Solar Thermal Storage and Room-Temperature Fast Release Using a Uniform Flexible Azobenzene-Grafted Polynorborene Film Enhanced by Stretching

Azopyridine Polymers in Organic Phase Change Materials for High Energy Density Photothermal Storage and Controlled Release

Azo-compounds molecules and phase change materials offer potential applications for sustainable energy systems through the storage and controllable release photochemical and phase change energy. Developing novel and highly efficient Azo-based solar thermal fuels (STFs) for photothermal energy storage and synergistic cooperation with organic phase change materials present significant challenges. Herein, three types of (ortho-, meta-, and para-) azopyridine polymers hinged with flexible alkyl chain are synthesized, in which meta-azopyridine polymer exhibits striking photothermal storage capacity of 430 J/g, providing a feasibility solution for developing high energy density Azo-based STFs. Furthermore, a stable two-phase hybrid system was innovatively constructed by combining the meta-azopyridine polymer with organic phase change materials leveraging hydrogen bonds and van der Waals interactions to collectively harness phase change energy and photothermal energy. The organic phase change material not only supplies additional phase change latent heat but also serves as a solvent, offering abundant free volume for the photo-induced isomerization of the azopyridine chromophores, which successfully circumvents the low charging efficiency in the condensed state and reliance on solvent-assisted charging in traditional Azo-based STFs. This study demonstrates the energy distribution and utilization for household consumers and the photothermal-assisted insulation strategy, achieving more extensive potential implementation for STFs.

Water-Soluble Azobenzene-Based Solar Thermal Fuels with Improved Long-Term Energy Storage and Energy Density

Azobenzene (azo)-based solar thermal fuels (STFs) have been developed to harvest and store solar energy. However, due to the lipophilicity and low energy density of azo-based STFs, the derived devices demand a large amount of toxic organic solvents for continuous and scalable energy storage. Herein, we report an ionic strategy to prepare water-soluble azo-based STFs (WASTFs) with improved energy storage performance, which can be realized through a facile quaternization reaction using commercial reagents. A family of WASTFs were synthesized, and all of them showed good water solubility, long-term thermal half-life (>30 days), and high energy storage density (a highest energy density of ∼143.6 J g-1 corresponding to an energy storage enthalpy of ∼111.8 kJ mol-1). Compared to the electrically neutral azo-based STFs with similar chemical structures, ΔH and thermal half-life (τ1/2) of the WASTFs are 2.5 times higher and 7.3 times longer, respectively. Cation-π interactions between the quaternized moieties [N+(CHx)4] and benzene moieties of azo were confirmed, which could account for their improvement of the energy storage performance. Macroscale heat release with an average temperature difference of ∼2 °C was achieved for the WASTFs prepared in this work. Generally, a novel family of WASTFs are synthesized and show great applicable prospects in fabricating advanced solar energy storage devices.

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.

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.

Light-Controlled Triple-Shape-Memory, High-Permittivity Dynamic Elastomer for Wearable Multifunctional Information Encoding Devices

Self-powered information encoding devices (IEDs) have drawn considerable interest owing to their capability to process information without batteries. Next-generation IEDs should be reprogrammable, self-healing, and wearable to satisfy the emerging requirements for multifunctional IEDs; however, such devices have not been demonstrated. Herein, an integrated triboelectric nanogenerator-based IED with the aforementioned features was developed based on the designed light-responsive high-permittivity poly(sebacoyl diglyceride-co-4,4'-azodibenzoyl diglyceride) elastomer (PSeDAE) with a triple-shape-memory effect. The electrical memory feature was achieved through a microscale shape-memory property, enabling spatiotemporal information reprogramming for the IED. Macroscale shape-memory behavior afforded the IED shape-reprogramming ability, yielding wearable and detachable features. The dynamic transesterifications and light-heating groups in the PSeDAE afforded a remotely controlled rearrangement of its cross-linking network, producing the self-healing IED.

Liquid and Photoliquefiable Azobenzene Derivatives for Solvent-free Molecular Solar Thermal Fuels

A series of liquid and photoliquefiable azobenzene (Azo) derivatives (Azo-Cn-Br) have been synthesized for molecular solar thermal fuels. Each of the liquid and photoliquefiable azo derivatives shows a high degree of isomerization, a fast isomerization rate, a long half-life, an appropriate energy storage density, and a solvent-free "charging" and "discharging" process. The photoliquefied azo derivatives can isomerize upon UV light irradiation at low temperatures to give the "UV-charged" azo ones. Therefore, the phase transition enthalpy is stored simultaneously along with the isomerization enthalpy. The "UV-charged" azo derivatives are capable of releasing heat under the manipulation of blue light.

Metallic-Ion Controlled Dynamic Bonds to Co-Harvest Isomerization Energy and Bond Enthalpy for High-Energy Output of Flexible Self-Heated Textile

Molecular light-harvesting capabilities and the production of low-temperature heat output are essential for flexible self-heated textiles. An effective strategy to achieve these characteristics is to introduce photoresponsive molecular interactions (photodynamic bonds) to increase the energy storage capacity and optimize the low-temperature photochromic kinetics. In this study, a series of sulfonic-grafted azobenzene-based polymers interacted with different metal ions (PAzo-M, M = Mg, Ca, Ni, Zn, Cu, and Fe) to optimize the energy level and isomerization kinetics of these polymers is designed and prepared. Photoinduced formation and dissociation of MO dynamic bonds enlarge the energy gap (∆E) between trans and cis isomers for high-energy storage and favor a high rate of isomerization for low-temperature heat release. The suitable binding energy and high ∆E enable PAzo-M to store and release isomerization energy and bond enthalpy even in a low-temperature (-5 °C) environment. PAzo-Mg possesses the highest energy storage density of 408.6 J g-1 (113.5 Wh kg-1 ). A flexible textile coated with PAzo-Mg can provide a high rise in temperature of 7.7-12.5 °C in a low-temperature (-5.0 to 5.0 °C) environment by selectively self-releasing heat indoors and outdoors. The flexible textile provides a new pathway for wearable thermal management devices.

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Molecular Solar Thermal Systems towards Phase Change and Visible Light Photon Energy Storage

Molecular solar thermal (MOST) systems have attracted tremendous attention for solar energy conversion and storage, which can generate high-energy metastable isomers upon capturing photon energy, and release the stored energy as heat on demand during back conversion. However, the pristine molecular photoswitches are limited by low storage energy density and UV light photon energy storage. Recently, numerous pioneering works have been focused on the development of MOST systems towards phase change (PC) and visible light photon energy storage to increase their properties. On the one hand, the strategy of simultaneously capturing isomerization enthalpy and PC energy between solid and liquid can not only offer high latent heat, but also promote the development of sustainable energy systems. On the other hand, the efficient photon energy storage in the visible light range opens a tremendously fascinating avenue to fabricate MOST systems powered under natural sunlight. Here, the recent advances of MOST systems towards PC and visible light photon energy storage are systematically summarized, the most promising advantages and current challenges are analyzed, and emerging strategies and future research directions are proposed.

Absorption and Isomerization of Azobenzene Guest Molecules in Polymeric Nanoporous Crystalline Phases

PPO co-crystalline (CC) films including azobenzene guest molecules have been prepared and characterized by WAXD, FTIR and UV-Visible measurements. Isomerization reactions of azobenzene (photo-induced trans to cis and spontaneous cis to trans) included in α and β nanoporous-crystalline (NC) phases leading to CC phases, or simply absorbed in amorphous phase have been studied on thick and thin films. Spectroscopic analysis shows that photo-isomerization of azobenzene occurs without expulsion of azobenzene guest molecules from crystalline phases. Sorption studies of α and β NC films immersed into photo-isomerized azobenzene solution reveal a higher selectivity of the β NC phase toward cis azobenzene isomer than the α NC phase, inducing us to propose the β NC phase as particularly suitable for absorbing spherically bulky guest molecules.

Arylazopyrazole-Based Dendrimer Solar Thermal Fuels: Stable Visible Light Storage and Controllable Heat Release

Solar thermal fuels offer a closed cycle and a renewable energy storage strategy by harvesting photon energy within the chemical conformations of molecules and retrieving energy by an induced release of heat. However, the majority of reports are limited to the ultraviolet light storage, which potentially interferes with the surrounding environment and reduces the material lifetime. Here, we present a novel arylazopyrazole (AAP)-containing dendrimer that not only addresses the hindrance of visible light storage for solar thermal fuels but also exhibits outstanding performances of abundant energy conversion and stable storage, which are attributed to the substantial absorbance in visible wavelengths of para-thiomethyl-substituted AAP groups and the stability of cis isomers, respectively. The energy density of the dendrimer fuel after efficiently harvesting blue light (405 nm) is as high as 0.14 MJ kg^-1 (67 kJ mol^-1), and the storage half-life of the fabricated dendrimer film can reach up to 12.9 days. Moreover, the heat release of the dendrimer film can be triggered by different stimuli (light and heat). The dendrimer film displays a 6.5 °C temperature difference between trans isomers and cis isomers during green light irradiation. Our work provides a fascinating avenue to fabricate visible light storage solar thermal fuels and unlocks the possibility of developing natural sunlight storage in the future.

Synthesis and self-assembly of photo-responsive polypeptoid-based copolymers containing azobenzene side chains

Photo-responsive polypeptoid-based copolymers containing azobenzene side chains have been well synthesized and they could self-assemble into tunable nanostructures with reversible light-switched behaviors.

Photochromic Dendrimers for Photoswitched Solid-To-Liquid Transitions and Solar Thermal Fuels

Dendrimers are well-defined, highly branched macromolecules that have been widely applied in the fields of catalysis, sensing, and biomedicine. Here, we present a novel multifunctional photochromic dendrimer fabricated through grafting azobenzene units onto dendrimers, which not only enables controlled switching of adhesives and effective repair of coating scratches but also realizes high-performance solar energy storage and on-demand heat release. The switchable adhesives and healable coatings of azobenzene-containing dendrimers are attributed to the reversible solid-to-liquid transitions because trans-isomers and cis-isomers have different glass transition temperatures. The adhesion strengths increase significantly with the increase in dendrimer generations, wherein the adhesion strength of fifth-generation photochromic dendrimers (G5-Azo) can reach up to 1.62 MPa, five times higher than that of pristine azobenzenes. The solar energy storage and heat release of dendrimer solar thermal fuels, the isomers of which possess different chemical energies, can be also enhanced remarkably with the amplification of azobenzene groups on dendrimers. The storage energy density of G5-Azo can reach 59 W h kg^-1, which is much higher than that of pristine azobenzenes (36 W h kg^-1). The G5-Azo fuels exhibit a 5.2 °C temperature difference between cis-isomers and trans-isomers. These findings provide a new perspective and tremendously attractive avenue for the fabrication of photoswitchable adhesives and coatings and solar thermal fuels with dendrimer structures.

Metathesis Cyclopolymerization Triggered Self-Assembly of Azobenzene-Containing Nanostructure

Azobenzene (AB) units were successfully introduced into poly(1,6-heptadiyne)s in order to ensure smooth synthesis of double- and single-stranded poly(1,6-heptadiyne)s (P1 and P2) and simultaneously realize the self-assembly by Grubbs-III catalyst-mediated metathesis cyclopolymerization (CP) of AB-functionalized bis(1,6-heptadiyne) and 1,6-heptadiyne monomers (M1 and M2). Monomers and polymers were characterized by 1H NMR, mass spectroscopy, and GPC techniques. The double-stranded poly(1,6-heptadiyne)s exhibited a large scale of ordered ladder nanostructure. This result was attributed to the π-π attractions between end groups along the longitudinal axis of the polymers and van der Waals interactions between the neighboring polymeric backbones. While the Azo chromophore connected in the side chain of P2 induced conformation of micelles nanostructure during the CP process without any post-treatment. Furthermore, the photoisomerization of Azo units had an obviously different regulatory effect on the conjugated degree of the polymer backbone, especially for the single-stranded P2, which was attributed to the structural differences and the interaction between AB chromophores in the polymers.

Enlightening Materials with Photoswitches

Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenges in harnessing geometrical and electronic changes on the molecular level to modulate macroscopic and bulk material properties. Herein, progress made during the past decade in the field of photoswitchable materials is highlighted. After pointing to some general design principles, materials with an increasing order of the integrated photoswitchable units are discussed, spanning the range from amorphous settings over surfaces/interfaces and supramolecular ensembles, to liquid crystalline and crystalline phases. Finally, some potential future directions are pointed out in the conclusion. In view of the exciting recent achievements in the field, the future emergence and further development of light-driven and optically programmable (inter)active materials and systems are eagerly anticipated.

Solar-Thermal Energy Conversion and Storage Using Photoresponsive Azobenzene-Containing Polymers

Photoswitchable compounds are promising materials for solar-thermal energy conversion and storage. In particular, photoresponsive azobenzene-containing compounds are proposed as materials for solar-thermal fuels. In this feature article, solar-thermal fuels based on azobenzene-containing polymers (azopolymers) are reviewed. The mechanism of azopolymer-based solar-thermal fuels is introduced, and computer simulations and experimental results on azopolymer-based solar-thermal fuels are highlighted. Different types of azopolymers such as linear azopolymers, 2D azopolymers, and conjugated azopolymers are addressed. The advantages and limitations of these azopolymers for solar-thermal energy conversion and storage, along with the remaining challenges of azopolymer-based solar-thermal fuels, are discussed.

Responsive Smart Windows Enabled by the Azobenzene Copolymer Brush with Photothermal Effect

An azobenzene side chain liquid crystalline copolymer (MAzo-co-GMA) is successfully synthesized through copolymerizing the monomer 6-(4-((4-butylphenyl)diazenyl)phenoxy)hexyl methacrylate (MAzo) with glycidyl methacrylate (GMA). The obtained MAzo-co-GMA copolymer can form stabilized polymer brush on the surface after thermal annealing. The obtained polymer brush not only induces the alignment of liquid crystals but also shows a photothermal effect under UV light irradiation due to the azobenzene side group. On basis of these results, the LC cell with this polymer brush as the substrate is further used to fabricate the polymer-stabilized liquid crystal (PSLC) smart window. The resultant PSLC smart window shows the transparent state because the homeotropic alignment in the SmA* phase of PSLC is induced by the polymer brush on the surface of the LC cell. The opaque state can be achieved in the scattering N* phase by UV light irradiation or heating. The response time of the PSLC smart window can be regulated by adjusting the concentration of MAzo-co-GMA copolymer brush and the intensity of UV light. This kind of PSLC smart window with both thermal and UV response shows good reversibility and stability, which endows enormous promising applications in energy-saving devices.