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

Abstract

Molecular solar thermal (MOST) fuels offer a closed-cycle and renewable energy storage strategy that can harvest photons within the chemical conformations and release heat on demand through reversible isomerization of molecular photoswitches. However, most reports rely on the ultraviolet (UV) light storage at room temperature, which significantly restricts the application of MOST fuels. Here, we present a novel fluorochloroazobenzene-containing dendrimer that can not only efficiently store photon energy and release heat in the visible light range but also exhibit excellent application potential below 0 °C. These excellent properties are attributed to the neighbouring halogen atoms modulating the intramolecular interactions of N=N groups and the favourable chain mobility of the dendrimer at low temperature. The energy density of the dendrimer fuel after harvesting green light (520 nm) can reach 0.046 MJ kg⁻¹ (19.0 kJ mol⁻¹) accompanied by a storage half-life of up to approximately 20.6 days. Moreover, blue light-triggered heat release from the MOST film in low-temperature environments (−2 °C) can increase the temperature by 3.7 °C, exhibiting significant de-icing effects. Our work provides a fascinating avenue to fabricate visible light activated MOST fuels and unlocks the possibility of developing natural sunlight storage.

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Introduction

Developing sustainable and renewable energy technologies is essential to meet the escalating global energy demand and address the environmental pollution caused by the combustion of conventional fossil fuels.^1–4 As a renewable energy source, solar energy exhibits tremendous application potential in the areas of photovoltaic conversion and photothermal conversion due to its unlimited availability.^5,6 However, efficient conversion of diffuse sunlight into useful forms of energy is quite a challenge.⁷ Hence, it is an urgent demand to develop effective solar energy conversion, storage and utilization technologies. Molecular solar thermal (MOST) fuels relying on photoswitchable compounds offer an appealing strategy for solar energy conversion and storage, which can generate high-energy metastable isomers upon absorbing photons and release the stored energy as heat during back conversion upon external triggering.^8–10 Various photoswitches, including norbornadiene/quadricyclane couples,^11,12 hydrazones,¹³ anthracene,¹⁴ azo(hetero)arenes,^15–17 and dihydroazulene/vinyl-heptafulvene systems,^18,19 are currently investigated for their potential as energy storage molecules. In particular, azo(hetero)arenes are well studied as MOST fuels due to the high photosensitivity, remarkable chemical stability and reversible thermal reduction.

Various forms of azobenzene-based MOST fuels, including azobenzene derivatives, azopolymers, and azobenzene carbon nanomaterials, have been exploited in recent years, and certain crucial parameters such as energy density (ΔH, energy difference between Z and E isomers) and storage half-life (τ_1/2, time taken for 50% of the Z isomer to decay back to the E isomer) have also been intensively investigated.^20–23 Han and co-workers developed several azobispyrazole photoswitches featuring photo-controllable reversible phase change, whose effective light penetration depth under UV light was more than 1400 μm and energy densities were higher than 0.3 MJ kg⁻¹.²⁴ Li and colleagues designed a series of azobipyrazole photoswitches that displayed (nearly-) quantitative bidirectional photoconversion and widely tunable thermal half-lives of the Z-isomer from hours to years due to the two five-membered rings remarkably weakening the intramolecular steric hindrance.^25–28 A series of solar thermal fabrics were fabricated by Feng and co-workers via combining azopolymers with dynamic covalent bonding, where reversible isomerization induced supramolecular bond formation and dissociation significantly enhanced the energy densities (0.41 MJ kg⁻¹).²⁹ Most recently, Moth-Poulsen and co-workers proposed an emerging strategy of coupling two or more photoswitching units to fabricate MOST fuels that not only could redshift the absorption spectra by extending the conjugate system but also may facilitate the enhancement of storage energy densities.³⁰ Despite the discovery of various azobenzene-based MOST fuels that have been proven to be a promising and efficient pathway for solar thermal conversion, but most reported MOST fuels depend on UV light storage, which not only limits the material lifetime by degrading the azobenzene but also potentially interferes with and destroys the surrounding environment.^31–33 Moreover, UV light can be scattered usually by aggregate nanostructures and therefore has poor penetration.³⁴ As a consequence, there has been considerable interest in developing MOST fuels that can store photons and release heat in the visible light range. It is noted that visible light commonly induces the reversion process by converting the high-energy Z-isomers back to low-energy E-isomers.³⁵ Thus, it is immensely difficult to fabricate MOST fuels that can store photons of visible light. Fortunately, several emerging molecular photoswitches such as Azo-BF₂ switches, azobispyrazole, and ortho-substituted azobenzene derivatives have recently been successfully designed and probed for their energy storage and release in the visible light range.^36–38 Nevertheless, there are still scarce studies on the solar thermal energy storage and release in the visible light range in low-temperature environments. Therefore, tireless efforts are still necessary to explore the challenging but fascinating area.

Herein, we report one fluorochloroazobenzene (FClAzo)-containing dendrimer that not only can store photon energy and release heat below 0 °C in the visible light range but also exhibit excellent storage stability. The excellent visible light activation ability is assigned to the neighbouring halogen atoms red-shifting the n–π* band absorption band. The favourable chain mobility of the dendrimer allows the photoswitches to accomplish solar thermal energy storage and release in low-temperature environments. The FClAzo-containing dendrimer (G3-FClAzo) displays a storage energy density of 0.04 MJ kg⁻¹ (19.0 kJ mol⁻¹) after capturing sufficiently green light (520 nm). The Z-rich dendrimer fuel exhibits a half-life of 20.6 days, demonstrating its stable energy storage capacity. In addition, blue light-triggered (420 nm) heat release from its MOST film can increase the temperature by 3.7 °C under ice-cold conditions, exhibiting a significant de-icing effect. This work paves the way for MOST fuels that can store natural sunlight energy and release heat over a wide temperature range.

Results and discussion

Dendrimers are well-defined, highly branched macromolecules with a large number of highly reactive surface functional groups that have been widely applied in catalysis, sensing and biomedicine.^39,40 These unique structural advantages offer opportunities for covalent grafting of azo groups on the surface of the dendrimer, which facilitates the enhancement of intermolecular interactions between neighbouring azo groups to improve storage performances.^41–43 Besides, the dendrimer has the benefits of low solution viscosity, high solubility, and good film formation compared to traditional linear polymers, which enables it to be better applied as an existing device coating to fabricate dendrimer MOST devices such as flexible fabrics and smart films.^44,45 The FClAzo-containing dendrimers presented here were demonstrated to harvest photon energy of green light and store energy in the conformation of the metastable Z-isomers and then release the stored energy in the form of heat by overcoming the thermal barrier when stimulated by blue light, as illustrated in Fig. 1. The visible light activated dendrimers (G3-FClAzo) were successfully fabricated through the Michael addition reaction between one poly(amidoamine) dendrimer (G3) and FClAzo. The chemical structures of FClAzo and G3-FClAzo were characterized and verified by ¹H-NMR spectra, FT-IR spectra, and UV-vis spectra, as shown in Fig. S1–S7.† The numbers of FClAzo groups on the dendrimer were calculated to be 17, by contrasting the integral values of the chemical shifts of FClAzo (Ar–H) at 7.36 and 7.06 ppm, and poly(amidoamine) dendrimers at 3.28 ppm (–CH̲₂NHCO–) in ¹H NMR.^46,47 The molecular weights of FClAzo and G3-FClAzo were measured by Mass spectrometric analyses (ESI†) and Gel Permeation Chromatography (GPC), respectively, as shown in Fig. S8–S10.† This new generation of dendrimer fuels exhibits superior solar thermal energy storage and release properties in the low-temperature visible light range, which can further enrich the molecular photoswitching system.

Fig. 1 Schematic diagram of FClAzo-containing dendrimers storing photon energy of green light (520 nm) and releasing heat for de-icing under blue light (420 nm) irradiation below 0 °C.

Photoisomerization properties

The photoisomerization properties of FClAzo-containing dendrimers are essential for solar thermal energy storage and release, which could be studied using UV-vis absorption spectroscopy. Fig. 2a shows the UV-vis spectra of G3-FClAzo in ethyl acetate (3 × 10⁻⁵ M) before and after UV, blue, and green light irradiation. The G3-FClAzo displayed an intense π–π* absorption band at 330 nm and a weak n–π* band at 450 nm. It is noted that the n–π* band could extend beyond 550 nm, suggesting that the FClAzo-containing dendrimer could be isomerized in the visible light range.⁴⁸ Although the previously reported G3-4FAzo can also be isomerized by green light, the higher molar extinction coefficient in green light range indicates that G3-FClAzo is more efficient to absorb green light, as shown in Fig. S12 and Table S2.†⁴⁹ The fractions of Z-isomers of G3-FClAzo after UV, blue, and green light irradiation to the photostationary state (PSS) were about 70%, 5%, and 66%, respectively, which were calculated according to the absorbance at 330 nm, as shown in Fig. 2b. The visible light activation property is attributed to the neighbouring halogen atoms that modulate the intramolecular interactions of the N=N groups, leading to a significant redshift of the n–π* absorption band.⁵⁰Fig. 2c shows that green light irradiation led to a blueshift and intensity decrease of the π–π* absorbance, whereas the intensity of the n–π* absorbance increases, which is characteristic for isomerization from the thermodynamically stable E-isomers to the Z-isomers. Fig. 2d shows that subsequent blue light irradiation reverted the spectral changes back, indicating that the Z-isomers were switched back to the E-isomers. The reversible photoisomerization of FClAzo and G3-FClAzo under the alternating irradiation of green light and blue light could also occur in the thin-film state, which can be determined by UV-vis absorption spectroscopy, as shown in Fig. 2e and S13–S16.† It should be noted that FClAzo and G3-FClAzo also exhibited excellent cycling stabilities of isomerization in solution and the film under the alternating irradiation of green light and blue light, as shown in Fig. 2f and S17.† Such efficient photoisomerization properties of FClAzo-containing dendrimers offer opportunities for solar energy storage and heat release in the visible light range.

Fig. 2 (a) UV-vis absorption spectra of G3-FClAzo in ethyl acetate (3 × 10⁻⁵ M) before and after UV, blue, and green light irradiation. (b) The fraction of isomers of G3-FClAzo in solution after UV, blue, and green light irradiation. (c) UV-vis absorption spectra of G3-FClAzo in ethyl acetate under green light (520 nm, 20 mW cm⁻²) irradiation. (d) UV-vis absorption spectra of G3-FClAzo in ethyl acetate under blue light (420 nm, 20 mW cm⁻²) irradiation. (e) The fraction of Z isomers of FClAzo and G3-FClAzo in solution and the film after the alternating light irradiation. (f) Cycle stability of G3-FClAzo in ethyl acetate under the alternating light irradiation (20 mW cm⁻², 10 min).

Thermal relaxation behaviour

The thermal relaxation behaviours represent the energy storage time of metastable state solar thermal fuels, which were investigated by observing a series of UV-vis spectra during the reversion process under darkness. Fig. 3a shows that the π–π* absorbance of G3-FClAzo in solution increased slowly over one month, which suggests that the content of E-isomers gradually increased.⁵¹ The thermal relaxation behaviours of FClAzo and G3-FClAzo in solution and the film state are shown in Fig. S18.† The thermal relaxation behaviours of visible light activated dendrimer solar thermal fuels under darkness follow first-order kinetics, which can be described by eqn (1),⁵²wherein, A_∞ is the absorption intensity of the solar thermal fuels after undergoing Z-to-E reversion, A_t is the absorption intensity of the samples kept under darkness for a “t” time, and A₀ is the absorption intensity of samples at the metastable state after green light irradiation. Fig. 3b shows the first-order rate constants (k_rev) of the FClAzo solution, G3-FClAzo solution, FClAzo film and G3-FClAzo film, which were calculated to be 6.69 × 10⁻⁷ s⁻¹, 5.49 × 10⁻⁷ s⁻¹, 4.29 × 10⁻⁷ s⁻¹, and 3.61 × 10⁻⁷ s⁻¹, respectively. The corresponding half-lives of Z-rich solar thermal fuels in solution and the film state were about 11.9 days, 14.7 days, 17.3 days and 20.6 days, respectively, as shown in Fig. 3c. The long half-lives of Z-rich solar thermal fuels suggest that the energy in the high-energy state can be stored stably. The thermal barrier (E_a) for isomerization can be calculated using eqn (2):⁵³ where T is the storage temperature and τ_1/2 is the storage half-life. k_B, R, and h are the Boltzmann, gas, and Planck constants, respectively.

Fig. 3 (a) Reversion process of G3-FClAzo in ethyl acetate under darkness. (b) Reversion rate lines of visible light activated solar thermal fuels in solution and the film state under darkness. (c) Half-lives of visible light activated solar thermal fuels in solution and the film state. (d) Thermal barriers of visible light activated solar thermal fuels in solution and the film state.

Fig. 3d shows the E_a values of the FClAzo solution, G3-FClAzo solution, FClAzo film and G3-FClAzo film, which were calculated to be 1.11 eV, 1.12 eV, 1.13 eV and 1.14 eV, respectively. The k_rev, τ_1/2 and E_a of FClAzo and G3-FClAzo fuels are summarized in Table 1, it can be seen that the k_rev of G3-FClAzo is slower, τ_1/2 is longer and E_a is higher than that of FClAzo. These superior performances of G3-FClAzo may be attributed to the template self-assembly strategy that endows the azobenzene groups with stronger steric hindrance and intermolecular interaction.^54–56 These results demonstrate that the visible light activated MOST fuels could not only allow for stable storage of photon energy, but also provide an attractive avenue for the long-term efficient utilization of renewable energy.

Table 1 The fraction of Z-isomers, first-order rate constants (k_rev), half-life (τ_1/2) and thermal barrier (E_a) of visible light activated MOST fuels in solution and the film state

	Fraction of Z-isomers (%)		k rev (s^−1)	τ 1/2 (day)	E a (eV)
	520 nm	420 nm
FClAzo solution	72	3	6.69 × 10^−7	11.9	1.11
G3-FClAzo solution	70	5	5.49 × 10^−7	14.7	1.12
FClAzo film	27	5	4.29 × 10^−7	17.3	1.13
G3-FClAzo film	25	8	3.61 × 10^−7	20.6	1.14

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Storage energy densities

The storage energy densities of visible light activated solar thermal fuels were measured by differential scanning calorimetry (DSC). Fig. 4a shows the heating and subsequent cooling curves of Z-rich FClAzo fuels. The metastable state FClAzo fuels exhibited a remarkable melting point (T_m) at around −20 °C and a clear crystallization peak at −13 °C, indicating that the metastable state fuels are in the liquid state below 0 °C. It is noted that the metastable state FClAzo fuels also displayed a broad exothermic heat flow over 110–160 °C due to the thermal Z-to-E isomerization during the heating process, whereas the E-rich FClAzo fuels in the heating process only showed the T_m, as shown in Fig. S19.† The storage energy density (ΔH) of the Z-rich FClAzo fuel was calculated to be about 0.12 MJ kg⁻¹ by integration of the exothermic peak area, where the isomerization enthalpy (ΔH_iso) is 0.037 MJ kg⁻¹ (16.7 kJ mol⁻¹). Fig. 4b exhibits the heating and subsequent cooling curves of Z-rich G3-FClAzo fuels, from which the Z-rich G3-FClAzo fuels also displayed a broader exothermic heat flow during the heating process due to the sufficient Z-to-E isomerization while they did not exhibit exothermic heat flow but a glass transition temperature (T_g) at about 0 °C. It is noted that the T_g of the E-rich G3-FClAzo fuel is about 10 °C, as shown in Fig. S19.† This adjustable T_g offers the possibility of storing photon energy of the dendrimer fuel in low-temperature environments. The storage energy density (ΔH_iso) of the G3-FClAzo fuel was calculated to be approximately 0.046 MJ kg⁻¹ (19.0 kJ mol⁻¹) based on Z-to-E isomerization. The isomerization enthalpy (ΔH_iso) of the G3-FClAzo fuels was higher than that of FClAzo fuels and closer to the computational value (25.2 kJ mol⁻¹), which may be due to the amplification of FClAzo groups enhancing the intermolecular interactions.^57,58 Furthermore, FClAzo and G3-FClAzo also exhibited effective solar energy storage capability after filtered sunlight irradiation (allowing photons with wavelength ≥490 nm to pass through). The ΔH_iso values of FClAzo and G3-FClAzo after filtered sunlight irradiation were about 0.026 MJ kg⁻¹ and 0.032 MJ kg⁻¹, respectively, as shown in Fig. S21.†

Fig. 4 (a) The heating (red, up) and subsequent cooling curves (blue, down) of FClAzo fuels after green light irradiation (520 nm, 100 mW cm⁻², 20 min) with at heating rate of 10 °C min⁻¹. (b) The heating (red, up) and subsequent cooling traces (blue, down) of G3-FClAzo fuels after green light irradiation (520 nm, 100 mW cm⁻², 20 min) with a heating rate of 10 °C min⁻¹. (c) DSC heating curves of Z-rich FClAzo fuels under darkness for different times (10 °C min⁻¹). (d) Relationship between the residual energy densities and storage time of Z-rich FClAzo fuels under darkness. (e) DSC heating curves of Z-rich G3-FClAzo fuels under darkness for different times (10 °C min⁻¹). (f) Relationship between the residual energy densities and storage time of Z-rich G3-FClAzo fuels under darkness.

To confirm the storage stability of the visible light activated solar thermal fuels, the DSC heating curves of MOST fuels in the dark for different times were determined, reflecting the slow decreasing process of residual energy densities due to the spontaneous Z-to-E isomerization. Fig. 4c shows that the residual energy density of FClAzo fuels decreased slowly from 0.03 MJ kg⁻¹ to 0.009 MJ kg⁻¹ as the storage time under darkness increased from 0 days to one month. The relationship between the residual energy density and storage time of Z-rich FClAzo suggests that approximately 82% stored energy of the Z-rich FClAzo was released after being left under darkness for one month, as shown in Fig. 4d. The DSC heating curves of Z-rich G3-FClAzo fuels under darkness for different times and the relationship between the residual energy density and storage time are shown in Fig. 4e and f. A summary of the residual energy densities in the dark for different times of Z-rich MOST fuels is presented in Table S2.† These results demonstrated that the FClAzo and G3-FClAzo are capable of absorbing and converting photon energy efficiently in the visible light range, whereas dendrimer MOST fuels exhibit stronger isomerization enthalpy storage capability and longer energy storage stability, which offers insights for the further development of molecular solar thermal systems with excellent performances.

Controllable heat release

The heat release of MOST fuels is usually spontaneous and slow due to the Z-to-E reversion under darkness, which is consistent with long-term storage half-lives.⁴⁶ External stimuli can help Z-rich MOST fuels climb the energy barrier to accelerate heat release.⁵⁹ Currently, most studies utilize external light stimulation in room- or high-temperature environments for rapid heat release. Indeed, the visible light activated MOST films fabricated on polyethylene terephthalate (PET) can also increase their temperature by 4.4 °C at room temperature under blue light irradiation (420 nm, 80 mW cm⁻²), as shown in Fig. S22–S23.† More interestingly, MOST films can also rapidly release heat under blue light irradiation in cold environments (below 0 °C). It is noted that the T_m values of the designed Z-rich FClAzo and Z-rich G3-FClAzo were about −20 °C and 0 °C, respectively, which is the critical condition for MOST films to store photons and release heat in low-temperature environments. Fig. 5a illustrates the schematic diagram of heat release from Z-rich MOST fuels under blue light irradiation at −2 °C. The heat release of MOST films was measured by an infrared camera. The temperature of the E-rich G3-FClAzo film was increased by 1.1 °C during the blue light irradiation due to the photothermal effect, whereas the temperature difference (ΔT) of the Z-rich G3-FClAzo film was 2.6 °C due to the Z-to-E isomerization, as shown in Fig. 5b. Note that the ΔT for the Z-rich FClAzo film is slightly higher than that of the Z-rich G3-FClAzo film, which may be due to the partial release of phase transition energy from Z-rich FClAzo. The temperature change curves, temperature difference histograms and cycle release for visible light activated MOST fuels are shown in Fig. S24 and S25.† The energy density, heat release and conversion efficiency of the visible light activated MOST fuels are summarized in Table 2. It can be seen that the solar energy conversion efficiencies of FClAzo and G3-FClAzo are 0.07% and 0.09%, respectively, which are comparable to the reported studies.^61–63 Besides, although this work exhibits a lower energy density and a lower exothermic temperature difference compared to reported studies, the benefit of the dendrimer MOST film is the ability to realize solar energy storage and heat release in the low-temperature visible light range, which provides an avenue for the further design of more advanced MOST devices.⁶⁴

Fig. 5 (a) Schematic diagram of heat release from Z-rich MOST fuels under blue light irradiation at low-temperature (−2 °C). (b) Infrared images of heat release of E-rich and Z-rich G3-FClAzo films under blue light irradiation. (c) Schematic diagram of exothermic de-icing of Z-rich MOST fuels under blue light irradiation. (d) Optical photographs of the exothermic deicing effect of E-rich and Z-rich FClAzo films under blue light irradiation.

Table 2 Energy density, heat release and conversion efficiency of visible light activated MOST fuel below 0 °C

	ΔHiso^a (MJ kg^−1)	ΔHiso (kJ mol^−1)	ΔT (°C)	Energy conversion efficiency^b	Solar energy conversion efficiency^c
a The isomerization enthalpy (ΔHiso) of MOST fuels (3 mg) was obtained by DSC measurements after green light irradiation (100 mW cm^−2, 20 min). DSC measurement with a heating rate of 10 °C min^−1. Energy density = DSC result of exothermic value × (mass of MOST)^−1. b Energy conversion efficiency = DSC result of exothermic value × (irradiation time × light intensity × incident area)^−1, the incident area = πr^2 = 20.27 mm^2. c Solar energy conversion efficiency = energy conversion efficiency × 1.74%, 1.74% was determined by the integral result according to ref. 60.
FClAzo	0.037	16.7	3.7	4.4%	0.07%
G3-FClAzo	0.046	19.0	2.6	5.5%	0.09%

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De-icing experiments of visible light activated MOST films were performed in cold environments (−2 °C). Fig. 5c shows the de-icing schematic of Z-rich MOST fuels under blue light irradiation (420 nm, 80 mW cm⁻²). The ice flakes were scattered on the surface of the MOST films with E-rich and Z-rich isomers, respectively. The ice flakes on the surface of the E-rich FClAzo film were intact after 6 min of blue light irradiation, whereas most of the ice flakes were found to be melted in the light-irradiated region of the Z-rich FClAzo film because of the thermal energy release, as shown in Fig. 5d. Extending the irradiation time broadens the heat release area and thus melts more ice flakes. As heat diffuses laterally within the film, melting ice can also be observed in the surrounding area as heat diffuses laterally within the film, as shown in Fig. S26.† The de-icing capability of MOST fuels is attributed to the blue light-triggered heat release that keeps the film surface temperature above the melting point of the ice flakes. The above results demonstrate that the MOST films not only can effectively store photon energy and rapidly release heat in the visible light range but also exhibit excellent de-icing effect in low temperatures environments. Such visible light activated MOST fuels provide wider application opportunities for the thermal management and thermal control of molecular solar thermal systems.

Conclusions

In summary, we have successfully fabricated visible light activated dendrimer solar thermal fuels through the Michael addition reaction between poly(amidoamine) dendrimers and FClAzo derivatives. The novel dendrimer solar thermal fuels not only overcome the challenge of storing photon energy in the visible light range in low-temperature environments but also display the outstanding effect of stable storage. These excellent properties are due to the neighbouring halogen atoms modulating the intramolecular interactions of N=N groups and the favourable chain mobility of the dendrimer at low temperature. The energy density of the G3-FClAzo fuel was about 0.046 MJ kg⁻¹ after harvesting green light, and the storage half-life of the fabricated solar thermal film could reach up to 20.6 days. In addition, the heat release of MOST films under blue light irradiation was visually demonstrated by an infrared imager. The MOST film in low temperature environments (−2 °C) could increase the temperature by 3.7 °C, exhibiting significant de-icing effects. The FClAzo-based dendrimer MOST fuels reported here for stable visible light storage and release in cold environments offer fascinating avenues for the development of high-performance MOST fuels powered under natural sunlight.

Experimental

Synthesis of fluorochloroazobenzene derivatives

The synthesized intermediate (FClAzoC12-OH, 5.00 g, 10.00 mmol) and triethylamine (5.00 mL, 36.00 mmol) were added to a flask with 100 mL of anhydrous tetrahydrofuran and stirred at 0 °C for 30 min, and then a mixed solution of acryloyl chloride (2.50 mL, 30.00 mmol) and anhydrous tetrahydrofuran (10 mL) was slowly added to the solution. The mixed solution was stirred for 6 h at 0–5 °C. After the reaction was completed, the triethylamine salt was removed by filtration, the filtrate was concentrated and the solid was dissolved in dichloromethane and extracted with saturated brine. After the organic layer was dried with anhydrous sodium sulfate, the solvent was concentrated to obtain the crude product. The crude product was purified by column chromatography (ethyl acetate/petroleum ether = 1/3, V/V) to obtain high-purity FClAzo. Yield: 66.2%. ¹H NMR (400 MHz, CDCl₃, δ): 7.42–7.32 (m, 2H, Ar–H), 7.17 (t, 1H, Ar–H), 6.69 (s, 1H, Ar–H), 6.67 (1H, d, Ar–H), 6.44 (d, 1H, CH=CH₂), 6.15 (d, 1H, CH=CH₂), 5.82 (d, 1H, CH=CH₂), 4.17 (d, 2H, –CH₂), 4.05 (d, 2H, –CH₂), 1.46–1.21 (20H, m, CH₂). MS (ESI): [M + H]⁺ = 541.01.

Synthesis of visible light activated dendrimer MOST fuels

FClAzo derivatives (0.78 g, 1.45 mmol) were dissolved in methanol (50 mL), and then the poly(amidoamine) dendrimer (G3, 200 mg, 0.03 mmol) in methanol (5 mL) was slowly added. The reaction was left stirring at 50 °C under a nitrogen atmosphere for 36 h. The solvent was removed by vacuum distillation. The residue was washed several times with 200 mL of ethyl ether. The filtered yellow products were dried thoroughly at 40 °C in a vacuum for 24 h. Yield: 50.2%. ¹H NMR (400 MHz, CDCl₃, δ): 7.42–7.32 (2H, m, Ar–H), 7.17 (1H, t, Ar–H), 6.69 (1H, s, Ar–H), 6.67 (1H, d, Ar–H), 4.05 (34H, d, –CH₂), 3.68 (34H, d, –CH₂), 3.25 (64H, m, CH₂NHCO), 2.80–2.25 (380H, t, –CH₂), 1.92–1.21 (204H, m, –CH₂–). The molecular weight (M_n) and the polydispersity index (PDI) of the fabricated dendrimer MOST fuel (G3-FClAzo) measured by GPC were 1.53 × 10⁴ g mol⁻¹ and 1.67, respectively.

UV-vis absorption spectroscopy

The sample in ethyl acetate (3 × 10⁻⁵ M) was induced by green light (520 nm, 20 mW cm⁻²) to undergo E-to-Z isomerization, which was monitored by using a UV-vis spectrophotometer. The reversion was induced by blue light (420 nm, 20 mW cm⁻²) or under darkness to undergo Z-to-E isomerization. The sample film used for UV-vis absorption spectroscopy was fabricated by a drop-casting method. The sample solution was pressed between two pieces of quartz plates, and the residual solvent was removed by drying overnight in a vacuum at 50 °C. The E-to-Z isomerization and the reversion process were consistent with the samples in solution.

DSC measurement

The high-purity Z-rich isomers were prepared by a solvent-assisted method. 20–30 mg of E-rich sample was dissolved in 1.5 mL of dichloromethane in a quartz cuvette, which was irradiated with green light (520 nm 50 mW cm⁻²) until a metastable state was reached. This E-to-Z isomerization to the photostationary state process and the percentage of Z isomers were determined by UV-visible absorption spectroscopy, as shown in Fig. S20.† Then the solution was dried under reduced pressure in the dark, and the resulting samples were transferred to DSC pans for heat release. The experiments were performed under a N₂ atmosphere from −30 °C to 180 °C with a heating and cooling rate of 10 °C min⁻¹, respectively.

Theoretical calculation

Calculations were performed using the ORCA 5.0 quantum chemistry program package.^65–68 Ground state geometry optimizations and excited states calculations were carried out at the B3LYP-D3/def2-TZVP level.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†].

Author contributions

Xingtang Xu: conceptualization, writing – original draft, methodology. Chonghua Li: data curation, editing, formal analysis. Wenjing Chen: editing, visualization, formal analysis. Jie Feng: data curation, editing, formal analysis. Wen-Ying Li: resources, conceptualization, validation, supervision. Guojie Wang: resources, conceptualization, validation, supervision. Haifeng Yu: resources, conceptualization, validation, supervision.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22308241), Shanxi Province Basic Research Program (Free Exploration Category) Youth Project (202303021222206), and China Postdoctoral Science Foundation (2024T170633 and 2022M722345).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04022h

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