Molecular Photoswitches Research Articles

Boosting the Photoresponse of Azobenzene Single-Molecule Junctions via Mechanical Interlock and Dynamic Anchor.

As the most classic photoisomerization system, azobenzene has been widely utilized as a building unit in various photoswitching applications. However, attempts to build azobenzene-based single-molecule photoswitches have met with limited success, giving low on/off ratios. Herein, we demonstrate two designs of azobenzene-based photoresponsive single-molecule junctions, based on mechanically interlocked diazocine and azobenzene-based dynamic anchors, respectively. Molecular conductance measurements using the scanning tunneling microscope breaking junction (STMBJ) technique revealed dramatic conductance changes upon photoillumination, achieving a high on/off ratio of ∼3.7. Using density functional theory (DFT), we revealed peculiar quantum interference (QI) effects in the diazocine molecular switch, indicating that diazocine is an excellent candidate for molecular photoswitches. The asymmetric azobenzene devices with a dynamic anchor exhibit switching behavior between a fully off state and a highly conductive state associated with the trans/cis conformation transition. The findings of this work not only present the design and development of functional molecular devices based on azobenzene units but also provide insight into the fundamental properties of light-induced quantum interference in azobenzene-based molecular devices.

Aza‐Diarylethenes Undergoing Both Photochemically and Thermally Reversible Electrocyclic Reactions

AbstractExploring novel molecular photoswitches plays a crucial role in the field of photo‐functional materials chemistry. In this study, we synthesized aza‐diarylethenes with benzothiophene‐S,S‐dioxide as a part of the hexatriene structure and investigated their photochromic properties. Unlike previously reported aza‐diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo‐photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X‐ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed‐ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza‐diarylethenes exhibited not only photochemical ring‐closing and ring‐opening reactions but also thermal ring‐closing and ring‐opening reactions, driven by a thermal equilibrium between the open‐ and closed‐ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy‐level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed‐ring isomers adopt the same molecular structure, which was well explained based on the reaction mechanism of photochemical and thermal ring‐closing reactions. These findings not only advance the field of aza‐diarylethenes but also inspire future research in the development of new photoswitches.

Advances in Molecular Photoswitches: From Azobenzenes to Azoquinolines

Molecular photoswitches represent a dynamic and ever-growing research area based on the ability of molecules to convert (switch) between cis- (Z) and trans- (E) isomers. Azobenzenes are the most popular and widely employed Z-E photoswitchable molecules in the development of photoresponsive, multifunctional smart materials for various applications. The promising avenues in this field include molecular fine-tuning of azobenzene- based photoswitches and the creation of single or dual-functional probes. This short overview highlights recent advances in the design of molecular photoswitches, particularly the molecular design strategies of azobenzene-based photoswitches with their structural and electronic features. Particular attention is paid to azoquinolines, which seem to be a promising alternative to azobenzenes in the design of novel multifunctional photoswitches with improved photochromic properties. Here, we have also developed the novel star-shaped multiazoquinoline photoswitch comprising individual azoquinoline-based photochromes connected to a central trisubstituted 1,3,5-triformylphloroglucinol core by quantum chemical calculations. This unique structure is favorable for independent Z-E isomerization of each azoquinoline-based photochrome within one macromolecule.

Converting second-order saddle points to transition states: New principles for the design of 4π photoswitches.

Molecular photoswitches have demonstrated potential for storing solar energy at the molecular level, with power densities comparable to commercial batteries and hydroelectric energy storage. However, development of efficient photoswitches is hindered by limitations in cyclability and optical properties of existing materials. We here demonstrate that certain limitations in photoswitches based on electrocyclizations stem from the issue of controlling competition between Woodward-Hoffmann allowed and forbidden pathways. Our approach moves beyond the traditional view of activation barriers and reveals that second-order saddle points are crucial in dictating the competition between disrotatory and conrotatory pathways. These insights suggest new opportunities to manipulate the competition between these pathways through geometric constraints, fundamentally altering the connectivity of the potential energy surface. Our study also emphasizes the necessity of multi-reference methods and the need to conduct higher-dimensional explorations for competing pathways beyond photoswitch design.

New opportunities for molecular photoswitches as wearable ultraviolet radiation dosimeters

Australians have the highest incidence of melanoma globally, despite increasing awareness of the risks of excessive sun exposure. Although excess ultraviolet radiation (UVR) can cause irreparable cell damage and lead to cancer, some exposure is vital to maintain bodily processes such as vitamin D production. For an individual, finding the balance between healthy exposure and skin damage is largely guesswork. The ability to provide a simple, individualised indicator of cumulative UVR dosage could be transformative in preventing skin cancer. This review will provide a brief overview of the variety of UVR sensor technologies and explain the important role of colourimetric dosimeters. The chemistry behind some recent examples of colourimetric dosimeters will be discussed, identifying that molecular photoswitches are ideal candidates to enable this technology. We discuss the chemical mechanisms of photoswitches and how to modify their chemical structure to optimise their properties for use as dosimeters. Through this lens, diarylethenes have been identified as prime dosimeter candidates, owing to their sensitivity, stability, adaptability and the variety of visually striking colours possible. Finally, some specific challenges are identified in the design and fabrication of personalised colourimetric dosimeters that can equitably meet the requirements of all users in our community.

Triptycene-Based Tripodal Molecular Platforms.

Molecular platforms are essential components of various surface-mounted molecular devices. Here, we document the synthesis of two universal triptycene-based tripodal pedestals featuring terminal alkynes in the axial position. We showcase their versatility by incorporating them into the structures of diverse functional molecules such as unidirectional light-driven molecular motors, photoswitches, and Brownian molecular rotors using standard cross-coupling reactions. We also present their fundamental physical properties, including acidity constants, data from differential scanning calorimetry, and crystallographic analysis of two parent and five derived structures. Finally, and importantly, we demonstrate that the photochemical properties of selected photoswitch representatives remain uncompromised when fused with tripods.

Engineering the novel azobenzene-based molecular photoswitches for suppressing bacterial infection through dynamic regulation of biofilm formation.

Bacterial biofilm is a strong fortress for bacteria to resist harsh external environments, which can enhance their tolerance and exacerbate the drug/pesticide resistance risk. Currently, photopharmacology provides an advanced approach via precise spatiotemporal control for regulating biological activities by light-controlling the molecular configurations, thereby having enormous potential in the development of drug/pesticides. To further expand the photopharmacology application for discovering new antibiofilm agents, we prepared a series of light-controlled azo-active molecules and explored their photo isomerization, fatigue resistance, and anti-biofilm performance. Furthermore, their mechanisms of inhibiting biofilm formation were systematically investigated. Overall, designed azo-derivative A11 featured excellent anti-Xoo activity with an half-maximal effective concentration (EC50) value of 5.45 μg mL-1, and the EC50 value could be further elevated to 2.19 μg mL-1 after ultraviolet irradiation (converted as cis-configuration). The photo-switching behavior showed that A11 had outstanding anti-fatigue properties. An in-depth analysis of the action mechanism showed that A11 could effectively inhibit biofilm formation and the expression of relevant virulence factors. This performance could be dynamically regulated via loading with private light-switch property. In this work, designed light-controlled azo molecules provide a new model for resisting bacterial infection via dynamic regulation of bacterial biofilm formation. © 2024 Society of Chemical Industry.

Pfadunabhängiges mit sichtbarem Licht adressierbares orthogonales Photoschalten für Anwendungen in multi‐photochromomen Polymeren und molekularen Computern

AbstractSynthetische molekulare Photoschalter sind als hochpräzise Werkzeuge zur Einführung von Lichtadressierbarkeit auf kleinster Ebene in den Mittelpunkt der Forschung gerückt. Heute sind sie in allen Bereichen der angewandten Chemie zu finden, sei es in der Materialforschung, chemischen Biologie, Katalyse oder Nanotechnologie. Für einen nächsten Schritt in der Anwendbarkeit ist vollständig orthogonales Photoschalten besonders wünschenswert, aber bis heute ist eine solche unabhängige Adressierbarkeit verschiedener Photoschalter eine große Herausforderung. In dieser Arbeit präsentieren wir das erste Beispiel für ein vollständig durch sichtbares Licht adressierbares und pfadunabhängiges orthogonales Photoschalten. Durch die Kombination von zwei kürzlich entwickelten indigoiden Photoschaltern – ein peri‐Anthracenthioindigo und ein auf Rhodanin‐basiertes Chromophor – wird ein Schaltsystem mit vier Zuständen geschaffen, wobei jeder Zustand in hoher Ausbeute völlig unabhängig und ausnahmslos mit sichtbarem Licht angesteuert werden kann. Die vier Zustände führen zu vier verschiedenen Farben, die auf eine feste Polymermatrix übertragen werden können, wodurch ein vielseitiges photochromes Material mit multiplen Zuständen entsteht. Darüber hinaus wird die Kombination mit einem Fluoreszenzfarbstoff als dritter Komponente gezeigt, was die Anwendbarkeit dieses orthogonalen Photoschaltersystems in der molekularen Logik und Informationsverarbeitung ermöglicht.

Path-Independent All-Visible Orthogonal Photoswitching for Applications in Multi-Photochromic Polymers and Molecular Computing.

Synthetic molecular photoswitches have taken center stage as high-precision tools to introduce light-responsiveness at the smallest scales. Today they are found in all areas of applied chemistry, covering materials research, chemical biology, catalysis, or nanotechnology. For a next step of applicability truly orthogonal photoswitching is highly desirable but to date such independent addressability of different photoswitches remains to be highly challenging. In this work we present the first example of all-visible, all-light responsive, and path independent orthogonal photoswitching. By combining two recently developed indigoid photoswitches -peri-anthracenethioindigo and a rhodanine-based chromophore - a four-state system is established and each state can be accessed in high yields completely independently and also with visible light irradiation only. The four states give rise to four different colors, which can be transferred to a solid polymer matrix yielding a versatile multi-state photochromic material. Further, combination with a fluorescent dye as third component is possible, demonstrating applicability of this orthogonal photoswitching system in all-photonic molecular logic behavior and information processing.

Theoretical Investigation on the Reversible Photoswitch Mechanism of Benzylidene-Oxazolone System.

The design and application of molecular photoswitches have attracted much attention. Herein, we performed a detailed computational study on the photoswitch benzylidene-oxazolone system based on static electronic structure calculations and on-the-fly excited-state dynamic simulations. For the Z and E isomer, we located six and four minimum energy conical intersections (MECIs) between the first excited state (S1) and the ground state (S0), respectively. Among them, the relaxation pathway driven by ring-puckering motion is the most competitive channel with the photoisomeization process, leading to the low photoisomerization quantum yield. In the dynamic simulations, about 88 % and 66 % trajectories decay from S1 to S0 for Z and E isomer, respectively, within the total simulation time of ~2 ps. The photoisomeization quantum yields obtained in our study (0.20 for Z→E and 0.12 for E→Z) agree well with the experimental measured values (0.25 and 0.11), even though the number of trajectories is limited to 50. Our study sheds light on the complexity of the benzylidene-oxazolone system 's deactivation process and the competitive mechanisms among different reaction channels, which provides theoretical guidance for further design and development of benzylidene-oxazolone based molecular photoswitches.

Heterocyclic Hemipiperazines: Multistimuli-Responsive Switches and Sensors for Zinc or Cadmium Ions.

Advance in the design of molecular photoswitches - adapters that convert light into changes at molecular level - opens up exciting possibilities in preparing smart polymers, drugs photoactivated inside humans, or light-fueled nanomachines that might in the future operate in our bloodstream. Hemipiperazines are recently reported biocompatible molecular photoswitches based on cyclic dipeptides. Here we report a multistimuli-responsive hemipiperazine-based switch that reacts on light, solvents, acidity, or metal ions. Its photoequilibration is controlled by the intramolecular hydrogen bonding pattern. The compound can be used as a mid-nanomolar photoswitchable fluorescent sensor for zinc and cadmium ions, applicable to monitor environmental pollution in real time.

Tuning the Thermal Stability of Tetra-o-chloroazobenzene Derivatives by Transforming Push-Pull to Push-Push Systems.

Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible-light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra-o-chloroazobenzenes, we synthesized push-pull systems, by introducing dialkyl amine and nitro groups as strong electron-donating and electron-withdrawing groups on the para-positions, and then transformed to push-push systems by a simple reduction step. The developed push-pull and push-push tetra-o-chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half-life of the Z-isomers can be tuned from milliseconds for the push-pull system to several hours for the push-push system. The n-π* and π-π* transitions have better resolution in the push-push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push-pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push-push counterpart.

Aza-Diarylethenes Undergoing Both Photochemically and Thermally Reversible Electrocyclic Reactions.

Exploring novel molecular photoswitches plays a crucial role in the field of photo-functional materials chemistry. In this study, we synthesized aza-diarylethenes with benzothiophene-S,S-dioxide as apart of hexatriene structureand investigated their photochromic properties. Unlike previously reported aza-diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo-photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X-ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed-ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza-diarylethenes exhibited not only photochemical ring-closing and ring-opening reactions but also thermal ring-closing and ring-opening reactions, driven by a thermal equilibrium between the open- and closed-ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy-level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed-ring isomers adopt the same structure, which was well explained based on the reaction mechanism of photochemical and thermal ring-closing reactions. These findings not only advance the field of aza-diarylethenes but also inspire future research in the development of new photoswitches.

Complementing photothermal to photochemical actuation of assembled liquid crystal networks for wavelength-selective multimodal locomotion

The incorporation of molecular photoswitches into liquid crystal networks (LCNs) endows spatiotemporal control of their macroscale deformation and mechanical actuation in a noninvasive manner using light. Extending the photoactuation modes of LCNs to a higher level of life-like complexity and intelligence relies significantly on the rational collective utilization of multiple photoswitchable compounds. Here, we report the development of heterogeneously assembled LCN photoactuators that display well-defined stepwise actuation by assembly of new arylazopyrazole (azp)-based and conventional azobenzene (azb)-based LCN segments. Due to their activation wavelength selectivity, where azp is photochemically driven by UV light while azb is photothermally driven by green light, the assembled actuators show a high degree of photocontrollability of the overall actuation behaviors. A series of assembled actuators composed of azp and azb segments ranging from linear to branched structures are constructed to perform programmable wavelength-selective shape transformation. Bioinspired multimodal locomotion including rotating, crawling and rolling as well as optical switching between motion modes, is developed through selective irradiation to generate the robotic function for cargo transportation and release. Our work provides a useful molecular design strategy for the construction of light-driven actuators with precise photocontrollability and expanded actuation modes to better mimic the intelligent behaviors observed in nature.

Study on Thermophysical Properties and Phase Change Regulation Mechanism of Optically-Controlled Phase Change Materials: Synthesis, Crystal Structure and Molecular Dynamics.

Optically-controlled phase change materials, which are prepared by introducing molecular photoswitches into traditional phase change materials (PCMs), can convert and store solar energy into photochemical enthalpy and phase change enthalpy. However, the thermophysical properties of optically controlled PCMs, which are crucial in the practical, are rarely paid attention to. 4-(phenyldiazenyl)phenyl decanoate (Azo-A-10) is experimentally prepared as an optically-controlled PCMs, whose energy storage density is 210.0 kJ·kg-1, and the trans single crystal structure is obtained. The density, phase transition temperature, thermal conductivity, and other parameters in trans state are measured experimentally. Furthermore, a microscopic model of Azo-A-10 is established, and the thermophysical properties are analyzed based on molecular dynamics. The results show that the microstructure parameter (order parameters) and thermophysical properties (density, radial distribution function, self-diffusion coefficient, phase change temperature, and thermal conductivity) of partially or completely isomerized Azo-A-10, which are challenging to observe in experiments, can be predicted by molecular dynamics simulation. The optically-controlled phase change mechanism can be clarified according to the differences in microstructure. The optically-controlled switchability of thermophysical properties of an optically-controlled PCM is analyzed. This study provides ideas for the improvement, development, and application of optically-controlled PCMs in the future.

Photoresponsive Block Copolymer Nanostructures through Implementation of Arylazopyrazoles.

Responsive nanomaterials that can undergo reversible changes in morphology are interesting for the development of functional materials that interact with and respond to their environment. Amphiphilic block copolymers are well-known for their ability to create a wide range of supramolecular nanostructures in solution. Arylazopyrazoles (AAPs) are versatile molecular photoswitches, which change their configuration and hydrophobicity upon irradiation with UV light (365 nm, Z isomer, less hydrophobic) and green light (520 nm, E isomer, more hydrophobic). In this work, photoswitchable block copolymers containing arylazopyrazole tetraethylene glycol methacrylate (AAPMA) and oligo(ethylene glycol) methacrylate (OEGMA) forming amphiphilic POEGMA-b-PAAPMA with varying block lengths are prepared by RAFT polymerization. The photochemical properties of AAP persist in the polymers. Due to their amphiphilic structure, the polymers self-assemble into supramolecular morphologies in water. Remarkably, photoisomerization results in a reversible change in the self-assembly behavior. Specifically, spherical and cylindrical micelles are observed for POEGMA33-b-PAAPMA47 when illuminated with green or UV light during assembly. Furthermore, the morphology of assembled structures can be reversibly switched by subsequent irradiation with UV and green light.

A Halogen‐Bonded Fluorescent Molecular Photoswitch: Transition from 3D Cubic Lattice to 1D Helical Superstructure for Polarization Inversion of Circularly Polarized Luminescence

AbstractThe fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen‐bonded fluorescent molecular photoswitches, namely, HB‐switch 1 and HB‐switch 2, containing α‐cyano‐substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue‐phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB‐switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual‐response behavior of the CPL signals can be attributed to the phase transition from a high‐symmetry 3D BP Icubic lattice to a low‐symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB‐switch 1 and temperature changes. This study underscores the significance of employing halogen‐bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL‐active systems.

A Halogen-Bonded Fluorescent Molecular Photoswitch: Transition from 3D Cubic Lattice to 1D Helical Superstructure for Polarization Inversion of Circularly Polarized Luminescence.

The fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen-bonded fluorescent molecular photoswitches, namely, HB-switch 1 and HB-switch 2, containing α-cyano-substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue-phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB-switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual-response behavior of the CPL signals can be attributed to the phase transition from a high-symmetry 3D BP Icubic lattice to a low-symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB-switch 1 and temperature changes. This study underscores the significance of employing halogen-bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL-active systems.

Predictive Guidelines for Electrocyclization of Dithienylethenes.

Molecular photoswitches undergo a structural transformation upon photoexcitation to interconvert between two or more stable forms. In some cases, the structural rearrangement involves the transition between ring-open and ring-closed forms. In this work, we develop simple guidelines for the design of organic molecules able to undergo photochemical electrocyclization, using electronic structure calculations on dithienylethene (DTE)-based compounds. We conclude that the ability to photocyclize can be predicted from the localization and symmetry of frontier molecular orbitals. These ideas are based on the Woodward-Hoffmann rules but go beyond them, allowing us to address a more general family of chromophores. Our methodology has been experimentally validated in a novel series of quinoline-based DTEs, where the ability for photocyclization depends on specific chemical modifications. We use the DTE scaffold as the workhorse model in our study; however, these simple yet powerful guidelines are expected to be generally valid to guide the design of other diarylethenes.

Bis-azopyrazole Photoswitches for Efficient Solar Light Harvesting.

Although natural sunlight is one of the most abundant and sustainable energy resources, only a fraction of its energy is currently harnessed and utilized in photoactive systems. The development of molecular photoswitches that can be directly activated by sunlight is imperative for unlocking the full potential of solar energy and addressing the growing energy demands. Herein, we designed a series of 2-amino-1,3-bis-azopyrazoles featuring a coupled πn system, resulting in a pronounced redshift in their spectral absorption, reaching up to 661 nm in the red region. By varying the amino substituents of these molecules, highly efficient E→Z photoisomerization under unfiltered sunlight can be achieved, with yields of up to 88.4 %. Moreover, the Z,Z-isomers have high thermal stability with half-lives from days to years at room temperature. The introduction of ortho-amino substitutions and meta-bisazo units leads to a reversal of the n-π* and πn-π* transitions on the energy scale. This change provides a new perspective for further tuning the visible absorption of azo-switches by utilizing the πn-π* band instead of the conventional n-π* band. These results suggest that photoresponsive systems can be powered by sunlight instead of traditional artificial lights, thereby paving the way for sustainable smart materials and devices.