Photomechanical Azobenzene Crystals
Round 1
Reviewer 1 Report
This review describes the photomecanically responsive photochromic crystals especially in azobenzene derivatives. This review contains excellent collections of original papers of recent advances in this scientific field. This article certainly contributes to the scientific community of specialists as well as beginners in this field. I recommend this review to be published. There are some points that I have comments to be considered:
Line 102: "trans-cis photoisomerization in the crystalline state... not to occur at all," there is a report of experiment that trans-azobenzene does not show the isomerization in the crystalline state. See: M. Tsuda, K. Kuratani, Bull. Chem. Soc. Jpn., 1964, 37, 1284-1288. Line 173: "inner-system crossing" should be "intersystem crossing"? As for the ref. 42: Recently, the crawling motion was achieved by using only one visible light source in 4-(methylamino)azobenzene. See: Saito, et al., Chem. Commun., 2019, 55, 9303-9306.Author Response
Thank you for the high evaluation and important comments for our paper. We have revised the manuscript according to the comments.
Reviewer’s Comment 1:
Line 102: trans-cis photoisomerization in the crystalline state... not to occur at all," there is a report of experiment that trans-azobenzene does not show the isomerization in the crystalline state. See: M. Tsuda, K. Kuratani, Bull. Chem. Soc. Jpn., 1964, 37, 1284-1288.
Author’s Response 1:
Line 102 on page 4: According to the comment, we added the reference as [25], "trans-cis photoisomerization in the crystalline state... not to occur at all [25],".
Reviewer’s Comment 2:
Line 173: "inner-system crossing" should be "intersystem crossing"?
Author’s Response 2:
Line 175 on page 6: Yes, we changed "inner-system crossing" into "intersystem crossing".
Reviewer’s Comment 3:
As for the ref. 42: Recently, the crawling motion was achieved by using only one visible light source in 4-(methylamino)azobenzene. See: Saito, et al., Chem. Commun., 2019, 55, 9303-9306.
Author’s Response 3:
Line 284 on page 9: According to the comment, we added a sentence with the reference, “Recently, the crawling motion of crystals of 27 was achieved by using only visible light [47].” Also, the molecular structure was added in Figure 1 as 27.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors review an interesting area of research, mechanical motion in azobenzene crystals. There are many reviews on azo polymers and gels but to my knowledge not the one focusing on crystals. Therefore, I think the review will gain significant attention of the community. It is well written and systematic in treatment of the sub-topics within the main topic. I enjoyed reading it and I am sure that it will become a primary citation in the field. I, therefore, recommend acceptance with two minor comments. First, the readability of Figure 1 can be enhanced (its difficult to read the structures. The authors can perhaps change the line thickness). Second, in the introduction where the authors (line 40-42, page 1) state different stimuli, it will be worthwhile to add biological enzymes in the list of stimuli to which azo bond responds. This facet is overlooked by the community and worth mentioning along with the references (J. Am. Chem. Soc.2013, 135, 14056,J. Am. Chem. Soc.2014, 136, 5872, Polym. Chem.2015, 6, 686, Biomaterials2018, 185, 333).
Author Response
Thank you for the high evaluation and important comments for our paper. We have revised the manuscript according to the comments.
Reviewer’s Comment 1:
First, the readability of Figure 1 can be enhanced (its difficult to read the structures. The authors can perhaps change the line thickness).
Author’s Response 1:
On page 3: According to the comment, we changed the line thickness for the readability of Figure 1.
Reviewer’s Comment 2:
Second, in the introduction where the authors (line 40-42, page 1) state different stimuli, it will be worthwhile to add biological enzymes in the list of stimuli to which azo bond responds. This facet is overlooked by the community and worth mentioning along with the references (J. Am. Chem. Soc.2013, 135, 14056, J. Am. Chem. Soc.2014, 136, 5872, Polym. Chem.2015, 6, 686, Biomaterials2018, 185, 333).
Author’s Response 2:
Line 41-42 on page 1: According to the comment, we added biological enzymes to the sentence with a recent paper, “Responses to various kinds of stimuli, including light, heat, voltage, magnetic fields, pressure, humidity, and biological enzymes have been achieved [2,3].”
[3] Eom, T.; Yoo, W.; Kim, S.; Khan, A. Biologically activatable azobenzene polymers targeted at drug delivery and imaging applications. Biomaterials 2018, 185, 333–347.
Author Response File: Author Response.pdf
Reviewer 3 Report
This review paper of photomechanical azobenzene crystals is overall a nice review and it summaries the recent research progress on the work of solid-state photomechanical azobenzene crystals. Azobenzene is a star-level molecule that can work in both crystal state and when embedded in a polymer matrix. It is of importance to illustrate the photomechanical properties when the molecule function in crystal state in order to get a better understanding of how to design such molecules. I indeed recommend it to be accepted and published in the Crystals after some minor revision.
On page 5, from line 147 to line 153, the author mentioned electron-poor groups and electron-pull substituents, a more standard way to name these groups could be electron-withdrawing groups
On page 5, in the model of dynamical photo-bending section, the author first introduced the classical bimetal model described by Timoshenko, which is based on the difference of coefficient of thermal expansion in metals. This model was further explored and used to study the properties of photomechanical diarylethene crystals later by Kobatake and Kitagawa. It would be more relevant if the author can also cite their papers such as:
Kitagawa D, Iwaihara C, Nishi H, Kobatake S. Quantitative evaluation of photoinduced bending speed of diarylethene crystals. Crystals. 2015 Nov 6;5(4):551-61.
On page 5 and 6, in the model of dynamic photo-bending section, the author mentioned the kinematic models constructed by Naumov et al. with one reference cited which is reference 29. Actually, the models was extended in Naumov’s following paper, which is reference 18 this paper. It would be more solid if the author can also cite reference 18 here.
On page 6, in the chirality induction section, the author should cite the twisting crystal of 9-anthracene carboxylic acid by Bardeen et al, which is an early and nice work on twisting crystals.
Zhu L, Al-Kaysi RO, Bardeen CJ. Reversible photoinduced twisting of molecular crystal microribbons. Journal of the American Chemical Society. 2011 Jul 27;133(32):12569-75.
The other paper that I recommend to cite is the twisting crystal based on the chirality which might be complementary to the topic of this section:
Zhu L, Al‐Kaysi RO, Bardeen CJ. Photoinduced Ratchet‐Like Rotational Motion of Branched Molecular Crystals. Angewandte Chemie International Edition. 2016 Jun 13;55(25):7073-6.
On page 7, from line 230 to line 240, the author mentioned the cocrystal of halogenated azobenzene and put reference 40 as the reference. The author mentioned halogen I- or Br- at the para-position of the fluorinated azobenzene is an electron-rich state that works as a donor, which is a little confusing. Actually, the I- or Br- here is highly polarizable and the fluorinated azobenzene which is normally called perfluorphenyl moiety or group is an electron-deficient group, making them potentially suitable as halogen bond donor.
On page 11, the author mentioned the maximum stress of photomechanical molecular crystals is in a range of 1-50 MPa compared to typical human muscle (0.3 MPa). It would be better if the author can put some references to support these numbers.
On page 11, from line 351 to line 357, the author mentioned the hybridization of crystals and polymers is a promising approach to overcome the limitations. Actually, there are some papers published recently that incorporate molecular crystals with polymers, the author can cite these papers, such as:
Yu Q, Yang X, Chen Y, Yu K, Gao J, Liu Z, Cheng P, Zhang Z, Aguila B, Ma S. Fabrication of Light‐Triggered Soft Artificial Muscles via a Mixed‐Matrix Membrane Strategy. Angewandte Chemie International Edition. 2018 Aug 6;57(32):10192-6. Shi YX, Zhang WH, Abrahams BF, Braunstein P, Lang JP. Fabrication of Photoactuators: Macroscopic Photomechanical Responses of Metal-Organic Frameworks to Irradiation by UV Light. Angewandte Chemie International Edition. 2019 Jul 8;58(28):9453-8.
For the reference citation, I am not asking for citing my papers. Actually, I am not coauthored in any of the references that I suggested above. And also, those papers are widely distributed in different groups. I just would like to try to make this review become better.
Comments for author File: Comments.pdf
Author Response
Thank you for the good evaluation of our paper. We have revised the manuscript according to the comments.
Reviewer’s Comment 1:
On page 5, from line 147 to line 153, the author mentioned electron-poor groups and electron-pull substituents, a more standard way to name these groups could be electron-withdrawing groups
Author’s Response 1:
Line 147–153 on page 5: According to the comment, we changed “electron-poor” and “electron-pull” into “electron-withdrawing”.
Reviewer’s Comment 2:
On page 5, in the model of dynamical photo-bending section, the author first introduced the classical bimetal model described by Timoshenko, which is based on the difference of coefficient of thermal expansion in metals. This model was further explored and used to study the properties of photomechanical diarylethene crystals later by Kobatake and Kitagawa. It would be more relevant if the author can also cite their papers such as:
Kitagawa D, Iwaihara C, Nishi H, Kobatake S. Quantitative evaluation of photoinduced bending speed of diarylethene crystals. Crystals. 2015 Nov 6;5(4):551-61.
Author’s Response 2:
Line 167 on page 5: We added the reference as [31], and modified the sentence as following, “Although Timoshenko’s bimetal model can explain the bending speed of photomechanical crystals [31], it is essential to establish a mathematical model that provides a kinematic explanation of the macroscopic reshaping.”
Reviewer’s Comment 3:
On page 5 and 6, in the model of dynamic photo-bending section, the author mentioned the kinematic models constructed by Naumov et al. with one reference cited which is reference 29. Actually, the models was extended in Naumov’s following paper, which is reference 18 this paper. It would be more solid if the author can also cite reference 18 here.
Author’s Response 3:
Line 179 on page 6: According to the comment, we added a sentence with the reference, “Such kinematic model was further extended as described in their review [19].”
Reviewer’s Comment 4:
On page 6, in the chirality induction section, the author should cite the twisting crystal of 9-anthracene carboxylic acid by Bardeen et al, which is an early and nice work on twisting crystals.
Zhu L, Al-Kaysi RO, Bardeen CJ. Reversible photoinduced twisting of molecular crystal microribbons. Journal of the American Chemical Society. 2011 Jul 27;133(32):12569-75.
The other paper that I recommend to cite is the twisting crystal based on the chirality which might be complementary to the topic of this section:
Zhu L, Al‐Kaysi RO, Bardeen CJ. Photoinduced Ratchet‐Like Rotational Motion of Branched Molecular Crystals. Angewandte Chemie International Edition. 2016 Jun 13;55(25):7073-6.
Author’s Response 4:
Line 198 on page 6: We referred the paper “Reversible photoinduced …”as [34] for the description of twist.
The latter paper, “Photoinduced Ratchet‐Like Rotational Motion of Branched Molecular Crystals”, mentions that the origin of rotational motion is the chirality of branched shape, not the chirality of molecular or crystal structure. We think this paper is a bit out of focus of our review.
Reviewer’s Comment 5:
On page 7, from line 230 to line 240, the author mentioned the cocrystal of halogenated azobenzene and put reference 40 as the reference. The author mentioned halogen I- or Br- at the para-position of the fluorinated azobenzene is an electron-rich state that works as a donor, which is a little confusing. Actually, the I- or Br- here is highly polarizable and the fluorinated azobenzene which is normally called perfluorphenyl moiety or group is an electron-deficient group, making them potentially suitable as halogen bond donor.
Author’s Response 5:
Line 236 on page 7: According to the comment, we changed into “The halogen I- or Br- at the para-position of the electron-deficient perfluorphenyl group is highly polarizable and works as a donor, forming the linear interaction between an accepter of the halogen bond, pyridine.”
Reviewer’s Comment 6:
On page 11, the author mentioned the maximum stress of photomechanical molecular crystals is in a range of 1-50 MPa compared to typical human muscle (0.3 MPa). It would be better if the author can put some references to support these numbers.
Author’s Response 6:
Line 351 on page 11: We added several references to support these numbers as follows, “Regarding the first one, mechanically responsive molecular crystals typically generate a maximum stress in the range of 1–50 MPa at actuation [50, 56, 57]. These values are generally 10–100 times larger than the maximum stress of typical human muscle (0.3 MPa) [58].”
Reviewer’s Comment 7:
On page 11, from line 351 to line 357, the author mentioned the hybridization of crystals and polymers is a promising approach to overcome the limitations. Actually, there are some papers published recently that incorporate molecular crystals with polymers, the author can cite these papers, such as:
Yu Q, Yang X, Chen Y, Yu K, Gao J, Liu Z, Cheng P, Zhang Z, Aguila B, Ma S. Fabrication of Light‐Triggered Soft Artificial Muscles via a Mixed‐Matrix Membrane Strategy. Angewandte Chemie International Edition. 2018 Aug 6;57(32):10192-6. Shi YX, Zhang WH, Abrahams BF, Braunstein P, Lang JP. Fabrication of Photoactuators: Macroscopic Photomechanical Responses of Metal-Organic Frameworks to Irradiation by UV Light. Angewandte Chemie International Edition. 2019 Jul 8;58(28):9453-8.
Author’s Response 7:
Line 355 on page 11: According to the comment, we added the reference as follows, “The hybridization of crystals and polymers is a promising approach to overcome the limitations discussed above [59].”
[59] Yu, Q.; Yang, X.; Chen, Y.; Yu, K.; Gao, J.; Liu, Z.; Cheng, P.; Zhang, Z.; Aguila, B.; Ma, S. Fabrication of light‐triggered soft artificial muscles via a mixed‐matrix membrane strategy. Angew. Chem. Int. Ed. 2018, 57, 10192–10196.
Author Response File: Author Response.pdf
