Ultrafast mechanosynthesis of hydrogen-bonded organic frameworks with UV and NIR photoswitching of photochromic/photothermal behavior†

Yanfeng Gao ^a, Zhe Wang ^a, Tieqiang Wang ^a, Junbiao Wu *^a, Zhuopeng Wang ^a, Zhiqiang Liang ^b and Jiyang Li ^b
aDepartment of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, P. R. China. E-mail: wujunbiao@mail.neu.edu.cn
^bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, P. R. China

First published on 19th August 2024

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Photochromic materials exhibit reversible color changes and a variety of synergistic tuneable properties in response to external light stimuli of different wavelengths.¹ In recent years, photochromic materials have extended from inorganic and organic to inorganic–organic hybrid materials, which has facilitated their application in displays, ink-free printing, decoration and anti-counterfeiting.² However, the majority of crystalline photochromic materials have not yet gained widespread acceptance in the market due to the intricate structural design, complex synthesis, and high cost associated with their production, which make large-scale synthesis unfeasible.³ It is challenging and essential to develop a facile and scalable strategy to synthesize photochromic materials.

Hydrogen-bonded organic frameworks (HOFs) are porous solids that are constructed mainly by non-covalent interactions such as hydrogen bonding, π–π stacking and van der Waals interactions.⁴ These materials have attracted considerable interest due to their large surface area,⁵ solution processability,⁶ washability, regeneration and tuneable structure.⁷ Non-covalent interactions are dramatically altered and combined to create flexible HOFs with practical functions, which have significant potential in various fields such as adsorption, gas separation, proton conduction, catalysis, optics and biological applications.⁸ Looking at the previous synthesis processes of photochromic HOFs, Zhang et al.⁹ used the solvothermal method to heat a zwitterionic viologen derivative and 2,7-naphthalene disulfonate in a mixed solvent of MeOH and H₂O at 40 °C for 24 h, resulting in HOF-FJU-36; Wang et al.¹⁰ employed a hydrothermal method to react 1,4-bis(tetrapyridyl)benzene (bpyb), phosphoric acid, DMF and water in a high-pressure sterilized oven at 150 °C for 2 days to crystallize [H₂(bpyb)](H₂PO₄)₂·2H₂O; Zhou et al.¹¹ dissolved organosulfonic acids and amidine salts in water, then reacted them in a reaction vessel at 70 °C for 2 days to obtain iHOF-18. These processes are often confined to time-consuming solution-based syntheses.

Solvent-free mechanosynthesis has been developed for the scalable fabrication of functional materials, e.g. nanoparticles, graphene, perovskites and metal–organic frameworks (MOFs),¹² and exhibits broad applicability in the formation of various chemical bonds such as metal bonds, covalent bonds, coordination bonds and weak supramolecular interactions. Cao et al.¹³ reported the reticular synthesis of HOFs with predictable structures through mechanochemical methods; Wang et al.¹⁴ synthesized Ni-MOF in 65.57% yield in only 1 min without using any co-solvent. These efforts demonstrate the broad prospects and high efficiency of mechanochemical synthesis in the green research and development of MOFs/HOFs. However, there is still no report on photochromic HOFs.

A known compound |C₁₀N₂H₁₀‖HC₂O₄|₂ (CCDC 846634) is selected to assess the viability of mechanically synthesizing photochromic HOFs. It is first hydrothermally synthesized by Jurić et al.,¹⁵ and then assembled by charge-assisted hydrogen bonding.¹⁶ Wang et al. investigated the electron transfer photochromism of this structure for the first time, showing that hydrogen bonding is the critical factor affecting photochromism.¹⁷ Subsequently, they discovered that its semiconductor features can break the intrinsic positive relation between conductance and temperature.¹⁸ Recently, this structure has been proven to be capable of achieving bidirectional photoswitchable electron transfer under a high-energy NIR laser.¹⁹

Herein, we present a mechanosynthesis strategy to synthesize the photochromic hydrogen-bonded organic framework |C₁₀N₂H₁₀‖HC₂O₄|₂ (denoted as MCS-HOF) composed of 4,4-bipyridine and hydrogen oxalate ions in only 10 seconds without any solvent, and the method is both high-yield (>94%) and scalable (up to 600 g). A systematic investigation was conducted to determine the effect of lapping time and lapping machine frequency on the synthesis and morphology of MCS-HOF. The intrinsic synergy of N–H⋯O hydrogen bonding interaction and π–π stacking interactions endows MCS-HOF with excellent photochromic behavior: insensitivity to visible light, UV specific response, outstanding color contrast, rapid responsiveness and good reversibility. Benefitting from the tunable photogenerated π-aggregate radicals, MCS-HOF exhibits a modulated near-infrared (NIR) photothermal effect at low constant NIR laser (808 nm, 0.27–2.53 W cm⁻²) while rapid photoinduced decoloration at high energy NIR laser (6.5–13 W cm⁻²) owing to the thermal quenching features of radicals. In addition, the photothermal stability and conversion efficiency of MCS-HOF have been further investigated.

The crystal structure of MCS-HOF has been fully elucidated and defined in the literature,^15,16 and consists of a diprotonated 4,4-bipyridine (H₂bpy²⁺) and two hydrogen oxalates (HC₂O₄⁻) arranged alternately and linked by multiple hydrogen bonds (O–H⋯O, N–H⋯O and C–H⋯O hydrogen bonds) to form a two-dimensional network, while a three-dimensional supramolecular network is formed by stacked interactions of the H₂bpy²⁺ (Fig. 1). The shortest N–H⋯O bond in this compound is 2.6436 Å, which provides a possible pathway for photoinduced electron transfer.¹⁷ The infinite π–π stacking interactions (face-to-face distances between adjacent H₂bpy²⁺ from 3.39 Å to 3.76 Å) comprise the π-aggregates that can stabilize radicals and hinder the electron retransfer between the radicals and the HC₂O₄⁻ donors.

The scheme of mechanosynthesis of MCS-HOF is illustrated in Fig. 1. The effects of milling time and milling frequency on the synthesis of MCS-HOF were systematically investigated and optimized (ESI†). Initially, we found that the MCS-HOF could be synthesized very quickly at a milling frequency of 50 Hz for 1 min, so we reduced the synthesis time from 60 to 10 s and still obtained high yields. The average yield was calculated to be between 92% and 96% with some minor fluctuations after repeating three experiments at 50 Hz mechanical frequency and different times (10–60 s) (Table S1, ESI†). Then, we optimized the milling frequency from 10 Hz to 50 Hz with a milling time of 10 s. The yield of MCS-HOF increased from 85% to 95% with the increase of the lapping frequency at different mechanical frequencies (10–50 Hz) (Table S2, ESI†). Powder X-ray diffraction (PXRD) analysis (Fig. S1 and S2, ESI†) shows that the characteristic diffraction peaks of MCS-HOF synthesized at different milling times and mechanical frequencies are in good agreement with the simulated peaks of single crystal data (CCDC 846634). Scanning electron microscope (SEM) images (Fig. S3 and S4, ESI†) show that the morphologies of HOF prepared by ball milling are irregular micron prisms with a size of about 1 μm to 25 μm. In addition, prolonging the milling time and increasing the mechanical frequency cannot significantly change their shape or size. The optimized synthesis of MCS-HOF at 50 Hz mechanical frequency within a milling time of 10 s is successful and ultrafast and the yield is as high as 95%. Compared with the milling time and yield of the HOFs or MOFs in previous literature (Table S3, ESI†), the mechanosynthesis of MCS-HOF is solvent-free, ultrafast and high-yield. With regard to the large-scale synthesis of MCS-HOF, a scale-up synthesis experiment with a total amount of 600 g was carried out by increasing the feed amount of the two starting reactants, H₂C₂O₄ and C₁₀H₈N₂, and the product has good crystallinity and high yield (90%), as well as excellent and recyclable photochromic behavior (Fig. S5, S6 and Table S4, ESI†). This method offers a rapid effective synthesis approach and provides a promising foundation for the large-scale production of HOFs in industrial contexts.

MCS-HOF-50Hz-10s was exposed to different light sources to investigate its photochromic features in detail. Interestingly, no obvious photochromic behavior is observed under visible light irradiation (300 W xenon lamp with 420 nm cut-off filter) (Fig. 2b), which ensures that MCS-HOF can be used as a visible-blind photo-sensor. The subsequent light response test showed that MCS-HOF-50Hz-10s is photoactive to UVA (365 nm, 24 W), UVB (311 nm, 20 W) and UVC (254 nm, 16 W) (Fig. 2a and b) and exhibits a high contrast color change from colorless to purple, the color getting deeper accompanied with shorter UV light wavelength at the same irradiation time.

In situ time-dependent UV-vis spectra reveal three new adsorption bands at 385 nm, 552 nm and 593 nm upon 20 s UVC irradiation (Fig. 2c), and the intensity of these peaks increases with prolonged irradiation time, accompanied by the gradual color change of the irradiated samples (denoted as MCS-HOF-P) from colorless to deeper purple, similar to the literature and attributed to the photoinduced electron transfer (PIET) generated H₂bpy˙⁺ radicals.¹⁸In situ time-dependent UV-vis spectra monitored at 552 nm indicated that the PIET process follows first-order reaction kinetics, with a rate constant k_obs of 0.0063 s⁻¹ (Fig. S9, ESI†). MCS-HOF-P can be maintained under ambient conditions for five weeks with no detectable color change and only a slight decrease in absorbance intensity (Fig. S10, ESI†), indicating an ultralong-lived charge-separated state and excellent color stability upon UV irradiation, which can be attributed to the dense packing mode of π–π stacking interactions that stabilizes the generated radicals and prevents the electron retransfer between the radicals and oxalates. The colored samples can revert to the original colourless state upon heating at 100 °C for 10–60 min (Fig. 2a and Fig. S11, ESI†). UV-vis absorption spectra demonstrated that the coloration–decoloration process can be repeated at least 5 times with repeated UV irradiation and heating of the sample (Fig. S12, ESI†). Electron paramagnetic resonance (EPR) spectra further confirmed the PIET process during the photochromic transformation, showing no detectable signal from the original samples but an obvious signal with a g-value of 2.0032 (close to the g-value of free electrons at 2.0023) is observed after UVC irradiation and it disappears after thermal bleaching (Fig. 2d). UV-vis spectra (Fig. S11, ESI†), XRD patterns (Fig. S13, ESI†) and IR spectra (Fig. S14, ESI†) of the colored MCS-HOF-P and faded MCS-HOF-H are good agreement with the original sample, indicating that the coloration–decolorization process does not change the structure with no photolysis or rearrangement before and after irradiation.

Wang et al.¹⁹ found the photoinduced excitation of π-aggregate radicals retransfer quenching under high-energy NIR laser (5 W cm⁻²) and achieved the rapid NIR trigger decoloration. We have reported the modulated NIR photothermal conversion based on the photoinduced π-aggregate radical generated absorption bands in the red and IR spectra under low constant NIR laser density (0.23–1.27 W cm⁻²).²⁰ In view of the UV-vis-IR spectra, the colored sample emerges new absorption bands in the red and IR region (738 nm, 821 nm and 1360 nm) compared with the original sample (Fig. S15, ESI†), accompanied by the formation of dense packing H₂bpy˙⁺ π-aggregate radicals. This confers MCS-HOF-P possibility to NIR photothermal conversion, which can be modulated by tunable π-aggregate radicals generated through a UV-induced coloration process. MCS-HOF-P under UVC irradiation was chosen to evaluate the photothermal performance. MCS-HOF-P is ground into a powder and then fixed on a piece of quartz glass to form a uniform film (1 cm × 1 cm). Under the irradiation of an 808 nm laser (1.07 W cm⁻²), the temperature of the blank quartz glass only increased by 0.6 °C, and the temperature of a piece of quartz glass coated with a film of uncolored sample just increased by 1.6 °C. As expected, the MCS-HOF-P displays a gradually rising trend of temperature with the extension of the UVC irradiation time because of the gradually increasing content of H₂bpy˙⁺ π-aggregate radicals and the temperature gets saturated at 48.9 °C (ΔT_max = 26.3 °C) (Fig. 3a). This means that the photothermal conversion temperature can be modulated through the photoinduced time by generating tuneable π-aggregate radicals and the radicals remain stable in a certain temperature range. Moreover, the cyclic tests show that MCS-HOF-P has negligible attenuation of the photothermal conversion performance for at least five repeated measurements at 808 nm laser irradiation (1.07 W cm⁻²), further confirming the high stability of the H₂bpy˙⁺ π-aggregate radicals within a certain temperature range (Fig. 3b). The photothermal conversion temperature of MCS-HOF-P has a linear relationship with the photoresponse time under the constant NIR laser power (Fig. S16, ESI†), which provides the possibility of precise temperature control within 30–49 °C. The photothermal conversion performance of the MCS-HOF-P increases linearly with NIR laser power from 0.27 to 2.53 W cm⁻² (Fig. 3c), indicating that the temperature can also be controlled by altering the laser power and making the MCS-HOF-P capable of bidirectional control of the photothermal conversion temperature via NIR laser power and UVC exposure time. According to the cooling curves (Fig. S17 and S18, ESI†) and the relative absorbance value (A_808nm = 0.41073†) of MCS-HOF-P, the conversion efficiency η can be calculated to be 61.86%, which is comparable to the reported photothermal MOFs.^20,21

As a proof of concept, the “MCS” letter pattern was fabricated with MCS-HOF powder and the temperature images of the samples were captured by an IR thermal camera (Fig. 3d). Under 808 nm NIR laser irradiation (1.07 W cm⁻²), the “MCS” pattern becomes brighter and brighter with increasing UVC irradiation time, confirming that the photothermal conversion temperature can be adjusted by the UVC irradiation time and indicating that the MCS-HOF-P possesses high quality imaging ability, which provides potential applications for bioimaging in photothermal therapy and smart windows.

To verify whether MCS-HOF-P can be bleached under the irradiation of a high-power NIR laser, the temperatures of MCS-HOF-P under high-power NIR laser irradiation (6.5–13 W cm⁻²) were recorded (Fig. S19, ESI†), and it can be seen that the temperature of MCS-HOF-P was sharply increased up to 106–120 °C within only 1 s, and then decreased to 71–85 °C within 5 min, accompanied by an obvious bleaching of the sample color consistent with the thermal quenching of radials. In situ UV-vis absorption spectra show that the absorbance at 593 nm of the purple samples obtained after 80 s of UVC irradiation decreased significantly, and the absorbance under 808 nm laser irradiation (6.5 W cm⁻²) for only 1 s decreased by 60%. After continuous stimulation for 30 s, the absorbance decreased by 84% (Fig. 3e), and the color has almost completely faded, while the structure remained unchanged (Fig. S20, ESI†). Noticeably, the complete decoloration of the sample utilized for an absorption test is difficult because the sample cannot be thoroughly irradiated in all areas, so the absorbance of the sample is obviously changed but cannot be restored to the original state (Fig. 3f).

In conclusion, we present a facile and ultrafast solvent-free mechanosynthesis of hydrogen-bonded organic frameworks |C₁₀N₂H₁₀‖HC₂O₄|₂ with UV and NIR bidirectional photoswitching of photochromic/photothermal behavior. The reaction time can be reduced to 10 s, and the method is both high-yield (>94%) and scalable (up to 600 g). Owing to the tunable photogenerated π-aggregate radicals through photochromic behavior, MCS-HOF exhibits a modulated NIR photothermal effect under a low constant NIR laser (808 nm, 0.27–2.53 W cm⁻²) while rapid photoinduced decoloration under a high-energy NIR laser (6.5–13 W cm⁻²) due to the fast thermal quenching of radicals. This work not only provides a novel strategy for the large-scale synthesis of photochromic HOFs but also sheds light on exploring materials with UV and NIR photoswitching of photochromic/photothermal behavior.

This work was financially supported by the Natural Science Foundation of Liaoning Province (No. 2022-MS-117), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (No. 2023-13), and China Postdoctoral Science Foundation (No. 2021MD703898). Special thanks are due to the instrumental analysis from the Analytical and Testing Center, Northeastern University.

Data availability

All data supporting this research are included in the main article and/or ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, supporting figures and tables. See DOI: https://doi.org/10.1039/d4cc03594a

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