Design and Synthesis of a New Photoluminescent 2D Coordination Polymer Employing a Ligand Derived from Quinoline and Pyridine

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Kochel, Holynska, et al. describes the synthesis of a two-dimensional (2D) coordination polymer containing the 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate ligand. The prepared Cu(II) compound and its organic ligand were characterized using single-crystal X-ray diffraction analysis, elemental analysis, and infrared spectroscopy. Additionally, the authors investigated its photoluminescent and magnetic properties and performed theoretical calculations. However, after a thorough review, it is clear that the manuscript requires revisions before it can be published in Crystals.

1. It is recommended that the prepared ligand be written with the abbreviation HL, with the understanding that HL = the name of the ligand.

2. The formula of the prepared coordination polymer should be written according to IUPAC nomenclature, for example, as {[Cu(L)]BF4}n.

3. In the Introduction, the authors should decipher the abbreviations of the ligands in the formulas of the mentioned compounds, such as 4,4'-bpy, pzdc, dpyg4,4’–bpy, pzdc, dpyg, etc.

4. Furthermore, the experimental MS section does not provide an adequate characterization of the new Cu(II) coordination polymer, including its identity and purity degree. This is evidenced by the elemental analysis of the sample, which shows a deviation from theoretical and practical values of more than 0.4%. This deviation requires further investigation to establish its bulk composition. Furthermore, it is strongly recommended that powder X-ray measurements be performed in order to exclude the presence of additional products, especially in the case of magnetic measurements.

5. In the section entitled "Results and discussion," the authors introduce the number for the ligand as "1" and for the coordination polymer as "2." It would have been preferable to introduce these numbers at an earlier point in the text.

Author Response

Responses to Reviewers
Reviewer 1

The manuscript by Kochel, Holynska, et al. describes the synthesis of a two-dimensional (2D) coordination polymer containing the 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate ligand. The prepared Cu(II) compound and its organic ligand were characterized using single-crystal X-ray diffraction analysis, elemental analysis, and infrared spectroscopy. Additionally, the authors investigated its photoluminescent and magnetic properties and performed theoretical calculations. However, after a thorough review, it is clear that the manuscript requires revisions before it can be published in Crystals.

1. It is recommended that the prepared ligand be written with the abbreviation HL, with the understanding that HL = the name of the ligand.

Response: Thank you, the suggested notation is now implemented in the revised manuscript.

2. The formula of the prepared coordination polymer should be written according to IUPAC nomenclature, for example, as {[Cu(L)]BF4}n.

Response: Thank you, the suggested notation is now implemented in the revised manuscript.

3. In the Introduction, the authors should decipher the abbreviations of the ligands in the formulas of the mentioned compounds, such as 4,4'-bpy, pzdc, dpyg4,4’–bpy, pzdc, dpyg, etc.

Response: Thank you, the suggested correction is now implemented in the revised manuscript.

4. Furthermore, the experimental MS section does not provide an adequate characterization of the new Cu(II) coordination polymer, including its identity and purity degree. This is evidenced by the elemental analysis of the sample, which shows a deviation from theoretical and practical values of more than 0.4%. This deviation requires further investigation to establish its bulk composition. Furthermore, it is strongly recommended that powder X-ray measurements be performed in order to exclude the presence of additional products, especially in the case of magnetic measurements.

Response: Thank you for this remark, we understand your concern. However, our experience and reference testing show that this is a normal deviation achievable on our device for elemental analyses. Please note that similar deviations are observed for the organic ligand results. Moreover, powder diffraction could give us an idea only on the presence of crystalline phases, while for amorphous content we would not get any information. Currently we have limited access to this characterization technique and we hope that the quoted arguments justify our choice of characterization techniques.

5. In the section entitled "Results and discussion," the authors introduce the number for the ligand as "1" and for the coordination polymer as "2." It would have been preferable to introduce these numbers at an earlier point in the text.

Response: Thank you for your suggestion which is now implemented in the revised manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors report on a new ligand and Cu(II) coordination polymer displaying photoluminescence and antiferromagnetic behavior, together with insights on the conformational preferences based on molecular modeling. This is a peculiar and interesting crystal structure, due to the coordination of Cu(II) ions to both pyrazyl, quinoline and carboxylate groups of the ligand. I recommend the publication of the manuscript after some additional information and corrections listed below.

Experimental part: Please correct the numbering of paragraphs (2.3, 2.3.1 and 2.3.2 increment for X-ray crystal studies)

Crystal structure description: Did the authors analyze the possible void space in the crystal structure of the polymer, and confirm the absence of solvent molecule? Instead of the tables of selected hydrogen bond distances (which in my opinion could be moved to the SI), can the authors display a figure with a more extended view of the crystal packing in the coordination polymer?

Figure 4: only the view along the (100) direction is represented, the authors should add in the same figure the view along the (010) direction.

Please increase the size of the coordinate system in each figure of crystal structures.

Photoluminescence study: the excitation spectra should be recorded and displayed together with emission spectra with normalized intensity. Selected excitation and emission wavelength should be indicated in the caption.

Can the authors measure the quantum yield for both ligand and coordination polymer?

Magnetic characterization: Figure 9: in the caption, the authors label Cu(1) and Cu(2), which is rather confusing since there is only one independent Cu(II) metal ion in the structure. It should be written “Cu(1)...Cu(1) [x, 0.5-y, 0.5+z).

Can the authors report also the Cu...Cu distance between Cu(II) ions from both sides of the pyrazyl bridge, i. e. where the antiferromagnetic interactions take place? I assume there is an error in the model of antiferromagnetic interactions displayed in figure 10, please double check and confirm.

Supporting information: ¹H NMR spectrum of the ligand: what do the signals between 3.5-3.7 ppm correspond to?

Author Response

Review 2
The authors report on a new ligand and Cu(II) coordination polymer displaying photoluminescence and antiferromagnetic behavior, together with insights on the conformational preferences based on molecular modeling. This is a peculiar and interesting crystal structure, due to the coordination of Cu(II) ions to both pyrazyl, quinoline and carboxylate groups of the ligand. I recommend the publication of the manuscript after some additional information and corrections listed below.

Experimental part: Please correct the numbering of paragraphs (2.3, 2.3.1 and 2.3.2 increment for X-ray crystal studies)

Response: Thank you, this is now corrected in the revised manuscript.

Crystal structure description: Did the authors analyze the possible void space in the crystal structure of the polymer, and confirm the absence of solvent molecule? Instead of the tables of selected hydrogen bond distances (which in my opinion could be moved to the SI), can the authors display a figure with a more extended view of the crystal packing in the coordination polymer?

Response: Thank you for your remark, crystal structure voids were analyzed with PLATO software and absence of solvent molecules was confirmed. Table with hydrogen bonding parameters is now moved to SI, Figures 7, 8 are added with more extensive views of crystal structure packing.

Figure 4: only the view along the (100) direction is represented, the authors should add in the same figure the view along the (010) direction.
Response : Thank you, the suggested figure is now included in the revised manuscript.
Please increase the size of the coordinate system in each figure of crystal structures.

Response : Thank you, this is now implemented.

Photoluminescence study: the excitation spectra should be recorded and displayed together with emission spectra with normalized intensity. Selected excitation and emission wavelength should be indicated in the caption.

Response : Thank you, this is now implemented.

Can the authors measure the quantum yield for both ligand and coordination polymer?

Response: Thank you for your suggestion. Unfortunately currently we do not have access to this capability but will consider it for future studies.

Magnetic characterization: Figure 9: in the caption, the authors label Cu(1) and Cu(2), which is rather confusing since there is only one independent Cu(II) metal ion in the structure. It should be written “Cu(1)...Cu(1) [x, 0.5-y, 0.5+z).

Response: Thank you, this is now corrected in the revised manuscript.

Can the authors report also the Cu...Cu distance between Cu(II) ions from both sides of the pyrazyl bridge, i. e. where the antiferromagnetic interactions take place? I assume there is an error in the model of antiferromagnetic interactions displayed in figure 10, please double check and confirm.

Response: Thank you, we double-checked and corrected the Figure.

Supporting information: ¹H NMR spectrum of the ligand: what do the signals between 3.5-3.7 ppm correspond to?
Response: Unfortunately there are minor impurities in our solvents which result in these signals, these cannot be plausibly identified.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors report a 2D Cu(II) coordination polymer with quinoline derivatives ligand, which displays photoluminescent and paramagnetic properties. Density Functional Theory was employed to describe the conformational changes within the ligand and investigate the electronic structure and non-covalent interactions in polymer fragments.

However, the manuscript does not meet the journal’s standards for publication.

 

Synthesis, photoluminescence properties, magnetic properties, and DFT calculations are reported. However, the manuscript is poorly structured; the description of the experimental results is just a collection of each experimental result. The novelty of this study that the authors wish to report is unclear.    

 

The manuscript is difficult to read. One reason for this is that some figures are not cited in the main text.

 

In chapter “3.3. Magnetic properties”, Figure 11 is not cited.

Furthermore, the authors stated, “Based on c_mT experimental curve, at 300 K the corresponding value is 1.405 cm³ K mol^-1”. However, Figure 11 shows that c_mT value at 300 K is about 0.42 cm³ K mol^-1.

Author Response

Review 3
The authors report a 2D Cu(II) coordination polymer with quinoline derivatives ligand, which displays photoluminescent and paramagnetic properties. Density Functional Theory was employed to describe the conformational changes within the ligand and investigate the electronic structure and non-covalent interactions in polymer fragments.

However, the manuscript does not meet the journal’s standards for publication.

Synthesis, photoluminescence properties, magnetic properties, and DFT calculations are reported. However, the manuscript is poorly structured; the description of the experimental results is just a collection of each experimental result. The novelty of this study that the authors wish to report is unclear.    

Response: The reported compound is an original contribution to the field of photoluminescent coordination polymers, quoting another Referee of this work “a peculiar and interesting crystal structure, due to the coordination of Cu(II) ions to both pyrazyl, quinoline and carboxylate groups of the ligand.” We tried to underline this in our manuscript.

The manuscript is difficult to read. One reason for this is that some figures are not cited in the main text.

Response: Apologies for this, we did our best to fix this issue in the revised manuscript.

In chapter “3.3. Magnetic properties”, Figure 11 is not cited.

Response: Thank you, this is now corrected in the revised manuscript.

Furthermore, the authors stated, “Based on c_mT experimental curve, at 300 K the corresponding value is 1.405 cm³ K mol^-1”. However, Figure 11 shows that c_mT value at 300 K is about 0.42 cm³ K mol^-1.

Response: This is a typo, it is now corrected in the revised manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept in present form

Author Response

Thank you

Reviewer 2 Report

Comments and Suggestions for Authors

Two figures with extended views of the crystal packing in the coordination polymer have been added (Figures 7 and 8), but the color code for the atoms is not specified and is clearly different from the color code used in the previous figures.

The authors didn’t understand my request about photoluminescence spectra. While I was suggesting to measure both excitation and emission spectra, then to normalize these spectra and to report the excitation wavelength (for emission spectrum measurement) and emission wavelength (for excitation spectrum measurement), the authors have now added a second spectrum (in green) that I don’t understand (captions have no meaning).

The authors still labeled Cu(1) and Cu(2) in the caption of Figure 11.

Author Response

The responses to the Reviewer are below:

Two figures with extended views of the crystal packing in the coordination polymer have been added (Figures 7 and 8), but the color code for the atoms is not specified and is clearly different from the color code used in the previous figures.

Response: In the revised manuscript we tried to keep the same atom style for all figures.

The authors didn’t understand my request about photoluminescence spectra. While I was suggesting to measure both excitation and emission spectra, then to normalize these spectra and to report the excitation wavelength (for emission spectrum measurement) and emission wavelength (for excitation spectrum measurement), the authors have now added a second spectrum (in green) that I don’t understand (captions have no meaning).

Response: Apologies for this misunderstanding, we believe this is now implemented in the revised manuscript. For ligand and for polymer separate figures with the requested spectra are presented.

The authors still labeled Cu(1) and Cu(2) in the caption of Figure 11.

Response: Indeed this was a typo from our side, again we did not understand the original question, this is now corrected.

We hope now the revised manuscript fulfils your standards.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript has been appropriately revised.

Author Response

Thank you

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

I still don't understand the PL characterization. Why do the authors have selected the 455 nm emission wavelength for the excitation spectrum measurement of ligand 1? According to the emission spectrum, the compound is not emitting at such wavelength. Same for polymer 2 (the compound emits from 400 to 540 nm, with maximum at around 470 nm, and for the excitation spectrum measurement the selected wavelength is 550 nm?). Both excitation and emission spectra should be displayed in the same graph, but with normalized intensities (i.e. from 0 to 1).

Author Response

I still don't understand the PL characterization. Why do the authors have selected the 455 nm emission wavelength for the excitation spectrum measurement of ligand 1? According to the emission spectrum, the compound is not emitting at such wavelength. Same for polymer 2 (the compound emits from 400 to 540 nm, with maximum at around 470 nm, and for the excitation spectrum measurement the selected wavelength is 550 nm?).

 

Response: Indeed wrong numbers were indicated in figure captions, this is now corrected: λexc=260 nm for ligand and λexc=365 nm for polymer.

 

Both excitation and emission spectra should be displayed in the same graph, but with normalized intensities (i.e. from 0 to 1).

 

Response: Both spectra are now displayed on one graph with normalized intensities. Curve colours are explained in the captions.