Next Article in Journal
Integration of a Double-Concentrated Solar Cooking System Operable from Inside a Home for Energy Sustainability
Previous Article in Journal
Beyond Traditional Grid: A Novel Quantitative Framework for Assessing Automation’s Impact on System Average Interruption Duration Index and System Average Interruption Frequency Index
Previous Article in Special Issue
Direct Current Photovoltaic Solar Energy for Water Heating
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Energies
Volume 18
Issue 11
10.3390/en18112672
energies-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Molecular Solar Thermal Fuels with High Energy Density Based on Azobenzene Derivatives

by
Yan Jiang
1,
Rui Liu
1,2,
Yupeng Guo
1,2,
Hai Wang
1,*,
Wen Luo
3,* and
Jin Huang
3,4
1
School of Mechanical and Automotive Engineering, Zhaoqing University, Zhaoqing 526061, China
2
School of Electronic and Electrical Engineering, Zhaoqing University, Zhaoqing 526061, China
3
School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China
4
School of Electric Engineering, Guangdong Polytechnic of Water Resources and Electric Engineering, Guangzhou 510635, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(11), 2672; https://doi.org/10.3390/en18112672
Submission received: 14 March 2025 / Revised: 1 May 2025 / Accepted: 16 May 2025 / Published: 22 May 2025
(This article belongs to the Special Issue Solar Energy and Photovoltaic Systems: Prospect Development and Challenges)
Browse Figures
Versions Notes

Abstract

:
Molecular solar thermal fuels (MOSTs) based on azobenzene derivatives have become one of the research hotspots for solar thermal conversion and storage due to their excellent cycling stability, resistance to photodegradation, and the capability to precisely adjust their absorption wavelengths, and other merits. Here, a novel MOST with connecting two azobenzene molecules by a short linkage (bis-AZO) has been proposed; the photoisomerization regulation and energy storage performance are studied experimentally in detail. The photoisomerization rate of the resultant MOST could be controlled by diverse irradiation intensities. The energy density for bis-AZO was 275.03 J g−1 at 100% isomerization degree, with excellent thermal and photochemical cycling stability. The macroscale heat release of bis-AZO loaded on fabric reached a temperature increase of about 4.3 °C. This research offers a new design strategy for increasing the energy density in azobenzene-based molecular solar thermal fuels.

1. Introduction

Solar energy represents a clean, pollution-free, abundant, and sustainable energy source that holds broad potential for diverse applications [1,2]. In recent years, a solar thermal conversion and storage technology known as molecular solar thermal fuels (MOSTs) has garnered widespread attention [3,4,5,6]. Commonly studied MOSTs systems are norbornadiene/quadricyclane systems [7,8,9], dihydroazulene/vinylheptafulvene systems [10,11,12,13], fulvalene dimetal complex systems [14,15], and trans-azobenzene/cis-azobenzene systems [16,17,18,19,20,21]. Among them, azobenzene has been extensively researched as MOSTs due to its high thermal stability, great molecular design possibilities, and ease of synthesis [18,22,23].
Generally, trans-azobenzene is relatively thermally stable and can be efficiently induced to undergo trans-to-cis isomerization upon exposure to UV light [24,25], which transforms trans-azobenzene into a high-energy metastable cis-azobenzene. Subsequently, cis-azobenzene can be transformed back into trans-azobenzene through external stimuli (such as light, heat, or catalysts), releasing the stored energy in the process without generating harmful substances [5,22,26,27]. However, azobenzene as MOSTs encounter a challenge concerning their energy density, limiting their suitability for practical applications [28,29].
To address the aforementioned challenge, scientists have made many efforts to improve the energy density, such as azobenzene grafted onto nanocarbon template and polymer template [30,31,32,33,34], and azobenzene small molecule derivatives [35,36,37]. The formers show a significant effect on enhancing the energy storage performance of azobenzene, but the limited improvement of the energy density for azobenzene grafted onto polymer template, along with the complex preparation process and the lack of scalability associated with azobenzene grafted onto nanocarbon templates, have collectively hindered their practical application. However, the latter, especially bis-azobenzene molecules, have a relatively low molecular weight, often exhibiting high mass energy density and unique properties. Some researchers have recently begun to pay attention to [37].
Generally, a bis-azobenzene molecule is composed of two azobenzene molecules. According to the different ways of connecting the two azobenzenes, the bis-azobenzene molecules can be divided into o-bis-azobenzene, m-bis-azobenzene, and p-bis-azobenzene. Ortho-bis-azobenzene cannot undergo isomerization due to its small free volume, so it has been studied less as a MOSTs [38,39]. On the contrary, m-bis-azobenzene and p-bis-azobenzene are particularly suitable for the development of MOST materials because their behavior is similar to that of independent azobenzene units, and their absorption spectra can be represented by the sum of the spectra without a shift in the maximum absorption wavelength. Several studies [35,36,37] have focused on m-bis-azobenzene and p-bis-azobenzene to improve molecular solar thermal energy storage. Morikawa et al. [35] first reported the m-bis-azobenzene molecule for enhancing the energy density of MOSTs. The two azobenzenes of the m-bis-azobenzene molecule are connected by a benzene ring, which maximizes the energy density of m-bis-azobenzene while maintaining high isomerization efficiency and minimizing the molecular weight. The results show that its energy density is estimated to be 230~262 J g−1 when the degree of isomerization is 100%. Sun et al. [36] also prepared a class of m-bis-azobenzene molecules for energy storage, in which the two azobenzene molecules are also connected by a benzene ring. The experimental results show that the m-bis-azobenzene molecule exhibits excellent energy storage performance, with an energy density of 272 J g−1. Gonzalez et al. [37] reported a series of p-bis-azobenzene molecules. The two azobenzene molecules of this pair of p-bis-azobenzene molecules are linked by a short connecting group (diethylmethylamine). Research on its energy storage performance shows that its energy density is significantly increased to 255 J g−1 compared with azobenzene. However, practical energy storage applications are still limited by low energy density and complex synthesis processes. In addition, there are few studies on the photoisomerization rate regulation of bis-azobenzene molecules used for MOSTs, which have mainly focused on surfactants and photochromism in previous studies [40,41,42].
In this study, a short alkyl group was introduced to link two azobenzene molecules via two ether linkages (bis-AZO) (Figure 1). Introduction of such a short alkyl group via two ether linkages was primarily selected for the synthetic ease of one step, while most syntheses of bis-azobenzenes require complicated multiple steps and have low yields [35,36,37]. The short alkyl group via two ether linkages also serves to electronically decouple from each other and essentially behave like independent azobenzene molecules [39,43]. Also, the small molecular weight of short alkyl groups via two ether linkages is considered to be beneficial for achieving high energy density. The chemical structure and crystalline were analyzed by FT-IR, NMR, XRD, and HRMS. The light absorption of the bis-AZO is analyzed by UV–Vis spectroscopy. In addition, the influences of irradiation intensities on trans↔cis photoisomerization rate are further discussed. The energy storage performance and stability of this bis-azobenzene compound in dichloromethane solution are further investigated. Moreover, the temperature change of charged bis-AZO in a porous fabric was monitored by a thermal imager to observe the heat release process and obtain the maximum temperature difference for future solar thermal applications. The findings provide inspiration for the design of novel MOSTs with high energy densities.

2. Experimental

2.1. Materials

Bis-AZO was synthesized using 4-hydroxyazobenzene and 1,6-dibromohexane through a substitution reaction. All the main materials used without further purification were obtained from commercial sources, unless otherwise mentioned. Specifically, the 4-hydroxyazobenzene and 1,6-dibromohexane were bought from Sigma-Aldrich (St. Louis, MO, USA). Potassium carbonate, potassium iodide, N, N-dimethylformamide (DMF), and dichloromethane (DCM) were purchased from Tianjin Zhiyuan (Tianjin, China).

2.2. Synthesis of bis-AZO

To a 250 mL round-bottom flask, 10 mmol of 4-hydroxyazobenzene, 15 mmol of potassium carbonate, 1 mmol of potassium iodide, and 40 mL of DMF were added, and the mixture was stirred magnetically at room temperature for several minutes to ensure complete dissolution. Subsequently, 4.5 mmol of 1,6-dibromohexane was added dropwise to the mixture while maintaining continuous stirring. The reaction mixture was then heated to 120 °C and allowed to react for 24 h under reflux conditions. Upon cooling to room temperature, 150 mL of DCM was added under stirring, resulting in the precipitation of an insoluble product. The resulting solid was isolated by filtration, washed thoroughly with deionized water to remove any soluble impurities, and recrystallized. The final product was then vacuum-dried at 60 °C to obtain the desired compound with a yield of 61.6%.

2.3. Characterizations

The chemical compositions and structures of bis-AZO were determined through the analysis of Fourier transform infrared (FT-IR) spectra recorded with a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA USA) using a pure KBr pellet, proton, and carbon nuclear magnetic resonance (1H NMR and 13C NMR) spectra obtained from a Bruker AVANCE III HD400 using deuterated chloroform (CDCl3) as the solvent, X-ray diffraction (XRD) patterns recorded on Rigaku Ultima IV, as well as high-resolution mass spectra (HRMS) recorded by Thermoscientific Q EXACTIVE FOCUS.
Photoisomerization characteristics of bis-AZO were studied using a UV–Vis spectrophotometer (UV-1900i, Shimadzu, Japan) based on UV–visible (UV–Vis) absorption spectra. Measurements were conducted in quartz glass cuvettes with a path length of 1 cm. The samples, dissolved in DCM solution, were subjected to light irradiation at wavelengths of 365 nm (UV light) and 450 nm (blue light). The light intensities for UV and blue irradiation were determined using a photometer (CEL-NP2000-10, Beijing China Education Au-light Co., Ltd., Beijing, China).
Thermal stability of bis-AZO was corroborated by the thermogravimetric curve recorded using an SDT Q600 simultaneous thermogravimetric analyzer (TGA, TA Instruments, Newcastle, WA USA). The samples, placed in an alumina crucible, were heated from 30 to 600 °C at a constant heating rate of 10 °C min−1 under a nitrogen atmosphere with a flow rate of 50 mL min−1.
The storage energy density of bis-AZO was measured by differential scanning calorimeter (DSC, DSC3, Mettler Toledo, Zurich, Switzerland) under a nitrogen atmosphere. The typical heating/cooling procedures were as follows: equilibrate at 0 °C; ramp up to 200 °C at a constant heating rate of 20 °C min−1 (10 °C min−1 or 30 °C min−1); cool the sample to 0 °C at a constant cooling rate of 20 °C min−1 (10 °C min−1 or 30 °C min−1); finally, re-heat the sample to 200 °C at a constant heating rate of 20 °C min−1 (10 °C min−1 or 30 °C min−1). The absence of heat flow during the subsequent heating cycle confirmed that the energy released during the initial heating was derived from the stored solar energy.

3. Results and Discussion

3.1. Chemical Composition

The structure of bis-AZO was characterized using FTIR, NMR, and HRMS. The FTIR spectrum of bis-AZO is shown in Figure 2a. The absorption peaks at 2939 and 2868 cm−1 are assigned to the -CH2- group, indicating the presence of aliphatic methylene groups. Between 1602 and 1580 cm−1, peaks are observed that are associated with the C-C and C=C groups of the benzene ring, confirming the presence of aromatic structures. The peaks at 1497 and 1473 cm−1 correspond to the -CH2- group, further supporting the presence of methylene groups in the structure. A peak near 1394 cm−1 is attributed to the N=N characteristic of the azobenzene. Additionally, the peaks at 1245 cm−1 and 1019 cm−1 are assigned to the C-O group, with the former corresponding to the aromatic C-O bond in the benzene ring and the latter to the C-O bond in the alkoxy group of the bis-AZO. These observations preliminarily indicate that bis-AZO has been successfully synthesized. In addition, the 1H NMR, 13C NMR spectrums, and HRMS were also applied to demonstrate the chemical composition, as shown in Figure 2b and Figure S1. The chemical shifts of hydrogen atoms appear at δ 7.91, 7.48, 7.01, 4.08, 1.89, and 1.64, and the corresponding ratio of the integral area is 8:6:4:4:4:4, which is consistent with the theoretical value. The chemical shifts of carbon atoms are 161.68, 152.75, 146.89, 130.34, 129.04, 124.82, 122.55, 114.72, 68.17, 29.15, and 25.87. The m/z is found to be 479.2421, which is equivalent to the theoretically calculated value (479.5876). The above results confirm the successful synthesis of bis-AZO.

3.2. Photoisomerization Properties

The photoisomerization properties of azobenzene are fundamental to energy storage and release, which can be fully comprehensive insights through UV–Vis absorption spectroscopy. As illustrated in Figure 3, the UV–Vis absorption spectrum of bis-AZO reveals that trans-bis-AZO exhibits a maximum absorption peak at 349 nm before 365 nm irradiation, which corresponds to the π-π* transition of the trans-bis-AZO chromophore. Upon exposure to UV light at 365 nm, the absorption intensity of the π-π* transition at 349 nm significantly diminishes and changes to maximum absorption peaks at 308 and 441 nm, the latter being associated with the n-π* transition of cis-bis-AZO. When irradiated with 450 nm blue light or in the dark, the absorption spectrum reverts to its original spectral pattern due to cis→trans isomerization. This spectral change is characteristic of the trans-to-cis-to-trans photoisomerization reactions of azobenzene.
To investigate the influence of light irradiation intensity on the photoisomerization of bis-AZO in DCM, the UV–Vis absorption spectra were used to determine the time required for cis-bis-AZO and trans-bis-AZO to reach the photostationary states and the isomerization degree that can be achieved. Figure 4 shows the UV–Vis absorption spectra of bis-AZO as it evolves over time under different light intensities: 365 nm UV light intensities of 30 (Figure 4a), 20 (Figure 4c), and 10 mW cm−2 (Figure 4e), and 450 nm blue light intensities of 120 (Figure 4b), 80 (Figure 4d), and 40 mW cm−2 (Figure 4f). It can be observed that as the UV light irradiation intensity increases, the time required for the trans-bis-AZO to isomerize into the cis form decreases from 30 s to 7 s. Similarly, with increasing intensity of the 450 nm blue light, the time needed for the cis-bis-AZO to revert back to the trans form is also shortened from 20 s to 7 s. These results indicate that the photoisomerization rate of bis-AZO can be effectively controlled by adjusting the irradiation intensity.
Additionally, the isomerization degree of bis-AZO was calculated using the formula from ref. [44] and plotted as shown in Figure 5. From the figure, it is evident that under diverse irradiation intensities, the amount of cis-bis-AZO increases with increasing irradiation time, eventually reaching an isomerization degree of 68%, 73%, and 68% (approximately 70%). Furthermore, the similar isomerization degree (71%) of trans-cis bis-AZO can also be quantified by 1H NMR through integration of the characteristic signals before and after UV irradiation (Figure S2), which aligns well with values reported in the recent literature (70–77%) [35,37,39]. The above results exhibit that the bis-AZO has great potential for controllable energy storing and releasing.
Given the relatively high isomerization degree and excellent photoreversibility of bis-AZO, a quantitative study was conducted to investigate the kinetics of the photoinduced trans-to-cis and cis-to-trans isomerization processes of bis-AZO under 365 nm and 450 nm light irradiation in DCM solution. The first-order rate constants for isomerization were calculated using Equation (1) [45], providing insight into the dynamic behavior of these photoisomerization reactions.
ln[(A∞ − At)/(A∞ − A0)] = −kt
where A0, At, and A∞ are the absorbance of the π-π* transition at 349 nm at time zero, time t, and infinite time, respectively. k represents the first-order rate constant for photoinduced isomerization.
Figure 6 illustrates the first-order kinetics plots for the photoinduced trans-to-cis and cis-to-trans isomerization of bis-AZO in DCM solution under varying irradiation intensities. The plots exhibit similar linear trends for bis-AZO across different irradiation intensities, confirming its adherence to first-order kinetics. Notably, the isomerization rates increase as light intensity rises, with higher intensities accelerating the reaction compared to lower intensities. Table 1 is presented with the corresponding first-order rate constants, kt-c (from trans to cis) and kc-t (from cis to trans), highlighting the direct influence of light intensity on the kinetics of bis-AZO isomerization.

3.3. Thermal Stability and Cyclic Stability

Stability is one of the crucial characteristics of MOSTs. To evaluate the stability of bis-AZO, thermal stability and cycling stability analyses were conducted. As shown in Figure 7a, the TGA of bis-AZO indicates that it undergoes a one-step degradation, with a degradation temperature of 369 °C and the maximum weight loss occurring around 377 °C. Subsequently, when the temperature reaches 600 °C, the final weight loss is 74%, demonstrating good thermal stability. Additionally, the repetitive photoisomerization of bis-AZO between trans and cis states was studied. The sample was irradiated with UV light to reach a cis-rich state and then immediately irradiated with blue light to revert to a trans-rich state. The change in absorbance of bis-AZO at 349 nm is shown in Figure 7b. It is evident that the change in absorbance at 349 nm is negligible after 10 cycles of photoisomerization, confirming that bis-AZO possesses excellent cyclic stability.

3.4. Energy Storage Performance

To assess the energy storage performance of the bis-AZO, the isomerization enthalpy (energy density) released during the heat-induced cis-to-trans isomerization was measured using DSC. As depicted in Figure 8a, the DSC traces of cis-bis-AZO exhibit an exothermic peak within the temperature range of 90–130 °C in the first heating cycle because of cis-to-trans isomerization. However, the heat flow vanishes during the second heating cycle, suggesting that the heat released in the first phase corresponds to the energy stored from light absorption [31]. Additionally, it should be noted that DSC is used to obtain thermodynamic parameters related to thermal equilibrium. Therefore, setting an appropriate heating rate is crucial for accurately determining the heat storage and release using DSC [18,22,46]. To explore the optimal heating rate, DSC measurements were performed at three different rates: 10, 20, and 30 °C min−1. Figure 8b shows the DSC curves of cis-rich bis-AZO at varying heating rates. It can be clearly seen that the cis-bis-AZO exhibits a broad exothermic peak at all heating rates, with the peak temperature shifting toward higher temperatures as the heating rate increases. Furthermore, upon integrating the peak area, it was found that the maximum exothermic energy, 198.34 J g−1, occurred at a heating rate of 20 °C min−1 due to the facile formation of trans-bis-AZO nucleators by thermally induced cis→trans conversion process [46,47], whose crystalline structure is verified as in the XRD pattern (Figure S3), indicating that the optimal heating rate is recommended to be 20 °C min−1.
Additionally, DSC curves of bis-AZO at different degrees of isomerization were studied with a heating rate set to 20 °C min−1, as presented in Figure 8c. It is clear that the energy density of bis-AZO increases as the isomerization degree increases, showing a linear dependence on the isomerization degree [35]. Specifically, the energy density of bis-AZO is 198.34 J g−1 at an isomerization degree of 73%. Given that the total energy density has a linear relationship with the isomerization degree, the total theoretical energy density corresponding to 100% degree of isomerization was calculated as 275.03 J g−1 through extrapolating the plot (Figure 8d). The energy density obtained for the bis-AZO is higher than that of other bis-azobenzene compound previously reported with energy density of 230~262 J g−1 [35] and 255 J g−1 [37], which is comparable to that of the compound with an energy density of 272 J g−1 [36] and almost double that of monofunctional azobenzene derivatives [35,48,49]. The relatively high and enhanced energy density can be attributed to the short alkyl linkage of bis-azobenzene, enabling electronically decoupling of bis-azobenzene, resulting in independent and highly reactive photochemical isomerization [35,36]. This relatively high energy density underscores the potential of bis-azobenzene for practical applications in MOST systems, suggesting its promising future in energy storage technologies.

3.5. Macroscale Heat Release

It is necessary to macroscopic heat release for large-scale applications. Here, polyester fabric is used as a carrier. The samples were prepared according to these steps. Bis-AZO was added to the DCM solution. Subsequently, it was charged with UV light for 10 min to isomerize to rich cis-bis-AZO. Following this, the polyester fabric (1 cm2) was fully immersed in the DCM solution of rich-cis bis-AZO, taken out, and weighed after cold air drying. 1 mg of cis-bis-AZO was loaded on the fabric (named as a charged sample). The sample loaded with rich trans-bis-AZO without UV light charging was also prepared as above (named as an uncharged sample). Then, two samples were heated on a heating stage. The macroscale heat release temperature changes of the fabric loaded with bis-AZO were tracked by a thermal imager. For comparison, the temperature change (DT) between the charged and uncharged samples was studied to demonstrate the heat release ability of bis-AZO. Figure 9 illustrates the surface temperature distribution of the fabric loaded with bis-AZO, showing the color changes represent corresponding temperature changes. It can be observed that the temperature of the charged sample is significantly higher than that of the uncharged sample, and there is minimal change in DT during the heat release for uncharged samples, suggesting that the DT is primarily attributed to heat release from cis→trans isomerization rather than other effects. The maximum DT reaches up to 4.3 °C. The results indicate that bis-AZO has good heat release behavior and is expected to be used in large-scale MOSTs.

4. Conclusions

In this study, a novel bis-AZO was synthesized, and its isomerization performance, stability, energy storage density, and macroscale heat release were investigated in depth. The following conclusions can be drawn:
(1)
The synthesized bis-AZO exhibited efficient and reversible photoisomerization properties in DCM, with its isomerization rate effectively regulated by irradiation intensity. Higher irradiation intensities resulted in faster isomerization, while lower intensities led to slower isomerization.
(2)
Due to the introduction of two azobenzene molecules, bis-AZO shows a remarkable energy density of 275.03 J g−1 with excellent thermal stability and cycling stability.
(3)
Bis-AZO exhibits excellent heat release behavior, achieving a significant temperature increase of approximately 4.3 °C.
Therefore, this bis-AZO holds promise for applications in solar energy storage technology as a MOST.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en18112672/s1, Figure S1: (a) 1H NMR and (b) 13C NMR spectrums of the bis-AZO. Figure S2: 1H NMR of bis-AZO (a) before UV charging and (b) after UV charging for 10 min. Figure S3: XRD of trans-bis-AZO.

Author Contributions

Y.J.: conceptualization, methodology, investigation, formal analysis, visualization, writing—original draft, writing—review and editing. R.L. and Y.G.: investigation, formal analysis. H.W.: writing—review and editing, formal analysis. W.L.: writing—review and editing, resources, formal analysis. J.H.: resources, methodology, conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Guangdong Basic and Applied Basic Research Foundation (No. 2022A1515110775); the Characteristic Innovation Project of Universities in Guangdong (No. 2024KTSCX031; No. 2023KTSCX154); the Zhaoqing University High-level Project Training Program Funding Project (No. GCCZK202414); the Zhaoqing University Research Funding Project (No. 2019010127, No. 190128); the Natural Science Foundation of Guangdong Province (No. 2025A1515011649); the Science and Technology Program of Guangzhou City (No. 202201010292).

Data Availability Statement

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

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

MOSTsMolecular solar thermal fuels
AZOAzobenzene
DMFN, N-dimethylformamide
DCMDichloromethane
FT-IRFourier transform infrared
1HNMRProton nuclear magnetic resonance
13CNMRCarbon nuclear magnetic resonance
XRDX-ray diffraction
HRMSHigh-resolution mass spectra
UV–VisUV–visible
TGAThermogravimetric analyzer
DSCDifferential scanning calorimeter
kFirst-order kinetic constants
kt-cFirst-order rate constants from trans to cis
kc-tFirst-order rate constants from cis to trans

References

  1. Wang, H.; Huang, J.; Song, M.J.; Hu, Y.X.; Wang, Y.F.; Lu, Z.J. Simulation and experimental study on the optical performance of a fixed-focus Fresnel lens solar concentrator using polar-axis tracking. Energies 2018, 11, 887. [Google Scholar] [CrossRef]
  2. Wang, G.; Zhang, Z.; Lin, J.Q. Multi-energy complementary power systems based on solar energy: A review. Renew. Sustain. Energy Rev. 2024, 199, 114464. [Google Scholar] [CrossRef]
  3. Börjesson, K.; Lennartson, A.; Moth-Poulsen, K. Efficiency limit of molecular solar thermal energy collecting devices. ACS Sustain. Chem. Eng. 2013, 1, 585–590. [Google Scholar] [CrossRef]
  4. Dong, L.Q.; Feng, Y.Y.; Wang, L.; Feng, W. Azobenzene-based solar thermal fuels: Design, properties, and applications. Chem. Soc. Rev. 2018, 47, 7339–7368. [Google Scholar] [CrossRef] [PubMed]
  5. Kucharski, T.J.; Tian, Y.C.; Akbulatov, S.; Boulatov, R. Chemical solutions for the closed-cycle storage of solar energy. Energy Environ. Sci. 2011, 4, 4449–4472. [Google Scholar] [CrossRef]
  6. Hu, J.; Huang, S.; Yu, M.M.; Yu, H.F. Flexible solar thermal fuel devices: Composites of fabric and a photoliquefiable azobenzene derivative. Adv. Energy Mater. 2019, 9, 1901363. [Google Scholar] [CrossRef]
  7. Quant, M.; Hillers-Bendtsen, A.E.; Ghasemi, S.; Erdelyi, M.; Wang, Z.H.; Muhammad, L.M.; Kann, N.; Mikkelsen, K.V.; Moth-Poulsen, K. Synthesis, characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage. Chem. Sci. 2022, 13, 834–841. [Google Scholar] [CrossRef]
  8. Wang, Z.H.; Moïse, H.; Cacciarini, M.; Nielsen, M.B.; Morikawa, M.; Kimizuka, N.; Moth-Poulsen, K. Liquid-based multijunction molecular solar thermal energy collection device. Adv. Sci. 2021, 8, 2103060. [Google Scholar] [CrossRef]
  9. Orrego-Hernández, J.; Dreos, A.; Moth-Poulsen, K. Engineering of norbornadiene/quadricyclane photoswitches for molecular solar thermal energy storage applications. Acc. Chem. Res. 2020, 53, 1478–1487. [Google Scholar] [CrossRef]
  10. Kjeldsen, I.L.H.; Høvring, J.F.; von Buchwald, T.J.; Hillers-Bendtsen, A.E.; Mikkelsen, K.V. The effects of solvation on the back reaction and storage capabilities of solar thermal energy storage systems. Phys. Chem. Chem. Phys. 2022, 24, 5564–5577. [Google Scholar] [CrossRef]
  11. Schøttler, C.; Vegge, S.K.; Cacciarini, M.; Nielsen, M.B. Long-term energy storage systems based on the dihydroazulene/vinylheptafulvene photo-/thermoswitch. ChemPhotoChem 2022, 6, e202200037. [Google Scholar] [CrossRef]
  12. Berdnikova, D.V.; Kriesche, B.M.; Paululat, T.; Hofer, T.S. Fused bis (hemi-indigo): Broad-range wavelength-independent photoswitches. Chem. Eur. J. 2022, 28, e202202752. [Google Scholar] [CrossRef] [PubMed]
  13. Doronina, E.P.; Jouikov, V.; Belogolova, E.F.; Sidorkin, V.F. Isomer-selective dative bond O→M (M= Si, Ge) for designing new photochromic hemi-indigo systems. J. Organomet. Chem. 2022, 958, 122189. [Google Scholar] [CrossRef]
  14. Zhang, B.; Feng, Y.Y.; Feng, W. Azobenzene-based solar thermal fuels: A review. Nano-Micro Lett. 2022, 14, 138. [Google Scholar] [CrossRef]
  15. Duvva, N.; Chilakamarthi, U.; Giribabu, L. Recent developments in tetrathiafulvalene and dithiafulvalene based metal-free organic sensitizers for dye-sensitized solar cells: A mini-review. Sustain. Energy Fuels 2017, 1, 678–688. [Google Scholar] [CrossRef]
  16. Wu, S.; Li, T.X.; Zhang, Z.Y.; Li, T.; Wang, R.Z. Photoswitchable phase change materials for unconventional thermal energy storage and upgrade. Matter 2021, 4, 3385–3399. [Google Scholar] [CrossRef]
  17. Lennartson, A.; Roffey, A.; Moth-Poulsen, K. Designing photoswitches for molecular solar thermal energy storage. Tetrahedron Lett. 2015, 56, 1457–1465. [Google Scholar] [CrossRef]
  18. Masutani, K.; Morikawa, M.A.; Kimizuka, N. A liquid azobenzene derivative as a solvent-free solar thermal fuel. Chem. Commun. 2014, 50, 15803–15806. [Google Scholar] [CrossRef]
  19. Ishiba, K.; Morikawa, M.A.; Chikara, C.; Yamada, T.; Iwase, K.; Kawakita, M.; Kimizuka, N. Photoliquefiable ionic crystals: A phase crossover approach for photon energy storage materials with functional multiplicity. Angew. Chem. Int. Ed. 2015, 54, 1532–1536. [Google Scholar] [CrossRef]
  20. Cho, E.N.; Zhitomirsky, D.; Han, G.G.D.; Liu, Y.; Grossman, J.C. Molecularly engineered azobenzene derivatives for high energy density solid-state solar thermal fuels. ACS Appl. Mater. Interfaces 2017, 9, 8679–8687. [Google Scholar] [CrossRef]
  21. Han, G.G.D.; Li, H.S.; Grossman, J.C. Optically-controlled long-term storage and release of thermal energy in phase-change materials. Nat. Commun. 2017, 8, 1446. [Google Scholar] [CrossRef] [PubMed]
  22. Morikawa, M.A.; Yamanaka, Y.; Hui, J.K.H.; Kimizuka, N. Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density. RSC Adv. 2023, 13, 24031–24037. [Google Scholar] [CrossRef]
  23. Durgun, E.; Grossman, J.C. Photoswitchable molecular rings for solar-thermal energy storage. J. Phys. Chem. Lett. 2013, 4, 854–860. [Google Scholar] [CrossRef] [PubMed]
  24. Piwowarczyk, M.; Deptuch, A.; Drzewicz, A.; Juszyńska-Gałązka, E.; Gałązka, M.; Galewski, Z. Mesomorphism of (E)-4-[(4-propyloxyphenyl) diazenyl] alkanoates (3OABOOCm). Thermochim. Acta 2024, 731, 179649. [Google Scholar] [CrossRef]
  25. Yu, X.J.; Wang, Z.B.; Buchholz, M.; Füllgrabe, N.; Grosjean, S.; Bebensee, F.; Bräse, S.; Wöll, C.; Heinke, L. Cis-to-trans isomerization of azobenzene investigated by using thin films of metal-organic frameworks. Phys. Chem. Chem. Phys. 2015, 17, 22721–22725. [Google Scholar] [CrossRef]
  26. Wu, S.; Butt, H.J. Solar-thermal energy conversion and storage using photoresponsive azobenzene-containing polymers. Macromol. Rapid Commun. 2020, 41, 1900413. [Google Scholar] [CrossRef] [PubMed]
  27. Dong, L.Q.; Chen, Y.H.; Zhai, F.; Tang, L.; Gao, W.C.; Tang, J.W.; Feng, Y.Y.; Feng, W. Azobenzene-based solar thermal energy storage enhanced by gold nanoparticles for rapid, optically-triggered heat release at room temperature. J. Mater. Chem. A 2020, 8, 18668–18676. [Google Scholar] [CrossRef]
  28. Xu, X.; Feng, J.; Li, W.Y.; Wang, G.; Feng, W.; Yu, H.F. Azobenzene-containing polymer for solar thermal energy storage and release: Advances, challenges, and opportunities. Prog. Polym. Sci. 2024, 149, 101782. [Google Scholar] [CrossRef]
  29. Cai, F.; Song, T.; Yang, B.; Lv, X.; Zhang, L.; Yu, H.F. Enhancement of solar thermal fuel by microphase separation and nanoconfinement of a block copolymer. Chem. Mater. 2021, 33, 9750–9759. [Google Scholar] [CrossRef]
  30. Kucharski, T.J.; Ferralis, N.; Kolpak, A.M.; Zheng, J.O.; Nocera, D.G.; Grossman, J.C. Templated assembly of photoswitches significantly increases the energy-storage capacity of solar thermal fuels. Nat. Chem. 2014, 6, 441–447. [Google Scholar] [CrossRef]
  31. Jiang, Y.; Huang, J.; Feng, W.; Zhao, X.; Wang, T.; Li, C.H.; Luo, W. Molecular regulation of nano-structured solid-state AZO-SWCNTs assembly film for the high-energy and short-term solar thermal storage. Sol. Energy Mater. Sol. Cells 2019, 193, 198–205. [Google Scholar] [CrossRef]
  32. Yang, W.X.; Feng, Y.Y.; Si, Q.Y.; Yan, Q.H.; Long, P.; Dong, L.Q.; Fu, L.X.; Feng, W. Efficient cycling utilization of solar-thermal energy for thermochromic displays with controllable heat output. J. Mater. Chem. A 2019, 7, 97–106. [Google Scholar] [CrossRef]
  33. Han, G.D.; Park, S.S.; Liu, Y.; Zhitomirsky, D.; Cho, E.; Dincă, M.; Grossman, J.C. Photon energy storage materials with high energy densities based on diacetylene-azobenzene derivatives. J. Mater. Chem. A 2016, 4, 16157–16165. [Google Scholar] [CrossRef]
  34. Xu, X.T.; Zhang, P.; Wu, B.; Xing, Y.M.; Shi, K.; Fang, W.H.; Yu, H.F.; Wang, G.J. Photochromic dendrimers for photoswitched solid-to-liquid transitions and solar thermal fuels. ACS Appl. Mater. Interfaces 2020, 12, 50135–50142. [Google Scholar] [CrossRef]
  35. Morikawa, M.; Yamanaka, Y.; Kimizuka, N. Liquid bisazobenzenes as molecular solar thermal fuel with enhanced energy density. Chem. Lett. 2022, 51, 402–406. [Google Scholar] [CrossRef]
  36. Sun, S.D.; Liang, S.F.; Xu, W.C.; Wang, M.H.; Gao, J.G.; Zhang, Q.J.; Wu, S. Photoswitches with different numbers of azo chromophores for molecular solar thermal storage. Soft Matt. 2022, 18, 8840–8849. [Google Scholar] [CrossRef] [PubMed]
  37. Gonzalez, A.; Qiu, Q.F.; Usuba, J.; Wan, J.; Han, G.G.D. Photoinduced solid-liquid phase transition and energy storage enabled by the design of linked double photoswitches. ACS Mater. Au 2023, 4, 30–34. [Google Scholar] [CrossRef]
  38. Slavov, C.; Yang, C.; Schweighauser, L.; Boumrifak, C.; Dreuw, A.; Wegner, H.A.; Wachtveitl, J. Connectivity matters-ultrafast isomerization dynamics of bisazobenzene photoswitches. Phys. Chem. Chem. Phys. 2016, 18, 14795–14804. [Google Scholar] [CrossRef]
  39. Dong, D.F.; Li, T. Multi-azo photoswitches for improved molecular solar thermal energy storage. ChemPhotoChem 2024, 8, e202400007. [Google Scholar] [CrossRef]
  40. Sharma, A.; Bekir, M.; Lomadze, N.; Santer, S. Photo-isomerization kinetics of azobenzene containing surfactant conjugated with polyelectrolyte. Molecules 2020, 26, 19. [Google Scholar] [CrossRef]
  41. Arya, P.; Jelken, J.; Lomadze, N.; Santer, S.; Bekir, M. Kinetics of photo-isomerization of azobenzene containing surfactants. J. Chem. Phys. 2020, 152, 024904. [Google Scholar] [CrossRef] [PubMed]
  42. Tamaoki, N.; Yamaoka, T. Light-intensity dependence in the photochromism of dibenzo [2.2](4, 4′)-azobenzenophane. J. Chem. Soc. Perkin Trans. 2 1991, 6, 873–878. [Google Scholar] [CrossRef]
  43. Bleger, D.; Dokic, J.; Peters, M.V.; Grubert, L.; Saalfrank, P.; Hecht, S. Electronic decoupling approach to quantitative photoswitching in linear multiazobenzene architectures. J. Phys. Chem. B 2011, 115, 9930–9940. [Google Scholar] [CrossRef]
  44. Wang, G.J.; Yuan, D.; Yuan, T.T.; Dong, J.; Feng, N.; Han, G.X. A visible light responsive azobenzene-functionalized polymer: Synthesis, self-assembly, and photoresponsive properties. J. Polym. Sci. Pol. Chem. 2015, 53, 2768–2775. [Google Scholar] [CrossRef]
  45. Sin, S.L.; Gan, L.H.; Hu, X.; Tam, K.C.; Gan, Y.Y. Photochemical and thermal isomerizations of azobenzene-containing amphiphilic diblock copolymers in aqueous micellar aggregates and in film. Macromolecules 2005, 38, 3943–3948. [Google Scholar] [CrossRef]
  46. Jiang, Y.; Liu, J.X.; Luo, W.; Quan, X.Q.; Li, H.J.; Huang, J.; Feng, W. High-energy and light-actuated phase change composite for solar energy storage and heat release. Surf. Interfaces 2021, 24, 101071. [Google Scholar] [CrossRef]
  47. Jiang, Y.; Liu, R.; Luo, W.; Hu, Y.; Wang, H.; Wang, C.H.; Li, X.X.; Huang, J. Photoguided AZO-phase change composite for high-energy solar storage and heat release at near ambient temperature. J. Energy Storage 2024, 101, 113974. [Google Scholar] [CrossRef]
  48. Taoda, H.; Hayakawa, K.; Kawase, K.; Yamakita, H. Photochemical conversion and storage of solar energy by azobenzene. J. Chem. Eng. Japan 1987, 20, 265–270. [Google Scholar] [CrossRef]
  49. Kunz, A.; Heindl, A.H.; Dreos, A.; Wang, Z.H.; Moth-Poulsen, K.; Becker, J.; Wegner, H.A. Intermolecular London dispersion interactions of azobenzene switches for tuning molecular solar thermal energy storage systems. ChemPlusChem 2019, 84, 1145–1148. [Google Scholar] [CrossRef]
Energies 18 02672 g001
Figure 1. Solar energy storage and heat release of bis-AZO as a MOST.
Figure 1. Solar energy storage and heat release of bis-AZO as a MOST.
Energies 18 02672 g001
Energies 18 02672 g002
Figure 2. (a) FT-IR spectra and (b) 1H NMR of bis-AZO.
Figure 2. (a) FT-IR spectra and (b) 1H NMR of bis-AZO.
Energies 18 02672 g002
Energies 18 02672 g003
Figure 3. Photoisomerization properties of bis-AZO in DCM solution. UV–Vis absorption spectra of trans-bis-AZO before (black solid line) and after irradiation at 365 nm (red solid line) and 450 nm (blue solid line).
Figure 3. Photoisomerization properties of bis-AZO in DCM solution. UV–Vis absorption spectra of trans-bis-AZO before (black solid line) and after irradiation at 365 nm (red solid line) and 450 nm (blue solid line).
Energies 18 02672 g003
Energies 18 02672 g004aEnergies 18 02672 g004b
Figure 4. UV–Vis absorption spectra of bis-AZO with diverse irradiation intensities: 365 nm UV light at (a) 30, (c) 20, and (e) 10 mW cm−2. 450 nm blue light at (b) 120, (d) 80, and (f) 40 mW cm−2.
Figure 4. UV–Vis absorption spectra of bis-AZO with diverse irradiation intensities: 365 nm UV light at (a) 30, (c) 20, and (e) 10 mW cm−2. 450 nm blue light at (b) 120, (d) 80, and (f) 40 mW cm−2.
Energies 18 02672 g004aEnergies 18 02672 g004b
Energies 18 02672 g005
Figure 5. Isomerization degree of bis-AZO with diverse irradiation intensities: 365 nm UV light at (a) 30, (b) 20, and (c) 10 mW cm−2.
Figure 5. Isomerization degree of bis-AZO with diverse irradiation intensities: 365 nm UV light at (a) 30, (b) 20, and (c) 10 mW cm−2.
Energies 18 02672 g005
Energies 18 02672 g006
Figure 6. First-order plots for (a) trans-to-cis and (b) cis-to-trans isomerization processes of bis-AZO with diverse irradiation intensities.
Figure 6. First-order plots for (a) trans-to-cis and (b) cis-to-trans isomerization processes of bis-AZO with diverse irradiation intensities.
Energies 18 02672 g006
Energies 18 02672 g007
Figure 7. (a) TGA curves and (b) cyclic stability of bis-AZO.
Figure 7. (a) TGA curves and (b) cyclic stability of bis-AZO.
Energies 18 02672 g007
Energies 18 02672 g008
Figure 8. Energy storage performance of bis-AZO. (a) DSC traces of bis-AZO with first-heating and second-heating stage at heating rates of 10, 20, and 30 °C min−1 within the temperature range of 30–150 °C. (b) DSC curves at heating rate of 10, 20, and 30 °C min−1, with first-heating stage within the temperature range of 80–150 °C. (c) DSC curves of cis-bis-AZO at a heating rate of 20 °C min−1 with isomerization degrees of 73, 53, 30, and 0%. (d) Dependence of energy density as a function of the isomerization degree.
Figure 8. Energy storage performance of bis-AZO. (a) DSC traces of bis-AZO with first-heating and second-heating stage at heating rates of 10, 20, and 30 °C min−1 within the temperature range of 30–150 °C. (b) DSC curves at heating rate of 10, 20, and 30 °C min−1, with first-heating stage within the temperature range of 80–150 °C. (c) DSC curves of cis-bis-AZO at a heating rate of 20 °C min−1 with isomerization degrees of 73, 53, 30, and 0%. (d) Dependence of energy density as a function of the isomerization degree.
Energies 18 02672 g008
Energies 18 02672 g009
Figure 9. IR thermal imaging of the charged (fabric loaded with cis-bis-AZO) and uncharged samples (fabric loaded with trans-bis-AZO).
Figure 9. IR thermal imaging of the charged (fabric loaded with cis-bis-AZO) and uncharged samples (fabric loaded with trans-bis-AZO).
Energies 18 02672 g009
Table 1. First-order kinetic constants (k) for the isomerization of bis-AZO.
Table 1. First-order kinetic constants (k) for the isomerization of bis-AZO.
Light Wavelength (nm)Irradiation Intensity (mW cm−2)k(t-c) (s−1)k(c-t) (s−1)
365300.59069
200.54457
100.29892
450120 0.3868
800.22115
400.16731
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Jiang, Y.; Liu, R.; Guo, Y.; Wang, H.; Luo, W.; Huang, J. Molecular Solar Thermal Fuels with High Energy Density Based on Azobenzene Derivatives. Energies 2025, 18, 2672. https://doi.org/10.3390/en18112672

AMA Style

Jiang Y, Liu R, Guo Y, Wang H, Luo W, Huang J. Molecular Solar Thermal Fuels with High Energy Density Based on Azobenzene Derivatives. Energies. 2025; 18(11):2672. https://doi.org/10.3390/en18112672

Chicago/Turabian Style

Jiang, Yan, Rui Liu, Yupeng Guo, Hai Wang, Wen Luo, and Jin Huang. 2025. "Molecular Solar Thermal Fuels with High Energy Density Based on Azobenzene Derivatives" Energies 18, no. 11: 2672. https://doi.org/10.3390/en18112672

APA Style

Jiang, Y., Liu, R., Guo, Y., Wang, H., Luo, W., & Huang, J. (2025). Molecular Solar Thermal Fuels with High Energy Density Based on Azobenzene Derivatives. Energies, 18(11), 2672. https://doi.org/10.3390/en18112672

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop