Photocontrolled Energy Storage in Azobispyrazoles with Exceptionally Large Light Penetration Depths

Systematic investigation of the structure-property relationship of substituted p-alkoxy-azothiophenes

Differently substituted p-alkoxy azothiophenes with increasing alkoxy chains were systematically investigated in terms of their structure-property relationships. In particular, it was observed that increasing the length of the alkoxy chain had an unusual effect on the melting point, which did not follow the expected odd-even effect. It was also shown that changing the length of the alkoxy chain did not significantly affect the thermal half-life, a finding that disagrees with results reported in other studies. These observations provide valuable insights into structure-property relationships with important implications for the design and development of azobenzenes as molecular materials for various applications. Furthermore, each p-alkoxy azothiophene was investigated in terms of neat solid-state photoisomerisation or photoinduced liquefaction, which is a critical parameter for application as a molecular solar thermal phase-change (MOST-PCM) energy storage system.

Advancing Light-Driven Reactions with Surface-Modified Optical Fibers

ConspectusThe challenge of optimizing decentralized water, wastewater, and reuse treatment systems calls for innovative, efficient technologies. One advancement involves surface-modified side-emitting optical fibers (SEOFs), which enhance biochemical and chemical light-driven reactions. SEOFs are thin glass or polymeric optical fibers with functionalized surfaces that can be used individually or bundled together. They can be attached to various light sources, such as light-emitting diodes (LEDs) or lasers, which launch ultraviolet (UV) or visible light into the fibers. This light is then emitted along the fiber's surface, creating irradiance similar to a glow stick. The resulting SEOFs uniquely deliver light energy to complex environments while maximizing photon utilization and minimizing energy loss, addressing long-standing inefficiencies in photolysis and photocatalysis systems. SEOFs generate and leverage refracted light and evanescent waves to achieve continuous irradiation of their cladding, wherein photocatalysts are embedded. This method contrasts with traditional slurry-based systems, where light energy is often scattered or absorbed before reaching the reaction sites. Such scattering typically reduces quantum yields and reaction kinetics. In contrast, SEOFs create a controlled light delivery system that enhances reaction efficiency and adaptability to diverse applications.Important chemical and physical concepts are explored when scaling up SEOFs for three potential engineered applications. The selection of polymer materials and nanoparticle compositions is crucial for optimizing SEOFs as waveguides for visible to UV-C wavelengths and for embedding surface-accessible photocatalysts within porous polymer coatings on SEOF surfaces. Additionally, understanding how light propagates within SEOFs and emits along their exterior surface and length is essential for influencing the quantum yields of chemical products and enhancing biochemical sensitivity to low UV-C exposure. UV-C SEOFs are employed for germicidal disinfection, inactivating biofilms and pathogens in water systems. By overcoming UV light attenuation issues in traditional methods, SEOFs facilitate uniform distribution of UV-C energy, disrupting biofilm formation at early stages. SEOFs enhance UV-A and visible-light photocatalytic degradation of pollutants. Embedding photocatalysts in porous polymer cladding enables simultaneous improvements in reaction kinetics and quantum yields. SEOFs enable decentralized light-driven production of clean energy resources such as hydrogen, hydrogen peroxide, and formic acid, offering sustainable alternatives for off-grid systems.The design principles of SEOFs emphasize scalability, flexibility, and efficiency. Recent innovations in polymer chemistry, nanoparticle coatings, and surface roughness engineering have further optimized light delivery and side-emission. Tailoring the refractive index and nanoparticle distribution on fiber surfaces ensures precise evanescent wave propagation, enhancing photocatalytic performance. These advancements, coupled with scalable fabrication techniques, have positioned SEOFs as promising platforms for broad photochemical applications.By summarizing recent advances and identifying future needs, this Account positions SEOFs as a transformative approach to light-driven reactions, merging cutting-edge materials science with sustainable water treatment and energy production goals. This emerging technology offers immense potential to reshape photochemical processes for decentralized applications. Despite significant progress, challenges remain. Future research should focus on optimizing catalyst loading, improving uniformity in side emissions, and enhancing polymer durability for long-term operational stability. Additionally, scaling SEOF configurations to multifiber bundles and integrating them into decentralized water systems will be critical for broader adoption.

Heteroarylstilbenes: Visible-Light-Tunable Photochromic Systems in Water

Visible-light switchable stilbenes with a stable Z isomer that can function in aqueous media are notably rare in the literature. The synthesis and characterization of a set of 12 novel molecules featuring an aryl-substituted heterostilbene motif, a class of visible-light switchable stilbenes, are presented here. Leveraging an electronic push-pull mechanism, strategic selection of electron donors, and an indolinium moiety as the acceptor, we achieved precise tuning of the switching wavelengths.

Resolving the Ambiguity of Thermal Reversion in a Nonconjugated Monocyclic Diene-Based Photoswitch for Rechargeable Solar Thermal Batteries

We report nonconjugated monocyclic dienes (nCMDs) as unique photoswitchable molecules that hold promise for harvesting substantial solar energy and storing it for extended durations. Herein, cyclohepta-1,4-diene 1a and its N-heterocyclic analogue 2a have been considered as prototypical models for investigating photoswitching behavior in nCMDs. Initially, the nonradiative deactivation pathway of nCMD from the low-lying excited state to the [2 + 2]-cycloadduct has been evaluated. This work resolves the ambiguity and rationalizes the kinetically preferred route for the regeneration of parent diene from a photoisomer that enables the thermo-reversible photoswitching cycle. Moreover, it sheds light on how the syn-anti-isomerism upon thermolysis facilitates the long-term storage of harvested solar energy. Extensive electronic structure analysis reveals that charge transfer from a lone pair on N restricts syn-anti-isomerism while promoting direct reversal of the photoproduct into diene and is highlighted as a pivotal factor for relatively short-term energy storage. The critical physical insights gained in this work are crucial for achieving efficient and sustainable solutions for solar thermal energy storage using novel photoswitches which can contribute to the global transition toward renewable energy sources.

Switching Sides: Regiochemistry and Functionalization Dictate the Photoswitching Properties of Imines

Photoswitchable imines demonstrate light-dependent dynamic covalent chemistry and can function as molecular ratchets. However, the design of aryliminopyrazoles (AIPs) has been limited to N-pyrazole derivatives with ortho-pyrrolidine motifs. The impact of other functionalization patterns on the photoswitching properties remains unknown. Here, we present a systematic structure-property analysis and study how the photoswitching properties can be tuned through ortho- and para-functionalization of the phenyl ring in N-pyrazole and N-phenyl AIPs. This study establishes the first set of design rules for these AIP photoswitches and reports the most stable Z-isomer of an AIP to date, enabling its crystallization and resulting in the first reported crystal structure of a metastable Z-aldimine. Finally, we demonstrate that the AIPs are promising candidates for photoswitching in the condensed phase.

Being Smarter, Azobenzene-Containing Biomaterial Showing Triple Stimuli-Responsive Phase Change Property to Light, Humidity and Force at Room Temperature

Multiple stimuli-responsiveness is an attractive property that is studied in physical chemistry and materials chemistry. While, multiple stimuli-responsive phase change in an isothermal way is rarely addressed for functional materials at room temperature. In this study, one azobenzene-containing surfactant AZO is designed for the fabrication of triple stimuli-responsive phase change biomaterial (Alg-AZO) through the electrostatic complexation with natural alginate. Thanks to the photoisomerization ability, molecular flexibility and hydrophilicity of AZO, together with the tailoring effect of alginate on AZO, Alg-AZO could perform reversible isothermal phase transition between liquid crystalline and isotropic liquid states under the stimuli of either light or humidity at room temperature. Furthermore, the humidity-induced isotropic state can also fast transit to ordered state under shear force, owing to the π-π interactions between planar trans-AZO in Alg-AZO material. With good biocompatibility, self-healing property and in vivo wound healing promoting capacity that is promoted by light, humidity and force, Alg-AZO would be suitable for working as a new smart biomaterial in biological and biomedical areas. This work provides a designing strategy for gaining multiple stimuli-responsive smart materials based on biomacromolecules, and also opening a new opportunity for gaining self-healing biomaterials capable of working in various conditions.

Self-Activated Energy Release Cascade from Anthracene-Based Solid-State Molecular Solar Thermal Energy Storage Systems

We introduce donor-acceptor substituted anthracenes as effective molecular solar thermal energy storage compounds that operate exclusively in the solid state. The donor-acceptor anthracenes undergo visible light-induced [4+4] cycloaddition reaction, producing metastable cycloadducts, dianthracenes with quaternary carbons, and storing photon energy. The triggered cycloreversion of dianthracenes to anthracenes discharges the stored energy as heat in the order of 100 kJ/mol (200 J/g). The series of compounds displays remarkable self-heating, or cascading heat release, upon the initial triggering. Such self-activated energy release is enabled by the large energy storage in dianthracenes, low activation energy for their thermal reversion, and effective heat transfer to unreacted molecules in the solid state. This process mirroring the self-ignition of fossil fuels opens up opportunities to use dianthracenes as effective and renewable solid-state fuels that can release energy rapidly and completely upon initial activation.

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.

Photoswitchable Azobispyrazole Crystals Achieving Near-Quantitative Crystalline-State Bidirectional E ⇆ Z Conversions

Azo molecules, being extensively studied as photoswitches, have demonstrated versatile photoswitching performance and applications in solution-phase systems. However, the dense molecular packing and insufficient conformational freedom in the solid/crystalline state typically pose a challenge to their E ⇆ Z isomerization. This study presents a breakthrough in solid-state azo chemistry, where the investigated azobispyrazole molecules are capable of achieving high E → Z photoconversion, ranging from 85% to nearly quantitative (96%), and quantitative Z → E photoswitching in their crystalline states. To the best of our knowledge, azobispyrazoles are the first photoswitchable azo crystals that achieve high-yield bidirectional conversions, particularly the challenging thermodynamically stable-to-metastable E → Z transformation. Crystallographic and computational analyses provide in-depth insights into the photoswitching mechanism and propose that locally distributed free spaces and weak intermolecular interactions within the crystal structures are key factors contributing to the crystalline-state conversion. This work opens up new avenues for the development of promising photoswitchable azo crystals and also underscores the potential application of azobispyrazole crystals as light-responsive materials.

Iminobispyrazole (IBP) photoswitches: two pyrazole rings can be better than one

We recently demonstrated that suitably functionalised aryliminopyrazoles can exhibit useful photoswitching properties. This study investigates the photoswitching potential of iminobispyrazoles (IBPs). We find that the regiochemistry of the IBPs strongly dictates their photoswitching properties, most notably, the λmax, the photostationary state, and the thermal half-life of the Z-isomer.

Large and long-term photon energy storage in diazetidines via [2+2] photocycloaddition

We report a series of p-functionalized phenylbenzoxazoles that offer remarkable energy storage, exceeding 300 J g-1, for the first time among intermolecular cycloaddition-based molecular solar thermal energy storage systems. The [2 + 2] photocycloaddition of phenylbenzoxazoles generates diazetidine cycloadducts that store energy for up to 23 years in the solid state and release energy upon triggered cycloreversion. The solid-state phase transition contributes to increasing overall energy storage densities, and the dearomative cycloaddition process is revealed to be critical for maximizing the intrinsic energy storage capacities. The solvent-assisted cycloreversion is also used to accelerate the energy release from the emerging molecular scaffold.

Switching from Molecules to Functional Materials: Breakthroughs in Photochromism With MOFs

Photochromic materials with properties that can be dynamically tailored as a function of external stimuli are a rapidly expanding field driven by applications in areas ranging from molecular computing, nanotechnology, or photopharmacology to programable heterogeneous catalysis. Challenges arise, however, when translating the rapid, solution-like response of stimuli-responsive moieties to solid-state materials due to the intermolecular interactions imposed through close molecular packing in bulk solids. As a result, the integration of photochromic compounds into synthetically programable porous matrices, such as metal-organic frameworks (MOFs), has come to the forefront as an emerging strategy for photochromic material development. This review highlights how the core principles of reticular chemistry (on the example of MOFs) play a critical role in the photochromic material performance, surpassing the limitations previously observed in solution or solid state. The symbiotic relationship between photoresponsive compounds and porous frameworks with a focus on how reticular synthesis creates avenues toward tailorable photoisomerization kinetics, directional energy and charge transfer, switchable gas sorption, and synergistic chromophore communication is discussed. This review not only focuses on the recent cutting-edge advancements in photochromic material development, but also highlights novel, vital-to-pursue pathways for multifaceted functional materials in the realms of energy, technology, and biomedicine.

Two-way photoswitching norbornadiene derivatives for solar energy storage

Molecular photoswitches of norbornadiene (NBD) derivatives have been effectively applied in molecular solar-thermal energy storage (MOST) by photoisomerization of NBD to a quadricyclane (QC) state. However, a challenge of the NBD-based MOST system is the lack of a reversible two-way photoswitching process, limiting conversion from QC to thermal and catalytic methods. Here we design a series of NBD derivatives with a combination of acceptor and donor units to achieve two-way photoswitching, which can optically release energy by back-conversion from QC to NBD. Highly efficient photoconversion yields from NBD to QC and QC to NBD are up to 99% and 82%, respectively. The energy storage density of two-way photoswitching NBD is up to 312 J g-1 and optically controlled two-way photoswitching devices are demonstrated for the first time both in flow and in thin films, which illustrate a promising approach for fast and robust energy release in both solution and solid state.

Photoresponsive heparin ionic complexes toward controllable therapeutic efficacy of anticoagulation

Controllable heparin-release is of great importance and necessity for the precise anticoagulant regulation. Efforts have been made on designing heparin-releasing systems, while, it remains a great challenge for gaining the external-stimuli responsive heparin-release in either intravenous or catheter delivery. In this study, an azobenzene-containing ammonium surfactant is designed and synthesized for the fabrication of photoresponsive heparin ionic complexes through the electrostatic complexation with heparin. Under the assistance of photoinduced trans-cis isomerization of azobenzene, the obtained heparin materials perform reversible athermal phase transition between ordered crystalline and isotropic liquid state at room temperature. Compared to the ordered state, the formation of isotropic state can effectively improve the dissolving of heparin from ionic materials in aqueous condition, which realizes the photo-modulation on the concentration of free heparin molecules. With good biocompatibility, such a heparin-releasing system addresses photoresponsive anticoagulation in both in vitro and in vivo biological studies, confirming its great potential clinical values. This work provides a new designing strategy for gaining anticoagulant regulation by light, also opening new opportunities for the development of photoresponsive drugs and biomedical materials based on biomolecules.

Visible-Light-Activated Heteroaryl Azoswitches: Toward a More Colorful Future

Azobenzenes (Ph-N═N-Ph) are known as the most widely studied molecular photoswitches, and the recent rise of azoheteroarenes (Het-N═N-Ph or Het-N═N-Het) offers great opportunities to advance this already mature field. A common limitation is that azo-switches generally require harmful UV light for activation, which hinders their application across various fields. Despite great efforts in developing visible-light azobenzenes over the past few decades, the potential of visible-light heteroaryl azoswitches remains largely unexplored. This Perspective summarizes the state-of-the-art advancements in visible-light heteroaryl azoswitches, covering molecular design strategies, the structure-property relationship, and potential applications. We highlight the distinctive advantages of azoheteroarenes over azobenzenes in the research and development of visible-light switches. Furthermore, we discuss the opportunities and challenges in this emerging field and propose potential solutions to address crucial issues such as spectral red-shift and thermal half-life. Through this Perspective paper, we aim to provide inspiration for further exploration in this field, in anticipation of the growing prosperity and bright future of visible-light azoheteroarene photoswitches.

Arylazo-3,5-diphenylpyrazole Derivatives: Molecular Probes Exhibiting Reversible Light-induced Phase Transitions for Energy Storage and Direct Photolithographic Patterning

We report azopyrazole photoswitches decorated with variable N-alkyl and alkoxy chains (for hydrophobic interactions) and phenyl substituents on the pyrazoles (enabling π-π stacking), showing efficient bidirectional photoswitching and reversible light-induced phase transition (LIPT). Extensive spectroscopic, microscopic, and diffraction studies and computations confirmed the manifestation of molecular-level interactions and photoisomerization into macroscopic changes leading to the LIPT phenomena. Using differential scanning calorimetric (DSC) studies, the energetics associated with those accompanying processes were estimated. The long half-lives of Z isomers, high energy contents for isomerization and phase transitions, and the stability of phases over an extended temperature range (-60 to 80 °C) make them excellent candidates for energy storage and release applications. Remarkably, the difference in the solubility of the distinct phases in one of the derivatives allowed us to utilize it as a photoresist in photolithography applications on diverse substrates.

Novel route to enhance the thermo-optical performance of bicyclic diene photoswitches for solar thermal batteries

Harnessing solar energy by employing chemical photoswitches in molecular solar thermal (MOST) energy storage systems is a topic of appealing research interest. However, incorporating all the features desired for an ideal MOST system in a single photoswitching couple is challenging. Inspired by experimental synthesis, herein we report our attempt to enhance both the thermochemical and photophysical properties in a single-bridged bicyclic diene (BBD)-based photoswitch by elongating the unsaturated bridge with different heteroatomic units. To elucidate the best elongation unit, the energy storage capacity and the TBR barriers were accounted using the DLPNO-CCSD(T) and (8,8)-CASPT2 methods, respectively. The photophysical properties including the absorption onset, excitation wavelengths, and the absorption intensities were extensively investigated with the time-dependent calculations. The result provides information on the most versatile solvent to exhibit the best photoswitching behaviour which is beneficial for real-life energy storage applications. Additionally, the stability and reversibility of the photoswitching system with elongated unsaturated bridges have also been assessed. By means of the studied modification, the storage energy of 158.57 kJ/mol, energy storage density of 1.48 MJ/kg, TBR barrier of 136.36 kJ/mol, and the absorption onset of 305.00 nm is achieved in acetonitrile. These values are substantially higher when compared with the storage energy (96.06 kJ/mol), energy storage density (1.04 MJ/kg), and TBR barrier (121.76 kJ/mol) of prototype NBD/QC in the gas phase. The outcomes render useful insights into the stability and properties of bicyclic diene-based photoswitches having elongated unsaturated bridges and indeed paves the way for the rational design of practical MOST systems.

Photoswitchable imines: aryliminopyrazoles quantitatively convert to long-lived Z-isomers with visible light

Arylimines offer promise in dynamic-covalent materials due to their recyclability and ease of synthesis. However, their light-triggered E/Z isomerism has received little attention. This is attributed to challenges that include low thermal stability of their metastable state (<60 s at 20 °C), incomplete photoswitching (<50% to the metastable state), and the need for UV light (≤365 nm). We overcome these limitations with a novel class of imine photoswitch, the aryliminopyrazoles (AIPs). These AIPs can be switched using visible light (470 nm), attain photostationary states with over 95% of the Z-isomer, exhibit great resistance to fatigue, and have thermal half-lives up to 19.2 hours at room temperature. Additionally, they display T-type and negative photochromism under visible light irradiation-a useful property. The photochromic properties, quantitative assembly and accessibility of precursors set these photoswitches apart from their azo-based analogues. These findings open avenues for next-generation photoresponsive dynamic-covalent materials driven solely by these new photochromic linkages and further exploration of photocontrolled dynamic combinatorial chemistry.

Multichromophoric photoswitches for solar energy storage: from azobenzene to norbornadiene, and MOST things in between

The ever-increasing global demands for energy supply and storage have led to numerous research efforts into finding and developing renewable energy technologies. Molecular solar thermal energy storage (MOST) systems utilise molecular photoswitches that can be isomerized to a metastable high-energy state upon solar irradiation. These high-energy isomers can then be thermally or catalytically converted back to their original state, releasing the stored energy as heat on-demand, offering a means of emission-free energy storage from a closed system, often from only organic materials. In this context, multichromophoric systems which incorporate two or more photochromic units may offer additional functionality over monosubstituted analogues, due to their potential to access multiple states as well as having more attractive physical properties. The extended conjugation offered by these systems can lead to a red shift in the absorption profile and hence a better overlap with the solar spectrum. Additionally, the multichromophoric design may lead to increased energy storage densities due to some of the molecular weight being 'shared' across several energy storage units. This review provides an overview and analysis of multichromophoric photoswitches incorporating the norbornadiene/quadricyclane (NBD/QC) couple, azobenzene (AZB), dihydroazulene (DHA) and diarylethene (DAE) systems, in the context of energy storage applications. Mixed systems, where two or more different chromophores are linked together in one molecule, are also discussed, as well as limitations such as the loss of photochromism due to inner filter effects or self-quenching, and how these challenges may be overcome in future designs of multichromophoric systems.

Photoinduced Solid-Liquid Phase Transition and Energy Storage Enabled by the Design of Linked Double Photoswitches

We demonstrate an effective design strategy of photoswitchable phase change materials based on the bis-azobenzene scaffold. These compounds display a solid phase in the E,E state and a liquid phase in the Z,Z state, in contrast to their monoazobenzene counterparts that exhibit less controlled phase transition behaviors that are largely influenced by their functional groups.

Visible light activated dendrimers for solar thermal energy storage and release below 0 °C

Molecular solar thermal (MOST) fuels offer a closed-cycle and renewable energy storage strategy that can harvest photons within the chemical conformations and release heat on demand through reversible isomerization of molecular photoswitches.

Breaking the photoswitch speed limit

The forthcoming generation of materials, including artificial muscles, recyclable and healable systems, photochromic heterogeneous catalysts, or tailorable supercapacitors, relies on the fundamental concept of rapid switching between two or more discrete forms in the solid state. Herein, we report a breakthrough in the "speed limit" of photochromic molecules on the example of sterically-demanding spiropyran derivatives through their integration within solvent-free confined space, allowing for engineering of the photoresponsive moiety environment and tailoring their photoisomerization rates. The presented conceptual approach realized through construction of the spiropyran environment results in ~1000 times switching enhancement even in the solid state compared to its behavior in solution, setting a record in the field of photochromic compounds. Moreover, integration of two distinct photochromic moieties in the same framework provided access to a dynamic range of rates as well as complementary switching in the material's optical profile, uncovering a previously inaccessible pathway for interstate rapid photoisomerization.

Exploiting benzilic acid as a modular template: controlling photoreactivity and solid to liquid transition during photodimerization

A well-known molecule, benzilic acid, is used as a [2+2] photodimerization template by using third-generation crystal engineering principles. This template utilizes orthogonality and non-covalent interactions in an optimized way and is shown to be effective in tuning the photoreactivity of styryl pyridine derivatives. The photo-induced crystal-to-liquid transformation was observed during photodimerization. This phenomenon is explained based on slip plane and energy framework analysis.

Disequilibrating azobenzenes by visible-light sensitization under confinement

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

Solar Efficiency of Azo-Photoswitches for Energy Conversion: A Comprehensive Assessment

Photoswitches can absorb solar photons and store them as chemical energy by photoisomerization, which is regarded as a promising strategy for photochemical solar energy storage. Although many efforts have been devoted to photoswitch discovery, the solar efficiency, a critical fundamental parameter assessing the solar energy conversion ability, has attracted little attention and remains to be studied comprehensively. Here we provide a systematic evaluation of the solar efficiency of typical azo-switches including azobenzenes and azopyrazoles, and gain a comprehensive understanding on its decisive factors. All the efficiencies are found below 1.0 %, far from the proposed limits for molecular solar thermal energy storage systems. Azopyrazoles exhibit remarkably higher solar efficiencies (0.59-0.94 %) than azobenzenes (0.11-0.43 %), benefiting from largely improved quantum yield and photoisomerization yield. Light filters can be used to improve the isomerization yield but inevitably narrow the usable range of solar spectrum, and these two contradictory effects ultimately reduce solar efficiencies. We envision this conflict could be resolved through developing azo-switches that afford high isomerization yields by absorbing wide-spectrum solar energy. We hope this work could promote more efforts to improve the solar efficiency of photoswitches, which is highly relevant to the prospect for future applications.

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.

A Photopatternable Conjugated Polymer with Thermal-Annealing-Promoted Interchain Stacking for Highly Stable Anti-Counterfeiting Materials

Photoresponsive polymers can be conveniently used to fabricate anti-counterfeiting materials through photopatterning. However, an unsolved problem is that ambient light and heat can damage anti-counterfeiting patterns on photoresponsive polymers. Herein, photo- and thermostable anti-counterfeiting materials are developed by photopatterning and thermal annealing of a photoresponsive conjugated polymer (MC-Azo). MC-Azo contains alternating azobenzene and fluorene units in the polymer backbone. To prepare an anti-counterfeiting material, an MC-Azo film is irradiated with polarized blue light through a photomask, and then thermally annealed under the pressure of a photonic stamp. This strategy generates a highly secure anti-counterfeiting material with dual patterns, which is stable to sunlight and heat over 200 °C. A key for the stability is that thermal annealing promotes interchain stacking, which converts photoresponsive MC-Azo to a photostable material. Another key for the stability is that the conjugated structure endows MC-Azo with desirable thermal properties. This study shows that the design of photopatternable conjugated polymers with thermal-annealing-promoted interchain stacking provides a new strategy for the development of highly stable and secure anti-counterfeiting materials.

Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1-4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g-1 that exceeds previous records for well-defined molecular STFs.

Optically Controlled Recovery and Recycling of Homogeneous Organocatalysts Enabled by Photoswitches

We address a critical challenge of recovering and recycling homogeneous organocatalysts by designing photoswitchable catalyst structures that display a reversible solubility change in response to light. Initially insoluble catalysts are UV-switched to a soluble isomeric state, which catalyzes the reaction, then back-isomerizes to the insoluble state upon completion of the reaction to be filtered and recycled. The molecular design principles that allow for the drastic solubility change over 10 times between the isomeric states, 87 % recovery by the light-induced precipitation, and multiple rounds of catalyst recycling are revealed. This proof of concept will open up opportunities to develop highly recyclable homogeneous catalysts that are important for the synthesis of critical compounds in various industries, which is anticipated to significantly reduce environmental impact and costs.

Photoliquefiable Azobenzene Surfactants toward Solar Thermal Fuels that Upgrade Photon Energy Storage via Molecular Design

Photoresponsive phase change materials (PPCMs) are capable of storing photon and heat energy simultaneously and releasing the stored energy as heat in a controllable way. While, the azobenzene-based PPCMs exhibit a contradiction between gravimetric energy storage density and photoinduced phase change. Here, a type of azobenzene surfactants with balance between molecular free volume and intermolecular interaction is designed in molecular level, which can address the coharvest of photon energy and low-grade heat energy at room temperature. Such PPCMs gain the total gravimetric energy density up to 131.18 J g^-1 by charging solid sample and 160.50 J g^-1 by charging solution. Notably, the molar isomerization enthalpy upgrades by a factor of up to 2.4 compared to azobenzene. The working mechanism is explained by the computational studies. All the stored energy can release out as heat under Vis light, causing a fast surface temperature rise. This study demonstrates a new molecular designing strategy for developing azobenzene-based PPCMs with high gravimetric energy density by improving the photon energy storage.