Controlling Heat Release from a Close-Packed Bisazobenzene-Reduced-Graphene-Oxide Assembly Film for High-Energy Solid-State Photothermal Fuels

Photochromic Carbon Nanomaterials: An Emerging Class of Light-Driven Hybrid Functional Materials

Photochromic molecules have remarkable potential in memory and optical devices, as well as in driving and manipulating molecular motors or actuators and many other systems using light. When photochromic molecules are introduced into carbon nanomaterials (CNMs), the resulting hybrids provide unique advantages and create new functions that can be employed in specific applications and devices. This review highlights the recent developments in diverse photochromic CNMs. Photochromic molecules and CNMs are also introduced. The fundamentals of different photochromic CNMs are discussed, including design principles and the types of interactions between CNMs and photochromic molecules via covalent interactions and non-covalent bonding such as π-π stacking, amphiphilic, electrostatic, and hydrogen bonding. Then the properties of photochromic CNMs, e.g., in photopatterning, fluorescence modulation, actuation, and photoinduced surface-relief gratings, and their applications in energy storage (solar thermal fuels, photothermal batteries, and supercapacitors), nanoelectronics (transistors, molecular junctions, photo-switchable conductance, and photoinduced electron transfer), sensors, and bioimaging are highlighted. Finally, an outlook on the challenges and opportunities in the future of photochromic CNMs is presented. This review discusses a vibrant interdisciplinary research field and is expected to stimulate further developments in nanoscience, advanced nanotechnology, intelligently responsive materials, and devices.

Sensor to Electronics Applications of Graphene Oxide through AZO Grafting

Graphene is a two-dimensional (2D) material with a single atomic crystal structure of carbon that has the potential to create next-generation devices for photonic, optoelectronic, thermoelectric, sensing, wearable electronics, etc., owing to its excellent electron mobility, large surface-to-volume ratio, adjustable optics, and high mechanical strength. In contrast, owing to their light-induced conformations, fast response, photochemical stability, and surface-relief structures, azobenzene (AZO) polymers have been used as temperature sensors and photo-switchable molecules and are recognized as excellent candidates for a new generation of light-controllable molecular electronics. They can withstand trans-cis isomerization by conducting light irradiation or heating but have poor photon lifetime and energy density and are prone to agglomeration even at mild doping levels, reducing their optical sensitivity. Graphene derivatives, including graphene oxide (GO) and reduced graphene oxide (RGO), are an excellent platform that, combined with AZO-based polymers, could generate a new type of hybrid structure with interesting properties of ordered molecules. AZO derivatives may modify the energy density, optical responsiveness, and photon storage capacity, potentially preventing aggregation and strengthening the AZO complexes. They are potential candidates for sensors, photocatalysts, photodetectors, photocurrent switching, and other optical applications. This review aimed to provide an overview of the recent progress in graphene-related 2D materials (Gr2MS) and AZO polymer AZO-GO/RGO hybrid structures and their synthesis and applications. The review concludes with remarks based on the findings of this study.

Multiple Azoarenes Based Systems - Photoswitching, Supramolecular Chemistry and Application Prospects

In the recent decades, the investigations on photoresponsive molecular systems with multiple azoarenes are quite popular in diverse perspectives ranging from fundamental understanding of multiple photoswitches, supramolecular chemistry, and various application prospects. In fact, several insightful and conceptual designs of such systems were investigated with architectural distinctions. In particular, the demonstration of applications such as data storage with the help of multistate or orthogonal photoswitches, light modulation of catalysis via cooperative switching, sensors using supramolecular host-guest interactions, and materials such as liquid crystals, grating, actuators, etc. are some of the milestones in this area. Herein, we cover the recent advancements in the research areas of multiazoarenes containing systems that have been classified into Type-1 {linear, non-linear, and core-based (A)}, Type-2 {tripodal C₃ -symmetric (C3)} and Type-3 {macrocyclic (M)} structural motifs.

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.

Efficient solid-state photoswitching of methoxyazobenzene in a metal-organic framework for thermal energy storage

Efficient photoswitching in the solid-state remains rare, yet is highly desirable for the design of functional solid materials. In particular, for molecular solar thermal energy storage materials high conversion to the metastable isomer is crucial to achieve high energy density. Herein, we report that 4-methoxyazobenzene (MOAB) can be occluded into the pores of a metal-organic framework Zn2(BDC)2(DABCO), where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane. The occluded MOAB guest molecules show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy stored within the metastable Z-MOAB molecules can be retrieved as heat during thermally-driven relaxation to the ground-state E-isomer. The energy density of the composite is 101 J g-1 and the half-life of the Z-isomer is 6 days when stored in the dark at ambient temperature.

Strategies for Incorporating Graphene Oxides and Quantum Dots into Photoresponsive Azobenzenes for Photonics and Thermal Applications

Graphene represents a new generation of materials which exhibit unique physicochemical properties such as high electron mobility, tunable optics, a large surface to volume ratio, and robust mechanical strength. These properties make graphene an ideal candidate for various optoelectronic, photonics, and sensing applications. In recent years, numerous efforts have been focused on azobenzene polymers (AZO-polymers) as photochromic molecular switches and thermal sensors because of their light-induced conformations and surface-relief structures. However, these polymers often exhibit drawbacks such as low photon storage lifetime and energy density. Additionally, AZO-polymers tend to aggregate even at moderate doping levels, which is detrimental to their optical response. These issues can be alleviated by incorporating graphene derivatives (GDs) into AZO-polymers to form orderly arranged molecules. GDs such as graphene oxide (GO), reduced graphene oxide (RGO), and graphene quantum dots (GQDs) can modulate the optical response, energy density, and photon storage capacity of these composites. Moreover, they have the potential to prevent aggregation and increase the mechanical strength of the azobenzene complexes. This review article summarizes and assesses literature on various strategies that may be used to incorporate GDs into azobenzene complexes. The review begins with a detailed analysis of structures and properties of GDs and azobenzene complexes. Then, important aspects of GD-azobenzene composites are discussed, including: (1) synthesis methods for GD-azobenzene composites, (2) structure and physicochemical properties of GD-azobenzene composites, (3) characterization techniques employed to analyze GD-azobenzene composites, and most importantly, (4) applications of these composites in various photonics and thermal devices. Finally, a conclusion and future scope are given to discuss remaining challenges facing GD-azobenzene composites in functional science engineering.

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.

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.

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.

Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems

The performance of molecular solar thermal energy storage systems (MOST) depends amongst others on the amount of energy stored. Azobenzenes have been investigated as high-potential materials for MOST applications. In the present study it could be shown that intermolecular attractive London dispersion interactions stabilize the (E)-isomer in bisazobenzene that is linked by different alkyl bridges. Differential scanning calorimetry (DSC) measurements revealed, that this interaction leads to an increased storage energy per azo-unit of more than 3 kcal/mol compared to the parent azobenzene. The origin of this effect has been supported by computation as well as X-ray analysis. In the solid state structure attractive London dispersion interactions between the C-H of the alkyl bridge and the π-system of the azobenzene could be clearly assigned. This concept will be highly useful in designing more effective MOST systems in the future.

Solar Energy Storage by Molecular Norbornadiene-Quadricyclane Photoswitches: Polymer Film Devices

Devices that can capture and convert sunlight into stored chemical energy are attractive candidates for future energy technologies. A general challenge is to combine efficient solar energy capture with high energy densities and energy storage time into a processable composite for device application. Here, norbornadiene (NBD)-quadricyclane (QC) molecular photoswitches are embedded into polymer matrices, with possible applications in energy storing coatings. The NBD-QC photoswitches that are capable of absorbing sunlight with estimated solar energy storage efficiencies of up to 3.8% combined with attractive energy storage densities of up to 0.48 MJ kg-1. The combination of donor and acceptor units leads to an improved solar spectrum match with an onset of absorption of up to 529 nm and a lifetime (t 1/2) of up to 10 months. The NBD-QC systems with properties matched to a daily energy storage cycle are further investigated in the solid state by embedding the molecules into a series of polymer matrices revealing that polystyrene is the preferred choice of matrix. These polymer devices, which can absorb sunlight and over a daily cycle release the energy as heat, are investigated for their cyclability, showing multicycle reusability with limited degradation that might allow them to be applied as window laminates.

A High Energy Density Azobenzene/Graphene Oxide Hybrid with Weak Nonbonding Interactions for Solar Thermal Storage

Incorporating photochromic chromophores into polymer composites provides the possibility of a reversible photoswitch of the intrinsic properties of these materials. In this paper we report a route to attach azobenzene (AZO) moiety covalently to graphene oxide (GO) to create chromophore/graphene oxide (AZO-GO) hybrid, in which GO is both part of the chromophore and the template. Due to the high grafting density of AZO moiety and the low mass of the novel structure, the hybrid is a potential solar thermal storage material with high energy density of about 240 Wh·kg-1. It is found that C-H···π interaction between the cis-AZO chromophores and the aromatic rings of the substrate induces collective electronic modifications of GO at critical percentage of cis-isomers and reduce the thermal barrier of π-π* transition of the chromophores directly, which results in two sections of first-order reactions during the photoisomerization of trans- to cis-hybrid and also thermally stabilizes the cis-hybrid. Our findings demonstrate that high-performance AZO-GO hybrid can be manipulated by optimizing intermolecular nonbonding interactions.

Form-Stable Solar Thermal Heat Packs Prepared by Impregnating Phase-Changing Materials within Carbon-Coated Copper Foams

The heat packs that are based on solid-liquid transition of phase-changing materials (PCMs) have been pursued as a promising way to provide heating for human body comfort and thermotherapy owning to their large heat storage capacity and near-constant heat-release temperature. Current heat packs, however, suffer from leakage, slow charging, and poor heat-release performance due to the flow of liquid PCMs and their low thermal conductivity. Here, we report a strategy for preparing high-performance PCM-based solar thermal heat packs through impregnating organic PCMs within carbon-coated copper foams (CCFs). The porous structure and hydrophobic surface of CCF help to effectively confine the melted liquid PCM within the composite heat pack without leakage. The carbon coating layer efficiently converts the incident solar light into heat, which is rapidly transferred along the three-dimensional thermal conductive network of CCF and stored within the PCM. In the discharging process, the CCF network facilitates the extraction of the heat stored within the PCM. In contrast to neat PCM pack within which only a small portion of PCM that is in contact with human skin contributes to thermal comfort, all PCMs within the CCF-based composite heat pack concertedly release the stored heat. Such release significantly increases the extractable thermal energy and prolongs the usable healing duration for thermotherapy.

Electrorheology of SI-ATRP-modified graphene oxide particles with poly(butyl methacrylate): effect of reduction and compatibility with silicone oil

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to modify graphene oxide (GO) particles with poly(butyl methacrylate) (PBMA) chains. This procedure facilitated the single-step fabrication of a hybrid material with tailored conductivity for the preparation of a suspension in silicone oil with enhanced sedimentation stability and improved electrorheological (ER) activity. PBMA was characterized using various techniques, such as gel permeation chromatography (GPC) and ¹H NMR spectroscopy. Thermogravimetric analysis through on-line investigation of the Fourier transform infrared spectra, together with transmission electron microscopy, X-ray photoelectron microscopy, and atomic force microscopy, were successfully used to confirm GO surface modification. The ER performance was investigated using optical microscopy images and steady shear rheometry, and the mechanism of the internal chain-like structure formation was elucidated. The dielectric properties confirmed enhanced ER performance owing to an increase in relaxation strength to 1.36 and decrease in relaxation time to 5 × 10⁻³ s. The compatibility between GO and silicone oil was significantly influenced by covalently bonded PBMA polymer brushes on the GO surface, showing enhanced compatibility with silicone oil, which resulted in the considerably improved sedimentation stability. Furthermore, a controlled degree of reduction of the GO surface ensured that the suspension had improved ER properties.

Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to modify graphene oxide (GO) particles with poly(butyl methacrylate) (PBMA) chains.

Azobenzene-based solar thermal fuels: design, properties, and applications

Development of renewable energy technologies has been a significant area of research amongst scientists with the aim of attaining a sustainable world society. Solar thermal fuels that can capture, convert, store, and release solar energy in the form of heat through reversible photoisomerization of molecular photoswitches such as azobenzene derivatives are currently in the limelight of research. Herein, we provide a state-of-the-art account on the recent advancements in solar thermal fuels based on azobenzene photoswitches. We begin with an overview on the importance of azobenzene-based solar thermal fuels and their fundamentals. Then, we highlight the recent advances in diverse azobenzene materials for solar thermal fuels such as pure azobenzene derivatives, nanocarbon-templated azobenzene, and polymer-templated azobenzene. The basic design concepts of these advanced solar energy storage materials are discussed, and their promising applications are highlighted. We then introduce the recent endeavors in the molecular design of azobenzene derivatives toward efficient solar thermal fuels, and conclude with new perspectives on the future scope, opportunities and challenges. It is expected that continuous pioneering research involving scientists and engineers from diverse technological backgrounds could trigger the rapid advancement of this important interdisciplinary field, which embraces chemistry, physics, engineering, nanoscience, nanotechnology, materials science, polymer science, etc.

Controllable and Stable Deformation of a Self-Healing Photo-Responsive Supramolecular Assembly for an Optically Actuated Manipulator Arm

It is highly challenging to achieve an optically deformable polymer with good controllability, stability, and self-healability for fabricating an optically controlled microrobotics. Here, we present a photo-responsive self-healing supramolecular assembly cross-linked by 3,3',5,5'-azobenzenetetracarboxylic acid (t-Azo) enabling the controllable and stable deformation. The network (PAA-u) of polyacrylic acid (PAA) grafted with 2-ureido-4[1 H]-pyrimidinone (UPy) is formed via multiple intermolecular hydrogen bonds (H-bonds) between UPy and t-Azo moieties. Molecular H-bonds stabilize the cis-isomer, enables stress transfer at the interface, and also contributes to fast healability. The PAA-u/t-Azo assembly shows a green-light-induced bending deformation, which recovers its shape under the irradiation of UV light. On the basis of this controllable and reversible deformation, the PAA-u/t-Azo "hand" realizes reversible light-driven grabbing and releasing of an object by optimizing bending and recovery. The assembly also shows a fast and excellent self-healing performance irradiated by green light during deformation. The multiple-H-bonding-cross-linked assembly with stable deformation and fast self-healability can be used for the development of a multitude of advanced microrobotics.

Heteroaryl-linked norbornadiene dimers with redshifted absorptions

The longest-wavelength absorption maximum of norbornadiene dimers with potential for molecular solar thermal systems can be finely tuned by varying the electronic nature of a heteroaryl spacer.