl nitroxide radicals connected by a diarylethene unit was photocontrolled reversibly along with photochromism. The switching efficiency is more than 150-fold and thiophene spacer transmits the interaction more efficiently. When diarylethene dimer is used for the switching unit, the electric circuit-like behavior was observed. Moreover, reversed switching is achieved by reversing the thiophene ring. Magnetochemical and photochemical behavior of the radical-substituted diarylethene is described extensively.     

Photoresponsive dithienylethene-urea-based organogels with "reversed" behavior

Dithienylperhydrocyclopentene-bisurea-based low molecular weight gelators are described that function as photoresponsive organogels that show a remarkable gel-to-liquid transition upon irradiation. The two series of derivatives, with and without alkyl spacers between the urea hydrogen bonding groups and the photochromic unit, show different gelation behavior. Upon UV irradiation of the gels, a gel liquified at only 1.4% conversion of the photochromic unit. Transmission electron microscopy (TEM) shows that the gel fibres consist of thin ribbons. Semi-empirical (PM3) calculations indicate that the hydrogen bonding between the open-ring isomer (o) molecules is weak, and that formation of the closed-ring isomer (c) destabilises the hydrogen bonding further. The results indicate that a small amount of the closed-ring isomer will disrupt the intermolecular hydrogen-bonding, leading to disintegration of the gel fibre ribbons and hence reversible liquification.     

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.     

Geometrical Evolution and Formation of the Photoproduct in the Cycloreversion Reaction of a Diarylethene Derivative Probed by Vibrational Spectroscopy

The geometrical evolution of the reactant and formation of the photoproduct in the cycloreversion reaction of a diarylethene derivative were probed using time-resolved absorption spectroscopies in the visible to near-infrared and mid-infrared regions. The time-domain vibrational data in the visible region show that the initially formed Franck-Condon state is geometrically relaxed into the minimum in the excited state potential energy surface, concomitantly with the low-frequency coherent vibrations. Theoretical calculations indicate that the nuclear displacement in this coherent vibration is nearly parallel to that in the geometrical relaxation. Time-resolved mid-infrared spectroscopy directly detected the formation of the open-ring isomer with the same time constant as the decrease of the closed-ring isomer in the excited state minimum. This observation reveals that no detectable intermediate, in which the population is accumulated, is present between the excited closed-ring isomer and the open-ring isomer in the ground state.     

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.     

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.     

In situ preparation of highly fluorescent dyes upon photoirradiation

Photoswitchable or photoactivatable fluorescent dyes are potentially applicable to ultrahigh density optical memory media as well as super-resolution fluorescence imaging when the dyes are highly fluorescent and have large absorption coefficients. Here, we report on highly fluorescent photochromic dyes, which are initially nonluminous in solution under irradiation with visible light but activated to emit green or red fluorescence upon irradiation with ultraviolet (UV) light. The dyes 5a-9a are sulfone derivatives of 1,2-bis(2-ethyl-6-phenyl(or thienyl)-1-benzothiophen-3-yl)perfluorocyclopentene. It was found that substitution of phenyl or thiophene rings at 6 and 6' positions of the benzothiophene-1,1-dioxide groups is effective to increase the fluorescence quantum yields of the closed-ring isomers over 0.7 and absorption coefficients over 4 × 10(4) M(-1) cm(-1). The phenyl-substituted derivatives 5a-7a undergo photocyclization reactions to produce yellow closed-ring isomers 5b-7b, which emit brilliant green fluorescence at around 550 nm (Φ(F) = 0.87-0.88) under irradiation with 488 nm light. Any absorption intensity change of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light. The ring-opening quantum yields (Φ(C→O)) were measured to be (1.6-4.0) × 10(-4). When the phenyl substituents are replaced with thiophene rings, such as compounds 8a and 9a, the absorption bands of the closed-ring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red fluorescences at around 620 nm (Φ(F) = 0.61-0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow (Φ(C→O) < 1 × 10(-5)). The fluorescence lifetimes of these sulfone derivatives were measured to be around 2-3 ns, which is much longer than the value of the closed-ring isomer of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (τ(F) = 4 and 22 ps). The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light (λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol.     

Study on photochromism of diarylethenes with a 2,5-dihydropyrrole bridging unit: a convenient preparation of 3,4-diarylpyrroles from 3,4-diaryl-2,5-dihydropyrroles

Symmetric and nonsymmetric diarylethenes with a 2,5-dihydropyrrole bridging unit have been prepared, and the photochromic properties are investigated. Both symmetric and nonsymmetric diarylethenes with 2,5-dihydropyrrole bridging units undergo reversible ring-opening and ring-closing photoisomerization reactions in nonpolar solvents with UV/vis light, and some of them exhibit good fatigue resistance and no marked degradation detected after 10 cycles via an on/off switch. In polar solvents, however, photochromic diarylethenes with 2,5-dihydropyrrole bridging units produce 3,4-diarylpyrrole derivatives instead of the ring-closing isomer of diarylethenes with UV light irradiation. A class of N-substituted 3,4-diphenylethenes with 2,5-dihydropyrrole bridging units were prepared and used as templates to investigate the conversion reactions. The mechanism of photoconversion of 3,4-diaryl-2,5-dihydropyrroles to 3,4-diarylpyrroles was explored as well.     

Synthesis and characterisation of photochromic dithienylcyclopentene liquid crystal with thermal irreversibility

A series of photochromic liquid crystal dithienylcyclopentene derivatives were synthesised from the initial material 2-chloro-5-methylthiophene. They were characterised by ¹H NMR, high resolution mass spectrometer, differential scanning calorimetry (DSC) and polarised optical microscopy (POM) with a hot stage. All of them exhibited excellent photochemically reversible isomerisation between the open-ring form and the closed-ring form in both organic solvents and LC media, resulting in the colour change between colourless and purple under different light irradiations. The typical textures from POM showed that their isomers both had nematic phase and smectic phase with different colours. These compounds with bent shape were more sensitive to light than those of diarylethenes reported.     

A highly selective fluorescent probe for Sn based on a new photochromic diarylethene with a phenol-containing Schiff base unit

A novel diarylethene with a phenol-containing Schiff base unit was successfully synthesized and its multi-responsive photoswitching property was investigated by the stimulation of base/acid and light. The diarylethene showed not only normal photochromism by photoirradiation, but also abnormal photochromism by base and light stimuli. By the stimulation of sodium hydroxide, the absorption maximum of its open-ring isomer showed extraordinarily large redshift of 60 nm, accompanied with a notable color change from colorless to yellow. Subsequent irradiation with 297 nm light further caused a dramatic change in its structure transformation from the deprotonated open-ring isomer to the original closed-ring isomer. Similarly, its deprotonated closed-ring isomer returned to its original open-ring isomer upon irradiation with appropriate visible light. Its deprotonated forms could be restored by the stimulation of hydrochloric acid. Moreover, the diarylethene could be served as a fluorescence probe for recognition of Sn²⁺ with high selectivity.     

Photochromism of diarylethenes with two nitronyl nitroxides: photoswitching of an intramolecular magnetic interaction

Photochromic diarylethenes that have p-phenylene-substituted benzothiophene aryl groups with and without nitronyl nitroxide radicals at both ends of the molecules were synthesized. The absorption maxima of the closed-ring isomers showed a hypsochromic shift with the increase in the pi-conjugated chain length. The unique behavior was attributed to the stabilization by the resonant quinoid structures. Both photocyclization and photocycloreversion quantum yields of the diarylethene with nitronyl nitroxide radicals were found to increase with the increase in the pi-conjugated chain length. Photoswitching of the magnetic interaction between two nitronyl nitroxide radicals was studied by means of ESR spectroscopy. The change in exchange interaction between open- and closed-ring isomers of 1,2-bis[6-[4-[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]phenyl]-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene was determined to be more than 30-fold.     

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.     

Three bits eight states photochromic recording and nondestructive readout by using IR light

A photochromic polymer film containing three different diarylethene derivatives, that is, 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (1), 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2), and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (3) was prepared. Upon UV irradiation, the three derivatives changed to their closed-ring isomers having different colors, yellow, red, and blue. They showed different spectra not only in UV/Vis region but also in the IR spectral region. Upon irradiation with visible light of appropriate wavelengths, each closed-ring isomer was selectively bleached, and three bits eight states recording was performed. The eight states could be read out nondestructively by using IR light of appropriate wavenumbers.     

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.     

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.     

Synthesis of silsesquioxanes having photochromic diarylethene pendant groups

Silsesquioxanes having 1-(2-methylbenzo[b]thien-3-yl)-2-[5-(4-butylphenyl)-2,4-dimethylthien-3-yl]perfluorocyclopentenes as the pendant groups were synthesized. Photochemical conversion from the open-ring to the closed-ring form of diarylethenes chemically bonded to polymers was by 15–25% lower than the conversion of the chromophores dispersed in the same matrix. No appreciable difference in the photoconversion of the fixed chromophores was observed below and above the glass transition temperature (T_g) of the polymer films.