Warm/cool-tone switchable thermochromic material for smart windows by orthogonally integrating properties of pillar[6]arene and ferrocene

Functional materials play a vital role in the fabrication of smart windows, which can provide a more comfortable indoor environment for humans to enjoy a better lifestyle. Traditional materials for smart windows tend to possess only a single functionality with the purpose of regulating the input of solar energy. However, different color tones also have great influences on human emotions. Herein, a strategy for orthogonal integration of different properties is proposed, namely the thermo-responsiveness of ethylene glycol-modified pillar[6]arene (EGP6) and the redox-induced reversible color switching of ferrocene/ferrocenium groups are orthogonally integrated into one system. This gives rise to a material with cooperative and non-interfering dual functions, featuring both thermochromism and warm/cool tone-switchability. Consequently, the obtained bifunctional material for fabricating smart windows can not only regulate the input of solar energy but also can provide a more comfortable color tone to improve the feelings and emotions of people in indoor environments.

In this very nice manuscript, the thermo-responsiveness of ethylene glycol chain-modified pillar[6]arene (EGP6) and the redox-induced reversible color switching between warm and cool tune of ferrocene/ferrocenium groups are orthogonally integrated into a system to achieve the fabrication of warm/cool tone-switchable thermochromic materials with cooperative and noninterfering dual functions. The obtained bifunctional material for fabricating smart windows can not only regulate the input of solar energy but also can provide a more comfortable in-door environment to improve the feelings and emotions of indoor people. Overall, the results presented in this manuscript are of interest to the readers. Furthermore, they have good scientific value and novelty. Therefore, I suggest the acceptance for publication after the following minor points are addressed: 1. One of the following important paper about pillararenes was missed: Acc. Chem. Res. 2012Res. , 45, 1294Res. -1308. In this manuscript, the authors use ethylene glycol chain-modified pillar[6]arene (EGP6) to fabricate the hydrogel Fc-gel•EGP6 and claimed that the water absorption capacity of the generated Fc-gel•EGP6 hydrogel was significantly improved which resulted in the drastically swelling property and enhanced transparency of Fc-gel•EGP6 hydrogel than that of the dry Fc-gel. But there is no reasonable explanation about this dramatic phenomenon. So, a detailed investigation of the water absorption capacity and the enhancement of transparency of the Fcgel•EGP6 hydrogel should be provided. 3. Ethylene glycol chain-modified pillar[6]arene (EGP6) shows thermo-responsiveness and the ferrocene moiety exhibits reversible transformation between orange and green ferrocene/ferrocenium groups under redox-stimuli in the Fc-gel•EGP6 hydrogel. Furthermore, the author said that EGP6 was used in this system to obtain the reported hydrogel because the hostguest interaction property between EGP6 and ferrocene. However, from the observation of present situation, the host-guest interaction between EGP6 and ferrocene made no contribution for the fabrication of the hydrogel. So, the review just wonders what the role of the host-guest interaction between EGP6 and ferrocene for the fabrication of the hydrogel is in this system. 4. Variable-temperature 1H-NMR study should be performed to investigate the thermoresponsiveness of the host-guest interaction between EGP6 and ferrocene. 5. The authors investigated the optical transparency of Fc-gel•EGP6 hydrogel by UV-vis transmission spectrum and the hydrogel showed a high transmittance from 79.4% to 89.6% in the near infrared region. As we known, the transparency is proportional to the concentration of the compound contained in the material. So, the transparency of Fc-gel•EGP6 hydrogel at different concentrations of the gelators should be investigated here. 6. To construct smart windows, the stability of the hydrogel is one of the most important performance. So, the investigation of the stability of the hydrogel under different temperatures (25 ºC-45ºC as mentioned by the authors in the manuscript) was strongly recommended. 7. The host-guest interaction between Fc+ and EGP6 was investigated in Supplementary Figures 9-11, but the data presented here was about the study of EGP6 and Cob + PF6−, please check! Reviewer #2 (Remarks to the Author): The authors reported a new type of coating material potentially for smart window applications, which can switch color and transparency in response to temperature and redox spices. While the integration of two stimuli-responsive materials to create dual-functional materials looks interesting, this work lacks of enough significance and novelty to be published in Nature Communication. First of all, the idea of integrating two functional materials to develop multi-responsive materials is not new. For example, in a paper recently published in Advanced Functional Materials (Adv. Funct. Mater. 2017, 27, 1702784), the authors also combined ferrocene derivative with a thermoresponsive polymer to develop a thermal/redox dual-functional material for smart windows. Depending on the salt concentration, the material becomes opaque at a temperature ranging from 33-66 C, demonstrating a better temperature tunabilty than the material reported in this manuscript. Second, for smart window applications, the change of color by adding/immersing into redox spices is not quite feasible. Also, the color change time (8 minutes) seems too long for real life applications. If the material can be electrochromic and can change its color at a faster speed, the importance of this work would be largely enhanced. Lastly, although the authors showed some reproducibility result, the number of cycles is not enough for applications like smart windows. Reproducibility experiments with more cycles are encouraged. Given the concerns mentioned above, I could not recommend publication of this work on Nature Communication at the present form.
Reviewer #3 (Remarks to the Author): Manuscript by Sai Wang et al., " Warm/cool tone switchable thermochromic material for smart windows by orthogonally integrating the unique properties of pillar[6]arene and ferrocene " presents a novel orthogonal integration strategy of different properties for smart windows. The thermo-responsiveness of ethylene glycol chain-modified pillar[6]arene (EGP6) and the redoxinduced reversible color switching between warm and cool tune of ferrocene/ferrocenium groups are integrated into a system to achieve the fabrication of warm/cool tone-switchable thermochromic materials with cooperative and non-interfering dual functions. However, some details are not sufficient for publishing in the present form and major revision are needed. The following questions should be addressed before publishing. 1. The UV-Vis-NMR measurement is not in the full spectrum of solar energy (to 2500nm). Energy modulation need to be considered from visible range to NIR range. 2. From the paper, researchers only mentioned the thickness of dry Fc-gel, can you provide the thickness of samples in Fig 4? The very low luminous transmission is a concern. Reducing the thickness should help to enhance the luminous transmission to be used in thermochromic materials. 3. Durability is always a problem for organic smart window, for the durability test, 10 cycles were not enough for both transmittance change and redox reaction, is it possible to test at least 50 cycles or its better to test until the gel will not change color and transmittance to understand the limitation of this material (Fig 6) 4. Since it is a new type of thermochromics material, please characterize it in a standard way, please calculate the Tlum and detalTsol. and the calculation details is listed in the paper https://doi.org/10.1016/j.jallcom.2017.10.045 and compare with other thermochromic materials as summerized in 10.1039/C6TC02694J. 5. The authors provided a new idea to integrate the thermochromic composite with the color switchable compound. However, the idea is lack of proofing-of-concept. The authors demonstrated the gels with different oxidation state by soaking into different precursors. However, the capability of such a color-switch approaching to the real-life application has not been investigated. I suggest that the authors could fabricate device to show a demo that based on this new material to proof the concept. 6. To continue from previous point, How to immerse the hydrogel film into the respective solution for redox reactions in real application? Whereas, since this is a research paper, these concerns are less critical. Maybe share a few strategies to put this material in real appcliations.
The following is a point-to-point response to the reviewers' comments (for your convenience, we repeat the referee's comments below in black, followed by our reply in blue).

Comments to the Author
"In this very nice manuscript, the thermo-responsiveness of ethylene glycol chain-modified pillar [6]arene (EGP6) and the redox-induced reversible color switching between warm and cool tune of ferrocene/ferrocenium groups are orthogonally integrated into a system to achieve the fabrication of warm/cool tone-switchable thermochromic materials with cooperative and non-interfering dual functions. The obtained bifunctional material for fabricating smart windows can not only regulate the input of solar energy but also can provide a more comfortable in-door environment to improve the feelings and emotions of indoor people. Overall, the results presented in this manuscript are of interest to the readers. Furthermore, they have good scientific value and novelty. Therefore, I suggest the acceptance for publication after the following minor points are addressed:"

Response:
We greatly appreciate Reviewer 1 for his/her positive evaluation and kind recommendation of publishing our manuscript in Nature Communications after minor revisions. And all of the issues suggested by Reviewer 1 have been addressed accordingly in detail as follows, and the revisions have also been made in the Revised Manuscript and Supplementary Information.

"In this manuscript, the authors use ethylene glycol chain-modified pillar[6]arene (EGP6) to fabricate the hydrogel Fc-gel•EGP6 and claimed that the water absorption capacity of the generated Fc-gel•EGP6 hydrogel was significantly improved which resulted in the drastically swelling property and enhanced transparency of Fc-gel•EGP6 hydrogel than that of the dry Fc-gel. But there is no reasonable explanation about this dramatic phenomenon. So, a detailed investigation of the water absorption capacity and the enhancement of transparency of the Fc-gel•EGP6
hydrogel should be provided."

Response:
Many thanks for the profound suggestions. According to reviewer 1's suggestion, we have provided more detailed explanations about this dramatic phenomenon in the revised Manuscript. Upon adding EGP6 to the Fc-gel, due to the formation of hydrophilic EGP6-ferrocene inclusion complexes, the original hydrophobic ferrocene groups were transformed into hydrophilic moieties, which resulted in the obviously improved water absorption capacity and drastically swelling property. On the other hand, the original hydrophobic domains of Fc-gel were formed by physical cross-linking of the hydrophobic ferrocene groups and the gel exhibited a heterogeneous structure as confirmed by SEM image, which resulted in the light scattering, light 4 refraction, and light reflection. However, with respect to the formed Fc-gel•EGP6 hydrogel, the hydrophilic EGP6-ferrocene inclusion complexes eliminated the original hydrophobic domains and the whole polymer chains could be well integrated into water, so that the polymer chains had hardly any effects on the transmittance of the generated Fc-gel•EGP6 hydrogel. Thus, the transparency of Fc-gel•EGP6 hydrogel was enhanced compared to that of the dry Fc-gel. In addition, the homogeneous morphology with uniform porous network structure of the formed Fc-gel•EGP6 hydrogel further confirmed the above point.

"Ethylene glycol chain-modified pillar[6]arene (EGP6) shows thermo-responsiveness and the ferrocene moiety exhibits reversible transformation between orange and green ferrocene/ferrocenium groups under redox-stimuli in the
Fc-gel•EGP6 hydrogel. Furthermore, the author said that EGP6 was used in this system to obtain the reported hydrogel because the host-guest interaction property between EGP6 and ferrocene. However, from the observation of present situation, the host-guest interaction between EGP6 and ferrocene made no contribution for the fabrication of the hydrogel. So, the review just wonders what the role of the host-guest interaction between EGP6 and ferrocene for the fabrication of the hydrogel is in this system."

Response:
Thanks a lot for the profound comments. In this system, the host-guest interaction between EGP6 and ferrocene was a necessary factor for the fabrication of the swollen hydrogel. Firstly, the host-guest interaction between EGP6 and ferrocene moiety was confirmed in aqueous solution by using a water-soluble linear polymer mPEG-Fc as a model guest. And then, when dry Fc-gel was immersed in the EGP6 aqueous solution, EGP6 could interact with the ferrocene moiety in the Fc-gel to form hydrophilic EGP6-ferrocene inclusion complexes. Thus, the original hydrophobic domains formed by ferrocene groups disappeared, and accordingly, the Fc-gel could absorb a large amount of water and swelled dramatically to achieve the

Response:
Thanks a lot for the helpful suggestions. The thermo-responsiveness of the host-guest interaction between EGP6 and ferrocene moiety were investigated by variable-temperature 1 H-NMR spectroscopy ( Figure R1).
The results showed that when an aqueous solution of EGP6 and mPEG-Fc was heated up to 45 °C, the 5 chemical shifts of ferrocene signals of the model guest mPEG-Fc returned to the uncomplexed state.
However, the complexation between EGP6 and mPEG-Fc re-formed after decreasing the solution temperature to 25 °C. Therefore, the complexation between EGP6 and mPEG-Fc can be reversibly controlled by heating and cooling. In addition, Figure R1 and the corresponding descriptions were also added in the revised Supplementary Information as Supplementary Figure 12.  Figure   R2, and the results were also added in the revised Supplementary Information as Fig. 7 instead of the original Supplementary Fig. 7. For the Fc-gel, the original hydrophobic domains were formed by physical crosslinking of the hydrophobic ferrocene groups and the gel exhibited a heterogeneous structure as confirmed by 6 SEM image, which led to the light scattering, light refraction and reflection. When the Fc-gel was immersed in pure water, the polyacrylamide-based main chains could absorb water, which caused that the heterogeneous structure was improved and the transmittance of the hydrogel was enhanced. Upon adding EGP6 solution to the Fc-gel, the original hydrophobic domains composed by ferrocene groups gradually transformed into hydrophilic EGP6-ferrocene inclusion complexes, which dramatically improved the water absorption ability of Fc-gel and the obtained well-swollen Fc-gel•EGP6 hydrogel showed enhanced transparency. From Figure R2, it was found that the transmittance of hydrogel was enhanced more efficiently with a low concentration of EGP6 than that of with a higher concentration. And at higher concentrations of 15 mM and 20 mM, there was hardly any enhancement in the transmittances. The main reason might be that the heterogeneous structure could be improved more efficiently at beginning upon adding EGP6, while the remained a small portion of heterogeneous structure was too less to be transformed for enhancing the transmittance of the hydrogel. In addition, the above investigations have been added in the revised Supplementary Information as Supplementary Fig. 7 in Part S4. Response: Many thanks for the profound suggestions. The main factor determining the stability of Fc-gel•EGP6 hydrogel is its capability to hold EGP6 within the hydrogel without leaking out from the hydrogel backbone. 7 To investigate the stability of Fc-gel•EGP6 hydrogel, we alternately exposed the hydrogel to air at 25 °C and then to water at 40 °C for 50 and 100 cycles, respectively. Then the water medium was measured by UV-Vis Spectroscopy. As shown in Figure R3, there was almost no absorption signal at 290 nm (a characteristic absorption peak of EGP6), which revealed that almost no EGP6 had leaked out from the hydrogel. So the stability of the Fc-gel•EGP6 hydrogel is very good. In addition, the above investigation of the stability of the hydrogel under different temperatures was also added in the revised Supplementary Supplementary Figures 9-11 Response:

"The host-guest interaction between Fc+ and EGP6 was investigated in
We greatly appreciate Reviewer 2's comments. However, we couldn't bring ourselves to agree with his/her view that "this work lacks of enough significance and novelty". The idea proposed in our work is based on an orthogonal integration strategy of different properties, which was proposed for the first time. In this work, the thermo-responsiveness of EGP6 and the redox-induced reversible color switching between warm and cool tune of ferrocene/ferrocenium units were orthogonally integrated into a supramolecular system to achieve the fabrication of warm/cool tone-switchable thermochromic materials with cooperative and non-interfering dual functions. The obtained hydrogel material could be potentially applied for fabricating smart windows which can not only regulate the input of solar energy but also can provide a more comfortable indoor environment to improve the feelings and emotions of indoor people. Meanwhile, both in warm and cool tones, temperature regulation could be achieved by switching the transmittance of Fcgel•EGP6 hydrogel between transparency and opacity. Notably, these two functions were cooperative and non-interfering. So we believe that this orthogonal integration strategy of different properties could provide a new direction for design and development of novel functional materials. In addition, according to the profound suggestions of Reviewer 2, we have further improved the quality of our manuscript and demonstrated the significance and novelty of this work in details in the following parts. Thanks a lot for the profound comments. The paper recently published in Advanced Functional Materials (Adv. Funct. Mater. 2017, 27, 1702784) reported a very nice work. In their work, a thermal/redox dualfunctional material for smart windows was successfully fabricated based on organometallic poly(ionic liquid)s. The switching of light transmittance could be achieved by temperature regulation or redox stimuli.
That's a very delicate design. However, redox stimuli in our work were used to switch between warm color and cool color, which could provide a more comfortable in-door environment to improve the feelings and emotions of indoor people. Meanwhile, both in warm and cool tones, temperature regulation could be achieved by switching the transmittance of Fc-gel•EGP6 hydrogel between transparency and opacity.
Notably, these two functions were cooperative and non-interfering.
Moreover, although ferrocene was used both in their paper and our work, the designs and applications of ferrocene were quite different. In their paper, ferrocene moiety was used to synthesize organometallic poly(ionic liquid)s and change the LCST-type transition temperature via redox stimuli, which could achieve the switching of light transmittance. However, in our work, the ferrocene group was not only applied to construct EGP6-ferrocene supramolecular inclusion complexes based on host-guest interaction to achieve the dramatically enhanced swelling and transparency of the hydrogel material, but also was used to switch the color of the material between warm and cool tune to provide a more comfortable in-door environment for improving the feelings and emotions of indoor people.
Furthermore, an orthogonal integration strategy of different properties was proposed for the first time. In our work, the thermo-responsiveness of EGP6 and the redox-induced reversible color switching between warm and cool tune of ferrocene/ferrocenium units are orthogonally integrated into a supramolecular system to achieve the fabrication of warm/cool tone-switchable thermochromic materials with cooperative and noninterfering dual functions. So we believe that this orthogonal integration strategy of different properties could provide a new direction for design and development of novel functional materials. Response: Many thanks for the profound comments and helpful suggestions. Just as Reviewer 2 said, a feasible and practical way for smart window is very important for the real-life applications, such as electrochemical triggers. In this Fc-gel•EGP6 hydrogel system, thermochromic performance depends on the environmental 10 temperature, while color switching between warm and cool tune mostly depends on the human's comfort demands and controlled by a feasible redox stimuli. According to the previous work (Chem. Commun., 2013, 49, 5085-5087;Macromolecules 2015, 48, 4403-4409;J. Am. Chem Soc. 2010, 132, 9268-9270), electrochemistry methods including cyclic voltammetry (CV) and square-wave voltammetry (SWV) studies indicated that the pillararene-ferrocene inclusion complexes have stable oxidation and reduction peaks.
Therefore, we have fabricated an electrochemical device based on Fc-gel•EGP6 hydrogel to achieve the color switching between warm and cool tune by electrochemical triggers. As shown in Figure R4, after being immersed in 0.1 mol/L KCl aqueous solution, the Fc-gel•EGP6 hydrogel was sandwiched between two pieces of ITO conductive glass to form a simple device. By holding the potential at +4.0 V for 6 min, the color of the material in the device could be completely transformed from orange to green due to the electrochemical oxidation of ferrocene to ferricenium cation. After further holding the potential at -4.0 V for 6 min, Fc-gel•EGP6 hydrogel was reduced and the color of the material returned to orange again. The above investigations have been added in the revised Manuscript as Fig. 7.  Response: Thanks a lot for the profound suggestions. Actually, we have investigated much more cycles, the original repeatability experiments were only the first ten cycles. According to Reviewer 2' suggestion, we have provided the whole cyclic experiments in the revised Manuscript as Fig. 6. The warm/cool tone-switchable function and optical performance of Fc-gel•EGP6 hydrogel were hardly altered after more than 100 thermal or redox cycles as shown in Figure R5. Thus, the Fc-gel•EGP6 hydrogel exhibited high reproducibility and good reversibility in the processes of warm/cool tone-switching and thermochromism..  Response: Many thanks for the profound suggestions. Transmittance spectra of Fc-gel•EGP6 hydrogel with the wavelength range from 300 to 2000 nm were measured at 25 °C and 40 °C, which were provided in the revised Supplementary Information as Supplementary Fig. 14. The result also revealed that the Fc-gel•EGP6 hydrogel had an excellent property of thermochromism. However, due to the limitation of our current instrument, the transmittance spectra could only be measured from 300 to 2000 nm, but it had almost no influence on our results. In addition, we found that some reported work (Adv. 2. " From the paper, researchers only mentioned the thickness of dry Fc-gel, can you provide the thickness of samples in Fig 4? The very low luminous transmission is a concern. Reducing the thickness should help to enhance the luminous transmission to be used in thermochromic materials." Response: Many thanks for the helpful suggestions. The thickness of Fc-gel immersed in pure water is about 1 mm, and the thickness of the formed Fc-gel•EGP6 hydrogel in Fig. 4 is about 2.8 mm. The above data were also provided in the Figure Caption of Fig. 4 in the revised Manuscript. Moreover, we will adopt Reviewer 3's nice suggestion to further reduce the thickness of the hydrogel, and even try to get a membrane-like hydrogel material in the future study. Hopefully, this will greatly improve the luminous transmission of the obtained material for its real applications as thermochromic materials. 3. "Durability is always a problem for organic smart window, for the durability test, 10 cycles were not enough for both transmittance change and redox reaction, is it possible to test at least 50 cycles or its better to test until the gel will not change color and transmittance to understand the limitation of this material (Fig 6)" Response: Many thanks for the profound suggestions. Just as aforementioned ( Figure R5), we have investigated the durability of Fc-gel•EGP6 hydrogel for more than 100 thermal or redox cycles. The results showed that Fcgel•EGP6 hydrogel exhibited high reproducibility and good reversibility in the processes of transmittance change and warm/cool tone-switching, which were provided in the revised Manuscript as Fig. 6.