Construction of Multi‐Stimuli Responsive Highly Porous Switchable Frameworks by In Situ Solid‐State Generation of Spiropyran Switches

Stimuli‐responsive molecular systems support within permanently porous materials offer the opportunity to host dynamic functions in multifunctional smart materials. However, the construction of highly porous frameworks featuring external‐stimuli responsiveness, for example by light excitation, is still in its infancy. Here a general strategy is presented to construct spiropyran‐functionalized highly porous switchable aromatic frameworks by modular and high‐precision anchoring of molecular hooks and an innovative in situ solid‐state grafting approach. Three spiropyran‐grafted frameworks bearing distinct functional groups exhibiting various stimuli‐responsiveness are generated by two‐step post‐solid‐state synthesis of a parent indole‐based material. The quantitative transformation and preservation of high porosity are demonstrated by spectroscopic and gas adsorption techniques. For the first time, a highly efficient strategy is provided to construct multi‐stimuli‐responsive, yet structurally robust, spiropyran materials with high pore capacity which is proved essential for the reversible and quantitative isomerization in the bulk as demonstrated by solid‐state NMR spectroscopy. The overall strategy allows to construct dynamic materials that undergoes reversible transformation of spiropyran to zwitterionic merocyanine, by chemical and physical stimulation, showing potential for pH active control, responsive gas uptake and release, contaminant removal, and water harvesting.


Introduction
6][7][8][9][10] In this context, spiropyrans (SPs) [11][12][13] are among the most prolific representatives of molecular switches due to their prominent multistimuli responsiveness encompassing light, mechanical force, pH, temperature, the polarity of media, metal ions, and redox potential. [11,14,15]][17] In addition, a careful choice of substituents allows for the regulation of the thermal stability of the MC form, the absorption wavelength, and their photostability as well as the robustness to hydrolysis. [14,17]ue to these versatile features, SPs were widely used for the construction of responsive materials including polymers, [17][18][19] Figure 1.Schematic representation of in situ generation of SP switches and their transformations in PSFs.Formation of Indole-PAF by direct insertion of indole moieties covalently connected to the framework (Indole-PAF), followed by post-synthetic modification of the indole moiety and in situ generation of the SP switch in the framework (SP-PSFs), and responsive behavior of SP switch group triggered by different stimuli to form MC-PSF (from left to right).
32][33][34][35][36][37][38][39] The development of porous solids capable of reversibly interconverting between two or more states in response to one or more stimuli offers opportunities to manipulate noninvasively the properties of the material enabling new functions with the advantage of broad flexibility with the choice of the applied stimulus.In this context, the incorporation of SPs into porous systems offers a general and convenient approach to develop multi-stimuli responsive porous materials for applications ranging from cargo capture, delivery, and release to chemical sensing. [13,40]ioneering studies by Klajn and coworkers on the incorporation of SPs in nanoporous materials indicated the partiallypreserved multi-stimuli responsiveness of the SP-moiety with respect to that in solution. [30]Recently, major attention was focused on the incorporation of SPs as guests or pendant moieties in metal-organic frameworks proving a suitable strategy to achieve fast photoisomerization, [31] modulation of conductivity, [33][34][35] capability of water desalination [36] and tuning of gas adsorption. [41,42]However, the effective integration of SPs in highly porous covalent frameworks and the preservation of their isomerization behavior remains a major challenge. [43]In addition, the fabrication of SP-functionalized porous switchable frameworks with large accessible surface areas is unexplored.A typical strategy to construct highly porous materials is to exploit the efficient Yamamoto Ullmann-type coupling reaction, which unlike Suzuki-type coupling, forms highly porous aromatic frameworks (PAFs) with unparalleled surface areas. [44,45]y contrast, as a result of the high reactivity of the Ni(0) complex, the number of functional groups tolerated in these reactions is severely limited.Hence, apart from rare examples of overcrowded alkenes, [46,47] molecular switches are largely incompatible with the Yamamoto coupling reaction, consequently compromising the fabrication methods of switchable aromatic frameworks.
Here, we provide a judicious post-synthetic strategy for the construction of SP-grafted highly porous switchable frameworks (SP-PSFs).We exploit in situ solid-state synthesis of the SPs moieties in the pores of the framework via a pre-anchored indole precursor inside a PAF followed by methylation of the indole and finally the grafting of the SP moiety by condensation of the indoline Fisher-base with a variety of salicylaldehydes.This approach bypasses the limited functional group-tolerance of Yamamoto coupling as a result of the high stability of indole moiety and allows for the fabrication of materials decorated with SP moieties exhibiting Brunauer-Emmett-Teller (BET) surface area as high as 1608 m 2 g −1 and 1.21 cm 3 g −1 total pore volume.Due to the modularity and high precision of this approach, three novel SP-PSFs with distinct properties and functions were fabricated illustrating the scope and versatility of SP switches.In addition to the high porosity, all the synthesized porous materials possess multistimuli responsiveness comparable to that of the SPs in solution, owing to the high porosity minimizing the interactions of the responsive moieties with the aromatic framework.These highly porous and responsive materials were tested for various applications, including acidic gas adsorption and detection, as well as the removal of metal ions from the solution (Figure 1).

Synthesis and Characterization of PAFs and NSP-PSF
The parent indole-grafted porous aromatic framework (indole-PAF) was synthesized by a Yamamoto-Ullmann cross-coupling reaction.Since monofunctional or linear co-monomers are expected to drastically decrease the porosity of the frameworks, [46,48] here a tritopic indole-appended building block (1-Br 3 , Figure 2a) was designed.The copolymerization of tetratopic and dior tritopic monomers by Yamamoto coupling is recognized for yielding materials with reduced surface area in comparison to the frameworks constructed exclusively from the tetratopic monomers. [46,48]With the expectation that a higher porosity would facilitate subsequent post-synthetic transformations inside the pore space while still preventing the tight packing of the spiropyrans, [17] the 4:1 proportion of porogenic tetra-p-bromophenylmethane (TPM-Br 4 ) and functional monomers (1-Br 3 ) was selected to balance the porosity of the materials and the concentration of indole moieties in the framework. [46,48]As fabricated indole-PAF material exhibits high pore capacity as established by N 2 adsorption measurements at 77 K.A typical type IV isotherm with large hysteresis is observed for this material (Figure 2b, grey) indicating the presence of micropores and expanded capacity in the mesopore region at high gas loading.The Langmuir and BET surface areas were as large as 1913 and 1708 m 2 g −1 , respectively.The pore-size distribution was calculated using non-local density functional theory, which indicated that micropores constitute ≈32% of pore volume, with the total pore volume of 1.26 cm 3 g −1 , highlighting the significant tendency of the material to generate mesopores.Subsequent Nmethylation of indole-PAF with methyl iodide afforded indolium-PAF (Figure 2a) with reduced BET surface area and total pore volume to 1310 m 2 g −1 and 0.81 cm 3 g −1 , respectively.Such a significant drop in the available voids is expected given the presence of bulky iodide anions in the structure, as confirmed by energydispersive X-ray spectroscopy (EDS) (Figure S34, Supporting Information).Interestingly, while the microporosity dropped from 0.54 cm 3 g −1 to 0.44 cm 3 g −1 (percentage decrease = 18%), the mesoporosity was more affected with a 48% decrease in mesopore volume.Subsequent condensation of iodolium-PAF with 5-nitro-salicylaldehyde in the presence of base yielded a nitroappended spiropyran porous switchable framework (NSP-PSF).The NSP-PSF material exhibits higher porosity than the preceding indolium-PAF with BET surface area and total pore volume of 1606 m 2 g −1 and 1.08 cm 3 g −1 , respectively (Table S1, Supporting Information, see Section 4.1 for the discussion on the impact of changes in molecular weight of the monomeric units on the porosity of the framework), which is consistent with removal of the loosely-bound bulky iodides from the pores of the framework.The lower BET surface area of the NSP-PSF with respect to the parent indole-PAF framework is in accordance with the formation of the SP pendants, which are bulkier and heavier than the indole moieties, thus occupying more volume in the material and leading to the increase of the molecular weight of the monomeric unit.
These solid-state functionality transformations were further confirmed by diffuse-reflectance UV/Vis (DR UV/Vis) and diffuse-reflectance infrared Fourier-transform (DRIFT) spectroscopies.The NSP-PSF material showed a clearly distinguishable spectrum compared to the predecessor material indolium-PAF and parent material indole-PAF (Figure 2c), indicating the step-wise transformation of functional groups during the post-synthetic solid-state transformations within the solid framework.DRIFT spectroscopy displays the appearance of two additional intense bands for NSP-PSF material centered at 1530 and 1340 cm −1 associated with the N-O asymmetric and symmetric stretching of the nitro-group (Figure 2d), respectively.The two additional bands ≈1100 cm −1 (Figure 2d, blue spectrum) were ascribed to the C─H out-of-plane asymmetric bending modes. [49]ogether with a C spiro -O stretching band centered at 1260 cm −1 , the combined data strongly support the successful formation of the NSP moiety. [50]Furthermore, the presence of characteristic 13 C resonances in 13 C cross-polarization magic angle spinning (CP MAS NMR) allowed to unambiguously confirm the chemical structures of the framework at each stage of the solid-state synthesis (Figure 2e, Tables S2-S7, Figure S29, Supporting Information for the comparison of solid-state (ss) 13 C CP MAS NMR of indole-PAF with monomers). [51]The formation of indolium-PAF was supported by the presence of a signal at 43.8 ppm characteristic of the methyl group adjacent to the charged quaternary nitrogen (CH 3 ─N + ) moiety (Figure 2e, middle spectrum).Accordingly, the disappearance of the resonance at 196.0 ppm ascribed to the iodolium carbon in 2-position (C a , see Figure 2e, middle panel) and the emergence of the resonance at 106.7 ppm characteristic of quaternary spiro carbon (C a , Figure 2e, bottom panel) along with two distinct resonances for diastereotopic methyl groups (e and e') indicated the successful conversion to spiropyran derivative (NSP-PSF) (Figure 2e, bottom spectrum).Importantly, characteristic shifts of the diagnostic "C a " resonance (Figure 2e, see Section 4.2, Supporting Information for further discussion) in the 13 C CP MAS NMR spectra showed quantitative conversion, within the sensitivity limit of the technique, at each stage of the transformation, indicating the remarkably high efficiency of this approach.These results support the fact that the reaction does not generate side products and residues that are irreversibly encapsulated within the pores.The exceptionally high efficiency of this solid-state SP synthesis most likely stems from the high porosity of the material and its hierarchical micro-/meso-porous structure which facilitates mass transport through the solid during the whole process.It should be emphasized that this post-solid-state synthesis strategy successfully proved the formation of NSP-PSF materials not accessible via direct synthetic methods.Thermal gravimetric analysis revealed the high thermal stability of these porous materials, an initial weight loss is observed above 300 °C due to the decomposition of the SP moiety, while the framework is stable up to 450 °C (Figure S36, Supporting Information).Powder X-ray diffraction (PXRD) data confirmed the amorphous nature of the materials (Figure S38, Supporting Information) and scanning electron microscopy (SEM) images (Figures S39-S41, Supporting Information) showed that the porous materials comprise of submicrometer-size particles, common for the frameworks formed via irreversible reactions.

Extension of Strategy for the Synthesis of SP-PSF and SSP-PSF Materials
SP motifs with various substituents at the 6-position of the benzopyran part show dramatic differences in properties, leading to a  variety of functions such as photoacidity, [52][53][54] actuation [18,20,23,55] and multi-stimuli responsiveness. [30,56,57]Therefore, we further extended the generality of our strategy to the fabrication of nonsubstituted and sulfonated SP functionalized materials (denoted as SP-PSF and SSP-PSF, respectively).By the condensation reaction of salicylaldehyde or sodium salicylaldehyde-5-sulfonate with indolium-PAF, we successfully constructed SP-PSF and SSP-PSF materials (Figure 3a), respectively.These two materials showed similar features as their sister NSP-PSF counterparts as both materials maintained high BET surface areas and pore volumes of 1608 m 2 g −1 and 1.21 cm 3 g −1 for SP-PSF and 1440 m 2 g −1 and 0.76 cm 3 g −1 for SSP-PSF, respectively (Figure 3b).At higher partial pressure, both materials considerably differ regarding the polarity of the framework: the less polar SP-PSF displays a swelling behavior, [58] as confirmed by the wide hysteresis in the desorption curve upon nitrogen adsorption at 77 K, SSP-PSF showed much lower uptake and narrower hysteresis loop between adsorption-desorption branches (Figure 3b).Indeed, CO 2 adsorption isotherms collected at 195 K show a comparable behavior owing to stronger interactions of CO 2 with the frameworks which induce similar pore expansion in both frameworks, indicating that the swellability of these PSFs strongly depends on the polarity of the environment and adsorbate (Figure 3b, inset, see Figure S27, Supporting Information and discussion therein).DR UV/Vis spectra of both materials indicate that the responsive pendants are initially in the cyclized SP form as no bands characteristic of the opened MC moieties were observed in the visible region of the spectra (Figure 3c).In both cases, clear C spiro ─O bond stretching bands were observed at 1260 cm −1 in the DRIFT spectra, indicating the successful grafting of the SP motifs in the materials (Figure 3d).An additional band at ≈1095 cm −1 characteristic of the SO 3 − group was observed in SSP-PSF, indicating the successful construction of the sulfonated spiropyran (Figure 3d, pink spectrum). [35,59]Furthermore, in the case of SSP-PSF, EDS spectroscopy proved the presence of both sulfur and sodium elements in the material with concomitant absence of iodine (Figure S35, Supporting Information), indicating the complete transformation of functionalities.Finally, 13 C CP MAS NMR spectroscopy demonstrates the quantitative transformation of functionalities, within the sensitivity limit of the analysis (Figure S31, Supporting Information).SEM images and PXRD (Figures S42,S43,S38, Supporting Information) confirm that both SP-PSF and SSP-PSF materials have similar morphology and are amorphous in nature.

Stimuli-Induced Isomerization of SP-PSFs in the Solid State
The photochemical isomerization behavior of the solid NSP-PSF material was studied by DRUV/Vis and DRIFT spectroscopies (see Figures S4-S8, Supporting Information for the studies of NSP in solution).The initial DRUV/Vis spectrum (Figure 4b, blue spectrum) showed a small absorption band centered at 600 nm which was ascribed to a small amount of opened MC form present in the pristine sample of NSP-PSF material.Upon the irradiation of the material at 365 nm light for 10 s, a dramatic color change from grey to bright blue was observed (Figure 4a), in line with the isomerization of closed SP form to opened MC form via C spiro ─O bond cleavage.This change was accompanied by the drastic increase of the intensity of the broad band centered at 600 nm, characteristic of the open MC-form (Figure 4b, green spectrum).Subsequent illumination at 617 nm for 15 min led to the recovery of the initial spectrum (Figure 4b, light blue spectrum), in line with the reversible photoisomerization between the two forms.DRIFT spectroscopy further confirms the isomerization behavior of NSP-PSF (Figures S45,S46, Supporting Information).The reversible changes in the DRIFT spectra were observed ≈1260 cm −1 upon consecutive UV and Visible light irradiation which can be ascribed to the scission/reformation of C spiro ─O bond, [33] further corroborating the isomerization between closed and open forms.Like the NSP photoisomerization behavior in solution (Figures S6,S8, Supporting Information), the NMC form in the solid state was found to be thermally unstable, and a fast equilibration of the NMC to NSP form was observed at ambient temperature (Figure S51a, Supporting Information).The kinetics of the thermal isomerization of the NMC was followed by DRUV/Vis spectroscopy and found to be almost completed within a few hours at ambient temperature.(Figure S51b, Supporting Information).
Similar to the typical 6-H SP behavior in solution, the SP-PSF material does not show any photo-response under ambient conditions due to the low thermal stability of the 6-H MC that quickly thermalizes back to the SP form. [60]Instead, the switching behavior of SP-PSF material could be triggered upon spontaneous isomerization to the protonated merocyanine (MCH + -PSF) by exposure to gaseous or an aqueous HCl solution (1 m).An obvious color change from grey to orange could be observed by the naked eye (Figure 4c), indicative of the formation of MCH + .DRUV/Vis and DRIFT spectroscopies indicate that illumination of the MCH + -PSF material at 365 nm or in the visible light region leads to the recovery of SP-PSF (Figure 4d; Figure S52, Supporting Information), likely with concomitant expulsion of the volatile acid from the framework.This behavior differs from the one in solution as 6-H spiropyrans typically open to Z-MCH + upon protonation and isomerize to E-MCH + upon irradiation at 365 nm (see Figures S12-S15, Supporting Information).Further experiments proved that the SP-PSF material could be also regenerated from MCH + -PSF in either vacuum or upon heating conditions, which is consistent with the volatility of hydrochloric acid (Figure S54, Supporting Information).This pH-responsive behavior was also observed for the SSP-PSF material upon treatment with gaseous HCl (g) (Figure S53, Supporting Information).
In addition to the pH-responsive behavior, the SSP-PSF material shows humidity-induced isomerization.This unique property stems from the SSP pendant, which is known to be stable in the opened MC form in aqueous media in the dark (see Figures S17-S21, Supporting Information for studies in solutions). [61]After storing under ambient conditions in a sealed vial in the dark for 4 weeks, the greyish SSP-PSF material changed color to purple, indicating isomerization of the SSP pendants (Figure 4e).DRUV/Vis and DRIFT spectra supported the isomerization from SP to MC form in the solid state (Figure 4f, violet UV-vis spectrum), both showing features characteristic of the MC isomer. [35]ue to the presence of the polar sulfonate side group in the material, moisture under ambient conditions may enter the pores of the framework resulting in the increase of the polarity of the environment and therefore leading to the shift in the position of the SSP/SMC thermal equilibrium in favor of the stabilization of the SMC isomer.Indeed, after soaking of SSP-PSF in water, the 13 C CP MAS spectrum showed the complete disappearance of the signal at 106.6 ppm of C spiro ─O (closed form) and the emergence of two new signals at 196.9 and 183.9 ppm revealing the formation of both Z and E isomers of SMC, respectively (Figure S33, Supporting Information).However, upon thermal treatment and water removal, only the Z-SMC reversed back quantitatively to SSP owing to the arrangement of the Z-SMC isomer which favors the formation of the closed form (Figure S33, Supporting Information).Nevertheless, the N 2 adsorption isotherms collected at 77 K of SSP-PSF in its pristine state and after water soaking and activation cycle was almost superimposable, indicating the reversibility of the water uptake without structural changes of the porous material (Figure S28, Supporting Information).4][65]

Multi-Applications with SPs Materials
To showcase the broad applicability of the PSF materials synthesized with this modular approach, a series of experiments testing the utility of these materials under various conditions were performed.Due to the large pore capacity, SP-PSF could find potential applications as an acidic gas sensor or sorbent that could be used to neutralize gas mixtures (Figure 5a).Indeed, when dry gaseous HCl was passed through the column packed with 10 mg of SP-PSF under N 2 flow, a dramatic color change from slight grey to deep orange was observed, indicating the formation of MCH + -PSF.Subsequent irradiation at 365 nm on the column induced the release of the gaseous HCl in N 2 flow and therefore regeneration of the pristine SP-PSF material.The ultimate confirmation of this structural transformation upon HCl (g) exposure from the close-to-open form was provided by 13 C CP MAS NMR spectra which showed the acid-induced opening of the SP-PSF as demonstrated by the disappearance of the resonance characteristic of the SP isomer at 106.2 ppm (C a , Figure 5b left) and the appearance of the new resonance at 182.3 ppm (C a , Figure 5b, middle) characteristic of the MCH + form, further corroborated by an emergence of a single resonance corresponding the chemically equivalent methyl groups (C e ) in flat MC (Figure 5b, middle).Notably, the downfield resonance at 182.3 ppm is diagnostic of the formation of the E-MCH + isomer. [53]Vice versa, after N 2 -flow treatment the 13 C CP MAS NMR spectrum showed recovery of all the features of the closed-form SP-PSF (Figure 5b, right).Thus, solid-state NMR spectroscopy highlights the quantitative (within the accuracy of the method) switching between the SP and the MCH + isomers, demonstrating the full accessibility of the SP switches decorating the pore surface.This process was followed by DR UV/Vis spectroscopy for two cycles which showed limited reversibility on account of the inhomogeneity in the gas flow and illumination of the material (Figure S56, Supporting Information).Nevertheless, this may be a question of experimental setup and in principle, upon optimization of the setup, this material can potentially be used as an acidic gas scavenger, which can be easily reactivated by light-and N 2 -assisted acidic gas removal or when acidified as a release agent for acidic gas upon light irradiation. [66]ue to the stability of the sulfonated MC form in polar media and the well-known capability of MC to bind metal ions, a metal ion binding experiment of the SMC-PSF material was investigated. [30]The SSP-PSF material was found to spontaneously undergo thermal isomerization to the open form in a highly-polar acetonitrile suspension.The presence of SSP-PSF in the CuCl 2 acetonitrile solution led to a significant drop of the adsorption at 460 nm, and a visible color change of the solid, showing the successful removal of Cu 2+ from solution (Figure 5c; Figure S57, Supporting Information, grey spectrum).As observed from the UV/Vis spectra, the absorption band centered at 460 nm is related to the Cu 2+ in acetonitrile.In contrast, the control solution without SSP-PSF showed a much higher concentration of Cu 2+ (Figure S57, Supporting Information, orange spectrum), proving the ability of SSP-PSF material to remove Cu 2+ and serve as a potential scavenger of transition-metal contaminants.Subsequent irradiation of the sample with visible light led to the release of the Cu 2+ ions (Figure S57, Supporting Information, light orange spectrum), corroborating the reversibility of the process.

Conclusion
In summary, by Yamamoto-type cross-coupling reaction, an unprecedented indole-functionalized porous aromatic framework with ultra-high pore capacity in both micropore and mesopore regions was fabricated.Notably, the modular post-synthetic grafting to the pre-anchored indole sites allowed us to obtain SP functionalities as pendants in the nano-porous materials.Solid-state 13 C NMR confirmed the quantitative transformation of functionalities at each step during the solid-state synthesis and N 2 sorption data indicated the post-solid-state synthesis occurring in the entire framework without its collapse.Importantly, the high level of porosity maintained by this approach allows us to fabricate PSF based on SP, which is crucial for the responsive functions as well as the capability of controlling material properties.It is worth to highlight that this is the first time that SP-based aromatic frameworks were constructed with remarkably high pore capacities.We furthermore demonstrate by DRUV-Vis, DRIFT spectroscopies, and solid-state NMR studies, the reversible photochromism of NSP-PSF, acidochromism of SP-PSF and SSP-PSF, ionochromism and humidity-responsiveness of SSP-PSF.The preservation of the multi-stimuli responsive behavior of the SP in the constructed frameworks allowed us to show several areas in which these responsive materials can meet potential applications.Given the simplicity and efficiency of this modular synthetic approach, our work established an elegant strategy for the construction of highly porous yet chemically and structurally stable switchable materials with multi-stimuli responsive behavior.We also provide a more general synthesis strategy useful for other switchable materials with high pore capacity which cannot be accessed via de novo syntheses.Furthermore, we envision that this strategy will allow to graft multiple functionalities within the same material, further increasing the functionality toward desired properties and applications.We are convinced that these switchable materials have the potential to manifest more powerful functions beyond the examples illustrated herein transforming them into a crucial platform toward responsiveness and actuation on the molecular level in the solid state.

Figure 4 .
Figure 4. Multi-stimuli responsiveness of the NSP-PSF, SP-PSF, and SSP-PSF materials.a) Schematic representation and visual changes of reversible photochromism of NSP-PSF.b) DR UV/Vis spectra showing the reversible photochromism of NSP-PSF.c) Schematic representation and visual changes of reversible acid-/photo-modulation of SP-PSF.d) DR UV/Vis spectra of reversible acid-/photo-modulation of SP-PSF.e) Schematic representation and visual changes of reversible moisture/heat modulation of SSP-PSF.f) DR UV/Vis spectra of reversible moisture/heat modulation of SSP-PSF.

Figure 5 .
Figure 5. a) Demonstration of the reversible uptake and release of HCl (g) by SP-PSF material for acidic gas removal.b) 13 C{ 1 H} CP MAS spectra performed at 298 K at a spinning speed of 12.5 kHz with a contact time of 2 ms of pristine SP-PSF (left panel), E-MCH + -PSF (medium panel) and SP-PSF material (right panel) after complete regeneration under nitrogen flow.c) Demonstration of copper ion-induced ring opening of SSF-PSF for metal ion capture.