Photochromic Molecules Research Articles

Assembly of Photochromic Molecules into Multiple Stimuli‐Responsive Hollow Microparticles via Templating Coordination

AbstractSupramolecular multifunctional metal–organic networks offer immense potential in drug delivery, bioimaging, catalysis, and therapeutics. However, the creation of multiple stimuli‐responsive fluorescent hollow nano/microparticles is a formidable challenge in this field. In this study, a novel strategy is reported for synthesizing photochromic hollow microparticles that exhibit responsiveness to light, pH, and glutathione (GSH). With the assistance of templating coordination from zeolitic imidazolate framework‐8 microparticles, bisphosphonate‐merocyanine ligands are successfully assembled to create hollow bisphosphonate‐spiropyran (BPSP)‐Zn2+ microparticles. The photochromic and pH responses stem from the reversible conversion between spiropyran and zwitterionic merocyanine components and between merocyanine and protonated merocyanine, respectively. The GSH response is attributed to the stronger affinity of Zn2+ to thiol groups than to phosphonate groups. These remarkable features render the hollow BPSP‐Zn2+ microparticles highly suitable for applications in stimulus‐triggered drug delivery, and these findings provide a platform for developing innovative materials that have the potential to transform the biomedical field.

(Invited) Stimuli-Responsive Metal-Organic Frameworks

Development of stimuli-responsive materials is crucial for the next advancements in the technology and energy sectors. For instance, reversible tuning photophysical profiles of materials or modulation of energy transfer processes are key aspects for the development of next generation of logic gates, spatially- and temporally-resolved sensors, and on-demand drug delivery systems. Our recent efforts have focused on employment of metal-organic frameworks (MOFs) as a versatile platform for the material development, which contain photochromic moieties, allowing for tailoring MOF electronic and photophysical properties. In our work, confinement-imposed photophysics was probed for a variety of photoresponsive moieties including a not-well studied class of hydrazone-based photoswitches integrated into topologically distinct porous scaffolds. The performed studies involved a comprehensive analysis of photophysics-structure correlation performed for the first time in two-dimensional and three-dimensional porous frameworks using steady-state and time-resolved photoluminescence spectroscopy. This presentation will delineate our findings on a series of different frameworks with coordinatively immobilized photochromic molecules. Specifically, we will discuss static and dynamic tuning of photophysical and electronic properties through metal incorporation, guest encapsulation, and photoresponsive linker embedment. Our findings were employed to develop the first example of the field-effect transistor built upon photochromic MOFs foreshadowing framework implementation in the practical realm.

Light-induced Photoswitching of 4-(Phenylazo)benzoic Acid on Au(111).

Photochromic molecules can undergo a reversible conversion between two isomeric forms upon exposure to external stimuli such as electromagnetic radiation. A significant physical transformation accompanying the photoisomerization process defines them as photoswitches, with potential applications in various molecular electronic devices. As such, a detailed understanding of the photoisomerization process on surfaces and the influence of the local chemical environment on switching efficiency is essential. Herein, we use scanning tunneling microscopy to observe the photoisomerization of 4-(phenylazo)benzoic acid (PABA) assembled on Au(111) in kinetically constrained metastable states guided by pulse deposition. Photoswitching is observed at low molecular density and is absent in tight-packed islands. Furthermore, switching events were noted in PABA molecules coadsorbed in a host octanethiol monolayer, suggesting an influence of the surrounding chemical environment on photoswitching efficiency.

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal-Organic Framework Confinement Leading to a Thermo-Photochromic Platform.

Cooperative behavior and orthogonal responses of two classes of coordinatively integrated photochromic molecules towards distinct external stimuli were demonstrated on the first example of a photo-thermo-responsive hierarchical platform. Synergetic and orthogonal responses to temperature and excitation wavelength are achieved by confining the stimuli-responsive moieties within a metal-organic framework (MOF), leading to the preparation of a novel photo-thermo-responsive spiropyran-diarylethene based material. Synergistic behavior of two photoswitches enables the study of stimuli-responsive resonance energy transfer as well as control of the photoinduced charge transfer processes, milestones required to advance optoelectronics development. Spectroscopic studies in combination with theoretical modeling revealed a nonlinear effect on the material electronic structure arising from the coordinative integration of photoresponsive molecules with distinct photoisomerization mechanisms. Thus, the reported work covers multivariable facets of not only fundamental aspects of photoswitch cooperativity, but also provides a pathway to modulate photophysics and electronics of multidimensional functional materials exhibiting thermo-photochromism.

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal‐Organic Framework Confinement Leading to a Thermo‐Photochromic Platform

AbstractCooperative behavior and orthogonal responses of two classes of coordinatively integrated photochromic molecules towards distinct external stimuli were demonstrated on the first example of a photo‐thermo‐responsive hierarchical platform. Synergetic and orthogonal responses to temperature and excitation wavelength are achieved by confining the stimuli‐responsive moieties within a metal–organic framework (MOF), leading to the preparation of a novel photo‐thermo‐responsive spiropyran‐diarylethene based material. Synergistic behavior of two photoswitches enables the study of stimuli‐responsive resonance energy transfer as well as control of the photoinduced charge transfer processes, milestones required to advance optoelectronics development. Spectroscopic studies in combination with theoretical modeling revealed a nonlinear effect on the material electronic structure arising from the coordinative integration of photoresponsive molecules with distinct photoisomerization mechanisms. Thus, the reported work covers multivariable facets of not only fundamental aspects of photoswitch cooperativity, but also provides a pathway to modulate photophysics and electronics of multidimensional functional materials exhibiting thermo‐photochromism.

Non-Conjugated Bis-(Dithienylethene)-Naphthalenediimide as a Dynamic Anti-Counterfeiting Agent: Driving the Wheel of Photoswitching Enactment.

Photochromic fluorescent molecules dramatically extend their fields of applications ranging from optical memories, bioimaging, photoswitches, photonic devices, anti-counterfeiting technology and many more. Here, we have logically designed and synthesized a triazole appended bis-(dithienylethene)-naphthalenediimide based photo-responsive material, 5, which demonstrated fluorescence enhancement property upon photocyclization (ΦF =0.42), with high photocyclization (44 s, ksolution =0.0355 s-1 , ksolid =0.0135 s-1 ) and photocycloreversion (160 s, ksolution =0.0181 s-1 , ksolid =0.0085 s-1 ) rate and decent photoreaction quantum yield (Φo→c =0.93 and Φc→o =0.11). The open isomer almost converted to the closed isomer at photo-stationary state (PSS) with distinct color change from colorless to blue with 92.85 % conversion yield. A reversible noninvasive modulation of fluorescence through efficient photoinduced electron transfer (PET) process was observed both in solution as well as in solid state. The fluorescence modulation through PET process was further corroborated with thermodynamic calculations using the Rehm-Weller equation and quantum chemical studies (DFT). The thermally stable compound 5 exhibits high fatigue resistance property (up to 50 cycles) both in solution and solid state. Furthermore, the compound 5 was successfully applied as erasable ink and in deciphering secret codes (Quick Response/bar code) portending potential promising application in anti-counterfeiting.

Quasi‐1D Polymer Semiconductor–Diarylethene Blends: High Performance Optically Switchable Transistors

AbstractOptically switchable field‐effect transistors (OSFETs) are non‐volatile photonic memory devices holding a great potential for applications in optical information storage and telecommunications. Solution processing of blends of photochromic molecules and π‐conjugated polymers is a low‐cost protocol to integrate simultaneously optical switching and charge transport functions in large‐area devices. However, the limited reversibility of the isomerization of photochromic molecules due to steric hindrance when embedded in ordered polymeric matrices represents a severe limitation and it obliges to incorporate as much as 20% in weight of the photochromic component, thereby drastically diluting the electronic function, limiting the device performance. Herein, a comparative study of the photoresponsivity of a suitably designed diarylethene molecule is reported when embedded in the matrix of six different polymer semiconductors displaying diverse charge transport properties. In particular, this study focuses on three semi‐crystalline polymers and three quasi‐1D polymers. It is found that 1% w/w of 1,2‐bis(5‐(3,5‐di‐tert‐butylphenyl)‐2‐methylthiophen‐3‐yl)cyclopent‐1‐ene in a blend with poly(indacenodithiophene‐co‐benzothiadiazole) is sufficient to fabricate OSFETs combining photo‐modulation efficiencies of 45.5%, mobilities >1 cm2 V−1s−1, and photo‐recovered efficiencies of 98.1%. These findings demonstrate that quasi‐1D polymer semiconductors, because of their charge transport dominated by intra‐molecular processes, epitomize the molecular design principles required for the fabrication of high‐performance OSFETs.

Fabrication of diarylethene-based reversible fluorescent switch encapsulated in silicone material and its photostimulus response mechanism

Photoresponsive materials, altering their properties such as color, luminescence, and luminescence lifetime under light as external stimulus, would be ideal candidates for application in fluorescent anti-counterfeiting and information encryption. In this paper, we develop a water-dispersive photo-stimuli response hybrid by encapsulating lanthanide complexes and photochromic molecules with an organosilicon material (OSM) via the reprecipitation-encapsulation method. Interestingly, repeated cycles of UV/Vis light irradiation switch the color and structure of photochromic molecules, which in turn influence the Förster resonance energy transfer (FRET) between photochromic molecules and lanthanide complexes, resulting in reversible luminescence ON/OFF switches, accompanied by reduction and recovery of luminescence lifetime of Eu3+ complexes. Considering the above-mentioned interesting characteristics, we realized the application of Eu(DBM)3·phen/BTHFC@OSM suspension as a light-driven fluorescent ink for fingerprint depth encryption. We expect our findings can expand the application scope of photochromic luminescence materials and provide a powerful approach for information encryption and anti-counterfeiting.

Super-resolution Reflection Microscopy via Absorbance Modulation.

In recent years, fluorescence microscopy has been revolutionized. Reversible switching of fluorophores has enabled circumventing the limits imposed by diffraction. Thus, resolution down to the molecular scale became possible. However, to the best of our knowledge, the application of the principles underlying super-resolution fluorescence microscopy to reflection microscopy has not been experimentally demonstrated. Here, we present the first evidence that this is indeed possible. A layer of photochromic molecules referred to as the absorbance modulation layer (AML) is applied to a sample under investigation. The AML-coated sample is then sequentially illuminated with a one-dimensional (1D) focal intensity distribution (similar to the transverse laser mode TEM01) at wavelength λ1 = 325 nm to create a subwavelength aperture within the AML, followed by illumination with a Gaussian focal spot at λ2 = 633 nm for high-resolution imaging. Using this method, called absorbance modulation imaging (AMI) in reflection, we demonstrate a 2.4-fold resolution enhancement over the diffraction limit for a numerical aperture (NA) of 0.65 and wavelength (λ) of 633 nm.

Novel class of photochromic molecules exhibiting photo-switching in the solid state.

Photo-switching compounds are widely used as super-resolution imaging agents, anti-counterfeiting dyes, and molecules that are able to control drug-receptor interactions. However, advancement of this field has been limited by the number of classes of molecules that exhibit this phenomenon, and thus there are growing activities to discover new photo-switching compounds that diversify and improve current applications and include the so-called donor-acceptor Stenhouse adducts. Herein, a new class of compounds, phenylindole alkene dimers, are presented as a novel class of photochromic molecules that exhibit photo-switching in the solid state. The synthesis of a small library of these compounds allowed the tuning of their optical properties. Surfaces coated with these photo-switches can be used as writable materials in a variety of applications.

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

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

Manipulating Photoisomerization Rate of Triphenylethylene Derivative through Metal Coordination for Irradiation Time‐Dependent Information Encryption

AbstractAchieving on‐demand control of responsive behaviors of photochromic molecules is important for advanced photonic applications, but it remains a challenge. In this work, a triphenylethylene‐based molecule, 5‐(2,2‐bis(4‐fluorophenyl)vinyl)‐5″‐methyl‐2,2″‐bipyridine (1), which can undergo open‐to‐closed isomerization under 365 nm irradiation, is designed and synthesized. Importantly, on‐demand acceleration or deceleration of photoisomerization rate over a wide range (kUV = 0.00584–0.06299 s−1) is achieved by coordination of 1 with different metal salts. Furthermore, more precise manipulation of the photoisomerization rate is achieved by changing the counterions of the 1‐based metal complex. Eventually, taking advantage of distinctly different photoisomerization rates of 1 and its corresponding metal complexes, high‐level irradiation time‐dependent information encryption and anticounterfeiting applications have been demonstrated.

Traffic Lights for Catalysis: Stimuli-Responsive Molecular and Extended Catalytic Systems.

The advances made in the field of stimuli-responsive catalysis during the last five years with a focus on the novel recently-emerged directions and applications have been surveyed. Metal-free catalysts and organometallic complexes, as well as biomimetic systems and extended structures, which display switchable catalytic activity for a variety of organic transformations, are discussed. Light-activated systems comprised of photochromic molecules capable of modulating reaction rate, yield, or enantioselectivity based on geometric and electronic changes associated with photoisomerization are the focus of the detailed discussion. Alternative stimuli, including pH and temperature, which could be applied either alone or in combination with light, are also addressed. Recent advances clearly demonstrate that the capability to finely tune catalyst behavior via an external stimulus is a powerful tool that could alter the landscape of sustainable chemistry.

Ampeln für die Katalyse: Stimulus‐reaktive molekulare und erweiterte katalytische Systeme

AbstractDie Fortschritte, die in den letzten fünf Jahren auf dem Gebiet der Stimuli‐reaktiven Katalyse erzielt wurden, mit dem Schwerpunkt auf neuen, kürzlich erschienenen Richtungen und Anwendungen, werden dargestellt. Diskutiert werden metallfreie Katalysatoren und metallorganische Komplexe sowie biomimetische Systeme und erweiterte Strukturen, die eine schaltbare katalytische Reaktivität für eine Vielzahl von organischen Reaktionen aufweisen. Lichtaktivierte Systeme, die aus photochromen Molekülen bestehen, welche in der Lage sind, Reaktionsgeschwindigkeit, Ausbeute oder Enantioselektivität auf Grundlage der geometrischen und elektronischen Veränderungen im Zusammenhang mit der Photoisomerisierung zu modulieren, stehen im Mittelpunkt der ausführlichen Diskussion. Alternative Stimuli wie pH‐Wert und Temperatur, die entweder allein oder in Kombination mit Licht angewendet werden können, werden ebenfalls behandelt. Die jüngsten Fortschritte zeigen deutlich, dass die Möglichkeit, das Verhalten eines Katalysators durch einen externen Stimulus fein abzustimmen, ein leistungsfähiges Instrument ist, das die Landschaft der nachhaltigen Chemie verändern könnte.

Recent Progress in Photoresponsive Biomaterials.

Photoresponsive biomaterials have garnered increasing attention recently due to their ability to dynamically regulate biological interactions and cellular behaviors in response to light. This review provides an overview of recent advances in the design, synthesis, and applications of photoresponsive biomaterials, including photochromic molecules, photocleavable linkers, and photoreactive polymers. We highlight the various approaches used to control the photoresponsive behavior of these materials, including modulation of light intensity, wavelength, and duration. Additionally, we discuss the applications of photoresponsive biomaterials in various fields, including drug delivery, tissue engineering, biosensing, and optical storage. A selection of significant cutting-edge articles collected in recent years has been discussed based on the structural pattern and light-responsive performance, focusing mainly on the photoactivity of azobenzene, hydrazone, diarylethenes, and spiropyrans, and the design of smart materials as the most targeted and desirable application. Overall, this review highlights the potential of photoresponsive biomaterials to enable spatiotemporal control of biological processes and opens up exciting opportunities for developing advanced biomaterials with enhanced functionality.

Achieving Enhanced Photochromism of Spiropyran in Pretreated Nanoporous Lanthanide Metal–Organic Frameworks for Information Storage Applications

Spiropyran and its derivatives are an important class of photochromic molecules that have a wide range of applications in information storage and anti-counterfeiting. However, the easy tight accumulation of molecules under the solid state of spiropyran affects its discoloration efficiency, which is still a challenge to improve the solid photochromic properties. In this paper, a method of loading spiropyran by pretreatment of a lanthanide metal–organic framework (MOF) to improve its photo-response ability is reported. The lanthanide MOF is pretreated by methanol activation and 330 °C calcination to remove the unwanted ligands and some unreacted impurities in the framework. Compared to the pore size of the untreated framework (2.06 nm), the pore sizes of the framework activated by methanol (2.5 nm) and sintering at 330 °C (4.23 nm) were increased. Then, the spiropyran was loaded into a lanthanide MOF by solvent adsorption. After pretreatment of the Tb-MOF, the loading content of spiropyran increased from the initial 4.3 to 9.08 and 12.13%. After 365 nm UV illumination, the difference value in UV absorbance of framework pretreated composites gradually increased from 0.24 to 0.36, accompanied by color transition from yellow to violet. Due to the fluorescence resonance energy transfer effect between the donor lanthanide ion and the acceptor spiropyran, the fluorescence emission transition of the composite material was realized within 10 min. Among them, the fluorescence emission of the untreated composite of the frame can only change from pale yellow to orange, and the maximum fluorescence intensity is 4.4 × 105 CPS, while the fluorescence emission can change from yellow to red after frameworks’ pretreatment, and the fluorescence intensity is significantly improved, which is 6.4 × 105 and 7.5 × 105 CPS, respectively. By applying a composite material to a filter paper, a pattern with high contrast can be made. The pattern can be irradiated with ultraviolet light to realize the physical and fluorescence changes and can be restored to the initial state after being irradiated with green light with a wavelength of 530 nm for 10 min. This not only improves the solid photochromic properties of spiropyran but also provides the application possibility for information storage.

Bridged-Imidazole Dimer Exhibiting Three-State Negative Photochromism with a Single Photochromic Unit.

Photochromic molecules that can exhibit multiple states of photochromism in a single photochromic unit are considered more attractive than traditional bistable photochromic molecules because they can offer more versatility and control in photoresponsive systems. We have synthesized a negative photochromic 1-(1-naphthyl)pyrenyl-bridged imidazole dimer (NPy-ImD) that has three different isomers: a colorless isomer, 6MR, a blue-colored isomer, 5MR-B, and a red-colored isomer, 5MR-R. NPy-ImD can interconvert between these isomers via a short-lived transient biradical, BR, upon photoirradiation. 5MR-R is the most stable isomer, and the energy levels of 6MR, 5MR-B, and BR are relatively close to each other. The colored isomers 5MR-R and 5MR-B are photochemically isomerized to 6MR via the short-lived BR upon irradiation with blue light and red light, respectively. The absorption bands of 5MR-R and 5MR-B are well separated by more than 150 nm, with a small overlap, which means they can be selectively excited with different light sources, visible light for 5MR-R and NIR light for 5MR-B. The colorless isomer 6MR is formed from the short-lived BR through a kinetically controlled reaction. 6MR and 5MR-B can then be converted to the more stable isomer 5MR-R through a thermodynamically controlled reaction, which is facilitated by the thermally accessible intermediate, BR. Notably, 5MR-R photoisomerizes to 6MR when irradiated with CW-UV light, whereas it photoisomerizes to 5MR-B by a two-photon process when irradiated with nanosecond UV laser pulses.

Evaluation of tight-binding DFT performance for the description of organic photochromes properties.

Photochromic molecules are widely studied and developed for their many potential applications. To optimize the required properties through theoretical models, a considerable chemical space is to be explored, and their environment in devices is to be accounted for.. To this end, cheap and reliable computational methods can be powerful tools to steer synthetic developments. As ab initio methods remain costly for extensive studies (in terms of the size of the system and/or number of molecules), semiempirical methods such as density functional tight-binding (TB) could offer a good compromise between accuracy computational cost. However, these approaches necessitate benchmarking on the families of compounds of interest. Thus, the aim of the present study is to evaluate the accuracy of several key features calculated with TB methods (DFTB2, DFTB3, GFN2-xTB, and LC-DFTB2) for three sets of photochromic organic molecules: azobenzene (AZO), norbornadiene/quadricyclane (NBD/QC), and dithienylethene (DTE) derivatives. The features considered here are the optimized geometries, the difference in energy between the two isomers (ΔE), and of the energies of the first relevant excited states. All the TB results are compared to those obtained with DFT methods and state-of-the-art electronic structure calculation methods: DLPNO-CCSD(T) for ground states and DLPNO-STEOM-CCSD for excited states. Our results show that, overall, DFTB3 is the TB method leading to the best results for the geometries and the ΔE values and can be used alone for these purposes for NBD/QC and DTE derivatives. Single point calculations at the r2SCAN-3c level using TB geometries allow circumventing the deficiencies of the TB methods in the AZO series. For electronic transition calculations, the range separated LC-DFTB2 method is the most accurate TB method tested for AZO and NBD/QC derivatives, in close agreement with the reference.

Negative Photochromic 3-Phenylperylenyl-Bridged Imidazole Dimer Offering Quantitative and Selective Bidirectional Photoisomerization with Visible and Near-Infrared Light

Selective bidirectional photoisomerization reactions with high conversion ratios between stable and metastable isomers by irradiation of photochromic molecules with visible light of different wavelengths have been an important issue for many years. For negative photochromic molecules known so far, metastable isomers also absorb UV or visible light in the same region as stable isomers, making it difficult to selectively achieve the reverse reaction by visible-light irradiation. We have demonstrated that the absorption bands of the stable and metastable isomers of 3-phenylperylenyl-bridged imidazole dimer are largely separated by more than 140 nm and that almost quantitative and selective bidirectional photoconversion can be achieved by 660 and 460 nm light. Furthermore, the forward reaction can be achieved completely with near-infrared light of 785 nm.

Designing Multistimuli-Responsive Anisotropic Bilayer Hydrogel Actuators by Integrating LCST Phase Transition and Photochromic Isomerization.

Stimuli-responsive hydrogel actuators have attracted tremendous interest in switches and microrobots. Based on N-isopropylacrylamide (NIPAM) monomers with LCST phase separation and photochromic molecule spiropyran which can respond to ultraviolet light and H+, we develop a novel multistimuli-responsive co-polymer anisotropic bilayer hydrogel, which can undergo complex deformation behavior under environmental stimuli. Diverse bending angles were achieved based on inhomogeneous swelling. By controlling the environmental temperature, the bilayer hydrogels achieved bending angles of 83.4° and -162.4° below and above the critical temperature of PNIPAM. Stimulated by ultraviolet light and H+, the bilayer hydrogels showed bending angles of -19.4° and -17.3°, respectively. In addition, we designed a strategy to enhance the mechanical properties of the hydrogel via double network (DN). The mechanical properties and microscopic Fourier transform infrared (micro-FTIR) spectrum showed that the bilayer hydrogel can be well bonded at the interfaces of such bilayers. This work will inspire the design and fabrication of novel soft actuators with synergistic functions.