Single-molecule fluorescence photoswitching of a diarylethene-perylenebisimide dyad: non-destructive fluorescence readout

Single-molecule fluorescence photoswitching plays an essential role in ultrahigh-density (Tbits/inch(2)) optical memories and super-high-resolution fluorescence imaging. Although several fluorescent photochromic molecules and fluorescent proteins have been applied, so far, to optical memories and super-high-resolution imaging, their performance is unsatisfactory because of the absence of "non-destructive fluorescence readout capability". Here we report on a new molecular design principle of a molecule having non-destructive readout capability. The molecule is composed of acceptor photochromic diarylethene and donor fluorescent perylenebisimide units. The fluorescence is reversibly quenched when the diarylethene unit converts between the open- and the closed-ring isomers upon irradiation with visible and UV light. The fluorescence quenching is based on an electron transfer from the donor to the acceptor units. The fluorescence photoswitching and non-destructive readout capability were demonstrated in solution (an ensemble state) and at the single-molecule level. Femtosecond time-resolved transient and fluorescent lifetime measurements confirmed that the fluorescence quenching is attributed to the intramolecular electron transfer.     

Unsymmetrical photochromic bithienylethene-bridge tetraphenylethene molecular switches: Synthesis,aggregation-induced emission and information storage

Aggregation-induced emission(AIE) active photochromic molecules have attracted growing attention for their versatile applications.Here we designed and synthesized five newly unsymmetrical photochromic diarylethene(DAE) dyads(BTE1-5) by connecting tetraphenylethene(TPE) and aromatic substituent via bithienylethene(BTE) bridge.The chemical structures of those compounds were identified by ^1H NMR,¹³C NMR and HRMS.The absorption and emission of these dyads were investigated by UV-vis and fluore scence spectroscopy,respectively.The results showed that all those compounds exhibited typically AIE or aggregation-induced emission enhancement(AIEE) characteristic.Particularly,when an aggregationcaused quenching(ACQ) fluorophore(triphenylamine) was grafted to the molecule,connecting with TPE via BTE-bridge,the ACQ phenomenon was dissipated and converted to an AIE luminophore,and those compounds exhibited photochromism upon irradiation with alternative UV and visible light.The solution or solid of those compounds showed distinctly fluorescence switching "ON" or "OFF" observation upon irradiation with alternative UV and visible light.It is interesting that BTE1 could be applied in recording and rewritable information storage,and the cyclization quantum yields could be affected by substituent significantly.     

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.     

Solvent effect of fluorescence on/off switching of diarylethene linked to excited-state intramolecular proton transfer fluorophore

Efficient fluorescence on/off switching of a dyad consisting of a photochromic diarylethene and a fluorescence dye based on excited state intramolecular proton transfer (ESIPT) was designed and demonstrated. Diarylethenes linked to (2-(2-methoxy-5-methylphenyl)benzothiazol-6-yl)- and (2-(2-hydroxy-5-methylphenyl)benzothiazol-6-yl)-9,9-dioctylfluorene moieties (1a and 2a, respectively) exhibited fluorescence on/off switching upon alternating irradiation with ultraviolet and visible light in various solvents. The fluorescence on/off contrast of 2a was found to be higher than that of 1a in n-hexane because the overlap integral between the absorption spectrum of the diarylethene closed-ring form and the fluorescence spectrum of 2a is larger than that of 1a. Diarylethene 2a exhibited green fluorescence with large Stokes shift in n-hexane, which is ascribed to the ESIPT process from the enol form to the keto form. In contrast, the fluorescence of 2a in N,N-dimethylsulfoxide (DMSO) was mainly observed as blue fluorescence from enol form, while diarylethene 1a exhibited blue fluorescence in n-hexane and DMSO. The fluorescence on/off contrast of 2a in n-hexane was higher than that in DMSO because of the difference in the spectral overlaps in n-hexane and DMSO.     

Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1(2)·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1(2)·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.     

Loading Photochromic Molecules into a Luminescent Metal–Organic Framework for Information Anticounterfeiting

Stimuli‐responsive photoluminescent materials have attracted considerable attention owing to their potential applications in security protection because the information recorded directly in materials with static luminescent outputs are usually visible under either ambient or UV light. Herein, we realize reversible information anticounterfeiting by loading a photoswitchable diarylethene derivative into a lanthanide metal–organic framework (MOF). Light triggers the open‐ and closed‐form isomerization of the diarylethene unit, which respectively regulates the inactivation and activation of the photochromic FRET process between the diarylethene acceptor and lanthanide donor, resulting in reversible luminescence on–off switching of the lanthanide emitting center in the MOF host. This photoresponsive host–guest system allows for reversible multiple information pattern visible/invisible transformation by simply alternating the exposure to UV and visible light.     

Photosalient Effect of Diarylethene Crystals of Thiazoyl and Thienyl Derivatives

The photoresponse of diarylethene crystals is found to depend on the intensity of UV light, that is, photoinduced bending is switched to photosalient phenomena by increasing the light intensity. The change in the size of the crystal unit cell upon UV irradiation is larger for asymmetric diarylethenes with thiazole and thiophene rings than that for the corresponding symmetric diarylethenes. As a result, the crystals of an asymmetric diarylethene show much more drastic photosalient effects than those of the corresponding symmetric diarylethene crystals upon UV irradiation. It is also found that the crystals of diarylethene, which have not previously been reported to exhibit a photosalient effect, show photosalient phenomena upon irradiation with strong UV light. Furthermore, the dependence of photosalient phenomena on the size and shape of the crystals is reported.     

Conjugated polymer nanoparticles as fluorescence switch for selective cell imaging

Fluorescence switch plays a vital role in bioelectronics and bioimaging.Herein,we presented a new kind of facile electrostatic complex nanoparticles(ECNs)for fluorescence switching in cells and marking of individual cell.The ECNs were prepared by mixing positively charged poly(6-(2-(thiophen-3-yl)ethoxy)hexyl trimethylammonium bromide)(PT)and negatively charged diarylethene sodium salt(DAECOONa).DAE-COONa is a photoswitchable molecule which can be transformed between the ring-closed fo rm and ring-open form under the irradiation of UV or visible light.The closed-form of DAE-COONa can efficie ntly quench the fluorescence of PT through intermolecular energy transfer,while the open form of DAE-COONa does not influence the emission of PT.Thus,the fluorescence of ECNs can be modulated by light irradiation,and the ECNs with good fluorescence switching performance have been employed for fluorescence imaging and individual cell lighting up process successfully.We demonstrate that the electrostatic complex strategy provides a facile method to construct fluorescence switch fo r selective cell marking and imaging applications.     

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.     

Synthesis, crystal structure and characterization of photochromic 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene

A novel photochromic diarylethene, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene (BEDTP), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its photochromic and fluorescent properties were also investigated. The results showed that this compound exhibited reversible photochromism, changing from colorless to magenta after irradiation with UV light both in solution and in the crystalline phase. In hexane solution, the open-ring isomer of BEDTP exhibited relatively strong fluorescence at 400 nm when excited at 282 nm. The fluorescence intensity decreased along with the photochromism upon irradiation with 254 nm light and its closed-ring isomer showed almost no fluorescence.     

Live Cell Imaging Using Photoswitchable Diarylethene‐Doped Fluorescent Polymer Dots

Fluorescence photoswitching using nanomaterials has recently emerged as a promising approach for the imaging of biological targets. However, despite intensive research efforts during the last decade, practical microscopy of biological targets using photoswitchable nanoparticles in real time remains challenging. To address this problem, we have developed live macrophage cell imaging and single particle imaging methods, using photoswitchable fluorescent diarylethene‐doped polymer nanoparticles (P‐dots) under Xe lamp irradiation. We established a 34‐times prolonged “off‐state”, using P‐dots doped with a diarylethene‐containing methoxy substituent, upon visible‐light irradiation using a Xe lamp and a green fluorescent protein filter cube. To demonstrate the practicality of doped P‐dots imaging, we imaged lysosomes in macrophage cells, and observed 11‐times slower recovery of the fluorescence from the “off‐state” to the “on‐state”, indicating their potential for cellular imaging.     

Fluorescence of single molecules in polymer films: sensitivity of blinking to local environment

The single-molecule fluorescence blinking behavior of the organic dye Atto647N in various polymer matrixes such as Zeonex, PVK, and PVA as well as aqueous media was investigated. Fluorescence blinking with off-times in the millisecond to second time range is assigned to dye radical ions formed by photoinduced electron transfer reactions from or to the environment. In Zeonex and PVK, the measured off-time distributions show power law dependence, whereas, in PVA, no such dependence is observed. Rather, in this polymer, off-time distributions can be best fitted to monoexponential or stretched exponential functions. Furthermore, treatment of PVA samples to mild heating and low pressure greatly reduces the frequency of blinking events. We tentatively ascribe this to the removal of water pockets within the polymer film itself. Measurements of the dye immobilized in water in the presence of methylviologen, a strongly oxidizing agent, reveal simple exponential on- and off-time distributions. Thus, our data suggest that the blinking behavior of single organic molecules is sensitive to their immediate environment and, moreover, that fluorescence blinking on- and off-time distributions do not inherently and uniquely obey a power law.     

Photochromism of diarylethene single molecules in polymer matrices

Robust fluorescent photoswitching molecules, having perylene bisimide as the fluorescent unit and diarylethene as the switching unit, were prepared, and their photochromic reactions were measured at the single-molecule level in various polymer matrices. The histograms of the fluorescent on and off times were found to deviate from normal exponential distribution and showed a peak when the molecules are embedded in rigid polymer matrices, such as Zeonex or poly(methyl methacrylate) (PMMA). In soft polymer matrices, such as poly(n-buthyl methacrylate) (PnBMA), exponential distribution was observed for the on and off times. The abnormal distribution suggests that the quantum yields of the photoreactions are not constant and the molecules undergo the reactions after absorbing a certain number of photons. A multilocal minima model was proposed to explain the environmental effect.     

Reversible Photomodulation of Electronic Communication in a π‐Conjugated Photoswitch‐Fluorophore Molecular Dyad

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S₀→S₁) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red‐shifted DAE behavior prevails.     

A highly selective fluorescence switch for Cu2+ and Fe3+ based on a new diarylethene with a triazole-linked rhodamine 6G unit

A new diarylethene compound with a triazole-linked rhodamine 6G unit attached to the imino group (1O) was designed and synthesized. According to the test results, the solution color and fluorescence color of diarylethene can be modulated by lights and metal ions. The solution color could change from colorless to light purple when irradiated with UV light. When Cu²⁺ was added to the diarylethene solution, the color of diarylethene solution became blue, the fluorescence color turned from dark to bright yellow. Although the solution color did not change by adding Fe³⁺, its fluorescence color varied from dark to yellow. Moreover, it was found that the complex ratio of the diarylethene to Cu²⁺ was 1:1 and the binding stoichiometry of the diarylethene to Fe³⁺ was also 1:1 based on the data of NMR, MS, and other experiments. Based on these findings, photochromic figure of the diarylethene with UV/Vis light, Cu²⁺ and Fe³⁺ was constructed. Furthermore, the logic circuit was designed by input signals (ultraviolet stimulus, visible light stimulus, Cu²⁺ (or Fe³⁺) and EDTA) and an output signal (fluorescent intensity at 566?nm (or 575?nm)).     

Photomobile Polymer Materials: Photoresponsive Behavior of Cross‐Linked Liquid‐Crystalline Polymers with Mesomorphic Diarylethenes

Cross‐linked liquid‐crystalline (LC) polymers with a mesomorphic diarylethene were prepared to demonstrate a versatile strategy for cross‐linked photochromic LC polymers as photomobile materials. Upon exposure to UV light to cause photocyclization of the diarylethene chromophore, the cross‐linked polymer films bend toward an actinic light source. By irradiation with visible light to cause a closed‐ring to open‐ring isomerization, the bent films revert to the initial flat state. Without visible‐light irradiation, the bent films remain bent even at 120 °C, indicating high thermal stability of the cross‐linked diarylethene LC polymers.     

Amphiphilic diarylethene as a photoswitchable probe for imaging living cells

This communication reports a unique example of water-soluble and fluorescent-switchable amphiphilic diarylethene. This compound performs stable vesicle aggregation in water and shows aggregation-dependent emission in its open form. The fluorescence can be effectively switched by alternating between UV and visible light irradiation. This compound thus can stain KB cells for switchable living cell imaging with excellent resistance to fatigue.     

Photoswitching of conductance of diarylethene-Au nanoparticle network

A network composed of gold nanoparticles covered with diarylethene dithiophenols was prepared on an interdigitated nanogapped gold electrode to show the reversible photoswitching of the conductance due to the photochromism of the diarylethene molecules induced by UV and visible light.     

Synthesis, crystal structure and optoelectronic properties of a new unsymmetrical photochromic diarylethene

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the solid state (PMMA film and the single-crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence at 470nm when excited at 300nm, and its emission intensity decreased along with the photochromism upon irradiation with 313nm light. Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic voltammetry experiment and its HOMO and LUMO energy level were calculated.     

Synthesis of a new diarylethene diradical which has extended π-conjugated chains from the 2,5-position of one thiophene ring

A diarylethene diradical having a new switching unit for intramolecular magnetic interaction was synthesized. The photoswitching unit has an extended π-conjugated chain in one aryl unit, and two nitronyl nitroxide radical are placed at both ends of the π-conjugated chain. The diarylethene moiety is located in the middle of the chain. This diarylethene is designed to change the hybrid orbital from sp² to sp³ at the 2-position of the thiophene ring when this diarylethene undergoes a photochromic reaction. But the new diradical compound did not undergo photocyclic reaction upon irradiation with UV light. The photochemical behavior is perturbed by a resonant quinoid structure which stabilizes the open-ring isomer.