Luminescent complexes of Eu3+,Tb3+ and Gd3+ nitrates with polytopic ligand 2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine

Article abstract of DOI:10.1016/j.ica.2020.119764

Three novel coordination compounds based on 2,4,6-tris(1H-pyrazolyl)-l,3,5-triazine ligand (PzTz), namely [Ln(PzTz)(NO₃)₃(H₂O)] (Ln = Eu, Gd, Tb) were synthesized and spectroscopically and structurally characterized. X-ray

Full text of DOI:10.1016/j.ica.2020.119764

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Research paper
Luminescent complexes of Eu3+,Tb3+ and Gd3+ nitrates with polytopic ligand
2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine
D.A. Kordeyro Magrino, V.M. Korshunov, K.A. Lyssenko, V.E.
Gontcharenko, Yu.A. Belousov, C. Pettinari, I.V. Taydakov
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DOI:
Reference:
S0020-1693(20)30608-3
https://doi.org/10.1016/j.ica.2020.119764
ICA 119764
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27 March 2020
11 May 2020
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Please cite this article as: D.A. Kordeyro Magrino, V.M. Korshunov, K.A. Lyssenko, V.E. Gontcharenko, Yu.A.
Belousov, C. Pettinari, I.V. Taydakov, Luminescent complexes of Eu3+,Tb3+ and Gd3+ nitrates with polytopic
ligand 2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/
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Luminescent complexes of Eu3+,Tb3+ and Gd3+ nitrates with polytopic
ligand 2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine
D.A. Kordeyro Magrinoa,b, V.M. Korshunova, K.A. Lyssenkoc,d, V.E.
Gontcharenkoc, Yu. A. Belousova,c, C. Pettinari,e,f,* I.V. Taydakova,d
5
a P. N. Lebedev Physical Institute of Russian Academy of Sciences, Leninsky
prospect 53, 119991 Moscow, Russia
b N. E. Bauman Moscow State Technical University, 2-ya Baumanskaya str. 5/1,
105005 Moscow, Russia
c
10
15
Chemistry Department, M.V. Lomonosov Moscow State University, 1
Leninskie Gory Str., Building 3, Moscow 119991, Russia
d
G.V. Plekhanov Russian University of Economics, 36 Stremyanny Per.,
Moscow 117997, Russia
e School of Pharmacy, University of Camerino, S. Agostino 1, 62032, MC, Italy
f ICCOM CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
Abstract
20
25
30
Three novel coordination compounds based on 2,4,6-tris(1H-pyrazolyl)-l,3,5-
triazine ligand (PzTz), namely [Ln(PzTz)(NO3)3(H2O)] (Ln = Eu, Gd, Tb)
were synthesized and spectroscopically and structurally characterized. X-ray
diffraction experiments revealed the monomeric nature of the complexes
exhibiting either “sphenocorona” or bicapped square antiprism distorted
coordination geometry, the lanthanide ions being ten-coordinated. The
relatively high energy of triplet state of the ligand (T1 = 23000 cm-1) has been
demonstrated to be favorable for sensitization of Tb3+ ion and this has been
proved by luminescence decay measurements as well as by experimental
determination of photoluminescence quantum yield (Φ = 34 %).
Keywords: Luminescence, Lanthanides, Pyrazoles, Triazines, X-ray

1. Introduction
Symmetrically substituted 1,3,5-triazines are unique platform for preparation
of many practically important compounds, including polydentate nitrogen donor
ligands for coordination chemistry [1,2], textile and plastics reactive dyes [3,4], optical
bleaching agents [5], pharmaceutical compounds [6] and for other applications as for
example luminescence [7,8]. Triazine-based bipolar host materials are prepared and
described for blue phosphorescent OLEDs [9], other such 2,4,6-tris(4-bromophenoxy)-
1,3,5-triazines whereas have been employed as host compounds for channel-type
inclusion of fullerenes [10] and ferrocene-based molecule [11]. As a rule, 2,4,6-
trichloro-1,3,5-triazine (cyanuric chloride, 1) is used as precursor for most of the
syntheses reported. Cyanuric chloride is produced on industrial scale by trimerization
of cyanogen chloride (CNCl), this compound being therefore inexpensive and readily
available commercially. All three Cl atoms in 1 can be easily replaced by various
nucleophiles (Scheme 1) like amines or N-containing heterocycles (NH-pyrazoles, -
imidazoles and -triazoles, for example). Noteworthy that substitution can be made
stepwise (depending on reaction temperature and stoichiometry), which led to
formation of unsymmetrical compounds [12,13].
35
40
45
N
N
Cl
N
N
N
N
N
H
N
N
N
N
Cl
N
Cl
N
N
Cl
1
50
Scheme 1. Nucleophilic substitution in 2,4,6-trichloro-1,3,5-triazine.
Among all, pyrazolyl-substituted 1,3,5-triazines are of interest as potential
multidentate ligands for simultaneous coordination of d- and f-block elements [14]. Up
to date only complexes with d-block metals or lanthanides were reported, but these
ligands have been also demonstrated to be useful linkers in heterometallic coordination
polymer gels, very important species applicable in catalysis, luminescence and new
types of sensing functionalities [15].
55
2

The simplest member of these symmetrically substituted triazines (Figure 1),
i.e. 2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine (PzTz, 2) was prepared in 22% yield by
interaction of the sodium salt of the unsubstituted pyrazole with cyanogen bromide in
THF solution [16]. An alternative synthesis for the methyl substituted analogue tris(4-
methylpyrazol-1-yl)triazine 3 replaces the toxic cyanogen bromide with the stable 1
[17], that upon reaction with potassium pyrazolate gave the expected compound in 28%
yield. The same authors reported the reaction of 2,4,6-tris(4-methylpyrazol-l-yl)- and
2,4,6-tris(3,5-dimethylpyrazol-l-yl)-l,3,5-triazine (3,4) with [ReX(CO)5] that gave both
1:1 fac-[ReX(CO)3L] (X = C1, Br or I, L = 3 or 4) and 2:1 [{ReX(CO)3}2(4)]
compounds.
60
65
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
3
4
2, PzTz
70
75
80
Figure 1. Symmetric tris(pyrazol-1-yl)-l,3,5-triazine ligands.
In a further synthetic improvement [18] NaH in THF solution was employed
as base to obtain a series of substituted azolyl-l,3,5-triazines. Some of the ligands were
utilized for preparation of Au(I) [19], Ag(I) [20], Pd(II) [19,21] Cu(II) [22] and Hg(II)
[23] derivatives.
As the hydrides MH (M = alkali metal) are potentially hazardous, other bases
were investigated to synthesize azolyl-l,3,5-triazines in high yields. Firstly, the
K2CO3/MeCN system has been explored in alkylation of iodo‐substituted pyrazoles by
cyanuric chloride [24] whereas more recently a convenient approach to azolyl-l,3,5-
triazines, based on the interaction of cyanuric chloride with azoles in refluxing THF
and in the presence of N,N’-diisopropylethylamine (DIPEA) as base, has been
developed [25]. The target compound 2,4,6-tris(1H-pyrazol-1-yl)-1,3,5-triazine (2) was
obtained by this method (Scheme 2) in high yield (83%) [26].
3

N
N
Cl
N
N
N
N
N
3 eq.
N
N
N
THF
Cl
N
Cl
H
N
N
N
N
reflux
1
2, PzTz
85
Scheme 2. Synthesis of ligand 2,4,6-tri(1H-pyrazol-1-yl)-1,3,5-triazine (PzTz)
Interestingly it has been also reported that cyclotrimerization of
pyrazolylcarbonitriles can yield tris(pyrazoly-1-yl)-1,3,5-triazines useful for crystal
engineering studies in solvent free conditions [27].
90
The coordination chemistry of PzTz has been to date not-widely explored.
Only [Cu(PzTz)2](BF4)2 [22], [Fe(PzTz)2](BF4)2, [Fe(PzTz)2](ClO4)2
[28],
[Ag(PzTz)]BF4 and [Ag(PzTz)]ClO4 [29] were reported in literature. The unique
characterized lanthanide is the Eu(III) complex containing the methyl substituted 4,
namely [Eu(4)(NO3)3(H2O)][30].
95
100
105
Here we report our recent studies on the reactivity of PzTz (2) toward selected
Ln(NO3)36H2O salts (Ln = Eu, Tb and Gd) as also the structural and spectroscopic
properties of the synthesized complexes with the aim to explore the ligand role on the
luminescence properties also by using theoretical calculations.
2. Experimental section
2.1 Synthesis and characterization of the ligand PzTz and of complexes 5-7.
Lanthanide nitrates hexahydrates (99.99 %) and other reagents were purchased
from Aldrich and used without additional purification. THF was distilled over sodium
– benzophenon ketyl prior use.
Elemental analyses were performed on the Elementar Vario MicroCube
CHNO(S) analyzer. Metal content of the samples was determined by complexometric
4

titration with Trilon B solution in the presence of Xylenol Orange as an indicator.
Complexes were decomposed before analysis by heating them in concentrated HNO3.
NMR spectra were recorded at 300 K on the Bruker AC-300 instrument
operating at 300.13 and 75.47 MHz for 1Н and 13C nucleus respectively. FTIR spectra,
recorded with a Bruker Tensor 27 instrument in KBr pellets, are presented in Figure
S1. Melting points were determined on a microscope hot stage apparatus and are
uncorrected.
110
115
120
125
2.1.1. Synthesis of 2,4,6-tri(1H-pyrazol-1-yl)-1,3,5-triazine (PzTz, 2)
PzTz was obtained with a modified procedure of that proposed in literature [25]. In
100 mL of dry THF, pyrazole (4.1 g, 60 mmol) and N,N’-diisopropylethylamine (15.5
g, 120 mmol) were dissolved. Cyanuric chloride (3.7 g, 20 mmol) was then added in
small portions to a vigorously stirred solution over a period of time 30 min. The solution
was then refluxed for 5 h in anhydrous conditions (CaCl2 tube). Upon cooling to room
temperature, the reaction mixture was evaporated to dryness under diminished pressure
and the oily residue triturated with 50 mL of ice-cold water. A colorless precipitate
formed which was filtered off, washed with cold water (10 mL) and dried to a constant
weight, then re-crystallized from hot EtOAc/MeOH mixture. 3.8 g of 2 were obtained
(68 % yield) as a light yellowish powder. M.p. 226-228 C [31]. 1H NMR (DMSO-d6)
 (ppm): 8.16 (s, 3H), 7.35 (s, 3H), 6.61 (s, 3H).13C NMR (DMSO-d6)  (ppm): 164.17,
146.23, 129.96, 111.01. IR (cm-1): 3600-3100 (br, s), 3150 (vw), 3115 (vw), 1889
(vw), 1792 (w), 1742 (w), 1654 (m), 1574 (vs), 1534 (s), 1466 (vs), 1396 (vs), 1303
(vw), 1229 (m), 1185 (m), 1150 (w), 1081 (m), 1036 (s), 946 (vs), 912 (vw), 887 (vw),
809 (s), 768 (s), 646 (w), 597 (m).
130
2.1.1. Synthesis of [Ln(NO3)3(PzTz)(H2O)], (5-7).
To a stirred solution of PzTz (0.279 g, 1 mmol) in 10 mL of hot EtOH a solution of
Ln(NO3)36H2O (1 mmol) in 10 mL of EtOH was added dropwise. The solution was
stirred at 60 C for 1 h and cooled to a room temperature. A colorless precipitate formed
which was filtered off, washed with cold EtOH (2 mL) and dried to constant weight at
135
5

40 C. Crystals of 5 and 7, suitable for X-ray diffraction experiments, were obtained by
slow crystallization of the powders from MeOH solutions.
140
2.1.1.1.
Europium(III)
(monoaqua)(2,4,6-tri(1H-pyrazol-1-yl)-1,3,5-triazine)
trinitrate (5).
5 has been obtained in 85% as a colorless powder (0.542 g) by using 0.428 g of
Eu(NO3)36H2O. Elemental analysis: Anal. Calc. for C12H11EuN12O10: C, 22.69; H,
1.75%; N, 26.46; Eu 23.92. Found: C, 22.98; H, 1.85; N, 26.23, Eu 24.11%.
IR (cm-1): 3600-3100 (br, s), 3159 (vw), 1610 (m), 1569 (w), 1524 (w), 1476 (vs),
1394 (vs), 1320 (w), 1287 (vw), 1195 (vw), 1146 (vw), 1086 (w), 1055 (m), 968 (m),
935 (vw), 814 (m), 748 (vw), 594 (vw)
145
2.1.1.2. Terbium(III) (monoaqua)(2,4,6-tri(1H-pyrazol-1-yl)-1,3,5-triazine) trinitrate
(6).
150
6 has been obtained in 79% as a colorless powder (0.505 g) by using 0.435 g of
Tb(NO3)36H2O. Elemental analysis: Anal. Calc. for C12H11N12O10Tb: C, 22.44; H,
1.73%; N, 26.17; Tb 24.75. Found: C, 22.63; H, 1.79; N, 26.24, Tb 24.97%.
IR (cm-1): 3600-3100 (br, s), 3147 (vw), 1617 (s), 1569 (s), 1522 (w), 1468 (vs), 1397
(vs), 1323 (w), 1290 (w), 1191 (m), 1149 (vw), 1084 (m), 1049 (s), 970 (s), 930 (m),
912 (vw), 813 (s), 793 (vw), 748 (w), 646 (vw), 594(w)
155
2.1.1.3. Gadolinium(III) (monoaqua)(2,4,6-tri(1H-pyrazol-1-yl)-1,3,5-triazine)
trinitrate (7).
160
7 has been obtained in 81% as a colorless powder (0.518 g) by using 0.451 g of
Gd(NO3)36H2O. Elemental analysis: Anal. Calc. for C12H11GdN12O10: C, 22.50; H,
1.73%; N, 26.24; Gd 24.55. Found: C, 22.41; H, 1.67; N, 26.07, Gd 25.08%.
IR (cm-1): 3600-3100 (br, s), 3146 (vw), 1616 (s), 1569 (s), 1523 (w), 1467 (vs), 1395
(vs), 1325 (w), 1292 (w), 1191 (m), 1083 (w), 1049 (s), 967 (s), 937 (m), 812 (m), 792
(s), 746 (vw), 596 (w)
165
6

2.2 Spectroscopic Methods
Optical absorption spectra were measured using a JASCO-V770 spectrophotometer
sensitive in spectral region 190-2500 nm. Compounds 5-7 were dissolved in acetonitrile
into quartz cells with 1 cm pathlength. The concentration of the solutions was
approximately 10-6 M.
170
175
180
High-resolution PL spectra at room temperature and 77K as well as optical excitation
spectra and the luminescence decays were recorded on a Fluorolog FL3-22
spectrofluorimeter equipped with a 450 W xenon arc lamp, xenon flash lamp and R-
928 photomultiplier sensitive within 300–850 nm.
Absolute luminescence quantum yields (QY) were measured with a Spectralone-
covered G8 integration sphere (GMP SA, Switzerland) coupled to the Fluorolog FL3-
22 spectrofluorimeter. The determination method of intrinsic quantum yield of the
europium complex is described in [32, 33].
Photoluminescence (PL) spectra of complex 7 were obtained at 77 K with an Ocean
Optics Maya 2000 Pro CCD spectrometer sensitive within 200–1100 nm spectral range.
A sample was mounted in quartz Dewar-type cryostat, connected to spectrometer via
optical fiber. A 365 nm CW laser was employed as the excitation source.
185
2.3 X-ray diffraction
X-ray diffraction data of 5 and 7 were collected on a APEX II CCD diffractometer
using molybdenum radiation [(MoKα) = 0.71072 Å, ω-scans]. The substantial
redundancy in data allowed empirical absorption correction to be applied with
SADABS by multiple measurements of equivalent reflections. The structure was solved
by direct methods and refined by the full-matrix least-squares technique against F2 in
the anisotropic-isotropic approximation. C-H hydrogen atoms in all structures were
located in calculated positions and refined within the riding model. In its turn for water
molecule the hydrogen atoms were located from the Fourier density synthesis. All
calculations were performed with the SHELXTL software package. Crystal data and
structure refinement parameters are listed in Table 1. Crystallographic data for the
structure reported in this paper have been deposited to the Cambridge Crystallographic
190
195
7

Data Centre as supplementary no.: CCDC- 1980735-1980736. These data can be
obtained free of charge from The Cambridge Crystallographic Data via
www.ccdc.cam.ac.uk/data_request/cif. According to X-ray structural studies, the
mononuclear complexes 5 and 7 are isostructural and connected through a network of
hydrogen bonds in a three-dimensional framework (Figure S2).
200
3. Results and discussion
3.1 Synthesis and characterization
205
The ligand PzTz was obtained by a modification of the procedures reported in
literature. At least two-fold excess of base DIPEA is necessary to achieve full
transformation of the starting materials. PzTz was pure enough to allow successful
preparation of complexes, however all analytical samples were additionally
recrystallized from boiling EtOH and dried at 0.1 torr.
O
N
O
N
N
O
N
N
N
N
N
O
O
Ln(NO3)3 6H2O
EtOH
N
N
N
N
Ln3+
N
N
O
H
N
N
N
N
O
H
N
O
N
O
N
2, PzTz
O
5
6
7
Ln = Eu,
Tb,
Gd,
210
Scheme 3. Synthesis of complexes [Ln(PzTz)(NO3)3(H2O)] (5-7).
The three [Ln(PzTz)(NO3)3(H2O)] derivatives (5-7) were synthesized by
215
reaction of equimolar quantities of the PzTz with corresponding lanthanide(III) nitrate
salt (Scheme 3). Analytical samples were purified by crystallization from hot MeOH
8

and dried at 40 C and 0.1 torr. No exchange of H2O molecules with MeOH during
recrystallization was observed. Complexes 5-7 are almost stable in air and do not
release H2O at least up to 70 C. It has been previously reported [30] that complexes
having a ligand to metal ratio ranging from 1:1 to 1:3 can be obtained when the reaction
between Eu salts and the ligand 4 was carried out, however in our reaction with the
ligand 2 only 1:1 species of formula [Ln(PzTz)(NO3)3(H2O)] formed, as confirmed by
X-ray diffraction and analytical data, also when different reactants quantities were
employed. The IR spectrum of the free ligand 2 show medium bands between 1600 and
1200 cm-1 that can be assigned to (C=N) and (C=C) stretching vibrations of both
azole and triazine rings. These bands remains almost unchanged in the spectra of
complexes 5-7 [34]. The absorption bands due to coordinated nitrate groups were
observed at ca 1520 (1), 1320 (4) 1050 and 810 cm-1, the separation of the two higher
frequency bands (1- 4) being approximately 200 cm-1, in accordance with a bidentate
nitrate coordination [35].
220
225
230
235
240
245
The crystal structure of complexes 5 and 7 (Figure 2) is composed of neutral
[Ln(PzTz)(NO3)3(H2O)] molecules and one reticular water molecule. The europium
and gadolinium complexes are isostructural, the metal center being ten-coordinated by
three nitrogen atoms of PzTz, six oxygen atoms by three bidentate NO3, groups and
one water molecule. Such coordination sphere is a common feature for all up to date
known complexes containing ligands analogous to PzTz. In particular such polyhedron
has been observed for Eu(III) complexes with 2,4-bis(3,5-dimethylpyrazol-1-yl)-6-
diethylamino-1,3,5-triazine [36]. 5 and7 have a distorted shape, the coordination
geometry around the Ln centers being described as “sphenocorona” (J86) or bicapped
square antiprism (J17) [32].
In 5 and 7 the ligand PzTz is virtually planar (with dihedral angles between
the triazine and pyrazolyl rings equal to 1.5-2.5). In Table 2 selected bond distances
in the coordination polyhedron of 5 and 6 were reported. PzTz is coordinated to Ln in
almost symmetrical manner, the longest Ln-N bond being found for N1 atom (2.5995(8)
and 2.600(2)) whereas Ln-N5 and LN-7 range from 2.5625(8) to 2.566(2) Å. The
pyrazolyl rings linked to triazine C1 and C2 atoms adopt a cis-cis conformation leading
to a planar meridional coordination of the Ln(III) ion that lies approximately in a basal
9

plane defined by N1, N3 and N5 atoms of PzTz and O7N atom of one of the nitrate
anion. The oxygen atom of the water molecule deviates from the above plane by 1.3 Å.
The shortest Eu-O bond is that between Eu and H2O (2.368(2) Å) while the bonds with
nitrates vary in the range 2.452(2)-2.540(2) Å. The average Eu-O bond with nitrates
(2.494(2) Å in our complexes are shorter with respect to those found in Eu(NO3)3
complexes with 1,10-phenantroline (2.512(1) Å) and 2,2’-bipyridine (2.518(1) Å) [37].
The analysis of the crystal packing of [Ln(PzTz)(NO3)3(H2O)] 5 and 7 has
revealed that despite of available surface of the -system the stacking interaction
between molecules in crystal is almost absent (there are some shortened C…C contacts
but their overlap is negligible). 5 and 7 are interconnected in a three-dimensional
framework by hydrogen bonds O-H…O (O…O: 2.74-2.94 Å) and O-H…N (O…N:
2.97-3.21Å) and also assembled through rather rare short O2N-O… interaction
(O…N: 3.019Å) (Figure S2).
250
255
260
Figure 2. General view of [Ln(PzTz)(NO3)3(H2O)] (5: Eu; 7: Gd). The atoms are
represented by thermal ellipsoids (except hydrogens, p = 50%).
265
10

3.2 Photophysical studies
Optical absorption spectra recorded for compounds 5 and 6 showed only one
characteristic band corresponding to ligand absorption. 5 and 6 exhibited high molar
10 ―4 l ∙ mol ―1 ∙ cm ―1
extinction
in the region 240-290 nm (see Figure 3). In order to
270
verify the formation of molecular aggregates we carried out additional experiments in
acetonitrile with different solution concentrations. As can be seen from Figure S3-S4
(Supplementary materials) changes of molar concentration in acetonitrile are not
affecting the structure of the absorption spectra. Moreover, a linear dependence of
optical density on the concentration is observed. Therefore, we claimed that there is no
formation of molecular aggregates.
275
PL spectra recorded for 5 and 6 revealed several bands corresponding to particular
Eu3 +
Tb3 +
ions, respectively, assigned on the basis of
electronic transitions of
and
7
literature data [33,38]. The bands are assigned to 5D0 FJ , where J = 0-4, for complex
→
7
5, and to 5D4 FJ , where J = 0-6 for complex 6, respectively. The photoluminescence
→
5
7
→
280
285
290
295
band associated to D0 F0 transition in the Eu3+ ion is not splitted, suggesting the
existence of only one emission site also in accordance with photoluminescence decay
measurements reported below.
The low degeneracy of the luminescence spectral bands of 5 makes evident that its
coordination polyhedron has high symmetry as supported by X-ray diffraction study.
In order to define more exactly the symmetry group of the coordination polyhedron
additional PL experiments at 77 K were carried out (see Figure S5). The D4d group was
estimated for 5 according to a procedure previously reported [38]. Unfortunately, it is
difficult to perform simple analysis of coordination environment symmetry based on
spectral data for Tb3+ ion due to the large number of energy levels. However, we
determined the value of the branching ratio as the integrated intensity of the electric-
dipole transition (5D4 → 7F6) referred to the integrated intensity of the magnetic dipole
transition (5D4 → F5). Thus, we achieved β equal to 0.35, that is comparable to the
7
Tb3+ complexes previously described [39]. Based on these data, one may only conclude,
that the symmetry of coordination environment of Tb3+ ion in 6 is rather high, but also
that an exact determination of the symmetry group is not possible.
11

In order to check the applicability of the Latva rule to 5 and 6 we defined the energy
of the excited triplet state of the ligand. The emission spectra of the Gd(III) complex
are appropriate to obtain information on the energy levels of the organic ligand,
6
considering that the first excited P7/2 level of the Gd+3 is around 32000 cm−1. The
300
305
310
energy of the triplet states of all organic ligands is located below this level, hindering
the intermolecular energy transfer from organic ligands to Gd3+ ion. Since the vibration
motion are mainly «frozen» at cryogenic temperatures (77 K) the triplet state energy of
the ligand can be estimated from the high-energy band (0-0 phonon transition) observed
in phosphorescence spectra. However, 0-0 transition energy is higher than the peak of
high-energy band of emission due to the reorganization of the radiation energy [40,41].
Therefore, triplet state energy can be obtained from high-energy edge of the
phosphorescence spectrum. We estimated the position of lowest triplet state as 23000
5
cm-1 (Figure S6). Since the energy of the resonant D4 level (20500 cm-1) of the Tb3+
ion is more than energy of 5D0 (17300 cm-1) state of Eu3+ ion we suggested that PzTz
ligand sensitizes Tb3+ much more than Eu3+ luminescence.
Luminescence excitation spectra were obtained for the Eu3+ and Tb3+ complexes
with emission measured at 615 nm and 545 nm respectively. We observed a wide PL
excitation band in spectral region 250-350 nm (see Figures 4 and 5) corresponding to
7
5
→
excitation through ligand. Additionally, narrow bands at 395 nm ( F0 L6) and 464
7
5
3+
→
315
320
325
nm ( F0 D2) corresponding to hypersensitive transitions of Eu , were detected. On
the contrary, hypersensitive transitions (5D4→7F6) of Tb3+ were not detected for 6 [42].
Absolute luminescence quantum yield (PLQY, ) values have been experimentally
determined for the complexes when excited at 315 nm wavelength. The PLQY value
obtained for the Eu3+ complex was 10%, which is three times less than for Tb3+ complex
(34%).
7
5
→
The electric field of ligand environment is not influencing the rate of F1
D0
Eu3 +
magnetic dipole transition of
ion corresponding to the PL peak at 590 nm [38].
We measured PL spectrum for 5 with excitation at 464 nm, thereby providing resonant
Eu3 +
→
ion via F0 D2 transition. Normalization of the emission spectra
7
5
absorption in
by dividing the integrated intensities of the overall spectrum and integrated intensity of
퐿푛
7
5
→
the F1 D transition allows us to measure the intrinsic quantum yield (
). Hence,

퐿푛
0
12

퐿푛
 퐿푛
we evaluated the intrinsic QY (for 5 is 28%). The ratio of absolute PLQY and
determines sensitization efficiency (SE, ) for 5 as 67 %.
The transient PL decay curves were measured for spectral bands corresponding to
the maxima in the PL spectra of 5 and 6. The Levenberg-Marquardt deconvolution
technique was used to delete the contribution from the instrument response function.
The resulting PL decays demonstrate single-exponential behavior (see Figure 6).
330
Figure 3. Optical absorption spectra recorded for [Eu(PzTz)(NO3)3(H2O)] (5, red) and
[Tb(PzTz)(NO3)3(H2O)] (6, green) in acetonitrile.
335
±
휇
± 휇
8 s for
The observed lifetimes (obs) were found to be 1052
5 s and 1679
Eu3+ and Tb3+ ions in 5 and 6 respectively. The obtained lifetimes values are comparable
with those reported in literature [39,43] for highly luminescent complexes. However,
the measured overall quantum yields (PLQY) for 5 and 6 are moderate. In the case of
5 it may be attributed to relatively large energy gap value (5700 cm-1) between triplet
and resonant levels of Eu3+, which causes non-radiative loses of energy due to
multiphonon relaxation. For the Tb3+ complex 6 relatively low PLQY may be ascribed
340
13

to inefficient ligand-to-metal energy transfer due to predominance of non-radiative
relaxation processes.
345
350
355
In the case of 5 the nature of the absorption band was estimated by means of TD-
DFT calculation. The electronic excitation energies for 5 were calculated with the
hybrid PBE functional and large-core energy-adjusted RECPs for Eu3+, developed by
the Stuttgart and Dresden groups, along with the accompanying basis set ECP54MWB
to describe the valence electron density [44]. Large-core energy-adjusted RECPs for
lanthanides put 5s, 5p, 6d, and 6s shells in the valence space, whereas 4f electrons
belong to the core pseudopotentials. For other atoms, a def-2-TZVP basis set was
employed.
The TD-DFT calculations have revealed that the lowest energy transitions (S0-S1)
and S0-T1 are mainly HOMO to LUMO and correspond to the charge transfer from the
triazine to the non-coordinated pyrazole heterocycle (see Figure 7). The energy value
of the S0-T1 transition 3.15 eV, in accordance with TD-DFT is relatively close to the
experimental value (2.85 eV). Furthermore, the energy of S0-S1 transition (4.28 eV) is
almost equal to maximum value observed in optical spectra (see Figure 3).
360
14

Figure 4. Excitation (blue) and photoluminescence (red) spectra for 5 in the solid state.
Excitation spectra were obtained with emission recorded at 615 nm for 5.
365
Figure 5. Excitation (blue) and photoluminescence (green) spectra for 6 in the solid
state. Excitation spectra were obtained with emission recorded at 545 nm for 6.
15

Figure 6. Luminescence decays measured within 615 ± 2 nm emission bands of
complexes 5 (A) and 6 (B) under 315 nm Xenon flash lamp excitation. Experiments
were carried out for solid-state samples.
370
HOMO
LUMO
16

375
Figure 7. View of the frontier orbitals (HOMO/LUMO) in 5 according to TD-DFT
calculations.
4. Conclusions
In the present work, we have reported the synthesis of three novel lanthanide
complexes containing the PzTz ligand. X-ray diffraction experiments demonstrated
that complexes have a monomeric structure, lanthanide coordination number being ten
due to a tri-coordinated PzTz, three bidentate nitrate anions and one H2O molecule.
The experimentally measured energy of triplet level of PzTz ligand in sufficiently high
(23000 cm-1). The PzTz ligand sensitizes the luminescence of the Tb3+ ion much better
than the Eu3+ ion. Absolute photoluminescence quantum yield (PLQY) were found to
be 34% for [Tb(PzTz)(NO3)3(H2O)] 6 and 10% for [Eu(PzTz)(NO3)3(H2O)] 5. An
observed luminescence lifetime (obs) was 1679 μs and 1052 μs, for Tb3+ and Eu3+
complexes respectively. Due to the presence of additional coordination sites, the
complexes here reported may be used as building blocks in synthesis of heterometallic
coordination compounds.
380
385
390
Acknowledgements
Photophysical measurements were made with the financial support of Russian Science
Foundation (Project № 19-13-00272). X-ray diffraction experiments were supported by
Russian Foundation for Basic Research (Project № 18-29-04029). Analytical support
was provided by Russian Foundation for Basic Research (Project 20-33-70208).
University of Camerino is also gratefully acknowledged.
395
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Table 1. Crystal data and structure refinement parameters.
5
7
Formula
FW
C12H13EuN12O11
653.30
C12H13GdN12O11
658.59
T, K
120
120
Crystal system
Space group
Z(Z’)
Monoclinic
P21/c
Monoclinic
P21/c
4(1)
4(1)
a, Å
7.4551(2)
12.5518(4)
22.1236(6)
90
7.4626(4)
12.5841(7)
22.1623(12)
90
b, Å
c, Å
, °
93.656(2)
90
93.632(3)
90
, °
, °
24

V, Å3
2066.00(10)
2.100
2077.1(2)
2.106
32.81
1284
dcalc, g.cm-3
, cm-1
31.25
F(000)
1280
60
99
2max, °
Measured refl.
117123
435614
6044
(0.0470)
21290
(0.0559)
Independent Refl. (Rint)
Observed refl. with
I >2(I)
5778
18603
Parameters
R1
329
329
0.0242
0.0611
1.066
0.0215
0.0480
1.019
wR2
GOF
ρmax, e.Å-3(dmin/dmax
)
0.957/-2.255
1.504/-2.956
25

Table 2. Interatomic distances in the coordination polyhedron of 5 and 7.
5
7
Bond, Å
Ln-O1N
Ln-O2N
Ln-O4N
Ln-O5N
Ln-O7N
Ln-O8N
Ln-O1W
2.482(2)
2.477(2)
2.497(2)
2.540(2)
2.522(2)
2.452(2)
2.368(2)
2.4806(8)
2.4734(7)
2.4908(8)
2.5356(8)
2.5296(8)
2.4460(8)
2.3663(8)
Ln-N1
Ln-N5
Ln-N7
2.600(2)
2.566(2)
2.566(2)
2.5995(8)
2.5628(8)
2.5625(8)
605
No data are present with exception of those attached
Conflicts of interest
The authors declare no conflicts of interest
610
615
[Ln(PzTz)(NO3)3(H2O)] (Ln = Eu, Gd, Tb) species spectroscopically and
structurally characterized in the very rare sphenocorona coordination geometry
exhibit very interesting optical and luminescence properties.
Luminescent compounds for sensing applications
26

Polytopic ligands useful for the synthesis of lanthanide species and able to sensitize
Tb3+ ions
Luminescence properties investigated by TD-DFT calculations
620
27

625
28

29