Structural Features of Eu3+ and Tb3+-Bipyridinedicarboxamide Complexes
, Konstantin V. Deriabin 1,†, Artem A. Rashevskii 1, Vitalii V. Suslonov 1, Alexander S. Novikov 1,3, Ivan S. Mukhin 1,2,4,5 and Regina M. Islamova 1,*Round 1
Reviewer 1 Report
The manuscripts reports on the synthesis of new lanthanide complexes of Eu3+ and Tb3+ as central atoms and N6 ,N6 '- 13 diisopropyl-[2,2'-bipyridine]-6,6'-dicarboxamides and their integration in PEG matrix. The fluorescence quantum yields of solid matrix of the complexes in PEG were rather reasonable and better that related ones in another polymer (PMC). The manuscript is well written and arranged. I only have a few comments that can improve the manuscript.
- The calculated HOMO–LUMO energy gaps (EH-L) in model structures [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 are 7.50 and 8.17 eV, respectively. ? Please better explain the relation with the optical gap (estimated HOMO – LUMO gap was found the double of optical gap).
- What about the absorption and emission spectra of QY of encapsulated complexes in PEG ? are they modified ? the spectra should also be shown to better characterize the optical properties
- In figure Figure 2b. D in yy axis (absorption) ?
- Please better discuss why fluorescence quantum yields in PMC can be lower.
Author Response
Point 1: the calculated HOMO–LUMO energy gaps (EH-L) in model structures [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 are 7.50 and 8.17 eV, respectively? Please better explain the relation with the optical gap (estimated HOMO–LUMO gap was found the double of optical gap).
Response 1: we are grateful to the Reviewer for the question. We have added the paragraph to the manuscript (page 5, lines 156–160):
“The calculated by the density-functional theory method (DFT) HOMO-LUMO gap gives only an approximation of the band gap and does not take into account the formation of electron-hole semi-particle in the excited state [28]. Thus, the optical gap obtained by Tauc’s relation from the UV-vis spectrum corresponds to the energy of the lowest electronic transition and could be substantially lower than EH-L.”
[28] Bredas, J.-L. Mind the Gap! Mater. Horiz. 2014, 1, 17–19, doi:10.1039/C3MH00098B.
Point 2: what about the absorption and emission spectra of QY of encapsulated complexes in PEG? Are they modified? The spectra should also be shown to better characterize the optical properties.
Response 2: according to the Reviewer's suggestion we have modified Figure 5 and Figure S10 along with the figures’ captions in the manuscript. We also added some discussion on emission and excitation spectra (page 6, lines 198–204):
“In comparison to the pure [Eu(BDCA)2(H2O)]Cl3, in the emission spectra signal at 580 nm, which corresponds to the forbidden transition 5D0→7F0, is disappearing after encapsulation in PEG. The intensity increases for the signals 5D0→7F0 (592 nm) and 5D4→7F6 (485 nm), which corresponds to energy transitions from excited state to ground state of Eu3+ and Tb3+, respectively. More information about excitation spectra and PL lifetime described in the Supplementary Material (see section 5 “Luminescent properties”).
Figure 5. PL emission spectra of [Eu(BDCA)2(H2O)]Cl3 (a) and [Tb(BDCA)2(H2O)]Cl3 (b) encapsulated in PEG at a 10% concentration of 10–3 mg·mL–1 under UV-light (340 nm).”
We have added the text to the Supplementary Material (page S17, lines 113–119):
“Both [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 show a broad excitation peak centered at 335 nm (Figure S10, a, b). However, for [Eu(BDCA)2(H2O)]Cl3 we observe a signal with low intensity at 400 nm, which could correspond to the ligand energy transition S1→T1 (Figure S10, a). After encapsulation in PEG excitation spectra of [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 show a narrow peak at 320 nm (Figure S10, c, d). This could be explained by influence of the polymer matrix (PEG) [12].”
Figure S10. Excitation spectra of raw [Eu(BDCA)2(H2O)]Cl3 (a), [Tb(BDCA)2(H2O)]Cl3 (b), and the encapsulated complexes in PEG (c, d).
Point 3: in Figure 2b D in yy axis (absorption)?
Response 3: we have corrected Figure 2 by replacing D with "absorption" in the manuscript (page 4).
Point 4: please better discuss why fluorescence quantum yields in PMC can be lower.
Response 4: according to the Reviewer’s suggestion, we have added the explanation (page 6, lines 190–193):
“The QY values for [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 are expectedly higher than that of corresponding reported Eu3+ and Tb3+-incorporating PMC [11] (Table 1). The increase in QY in complexes is associated with use of low-molecular-weight BDCA ligand in studied complexes instead of bipyridine-incorporating polymer ligand [11]. PMC contain polymer ligands with the number-average molecular weight of 45000–50000 [21] compared to BDCA ligand with a molecular weight of 326, which leads to luminescence quenching [20, 30].”
Reviewer 2 Report
The authors report the synthesis of Eu3+ and Tb3+- bipyridinedicarboxamide 2 complexes. Seems that the main advantage of these complexes when compared with previously complexes also prepared by the authors are the QY values. I think that the authors should compare the QY values not only with the values reported for the complexes previously prepared by them but also with the values reported for the complexes reported by other groups.
Other aspects that should be revised:
The abstract contains abbreviatures that are not defined (such as FTIR and TG).
A reference should be added to the phase that ends in line 49.
In line 89, the abbreviature HRESIMUS is used without being defined.
In Figure 2b, what means the “D, a.u.”?
How were the emission lifetimes obtained from Figure S10? What equation was fitted to the data?
In which wavelength were monitored the spectra in Figure S10?
Can the authors betters describe/discuss the spectra in Figure S10?
It should be given more details about the methods used for the PL measurements: what was the excitation source used for the steady state measurements?
What are the excitation wavelengths and monitoring wavelengths used for measuring the curves in Figure S11?
What are the error values of the QY values reported?
In Figure 5, a scale bar should be added.
Author Response
Reviewer 2: the authors report the synthesis of Eu3+ and Tb3+- bipyridinedicarboxamide 2 complexes. Seems that the main advantage of these complexes when compared with previously complexes also prepared by the authors are the QY values. I think that the authors should compare the QY values not only with the values reported for the complexes previously prepared by them but also with the values reported for the complexes reported by other groups. Other aspects that should be revised.
Response: we thank the Reviewer for the remark. There are reports on terbium bipyridinic complexes with thenoyltrifluoroacetone with a quantum yields about 23.4% (DOI: 10.1007/s10895-022-02920-7) and 37% (DOI: 10.1007/s10895-022-02956-9), which is comparable to quantum yields (36.5%) reported in our work. Europium bipyridinic complexes with carboxylate ligands (DOI: 10.1002/bio.4263) also shows quantum yield about 12.7%, which is comparable to our work (12.6%).
We have added the phrase to the manuscript (page 6, line 186–188):
“The QY values for [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 are expectedly higher than that of corresponding reported Eu3+ and Tb3+-incorporating PMC [11] (Table 1) and some other bipyridine derivatives of Eu3+ and Tb3+ [2,14,15].”
Point 1: the abstract contains abbreviatures that are not defined (such as FTIR and TG).
Response 1: according to the Reviewer’s remark, we have defined abbreviations in the abstract (lines 15–17):
“Structure of these complexes was established by single-crystal X-ray diffraction, mass spectrometry, 1H and 13C nuclear magnetic resonance, ultraviolet-visible, infrared spectroscopy, and thermogravimetry.”
Point 2: a reference should be added to the phase that ends in line 49.
Response 2: done accordingly.
Point 3: in line 89, the abbreviature HRESIMUS is used without being defined.
Response 3: the abbreviation HRESIMS is defined in the manuscript (page 2, line 65–66):
“Thus, the aims of the study are to (i) synthesize lanthanide complexes with a novel framework by complexation of Eu3+ and Tb3+ ions with N6,N6'-diisopropyl-[2,2'-bipyridine]-6,6'-dicarboxamide (BDCA) organic ligand, (ii) establish their structure by single-crystal X-ray diffraction (XRD), high-resolution electrospray ionization mass spectrometry (HRESIMS), 1H, 13C NMR, UV-vis, and FTIR spectroscopy, (iii) study their photoluminescent properties, (iv) incorporation of Eu, Tb-complexes as fillers in polymer films (polyethylene glycol, PEG) for creating luminescent composites, and (v) study their thermal stability. All our experimental data and the corresponding discussion are detailed in the following sections.”
Point 4: in Figure 2b, what means the “D, a.u.”?
Response 4: we have corrected Figure 2 by replacing D with "absorption" in the manuscript (page 4):
Figure 2. Spectra of [Eu(BDCA)2(H2O)]Cl3, [Tb(BDCA)2(H2O)]Cl3, and BDCA: FTIR in KBr (a) and UV-vis in DMSO (b).
Point 5: how were the emission lifetimes obtained from Figure S10? What equation was fitted to the data?
Response 5: we thank the Reviewer for the remark. We have added the text to the Supplementary Material (page S18, lines 130–132):
“Lifetime value was estimated by exponential approximation according to the Equation (1):
y = A1·exp(–x/t1) + y0 (1)
where y0=0, t1 - decay time constant, A1 - corresponding intensity.”
Point 6: in which wavelength were monitored the spectra in Figure S10?
Response 6: we thank the Reviewer for the question. We have added the text to the Supplementary Material (page S17, lines 113–114):
“Excitation spectra of [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 were measured at emission wavelengths of 617 nm and 554 nm, respectively.”
Point 7: can the authors betters describe/discuss the spectra in Figure S10?
Response 7: according the Reviewer’s suggestion we have added the text to the Supplementary Material and modified Figure S10 (page S17, lines 113–119):
“Both [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 show a broad excitation peak centered at 335 nm (Figure S10, a, b). However, for [Eu(BDCA)2(H2O)]Cl3 we observe a signal with low intensity at 400 nm, which could correspond to the ligand energy transition S1→T1 (Figure S10, a). After encapsulation in PEG excitation spectra of [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 show a narrow peak at 320 nm (Figure S10, c, d). This could be explained by influence of the polymer matrix (PEG) [12].”
Figure S10. Excitation spectra of raw [Eu(BDCA)2(H2O)]Cl3 (a), [Tb(BDCA)2(H2O)]Cl3 (b), and the encapsulated complexes in PEG (c, d).
Point 8: it should be given more details about the methods used for the PL measurements: what was the excitation source used for the steady state measurements?
Response 8: according to the Reviewer’s remark we have added the sentence to the section 3.2 “Methods” in the manuscript (page 8, lines 257–258):
“PL lifetime steady state measurements were performed via Horiba Fliorolog-3 spectrofluorometer with the impulse xenon lamp with a power of 150 W as an excitation source.”
Point 9: what are the excitation wavelengths and monitoring wavelengths used for measuring the curves in Figure S11?
Response 9: according to the Reviewer’s comment, we have added the phrase to the Supplementary Material (page S17, lines 124–126):
“PL lifetime (Figure S11) for both raw and encapsulated in PEG [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 were measured at emission wavelengths 617 nm and 554 nm, respectively. Excitation wavelength was 335 nm for the raw and 320 nm for the encapsulated complexes.”
We have added the graph of luminescence lifetime decay for [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3 encapsulated in PEG to Figure S11 in the Supplementary Material (page S17).
Figure S11. Luminescence lifetime decay for [Eu(BDCA)2(H2O)]Cl3 (a), [Tb(BDCA)2(H2O)]Cl3 (b), and the encapsulated complexes in PEG (c, d).
Point 10: what are the error values of the QY values reported?
Response 10: accuracy of the QY measurements is high and error values is less than 10% (DOI:10.1038/s41598-019-51718-4).
Point 11: in Figure 5, a scale bar should be added.
Response 11: done accordingly.
Reviewer 3 Report
The authors synthesized two lanthanide coordination complexes and characterized their structures and luminescent properties.
The referee believes that the manuscript can be accepted after addressing the following minor issues:
1. Figure 1：although in the caption, the authors claim that water molecules are omitted, the water ligands are still shown in the figure.
2. Regarding the single crystal X-ray diffraction experiments, the authors should also provide basic information of the experiment. For example, the name of the instrument, wavelength of the X-ray, upper limit of the resolution that data were collected, how good was the structural model and the diffraction data (Rint, Rwp, GOF, etc.)
3. For the theoretical calculations section:
How much difference is it between the crystal structure and the model optimized in the DFT calculation? An overlay between the two structure models should be provided.
Author Response
Point 1: Figure 1, although in the caption, the authors claim that water molecules are omitted, the water ligands are still shown in the figure.
Response 1: according to the Reviewer’s remark, we have corrected the caption in Figure 1 in the manuscript (page 3):
Figure 1. Molecular structures of the inner spheres of [Eu(BDCA)2(H2O)]Cl3 (a) and [Tb(BDCA)2(H2O)]Cl3 (b) with thermal ellipsoids shown at the 50% probability level. The Cl– anions and water molecules of the outer spheres of the complexes are omitted for better representability.
Point 2: regarding the single crystal X-ray diffraction experiments, the authors should also provide basic information of the experiment. For example, the name of the instrument, wavelength of the X-ray, upper limit of the resolution that data were collected, how good was the structural model and the diffraction data (Rint, Rwp, GOF, etc.).
Response 2: according to the Reviewer’s remark, we have added Table S1 to the Supplementary Material (page S3) and the sentence to the manuscript (page 8, lines 247–254):
“Crystallographic data for all crystals were obtained using Oxford Diffraction «SuperNova» (in the case of BDCA) and Rigaku Oxford Diffraction «Synergy XtaLAB» (in the case of [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3) diffractometers with monochromated micro-focus CuKα (λ = 1.54184) X-ray sources. All crystals were kept at 100 K during all data collection. Crystal structures were solved using ShelXT [31] structure solution program and refined by means using ShelXL [32] structure refinement program incorporated in Olex2 program package [33]. All crystallographic data for this paper can be obtained free of charge via Cambridge Crystallographic Database.”
Point 3: for the theoretical calculations section, how much difference is it between the crystal structure and the model optimized in the DFT calculation? An overlay between the two structure models should be provided.
Response 3: we thank the Reviewer for the question. The model structures fully correspond to the obtained X-ray data (DFT calculations were carried out based on the experimental X-ray structures of [Eu(BDCA)2(H2O)]Cl3 and [Tb(BDCA)2(H2O)]Cl3). We have noted about it in the Computational details section (Supplementary Material). The XYZ-files for model structures were also provided in the revised version of the Supplementary Material.
