Covalent Grafting of Eosin Y to the Giant Keplerate {Mo₁₃₂} through an Organosilicon Linker in Homogeneous Regime

Round 1

Reviewer 1 Report

The authors developed a novel synthetic approach to covalent grafting of xanthene dye eosin Y (EY) to nanoscale Keplerate POM {Mo132} via an organosilicon linker (3-aminopropyltrimethoxysilane, APTMS) in a homogeneous regime. The obtained compounds were characterized very well by IR and Raman spectroscopy, 1H NMR spectrum, UV-Vis and steady-state and time resolved fluorescence measurements. This work is writen very well. Therefore, I recommend its publication in this Journal after minor modification.

1.     Authors need to check the whole text for English revison. English needs to be improved, for example, in abstract,…a novel synthetic approach to covalent grafting……; …show case an amide bond formation between POM and EY… etc.

2.     It is better to do the electrochemical activity of Mo132 for checking if its activity is kept.

3.     It would be interesting if the electrochemistry of Mo132 can realize the fluorescent switching.

Author Response

Dear Referees,

Before I get to the questions, I would like to thank you for taking your time to review our manuscript on behalf of all authors. For your convenience, we list our answers in the same order as your questions.

Referee #1

1. Authors need to check the whole text for English revison. English needs to be improved, for example, in abstract,…a novel synthetic approach to covalent grafting……; …show case an amide bond formation between POM and EY… etc.

 

Thank you for the comment. We think that the manuscript has been sufficiently proofread by a native English speaker.

 

2. It is better to do the electrochemical activity of Mo132 for checking if its activity is kept.

 

I am not entirely positive that I understand what kind of electrochemical activity you refer to. If you are talking about whether the integrity of the Keplerate structure is intact after grafting, we affirmed the preservation of {Mo₁₃₂} with IR and Raman spectroscopy (Fig.2 and Fig.3 in the manuscript). We are also planning to investigate this system with cyclic voltammetry in the future. Thanks for your suggestion.

 

3. It would be interesting if the electrochemistry of Mo132 can realize the fluorescent switching

 

For eosin Y, the open-closed form equilibrium is affected by protonation of the nucleophile atom (oxygen or nitrogen, in the lactonic or lactamic species, respectively). Therefore, this switching is not conjugated with redox activities, and it is unlikely to affect the equilibrium directly via an electrochemical pathway. However, if the dye grafted to POM has a fluorescent response due to some ongoing redox process, one can switch electrochemically the luminophore state within the dye-POM conjugate.

Author Response File: Author Response.pdf

Reviewer 2 Report

 

This manuscript displays studies from the group of Grzhegorzhevskii and reports the synthesis and characterization of complex systems made of the covalent assembly of giant polyoxomolybdates Keplerate with xanthene dye eosin Y. The spectroscopic properties of these hybrid materials (UV-visible absorption and fluorescence) have also been studied in details, even if transient absorption spectra have not been recorded, which is now usual for such systems.

The post-functionalization strategy implemented for the construction of this assembly has also been often used in the literature, but it is also true that the originality of this work lies in the nature of the POM used here. It would have been interesting to develop the interest of using these giant POMs for this type of application. This is not evident from the reading of this manuscript.

In general, the experimental work is particularly solid and the physicochemical techniques used are relevant. However, in my opinion, there is no definitive proof of the formation of amide bonds. ¹³C NMR spectroscopy is probably the most suitable technique for determining the formation of this bond, as has been shown in the past (see Inorg. Chem. 2013, 52, 2958−2965, or Microporous and Mesoporous Materials, 2019, 278, 314–321) since it allows the differentiation of amide-type carbons from carboxylic acids in an unequivocal manner.

Furthermore, the bibliography is not sufficiently complete, and the references associating POMs and organic molecules have not been sufficiently announced. Without being exhaustive, the references from the Paris and Versailles groups could have been better cited. See for instance: Angew. Chem. Int. Ed. 2021, 60, 6518 – 6525, Angew. Chem. Int. Ed. 2017, 56, 4872 –4876, ACS Appl. Energy Mater. 2020, 3, 163−169, Electrochimica Acta, 2021, 368, 137635…

In conclusion I believe that this work deserves publication in Inorganics, after addressing the above mentioned issues.

No particular problems detected in the manuscript. The English is clear and well written, even if some usual typos can be found from time to time.

Author Response

Dear Referees,

Before I get to the questions, I would like to thank you for taking your time to review our manuscript on behalf of all authors. For your convenience, we list our answers in the same order as your questions.

Referee #2

1. …even if transient absorption spectra have not been recorded, which is now usual for such systems.

 

Thank you for the suggestion to use the transient absorption spectroscopy to investigate this system. We are currently in the process of collecting these data, and as we are synthesizing POM’s conjugates with a series of organic dyes to, we will be collecting more transient absorption spectra for all samples to compare the photophysical properties of all synthesized dye-POM systems in future.

 

2. It would have been interesting to develop the interest of using these giant POMs for this type of application. This is not evident from the reading of this manuscript.

 

Thank you for the comment. Additional information has been added to the manuscript.

 

3. However, in my opinion, there is no definitive proof of the formation of amide bonds. ¹³C NMR spectroscopy is probably the most suitable technique for determining the formation of this bond, as has been shown in the past.

 

Thank you for the comment, and I agree that ¹³C NMR would be a good technique to affirm the formation of the amide bond. However, ¹³C NMR needs fair amounts of sample given the POM:dye stoichiometry of 1:1. It is also evident from the ¹H NMR spectrum of {Mo₁₃₂}@Si₆@EY@TBA that the conjugation with giant POM leads to diminished signal strength when the protons are in proximity to the POM’s surface. This behavior was previously observed for the {Mo₁₃₂}-porphyrin system (DOI: 10.1039/D2CC05193A). Knowing that, we chose the anionic dye, eosin Y, to avoid any possibility of the dye sticking onto the POM’s surface during grafting. We can affirm that the amide bond has been formed with IR spectroscopy for {Mo₁₃₂}@Si₆@EY@TBA, but due to a low content of the amide bond within the structure, the signal is not as strong as one might expect. Since eosin-Y cannot be linked to {Mo₁₃₂} electrostatically and considering the observed downshift of the aromatic protons on the NMR spectrum for {Mo₁₃₂}@Si₆@EY@TBA, as well as the elemental analysis data, we can confirm that the grafting of eosin Y to POM has indeed occurred. There is only one possibility for this linking – through the amide bond formation. However, in the future, we will make sure to collect the ¹³C NMR data to present clear spectroscopic evidence of the amide bond formation.

 

4. Furthermore, the bibliography is not sufficiently complete, and the references associating POMs and organic molecules have not been sufficiently announced.

 

Thank you for the comment. All cited references were added to the bibliography, as well as the additional ones.

Author Response File: Author Response.pdf