New Carboxylate Anionic Sm-MOF: Synthesis, Structure and Effect of the Isomorphic Substitution of Sm³⁺ with Gd³⁺ and Tb³⁺ Ions on the Luminescent Properties

Round 1

Reviewer 1 Report

The manuscript submitted by Ivanova presents a very nice piece of work related to a topical filed of lanthanide-based luminophores. The research is well designed and obviously deserves publication. However, some critical notes should be considered by authors team before acceptance: 

1. Cif file for Sm compound must be resubmitted to CCDC after  structure re-refinement to be converged and filling missing crystal size, color and habit fields. 

2. ESI file needs to be added to submission. 

3. x and 1-x indices are confused in line 18

4.  intra-atmospheric is possibly needed in line 56

5. 2.1 → 2.2 in line 146

6. Some empty space is recommended to introduce between the figures in line 162

7. Spacebars between values and their deviations should be deleted throughout table 1. 

8. Please check lines 354-356 as the sense of this part seems a bit elusive. 

Author Response

The manuscript submitted by Ivanova presents a very nice piece of work related to a topical filed of lanthanide-based luminophores. The research is well designed and obviously deserves publication.

Thank you for your review, comments and good evaluation of our article!

However, some critical notes should be considered by authors team before acceptance: 

1. Cif file for Sm compound must be resubmitted to CCDC after  structure re-refinement to be converged and filling missing crystal size, color and habit fields.

Thanks for the comments! All necessary changes have been made;  corrected file was  re-deposited.

2. ESI file needs to be added to submission. 

Thank you for your comment! The ESI file is attached.

3. x and 1-x indices are confused in line 18
4. intra-atmospheric is possibly needed in line 56
5. 2.1 → 2.2 in line 146
6. Some empty space is recommended to introduce between the figures in line 162
7. Spacebars between values and their deviations should be deleted throughout table 1. 

Thank you for your comments!  All these remarks have been corrected in the text of the work.

8. Please check lines 354-356 as the sense of this part seems a bit elusive. 

Thank you for your comment! The error has been fixed.

in a teflon container (10 ml) was placed 78.5 mg (0.5 mmol) H₃TDA, 3.00 ml DMF and 2.5 ml 0.2 M Sm(NO₃)₃ solution or required volumes of Sm(NO₃)₃  (0.2 M) and Gd(NO₃)₃ (0.2 M) or Tb(NO₃)₃  (0.2 M) solutions.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript reported the synthesis, structure, and the luminescent properties of two new carboxylate anionic Sm-MOFs. In particular, the influence of isomorphic substitution of Sm³⁺with Gd³⁺and Tb³⁺ionson its luminescent properties are explained. When Gd³⁺ and Tb³⁺ ions with different optical activities are used for substitution, the spectra and decay kinetics of Sm-TDA and Gd-TDA are different, which is interesting. The experiment and characterization are comprehensive; however, many important problems need to be revised urgently and the manuscript is recommended for publication after some revisions as follows:

1.      "Isomorphyc" in the title is misspelled and should be "isomorphic".

2.      PXRD data (Fig. S1 and S2) can be considered in the main body of this paper as important evidence for proving the synthesis of Sm_xGd_1-x-TDA andSm_xTb_1-x-TDA pure phases. Please further explain the mechanism of pH on the formation of Sm-TDA single crystal.

3.      There is a minor error, label “2.1 Crystal structure” should be "2.2 …".

4.      It is very interesting that the results of the contribution of the phosphorescent band is significant only at high concentrations of gadolinium. But unfortunately, the author only understands it as the "paramagnetic effect" of Gd³⁺ ions on TDA^3- ligand, which lacks theoretical depth. Please further explain this phenomenon in combination with previous literature.

5.      Compared with Gd³⁺ doping, the emission spectra of Sm_xTb_1-x-TDAare completely different, especially the luminescence behavior of ligands. Please explain in detail the reason why Tb³⁺ doping leads to ligand luminescence quenching. Is it related to the optical activity of doped ions?

6.      Due to the optical activity of Tb³⁺ and the sensitization of Tb³⁺ to Sm³⁺, it shows complex decay kinetics. Moreover, in the case of low Tb³⁺ concentration, there is obviously phosphorescence of ligand in the spectrum. When using double exponential function fitting, it is difficult to explain τ₁ and τ₂clearly in physics. Please reviewed the rationality of using two-component fitting.

Author Response

This manuscript reported the synthesis, structure, and the luminescent properties of two new carboxylate anionic Sm-MOFs. In particular, the influence of isomorphic substitution of Sm³⁺with Gd³⁺and Tb³⁺ionson its luminescent properties are explained. When Gd³⁺ and Tb³⁺ ions with different optical activities are used for substitution, the spectra and decay kinetics of Sm-TDA and Gd-TDA are different, which is interesting. The experiment and characterization are comprehensive; however, many important problems need to be revised urgently and the manuscript is recommended for publication after some revisions as follows:

Thank you for your review, comments and good evaluation of our article!

1. "Isomorphyc" in the title is misspelled and should be "isomorphic".

Thank you for your comment! The error has been fixed.

2. PXRD data (Fig. S1 and S2) can be considered in the main body of this paper as important evidence for proving the synthesis of Sm_xGd_1-x-TDA andSm_xTb_1-x-TDA pure phases.

Thank you for your comment.

According to your recommendation, figures S1 and S2 have been moved to the article (Figure 3 a and b).

 Please further explain the mechanism of pH on the formation of Sm-TDA single crystal.

Thanks for the interesting question! According to our assumption, an increase in acidity leads to a decrease in the rate of formation of MOF crystals, which improves their quality. The corresponding explanation has been added to the text

An decrease of the pH of the solution contributes to a decrease in the concentration of the ligand anion TDA^3-, which slows down the MOF formation reaction and promotes the growth of larger crystals suitable for X-ray

3. There is a minor error, label “2.1 Crystal structure” should be "2.2 …".

Thank you for your comment! The error has been fixed.

4. It is very interesting that the results of the contribution of the phosphorescent band is significant only at high concentrations of gadolinium. But unfortunately, the author only understands it as the "paramagnetic effect" of Gd³⁺ ions on TDA^3- ligand, which lacks theoretical depth. Please further explain this phenomenon in combination with previous literature.

The paramagnetic atom effect, manifested in particular for paramagnetic lanthanide chelates, is associated with an increase in the rate of intersystem crossing (ISC) due to partial mixing of singlet and triplet levels. Due to this effect, when gadolinium ions are introduced, the relative intensity of ligand fluorescence decreases, and phosphorescence increases. The corresponding explanation has been added to the text.

The contribution of the fluorescent band remains significant for any Sm³⁺:Gd³⁺ ratios, while the contribution of the phosphorescent band is significant only at high concentrations of gadolinium. This is due to the role of Gd³⁺ ions in the "paramagnetic effect", which enhances the phosphorescence of and reduces the intensity of ligand fluorescence by increasing the rate of intersystem crossing (ISC) between the singlet and triple spin states [50-52]

50. Tobita, S.; Arakawa, M.; Tanaka, I. Electronic Relaxation Processes of Rare Earth Chelates of Benzoyltrifluoroacetone. J. Phys. Chem. 1984, 88, 2697–2702, doi:10.1021/j150657a006.
51. Tobita, S.; Arakawa, M.; Tanaka, I. The Paramagnetic Metal Effect on the Ligand Localized S1 .Apprx. .Fwdarw. T1 Intersystem Crossing in the Rare-Earth-Metal Complexes with Methyl Salicylate. J. Phys. Chem. 1985, 89, 5649–5654, doi:10.1021/j100272a015.
52. Huang, H.; Bu, Y. Effect of Paramagnetic Open-Shell Gadolinium(III) Texaphyrin on Its Kinetics and Electronic Structures in Fluorescence and Phosphorescence Emission States. J. Phys. Chem. C 2019, 123, 28327–28335, doi:10.1021/acs.jpcc.9b05453.

5.      Compared with Gd³⁺ doping, the emission spectra of Sm_xTb_1-x-TDAare completely different, especially the luminescence behavior of ligands. Please explain in detail the reason why Tb³⁺ doping leads to ligand luminescence quenching. Is it related to the optical activity of doped ions?

      The energy of the resonant level of Gd³⁺ ions is approximately 32,200 cm^-1, while for Tb³⁺ resonant energy is approximately 20,500 cm^-1. Due to this, gadolinium's own luminescence in complexes is usually not manifested, and for terbium it is extremely common. As a result, instead of ligand phosphorescence in the case of terbium-samarium complexes, energy transfer from the ligand to terbium ions and from the latter to samarium ions is observed, i.e., "stepwise" sensitization of luminescence (LigandàTb³⁺àSm³⁺. Relevant explanations have been added to the text of the work.

This is due to the efficient transfer of energy from the triplet level of the ligand to the resonance level of Tb³⁺ ions

6. Due to the optical activity of Tb³⁺ and the sensitization of Tb³⁺ to Sm³⁺, it shows complex decay kinetics. Moreover, in the case of low Tb³⁺ concentration, there is obviously phosphorescence of ligand in the spectrum. When using double exponential function fitting, it is difficult to explain τ₁ and τ₂clearly in physics. Please reviewed the rationality of using two-component fitting.

In the case of Tb³⁺ luminescence decay curves, registration was carried out at a wavelength of 545±5 nm corresponding to the terbium transition ⁵D₄-⁷F₅. In this region, neither ligand fluorescence nor Sm³⁺ emission not observed. In addition, the characteristic fluorescence time of ligands, as a rule, is significantly less (about 1-50 ns) than the values which can be measured on a device with a pulsed xenon lamp. Based on these assumptions, we believe that both components of the biexponential decay is related exclusively to Tb³⁺.

Biexponential decay is usually attributed to systems with 2 crystallographically independent luminescent centers, but this is not always obvious. In the case of our complexes, we assume that due to the presence of a large number of close metal-metal distances in the structure (about 8 "neighboring" Ln³⁺ ions at distances of 6-8Å), there may have 0, 1, 2 or more samarium ions among the "neighbors" of each terbium atom. Terbium cations containing samarium ions among their "neighbors" effectively sensitize the luminescence of the latter, as a result of which their τ_obs decreases. Those terbium cations that do not have such "neighbors" lose excitation energy less efficiently and exhibit greater τ_obs.

This explanation is one of many possible.  The literature describes some examples of compounds containing lanthanide cations with the only one crystallographically independent type, but exhibiting bi- or polyexponential decay (see, for example, https://doi.org/10.1021/acs.inorgchem.7b02071; https://doi.org/10.1016/j.jssc.2018.08.017 etc).

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have revised the manucsript acoording to the reviewer reports. The current version can be accept for publication.