Photochromic Switches Research Articles

Photo-responsive anticounterfeiting inks switching between colorless and cyan/magenta/yellow for flexible, full-color, and large-area printing

Inkjet printing of photo-responsive patterns capable of switching between colorless and full-color is regarded as an innovative anticounterfeiting technique with a higher security level. Therefore, colorless inks which can be changed to cyan (C), magenta (M), and yellow (Y) upon photochromism are highly required. Herein, three photochromic (PC) inks C, M, and Y switching between colorless and cyan/magenta/yellow, respectively, have been successfully prepared by dissolving viologen derivatives AV, AEV, and NOV in solvent mixtures of water, ethanol, and ethylene glycol, with sodium dodecyl sulfate (SDS) as surfactant. With a mass fraction of 0.5 wt% for SDS, an optimal rheological property with a viscosity of 3.48 mPa·s, a contact angle of 29.4° on poly (ethylene terephthalate) (PET) substrate, a surface tension of 23.16 mN m−1, and a Z value of 3.85, is realized for ink C. Under the same conditions, inks M and Y are prepared and display suitable rheological properties for inkjet printing. The films are invisible initially and become colorful upon UV irradiation with no significant decay after 20 repetitive PC switching. Subsequently, various colorful patterns with high resolutions were printed on conventional A4 paper and become visible only after irradiation by a UV lamp. Therefore, these photo-responsive tricolor inks can be used in anticounterfeiting field and two practical examples of a stamp and a personal check with cryptical texts and color information have been demonstrated. Overall, the developed photo-responsive C/M/Y inks can be utilized in flexible, full-color, and large-area printing, providing a novel approach towards innovative anticounterfeiting technology.

Zero-dimensional halide hybrid bulk glass exhibiting reversible photochromic ultralong phosphorescence

Dynamically responsive materials, capable of reversible changes in color appearance and/or photoemission upon external stimuli, have attracted substantial attention across various fields. This study presents an effective approach wherein switchable modulation of photochromism and ultralong phosphorescence can be achieved simultaneously in a zero-dimensional organic-inorganic halide hybrid glass doped with 4,4´-bipyridine. The facile fabrication of large-scale glasses is accomplished through a combined grinding-melting-quenching process. The persistent luminescence can be regulated through the photochromic switch induced by photo-generated radicals. Furthermore, the incorporation of the aggregation-induced chirality effect generates intriguing circularly polarized luminescence, with an optical dissymmetry factor (glum) reaching the order of 10–2. Exploiting the dynamic ultralong phosphorescence, this work further achieves promising applications, such as three-dimensional optical storage, rewritable photo-patterning, and multi-mode anti-counterfeiting with ease. Therefore, this study introduces a smart hybrid glass platform as a new photo-responsive switchable system, offering versatility for a wide array of photonic applications.

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies.

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.

Lanthanide-Polyoxometalate-Based Film with Reversible Photochromism and Luminescent Switching Properties for Erasable Inkless Security Printing.

Security printing is of the utmost importance in the information era. However, the excessive use of inks and paper still faces many economic and environmental issues. Thus, developing erasable inkless security printing materials is a remarkable strategy to save resources, protect the environment, and improve information security. To this endeavor, a photoresponsive lanthanide-polyoxometalate-doped gelatin film with high transparency was developed through the solution casting method. Attenuated total reflection Fourier-transform infrared spectroscopy confirmed the electrostatic and hydrogen bond interactions between gelatin and lanthanide-polyoxometalate. Absorption spectra, luminescent spectra, and digital images indicated that the film displayed reversible photochromism behavior and was accompanied by luminescent switching property upon exposure to UV irradiation and oxygen (in the dark) alternately, which allowed its potential application as a reprintable medium for inkless security printing. The printed information can be erased upon exposure to oxygen in the dark, and the film can be reused for printing again. The film exhibited excellent erasability, reprintability, renewability, and low toxicity. In addition, multiple encryption strategies were designed to improve information security. This work offers an attractive alternative strategy for constructing a reprintable film for inkless security printing in terms of simplifying the preparation process, saving resources, and protecting the environment.

Construction of dual-mode photochromic fluorescence switching system based on water-soluble rare earth phosphates and carboxyl-containing diarylethene

Photochromic fluorescent switches have been attracting great research interest owing to their optical color and fluorescence can be reversibly modulated by light stimulation. An efficient way to construct photochromic fluorescence switching system is to integrate photochromic molecule with an energy level-matching fluorophore by non-covalent assembly. In this paper, we reported the design of a water-soluble photochromic fluorescent switch (OF–CO2H@Ce6Tb4·P) with dual signal changes (optical color change and fluorescence ON/OFF) by combining Ce, Tb co-doped phosphate hybrid (Ce6Tb4·P) and diarylethene containing carboxyl groups (OF–CO2H) via hydrogen bonding. Ce6Tb4·P showed the typical green fluorescence of terbium ions due to the sensitization effect of cerium ions. OF-CO2H can undergo reversible photoisomerization reaction, accompanied by changes in optical color (colorless to pink) and UV–visible absorption. Moreover, controlling the photoisomerization of OF-CO2H can regulates the spectral overlap between its absorption and Ce6Tb4·P emission, thus achieving reversible modulation of the optical color and fluorescence of OF-CO2H@Ce6Tb4·P. The demonstrations of photocontrolled dual-mode signaling in photoswitchable patterning shows that OF-CO2H@Ce6Tb4·P is suited for surface-involved photocontrolled applications such as advanced optical anti-counterfeiting and information storage.

Three-dimensional blueprinting of molecular patterns in liquid crystalline polymers.

Exploiting the interplay of anisotropic diamagnetic susceptibility of liquid crystalline monomers and site selective photopolymerization enables the fabrication of 3D freeforms with highly refined microstructures. Utilizing chain transfer agents in the mesogenic inks presents a pathway for broadly tuning the mechanical properties of liquid crystalline polymers and their response to stimuli. In particular, the combination of 1,4-benzenedimethanethiol and tetrabromomethane is shown to enable voxelated blueprinting of molecular order, while allowing for a modulation of the crosslink density and the mechanical properties. The formulation of these monomers allows for the resolution of the voxels to approach the limits set by the coherence lengths defined by the anchoring from surfaces. These compositions demonstrate the expected thermotropic responses while allowing for their functionalization with photochromic switches to elicit photomechanical responses. Actuation strains are shown to outstrip that accomplished with prior systems that did not access chain transfer agents to modulate the structure of the macromolecular network. Test cases of this system are shown to create freeform actuators that exploit the refined director patterns during high-resolution printing. These include topological defects, hierarchically-structured light responsive grippers, and biomimetic flyers whose flight dynamics can be actively modulated via irradiation with light.

From Visible to Near-Infrared Light-Triggered Photochromism: Negative Photochromism.

Photochromic compounds, whose key molecular properties can be effectively modulated by light irradiation, have attracted significant attention for their potential applications in various research fields. The restriction of photoisomerization coloration induced by ultraviolet light limits their applications in the biomedical field and some other fields. Negative photochromism, wherein a relatively stable colored isomer transforms to a colorless metastable isomer under low-energy light irradiation, offers advantages in applications within materials science and life science. This review provides a summary of negatively photochromic compounds based on different molecular skeletons. Their corresponding design strategies and photochromic properties are presented to provide practical guidelines for future investigations. Negatively photochromic compounds can effectively expand the range of photochromic switches for future applications, offering unique properties such as responsiveness to visible to near-infrared light.

The solvation effect of photochromic nonlinear optical properties switch

In 2009, the interesting photochromic complex o-MeCN and its isomeric close form c-MeCN were synthesized and investigated, which would transform to each other according to the characterization of UV and visible irradiation. In our work, the excellent nonlinear optical (NLO) properties are obtained by theoretical calculating. Results show that the βtot value of c-MeCN is about 12 times as large as that of o-MeCN in acetonitrile solvent, which provide a basis for NLO switch. Further, the UV–visible absorption spectrum can confirm that the remarkable βtot values for c-MeCN are mainly attributed to the charge transfer induced by the photocyclization. Significantly, the βtot values reduced with the increasing of dielectric constant ε of the solvent, and the differences of βtot values between the open and close form, present a regularly change in different solvents. This reversible photochromic process and solvent effect may open up new opportunities for the discovery of attractive NLO switch.

Light-adaptable artificial iris with dynamically scalable pupil-aperture function by radially patterned photochromic transition control

The iris is an ocular organ that actively controls the size of the pupil-aperture in response to external light, thereby regulating the amount of light reaching the retina for better visual acquisition. Herein, we propose a light-adaptive pupil-scalable artificial iris for addressing human iris defects with biomimetic self-regulating light control similar to human iris actuation, which is realized by a radially gradient and reversible photoswitching of photochromic dyes doped within a biocompatible hydrogel matrix. The radial photochromic switching of light transmissions was achieved by the gradient patterning of the crosslinking density of the hydrogel matrix using a near-infrared light-absorbing photomask that generated radially thermal gradience during hydrogel matrix polymerization. With the effective pupil-aperture control, the proposed artificial iris exhibited a variation in the visible-light transmittance from ∼82 % at the ultraviolet light (UV) intensity of 0.5 mW/cm2 to ∼43 % at 3.0 mW/cm2 representing the transparent and colored states, respectively. The switching times for the transitions to the colored and transparent states were 27.42 and 112.25 s, respectively, at a UV intensity of 3.0 mW/cm2, which can be faster under the hydrated state. The artificial iris demonstrated potential in biomedical applications by offering reliable light-adaptive attenuation control through human-like pupil-aperture adjustments.

Waterproof organic-inorganic composite with fast reversible photochromic switch for UV indication and information storage

Design and preparation of photochromic materials with reversible color change under specific wavelength irradiation have been active research fields. However, these existing materials face a great challenge in rapid responsiveness and effective water management. Here, a waterproof composite with fast light response was successfully prepared by air-jet electrospinning and subsequent spray treatment. Specifically, its response time and saturation time are reduced to a few seconds and 1 min, respectively. Its reflectivity at 670 nm changes from 77.50% to 6.26%, which strongly indicates the large color contrast of this composite. Its natural fading process only takes 1 h, and infrared radiation can further reduce the erasure process to a few seconds. Concurrently, this material possesses hydrophobic properties and anti-adhesion performance, providing superior protection for its photochromism. Overall, this waterproof composite possesses rapid photochromism and appropriate spontaneous fading efficiency, enabling it to have a very broad application prospect in the fields of indicator materials, anti-counterfeiting materials, personalized labels, etc.

Optical Control of Invertebrate nAChR and Behaviors with Dithienylethene-Imidacloprid.

Photopharmacology involving azobenzene or dithienylethene has changed established methods of studying receptor functions, allowing for increasing the spatiotemporal resolution. There are no photopharmacological tools available for the invertebrate nicotinic acetylcholine receptor (nAChR). Here, we report a photochromic ligand, dithienylethene-imidacloprid (DitIMI), using a dithienylethene photoswitch embedded in the nAChR agonist imidacloprid. It was found that DitIMI displayed good photochromism and fluorescence switching behaviors upon irradiation with UV/vis light in aqueous solution. We demonstrated that open-DitIMI has low spontaneous in vitro and in vivo activity but can be photoisomerized to a highly active closed-form. Surprisingly, the photoswitchable DitIMI showed a large difference in insecticidal activity between the open and closed forms, in which the insecticidal activity against Aphis craccivora of the ring-closed isomers for DitIMI was 355 times that of the corresponding ring-open isomers. This photoisomerization can further be translated to photomodulation of neuron membrane potential and behavioral responses of living mosquito larvae and American cockroaches. The photomanipulation of nACh neurotransmission opens new avenues to understanding inhibitory circuits in intact animals.

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

Photochromic Tungsten Oxide Quantum Dots-based Fluorescent Photoswitches towards Dual-mode Anti-counterfeiting Application

Development of new anti-counterfeiting technology to increase the difficulty of imitation and decoding is becoming increasingly important, but still remains challenging yet. In this work, we report the design of new fluorescence photoswitches based on photochromic tungsten oxide quantum dots (WO3 QDs) for dual-mode anti-counterfeiting applications. Complexing photochromic WO3 QDs with fluorescent gold nanoclusters (AuNCs) enables the construction of a photoswitchable fluorescence system (WO3-AuNCs) based on fluorescence resonance energy transfer. Detailed spectral and photophysical characterization showed that WO3 QDs well-retain the photochromic properties within the WO3-AuNCs composite. Importantly, photoresponsive and highly reversible switching of both color and fluorescence signals was successfully achieved by simply alternating the irradiation with UV and visible light. Potential utility of photoswitchable WO3-AuNCs composite as novel dual-mode anti-counterfeiting materials has been successfully demonstrated, including photoswitchable ink, rewritable paper and number encryption. Compared with other anti-counterfeiting materials, the present photochromic WO3 QDs-based fluorescent switches are easily synthesized and handled, and they can provide dual security mode (color and fluorescence). This work provides a generable WO3 QDs-assisted strategy of fabricating advanced fluorescence photoswitches for versatile optical counterfeiting applications.

Pyrene-dithienylethene-tetra(tri)phenylethylene triads: Photocontrolled intramolecular energy transfer and photochromic fluorescence switching

Photochromic fluorescence switching with aggregation-induced emission (AIE) property has attracted great research interest due to their wide application potential in optical electronics and biomaterials. In this work, three novel pyrene-dithienylethene-tetra(tri)phenylethylene triads were synthesized by connecting triphenylethene (TrPE) or tetraphenylethylene (TPE) and pyrene unit via dithienlethene (DTE) bridge. Efficient photochromism and fluorescence switching were demonstrated in solution, aggregates, solid powder, and poly(methyl methacrylate) (PMMA) films under alternating irradiation with UV/visible light. The photocyclization yield of DTE-Py-3 was examined by 1H NMR spectra and calculated as 59%. Moreover, these three compounds exhibited typical AIE character in THF/water mixture, the photochromic and fluorescent switching behavior were explored in aggregated state with 90% water fraction. Furthermore, they could be utilized in application of optical information storage and ink with naked-eye color and fluorescence double channel, indicating the promising potential in anti-counterfeiting. Interestingly, DTE-Py-3 could be reversibly switched by visible light in both directions, which endowed it with the application in biological filed.

Light-driven modulation of electrical conductance with photochromic switches: bridging photochemistry with optoelectronics.

Photochromic conducting molecules have emerged because of their unique capacity to modulate electrical conductivity upon exposure to light, toggling between high and low conductive states. This unique amalgamation has unlocked novel avenues for the application of these materials across diverse areas in optoelectronics and smart materials. The fundamental mechanism underpinning this phenomenon is based on the light-driven isomerization of conjugated π-systems which influences the extent of conjugation. The photoisomerization process discussed here involves photochromic switches such as azobenzenes, diarylethenes, spiropyrans, dimethyldihydropyrenes, and norbornadiene. The change in the degree of conjugation alters the charge transport in both single molecules and bulk states in solid samples or solutions. This article discusses a number of recent examples of photochromic conducting systems and the challenges and potentials of the field.

Revisiting photocyclization of the donor-acceptor stenhouse adduct: missing pieces in the mechanistic jigsaw discovered.

Donor-acceptor Stenhouse adducts (DASA) have recently emerged as a class of visible-light-induced photochromic molecular switches, but their photocyclization mechanism remains puzzling and incomplete. In this work, we carried out MS-CASPT2//SA-CASSCF calculations to reveal the complete mechanism of the dominant channels and possible side reactions. We found that a new thermal-then-photo isomerization channel, i.e., EEZ → EZZ → EZE, other than the commonly accepted EEZ → EEE → EZE channel, is dominant in the initial step. Besides, our calculations rationalized why the expected byproducts ZEZ and ZEE are unobserved and proposed a competitive stepwise channel for the final ring-closure step. The findings here redraw the mechanistic picture of the DASA reaction by better accounting for experimental observations, and more importantly, provide critical physical insight in understanding the interplay between thermal- and photo-induced processes widely present in photochemical synthesis and reactions.

Functionalised organometallic photoswitches containing dihydropyrene units.

Functionalising organic molecular photoswitches with metal complexes has been shown to alter and enhance their switching states. These organometallic photoswitches provide a promising basis for novel smart molecular materials and molecular electronic devices. We have detailed the synthesis and characterisation of mono- and bimetallic half-sandwich ruthenium and iron complexes functionalised with alkynyl dihydropyrenes (DHP). Their electronic and photophysical properties were determined by the use of chemical, electrochemical and spectroelectrochemical techniques. The introduction of the metal alkynyl moiety allows access to additional redox and protonation states not accessible by the DHP alone. An additional metal alkynyl moiety inhibits observable photochromic switching. Analysis of the NIR and IR bands in the mixed valence complexes suggests there is a high degree of charge delocalisation across the DHP.

All‐Visible‐Light Triggered Photoswitch of Dithienylethene Derivatives with Molecular Conformation Changes Excess 5 Å

AbstractDeveloping new all‐visible‐light driven fluorescent molecular switches has attracted much interests due to their important application prospects in materials science and life science. Herein, three novel all‐visible‐light triggered fluorescent molecular photoswitches (2P3, 4F, and 4Cl) are designed and prepared, which are realized by prolonging π‐conjugation length with aggregated‐induced emission units. These photoswitches show reversible photochromic and fluorescent switching in dilute solutions and films under 420 nm and 560 nm irradiations. Meanwhile, upon alternating irradiations with 450 nm/560 nm light, effective photochromic and fluorescent switching in powder states are also achieved. More interestingly, two distinct conformations with different photochromic abilities can be altered by solvents polarity. The size difference of the two conformations is over 5 Å in the vertical direction which is one of the largest changes among photochromic molecules based on dithienylethene. The accompanying conformational changes are analysed to understand the relationship between solvent characteristics and crystal structure, as well as the intermolecular interaction in solid state for all‐visible‐light triggered photoswitching molecules.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400nm. Many of the modes that become enhanced in the 750nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.