Short Fluoroalkanes Suppress a Thermally-Induced Phase Transition in a Layered Organic Crystal

Round 1

Reviewer 1 Report

This is an interesting study that reports on a chemical modification of an organic material capable of suppressing a thermally induced phase transition. This is achieved by substituting the propyl chains with pentafluoropropyl groups, which create multiple fluorine-based interactions that effectively stabilize the crystal structure.

The findings are clearly presented. The claims made are substantiated by solid experimental evidence and well-supported by the accompanying models, rendering the text an enjoyable read. The evidence of a phase transition at approximately 175 K, as illustrated in Figure 2C through Raman spectroscopy, is very clear.

This study is relevant to the scientific community and warrants publication. However, having reviewed the manuscript, two pertinent questions have arisen, which, in my view, would enhance the manuscript's content. Two points may deserve some additional discussion.

Point 1

Figure 2C suggests the presence of a hysteresis effect when combining scanning down (cooling) and scanning up (heating) in a full loop. Could this hysteresis phenomenon be explicitly represented and discussed in the manuscript?

Point 2

The data presented in the manuscript is confined to the temperature range between 165 and 190 K. The behavior of the material between 200 and 300 K is not reported. Are there additional phase transitions or any other anomalous behaviors in this temperature range? It is somewhat surprising that only one phase transition is reported, and one might anticipate the presence of multiple transitions.

Incorporating discussions related to these aspects would enhance the manuscript's overall quality.

Author Response

Response to Reviewer 1

This is an interesting study that reports on a chemical modification of an organic material capable of suppressing a thermally induced phase transition. This is achieved by substituting the propyl chains with pentafluoropropyl groups, which create multiple fluorine-based interactions that effectively stabilize the crystal structure.

The findings are clearly presented. The claims made are substantiated by solid experimental evidence and well-supported by the accompanying models, rendering the text an enjoyable read. The evidence of a phase transition at approximately 175 K, as illustrated in Figure 2C through Raman spectroscopy, is very clear.

This study is relevant to the scientific community and warrants publication. However, having reviewed the manuscript, two pertinent questions have arisen, which, in my view, would enhance the manuscript's content. Two points may deserve some additional discussion.

 

Response: The authors thank Reviewer 1 for these helpful comments. We have addressed all the issues below and have updated the manuscript to reflect these changes.

 

Point 1

Figure 2C suggests the presence of a hysteresis effect when combining scanning down (cooling) and scanning up (heating) in a full loop. Could this hysteresis phenomenon be explicitly represented and discussed in the manuscript?

 

Response: We have supplemented the Raman data on this thermal transition with multiple DSC runs, including variable ramping rates that demonstrates that the phase change in this material is a kinetically reversable enantiotropic transition. This new data has been added to Figure 2, the SI, and the associated text updated in the methods (lines 117-124) and discussion (lines 196-208).

 

Point 2

The data presented in the manuscript is confined to the temperature range between 165 and 190 K. The behavior of the material between 200 and 300 K is not reported. Are there additional phase transitions or any other anomalous behaviors in this temperature range? It is somewhat surprising that only one phase transition is reported, and one might anticipate the presence of multiple transitions.

 

Response: We have updated the manuscript to clarify that our temperature range under consideration is from 150 K to room temperature (lines 196-208). Prior crystallography and variable-temperature XRD work has confirmed that there are only two phases in this temperature range, and this is supported by the Raman spectroscopy and the new DSC data. There may be additional phase changes above room temperature, but these have not been investigated.

 

Incorporating discussions related to these aspects would enhance the manuscript's overall quality.

 

Reviewer 2 Report

In this manuscript, Marjo and colleagues synthesized and characterized a chloro-s-triazine compound functionalized with the fluoropropyl group. Subsequently, the authors investigated and compared the layered crystal systems formed by hydrogen-bonded chloro-s-triazine rings functionalized with alkyl or fluoroalkyl chains. In this work, experimental and theoretical results have seamlessly been integrated and thoroughly discussed. This manuscript has made a significant contribution to crystal engineering and has left a favourable impression. However, I have a few notes and comments:

1.     The thermally-induced phase transition in the crystals was observed using Raman spectroscopy by varying the temperature. To obtain more precise measurements of the temperature changes during phase transitions, it is advisable to employ Differential Scanning Calorimetry (DSC). Utilizing the DSC method can provide a broader and more accurate understanding of the thermal characteristics of these crystals.

2.     In Figure 4a, the Hirshfeld analysis of the C3 and C3-F molecules is presented. I recommend comparing these surfaces with electrostatic potential maps of these molecules, which can be computed using Density Functional Theory (DFT).

3.     To calculate the strengths (energies) of the intermolecular interactions, the B3LYP hybrid functional was employed. However, for an accurate treatment of intermolecular interactions, it is recommended to use a dispersion-corrected potential approach. Functionals that incorporate dispersion or long-range corrections, such as ωB97XD or B3LYP-D3, are more suitable for this purpose.

In my opinion, the novelty and reliability of this work are of a sufficiently high standard to warrant publication in the Crystals journal.

Author Response

Response to Reviewer 2

 

In this manuscript, Marjo and colleagues synthesized and characterized a chloro-s-triazine compound functionalized with the fluoropropyl group. Subsequently, the authors investigated and compared the layered crystal systems formed by hydrogen-bonded chloro-s-triazine rings functionalized with alkyl or fluoroalkyl chains. In this work, experimental and theoretical results have seamlessly been integrated and thoroughly discussed. This manuscript has made a significant contribution to crystal engineering and has left a favourable impression. However, I have a few notes and comments:

 

Response: The authors thank Reviewer 2 for these helpful comments. We believe we have addressed all the issues below and have updated the manuscript to reflect these changes.

 

1. The thermally-induced phase transition in the crystals was observed using Raman spectroscopy by varying the temperature. To obtain more precise measurements of the temperature changes during phase transitions, it is advisable to employ Differential Scanning Calorimetry (DSC). Utilizing the DSC method can provide a broader and more accurate understanding of the thermal characteristics of these crystals.

 

Response: We agree and have presented low temperature DSC measurements in the temperature range of interest for this study (150 K to room temperature). This data is presented in Figure 2 and the associated text (methods lines 117-124 and discussion lines 196-208). Variable ramping rate DSC measurements confirmed that the phase change is a kinetically reversable enantiotropic transition. In addition, the fluorinated compound shows no transitions between 150 K and RT, supporting our other data that fluorination suppresses the phase change (lines 247-251).

 

2. In Figure 4a, the Hirshfeld analysis of the C3 and C3-F molecules is presented. I recommend comparing these surfaces with electrostatic potential maps of these molecules, which can be computed using Density Functional Theory (DFT).

 

Response: Thank you for this suggestion. We have prepared electrostatic potential maps using B3LYP calculations in Gaussian09 with 6-311G basis set. The resulting surfaces are quite informative about the distribution of charge across each molecule that has implications for the strength of the resulting intermolecular interactions. The methods are presented at lines 151-161 and the data is presented in Figure 4, the SI, and the associated text in the discussion (lines 274-281).

 

3. To calculate the strengths (energies) of the intermolecular interactions, the B3LYP hybrid functional was employed. However, for an accurate treatment of intermolecular interactions, it is recommended to use a dispersion-corrected potential approach. Functionals that incorporate dispersion or long-range corrections, such as ωB97XD or B3LYP-D3, are more suitable for this purpose.

 

Response: We recognise the greater expertise of the reviewer in this area. The authors relied on the standard computational parameters offered by CrystalExplorer for rapid calculation of intermolecular interaction energy calculations. The CE-B3LYP model used by CrystalExplorer is justified In Turner et al. J. Phys. Chem. Lett., 2014 5(24), 4249 as a model closely matching those from higher level calculations (within 2 kJ/mol) by using scale factors for each of the electrostatic, polarization, repulsion energies and, notably, Grimme’s D2 dispersion corrections that make up the interaction. We note that the interaction energy differences between the propyl and fluoropropyl derivatives is greater than the uncertainty of Spackman’s approach for these calculations (2 kJ/mol), so we believe this qualitatively shows the impact of the attractive fluorine-based interactions in shutting down the temperature induced phase transition in the propyl structure. We have updated the text at lines 151-161 and lines 298-300 to clarify this.

 

In my opinion, the novelty and reliability of this work are of a sufficiently high standard to warrant publication in the Crystals journal.

 

Round 2

Reviewer 2 Report

The authors addressed all of my questions and comments and even incorporated additional data into the manuscript. Consequently, I recommend accepting this paper.