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Electronic structure and optical properties of Sm(III) and Eu(III) complexes with hexamethylphosphoramide
Journal of Molecular Structure 1205 (2020) 127638
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Electronic structure and optical properties of Sm(III) and Eu(III) complexes with hexamethylphosphoramide A.V. Shurygin a, *, V.I. Vovna a, V.V. Korochentsev a, A.G. Mirochnik b, P.A. Zhikhareva b, V.I. Sergienko b a b
Far Eastern Federal University, 8 Sukhanova St., Vladivostok, Russia Institute of Chemistry FEB RAS, 159 Pr-t 100-letiya Vladivostoka, Vladivostok, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2019 Received in revised form 10 December 2019 Accepted 22 December 2019 Available online 24 December 2019
Nitrate complexes Ln(NO3)3 of Sm(III) and Eu(III) lanthanides with three neutral ligands HMPA (OP(NMe2)3) possessing triboluminescent properties were studied by X-ray photoelectron spectroscopy (XPS MgKa) and quantum chemistry (DFT/TDDFT). Interpretation of the bands of XPS spectra of the valence levels and core levels was carried out using the calculated energies and localization of the KohnSham orbitals. Using the DFT/TDDFT methods the influence of HMPA molecules on the electronic structure of lanthanide ion nitrate complexes was studied and the electronic effects of adduct formation are described. The nature of the bond of nitrate complexes with neutral molecules was revealed. The binding energy of Ln(NO3)3 (Ln ¼ Sm, Eu) with three HMPA molecules was determined theoretically (Sm(NO3)3(HMPA)3e5.17 eV, Eu(NO3)3(HMPA)3e4.11 eV). The relationship of the electronic structure with the absorption and excitation spectra, optical characteristics and its role in reducing the probability of the energy transfer from ligands to metal within the LMCT (ligand-to-metal charge-transfer) model is shown. The experimental and theoretical absorption spectra were obtained for the studied adducts possessing promising solubility properties in water. Prospects of further research of the intermolecular layers are shown for the crystals under study. © 2019 Elsevier B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy Density functional theory Nitrate complexes Ln(NO3)3(HMPA)3 Electronic structure OP(NMe2)3 Triboluminescence
1. Introduction Relevance of research of lanthanide complexes possessing unique photophysical properties is confirmed by an increase in the number of new publications [1e6], where they are studied by experimental and theoretical methods. Luminescence during destruction of the crystal lattice of some solids e triboluminescence is of considerable interest due to development of materials and devices capable to monitor destruction of various critical objects [7e10]. The main task in the triboluminescence research is to study the relationship of physico-chemical properties with the crystal lattice structure and the processes of energy transfer to lanthanide ions with the subsequent luminescence emission [11]. Several mechanisms are known that lead to the triboluminescence appearance: electrization at friction [12], electric discharge between the oppositely charged surfaces of crystal crack [13], piezoelectricity [14], and non-central symmetry of crystals [15]. One of
* Corresponding author. E-mail address: [email protected] (A.V. Shurygin). https://doi.org/10.1016/j.molstruc.2019.127638 0022-2860/© 2019 Elsevier B.V. All rights reserved.
the promising triboluminescent materials includes the complex compounds of lanthanide ions [16]. There is no unambiguous interpretation for the processes of excitation of lanthanide ions during triboluminescence, therefore, it is important to know not only the crystal structure but also the electronic one. The features of the electronic structure of the ligand environment and the model of charge transfer from ligands to metal (LMCT) are the determining factors in the luminescence efficiency of lanthanide ion complexes as it was shown in Refs. [17e19]. The electronic structure research of tris-b-diketonates of lanthanide ions made it possible to determine the bond type of ligands with a metal ion [20e22]. In the works [23,24], using the photoelectron spectra of vapors of the REE complexes and adducts, it was shown the sequence and structure of molecular orbitals involved in the excitation energy transfer processes and determined the LUMOHOMO energy gap. In Ref. [25] the reasons for luminescence lack were shown for compounds of nitrate complexes based on Ce, Nd, and Er with the addition of 1,10-phenanthroline molecule to them. Analogous knowledge about the geometric and electronic structure of complex compounds will help to understand the processes of formation of the triboluminescent properties.
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The work presents new research results of the electronic structure of Sm3þ and Eu3þ nitrate adducts with three molecules HMPA with the general formula Ln(NO3)3(HMPA)3. The methods of quantum chemical modeling DFT/TDDFT and X-ray photoelectron
spectroscopy with a MgKa radiation source were used. In order to analyze the relationship between the electronic structure and optical properties, it is shown the interpretation of the previously obtained absorption, excitation, and luminescence spectra [26,27]. 1.1. Experimental and calculation methods
Table 1 Some geometric parameters of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts. Parameters
Sm(NO3)3(HMPA)3
Eu(NO3)3(HMPA)3
Average r(Ln-O), Å Average r(Ln-OP), Å Average r(OP-P), Å :(Ln-OP1-P1), :(Ln-OP2-P2), :(Ln-OP3-P3), :(OP1-Ln-OP3), :(OP1-Ln-OP1), :(OP2-Ln-OP3),
2.58 (0.048) 2.33 (0.004) 1.49 (0.002) 146.8 160.1 172.4 153.8 86.1 85.6
2.52 (0.023) 2.32 (0.012) 1.49 (0.005) 158.3 169.7 165.9 150.3 93.6 108.2
Note: deviations for average value are given in brackets.
Sm(NO3)3(HMPA)3 CCDC: 1546567
Eu(NO3)3(HMPA)3 CCDC: 737633
Fig. 1. Geometric structure of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts with hidden H atoms. HMPA fragments are numbered and marked by a dashed line.
X-ray photoelectron spectra were obtained on an ultrahighvacuum photoelectron spectrometer (Omicron, Germany) with a hemispherical electrostatic analyzer (curvature radius of 125 mm) and a radiation source MgKa (1253.6 eV). The spectra were processed with the CASA XPS program [28]. Calibration of the electron binding energy scale was performed using the internal standard technique. In order to determine the chemical state of atoms, the spectral bands were decomposed into components with contours composed of a combination of Gaussian and Lorentz types. Quantum-chemical calculations were performed in the density functional theory (DFT) approximation using the FireFly 8.1.0 program [29]. The hybrid exchange correlation functional B3LYP5 was used, which proved to be successfully applied in the works [20e25,30,31]. For atoms Sm and Eu the basis set ECPnMWB was used with addition of the effective quasi-relativistic core potential developed by the Stuttgart/Cologne group [32]. The core potential for Sm includes 51 electrons (ECP51MWB) and for Eu it includes 52 electrons (ECP52MWB). The full-electron basis set 6-311G* was used for the rest atoms. The binding energy (E) of three ligands HMPA with the complex Ln(NO3)3 was determined by the formula: E ¼ EА e (ЕC þ 3 ЕL) where ЕА is the adduct total energy, ЕC is the total energy of the complex Ln(NO3)3, ЕL is the total energy of the free molecule HMPA. When modeling the X-ray photoelectron spectra, the photoionization cross section for the radiation source MgKa 1253.6 eV was taken into account [33]. 2. Results
Table 2 Mulliken atomic charges (a.u.) and bond orders (P). #
CI
I
CII
II
HMPA
Ln (NO3)1 (NO3)2 (NO3)3 HMPA1/O/P HMPA2/O/P HMPA3/O/P Avg. P(Ln-O) Avg. P(Ln-OP) Avg. P(OP-P)
1,91 0.64 0.64 0.64 e e e 0,264 e e
1.88 0.73 0.73 0.72 0.10/-0.91/1.94/ 0.11/-0.90/1.97 0.09/-0.92/1.97 0170 0,267 1174
1.90 0.63 0.63 0.63 e e e 0,286 e e
1.89 0.73 0.73 0.72 0.10/-0.93/1.95 0.09/-0.91/1.95 0.11/-0.91/1.95 0171 0,261 1178
e e e e 0/-0.69/1.59 e e e e e
Note: CI e Sm(NO3)3, I e Sm(NO3)3(HMPA)3, CII e Eu(NO3)3, II e Eu(NO3)3(HMPA)3; HMPA e free neutral molecule OP(NМе2)3.
OPN(CH3)2 in the free HMPA
Geometric structure of adducts. The adduct geometry was built on basis of the X-ray structural data obtained in Refs. [26,27]. The main differences in the geometric structure of the two adducts are mainly associated with the ionic radius of lanthanides, some geometric characteristics are shown in Table 1. Fig. 1 presents images of the structure of the adducts under study. All groups NO3 have a bond with an ion through two atoms O, every molecule OP(NМе2)3 is bound with an ion through an atom O, as a result, the coordination number of two adducts is 9. The binding energy (E) of three ligands HMPA with Ln(NO3)3 was determined using comparative calculations, for Sm(NO3)3(HMPA)3 it is 5.17 eV and for Eu(NO3)3(HMPA)3 it is 4.11 eV. Electronic structure of adducts. The charge distribution and
OPN(CH3)2 in the adduct I
Fig. 2. Mulliken charge distribution (a.u.) in OPN(CH3)2 in the free molecule HMPA and in the adduct I.
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Table 3 Energy (eV) and electron density localization (%) of 12 upper occupied MOs and 6 lower unoccupied MOs according to the DFT calculation of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts. Sm(NO3)3(HMPA)3
MO
ε, eV
Sm
205 204 203 202 201 200 199 198 197 196 195 194 193 192 191 190 189 188
L þ5 L þ4 L þ3 L þ2 L þ1 LUMO HOMO H 1 H 2 H 3 H 4 H 5 H 6 H 7 H 8 H 9 H 10 H 11
0.92 0.87 0.35 0.03 0.53 0.55 5.84 6.01 6.10 6.23 6.42 6.48 6.50 6.59 6.76 6.93 6.97 6.99
Eu(NO3)3(HMPA)3
МО localization, %
1 0 1 2 3 3 0 0 1 0 0 1 1 0 0 0 0 0
(NO3)3
0 0 2 95 93 92 2 7 87 2 9 93 87 4 2 11 12 2
ε, eV HMPA 1
2
3
12 57 3 1 1 1 98 0 3 97 1 1 2 0 0 1 85 98
36 23 83 1 2 2 0 0 0 1 88 3 7 2 1 87 3 0
51 20 11 1 1 2 0 92 9 0 1 2 2 94 97 1 0 0
1.03 0.83 0.50 0.44 0.10 0.67 5.82 5.96 6.14 6.25 6.28 6.47 6.52 6.62 6.66 6.79 6.82 6.85
МО localization, % Eu
(NO3)3
HMPA 1
2
3
1 1 1 1 6 2 2 0 1 0 1 0 0 0 1 0 1 0
1 1 32 64 90 94 95 2 83 4 13 1 9 10 83 13 82 79
47 23 6 2 0 2 2 98 3 95 0 0 0 0 0 0 4 19
24 30 45 26 4 0 0 0 0 0 0 4 88 11 6 79 10 2
27 45 16 7 0 2 1 0 12 1 86 96 3 79 11 8 3 0
Note: Numbers 1, 2, 3 indicate different fragments of HMPA. First column.
bond orders shown in Table 2 indicates the presence of ion-dipole interaction. At transition from the tris-complexes CI (Sm(NO3)3) and CII (Eu(NO3)3) to the complexes I (Sm(NO3)3(HMPA)3) and II (Eu(NO3)3(HMPA)3), the positive charge on Ln ions decreases, and the negative charge of ligands NO3 increases. Considering changes in charges in HMPA fragments of the adducts and comparing with charges in the free state of HMPA (Fig. 2), one can see an increase in the negative charge of the atom O, atoms P lose density (positive charge increases), and HMPA acts as an electron density donor. This distribution indicates an increase in the electron density on Ln(NO3)3 which is “pulled out” from the neutral molecules HMPA having a total positive charge in the adducts (from 0.08 to 0.11 a.e.). The analogous charge distribution was observed for the nitrate complexes with 1,10-phenanthroline molecules [24]. At that, after a comparison it was noted that the value of redistributed electron density is higher, that indicates a more “strong” ion-dipole bond of the nitrate complexes with 1,10-phenanthroline molecules than with HMPA molecules. The electronic structure of the adducts is presented in form of a table comprising localization of the electron density of MOs (Table 3) and the energy diagram of molecular orbitals (Fig. 3). The levels can be separated according to the localization type and correlated with the levels of Ln tris-nitrate and HMPA molecule. In order to demonstrate clearly the correlation of levels, the levels of the complexes Ln(NO3)3 and (HMPA)3 with the geometric structure corresponding to the state in the adduct Ln(NO3)3(HMPA)3 were additionally plotted in the diagram. When three neutral molecules HMPA are in the position corresponding to the position in the adduct (3L*), the MOs shift by an average of 1.2 eV relative to the free molecule HMPA (L). The first 9 HOMOs are localized on N2p-orbitals, followed by 6 orbitals with localization on O2p, then followed by the group of binding orbitals O2p-P3p. For the adduct I there is a difference from the adduct II MOs, so in the region of the group O2p-P3p of 3LI* orbitals there is a MO with an energy of 8.35 eV and localization Op2 e 13%, N2p e 6%, P3p e 7%, C2p e 47%, H1s e 27% on one HMPA fragment, and at transition to the adduct I this orbital completely retains its localization and has an energy of 9.27 eV. MOs of the nitrate complexes CI and CII under transition from C
Fig. 3. Correlation diagram of Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II) adducts in the range from 5 to 12 eV. HMPA localization is shown in red, (NO3)3 localization e in blue, and localization of lanthanide ions e in green. Designations: L e HMPA ligand, C e complex of Ln nitrate, * - position of atoms corresponding completely to the position in the adduct.
to C* have a positive shift, however, for the orbitals 7a1, 6a2, 12e, 5a2, 11e, and 10e it is observed an increase in the intervals that leads to a more uniform occupation of the energy region from 9.2 to 10.5 eV. MOs of the studied adducts possess close intervals and localization, therefore, description of the structure of valence electron orbitals is shown only for the adduct I with some remarks regarding differences in the structure of the two adducts. The upper group of MOs in the region of 5.5e7.8 eV includes 9 orbitals of HMPA and 12 orbitals of NO3 groups which do not undergo mixing, while 3 mixed
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orbitals are observed in this region for the adduct 2. Then the group of 6 mixed orbitals follows, after that mixed orbitals are completely absent. The rest correlation is shown well in the diagram (Fig. 3). In order to understand the structure of excited states, it is also necessary to analyze composition of the lower unoccupied orbitals. Table 3 presents localization of 6 LUMOs for the adducts I and II,
Fig. 4. XPS spectra of the valence region of Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II) adducts with assignment of bands to the Kohn-Sham orbital energies. (HMPA)3 localization is shown in red, (NO3)3 localization e in blue, localization of lanthanide ions Sm3þ and Eu3 e in green; the blue dotted line shows the modeled spectrum.
which shows that the 3 lower orbitals are completely localized on NO3 groups in both adducts. Then, for the adduct I, 3 orbitals with full localization on HMPA follow, and for the adduct II there is a mixed orbital (energy of 0.5 eV) with localization of 32% on NO3 groups and 67% on HMPA ligands, followed by MOs localized on HMPA. X-ray photoelectron spectra of adducts. X-ray photoelectron spectra of the valence region (Fig. 4) and core levels (Fig. 5, Table 4) were obtained for the studied adducts I and II. Using the obtained calculation data and taking into account the photoionization cross section for 1256.3 eV radiation, ligand levels were modeled (the blue dotted line shown in Fig. 4) and the bands in the XPS spectrum valence region were interpreted. The intense band 1 in both compounds is mainly due to 4felectrons of Ln3þ. The position of Sm4f band maximum is 6.52 eV, and that of Eu4f band maximum is 8.75 eV. For the compound of Eu, the band 0 is observed with the position of maximum of 2.16 eV. The difference between the positions of maxima of the bands 0 and 1 is 6.59 eV. The position of Eu2þ ions in methacrylate compounds was shown in work [34] where the band intensity of divalent europium increased with an increase in the irradiation time. The difference in the positions of maxima of the bands Eu2þ and Eu3þ in Ref. [34] was 6.87 eV. Taking that into account, there is a reason to guess that the band 0 is due to the presence of the divalent Eu2þ ion formed during the compound ionization. The bands 2, 3, and 4 are assigned to the ligand molecular orbitals 2p and 2s. C2p levels contribute to the bands 1, 2, 3, and 4, but taking into account the photoionization cross section, they do not make a significant contribution to the band intensity. The two bands of the spin-orbital doublet of 5p-electrons of Sm and Eu appears in the bands 5 and 6 of the ligand MOs of 2s-type from 20 to 30 eV. The splitting of the bands 5 and 6 in the spectra of Sm compound is 4.30 eV, in the spectra of Eu compound the splitting is 5.18 eV. The obtained XPS spectra of Ln4d levels are shown in Fig. 5, Table 4 presents the values of bond energies with the marked maximum position of the bands of core levels C1s, O1s, and N1s. For N1s levels, the values are shown for the positions of two bands corresponding to the positions of N atoms bonded to CH3 groups in HMPA and N atoms bonded to three O atoms in NO3. The Ln4d levels of lanthanide ions are shown in Fig. 4, and for Eu3þ the splitting is found to be 5.59 eV. The P2p levels are in the region of 4d electrons of Ln ions, that makes it difficult to determine the band splitting for Sm4d. The obtained data correspond to the data obtained in other works [24,34e36].
Fig. 5. XPS spectra of core 4d levels of Ln adducts I and II.
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Table 4 Binding energies (EBE) of core electrons of the adducts I and II. #
Sm(NO3)3(HMPA)3 (I) Eu(NO3)3(HMPA)3 (II)
ЕBE, eV C1s
O1s
N1s (N(CH3); NO3)
Ln4d (4d5/2;4d3/2)
288.81 288.59
532.17 532.37
399.71; 407.17 399.87; 407.38
e 136.9; 142.5
2.1. TDDFT modeling, absorption and excitation spectra The diagram of excited states (Fig. 6) shows the density of 50 calculated singlet and triplet states located in the region of Ln3þ ion levels. The charge transfer transitions (from the HOMO of HMPA to the LUMO of NO3) are shown in green, the transitions inside NO3 ligands are shown in blue, and the transitions inside HMPA are shown in red. In order to trace the effect of HMPA addition to nitrate complexes, the diagram additionally shows excited states of the nitrate complexes Ln(NO3)3. This diagram indicates that the ligand levels are located higher than the levels of Lnþ3 ions, that can be associated with the low probability of the energy transfer from ligands to ions within the LMCT model. Fig. 7 presents the experimental absorption spectra of aqueous
Fig. 7. Absorption spectra of the adducts Sm(NO3)3(HMPA)3 (red) and Eu(NO3)3(HMPA)3 (blue), the dotted line shows the TDDFT calculated spectrum.
solutions of the adducts I and II, and the TDDFT calculated spectra. The main band observed in the experiment is related to the absorption of HMPA ligands, that is confirmed by the plotted absorption spectrum of an individual HMPA molecule. In the region below 250 nm in the experimental spectrum, the very intense band is observed causing the device “scale exceeding”. The calculations also contain the strongly intense bands caused by the charge transfer transitions, that does not contradict the experimental data. Analyzing the excitation spectra of the adducts (Fig. 8), one can see that the luminescence excitation is caused only by f-f transitions of Sm3þ ions (transitions from 6H5/2) and Eu3þ (transitions from 7F0). An addition of three HMPA molecules to nitrate complexes does not reduce significantly the HOMO-LUMO energy gap (CI - 5.44 эВ, CII - 5.46 эВ) that leads to high values of this characteristic for both adducts (I - 5.56 eV, II - 5.15 eV). This indicates a decrease in the probability of luminescence appearance associated with the transfer of absorbed energy by ligands to a metal ion.
3. Conclusions
Fig. 6. Diagram of 50 excited singlet (S) and triplet (T) states of the nitrate complexes Sm(NO3)3 (CI) and Eu(NO3)3 (CII) and the adducts Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II). The transitions NO3 e NO3* МО are shown in blue, the transitions HMPA - HMPA* are in red, the charge transfer transitions HMPA e NO3* are in green.
The obtained experimental and theoretical results made it possible to show the influence of the electronic structure on the optical properties and reasons for the low probability of energy transfer from ligands to ions in the adducts Ln(NO3)3(HMPA)3 (Ln ¼ Sm, Eu). For the first time, the XPS spectra of the valence levels and core levels of the studied compounds were obtained and interpreted. The ion-dipole nature of the bond of complexes with neutral ligands was determined. All that makes it possible to conclude that the electronic structure determines the luminescence possibility and is directly related to the spatial arrangement of atoms of the molecular crystal. Slight distortions of the geometric structure cause rearrangement of electronic levels and, as a consequence, deterioration or improvement of the luminescent properties. As it was noted in the Introduction, the mechanism of the triboluminescence appearance is not exactly known, and that requires a detailed research of all features of this class of compounds. The results of research of the electronic structure obtained in this work make it possible to proceed to a study of the bond structure of intermolecular layers and to reveal their effect on the electronic structure.
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Sm(NO3)3(HMPA)3
Eu(NO3)3(HMPA)3
Fig. 8. Excitation spectra of the adducts Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3.
Author contributions section A.V. Shurygin: Software, Formal analysis, Investigation, Writing - Review & Editing, Visualization. V.I. Vovna: Conceptualization, Methodology, Project administration. V.V. Korochentsev: Investigation, Resources. A.G. Mirochnik: Resources. P.A. Zhikhareva: Resources. V.I. Sergienko: Supervision.
[10]
[11]
[12]
Acknowledgments [13]
This work was financially supported by the Ministry of Education and Science of the Russian Federation within the State Assignment for the Research Project No. 3.2168.2017/4.6 of Far Eastern Federal University.
[14]
[15]
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Electronic structure and optical properties of Sm(III) and Eu(III) complexes with hexamethylphosphoramide A.V. Shurygin a, *, V.I. Vovna a, V.V. Korochentsev a, A.G. Mirochnik b, P.A. Zhikhareva b, V.I. Sergienko b a b
Far Eastern Federal University, 8 Sukhanova St., Vladivostok, Russia Institute of Chemistry FEB RAS, 159 Pr-t 100-letiya Vladivostoka, Vladivostok, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2019 Received in revised form 10 December 2019 Accepted 22 December 2019 Available online 24 December 2019
Nitrate complexes Ln(NO3)3 of Sm(III) and Eu(III) lanthanides with three neutral ligands HMPA (OP(NMe2)3) possessing triboluminescent properties were studied by X-ray photoelectron spectroscopy (XPS MgKa) and quantum chemistry (DFT/TDDFT). Interpretation of the bands of XPS spectra of the valence levels and core levels was carried out using the calculated energies and localization of the KohnSham orbitals. Using the DFT/TDDFT methods the influence of HMPA molecules on the electronic structure of lanthanide ion nitrate complexes was studied and the electronic effects of adduct formation are described. The nature of the bond of nitrate complexes with neutral molecules was revealed. The binding energy of Ln(NO3)3 (Ln ¼ Sm, Eu) with three HMPA molecules was determined theoretically (Sm(NO3)3(HMPA)3e5.17 eV, Eu(NO3)3(HMPA)3e4.11 eV). The relationship of the electronic structure with the absorption and excitation spectra, optical characteristics and its role in reducing the probability of the energy transfer from ligands to metal within the LMCT (ligand-to-metal charge-transfer) model is shown. The experimental and theoretical absorption spectra were obtained for the studied adducts possessing promising solubility properties in water. Prospects of further research of the intermolecular layers are shown for the crystals under study. © 2019 Elsevier B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy Density functional theory Nitrate complexes Ln(NO3)3(HMPA)3 Electronic structure OP(NMe2)3 Triboluminescence
1. Introduction Relevance of research of lanthanide complexes possessing unique photophysical properties is confirmed by an increase in the number of new publications [1e6], where they are studied by experimental and theoretical methods. Luminescence during destruction of the crystal lattice of some solids e triboluminescence is of considerable interest due to development of materials and devices capable to monitor destruction of various critical objects [7e10]. The main task in the triboluminescence research is to study the relationship of physico-chemical properties with the crystal lattice structure and the processes of energy transfer to lanthanide ions with the subsequent luminescence emission [11]. Several mechanisms are known that lead to the triboluminescence appearance: electrization at friction [12], electric discharge between the oppositely charged surfaces of crystal crack [13], piezoelectricity [14], and non-central symmetry of crystals [15]. One of
* Corresponding author. E-mail address: [email protected] (A.V. Shurygin). https://doi.org/10.1016/j.molstruc.2019.127638 0022-2860/© 2019 Elsevier B.V. All rights reserved.
the promising triboluminescent materials includes the complex compounds of lanthanide ions [16]. There is no unambiguous interpretation for the processes of excitation of lanthanide ions during triboluminescence, therefore, it is important to know not only the crystal structure but also the electronic one. The features of the electronic structure of the ligand environment and the model of charge transfer from ligands to metal (LMCT) are the determining factors in the luminescence efficiency of lanthanide ion complexes as it was shown in Refs. [17e19]. The electronic structure research of tris-b-diketonates of lanthanide ions made it possible to determine the bond type of ligands with a metal ion [20e22]. In the works [23,24], using the photoelectron spectra of vapors of the REE complexes and adducts, it was shown the sequence and structure of molecular orbitals involved in the excitation energy transfer processes and determined the LUMOHOMO energy gap. In Ref. [25] the reasons for luminescence lack were shown for compounds of nitrate complexes based on Ce, Nd, and Er with the addition of 1,10-phenanthroline molecule to them. Analogous knowledge about the geometric and electronic structure of complex compounds will help to understand the processes of formation of the triboluminescent properties.
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The work presents new research results of the electronic structure of Sm3þ and Eu3þ nitrate adducts with three molecules HMPA with the general formula Ln(NO3)3(HMPA)3. The methods of quantum chemical modeling DFT/TDDFT and X-ray photoelectron
spectroscopy with a MgKa radiation source were used. In order to analyze the relationship between the electronic structure and optical properties, it is shown the interpretation of the previously obtained absorption, excitation, and luminescence spectra [26,27]. 1.1. Experimental and calculation methods
Table 1 Some geometric parameters of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts. Parameters
Sm(NO3)3(HMPA)3
Eu(NO3)3(HMPA)3
Average r(Ln-O), Å Average r(Ln-OP), Å Average r(OP-P), Å :(Ln-OP1-P1), :(Ln-OP2-P2), :(Ln-OP3-P3), :(OP1-Ln-OP3), :(OP1-Ln-OP1), :(OP2-Ln-OP3),
2.58 (0.048) 2.33 (0.004) 1.49 (0.002) 146.8 160.1 172.4 153.8 86.1 85.6
2.52 (0.023) 2.32 (0.012) 1.49 (0.005) 158.3 169.7 165.9 150.3 93.6 108.2
Note: deviations for average value are given in brackets.
Sm(NO3)3(HMPA)3 CCDC: 1546567
Eu(NO3)3(HMPA)3 CCDC: 737633
Fig. 1. Geometric structure of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts with hidden H atoms. HMPA fragments are numbered and marked by a dashed line.
X-ray photoelectron spectra were obtained on an ultrahighvacuum photoelectron spectrometer (Omicron, Germany) with a hemispherical electrostatic analyzer (curvature radius of 125 mm) and a radiation source MgKa (1253.6 eV). The spectra were processed with the CASA XPS program [28]. Calibration of the electron binding energy scale was performed using the internal standard technique. In order to determine the chemical state of atoms, the spectral bands were decomposed into components with contours composed of a combination of Gaussian and Lorentz types. Quantum-chemical calculations were performed in the density functional theory (DFT) approximation using the FireFly 8.1.0 program [29]. The hybrid exchange correlation functional B3LYP5 was used, which proved to be successfully applied in the works [20e25,30,31]. For atoms Sm and Eu the basis set ECPnMWB was used with addition of the effective quasi-relativistic core potential developed by the Stuttgart/Cologne group [32]. The core potential for Sm includes 51 electrons (ECP51MWB) and for Eu it includes 52 electrons (ECP52MWB). The full-electron basis set 6-311G* was used for the rest atoms. The binding energy (E) of three ligands HMPA with the complex Ln(NO3)3 was determined by the formula: E ¼ EА e (ЕC þ 3 ЕL) where ЕА is the adduct total energy, ЕC is the total energy of the complex Ln(NO3)3, ЕL is the total energy of the free molecule HMPA. When modeling the X-ray photoelectron spectra, the photoionization cross section for the radiation source MgKa 1253.6 eV was taken into account [33]. 2. Results
Table 2 Mulliken atomic charges (a.u.) and bond orders (P). #
CI
I
CII
II
HMPA
Ln (NO3)1 (NO3)2 (NO3)3 HMPA1/O/P HMPA2/O/P HMPA3/O/P Avg. P(Ln-O) Avg. P(Ln-OP) Avg. P(OP-P)
1,91 0.64 0.64 0.64 e e e 0,264 e e
1.88 0.73 0.73 0.72 0.10/-0.91/1.94/ 0.11/-0.90/1.97 0.09/-0.92/1.97 0170 0,267 1174
1.90 0.63 0.63 0.63 e e e 0,286 e e
1.89 0.73 0.73 0.72 0.10/-0.93/1.95 0.09/-0.91/1.95 0.11/-0.91/1.95 0171 0,261 1178
e e e e 0/-0.69/1.59 e e e e e
Note: CI e Sm(NO3)3, I e Sm(NO3)3(HMPA)3, CII e Eu(NO3)3, II e Eu(NO3)3(HMPA)3; HMPA e free neutral molecule OP(NМе2)3.
OPN(CH3)2 in the free HMPA
Geometric structure of adducts. The adduct geometry was built on basis of the X-ray structural data obtained in Refs. [26,27]. The main differences in the geometric structure of the two adducts are mainly associated with the ionic radius of lanthanides, some geometric characteristics are shown in Table 1. Fig. 1 presents images of the structure of the adducts under study. All groups NO3 have a bond with an ion through two atoms O, every molecule OP(NМе2)3 is bound with an ion through an atom O, as a result, the coordination number of two adducts is 9. The binding energy (E) of three ligands HMPA with Ln(NO3)3 was determined using comparative calculations, for Sm(NO3)3(HMPA)3 it is 5.17 eV and for Eu(NO3)3(HMPA)3 it is 4.11 eV. Electronic structure of adducts. The charge distribution and
OPN(CH3)2 in the adduct I
Fig. 2. Mulliken charge distribution (a.u.) in OPN(CH3)2 in the free molecule HMPA and in the adduct I.
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Table 3 Energy (eV) and electron density localization (%) of 12 upper occupied MOs and 6 lower unoccupied MOs according to the DFT calculation of Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3 adducts. Sm(NO3)3(HMPA)3
MO
ε, eV
Sm
205 204 203 202 201 200 199 198 197 196 195 194 193 192 191 190 189 188
L þ5 L þ4 L þ3 L þ2 L þ1 LUMO HOMO H 1 H 2 H 3 H 4 H 5 H 6 H 7 H 8 H 9 H 10 H 11
0.92 0.87 0.35 0.03 0.53 0.55 5.84 6.01 6.10 6.23 6.42 6.48 6.50 6.59 6.76 6.93 6.97 6.99
Eu(NO3)3(HMPA)3
МО localization, %
1 0 1 2 3 3 0 0 1 0 0 1 1 0 0 0 0 0
(NO3)3
0 0 2 95 93 92 2 7 87 2 9 93 87 4 2 11 12 2
ε, eV HMPA 1
2
3
12 57 3 1 1 1 98 0 3 97 1 1 2 0 0 1 85 98
36 23 83 1 2 2 0 0 0 1 88 3 7 2 1 87 3 0
51 20 11 1 1 2 0 92 9 0 1 2 2 94 97 1 0 0
1.03 0.83 0.50 0.44 0.10 0.67 5.82 5.96 6.14 6.25 6.28 6.47 6.52 6.62 6.66 6.79 6.82 6.85
МО localization, % Eu
(NO3)3
HMPA 1
2
3
1 1 1 1 6 2 2 0 1 0 1 0 0 0 1 0 1 0
1 1 32 64 90 94 95 2 83 4 13 1 9 10 83 13 82 79
47 23 6 2 0 2 2 98 3 95 0 0 0 0 0 0 4 19
24 30 45 26 4 0 0 0 0 0 0 4 88 11 6 79 10 2
27 45 16 7 0 2 1 0 12 1 86 96 3 79 11 8 3 0
Note: Numbers 1, 2, 3 indicate different fragments of HMPA. First column.
bond orders shown in Table 2 indicates the presence of ion-dipole interaction. At transition from the tris-complexes CI (Sm(NO3)3) and CII (Eu(NO3)3) to the complexes I (Sm(NO3)3(HMPA)3) and II (Eu(NO3)3(HMPA)3), the positive charge on Ln ions decreases, and the negative charge of ligands NO3 increases. Considering changes in charges in HMPA fragments of the adducts and comparing with charges in the free state of HMPA (Fig. 2), one can see an increase in the negative charge of the atom O, atoms P lose density (positive charge increases), and HMPA acts as an electron density donor. This distribution indicates an increase in the electron density on Ln(NO3)3 which is “pulled out” from the neutral molecules HMPA having a total positive charge in the adducts (from 0.08 to 0.11 a.e.). The analogous charge distribution was observed for the nitrate complexes with 1,10-phenanthroline molecules [24]. At that, after a comparison it was noted that the value of redistributed electron density is higher, that indicates a more “strong” ion-dipole bond of the nitrate complexes with 1,10-phenanthroline molecules than with HMPA molecules. The electronic structure of the adducts is presented in form of a table comprising localization of the electron density of MOs (Table 3) and the energy diagram of molecular orbitals (Fig. 3). The levels can be separated according to the localization type and correlated with the levels of Ln tris-nitrate and HMPA molecule. In order to demonstrate clearly the correlation of levels, the levels of the complexes Ln(NO3)3 and (HMPA)3 with the geometric structure corresponding to the state in the adduct Ln(NO3)3(HMPA)3 were additionally plotted in the diagram. When three neutral molecules HMPA are in the position corresponding to the position in the adduct (3L*), the MOs shift by an average of 1.2 eV relative to the free molecule HMPA (L). The first 9 HOMOs are localized on N2p-orbitals, followed by 6 orbitals with localization on O2p, then followed by the group of binding orbitals O2p-P3p. For the adduct I there is a difference from the adduct II MOs, so in the region of the group O2p-P3p of 3LI* orbitals there is a MO with an energy of 8.35 eV and localization Op2 e 13%, N2p e 6%, P3p e 7%, C2p e 47%, H1s e 27% on one HMPA fragment, and at transition to the adduct I this orbital completely retains its localization and has an energy of 9.27 eV. MOs of the nitrate complexes CI and CII under transition from C
Fig. 3. Correlation diagram of Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II) adducts in the range from 5 to 12 eV. HMPA localization is shown in red, (NO3)3 localization e in blue, and localization of lanthanide ions e in green. Designations: L e HMPA ligand, C e complex of Ln nitrate, * - position of atoms corresponding completely to the position in the adduct.
to C* have a positive shift, however, for the orbitals 7a1, 6a2, 12e, 5a2, 11e, and 10e it is observed an increase in the intervals that leads to a more uniform occupation of the energy region from 9.2 to 10.5 eV. MOs of the studied adducts possess close intervals and localization, therefore, description of the structure of valence electron orbitals is shown only for the adduct I with some remarks regarding differences in the structure of the two adducts. The upper group of MOs in the region of 5.5e7.8 eV includes 9 orbitals of HMPA and 12 orbitals of NO3 groups which do not undergo mixing, while 3 mixed
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orbitals are observed in this region for the adduct 2. Then the group of 6 mixed orbitals follows, after that mixed orbitals are completely absent. The rest correlation is shown well in the diagram (Fig. 3). In order to understand the structure of excited states, it is also necessary to analyze composition of the lower unoccupied orbitals. Table 3 presents localization of 6 LUMOs for the adducts I and II,
Fig. 4. XPS spectra of the valence region of Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II) adducts with assignment of bands to the Kohn-Sham orbital energies. (HMPA)3 localization is shown in red, (NO3)3 localization e in blue, localization of lanthanide ions Sm3þ and Eu3 e in green; the blue dotted line shows the modeled spectrum.
which shows that the 3 lower orbitals are completely localized on NO3 groups in both adducts. Then, for the adduct I, 3 orbitals with full localization on HMPA follow, and for the adduct II there is a mixed orbital (energy of 0.5 eV) with localization of 32% on NO3 groups and 67% on HMPA ligands, followed by MOs localized on HMPA. X-ray photoelectron spectra of adducts. X-ray photoelectron spectra of the valence region (Fig. 4) and core levels (Fig. 5, Table 4) were obtained for the studied adducts I and II. Using the obtained calculation data and taking into account the photoionization cross section for 1256.3 eV radiation, ligand levels were modeled (the blue dotted line shown in Fig. 4) and the bands in the XPS spectrum valence region were interpreted. The intense band 1 in both compounds is mainly due to 4felectrons of Ln3þ. The position of Sm4f band maximum is 6.52 eV, and that of Eu4f band maximum is 8.75 eV. For the compound of Eu, the band 0 is observed with the position of maximum of 2.16 eV. The difference between the positions of maxima of the bands 0 and 1 is 6.59 eV. The position of Eu2þ ions in methacrylate compounds was shown in work [34] where the band intensity of divalent europium increased with an increase in the irradiation time. The difference in the positions of maxima of the bands Eu2þ and Eu3þ in Ref. [34] was 6.87 eV. Taking that into account, there is a reason to guess that the band 0 is due to the presence of the divalent Eu2þ ion formed during the compound ionization. The bands 2, 3, and 4 are assigned to the ligand molecular orbitals 2p and 2s. C2p levels contribute to the bands 1, 2, 3, and 4, but taking into account the photoionization cross section, they do not make a significant contribution to the band intensity. The two bands of the spin-orbital doublet of 5p-electrons of Sm and Eu appears in the bands 5 and 6 of the ligand MOs of 2s-type from 20 to 30 eV. The splitting of the bands 5 and 6 in the spectra of Sm compound is 4.30 eV, in the spectra of Eu compound the splitting is 5.18 eV. The obtained XPS spectra of Ln4d levels are shown in Fig. 5, Table 4 presents the values of bond energies with the marked maximum position of the bands of core levels C1s, O1s, and N1s. For N1s levels, the values are shown for the positions of two bands corresponding to the positions of N atoms bonded to CH3 groups in HMPA and N atoms bonded to three O atoms in NO3. The Ln4d levels of lanthanide ions are shown in Fig. 4, and for Eu3þ the splitting is found to be 5.59 eV. The P2p levels are in the region of 4d electrons of Ln ions, that makes it difficult to determine the band splitting for Sm4d. The obtained data correspond to the data obtained in other works [24,34e36].
Fig. 5. XPS spectra of core 4d levels of Ln adducts I and II.
A.V. Shurygin et al. / Journal of Molecular Structure 1205 (2020) 127638
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Table 4 Binding energies (EBE) of core electrons of the adducts I and II. #
Sm(NO3)3(HMPA)3 (I) Eu(NO3)3(HMPA)3 (II)
ЕBE, eV C1s
O1s
N1s (N(CH3); NO3)
Ln4d (4d5/2;4d3/2)
288.81 288.59
532.17 532.37
399.71; 407.17 399.87; 407.38
e 136.9; 142.5
2.1. TDDFT modeling, absorption and excitation spectra The diagram of excited states (Fig. 6) shows the density of 50 calculated singlet and triplet states located in the region of Ln3þ ion levels. The charge transfer transitions (from the HOMO of HMPA to the LUMO of NO3) are shown in green, the transitions inside NO3 ligands are shown in blue, and the transitions inside HMPA are shown in red. In order to trace the effect of HMPA addition to nitrate complexes, the diagram additionally shows excited states of the nitrate complexes Ln(NO3)3. This diagram indicates that the ligand levels are located higher than the levels of Lnþ3 ions, that can be associated with the low probability of the energy transfer from ligands to ions within the LMCT model. Fig. 7 presents the experimental absorption spectra of aqueous
Fig. 7. Absorption spectra of the adducts Sm(NO3)3(HMPA)3 (red) and Eu(NO3)3(HMPA)3 (blue), the dotted line shows the TDDFT calculated spectrum.
solutions of the adducts I and II, and the TDDFT calculated spectra. The main band observed in the experiment is related to the absorption of HMPA ligands, that is confirmed by the plotted absorption spectrum of an individual HMPA molecule. In the region below 250 nm in the experimental spectrum, the very intense band is observed causing the device “scale exceeding”. The calculations also contain the strongly intense bands caused by the charge transfer transitions, that does not contradict the experimental data. Analyzing the excitation spectra of the adducts (Fig. 8), one can see that the luminescence excitation is caused only by f-f transitions of Sm3þ ions (transitions from 6H5/2) and Eu3þ (transitions from 7F0). An addition of three HMPA molecules to nitrate complexes does not reduce significantly the HOMO-LUMO energy gap (CI - 5.44 эВ, CII - 5.46 эВ) that leads to high values of this characteristic for both adducts (I - 5.56 eV, II - 5.15 eV). This indicates a decrease in the probability of luminescence appearance associated with the transfer of absorbed energy by ligands to a metal ion.
3. Conclusions
Fig. 6. Diagram of 50 excited singlet (S) and triplet (T) states of the nitrate complexes Sm(NO3)3 (CI) and Eu(NO3)3 (CII) and the adducts Sm(NO3)3(HMPA)3 (I) and Eu(NO3)3(HMPA)3 (II). The transitions NO3 e NO3* МО are shown in blue, the transitions HMPA - HMPA* are in red, the charge transfer transitions HMPA e NO3* are in green.
The obtained experimental and theoretical results made it possible to show the influence of the electronic structure on the optical properties and reasons for the low probability of energy transfer from ligands to ions in the adducts Ln(NO3)3(HMPA)3 (Ln ¼ Sm, Eu). For the first time, the XPS spectra of the valence levels and core levels of the studied compounds were obtained and interpreted. The ion-dipole nature of the bond of complexes with neutral ligands was determined. All that makes it possible to conclude that the electronic structure determines the luminescence possibility and is directly related to the spatial arrangement of atoms of the molecular crystal. Slight distortions of the geometric structure cause rearrangement of electronic levels and, as a consequence, deterioration or improvement of the luminescent properties. As it was noted in the Introduction, the mechanism of the triboluminescence appearance is not exactly known, and that requires a detailed research of all features of this class of compounds. The results of research of the electronic structure obtained in this work make it possible to proceed to a study of the bond structure of intermolecular layers and to reveal their effect on the electronic structure.
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Sm(NO3)3(HMPA)3
Eu(NO3)3(HMPA)3
Fig. 8. Excitation spectra of the adducts Sm(NO3)3(HMPA)3 and Eu(NO3)3(HMPA)3.
Author contributions section A.V. Shurygin: Software, Formal analysis, Investigation, Writing - Review & Editing, Visualization. V.I. Vovna: Conceptualization, Methodology, Project administration. V.V. Korochentsev: Investigation, Resources. A.G. Mirochnik: Resources. P.A. Zhikhareva: Resources. V.I. Sergienko: Supervision.
[10]
[11]
[12]
Acknowledgments [13]
This work was financially supported by the Ministry of Education and Science of the Russian Federation within the State Assignment for the Research Project No. 3.2168.2017/4.6 of Far Eastern Federal University.
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