Synthesis, Structure, and Luminescence Properties of Zinc(II) Complex with a Spacer-Armed Tetradentate N₂O₂-Donor Schiff Base

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study by Gusev et al. is aimed at solving an important and relevant scientific problem related to the search for new efficient blue emitters for OLED applications. To solve this problem, the authors synthesized and comprehensively studied a zinc complex with a pyrazolone based azomethine ligand. The work presents the photoluminescent and electroluminescent characteristics of the obtained complex. The ligand and zinc complex were previously obtained and described in the same scientific group (ref. 20). In this work, the authors succeeded in establishing the crystal structures for these compounds using single-crystal diffractometry and made progress in studying the photo- and electroluminescent properties of solid samples of the zinc complex. The study was performed at a high scientific level and can be recommended for publication after some revision.

1) Since the ligand and the zinc complex based on it have already been synthesized previously, the statement about obtaining new compounds in the abstract of the article should be abandoned and the title of the work should be corrected.

2) Figure 6 shows only one excitation spectrum. Do the excitation spectra of all the studied samples match? The crystalline sample exhibits a new band (shoulder at 470 nm). The authors should provide the excitation spectrum for the main band at 416 nm and at 470 nm for this sample.

3) The authors claim that the shoulder at 470 nm is due to the formation of excimers (exciplexes), based only on the data of the cited article, where a similar complex was reported. Experimental confirmation of this fact should be provided. For example, the emission spectrum in solution should be recorded — the shoulder should disappear; try to measure the lifetime for the main band at 416 and the shoulder at 470. In the case of an excimer contribution, the lifetime for the 470 nm band should be slightly longer.

4) The authors claim that grinding of the sample changes the emission spectrum. The diffractogram of the substance after grinding and its comparison with the diffractogram calculated for the discussed structure [ZnL(H2O)] are not presented. It is possible that grinding changes the phase composition. The phenomenon of luminescent polymorphism is well known.

5) Is it possible to record a diffractogram of a sample after heating and recrystallization from benzene (in the article this sample is called dehydrated). Is the dehydrated sample crystalline? What can be said about the reproducibility of this process?

6) The following lifetimes are mentioned in the article: τ1 = 6.8 ns, A1 = 0.32; τ2 = 12.0 ns, A2 = 0.68 for a zinc sample with water. Should these lifetimes be attributed to the crystals or to the ground complex? The contribution of both lifetimes is close. What explains the biexponential kinetics for this sample?

7) The authors state that they matched the HOMO-LUMO energy to the matrix into which the sample was embedded and to other layers in the device. How were the HOMO-LUMO values ​​obtained for the complex? Are these experimental or calculated values?

8) The authors mention several times that the ligand used is capable of photoproton transfer, but do not cite relevant literature on similar zinc(II) complexes.

See examples: 10.1021/acs.inorgchem.5c00222; Chem. Soc. Rev., 2016, 45, 169–202; Chem. Lett. 2018, 47, 1083–1089, doi:10.1246/cl.180495

Author Response

We thank the Referees for their interest in our work and for helpful comments and constructive suggestions that will greatly improve the quality of this manuscript. As indicated below, we have checked all the general and specific comments provided by the Referees and have made necessary changes according to their indications

Referee 1

The study by Gusev et al. is aimed at solving an important and relevant scientific problem related to the search for new efficient blue emitters for OLED applications. To solve this problem, the authors synthesized and comprehensively studied a zinc complex with a pyrazolone based azomethine ligand. The work presents the photoluminescent and electroluminescent characteristics of the obtained complex. The ligand and zinc complex were previously obtained and described in the same scientific group (ref. 20). In this work, the authors succeeded in establishing the crystal structures for these compounds using single-crystal diffractometry and made progress in studying the photo- and electroluminescent properties of solid samples of the zinc complex. The study was performed at a high scientific level and can be recommended for publication after some revision.

1) Since the ligand and the zinc complex based on it have already been synthesized previously, the statement about obtaining new compounds in the abstract of the article should be abandoned and the title of the work should be corrected.

Reply. The authors thank the reviewer for the valuable comment. Appropriate changes have been made to the text and title

 

2) Figure 6 shows only one excitation spectrum. Do the excitation spectra of all the studied samples match? The crystalline sample exhibits a new band (shoulder at 470 nm). The authors should provide the excitation spectrum for the main band at 416 nm and at 470 nm for this sample.

Reply. In all cases, the excitation spectrum has the same appearance: crystalline, ground and dehydrated samples. More monitoring pr 416 and 470 nm for crystals also does not affect the excitation spectrum

 

3) The authors claim that the shoulder at 470 nm is due to the formation of excimers (exciplexes), based only on the data of the cited article, where a similar complex was reported. Experimental confirmation of this fact should be provided. For example, the emission spectrum in solution should be recorded — the shoulder should disappear; try to measure the lifetime for the main band at 416 and the shoulder at 470. In the case of an excimer contribution, the lifetime for the 470 nm band should be slightly longer.

Reply. This proved to be a challenging question, which required additional measurements. To begin with, it should be pointed out that the spectrum of the complex in a highly polar solvent and at low concentration (less than 10-4 M) excluding intermolecular interaction between the molecules of the complex is observed one band at 484 nm ( https://doi.org/10.1016/j.dyepig.2020.108626). At concentrations above 10-3 M, two bands at 485 and 503 nm are registered, which is characteristic of excimer luminescence. As for the lifetime measurements of the crystalline sample with emission monitoring at 470 nm, in this case the lifetime is also approximated by a biexponential curve, but the contribution of the long component in this case is slightly higher τ1 = 6.3 ns, A1 = 0.15; τ2 = 12.6 ns, A2 = 0.85.

4) The authors claim that grinding of the sample changes the emission spectrum. The diffractogram of the substance after grinding and its comparison with the diffractogram calculated for the discussed structure [ZnL(H2O)] are not presented. It is possible that grinding changes the phase composition. The phenomenon of luminescent polymorphism is well known.

Reply. That's a valuable comment. We have made appropriate  measurements. The corresponding diffractogram is presented in ESI. As can be seen from the presented data, crystal grinding does not lead to the appearance of new phases

5) Is it possible to record a diffractogram of a sample after heating and recrystallization from benzene (in the article this sample is called dehydrated). Is the dehydrated sample crystalline? What can be said about the reproducibility of this process?

Reply. Dehydration of the complex by boiling in benzene does not allow us to isolate the crystalline phase - in all cases a broad halo is observed in the diffractograms. As for reproducibility, the approach consisting in the removal of coordinated water by boiling in benzene is well reproduced (at least three times), but the removal of water by heating is well reproduced only for small amounts, when using more than 100 mg - requires an individual selection of conditions

6) The following lifetimes are mentioned in the article: τ1 = 6.8 ns, A1 = 0.32; τ2 = 12.0 ns, A2 = 0.68 for a zinc sample with water. Should these lifetimes be attributed to the crystals or to the ground complex? The contribution of both lifetimes is close. What explains the biexponential kinetics for this sample?

Reply. In our opinion, this question is related to question 3. In general, the decay curve can be approximated by a single-exponential curve, but with fewer R-factors (0.9612 vs 0.9854). However, we сonsider that since the luminescence spectrum of a crystalline sample is a superposition of monomeric and excimer signals, it is more correct to use a bi-exponential curve reflecting two paths of deactivation of the excited state.

7) The authors state that they matched the HOMO-LUMO energy to the matrix into which the sample was embedded and to other layers in the device. How were the HOMO-LUMO values ​​obtained for the complex? Are these experimental or calculated values?

Reply. We used the previously described energy data of the HOMO/LUMO orbitals determined from the results of measurements of optical spectra and cyclic voltammetry. The corresponding indication has been added to the text

8) The authors mention several times that the ligand used is capable of photoproton transfer, but do not cite relevant literature on similar zinc(II) complexes. See examples: 10.1021/acs.inorgchem.5c00222; Chem. Soc. Rev., 2016, 45, 169–202; Chem. Lett. 2018, 47, 1083–1089, doi:10.1246/cl.180495

Reply.  We thank you for this comment. Relevant references to the literature are inserted in the text.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript reported a novel zinc complex bearing pyrazolone based azomethine ligand for blue-emitter in organic light-emitting diodes (OLEDs). This complex exhibits blue luminescence at 416 nm in the solid state (with a quantum yield of 2%), while the quantum yield significantly increases to 55.5% after dehydration. This work demonstrates the application potential of metal complexes in the field of optoelectronics through the combination of molecular design (hydration/dehydration regulation) and device engineering. Consequently, it is recommended that this manuscript be published following minor revisions.

 

1. The author attributed the increase in the quantum yield of the complex after dehydration to the quenching effect of water. What is the mechanism?
2. Why does the decay curve of the compound change from a biexponential function to a monoexponential function after dehydration, and further what is the physical meaning?
3. The author fabricated OLED devices based on this complex. Please add the related pictures to the manuscript.
4. The abbreviations lack full names and introductions when they first appear, such as CIE, EQE. Explanations should be added to make the subsequent discussion much clearer.
5. The article mentions that the complete removal of the coordinated water molecule was confirmed by elemental and thermal analyses and IR spectroscopy. However, the author did not present the specific results and conduct an analysis.
6. In Figure 5, it may be inaccurate to attribute the absorption at 282 nm to the n-π* transition. The attribution of the transition needs to be verified through theoretical calculation.
7. The description of the TGA and DSC test sections in the article is in units of ℃, while Figure 4 shows units of K. The units should be unified. In addition, the vertical coordinate should be "weight" instead of "weight lose".

Author Response

 

Referee2

We thank the Referees for their interest in our work and for helpful comments and constructive suggestions that will greatly improve the quality of this manuscript. As indicated below, we have checked all the general and specific comments provided by the Referees and have made necessary changes according to their indications

The manuscript reported a novel zinc complex bearing pyrazolone based azomethine ligand for blue-emitter in organic light-emitting diodes (OLEDs). This complex exhibits blue luminescence at 416 nm in the solid state (with a quantum yield of 2%), while the quantum yield significantly increases to 55.5% after dehydration. This work demonstrates the application potential of metal complexes in the field of optoelectronics through the combination of molecular design (hydration/dehydration regulation) and device engineering. Consequently, it is recommended that this manuscript be published following minor revisions.

 

1. The author attributed the increase in the quantum yield of the complex after dehydration to the quenching effect of water. What is the mechanism?

Reply: The authors thank the reviewer for the substantive comment. In our opinion, the low quantum yield of the hydrated complex is related to the vibrational effects of OH-oscillators, which increase the radiationless energy loss of the excited state. Accordingly, the removal of the coordinated water molecule contributes to the increase of the radiative constant

2. Why does the decay curve of the compound change from a biexponential function to a monoexponential function after dehydration, and further what is the physical meaning?

Reply: In general, the decay curve can be approximated by a single-exponential curve, but with fewer R-factors (0.9612 vs 0.9854). However, we сonsider that since the luminescence spectrum of a crystalline sample is a superposition of monomeric and excimer signals, it is more correct to use a bi-exponential curve reflecting two paths of deactivation of the excited state.

3. The author fabricated OLED devices based on this complex. Please add the related pictures to the manuscript.

Reply: As requested by the reviewer, we have added photos of running devices A and B to the Figure 8d

4. The abbreviations lack full names and introductions when they first appear, such as CIE, EQE. Explanations should be added to make the subsequent discussion much clearer.

Reply: As requested by the reviewer, we have added the abbreviation transcription after the first mention in the text

5. The article mentions that the complete removal of the coordinated water molecule was confirmed by elemental and thermal analyses and IR spectroscopy. However, the author did not present the specific results and conduct an analysis.

Reply: As requested by the reviewer, we have added corresponding data to ESI

6. In Figure 5, it may be inaccurate to attribute the absorption at 282 nm to the n-π* transition. The attribution of the transition needs to be verified through theoretical calculation.

Reply: We agree with the reviewer that this statement requires theoretical calculations. Similar calculations were made earlier for the zinc complex with dimethylene spacer. Taking into account the close structure of the complex described in our paper and the complex for which the calculations were performed (differences by one methylene group in the spacer), we consider it acceptable to use the interpretation of electronic transitions obtained earlier

7. The description of the TGA and DSC test sections in the article is in units of ℃, while coordinate should be "weight" instead of "weight lose".

Reply: The authors thank the reviewer for the substantive comment. Relevant changes are made.  

Reviewer 3 Report

Comments and Suggestions for Authors

The work entitled "Synthesis, structure and luminescence properties of a novel zinc(II) complex with a spacer-armed tetradentate N2O2-donor Schiff  base" fits well into the research topics of Inorganics and concerns practical issues related to OLEDs. In general, both the new ligand and the obtained zinc complex were well characterized. The authors carried out electroluminescence studies using the complex for the construction of blue OLED. The general impression of the article is good, but I have a few comments that the authors should address.

1) The introduction lacks information on whether studies have been conducted by other authors on the use of Schiff base complexes in OLEDs. If so, this should be included in the article.

2) Figures 2 and 3 should be corrected. Hydrogen atoms are practically invisible.

3) The axis captions in Figure 4 should be corrected. The text has different units (Celsius degrees) than the figure (K). There is no unit for DSC. For the TG curve, it should be mass (%). What do the authors mean by writing “Further heating leads to registration of exothermic effect in 287 obviously related to crystallization of the dehydrated sample.”

4) The authors use different abbreviations for the described complex ([ZnL(H2O)], ZnL, ZnL·H2O). This should be unified throughout the work

5) The authors mention that the luminescent properties of the complex were studied in solution early on. A comparison of the results for the compound in the liquid and solid phases would be welcome.

6) The authors often use the plural for zinc complexes. I understand that there was only one compound studied. Please review the work and correct it.

Author Response

Referee 3

We thank the Referees for their interest in our work and for helpful comments and constructive suggestions that will greatly improve the quality of this manuscript. As indicated below, we have checked all the general and specific comments provided by the Referees and have made necessary changes according to their indications

The work entitled "Synthesis, structure and luminescence properties of a novel zinc(II) complex with a spacer-armed tetradentate N2O2-donor Schiff  base" fits well into the research topics of Inorganics and concerns practical issues related to OLEDs. In general, both the new ligand and the obtained zinc complex were well characterized. The authors carried out electroluminescence studies using the complex for the construction of blue OLED. The general impression of the article is good, but I have a few comments that the authors should address.

 1) The introduction lacks information on whether studies have been conducted by other authors on the use of Schiff base complexes in OLEDs. If so, this should be included in the article.

Reply: The authors thank the reviewer for the substantive comment. Relevant references to the literature are inserted in the text.

2) Figures 2 and 3 should be corrected. Hydrogen atoms are practically invisible.

Reply: We have increased the size of the drawings for better visuals

3) The axis captions in Figure 4 should be corrected. The text has different units (Celsius degrees) than the figure (K). There is no unit for DSC. For the TG curve, it should be mass (%). What do the authors mean by writing “Further heating leads to registration of exothermic effect in 287 obviously related to crystallization of the dehydrated sample.”

Reply: The authors thank the reviewer for the substantive comment. Relevant changes are made. The dehydrated sample is amorphous in nature according to PXRD data. Therefore, the appearance of the maximum on the DSC curve at 287 we associate with crystallization of this sample. However, we agree that this requires more rigorous proof, so we omitted this phrase in the text

4) The authors use different abbreviations for the described complex ([ZnL(H2O)], ZnL, ZnL·H2O). This should be unified throughout the work

Reply: The authors thank the reviewer for the substantive comment. Relevant unifications are made

5) The authors mention that the luminescent properties of the complex were studied in solution early on. A comparison of the results for the compound in the liquid and solid phases would be welcome.

Reply: Comparison of luminescence in solutions and solid state has been added to the text

6) The authors often use the plural for zinc complexes. I understand that there was only one compound studied. Please review the work and correct it.

Reply: The reviewer is absolutely right. The discrepancy has been corrected

Reviewer 4 Report

Comments and Suggestions for Authors

In the manuscript entitled “Synthesis, structure and luminescence properties of a novel zinc(II) complex with a spacer-armed tetradentate N2O2-donor Schiff base” is described the azomethine ligand and its complex with Zn(II), their structure, spectral, photophysical and electroluminescent properties. The motivation and the objectives of the work are interesting, and the paper is well organized. The results and the conclusions are presented clearly. However, some corrections would enhance the clarity of the manuscript.

In the thermal part of the discussion the unit on the temperature axes in Figure 4. is K, but in the text, all temperatures are given in °C, which should be clarified. On the other hand, in this part, it is not mentioned that the loss of coordinated water was proved. I found it later in the text. Since the water loss is proven, it should be mentioned in the thermal part also. The °C unit is not properly written every time, it should be revised especially in this part of the manuscript. The direction of the endo- and exothermic peaks on the heat flow curve should be added to Figure 4.

Under the photophysical properties, Figure 6 is not placed below the corresponding part of the discussion nor cited in the text. The authors should revise its position, and it should be cited in the discussion.

What are CIE coordinates? They should be introduced at their first mention in the discussion.

Figure 7. is missing. Instead of it, there is Figure 8, but it contains too much different information. In my opinion information from Figure 8 should be presented in 2-3 separate figures. In Figure 8d is not indicated which symbols are on the brightness and which are on the current density curves. It should be added to the graph.

Taking into account all the abovementioned, I suggest the acceptance of this manuscript for publication after minor revision.

Author Response

Referee 4

We thank the Referees for their interest in our work and for helpful comments and constructive suggestions that will greatly improve the quality of this manuscript. As indicated below, we have checked all the general and specific comments provided by the Referees and have made necessary changes according to their indications

 

In the manuscript entitled “Synthesis, structure and luminescence properties of a novel zinc(II) complex with a spacer-armed tetradentate N2O2-donor Schiff base” is described the azomethine ligand and its complex with Zn(II), their structure, spectral, photophysical and electroluminescent properties. The motivation and the objectives of the work are interesting, and the paper is well organized. The results and the conclusions are presented clearly. However, some corrections would enhance the clarity of the manuscript.

In the thermal part of the discussion the unit on the temperature axes in Figure 4. is K, but in the text, all temperatures are given in °C, which should be clarified. On the other hand, in this part, it is not mentioned that the loss of coordinated water was proved. I found it later in the text. Since the water loss is proven, it should be mentioned in the thermal part also. The °C unit is not properly written every time, it should be revised especially in this part of the manuscript. The direction of the endo- and exothermic peaks on the heat flow curve should be added to Figure 4.

Reply: The authors thank the reviewer for the substantive comment. Relevant changes are made.

Under the photophysical properties, Figure 6 is not placed below the corresponding part of the discussion nor cited in the text. The authors should revise its position, and it should be cited in the discussion.

Reply: The authors thank the reviewer for the substantive comment. Relevant changes are made.

What are CIE coordinates? They should be introduced at their first mention in the discussion.

Reply: As requested by the reviewer, we have added the abbreviation transcription after the first mention in the text

Figure 7. is missing. Instead of it, there is Figure 8, but it contains too much different information. In my opinion information from Figure 8 should be presented in 2-3 separate figures. In Figure 8d is not indicated which symbols are on the brightness and which are on the current density curves. It should be added to the graph.

Reply  We agree with this comment. Figure 8 has been split into two figures numbered 7 and 8. As for the labeling of the brightness and current density on the corresponding curve, brightness is indicated by circles and current density by squares.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors took into account the reviewer's comments and revised the manuscript. In its current form, the manuscript is suitable for publication.

Author Response

The authors once again thank the reviewer for the valuable comments and are glad that the corrections really improved the paper 

Reviewer 2 Report

Comments and Suggestions for Authors

The AUTHORS claimed that they have added corresponding data to ESI (For instance, the FT-IR data). However they di nothing or they provide the wrong ESI.

Author Response

We apologize for the misunderstanding. We have indeed added three figures to the ESI: Figure S1. PXRD diffractograms of grounded crystals of zinc complexes
Figure S2. IR spectra of hydrated (black line) and unhydrated (red line) zinc complex and elemental analysis data for unhydrated sample
Figure S3. TGA curve of unhydrated zinc complex. The actual ESI and X-ray analysis data were presented in the archive file ESI.zip.  ESI is now loaded as a separate file 

 

Author Response File: Author Response.pdf