Molecular solar thermal energy storage devices: toward a more sustainable future

Abstract

The escalating demand for renewable energy is driving the rapid advancement of innovative energy storage and conversion technologies. Molecular solar thermal (MOST) systems, as a promising alternative energy solution, typically store photon energy as chemical energy in molecules via processes such as photoisomerization or cycloaddition reactions. This stored energy can then be released in the form of heat in a controlled manner upon external stimulation. Despite demonstrating tremendous potential under laboratory conditions, this technology still faces significant challenges in translation to functional devices. Recently, however, this dynamic field has begun to shift gradually from fundamental research toward functional applications, with notable progress being achieved. In this review, we systematically summarize the latest advances in functional devices based on MOST systems. We emphasize the key performance parameters and classification of MOST systems, and discuss the advantages and challenges of various MOST devices – with a particular focus on their significant potential for functionalized applications. Furthermore, we analyze emerging strategies and future opportunities for the development of MOST devices, aiming to facilitate their innovative application and propel further progress in MOST systems research.

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Xingtang Xu

Dr Xingtang Xu received his PhD degree from the University of Science and Technology Beijing in 2022. He spent the following two years as a postdoctoral researcher at the Taiyuan University of Technology (TYUT), supervised by Prof. Wen-Ying Li. He is currently an associate Professor at the School of Chemistry and Chemical Engineering, focusing on the molecular design, performance regulation and device development of MOST systems.

Chonghua Li

Mr Chonghua Li received the BS degree from the Xi'an University of Science and Technology in 2022. He is now a current PhD candidate at TYUT under the supervision of Prof. Wen-Ying Li. His current research focuses on the design and development of coal-based molecular solar thermal fuels.

Wang Li

Dr Wang Li received his PhD degree from the Taiyuan University of Technology (TYUT) in 2022. He is currently a research assistant at the Shanxi Research Institute of Huairou Laboratory, focusing on the basic research and technology development of direct coal conversion.

Jie Feng

Prof. Dr Jie Feng received his Bachelor's degree (1989) and PhD (1998) in Chemical Engineering and Technology from the Taiyuan University of Technology (TYUT). He has undertaken postdoctoral research at Monash University, Australia and served as a senior visiting scholar at the University of Florida, USA. Since 2004, he has been working as a Professor at TYUT. His research interests focus on three core areas: the design and optimization of coal-to-liquid upgrading processes, the scale-up of coal/biomass gasification reactors, and the development of emerging carbon-based energy storage materials.

Wen-Ying Li

Prof. Dr Wen-Ying Li received her PhD from the Dalian University of Technology in 1995. She was a postdoctoral fellow at Imperial College, London, UK, a Senior Visiting Scholar at the Max-Planck Institute for Coal Research, Germany, and a Senior Visiting Researcher at Princeton University and at Southern Illinois University, Carbondale, USA. Since then, she has been working as a Professor of Chemical Engineering in TYUT for about 30 years, and was awarded the Distinguished Professor of “Cheung Kong Scholars Program”. She is currently the director of the State Key Laboratory of Clean and Efficient Coal Utilization (TYUT). Her current research focuses entirely on enabling discovery and design of processes and catalysts for sustainable energy & fuels.

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Broader context

Global energy demand increases with societal development and population growth. As the most abundant renewable energy, solar energy can meet this demand while reducing ecological harm. Despite progress in solar capture, storage, and conversion (e.g., photovoltaics, solar collectors), efficient sunlight-to-usable energy conversion remains challenging. Molecular solar thermal (MOST) systems, a novel storage technology, store photon energy via reversible molecular conformational changes and release it as heat, and have gained research attention. Though promising, MOST systems face hurdles in functional device design and large-scale application. This review clarifies the performance metrics and classifications of MOST systems, and discusses the distinct advantages and technical bottlenecks of various MOST devices in functional applications. The proposed emerging strategies to overcome these technical barriers offer forward-looking guidance for optimizing MOST device performance and driving technological innovation, ultimately accelerating the leapfrog development of this emerging field.

1. Introduction

The development and commercialization of renewable energy sources have received widespread attention as means of addressing the energy crises and environmental problems caused by the large-scale use of fossil fuels, which dominate the global energy market.^1–5 The amount of solar energy reaching Earth each year is estimated to be about 5.5 × 10⁶ EJ, and has been used by more than 8 billion humans worldwide for about 9000 years.⁶ This highlights the importance of maximizing progress in the development and utilization of solar energy and minimizing the related adverse environmental impacts.⁷ Recent years have witnessed substantial advances in solar energy storage and conversion technologies, with notable examples including photovoltaic cells,^8,9 hydrogen energy storage,^10,11 solar collectors,^12,13 and artificial photosynthesis.^14,15 These technologies have substantially mitigated environmental pollution and contributed to sustainable energy development.^16–19 To further reduce reliance on fossil fuels, a novel solar energy storage technology, called molecular solar thermal (MOST) systems, has attracted considerable attention due to unique advantages including high cycling stability and environmental friendliness.^20–23 MOST systems can convert photoenergy into chemical energy when irradiated by sunlight at specific wavelengths and controllably release heat through reverse conversion in response to external triggers (Fig. 1). Various photo-switches, including anthracene,^24–26 norbornadiene/quadricyclane (NBD/QC),^27–29 azobenzene (E/Z-AZO),^30–32 dihydroazulene/vinylheptafulvene (DHA/VHF),^33–35 stilbene,^36,37 styrylpyrylium,³⁸ fulvalene dimetal complexes (FvRu₂(CO)₄),³⁹ and hydrazone,⁴⁰ have been studied as MOST systems. The raw materials for fabricating these photo-switches are abundant in coal-based liquid upgrading products.^41–43 Currently, anthracene, DHA/VHF, NBD/QC, and E/Z-AZO are considered to be the most promising for functionalization applications due to their simple synthesis, ease of functionalization, and tunable photochemical properties.^44–46

Fig. 1 Schematic of functionalized applications of MOST systems.

Since the discovery of anthracene dimerization as an effective method of solar energy storage by Luther and Weigert in the early 20th century, MOST systems have experienced rapid progress.⁴⁷ Yoshida et al. generalized the molecular characterization checklist in 1985–2002 and laid the foundation for the study of MOST systems.^48–50 In 2021, Moth-Poulsen et al. summarized the operating principle of molecular photo-isomers and discussed strategies for improving their storage properties.⁵¹ Ideally, MOST systems should meet the following criteria: high cycling stability, good solar spectrum matching, excellent energy storage capacity, and long-term storage stability.^52–55 To date, several studies have demonstrated that NBD/QC systems can withstand over 1000 storage and release cycles.^51,56 Heterocyclic azobenzene systems have been shown to achieve energy storage under sunlight with a half-life of up to 1000 days and an energy density of 0.43 MJ kg⁻¹.⁵⁷ Although few photo-switches meet all the abovementioned criteria, extensive efforts, such as designing novel molecular photo-switches and proposing template self-assembly strategies, have been made to enhance the energy density (ΔH), extend the storage half-life (τ_1/2), and broaden the solar spectrum matching of MOST systems.^58–60 The overall performance of MOST systems has substantially improved, and certain molecular photo-switches demonstrate excellent performance under sunlight.^23,61 However, the application of MOST systems is advancing slowly, which seriously hinders the further promotion of MOST technology. To address this problem, researchers have developed a series of MOST functional devices such as hybrid solar devices that generate power to improve their applicability. Wang et al. described the concept of MOST systems from surface science to functional devices, and discussed the current research status of MOST devices.⁶² Numerous studies have subsequently focused on novel MOST functional devices such as flexible wearable fabrics, smart films, and hybrid devices and explored their applications.^63–65 However, few reports have systematically summarized the recent development and application of MOST functional devices.

Herein, we systematically review the advances in MOST functional devices based on different molecular photo-switches, elaborating on several promising photo-switches and clarifying the fundamentals of MOST system operation. Subsequently, we present recent progress in MOST functional devices, emphasize their application potential, mention the challenges faced by MOST devices, and suggest emerging strategies and further development directions. We hope that this review will motivate researchers in the field of MOST systems to broaden the application areas of the related functional devices and achieve the efficient conversion of solar energy.

2. Types and parameters of MOST systems

2.1. Types of MOST systems

According to the current understanding of molecular photo-switching, MOST systems should have exhibit superior cycling stability, high ΔH, suitable τ_1/2 and solar spectrum matching.^66–68 Among the various photo-switching molecules proposed as MOST systems, anthracene, DHA/VHF, NBD/QC, and E/Z-Azo have the greatest promise.

Anthracene is a rigid aromatic hydrocarbon molecule that undergoes a [4+4] cycloaddition reaction under UV light irradiation above 300 nm to form a dimer, storing approximately 0.18 MJ kg⁻¹ of energy.⁶⁹ Subsequently, the dimer reverts to a monomer through heat treatment or UV light irradiation below 300 nm.⁷⁰ As one of the earliest MOST systems used for solar energy storage, the energy storage potential of anthracene has been explored as early as the late 20th century.⁷¹ Sarti-Fantoni et al. in 1972 and Bergmark et al. in 1978 studied the effect of functional group introduction on the energy storage capacity of anthracene.^72,73 They found that both electron withdrawing and electron donating groups increase the energy storage density, with the ΔH of cyano-substituted anthracene reaching 0.21 MJ kg⁻¹. However, anthracene derivatives usually only store and release energy in the UV light region, which has impeded research on anthracene for a long time.⁷⁴ In 2011, Paul et al. proved through theoretical calculations that anthracene cyclophanes have excellent solar energy storage properties, with a theoretical ΔH reaching 0.43 MJ kg⁻¹.⁷⁵ Recently, the potential of anthracene from coal tar in solar energy storage has gradually regained attention with the emergence of various molecular designs and functionalization strategies around the MOST systems.^76,77 Emerging strategies for anthracene-based MOST systems will be mainly discussed in Section 4.

The photo-reversible isomerization of DHA/VHF involves the photoactivated ring-opening reaction of DHA upon irradiation at specific wavelengths followed by the conversion of the thus generated s-Z-VHF into s-E-VHF. The reverse isomerization of VHF can be triggered by heating or catalysts, resulting in heat release.⁷⁸ DHA has multiple carbon sites suitable for molecular modification and therefore exhibits the benefits of easy functionalization, excellent structural tunability, high ΔH, and unique photochromic performance.^79–81 Currently, DHA derivatives are mainly designed by (i) removing the cyano group and introducing ketone, ester, or amides, which can double the ΔH (0.25 MJ kg⁻¹),⁸² and (ii) introducing push–pull electronic groups such as benzene rings and trifluoroacetyl chloride to increase ΔH and cycling stability.⁸³ Molecularly modified DHA/VHF derivatives can exhibit ΔH of up to 0.73 MJ kg⁻¹ and τ_1/2 ≈ 26 h.^84–86 Furthermore, DHA and VHF feature absorption spectra differing by about 150 nm, and the quantum yield (φ) from the parent isomer to the metastable isomer is approximately 0.55.⁸⁷ These advantages offer the possibility to develop advanced functional MOST systems.^88–90

NBD/QC is a photochromic bicyclic molecule containing two C=C bonds. Under UV light irradiation, NBD undergoes a [2+2] cycloaddition, affording QC, which can release the stored energy in response to external triggers such as catalysts and heating.^91–93 The cycloaddition reactions of most NBD derivatives can usually only be induced by irradiation with UV light, which leads to a low solar spectrum match and solar energy conversion efficiency.^94–97 The NBD absorption wavelength can be increased by introducing conjugated π-systems and push–pull electron systems to improve the solar spectrum match.^98–100 For example, Mikael et al. modelled the UV-vis spectra and thermal energy storage of NBD derivatives grafted with electron donating and electron withdrawing groups using single-and multi-reference methods and found that this grafting leads to a strong redshift.¹⁰¹ Moth-Poulsen et al. increased the absorption wavelength (456 nm) and ΔH (0.40 MJ kg⁻¹) of NBD derivatives by attaching low-molecular-weight electron donating and electron withdrawing groups onto the C=C bond.¹⁰² Currently, NBD/QC derivatives have a maximum absorption onset wavelength of 529 nm, ΔH of up to 0.97 MJ kg⁻¹, and τ_1/2 values ranging from months to years, indicating high application potential.^103–105

E/Z-Azo compounds are some of the most widely studied MOST systems, with E isomers undergoing isomerization to Z isomers, which release the stored energy via the reverse conversion induced by external triggers.^106–108 Pristine E/Z-AZO exhibits ΔH ≈ 0.27 MJ kg⁻¹ and τ_1/2 ≈ 4.2 days.¹⁰⁹ Although AZO compounds feature good photochemical indices and the advantages of simple synthesis and easy functionalization, all the solar energy conversion efficiencies are found below 1.0%, far from the proposed limits for MOST systems.¹¹⁰ Numerous attempts have aimed to enhance the MOST performances of E/Z-AZO. On the one hand, several high-performance azoheteroarenes that can be quantitatively isomerized in the visible light range have been engineered.^111–113 On the other hand, the covalent grafting of E/Z-AZO with carbon nanomaterials or polymer materials has been shown to improve performances of E/Z-AZO by enhancing the spatial site resistance and intermolecular forces.^114–116 These diverse molecular design strategies provide a material basis for the further development of Azo-based functional devices.

2.2. Key parameters of MOST systems

The key parameters involved in this reversible process include ΔH, τ_1/2, φ, cycling stability, and solar spectrum match. Describing these key performance parameters provides guidance and assistance in understanding and designing high-performance MOST devices.

ΔH: ΔH represents the energy gap between the parent isomer and the metastable high-energy isomer. As an energy storage system, the metastable high-energy isomer should have higher energy than its parent isomer. Previous studies have suggested that the ΔH of functional MOST systems should be more than 0.30 MJ kg⁻¹,^49,50 surpassing that of traditional energy storage materials such as molten salt (about 0.25 MJ kg⁻¹).^117–121

τ _1/2: τ_1/2 is the time required for half of the metastable high-energy isomers to voluntarily revert to the parent isomer under dark conditions at room temperature. The length of τ_1/2 depends on the thermal potential barrier (E_a) of the reverse conversion. The τ_1/2 of the ideal MOST system should be long enough to enable energy storage for days (E_a ≥ 110 kJ mol⁻¹), months (E_a ≥ 120 kJ mol⁻¹), and years (E_a ≥ 130 kJ mol⁻¹), and to enable controlled energy release upon external stimulation.^51,122

φ: φ is defined as the number of converted molecules divided by the number of photons absorbed (per unit).⁴⁵ Ideally, all photons induce an isomerization reaction and φ approaches unity.^51,123,124 Several studies have revealed that the Z → E quantum yield of azobispyrazole photo-switches can reach 0.76.^125,126

2.2.1. Cycling stability. Cycling stability is the number of times the MOST system can undergo a reversible transition between the parent isomer and metastable isomer. An ideal MOST system should sustain multiple reversible cycles without performance degradation. Numerous MOST systems exhibit high cycling stability, and some can be recycled thousands of times.^51,56 The excellent cycling stability has great promise for future solar energy conversion and storage applications.

2.2.2. Solar spectrum matching. Solar spectrum matching is critical for the practical applications of MOST systems or devices under sunlight. An ideal MOST system should absorb mostly in the UV-visible region (300–800 nm), which accounts for more than 50% of the total solar energy.⁵¹ However, many MOST systems can only store the UV or partially visible-light fraction of solar energy, which seriously hinders practical applications.^127–129 Fortunately, molecular photo-switches with excellent redshift capabilities have been developed recently, as exemplified by NBD derivatives with strong electron-donating groups and azo-aromatics.^130–132 Besides showing a high solar spectrum match, these MOST systems exhibit excellent energy storage performances, which provides an opportunity for the development of practically applicable MOST devices.

The development of molecular photo-switching and tuning of key parameters have attracted considerable attention, and key criteria for the construction of ideal MOST systems have been proposed.^51,52 However, MOST systems that fully meet all of the above design criteria are very rare. Considering the strong correlation between these key parameters, optimizing them simultaneously is challenging. Recently, researchers have proposed various effective strategies, such as optimizing a certain key indicator of MOST systems or constructing hybrid MOST systems, to develop MOST devices suitable for different application scenarios.¹³³

3. Advances in MOST functional devices

The concept of MOST was first proposed in 1909, when Weigert suggested using the photodimerization of the anthracene to store photoenergy.⁷¹ In 1979, Xuan et al. systematically overviewed the MOST working principle and identified standards for molecular photo switches.¹³⁴ Over the past century, researchers have focused on designing and optimizing laboratory-level MOST systems for energy storage such as DHA/VHF, NBD/QC, and E/Z-AZO, with the development and application of MOST functional devices remaining underexplored.¹³⁵ The most outstanding advantages of these three types of MOST systems are room-temperature energy transfer and potential for overall excellence in various performance metrics. Therefore, one should aim to utilize these advantages to extend the application scope of MOST systems from laboratory preparation to large-scale production. To date, MOST systems have been functionalized for diverse applications, such as fabric heating,¹³⁶ photovoltaic conversion,¹³⁷ de-icing and defrosting,^138–140 and information encryption. In this section, we elaborate on the applications of DHA/VHF, NBD/QC, E/Z-AZO, and hybrid MOST systems in functional devices, focusing on methods of addressing molecular properties and relating them to device performance.

3.1. DHA/VHF devices

In 2019, Mikkelsen et al. developed a simulation framework for determining the parameters of MOST systems. The model not only considered the microscopic parameters of the MOST system, including ΔH, φ, and τ_1/2, but also accounted for macroscopic parameters related to equipment installation engineering, such as equipment size, solar concentration coefficient, and fluid flow rate.¹⁴¹ This simulation framework could predict the thermal energy storage rate, conversion percentage, and temperature increase of the specific MOST system (Fig. 2A). The MOST system unit in the model could capture energy by using lenses to concentrate sunlight. Molecule A (parent isomer) was converted to molecule B (metastable isomer) by photoisomerization reaction. Subsequently, the thermal energy was released for heating cold water under an external trigger (Fig. 2B). By testing the macrosystem for the starting concentration range conversion percentage, fluid flow rate, and light intensity, it was concluded that an increase of starting concentration resulted in a decrease of conversion and requires a longer time to obtain a higher total conversion (≤50%) (Fig. 2C). The thermal insulation value was strongly positively correlated with the temperature in the channel (up to 95 °C). When the temperature exceeded 80 °C, the reverse conversion rate exceeded the forward conversion rate. In addition, the residence time hardly affected the change in fluid temperature (Fig. 2D). This study demonstrated that simulation frameworks can be used for theoretical evaluation prior to manufacturing equipment for MOST systems or for predicting MOST systems with promising applications.

Fig. 2 (A) Reaction process and schematic of the energy conversion of the DHA/VHF system. (B) Flowsheet of the hybrid device (MOST system with water). (C) Conversion percentage of different concentrations of DHA as a function of residence time. (D) Change curves of DHA concentration and reaction temperature for different channel lengths. Reproduced from ref. 141 Copyright 2019 Elsevier Inc.

Based on the above simulation framework, Moth-Poulsen et al. designed a DHA/VHF device that could be recycled more than 70 times in toluene, with a degradation rate of < 0.01% per cycle (Fig. 3A). The conversion percentage was determined by recording the UV-visible absorption spectra during the simulated sunlight (AM 1.5) irradiation of a microfluidic chip (optical path length = 85 μm) pumped with 1 mM and 4 mM toluene solutions of DHA.¹⁴² By optimizing the fluid residence time (3–43 s), the conversion of both different concentrations of DHA/VHF was increased from about 40% to 100%. Interestingly, the highest solar energy conversion efficiencies of 0.13% (4 mM) and 0.034% (1 mM) were achieved at a residence time of 3 s (Fig. 3B). Moreover, the spectrum of DHA in toluene displayed clear isosbestic points with increasing irradiation time, which indicated a clean conversion to VHF (Fig. 3C). A solar concentrator was constructed and used to determine the energy storage efficiency of the MOST system under sunlight (Fig. 3D). Although the conversion could reach 100% at a sufficiently high residence time, the practical solar energy conversion efficiency (0.02%) was markedly lower than the theoretical value (0.45%). The computed and measured efficiencies of the microfluidic device were in good mutual agreement, and the deviations observed for the amplification device (outdoor parabolic concentrator system) were attributed to the change in the device volume, increase in channel length, and competing absorptions between DHA and VHF amplifying the internal filtering effects (Fig. 3E). This study is the first account of a MOST system used in an outdoor environment, showing that DHA can achieve photochemical conversion processes under both simulated and natural light, and presenting a potential pathway for transitioning from laboratory to industrial applications.

Fig. 3 (A) DHA/VHF reversible reaction and a small-scale flow solar storage device. (B) Energy storage efficiency and conversion percentage of DHA with different residence times under simulated solar irradiation conditions. Red colour represents a concentration of 4 mM and blue colour represents a concentration of 1 mM. (C) UV-vis absorption spectra of the DHA/VHF system. (D) Outdoor parabolic concentrator device. (E) Conversion percentage and energy storage efficiency of DHA with different residence times under solar irradiation conditions. Reproduced from ref. 142 Copyright 2017 Wiley-VCH.

Research on DHA/VHF-based MOST devices reveals that they can not only effectively utilize solar energy but also inspire the development of small flowable MOST devices.^143–145 However, solar energy conversion efficiency of the DHA/VHF-based MOST device in scaled-up experiments (0.02%) is markedly lower than the theoretical value (0.45%). Hence, future functionalized device research and molecular optimization should consider shortening the device path length to eliminate the internal filtration effect, improving the performance of the DHA/VHF couple through structural modifications, and developing solvent-free systems to enable rapid thermal energy release.^146,147

3.2. NBD/QC devices

Generally, a closed MOST facility should include solar collectors for solar thermal conversion, storage equipment, thermal application equipment, and circulation piping (Fig. 4AI). Dreos et al. obtained NBD derivatives that could be liquefied at room temperature by introducing trifluoroacetyl and phenyl groups.¹⁴⁸ These derivatives had shear viscosities (0.03–0.07 Pa s) identical to that of colza oil, ΔH values of up to 0.58 MJ kg⁻¹, solar spectrum match scores of >10%, and good cycling stabilities (Fig. 4AII). Thus, these NBD derivatives with advantageous flow properties could be directly used for the rapid transportation of energy, at a notably reduced cost (Fig. 4AIII). Later, Franz et al. functionalized NBD with dimethoxyphenyl and cyano groups, realizing a MOST device with φ ≈ 0.68, ΔH ≈ 0.37 MJ kg⁻¹ and τ_1/2 ≈ 30 days. The employed Au (111) catalyst facilitated energy release at the solid–liquid interface, and an external field enabled control over the heterogeneous catalytic release process (Fig. 4BI). The device sustained more than 1 000 reversible isomerization processes under photochemical conversion and catalytic triggering.¹⁴⁹ The degradation was as low as 0.1% per storage cycle, implying the absence of substantial NBD decomposition (Fig. 4BII). This work demonstrated the possibility of using liquid NBD derivatives in a flow reactor and achieving controlled energy release in MOST systems through heterogeneous catalytic reaction. The rapid transport of thermal energy, simple separation of catalysts and controlled energy release in this system inspire the further research and development of MOST devices.

Fig. 4 (A) (I) A MOST system based on NBD and QC. (II) Shear rate as a function of shear viscosity for NBD derivatives. (III) Cycling stability and DSC curves of NBD derivatives. Reproduced from ref. 148 Copyright 2018 Wiley-VCH. (B) (I) Schematic of the experimental setup of the MOST/catalyst system. (II) Reversible cycle stability test of the NBD/QC system for 1000 cycles under catalyst triggering. Reproduced from ref. 149. Copyright 2022 American Chemical Society.

To further realize the application of the NBD/QC system in a closed-cycle device, Wang et al. grafted cyano and methoxyphenyl groups onto NBD and thus achieved a strong electron donating and electron withdrawing system, which resulted in an absorption wavelength increase with τ_1/2 = 30 days and ΔH = 0.40 MJ kg⁻¹ (Fig. 5A). By testing several candidate catalysts, the authors revealed that the reverse conversion of NBD/QC could be effectively triggered by cobalt phthalocyanine, and constructed the corresponding thermal release device (Fig. 5B). The temperature difference resulting from the reverse conversion was measured by immersing the catalyst into a solid activated carbon carrier and placing it in the center of a high-vacuum chamber for thermal insulation, with thermocouples placed on both sides of the catalyst bed.¹⁵⁰ To verify the cycling stability of this system, 43 complete photoconversion reactions were conducted using a NBD derivative solution at 85 °C with the degradation rate determined to be 0.14% per cycle (Fig. 5C). A 1.5 M solution of a QC derivative in toluene achieved rapid heat release upon catalytic activation, and the temperature difference was maximized at 63.4 °C after a reaction time of 2.5 min (Fig. 5D). During the heat release experiment, the simulated temperatures closely matched the measured values, and the energy storage performance of the neat compound was very similar to that of its solution (Fig. 5E). This MOST device based on a fixed-bed catalytic reactor not only demonstrated a high macroscopic heat release capability, but also validated the feasibility of the translation from the laboratory to an outdoor scale-up unit.

Fig. 5 (A) Photoisomerization and reaction mechanism diagram of NBD/QC. (B) Heat release test device for the NBD/QC system in a vacuum environment under catalytic triggering. (C) Cycling stability test of the NBD/QC system in toluene solution. (D) Heat release curve of QC in toluene solution. (E) Relationship between QC concentration and heat release temperature difference, theoretical values (lines) and tested values (dots). Reproduced from ref. 150. Copyright 2019 Royal Society of Chemistry.

Liquid NBD/QC systems not only enable heat release in closed-cycle devices but can also be combined with polymer matrices to form polymer film devices for glass deicing and defrosting as well as indoor insulating coatings.¹⁵¹ Inspired by the excellent properties of film devices, Refaa et al. constructed a simulation framework to assess the interaction between MOST films and solar energy and validated their model by analyzing the optical response of an NBD derivative film.¹⁵² The MOST film devices exhibited excellent UV shielding properties and optimized ΔH reaching 0.37 kWh m⁻² (Fig. 6A). The reverse conversion experiments of the NBD film were conducted at room temperature under dark conditions, revealing that absorbance increased with time. The theoretical and experimental values were rationalized by the polystyrene matrix presenting a glassy state at the experimental temperature. This state limited the mobility and flexibility of polystyrene, thus affecting the reversible reaction of the embedded NBD/QC system (Fig. 6B). Films containing 0.5 wt% MOST had lower temperatures, whereas other MOST-doped films absorbed incident light in the UV region, featuring a temperature of 26.8 °C (Fig. 6C). As MOST films could absorb solar radiation in the 460–2500 nm range and undergo photoconversion reactions, they reached higher temperatures under sunlight irradiation than polystyrene films (Fig. 6D). In addition, the optical and energy performances of MOST film devices were investigated in day–night cycles. The temperature difference of the MOST film devices could reach 9.1 °C and φ was close to 0.51. These findings pave the way for the fabrication of MOST film devices capable of controlled heat release.

Fig. 6 (A) Schematic application of the NBD/QC system and film devices for solar thermal energy storage. (B) Absorbance of MOST films under dark conditions as a function of time, simulated (lines) and experimental values (squares). (C) Effect of MOST system doping on MOST film temperature. (D) Temperature as a function of time for different materials under light irradiation. Reproduced from ref. 152 Copyright 2019 Elsevier Inc.

Although NBD derivatives enable effective energy conversion, the conversion of QC to NBD is hindered by bidirectional spectral overlap. To overcome this challenge, Fei et al. used NBD derivatives grafted with electron donating and electron withdrawing groups to overcome this challenge and realize a bidirectional reversible process, examining closed-cycle and polymer film devices based on the NBD/QC system (Fig. 7A). The isomerization degrees of forward and reverse conversions reached 99% and 82%, respectively, and ΔH was as high as 0.31 MJ kg⁻¹ (Fig. 7B). A bidirectional photo-switching microfluidic reactor device was constructed, in which all NBD solutions captured solar energy and were almost completely converted to QC, with the reverse process induced by irradiation at 265 nm.¹⁵³ The fabricated MOST devices also exhibited an excellent solar energy persistence (Fig. 7C). More importantly, the bi-directional photo-switching NBD could be mixed with polystyrene (PS) solution to prepare solid-state MOST films with thicknesses of about 40 μm. The films were hardly degraded after 20 cycles of alternating UV light irradiation at different wavelengths (Fig. 7D). This work not only realized a high solar spectrum coverage (5.4%) but also integrated NBD derivatives into two devices, highlighting the possibilities of the real-life applications of MOST systems.

Fig. 7 (A) NBD/QC system with two-way photo-switching properties. (B) DSC curves of energy release of different QC/NBD derivatives. (C) Schematic of the flow device for the NBD/QC system, and absorbance versus time curves for different NBD molecules at specific wavelengths. (D) Bidirectional conversion MOST film preparation process, UV-vis absorption spectra and cycling stability testing. Reproduced from ref. 153 Copyright 2024 Royal Society of Chemistry.

The above studies demonstrated that NBD/QC functional devices not only possess a higher ΔH (0.58 MJ kg⁻¹) and a controllable τ_1/2, but also exhibit a markedly higher solar energy conversion efficiency (0.5%) in practical applications compared to DHA/VHF functional devices (0.13%). Furthermore, τ_1/2 and φ could be enhanced by introducing aryl donors and a central phenyl bridge system with spatial effects.^154,155 In the future, the molecular design of NBD derivatives should concentrate on enhancing the redshift capability to improve solar spectrum matching and to enable two-way photo conversion.^156–158

3.3. E/Z-AZO devices

Despite the progress in modulating the storage performance of E/Z-AZO devices, the solid-state nature of E/Z-AZO derivatives at room temperature necessitates their dissolution in toluene to facilitate thermal energy transfer, which inevitably reduces the ΔH. Therefore, innovative strategies for improving the storage performance of E/Z-AZO devices are urgently required. In 2023, Feng et al. synthesized asymmetrical alkyl-grafted E/Z-AZO molecules capable of light-triggered phase transitions, enabling reversible transformations between solid and liquid states through photoreaction processes (Fig. 8AI). To investigate the heat release performance of E/Z-AZO devices, the authors designed and constructed an energy utilization device.¹⁵⁹ The sample flow was controlled using specific light wavelengths and heat release. The samples were sealed in a quartz tube, with quartz containers on both sides acting as light collectors. The sample flow was controlled by alternately irradiating the graphene aerogel within the containers, with a cooling plate used to lower the sample temperature. The device achieved a multi-stage controllable release of energy, with temperature increases of 4.3 °C and 6.5 °C obtained at ambient temperatures of 0 °C and −8 °C, respectively (Fig. 8AII). The authors subsequently prepared three E/Z-AZO derivatives, demonstrating efficient photoinduced heat release between −60.5 °C and 34.8 °C, with ΔH reaching 0.34 MJ kg⁻¹ (Fig. 8BI and II). A distributed annular energy utilization device was fabricated to achieve controlled energy utilization and delivery at low temperatures.¹⁶⁰ The device achieved a two-step controllable heat output, featuring a temperature increase of 6.3 °C at −5 °C (Fig. 8BIII). By optimizing the molecular structure and isomerization process, the authors not only increased the ΔH through the simultaneous capture of isomerization and phase transition energy, but also provided essential guidance for the development of energy-utilizing devices for optically switched microfluidic systems.

Fig. 8 (A) (I) Structural formulae and reversible isomerization reactions of E/Z-AZO derivatives. (II) Infrared thermography of E/Z-AZO derivatives. Reproduced from ref. 159 Copyright 2023 Wiley-VCH. (B) (I) Reversible conversion processes of E/Z-Azo derivatives. (II) DSC curves of Z-Azo derivatives. (III) Schematic illustration of the ring device and infrared thermography of Z-s-Azo-B. Reproduced from ref. 160 Copyright 2023 Science China Press.

Although liquid NBD/QC systems have been used in some polymer film devices, they have low solar spectrum match scores, which hinder the energy storage and release in visible light range. E/Z-AZO, which is easy to synthesize and functionalize, has promise for applications in the visible light range. Recently, our group presented an E/Z-AZO-containing dendrimer (G3-FClAzo) that not only efficiently stored visible-light energy but also rapidly released it as heat below 0 °C (Fig. 9A). After irradiation with green light, the ΔH of AZO molecule (FClAzo) and G3-FClAzo reached 0.12 MJ kg⁻¹ and 0.05 MJ kg⁻¹, respectively.¹⁶¹Fig. 9B and C show that Z-FClAzo featured melting point (T_m = −20 °C) while Z-G3-FClAzo featured glass transition temperature (T_g = 0 °C), which provided a solid foundation for storing energy below 0 °C. The introduction of halogen strengthened intramolecular interactions, allowing the phase transitions of AZO derivatives to be realized below 0 °C. The fabricated MOST films exhibited excellent deicing effects in low-temperature environments, with a temperature increase to 3.7 °C achieved at −2 °C (Fig. 9D). After 6 min of blue light irradiation, the ice in the irradiated area melted due to the exothermic isomerization of the MOST fuels. The properties of the film device remained almost unchanged during charge and discharge. Besides, lower light intensities or temperatures retarded charging, which reflected the photothermal coupling nature of the energy harvesting process (Fig. 9E). This work not only overcame the challenge of storing visible light below 0 °C but also provided a solid foundation for the study of sunlight-driven MOST systems.

Fig. 9 (A) Schematic of the Z-FClAzo film for deicing and defrosting at low temperature. (B) and (C) DSC curves of FClAzo and G3-FClAzo. (D) Optical photographs and infrared images of the Z-FClAzo film deicing and defrosting. (E) Cycling stability testing of Z-isomer films. Reproduced from ref. 161 Copyright 2024 Royal Society of Chemistry.

Although E/Z-AZO film devices can achieve energy storage and release at low temperatures, the lower ΔH limits the utilization efficiency of the film device. To further realize the reversible photoconversion of E/Z-AZO film devices, Li et al. designed a series of 4-methylthioarylazopyrazole derivatives capable of storing and releasing energy through alternate irradiation with visible light at an environmental temperature of −5 °C. To assess the ΔH of phase-change E/Z-AZO derivatives, the isomerization enthalpy for the reverse conversion (0.14 MJ kg⁻¹) and crystallization enthalpy for the E isomer (0.11 MJ kg⁻¹) were measured.¹⁶² By simultaneously harvesting isomerization and phase change energies, the authors achieved ΔH = 0.25 MJ kg⁻¹ and τ_1/2 = 22.4 days (Fig. 10A). The solar energy conversion efficiency was 1.3%, which was comparable to the highest E/Z-Azo system conversion efficiencies reported in the literature (0.2%−1.3%; Fig. 10B). Rechargeable films were prepared from a mixture of E isomers and chained silica and installed in a model house (ambient temperature of −5 °C) for real-world performance testing. Fig. 10C shows that the window surface temperature could increase to 21.7 °C during discharge (532 nm), which corresponds to a sustained thermopower output of 256.2 W m⁻² for 60 s. The film device exhibited promising durability under alternate blue/green light irradiation (Fig. 10D). The solar film devices prepared in this work exhibited unprecedentedly high performance, inspiring the functionalized applications of MOST devices.

Fig. 10 (A) Schematic energy storage and release of the E/Z-AZO system at 0 °C and the photoconversion process of 4-methylthioarylazopyrazole. (B) DSC curves for B7-S5. (C) Schematic of the information encryption storage process of the E/Z-AZO film and an infrared image of photo-triggered heat release. (D) Cycling stability testing of the E/Z-AZO film. Reproduced from ref. 162 Copyright 2022 Royal Society of Chemistry.

Besides being used for film preparation, MOST systems can also be composited with wearable fabrics for personal heat management. Most recently, our group designed an E/Z-AZO-based dendrimer polymer and applied it to fabric warming (Fig. 11A). This E/Z-AZO-based dendrimer could absorb photon energy from UV, green and red light and simultaneously store the photon energy, which was released as heat upon stimulation with blue light in a low-temperature, solvent-free environment.¹⁶³ This behavior was attributed to the excellent solar energy storage capability due to the n–π* redshift of E/Z-AZO and photothermal effect of the polymer (Fig. 11B). Considering that blue light irradiation caused a rapid reverse conversion, a long-pass filter with a cutoff of 490 nm was used to filter and thus achieve photon energy storage in wearable fabrics. Such wearable fabrics could store phase change and isomerization energies below 0 °C with a ΔH of 0.056 MJ kg⁻¹, τ_1/2 of 28.5 days and a solar energy conversion efficiency of 0.07% (Fig. 11C). The heat released from the wearable fabric triggered by blue light at 1 °C caused a temperature increase of 11.7 °C to 12.6 °C (Fig. 11D). The temperature difference could be maintained at 12 °C after 5 cycles. This work provides a simple and versatile method for designing superior thermal fabrics for energy storage and release under diverse conditions.

Fig. 11 (A) Illustration of a wearable fabric consisting of a dendritic polymer containing E/Z-AZO, polydopamine, and cotton fabric. (B) Photothermal conversion and photochemical storage processes. (C) Schematic of wearable fabric solar energy storage under simulated sunlight irradiation. (D) Image of a wearable fabric for body heating and infrared thermography at low temperatures. Reproduced from ref. 163 Copyright 2023 Elsevier Inc.

E/Z-AZO functional devices exhibit high ΔH (0.20–0.40 MJ kg⁻¹) and excellent solar energy conversion efficiency (1.3%), offering the advantages of storing and releasing phase change energy and isomerization enthalpy at low temperatures and reversible conversion in the visible light range.¹⁶⁴ To enhance energy storage and conversion performance, future research on E/Z-AZO functional devices should focus on extending the light absorption range into the red and near-infrared regions.^165–168 This extension would facilitate the development of efficient semi-transparent energy storage windows, closed reversible cycling devices, and fabrics with controlled heat dissipation functions.^169,170

3.4. Hybrid functional devices

Despite the progress in individual MOST systems, their applications remain limited by internal energy losses and low overlap with the solar spectrum.¹⁷¹ The fabrication of hybrid MOST functional devices with multijunction solar cell-inspired designs has attracted widespread interest as a means of increasing the efficiency limit.¹⁷² By transforming a single-component MOST system into a hybrid one, the theoretical maximum solar energy conversion efficiency could be increased to 21%.¹⁷³ Therefore, it is significant to explore how to fully utilize the solar energy to enhance solar energy conversion efficiency.

Saydjari et al. designed a hybrid functional device comprising coumarin 314, an azopolymer (PmAzo), a UV light transmission filter, and another azopolymer (PAzo) (Fig. 12A). The hybrid device could effectively store UV light and visible light energy.¹⁷⁴ Coumarin 314 could absorb blue light and permit UV and green light to pass through. PmAzo enabled the efficient storage of visible light, and a UV transparent filter allowed only UV light to pass through for the efficient storage of UV-light energy in the PAzo layer. Indeed, blue light would induce the reverse conversion of E/Z-AZO derivatives and thus hinder the efficient storage of solar energy. By selectively blocking the wavelengths inducing photoisomerization, the fraction of Z isomers could reach 60–70%. The rational device design and highly efficient solar spectrum utilization greatly enhanced the solar energy conversion efficiency, which reached 0.4%. This pioneering work provided novel ideas for the subsequent study of MOST hybrid devices. Compared with solid-state hybrid devices, liquid-based ones offer notable advantages in terms of energy transfer and are capable of automated cyclic operation in closed systems.^175,176 Kashyap et al. coupled a MOST system (NBD derivative) with phase-change materials to effectively utilize the solar spectrum.¹⁷⁷ In the daytime, the valve connecting to the MOST layer was closed, and the MOST system stored solar radiation energy. At night, the valve of the MOST layer was opened. The heat transfer fluid (HTF) was allowed to gain some thermal energy through the phase change material layer to trigger the MOST system to release the energy (Fig. 12BI). By compounding phase change materials with NBD-QC derivatives under 2 kW m⁻² of solar radiation, the hybrid device achieved energy harvesting efficiencies of up to 73% in the daytime, with limiting harvesting efficiencies reaching 90% (Fig. 12BII). The device not only achieved the complete absorption of the solar spectrum, but also inspired the efficient and cost-effective utilization of solar energy for desalination and other applications.

Fig. 12 (A) (I) Cyclic energy storage process of the MOST system. (II) Reversible reaction processes of E/Z-AZO and its derivatives. (III) Molecular formula of the E/Z-AZO polymer. (IV) Schematic of full spectra absorption of the E/Z-Azo device. Reproduced from ref. 174 Copyright 2017 Wiley. (B) (I) Illustration of a hybrid solar storage device (E/Z-AZO and L-PCM). (II) Efficiency of energy harvesting in hybrid systems during the daytime as a function of L-PCM surface temperature. Reproduced from ref. 177 Copyright 2019 Elsevier Inc.

To realize the full utilization of solar energy, Wang et al. developed a hybrid functional device comprising a triple-junction MOST device featuring a top-down structure (NBD, DHA and AZO derivatives).¹⁷³ The results of numerical modeling indicated that the solar energy conversion efficiency could be increased from 13% to 18.2% using a two-junction hybrid device, and an increase to 21% could be achieved by combining a triple-junction hybrid system with an ideal red shift (Fig. 13A). This hybrid device utilized multiple MOST systems including liquid NBD, DHA, and E/Z-AZO derivatives to improve the energy capture and storage performance (Fig. 13B). In single-layer microfluidic chips, the optimal solar energy conversion efficiencies of the three types of MOST derivatives were 0.005% (NBD), 0.013% (DHA), and 0.009% (AZO), respectively. The solar energy conversion efficiency of AZO initially increased and then decreased, possibly because the imperfect filtering capability of the bandpass filter used, allowed an increasing amount of visible light to pass through the transmissive lens (Fig. 13C). The optimal solar energy conversion efficiency of the upper two layers was 0.016%, and the overall optimal solar energy conversion efficiency was 0.02% (Fig. 13D). This study shows that apart from searching for ideal MOST molecules, one can increase energy storage efficiency by combining MOST systems with different wavelengths.

Fig. 13 (A) The triple-junction MOST device operating principle and experimental setup schematic. (B) Photoconversion processes of NBD/QC, DHA/VHF and E/Z-AZO derivatives in toluene. (C) The conversion percentage and energy storage efficiency of the MOST system as a function of residence time, respectively. (D) Conversion percentage and experimental energy storage efficiencies of two- and triple-junction MOST devices at different residence times. Reproduced from ref. 173 Copyright 2021 Wiley-VCH.

Wang et al. designed a solar thermal fuel operable within the visible-light range by combining a photochromic E/Z-AZO derivative with an organic phase change material.¹⁷⁸ Energy storage and release were realized by optical regulation, and the reversible charge/discharge process was realized continuously by light triggering (Fig. 14AI). The ΔH of this functional device reached 0.24 MJ kg⁻¹, and the phase change temperature difference exceeded 6 °C under visible light triggering, indicating that energy could be stored at low temperatures through optical modulation. Furthermore, as AZO derivatives could undergo photochromic transitions during the energy storage process, the relationship between the color change and the conversion percentage was investigated to calculate the ΔH change of the composites (Fig. 14AII). This work provides a unique strategy for the development of visualized MOST systems.

Fig. 14 (A) (I) Schematic of functionalized application of E/Z-AZO devices. (II) Schematic of a visualized solar thermal energy management device. Reproduced from ref. 178 Copyright 2020 Wiley-VCH. (B) (I) Illustration of the power generation concept for a MOST device. (II) Schematic of the MOST system structural and integrated generator device. Reproduced from ref. 179 Copyright 2020 Elsevier Inc.

Besides being employed for heat release, MOST hybrid devices can also be combined with thermoelectric generators (TEGs) for light energy–chemical energy–electricity conversion. Moth-Poulsen et al. designed a compact micro-functional device that combined MOST systems (liquid NBD and film AZO derivatives) with a newly designed MEMS-TEG (Micro–Electro–Mechanical Systems Thermoelectric Generator) (Fig. 14BI). The liquid NBD derivative exhibited a ΔH of 0.37 MJ kg⁻¹ and τ_1/2 of 1 month and achieved a high output power of 1.3 W m⁻³ in combination with the MEMS-TEG.¹⁷⁹ The AZO derivative film displayed a ΔH of 0.33 MJ kg⁻¹ accompanied by τ_1/2 of up to 3 months, producing an output power of 0.7 W m⁻³ when combined with the MEMS-TEG (Fig. 14BII). The above MOST device enabled the light energy–chemical energy–electricity conversion, which affirms that solar power generation is not limited by time and geography and opens a new path for diversified applications of MOST devices.

Following the development of the miniature photo-thermo-electric conversion functional device, Wang et al. developed a novel hybrid MOST device.¹⁸⁰ Three NBD derivatives capable of effectively storing light energy as chemical energy were chosen to study their effect on solar energy utilization efficiency. Simultaneously, NBD solution could effectively cool the photovoltaic (PV) cell through thermal conductivity. The solar energy conversion efficiency of the hybrid system was as high as 14.9% (Fig. 15A and B). The best-performing NBD derivative at a flow rate of 4 mL h⁻¹ exhibited a φ of 0.68 and solar energy conversion efficiency of 2.3%, setting a record for flow-type MOST devices. Furthermore, the hybrid device converted up to 97% of the NBD derivatives in outdoor tests, and the solar energy conversion efficiency was as high as 0.5% at a residence time of 13.5 min and flow rate of 4 mL h⁻¹ (Fig. 15C). Upon solar irradiation, the device maintained a surface temperature of 45 °C, whereas the temperature of the bare PV cells was maintained at 53 °C, which leads to a 0.2% increase in PV efficiency (Fig. 15D). In this system, 2.3% of the solar energy was used for energy storage through isomerization reactions, and integration of the MOST system with photovoltaic cells resulted in solar energy conversion efficiency of up to 14.9%. These findings not only help address the limitations of traditional photovoltaics, but also demonstrate the possibilities offered by the development of advanced hybrid photovoltaic solar systems.

Fig. 15 (A) Schematic of a hybrid MOST device (MOST system and photovoltaic cell). (B) Reversible reaction processes and UV-vis absorption spectra of NBD/QC derivatives. (C) Conversion percentage and energy storage efficiency of the NBD3 molecule (indoor) and NBD2 molecule (outdoor) at different residence times. (D) Temperature changes of NBD3 after different reaction times, and solar energy conversion efficiency under different reaction conditions. Reproduced from ref. 180 Copyright 2024 Elsevier Inc.

The above studies indicated that hybrid systems are beneficial not only for achieving full spectrum utilization, but also for human heating, building insulation and thermoelectric conversion development.^181,182 These diverse studies on hybrid devices provide experimental data and theoretical support for the further design of MOST systems with outstanding performances, and can facilitate the rapid development of MOST hybrid devices towards large-scale practical applications (Table 1).¹⁸³

Table 1 A comparison of parameters for the MOST function devices

MOST	φ	ΔH		τ 1/2 (day)	η (%)		Ref.
		MJ kg^−1	kJ mol^−1		η theo	η expt
a φ represents the quantum yield of molecular photo-switch from the parent isomer to the metastable high-energy isomer. b η represents the solar energy conversion efficiency of the MOST system. c η theo represents the theoretical solar energy conversion efficiency. d η expt represents the experimental solar energy conversion efficiency.
1	0.60	0.11	28	1.02	0.61	0.13	84 and 142
2	0.53	0.58	152	3.00	—	—	148
3	0.68	0.37	93	30.00	0.70	0.50	179
4	0.61	0.40	89	30.00	0.51	0.03	150
5	0.51	0.10	34	0.25	—	—	152
6a	0.62	0.08	22	317.00	—	—	184
6b	0.28	0.11	33	0.002	—	—	184
6c	0.37	0.12	26	48.00	—	—	184
6d	0.75	0.31	93	9772.00	—	—	184
7a	—	0.30	78	2.46	—	—	159
7b	—	0.29	80	2.50	—	—	159
7c	—	0.28	83	2.54	—	—	159
7d	—	0.28	85	2.58	—	—	159
7e	—	0.30	98	2.58	—	—	159
8a	—	—	—	2.17	—	—	159
8b	—	0.29	85	2.17	—	—	159
8c	—	0.28	85	2.17	—	—	159
8d	—	0.27	86	2.21	—	—	159
8e	—	0.26	88	2.25	—	—	159
9a	—	0.34	92	0.85	—	—	160
9b	—	0.32	86	0.64	—	—	160
9c	—	0.26	74	1.28	—	—	160
10a	0.72	0.12	54	11.90	—	0.04	161
10b	0.70	0.05	19	14.70	—	0.06	161
11	0.39	0.25	78	22.40	1.30	—	162
12	—	0.06	24	28.50	—	0.07	163
13a	0.20	—	12	0.67	—	0.40	174
13b	0.20	—	55	0.52	—	—	174
14a	0.82	0.40	95	1.96	0.79	0.02	173
14b	0.21	0.17	52	1.50	0.88	—	173
15	0.41	0.33	102	90.00	1.20	—	179
16a	0.60	0.30	72	240.00	0.40	0.40	180
16b	0.61	0.40	89	30.00	0.50	0.50	180
16c	0.68	0.36	105	0.29	2.90	2.30	180

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4. Emerging strategies for MOST devices

MOST systems are efficient multifunctional materials for converting and storing solar energy, and considerable advances have been achieved in solar spectrum matching, controlled heat release, and cycling stability. In addition, MOST systems have been comprehensively analyzed and simulated using theoretical calculations, which enable rapid predictions of the light conversion process.¹⁸⁵ These efforts provide a convenient pathway for the future development of high-energy and long-lasting MOST systems. Despite the progress in molecular modification and applications of MOST systems, the development of the corresponding functional devices is in its infancy. This field faces several limitations, such as the UV-dependent energy storage, single thermal triggering method, and a narrow range of application fields, which hinders the development and application of MOST devices. Therefore, novel strategies for addressing these problems and enabling the research and preparation of superior MOST devices are urgently required.

4.1. Sunlight-driven MOST devices

Solar radiation on Earth has a wide spectral range (100 nm to 1 mm), with UV light radiation having a share of only 7%.¹⁸¹ Most reported MOST systems can only store energy through UV light irradiation, which limits the efficiency of solar energy storage.¹⁸⁶ Therefore, the parent isomers of MOST systems should ideally undergo photoisomerization under irradiation with visible or near-infrared light, as these two light types collectively account for about 93% of the solar radiation.^187,188 However, visible light may make the photoisomerization reaction and reverse conversion occur simultaneously, which reduces the energy storage efficiency of MOST systems.^189,190 Therefore, high-efficiency MOST systems capable of utilizing the full-solar spectrum are challenging to realize.

Considerable advances have been achieved in expanding the solar spectrum overlap of MOST systems. Sunlight-driven MOST devices have promise for efficient photoenergy storage. Recently, Baggi et al. designed five ortho-dianthrylbenzenes with electron donating and electron withdrawing groups, achieving φ of 0.115–0.16, τ_1/2 of 9–37 years, ΔH of 0.14–0.20 MJ kg⁻¹, and solar energy conversion efficiencies of 0.38–0.66% in mesitylene solutions.¹⁹¹ The anthracene derivatives were rapidly converted into the corresponding isomers after 1.5 min of exposure to unfiltered sunlight, enabling the storage and conversion of sunlight energy without degradation (Fig. 16A). Due to their high ΔH and φ, these MOST systems pave the way for exploitation of anthracene-based photochromic agents in photoenergy storage.

Fig. 16 (A) Chemical structures of anthracene-based derivatives and spectral evolution under unfiltered sunlight irradiation. Reproduced from ref. 191 Copyright 2024 Royal Society of Chemistry. (B) Synthesis of sunlight-driven E/Z-AZO derivatives, and study of the heat release properties of films. Reproduced from ref. 192 Copyright 2024 Royal Society of Chemistry. (C) (I) Illustration of the spectral reshaping strategy for azo-switched high-yield solar E → Z isomerization and experimental setup for isomerization. (II) Molecular structure of 4–6 and their bidirectional photoconversion yield. Repetitive bidirectional photo-switching by alternate illumination with AM1.5 sunlight and 532 nm light. Reproduced from ref. 133 Copyright 2024 Wiley-VCH.

To realize sunlight-driven MOST systems absorbing in the visible range and address the low ΔH challenge, Gupta et al. covalently grafted tetraorthogonally substituted E/Z-AZO onto hexahydroxytriphenylene, achieving a high photostability, photo-cyclability and τ_1/2.¹⁹² It is worth noting that under sunlight with a bandpass filter, the fraction of Z isomer was 79.1%, and the solar energy conversion efficiency reached 1.87%. Thus, orthogonal halogen-substitution induced a red-shift in the absorption spectrum of E, rendering it responsive to visible light.¹⁹³ For instance, quadruple ortho fluorination induced a red shift of the n–π band of E-AZO through intramolecular interactions between the N=N group and ortho functional group.^194,195 The resulting compound effectively captured and stored the energy of green and red light (Fig. 16B).

Although the orthogonal halogen-substitution allowed E-AZO to absorb low-energy light by red-shifting its π–π band, it dramatically reduced the stability of the Z-isomer and could even impair the photoisomerization activity.¹³² Exposure to unfiltered sunlight induced backward Z–E photoisomerization, which prevented the efficient production of Z isomer. Zhang et al. designed a series of ortho-amino azopyrazoles to improve the visible light absorption ability of E isomers while suppressing that of the Z isomers.¹³³ This functional group, which strongly absorbs visible light, improved the absorption of the E isomer by favoring π–π electron leaps and suppressed the absorption of the Z isomer by utilizing the symmetry of the n–π transition.^196,197 The inability to realize the photoinduced Z–E conversion was overcome by enhancing the absorption peak of the Z isomer. Fig. 16CI show that sunlight irradiation could induce the E/Z-AZO photoswitch to reach the photostationary state, with a fraction of Z isomers exceeding 80%. The τ_1/2 of the Z isomer was as long as a few days to several months. An effective reverse photoconversion could be realized under irradiation with green light (532 nm), and the recovery of the E isomer exceeded 65%. The photo-switches were switched under alternating irradiation with AM1.5 and 532 nm light 4–6 times with an absorbance loss of <0.1%, showing good fatigue resistance (Fig. 16CII). This innovative work paves the way for the development and large-scale applications of sunlight-driven MOST systems.

Future research on sunlight-driven MOST devices may focus on two directions. On the one hand, balancing the conflict between redshift phenomenon and τ_1/2 and ΔH, and realizing MOST functional devices for full-solar-spectrum applications. On the other hand, introducing a strong π–π band via orthogonal amino substitution to enhance the visible absorption of the E isomer. By extending the π-conjugated system, one can red-shift the π–π band to wavelengths longer than that of the n–π band and thus extend absorption to more than 600 nm. Due to the decoupling between the π and π-systems, the Z isomer possesses a high thermal stability.^198,199 The above strategy not only enables the tuning of the photo-switching characteristics of MOST systems but also offers opportunities for fabricating MOST devices with ultra-high solar energy storage efficiencies.

4.2. Diverse methods of heat release in MOST devices

Despite its importance, the realization of MOST functional devices for full-solar-spectrum applications is not the only factor driving development in this field. The heat release of MOST functional devices is also critical. In fact, metastable isomers usually convert to parent isomers under visible or near-infrared light to rapidly release heat. However, the spontaneous conversion of metastable isomers back to parent isomers for heat release under dark conditions at room temperature lasts from hours to years.^200,201 Therefore, to deal with accidental situations that may occur during daily use, one should realize controlled energy release from MOST functional devices. Several external triggering methods (thermal, catalyst, electrocatalytic, and mechanical triggering) have been developed to meet the requirements for rapid energy release in different environments.^202–205

Most recently, Han et al. developed a series of anthracene-based derivatives that were grafted with electron donor-withdrawing groups and could undergo a solid-state cycloaddition reaction affording dianthrene, followed by a thermally triggered release of heat during cyclization (Fig. 17A). These compounds achieved a substantial self-heating release, which was attributed to the high ΔH of the anthracene dimers (up to 0.20 MJ kg⁻¹, τ_1/2 of several days) and the efficient thermal conductivity.²⁰⁶ Remarkably, the solid anthracene-based derivatives exhibited heat release up to 185 °C under 90 °C thermal triggering conditions. Although this solid-state anthracene-based MOST system is similar to conventional fossil fuels in terms of spontaneous combustion properties, its renewable and zero carbon dioxide emission advantages give it strong application potential. Beyond thermally induced heat release, Magson et al. designed a series of platinum-, copper- and nickel-based homogeneous catalysts to induce the heat release process of cyano-3-(4-methoxyphenyl)-NBD/QC (Fig. 17B). The porous supported metal catalysts exhibited extremely high catalytic reaction rates, and the platinum oxide catalysts achieved the highest QC to NBD reaction rates reported to date.²⁰⁷ The ΔH of the NBD derivatives could reach up to 0.40 MJ kg⁻¹, and the copper-based catalyst achieved 82% conversion within 1 h. This work not only eliminates the excessive need for precious metals in catalysts, but also provides a novel strategy for energy release in NBD/QC systems.

Fig. 17 (A) Schematic of the self-activated energy release cascade of solid-state anthracene-based MOST and the cycloaddition reaction. Reproduced from ref. 206 Copyright 2024 Elsevier Inc. (B) Schematic of the NBD derivative catalysed reverse conversion process. Reproduced from ref. 207 Copyright 2024 American Chemical Society. (C) Schematic of the electrocatalytic reverse conversion process of heterocyclic E/Z-AZO derivatives. Reproduced from ref. 208 Copyright 2022 Wiley-VCH. (D) Schematic of the mechanically triggered depolymerization process of anthracene-based derivatives. Reproduced from ref. 210 Copyright 2019 Royal Society of Chemistry.

Another interesting concept is electrocatalytically triggered energy release from MOST systems. Libuda et al. investigated the heat release properties of an azothiophene derivative under electrochemical departure using pyrolytic graphite as the working electrode (Fig. 17C). The high-energy isomer exhibited effective heat release upon an electrochemically triggered reverse conversion.²⁰⁸ The photoisomerization degree of the MOST system remained stable at 80% during 100 cycles, and the efficiency of the electrocatalytically triggered reverse restitution process reached 94%. This work demonstrates the advantages of triggering energy release in azothiophene-based MOST systems via electrochemical routes and establishes a foundation for the application of electrocatalytic triggering to other heterocyclic E/Z-AZO derivatives. Apart from thermal and (electro)catalytic triggering, mechanical triggering has also attracted attention. In general, force-induced chain cleavage usually occurs in stress bonds, which suggests that when the structure is subjected to an external force, the bonds usually break during mechanical fracture.²⁰⁹ Kan et al. synthesized anthracene-tailored trifunctional polyurethane prepolymers that formed cross-linked structures via the dimerization of anthracene groups under UV light irradiation (Fig. 17D). Under external pressure and UV light irradiation, anthracene dimers exhibited fluorescence, which evidenced the formation of anthracene monomers.²¹⁰ The synthesis of these anthracene derivatives not only expands the database of fluorescent materials, but also provides a new pathway for energy release from these derivatives.

Most recently reported MOST systems rely on light-triggered heat release. However, the penetration ability of light may be insufficient to achieve full energy release in practical applications.²¹¹ Fortunately, the diverse triggering strategies mentioned above enable not only efficient and controlled energy release but also the development of MOST devices operatable under harsh conditions.

4.3. Integration of MOST functional devices

As the performances of MOST devices continually improve, their combination with other types of materials may be a promising research direction. For example, combining MOST systems with Plexiglas enables de-icing and thawing through controlled energy release. Furthermore, MOST systems can be combined with thermoelectric generators to facilitate solar energy storage and thermoelectric conversion processes. These strategies not only broaden the application scope of MOST devices, but also provide a new method of efficient solar energy utilization and sustainable development.

Wang et al. integrated a MOST system with a fabric to achieve a warming effect (Fig. 18A). The fabric consisted of zolpidem-containing microcapsules and a deep-ultraviolet filtering shell that simultaneously harvested solar and ambient energy for highly efficient energy storage, with the photoisomerization degree exceeding 90%, τ_1/2 reaching three months, and ΔH reaching 2.5 kJ m⁻².²¹² Significantly, this high performance was largely retained after 2000 charges and 50 washes or rubs. The abovementioned study provides a new route to wearable heated fabrics and inspires the use of MOST systems in other personal thermal management applications. Moth-Poulsen et al. proposed a hybrid solar system (MOST system and water, Fig. 18B). The MOST system in the upper layer enabled photoenergy storage, and the remaining photoenergy was utilized to heat the water in the lower collector layer.²¹³ The MOST system exhibited a solar energy conversion efficiency of 1.1% and could be stably operated over 127 energy storage and release cycles, with the solar energy conversion efficiency of the solar water heating systems (SWH) layer reaching 80%. This work offers wide prospects for the development of integrated MOST devices to fully utilize solar energy.

Fig. 18 (A) Reversible reactions of heterocyclic azide-based derivatives and processes for the preparation of warming fabrics. Reproduced from ref. 212 Copyright 2023 Wiley-VCH. (B) Diagram of a hybrid MOST device. Reproduced from ref. 213 Copyright 2017 Royal Society of Chemistry. (C) Isomerization of Azo derivatives and cryptographic storage applications. Reproduced from ref. 214 Copyright 2020 Elsevier Inc. (D) Schematic of full spectrum utilization, and mechanism of reversible reactions in MOST systems. Reproduced from ref. 215 Copyright 2020 Elsevier Inc.

Beyond fabric warmth and thermal energy storage, MOST devices can also be used for the encrypted storage of information. Feng et al. fabricated a series of photothermal films by combining imidazolium-containing E/Z-AZO with graphene templates and explored their applications in information encryption (Fig. 18C). The photothermal film exhibited a ΔH of about 105 Wh kg⁻¹ and generated a temperature difference of 1.6–4.1 °C under thermal stimulation, or irradiation with green light (535 nm).²¹⁴ Time–resolved information encoding was achieved by selectively irradiating devices with different charge levels and reading the information at specific temperatures or by stimulation with light. This multilevel information encryption system based on thermochromic patterns with dynamic properties and excellent stability can contribute to the development of advanced information encryption strategies. Another intriguing strategy is the combination of MOST systems with photovoltaic cells and thermochemical reactions. Fang et al. proposed a concentrated photochemical–photovoltaic–thermochemical device (Fig. 18D). Some of the sunlight energy was stored in chemical bonds through photochemical processes, and the remainder was transmitted to photovoltaic cells and selectively converted into electrical energy.²¹⁵ The theoretical utilization efficiency of the hybrid MOST device (concentrated photochemical-photovoltaic-thermochemical system) was 67%. In actual testing, the daily average solar utilization efficiency of the device could reach 58% and 45% on typical summer and winter days (in the Hebei area, China), respectively. This research indicates that integrated strategies are an effective method for MOST devices to efficiently utilize solar energy. There is no doubt that strategies for combining MOST devices with other types of materials are highly promising for research. Even though these strategies have not been extensively studied, their emergence is expected to inspire the realization of efficient energy storage, stable recycling, superior performance, and sustainability.

4.4. Scaled synthesis of MOST systems

As the application of MOST systems in renewable energy storage gains increasing attention, a pressing challenge arises: how to achieve large-scale production of materials. Currently, MOST systems are largely produced using traditional intermittent synthesis methods, which are characterized by low production efficiency (milligram or gram level), substantial variability in quality, excessive reliance on manual intervention, and potential hazards to human health.^216,217 Therefore, new production equipment should be developed to meet the application demands of MOST systems across various fields. Fortunately, several strategies for the large-scale preparation of NBD derivatives and their precursors have been identified.

In 2023, Moth-Poulsen et al. developed a multistep-integrated-flow intermittent synthesis of NBD precursors, namely propynenitrile derivatives (Fig. 19A). A product yield of 94% was obtained by optimizing the flow rates of reactants and solvents, system pressure, and temperature and residence time in the reactor, and the atom economy (34.1%) exceeded that of traditional intermittent synthesis methods (20.8%).²¹⁸ Multi-step integrated flow intermittent synthesis is a sustainable and safe route to NBD precursors, not only improving mass transfer and the outcomes of single-step synthesis but also enabling the safer handling of toxic feedstocks and intermediates because of more efficient heat transfer. This research lays the foundation for the full-scale synthesis of NBD derivatives, positively responding to the concept of green chemistry. Although an intermittent synthesis technique enables the large-scale preparation of NBD derivatives, it still requires manual intervention to obtain the final product. Yoshida et al. developed an integrated continuous-flow route to 2,3-disubstituted NBDs, using a reaction apparatus consisting of a micro-mixer (500 μm inner diameter) and tubular reactor (1 mm inner diameter), with the reaction time in the tubular reactor depending on tube length (Fig. 19B).²¹⁹ NBD deprotonation was achieved by introducing electrophilic substituents at positions 2 and 3. At low temperatures (−40 °C to 15 °C), the reaction could be completed in less than 3 min, and the product yield of 2,3-disubstituted NBD exceeded 90%. This pioneering work demonstrates that the continuous-flow process eliminates the need for human intervention (unlike batch synthesis), improves production efficiency, avoids human injury at the source, and enables the large-scale production of MOST systems.

Fig. 19 (A) (I) Schematic of the multistep flow synthesis route for the preparation of NBD precursors from the corresponding acetophenones. (II) Flow reactor setup for the synthesis of NBD derivatives. Reproduced from ref. 218 Copyright 2024 Wiley-VCH. (B) Schematic of the integrated continuous flow synthetic route for 2,3-disubstituted-NBD. Reproduced from ref. 219 Copyright 2018 Royal Society of Chemistry. (C) Schematic for the synthesis of NBD derivatives using a tubular spiral flow reactor. Reproduced from ref. 220 Copyright 2021 Wiley-VCH.

To further facilitate the large-scale preparation of NBD derivatives, Moth-Poulsen et al. developed a tubular helical flow reactor, achieving an NBD derivative conversion of 95% at a synthesis rate of 9.71 g h⁻¹ (Fig. 19C). This technique has gained increasing interest due to its numerous advantages such as enhanced heat and mass transfer through the narrow channels of the reactor and improved reaction control and reproducibility.²²⁰ Moreover, such devices enable the upscaling of specific processes without being limited by equipment size, agitation or temperature control. Notably, this is the first successful use of the in situ cleavage of dicyclopentadiene combined with a Diels–Alder reaction for the large-scale synthesis of NBD derivatives. This research demonstrates the suitability of continuous-flow reactions for the efficient generation of multiple libraries of NBD derivatives and realization of real-time energy storage for large-scale applications.

Among all MOST systems, only certain NBD derivatives can be prepared on a large scale, whereas studies on the large-scale preparation of other types of MOST systems are scarce, possibly because of the ease of fluid transportation of NBD derivatives, simplicity of the synthesis process, and mild reaction conditions. Future studies on the industrialized preparation of MOST systems may focus on two aspects. On the one hand, continuous-flow reactors with higher flow rates or the parallel operation of multiple reactors can be exploited to improve the production efficiency of NBD derivatives. On the other hand, the industrialized preparation mechanism of NBD systems could be used to build a framework for other types of MOST systems. The joint efforts of numerous researchers are expected to enable the large-scale preparation and wide application of MOST systems. The strategies for functionalized MOST applications described above provide unlimited possibilities for the development of high-performance MOST thermal energy storage devices (Fig. 20). Continued concerted efforts are still needed to realize the diverse applications of these devices.

Fig. 20 Emerging strategies for future functionalized applications of MOST systems.

Apart from considering emerging strategies for developing MOST systems, it is also extremely crucial to evaluate the techno-economic feasibility, which is an essential factor in determining their commercial application. In fact, most of the research on MOST systems to date has focused on molecular design, performance optimization, and functional applications. Studies analysing economic factors such as material preparation and equipment costs are extremely scarce. In 1983, Constantine et al. studied the conversion efficiency and kinetics of the NBD/QC system and constructed a corresponding photochemical solar energy storage plant.²²¹ Process analysis and preliminary economic evaluation demonstrated that this system was technically feasible but economically uncompetitive, mainly due to the high equipment costs and steam production costs. In 2011, Roman studied the cost-economic analysis of the MOST system (E/Z-AZO derivatives), assuming that the wholesale price of E/Z-AZO derivatives was approximately $50 kg⁻¹, and no maintenance costs were incurred. The preparation cost of E/Z-AZO derivatives was obtained ($0.15 MJ⁻¹). Additionally, the production costs for gasoline ($0.03 MJ⁻¹), photovoltaic power generation ($0.06 – 0.12 MJ⁻¹), and batteries (lithium-ion batteries at $0.15 MJ⁻¹) in the United States for that year were obtained through data collection.²²² To further evaluate the economic feasibility of the MOST system, the emerging azopyrazole photoswitch is selected as the target. The ΔH of azopyrazole is 0.329 MJ kg⁻¹, enabling 2000 cycles of energy storage and release.²¹² Assuming that the production cost of azopyrazole is approximately equal to the raw material cost (approximately $379.4 kg⁻¹), the production cost is calculated to be approximately $0.577 MJ⁻¹. In 2025, the production costs of common energy sources in China (gasoline, diesel, coal, and natural gas) are estimated to be approximately $0.027 MJ⁻¹, $0.024 MJ⁻¹, $0.004 MJ⁻¹, and $0.012 MJ⁻¹, respectively.^223,224 The above preliminary economic analysis indicates that currently MOST systems do not have a competitive advantage in terms of production costs compared to common energy sources. Therefore, if the MOST system is to be used as an energy storage material and widely applied, it is necessary to further optimize the synthesis process, reduce the cost of upstream products, and precisely adjust the performance parameters. Future design of MOST systems and their functional applications should incorporate techno-economic analysis of materials and related equipment to ensure the industrial value and practicality.

5. Summary and outlook

Herein, we systematically summarize the key performance parameters of MOST systems, including τ_1/2, ΔH, φ, cycling stability, and solar spectrum matching, revealing directions for high-performance system development. These parameters not only serve as evaluation criteria for current systems but also provide clear guidance for the development of next-generation high-performance MOST systems. We analyse several molecular photo-switches that show the greatest potential for scaled-up applications, alongside strategies to enhance their ΔH and optimize other critical metrics. Additionally, we highlight state-of-the-art advances in functional devices based on MOST systems – encompassing DHA/VHF, NBD/QC, E/Z-AZO, and hybrid systems – with a focus on their feasibility for solar energy storage and release, while also identifying the core challenges limiting current MOST devices. Furthermore, we discuss emerging strategies for the future development of MOST functional devices, such as sunlight-driven energy storage, diversified induced heat release, integrated device development and scaled synthesis. While these strategies are still evolving, sustained research efforts are steadily advancing MOST functional devices toward practical sustainability. Although these emerging strategies may not be perfect, persistent efforts in this direction are expected to secure a sustainable future for MOST functional devices.

Notably, the theoretical maximum storage efficiency of an ideal MOST device is 21% – a value significantly higher than 0.1% – 0.3%.^173,225 However, the current maximum storage efficiency of existing MOST devices stands at only 2.3%, which remains far lower than the six-junction solar cell, at 47.1%.^226–228 Despite these gaps, MOST device design and application are still in their early stages, leaving substantial room for innovation. Thus, development of ultra-high-performance molecular photo-switches and integration into functional devices is becoming increasingly urgent to meet future sustainable development demands.

Future research could focus on two key directions. First, integrating existing high-performance, sunlight-responsive molecular photo-switches – either alone or in combination with other materials – into novel MOST devices using advanced techniques such as electrostatic self-assembly or bioinspired adhesion. These devices should then be tested under outdoor sunlight conditions and with diverse external triggers to characterize their performance parameters. Second, promoting the automated fabrication of various MOST systems to generate theoretical and experimental foundations for industrial production and scaled-up applications. While developing MOST functional devices with universally excellent performance may be challenging, tailoring devices to specific application scenarios – such as deicing, desalination, thermal clothing, thermodynamic vehicles, and deep-space probes – has significant promise. With ongoing research and the cumulative efforts of the scientific community, MOST functional devices are expected to find widespread use across multiple fields and make substantial contributions to global energy sustainability.

Author contributions

X. T. Xu: conceptualization, investigation, methodology, and writing – original draft; C. H. Li: data curation and investigation; W. Li: formal analysis; J. Feng: resources and validation; W.-Y, Li: funding acquisition, methodology, project administration, resources, supervision, and writing – review and editing.

Conflicts of interest

There are no conflicts of interest to declare.

Abbreviations

AM1.5	Air mass 1.5
B7-S5	4-Methylthioarylazopyrazoles
CoPc	Cobalt phthalocyanine catalyst
DHA	Dihydroazulene
DSC	Differential scanning calorimetry
E a	Thermal potential barrier
E/Z-AZO	Azobenzene
FClAzo	Fluorochloroazobenzene
FvRu2(CO)4	Fulvalene dimetal complexes
G3-FClAzo	Fluorochloroazobenzene-containing dendrimer
HTF	Heat transfer fluid
L-PCM	Localized phase-change material
LTMP	lithium tetramethylpiperidide
MeCN	Acetonitrile
MEMS	Microelectromechanical system
MOST	Molecular solar thermal fuel
NBD	Norbornadiene
Pazo	UV light responsive azopolymer
PmAzo	Visible-light-responsive azopolymer
PSS	Photostationary state
PS	Polystyrene
PU	Polyurethane
PV	Photovoltaic
QC	Quadricyclane
SWH	Solar water heating systems
TEGs	Thermoelectric generators
T g	Crystallization point
T m	Melting point
UV-vis	Ultraviolet-visible
VHF	Vinylheptafulvene
τ 1/2	Storage half-life
φ	Quantum yield
η	Solar energy conversion efficiency
η theo	Theoretical solar energy conversion efficiency
η expt	Experimental solar energy conversion efficiency
ΔH	Storage energy density
ΔT	Temperature difference

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22308241 and 22038008), the Shanxi Province Basic Research Program (Free Exploration Category) Youth Project (202303021212037), the National Key Research and Development Program of China (2022YFE0208400), the National Natural Science Foundation of China Enterprise Innovation and Development Joint Fund Integration Project (U24B6018) and the Program of Beijing Huairou Laboratory (ZD2023012A).

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