Efficient Photoswitching Research Articles

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.

The role of the correlated motion(s) of the chromophore in photoswitching of green and red forms of the photoconvertible fluorescent protein mSAASoti

Wild-type SAASoti and its monomeric variant mSAASoti can undergo phototransformations, including reversible photoswitching of the green form to a nonfluorescent state and irreversible green-to-red photoconversion. In this study, we extend the photochemistry of mSAASoti variants to enable reversible photoswitching of the red form. This result is achieved by rational and site-saturated mutagenesis of the M163 and F177 residues. In the case of mSAASoti it is M163T substitution that leads to the fastest switching and the most photostable variant, and reversible photoswitching can be observed for both green and red forms when expressed in eukaryotic cells. We obtained a 13-fold increase in the switching efficiency with the maximum switching contrast of the green form and the appearance of comparable switching of the red form for the C21N/M163T mSAASoti variant. The crystal structure of the C21N mSAASoti in its green on-state was obtained for the first time at 3.0 Å resolution, and it is in good agreement with previously calculated 3D-model. Dynamic network analysis reveals that efficient photoswitching occurs if motions of the 66H residue and phenyl fragment of chromophore are correlated and these moieties belong to the same community.

Gain of Function Recyclable Photoswitches: Reversible Simultaneous Substitution and Photochromism Generation.

The use of molecular photoswitches has spread to virtually every field of pure and applied chemistry because of the extraordinary level of control they provide over the behavior of matter at the smallest scales. Photoswitches possess at least two different states with distinct structures and/or electronics and further functionalization of their core chromophore structures is needed to tailor them for a specific application. In this work we present a different concept for the generation and use of molecular photoswitches. It allows not only simultaneous establishment of photochromism and functionalization, but also full recyclability of a non-photochromic precursor material. Using a high-yielding and reversible ammonium salt formation, a functional group is introduced into a symmetric precursor while at the same time a strong electronic push-pull character is established in the structure. The resulting desymmetrization leads to efficient photoswitching capacity and the functional group can be fully removed subsequently by a simple heating step recovering the precursor for another functionalization round. We finally demonstrate feasibility of this concept over two consecutive closed loop functionalization/photoswitching/recovery steps. This concept offers great potential in any chemical research and application driven area but especially for the creation of responsive reprogrammable materials, no-background photoswitch labeling, and sustainable chemistry.

Rhodanine-Based Chromophores: Fast Access to Capable Photoswitches and Application in Light-Induced Apoptosis.

Molecular photoswitches are highly desirable in all chemistry-related areas of research. They provide effective outside control over geometric and electronic changes at the nanoscale using an easy to apply, waste-free stimulus. However, simple and effective access to such molecular tools is typically not granted, and elaborate syntheses and substitution schemes are needed in order to obtain efficient photoswitching properties. Here we present a series of rhodanine-based photoswitches that can be prepared in one simple synthetic step without requiring elaborate purification. Photoswitching is induced by UV and visible light in both switching directions, and thermal stabilities of the metastable states as well as quantum yields are very high. An additional benefit is the hydrogen-bonding capacity of the rhodanine fragment, which enables applications in supramolecular or medicinal chemistry. We further show that the known rhodanine-based inhibitor SMI-16a is a photoswitchable apoptosis inducer. The biological activity of SMI-16a can effectively be switched ON or OFF by reversible photoisomerization between the inactive E and the active Z isomer. Rhodanine-based photoswitches therefore represent an easy to access and highly valuable molecular toolbox for implementing light responsiveness to the breadth of functional molecular systems.

Visible light mediated efficient photoswitching of dimethyldihydropyrenes in thin films for all-photonic logic gate applications and dynamic encryption/decryption capabilities

Dimethyldihydropyrene (DHP) photoswitches exhibit visible light-induced ring-opening photoisomerization, reversed by UV/visible light or heat in thin films.

Detour to success: photoswitching via indirect excitation.

Photoswitchable molecules that undergo nanoscopic changes upon photoisomerisation can be harnessed to control macroscopic properties such as colour, solubility, shape, and motion of the systems they are incorporated into. These molecules find applications in various fields of chemistry, physics, biology, and materials science. Until recently, research efforts have focused on the design of efficient photoswitches responsive to low-energy (red or near-infrared) irradiation, which however may compromise other molecular properties such as thermal stability and robustness. Indirect isomerisation methods enable photoisomerisation with low-energy photons without altering the photoswitch core, and also open up new avenues in controlling the thermal switching mechanism. In this perspective, we present the state of the art of five indirect excitation methods: two-photon excitation, triplet sensitisation, photon upconversion, photoinduced electron transfer, and indirect thermal methods. Each impacts our understanding of the fundamental physicochemical properties of photochemical switches, and offers unique application prospects in biomedical technologies and beyond.

Repurposing Bial’s Reaction to Engender Donor–Acceptor Stenhouse Adducts: A Computational Investigation

In pursuing novel photoresponsive materials, we computationally investigated a series of Bial’s reaction-derived Donor–Acceptor Stenhouse Adducts (BialDASAs), comparing them with a prototypical DASA compound, D1A1. We employed density functional theory to elucidate the electronic and geometric factors influencing their photochromic behaviour. Our study confirmed that BialDASA compounds display dual absorption peaks attributed to the π–π∗ transitions, distinct from the single peak of D1A1, reflecting the involvement of the acceptor group in their excited states. Moreover, minimal Bond Length Alternation (BLA) suggested a potential for extended conjugation, consistent across these compounds’ closed and open forms. Analysis of the Z to E isomerization in the excited state indicated a pathway for deactivation via conical intersection, hinting at efficient photoswitching capabilities. Furthermore, the thermodynamic profiles underscored feasible cyclization barrier heights in the ground state, augmented by solvent effects. These findings emphasis the promise of BialDASA molecules as photoswitchable entities and advance our understanding of photochemical mechanisms, providing a scaffold for further experimental validation and application development in molecular electronics and photonics.

Red-light photoswitching of indigos in polymer thin films.

Through simple synthetic derivatisation, the parent indigo dye becomes a red-light E-Z photoswitch exhibiting negative photochromism and tuneable thermal isomerisation kinetics. These attributes make indigo derivatives extremely attractive for applications related to materials and living systems. However, there is a lack of knowledge in translating indigo photoswitching dynamics from solution to solid state - the environment crucial for most applications. Herein, we study the photoswitching performance of six structurally distinct indigo derivatives in five polymers of varying rigidity. Three key strategies are identified to enable efficient photoswitching under red (660 nm) light: (i) choosing a soft polymer matrix to minimise its resistance toward the isomerisation, (ii) creating free volume around the indigo molecules through synthetic modifications, and (iii) applying low dye loading (<1% w/w) to inhibit aggregation. These strategies are shown to improve both photostationary state distributions and the thermal stability of the Z isomer. When all three strategies are implemented, the isomerisation performance (>80% Z form in the photostationary state) is nearly identical to that in solution. These findings thus pave the way for designing new red-light photochromic materials based on indigos.

All Visible Light Photoswitch Based on the Dimethyldihydropyrene Unit Operating in Aqueous Solutions with High Quantum Yields.

Molecular systems and devices whose properties can be modulated using light as an external stimulus are the subject of numerous research studies in the fields of materials and life sciences. In this context, the use of photochromic compounds that reversibly switch upon light irradiation is particularly attractive. However, for many envisioned applications, and in particular for biological purposes, illumination with harmful UV light must be avoided and these photoactivable systems must operate in aqueous media. In this context, we have designed a benzo[e]-fused dimethyldihydropyrene compound bearing a methyl-pyridinium electroacceptor group that meets these requirements. This compound (closed state) is able to reversibly isomerize under aerobic conditions into its corresponding cyclophanediene form (open isomer) through the opening of its central carbon-carbon bond. Both the photo-opening and the reverse photoclosing processes are triggered by visible light illumination and proceed with high quantum yields (respectively 14.5% yield at λ = 680 nm and quantitative quantum yield at λ = 470 nm, in water). This system has been investigated by nuclear magnetic resonance and absorption spectroscopy, and the efficient photoswitching behavior was rationalized by spin-flip time-dependent density functional theory calculations. In addition, it is demonstrated that the isomerization from the open to the closed form can be electrocatalytically triggered.

Photoswitching Properties of 5‐Methoxy‐2‐ (2‐phenyldiazenyl) Pyridine

AbstractHeteroaryl azobenzene molecular switches often have superior performance compared to their parent azobenzene derivatives. Herein, we report the efficient photoswitching of phenylazopyridine 5‐methoxy‐2‐(2‐phenyldiazenyl)pyridine using 1H NMR and UV/vis spectroscopy demonstrating &gt;92 % Z at the photostationary state and a thermal half‐life of 6.3 days at room temperature. Kinetic studies are in good agreement with the DFT predicted isomerization barrier.

Learning Matter: Materials Design with Machine Learning and Atomistic Simulations

ConspectusDesigning new materials is vital for addressing pressing societal challenges in health, energy, and sustainability. The combination of physicochemical laws and empirical trial and error has long guided material design, but this approach is limited by the cost of experiments and the difficulty of deriving complex guiding principles. The space of hypothetical materials to be considered is incredibly large, and only a small fraction of possible compounds can ever be tested experimentally. The computational techniques of atomistic simulation and machine learning (ML) offer an avenue to rapidly invent new materials and navigate this enormous space. Together, they can be used to infer complex design principles and identify high-quality candidates more rapidly than trial-and-error experimentation. In this Account, we review our group’s recent contributions to simulation and ML for materials design. We begin by discussing the numerical representation of materials for use in ML. Representations can be produced through deterministic algorithms, learnable encodings, or physics-based methods and lead to vector, graph, and matrix outputs. We describe how these different approaches offer distinct material- and application-specific advantages. We provide demonstrations from our own work on small-molecule drugs, macromolecules, dyes, electrolytes, and zeolites. In several cases, we show how the appropriate representation led to guiding principles that facilitated experimental materials design. Next, we highlight the development of ML methods for enhancing atomistic simulation. These advances help to improve simulation accuracy and expand the time and length scales that can be explored. They include differentiable atomistic simulations in which ensemble-averaged quantities are differentiated with respect to system parameters, and novel autoregressive methods for enhanced sampling of challenging physical distributions. Other developments include learnable coarse-grained models, which can accelerate molecular dynamics while minimizing the loss of all-atom information, and ML interatomic potentials, which can be trained on maximally informative quantum chemistry data through active learning and adversarial uncertainty attacks. Next, we show how these combined computational advances have enabled high-throughput virtual screening. This has led to the discovery of low-cost organic structure-directing agents for zeolite synthesis, polymer electrolytes, and efficient photoswitches for targeted medicine. We conclude by discussing the limitations of ML and simulation. These include the large data requirements and limited chemical transferability of the former and the speed–accuracy trade-offs of the latter. We predict that advancements in quantum chemistry will further accelerate simulations, while the incorporation of physical principles will improve the reliability of ML.

Toward real-world applications: promoting fast and efficient photoswitching in the solid state

This perspective focuses on the key challenges blocking the development of photoresponsive materials, and shows a guidance value for systematically designing solid-state photoresponsive materials.

Effect of incorporation of silole and phosphole heterocycles into photoswitchable diarylethenes

A reversible interconversion of diarylethenes between two isomers became a powerful means for the development of new materials with a photoswitching function. The versatility of this phenomenon allows a wide variety of heterocyclic systems with advanced properties to be applied. In this mini-review, we analyzed the effect of incorporation of silole and phosphole heterocycles into the diarylethene framework, in particular, its effect on absorbance, quantum yields and thermal stability. The elusive aromaticity and high photostability of silole and phosphole derivatives allowed new highly efficient photoswitches to be obtained. Moreover, the phosphorous atom of the latter displayed a unique ability to undergo chemical modifications that was used for fine-tuning of molecules and development of functional photoswitches. These results clearly demonstrated that incorporation of more exotic moieties into diarylethene remains a promising way to advanced molecules and materials.

Design of High-Performance Pyridine/Quinoline Hydrazone Photoswitches.

The design of P-type photoswitches with thermal stability of the metastable form of hundreds of years that would efficiently transform using excitation wavelengths above 350 nm remains a challenge in the field of photochromism. In this regard, we designed and synthesized an extended set of 13 pyridine/quinoline hydrazones and systematically investigated the structure-property relationships, defining their kinetic and photoswitching parameters. We show that the operational wavelengths of the pyridine hydrazone structural motif can be effectively shifted toward the visible region without simultaneous loss of their high thermal stability. Furthermore, we characterized the ground-state and excited-state potential energy surfaces with quantum-chemical calculations and ultrafast transient absorption spectroscopy, which allowed us to rationalize both the thermal and photochemical reaction mechanisms of the designed hydrazones. Whereas introducing an electron-withdrawing pyridyl moiety in benzoylpyridine hydrazones leads to thermal stabilities exceeding 200 years, extended π-conjugation in naphthoylquinoline hydrazones pushes the absorption maxima toward the visible spectral region. In either case, the compounds retain highly efficient photoswitching characteristics. Our findings open a route to the rational design of a new family of hydrazone-based P-type photoswitches with high application potential in photonics or photopharmacology.

Enhancement of Solid‐State Reversible Photochromism by Incorporation of Rigid Steric Hindrance Groups

AbstractSolid‐state materials with reversible properties are highly attractive, owing to versatile applications in security, sensors, bioimaging, and information storage. Unfortunately, obtaining these materials in the solid state remains a huge challenge. Efficient isomerization usually requires sufficient conformational freedom and cannot be achieved without solvents or matrices. In this study, four structurally simple spiropyran derivatives are prepared that show excellent reversible photochromism under solvent‐free conditions. The large free volumes induced by different rigid aromatic ring groups enable an efficient photoswitching between closed‐ring compounds and their photoisomers. Furthermore, these compounds exhibit mechanochromic properties upon application of a mechanical force. Their color can be maintained for at least 2 days without fading under visible light irradiation at 30 °C, and can be easily erased by heating at 60 °C for 5 min. At least 30 consecutive print–erase cycles can be performed without apparent fatigue. The excellent solid‐state photochromic properties of the present compounds can find application in both UV printing and rewritable paper.

Highly Efficient Photoswitch in Diarylethene-Based Molecular Junctions.

Thin layers of diarylethene oligomers (oligo(DAE)) were deposited by electrochemical reduction of a diazonium salt on glassy carbon and gold electrodes. The layers were fully characterized using electrochemistry, XPS, and AFM, and switching between open and closed forms using light was evidenced. Solid-state molecular junctions (MJs), in which a C-AFM tip is used as the top contact, were fabricated with total layer thicknesses fixed at 2-3 nm and 8-9 nm, i.e. below and above the direct tunneling limit. DAE was then photoswitched between its open and closed forms. Oligo(DAE) MJs using the open form of DAE are highly resistive while those with DAE in the closed form are more conductive. ON/OFF ratios of 2-3 and 200-400 were obtained for 3-nm- and 9-nm-thick DAE MJs, respectively.

A novel upconversion core–multishell nanoplatform for a highly efficient photoswitch

Red upconversion emission is significantly increased on the premise of ensuring optimal UV-blue upconversion emission.

A New Class of Rigid Multi(azobenzene) Switches Featuring Electronic Decoupling: Unravelling the Isomerization in Individual Photochromes.

We report a novel class of star-shaped multiazobenzene photoswitches comprising individual photochromes connected to a central trisubstituted 1,3,5-benzene core. The unique design of such C3-symmetric molecules, consisting of conformationally rigid and pseudoplanar scaffolds, made it possible to explore the role of electronic decoupling in the isomerization of the individual azobenzene units. The design of our tris-, bis-, and mono(azobenzene) compounds limits the π-conjugation between the switches belonging to the same molecule, thus enabling the efficient and independent isomerization of each photochrome. An in-depth experimental insight by making use of different complementary techniques such as UV-vis absorption spectroscopy, high performance liquid chromatography, and advanced mass spectrometry methods as ion mobility revealed an almost complete absence of electronic delocalization. Such evidence was further supported by both experimental (electrochemistry, kinetical analysis) and theoretical (DFT calculations) analyses. The electronic decoupling provided by this molecular design guarantees a remarkably efficient photoswitching of all azobenzenes, as evidenced by their photoisomerization quantum yields, as well as by the Z-rich UV photostationary states. Ion mobility mass spectrometry was exploited for the first time to study multiphotochromic compounds revealing the occurrence of a large molecular shape change in such rigid star-shaped azobenzene derivatives. In view of their high structural rigidity and efficient isomerization, our multiazobenzene photoswitches can be used as key components for the fabrication of complex stimuli-responsive porous materials.

Structure Property Analysis of the Solution and Solid-State Properties of Bistable Photochromic Hydrazones.

The development of new photochromic compounds and the optimization of their photophysical and switching properties are prerequisites for accessing new functions and opportunities that are not possible with currently available systems. To this end we recently developed a new bistable hydrazone switch that undergoes efficient photoswitching and emission ON/OFF toggling in both solution and solid-state. Here, we present a systematic structure-property analysis using a family of hydrazones and show how their properties, including activation wavelengths, photostationary states (PSSs), photoisomerization quantum yields, thermal half-lives (τ1/2), and solution/solid-state fluorescence characteristics vary as a function of electron donating (EDG) and/or withdrawing (EWG) substituents. These studies resulted in the red-shifting of the absorption profiles of the Z and E isomers of the switches, while maintaining excellent PSSs in almost all of the compounds. The introduction of para-NMe2 and/or para-NO2 groups improved the photoisomerization quantum yields, and the extremely long thermal half-lives (tens to thousands of years) were maintained in most cases, even in a push-pull system, which can be activated solely with visible light. Hydrazones bearing EDGs at the stator phenyl group are an exception and show up to 6 orders of magnitude acceleration in τ1/2 (i.e., days) because of a change in the isomerization mechanism. Moreover, we discovered that a para-NMe2 group is required to have reasonable fluorescence quantum yields in solution and that rigidification enhances the emission in the solid-state. Finally, X-ray crystallography analysis showed that the switching process is more efficient in the solid-state when the hydrazone is loosely packed.

Deciphering the grounds of the suitability of acylhydrazones as efficient photoswitches.

Many efforts are currently being devoted to designing molecular photoswitches with specific properties. In this sense, a recent publication [D. J. van Dijken et al., J. Am. Chem. Soc., 2015, 137, 14982-14991] has synthesized and analyzed the photochromic properties of a large set of acylhydrazones (ACHs), a relatively unexploited class of potential photoswitches with two stable E and Z isomers. This study has revealed a very diverse and complex pattern of the absorption/emission properties of ACHs depending on the substituents attached to the ACH motif. In this work, high level theoretical calculations are performed on a representative set of the experimentally studied ACHs in order to analyze, at the molecular level, the reasons behind the different photochemistries experimentally observed. This systematic study allows for the classification of the full set of ACHs into just four categories. The two more common groups display a small, either positive or negative, shift of the maximum wavelength of absorption between the E and Z isomers. Less common, but far more interesting from a practical point of view, are the compounds that show a large (>100 nm) Stokes shift. This behavior may arise from two different situations. The most common one implies the possibility of an intramolecular proton transfer in the excited electronic state of the less stable Z isomer. The less likely scenario would also involve a loss of the azidic proton through an intermolecular proton transfer that would take place with the aid of the solvent.