Bright photo- And triboluminescence of centrosymmetric Eu(iii) and Tb(iii) complexes with phosphine oxides containing azaheterocycles

Article abstract of DOI:10.1039/d1nj02441h

Six centrosymmetric mononuclear Eu³⁺and Tb³⁺complexes of the type [LnL₂(hfac)₃] have been synthesized employing diphenyl(pyridin-2-yl)phosphine oxide (Ph₂P(O)Py), diphenyl(pyridimin-2-yl)phosphine oxide (Ph₂P(O)Pym), and diphenyl(pyrazin-2-yl)phosphine oxide (Ph₂P(O)Pyr) as supporting ligands (L). The complexes [LnL₂(hfac)₃] (L = Ph₂P(O)Py and Ph₂P(O)Pyr) comprise an eight-coordinate Ln³⁺ion with two monodentate O-donor phosphine oxides and three bidentate hfac^?anions. In the [Ln{Ph₂P(O)Pym}₂(hfac)₃] complexes, the Ln³⁺ion is nine-coordinated by three bidentate hfac^?anions and two Ph₂P(O)Pym ligands, one of which is bound to Ln in a N,O-bidentate chelating mode, and the other acts as a monodentate O-donor ligand. The complexes display bright solid-state photoluminescence with emission quantum yields up to 56%. The integral intensity of⁵D₀→⁷F₂transition in the emission spectra of the Eu(iii) complexes strongly depends on the coordination environment of the Eu³⁺ion. The asymmetric coordination geometry around Eu³⁺results in the large radiative rate constants. The crystals of the Eu(iii) and Tb(iii) complexes exhibit triboluminescence upon breaking under ambient conditions. The relationship between the triboluminescent properties and crystal structures of the complexes is discussed.

Full text of DOI:10.1039/d1nj02441h

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PAPER
Bright photo- and triboluminescence of
centrosymmetric Eu(III) and Tb(III) complexes with
Cite this: New J. Chem., 2021,
45, 13869
phosphine oxides containing azaheterocycles†
Yuliya A. Bryleva, * Alexander V. Artem’ev,
Mariana I. Rakhmanova, Denis G. Samsonenko,
Maxim I. Rogovoy and Maria P. Davydova
Ludmila A. Glinskaya,
Vladislav Yu. Komarov,
3
+
3+
Six centrosymmetric mononuclear Eu and Tb complexes of the type [LnL
employing diphenyl(pyridin-2-yl)phosphine oxide (Ph P(O)Py), diphenyl(pyridimin-2-yl)phosphine oxide
Ph P(O)Pym), and diphenyl(pyrazin-2-yl)phosphine oxide (Ph P(O)Pyr) as supporting ligands (L). The
complexes [LnL
2 3
(hfac) ] have been synthesized
2
(
2
2
3+
2
(hfac)
3
] (L = Ph
2
P(O)Py and Ph
2
P(O)Pyr) comprise an eight-coordinate Ln ion with two
À
monodentate O-donor phosphine oxides and three bidentate hfac anions. In the [Ln{Ph
2
P(O)Pym}
2 3
(hfac) ]
3+
À
complexes, the Ln ion is nine-coordinated by three bidentate hfac anions and two Ph
2
P(O)Pym ligands,
one of which is bound to Ln in a N,O-bidentate chelating mode, and the other acts as a monodentate
O-donor ligand. The complexes display bright solid-state photoluminescence with emission quantum yields
5
7
Received 18th May 2021,
Accepted 25th June 2021
up to 56%. The integral intensity of
D
0
-
F
2
transition in the emission spectra of the Eu(III) complexes
3+
strongly depends on the coordination environment of the Eu ion. The asymmetric coordination geometry
3+
around Eu results in the large radiative rate constants. The crystals of the Eu(III) and Tb(III) complexes exhibit
triboluminescence upon breaking under ambient conditions. The relationship between the triboluminescent
properties and crystal structures of the complexes is discussed.
DOI: 10.1039/d1nj02441h
rsc.li/njc
the preparation of potentially polydentate N,O-donor ligands.
Phosphine oxides bearing N-heterocyclic rings attract attention
1
. Introduction
Phosphine oxides are widely used for the design and synthesis as ligands for Ln(III) complexes due to their distinguished
3
2,33
of luminescent lanthanide (Ln) coordination compounds due to photophysical properties,
and also due to their ability to
3+
1
their strong coordination ability to Ln ions, and the attractive discriminate ions of f-elements by their size. Some Ln(III)
1
–4
photophysical properties of complexes based on these ligands.
The phosphine oxides bearing fluorophore moieties have proven solvent and counter-ion dependent luminescence.
to be efficient sensitizers of the Ln(III) luminescence with high in most of these complexes the nitrogen atoms of N-heterocycle-
complexes with these ‘‘hard-and-soft’’ ligands show intense
3
4–37
However,
5–15
quantum yields and long luminescence lifetimes.
Moreover, based phosphine oxides are not involved in coordination with the
some Ln complexes based on phosphine oxides exhibit remark- Ln ions and bind to the metal centers only via the PQO
3+
1
6–23
33,38,39
able photophysical properties such as triboluminescence,
groups.
24–28
29,30
and luminescence depending on temperature,
solvent,
or
It should be noted that most phosphine oxide-based complexes
endowed with outstanding luminescence properties contain fluori-
3
1
counterion.
Importantly, the ability to incorporate a wide variety of nated b-diketonate anions, which are known as efficient photo-
3+
3+
1
groups into the structures of phosphine oxides gives enormous sensitizers of Ln ions, especially Eu ones. The presence of
scope to the tuning of the properties of the resulting complexes. fluorine atoms in these ligands significantly suppresses vibrational
In particular, the introduction of azaheterocyclic moieties into relaxation, thereby strongly enhancing the quantum performance
the phosphine oxides is one of the promising approaches for of the Ln(III) luminescence. The polyfluorinated moieties are also
responsible for enhanced volatility, lipophilicity and solubility of
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy
the complexes, and may play a role in the manifestation of
of Sciences, 3, Academician Lavrentiev Avenue, Novosibirsk 630090, Russian
Federation. E-mail: bryleva@niic.nsc.ru
18,19
triboluminescence.
The use of fluorinated b-diketonate
anions in combination with phosphine oxide ligands in the
Ln(III) coordination sphere is a promising approach for the
design of efficient emitters. Through this design strategy, a
†
Electronic supplementary information (ESI) available. CCDC 2078495–2078497.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1nj02441h
1
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1
31
number of Ln(III) b-diketonate complexes containing mono- or The H and P NMR spectra correspond to the literature
5
0
diphosphine dioxides showing photo- and triboluminescence data. Found: C, 73.3; H, 5.0. Calcd for C17
have been reported.
H
14NOP (279.27):
Triboluminescent activity makes C, 73.1; H, 5.1. IR (KBr pellets, n/cm ): 3521w, 3449w, 3228m,
such complexes very attractive for applications as damage and 3060w, 2822w, 1635w, 1591w, 1573m, 1482w, 1437s, 1374w,
1
6,18,19,22,33
À1
4
0
pressure sensors. The study of triboluminescent complexes is 1309w, 1276w, 1192s, 1141m, 1122s, 1099m, 1085w, 1070w,
also important from a fundamental viewpoint, because the exact 1044w, 1027w, 989w, 869w, 845w, 773w, 745s, 745s, 724s, 692s,
reasons for the origin of triboluminescence are not yet fully 620w, 542s, 501w, 455w, 443w (Fig. S1, ESI†).
3
+
understood. The ability of the Ln complexes to generate
triboluminescence is often associated with a contribution from 30% aqueous H
the piezoelectric effect upon breaking non-centrosymmetric 2-(diphenylphosphino)pyrazine (793 mg, 3 mmol) in acetone
Synthesis of diphenyl(pyrazin-2-yl)phosphine oxide (Ph
2
P(O)Pyr).
2 2
O
(0.3 mL) was added to a solution of
4
1–44
bulk crystals.
luminescent Ln(III) complexes were also reported.
been suggested that the disorder of ligands may be responsible with CH Cl (3 Â 30 mL). The combined extract was washed
Meanwhile, efficient centrosymmetric tribo- (30 mL) at room temperature. After stirring for 30 min, the
2
0,21
It has also mixture was diluted with distilled H O (20 mL) and extracted
2
2
2
4
5
for the triboluminescent activity of lanthanide complexes.
with distilled H
2
O (20 mL), dried over Na
2
SO
P(O)Pyr. Yield: 0.650 g
, 23 1C) d 9.41 (t, J = 1.5 Hz,
H in Pyr), 8.76–8.66 (m, 2H, C4,5H in Pyr), 7.93–7.80 (m,
properties of lanthanide coordination compounds are of great 4H, Ph), 7.59–7.52 (m, 2H, Ph), 7.47 (tdd, J = 8.2, 3.0, 1.3 Hz, 4H,
interest. Ph). Found: C, 68.5; H, 4.9. Calcd for C H N OP (280.26): C,
4
and evaporated in
Therefore, since there is not currently an unambiguous inter- vacuo to give an off-white powder of Ph
2
1
pretation of the Ln(III) excitation mechanism, studies of the (77%). H NMR (500.13 MHz, CDCl
relationships between the structure and triboluminescence 1H, C
3
3
1
6
13 2
À1
Here, we have designed and synthesized a series of bright 68.6; H, 4.7. IR (KBr pellets, n/cm ): 3525w, 3459w, 3258w,
photo- and triboluminescent [LnL (hfac) ] complexes (Ln = Eu, Tb) 3060w, 1642w, 1589w, 1573w, 1563w, 1482w, 1455w, 1438m,
2
3
supported by tertiary phosphine oxides (L) bearing pyridine, 1424w, 1313w, 1277w, 1201m, 1143w, 1118m, 1099w, 1081w,
pyrimidine, or pyrazine moieties. The effect of the nature of the 1069w, 1044w, 1027w, 990w, 925w, 903w, 862w, 845w, 771w,
N-heterocycle incorporated in phosphine oxide on the structure 757w, 744m, 735s, 722s, 619w, 540s, 497m, 447w (Fig. S1, ESI†).
and photoluminescence properties of the complexes has been
investigated. Moreover, the relationship between triboluminescence plexes 1–6. A solution of Ph
General synthetic procedure for the preparation of com-
P(O)Py, Ph P(O)Pym or Ph P(O)Pyr
2
2
2
properties and crystal structures of the complexes is discussed, (0.1 mmol, 0.028 g) in EtOH–Et O (2 :1, 5 mL) for 1 and 2 and in
2
which will contribute to the understanding of this type of MeOH (3 mL) for 3–6 was added to a solution of [Ln(hfac) (H O) ]
3
2
2
luminescence.
(0.05 mmol, 0.040 g) in EtOH–Et O (2: 1, 2 mL) for 1 and 2 and in
2
MeOH (2 mL) for 3–6 upon stirring. Slow evaporation of the
resulting solution at room temperature afforded colorless crystals
suitable for single crystal X-ray analysis after 2–3 days. The
crystals were filtered off, washed with EtOH for 1 and 2 and with
MeOH for 3–6, and were dried in vacuo at room temperature.
2
. Experimental part
2.1. Reagents
All reagents and solvents were commercially available and were
used without additional purification. The compounds were
[Eu{Ph
2
P(O)Py}
2
(hfac)
3
] (1). Yield: 0.045 g (67%). Found: C,
Eu
chemically pure), Eu(NO ) Á6H O (pure), Et O (pure for analysis), (1331.7): C, 44.2; H, 2.3; N, 2.1%; F, 25.7%. FT-IR (KBr pellets,
synthesized using Tb(NO ) Á6H O, acetone, CH Cl and MeOH 43.6; H, 2.4; N, 2.0%; F, 25.0%. Calcd for C49
31 2 18 8 2
H N F O P
3
3
2
2
2
(
3
3
2
2
À1
EtOH (rectified), and 30% aqueous H
Eu, Tb) were prepared according to ref. 46. The complexes 1496s, 1439m, 1427w, 1344w, 1320w, 1255s, 1199s, 1171s,
Ln(Ph PO) (hfac)
] (Ln = Eu, Tb) were prepared following a 1146s, 1098s, 1046w, 1029w, 998w, 991w, 950w, 848w, 797m,
2 2 3 2 2
O . [Ln(hfac) (H O)
] (Ln = n/cm ): 3420w, 3065w, 1653s, 1593w, 1576w, 1556m, 1528m,
[
3
2
3
4
7
known procedure. 2-(Diphenylphosphino)pyridine was used 773w, 740m, 726m, 693m, 660m, 618w, 583m, 543s, 460w,
as purchased (97%, Sigma-Aldrich), and 2-(diphenylphosphino)- 448w, 421w (Fig. S1, ESI†).
4
8
pyrazine was prepared according to the reported technique.
[
Tb{Ph
2 2 3
P(O)Py} (hfac) ] (2). Yield: 0.055 g (83%). Calcd for
Diphenyl(pyridimin-2-yl)phosphine oxide (Ph P(O)Pym) was
prepared by a procedure reported in ref. 49.
2
C H N F O P Tb (1338.6): C, 44.0; H, 2.3; N, 2.1%; F, 25.5%.
4
9
31 2 18 8 2
Found: C, 43.4; H, 2.3; N, 2.0%; F, 25.1%. FT-IR (KBr pellets,
n/cm ): 3420w, 3065w, 1650s, 1592w, 1575w, 1555m, 1516s,
À1
2.2. Synthesis
1
1
8
5
481s, 1472s, 1439s, 1427w, 1345w, 1320w, 1256s, 1125s, 1198s,
171s, 1146s, 1095s, 1045w, 1029w, 998w, 991w, 951w, 930w,
94w, 853w, 791m, 772w, 740s, 727s, 692m, 669s, 617w, 580m,
42s, 458w, 448w, 423w (Fig. S1, ESI†).
2
Synthesis of diphenyl(pyridin-2-yl)phosphine oxide (Ph P(O)Py).
3
2
(
0% aqueous H
2 2
O (0.2 mL) was added to a solution of
-(diphenylphosphino)pyridine (0.527 g, 2 mmol) in acetone
30 mL) at room temperature. After stirring for 20 min the
mixture was diluted with distilled H O (20 mL) and extracted
[Eu{Ph P(O)Pym} (hfac) ] (3). Yield: 0.039 g (54%). Found: C,
2
2
2
3
with CH
with distilled H
vacuo to give a white powder of Ph
2
Cl
2
(3 Â 30 mL), then the organic layer was washed 41.9; H, 2.9; N, 4.2%; F, 25.2%. Calcd for C47
O (20 mL), dried over Na SO and evaporated in (1333.6): C, 42.3; H, 2.2; N, 4.2%; F, 25.6%. FT-IR (KBr pellets,
P(O)Py. Yield: 0.450 g (80%). n/cm ): 3432w, 3065w, 1656m, 1614w, 1592w, 1556m, 1527m,
29 4 18 8 2
H N F O P Eu
2
2
4
À1
2
1
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1
506m, 1489w, 1440w, 1392w, 1345w, 1321w, 1256s, 1200s, 1180m, refined with anisotropic displacement parameters. The structures
53
1165s, 1148s, 1123m, 1099w, 1073w, 1029w, 998w, 990w, 950w, 1, 3, and 5 were solved using SHELXT and refined by the full-
2
54
806w, 794w, 765w, 746w, 727w, 692w, 661m, 635w, 615w, 585w, matrix least-squares method on F in SHELXL. The N atoms of
5
60m, 554m, 539m, 460w, 447w, 410w (Fig. S1, ESI†).
the heterocyclic moieties for all structures were localized on the
basis of a higher electron density compared to the neighboring C
atom positions, as well as the absence of the peaks from the H
atoms. The H atoms were localized from the electron density maps
and refined using a riding model. Selected interatomic distances
and bond angles are given in Table S2 (ESI†). CCDC 2078495–
[
Tb{Ph P(O)Pym} (hfac) ] (4). Yield: 0.038 g (57%). Found: C,
2
2
3
41.5; H, 2.0; N, 4.0%; F, 24.9%. Calcd for C H N F O P Tb
4
7
29 4 18 8 2
(1340.6): C, 42.1; H, 2.2; N, 4.2%; F, 25.5%. FT-IR (KBr pellets,
À1
n/cm ): 3430w, 3065w, 1660s, 1613w, 1592w, 1556s, 1528s,
1
1
9
5
506s, 1487m, 1440m, 1393m, 1345w, 1321w, 1257s, 1215s,
198s, 1165s, 1145s, 1121s, 1098m, 1071w, 1031w, 998w, 989w,
50w, 806w, 794m, 764w, 746m, 727m, 693m, 661s, 634w, 618w,
85m, 561m, 551m, 537s, 460w, 441w, 410w (Fig. S1, ESI†).
2078497 contain the supplementary crystallographic data for 1, 3,
and 5, respectively.†
Photoluminescence (PL) measurements were performed on
a Fluorolog 3 spectrometer (Horiba Jobin Yvon) with a cooled
PC177CE-010 photon detection module equipped with an R2658
photomultiplier. The photoluminescence quantum yields
[
Eu{Ph
2
P(O)Pyr}
2
(hfac)
3
] (5). Yield: 0.047 g (70%). Found: C,
Eu
42.0; H, 2.1; N, 4.0%; F, 25.1%. Calcd for C47H N F O P
29 4 18 8 2
(FTOT) were measured using a Fluorolog 3 Quanta-phi device.
(
(
1333.6): C, 42.3; H, 2.2; N, 4.2%; F, 25.6%. FT-IR
KBr pellets, cm ): 3440w, 3065w, 1654s, 1615w, 1594w,
À1
1557m, 1528m, 1498s, 1484s, 1439m, 1388w, 1345w, 1321w, 1256s,
1
221s, 1200s, 1185s, 1155s, 1146s, 1122s, 1097s, 1072w, 1041w, 3. Results and discussion
1
027w, 1014w, 998w, 950w, 854w, 798m, 776w, 752m, 728m, 693m,
3
.1. Synthesis and characterization
The reactions of stoichiometric amounts of hydrated lanthanide
] (6). Yield: 0.046 g (83%). Found: C, hexafluoroacetylacetonates with the tertiary phosphine oxides in
2.2; H, 2.2; N, 4.0%; F, 25.1%. Calcd for C47 Tb ethanol or methanol afforded mononuclear complexes with the
661m, 618w, 585m, 545s, 528w, 460w, 446w, 423w (Fig. S1, ESI†).
2 2 3
[Tb{Ph P(O)Pyr} (hfac)
4
29 4 18 8 2
H N F O P
(
1340.6): C, 42.1; H, 2.2; N, 4.2%; F, 25.5%. FT-IR (KBr pellets, general composition [LnL (hfac) ] (L = Ph P(O)Py, Ph P(O)Pym,
2 3 2 2
À1
n/cm ): 3445w, 3065w, 1655s, 1614w, 1592w, 1558m, 1528s, Ph P(O)Pyr; Ln = Eu, Tb). The use of methanol and ethanol in the
2
1
1
9
5
498s, 1483s, 1439m, 1389w, 1347w, 1322w, 1256s, 1220s, 1198s, syntheses results in the complexes of the same composition,
187s, 1156s, 1145s, 1124s, 1097s, 1072w, 1041w, 1029w, 1014m, however, crystals of a better quality were obtained from ethanol
99w, 950w, 854w, 798m, 777w, 753m, 729m, 693m, 661m, 617w, solution for 1 and 2 and from methanol solution for 3–6. Note that
3+
85m, 546s, 528w, 459w, 445w, 422w (Fig. S1, ESI†).
changing the Ln :L molar ratio from 1:2 to 1:1 and 1:3 did not
lead to the isolation of other products apart from 1–6.
2.3. Characterization of the compounds
According to the single-crystal X-ray diffraction analysis
The elemental analysis (C, H, and N) was performed with a Euro complexes 1, 3, and 5 crystallize in centrosymmetric space
EA 3000 analyzer (HEKAtech GmbH, Wegberg, Germany). groups (Table S1, ESI†). The crystal structures of the compounds
Fluorine determination was performed spectrophotometrically consist of isolated molecules of mononuclear complexes, whose
using a Varian 50 Scan spectrophotometer. The FT-IR spectra in atoms occupy the general positions (Fig. S2–S4, ESI†). The unit
À1
KBr (4000–400 cm ) were recorded using a Scimitar FTS2000 cell of crystal structure 1 contains two crystallographically
1
31
1
spectrophotometer. The H (500.13 MHz) and P{ H} NMR independent molecules.
202.47 MHz) spectra were recorded on a Bruker Advance 500 The phosphine oxides Ph
spectrometer. The X-ray powder diffraction (XRPD) analysis was are coordinated to the Eu atom in a monodentate mode via the
performed using a Shimadzu XRD-7000S diffractometer. PQO group, i.e., the nitrogen atoms of azaheterocycles are not
(
2 2
P(O)Py and Ph P(O)Pyr in 1 and 5
The unit cell parameters and intensities of reflections of 1 involved in the coordination (Fig. 1a and c). Thus, the coordi-
were measured at 100 K on a Bruker APEX Duo diffractometer nation sphere of the Eu atom in 1 and 5 includes two O atoms
equipped with a 4K CCD detector by standard techniques of two phosphine oxide ligands with the Eu–O distances in the
(
0
MoKa radiation, l = 0.71073 Å, graphite monochromator, region of 2.32–2.34 Å for 1 and 2.32, 2.33 Å for 5, and six O atoms
.51 j- and o-scanning). Integration of initial experimental of the three bidentate chelating hfac ligands with the Eu–O
À
data and absorption correction were carried out using the APEX2 distances in the region of 2.38–2.52 Å for 1 and 2.38–2.43 Å for 5
51
3+
program package. Diffraction data for crystals of 3 and 5 were (Table S2, ESI†). The geometrical symmetry around Eu ions was
collected at 150 K on an automated Agilent Xcalibur diffractometer determined to be an 8-coordinated square-antiprismatic struc-
equipped with an area AtlasS2 detector (MoKa radiation, l = ture without an inversion center. All three chelating EuO
2 3
C rings
0.71073 Å, graphite monochromator, 0.51 o-scanning). The are almost planar, and the average deviation of the atoms from
integration, absorption correction, and determination of the their root-mean-square plane does not exceed 0.03 Å in 1 and
unit cell parameters of 3 and 5 were performed using the 0.07 Å in 5. The dihedral angles between the EuO C rings in 1
2
3
5
2
CrysAlisPro program package. The crystallographic character- and 5 are significantly different: one of the dihedral angles is
istics, experimental data, and structure refinements of 1, 3, and 27.51/22.81 in 1, and 27.01 in 5, and the other two dihedral
5
are listed in Table S1 (ESI†). All non-hydrogen atoms were angles are in the region of 81.2–97.21 in 1 and are 84.61 and
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Fig. 1 Thermal ellipsoid diagram of complexes 1 (a), 3 (b), and 5 (c). Disordered atom positions and hydrogen atoms are omitted for clarity. The ellipsoids
are drawn at the 30% probability level.
8
5.71 in 5. The aromatic rings of phosphine oxides in both planes of three chelating EuO
complexes are nearly planar (the average deviation from the 69.01. The aromatic rings of the Ph
root-mean-square atomic plane is 0.01 Å). In 1, one of the six almost planar (the average deviation from the average planes does
CF groups is disordered, while all six CF groups are disordered not exceed 0.01 Å). Disordering over two positions in the pyrimidine
over two positions in 5. Note that 5 is isostructural to the known and in one of the two phenyl rings is observed at P(2). In addition,
2
C
3
rings are in the region of 53.9–
2
P(O)Pym ligand at P(1) are
3
3
4
7,55
complex [Eu(Ph
3 2 3
PO) (hfac) ].
3
Thus, the substitution of two all six CF groups are also disordered in two positions. Interestingly,
phenyl rings by pyrazine rings in 5 has no significant effect on in the previously reported nitrate complex [Eu{Ph P(O)Pym} (NO ) ],
2
2
3 3
the molecular structure and packing of the complex molecules both Ph P(O)Pym act as bidentate N,O-donor ligands, while in the
2
in the crystal.
2 2 3 3 2
[Tb{Ph P(O)Pym} (MeOH)(NO ) ] complex the Ph P(O)Pym ligands
49
The experimental X-ray diffraction patterns of 1 and 5 agree are monodentate. Thus, the nature of the counter-ion and the
well with the simulated patterns calculated from single-crystal lanthanide ion has a crucial effect on the coordination mode of
structure data (Fig. S5, ESI†). The XRPD pattern of 2 is similar Ph
to that of 1, indicating that they are isostructural. Compounds 5 tural (Fig. S5, ESI†).
and 6 are also isostructural (Fig. S5, ESI†). In 1, 3, and 5, the molecules of the complexes are assembled
2
P(O)Pym. The XRPD analysis reveals that 3 and 4 are isostruc-
3
+
In complex 3, the Eu ion is nine-coordinated by three through FÁÁÁF contacts (head-to-tail) between the disordered CF3
À
bidentate hfac anions (Eu–O 2.38–2.52 Å) and two ligands groups of two neighboring molecules with distances in the region of
2
Ph P(O)Pym, one of which is bound to Ln in a bidentate 2.53–2.97 Å (Fig. S2–S4, ESI†). Moreover, multiple intermolecular
chelating mode through the N atom of pyrimidine (Eu–N CHÁÁÁF contacts were observed for 1, 3, and 5 between the aromatic
2
.82 Å) and the PQO group (Eu–O 2.37 Å), and the other acts rings of the phosphine oxide ligands and the fluorine atoms of the
À
as a monodentate O-donor ligand (Eu–O 2.38 Å). Coordination hfac ligands, and the number of these contacts per unit cell (Z = 4)
polyhedron EuN
antiprism geometry (Fig. 1b). Two of the three chelating EuO
1
O
8
adopts a distorted single-cap tetragonal decreases in the series [Ln{Ph
2
P(O)Pym}
] (4 contacts) = [Ln{Ph
2
(hfac)
3
] (7 contacts) 4
P(O)Pyr} (hfac)
2
C
3
[Ln{Ph
2
P(O)Py}
2
(hfac)
3
2
2
3
]
À
rings formed upon the coordination of the hfac ligands to the (4 contacts). The shortest EuÁ Á ÁEu intermolecular distances are
europium ion have an envelope conformation: the Eu(1) atom 9.84 Å for 1, 9.80 Å for 3, and 9.78 Å for 5.
deviates from the O
ring and a five-membered chelating EuOPCN ring are almost
2 3 2 3
C planes by 0.70 and 0.81 Å. The third EuO C
3.2. Photoluminescence
planar: the average deviations of the atoms from their root-mean- The emission spectra of the Eu(III) complexes 1, 3, and 5 in the
square planes do not exceed 0.02. The dihedral angles between the solid state displayed in Fig. 2 contain the characteristic five
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3
+
a nine-coordinate Ln ion, is significantly different from the
emission spectra of 1 and 5. In particular, the integrated
5
7
intensity of the electronic dipole D - F transition of 3 is
0
2
more than 1.6 times lower than those of 1 and 5. Interestingly,
4
7,55
the emission spectrum of the complex [Eu(Ph PO) (hfac) ]
3
2
3
is similar to the emission spectra of 1 and 5 (Fig. 2). Thus, the
PL spectra of the Eu(III) complexes strongly depend on the
3
+
coordination environment of the Eu ion, and the substitution
of a phenyl ring in Ph PO by pyridine and pyrazine moieties
3
has no significant effect on the emission profile. It should be
noted that the shapes and the positions of the bands in
the excitation and emission spectra of the compounds
Ph P(O)Py and Ph P(O)Pyr are similar and differ significantly
2
2
2
from those of Ph P(O)Pym (Fig. S6 and S7, ESI†), which
could also lead to the observed differences in the photo-
luminescence properties of complexes 1 and 5 and complex 3.
Nevertheless, the photophysical parameters of all ligands are
similar (Table S3, ESI†).
The excitation spectra of the europium(III) complexes 1, 3,
and 5 obtained by monitoring the emission wavelength of the
3
+
Eu ions at 617 nm are similar (Fig. S8, ESI†). For all spectra, a
broad band ranging from 250 to 460 nm can be seen, which is
ascribed to the p–p* electron transition of the ligands. The
7
5
sharp lines at 465, 525, and 536 nm assigned to F
0
- D2,1 and
transitions, respectively, are also seen in the solid-
state excitation spectra of these complexes. However, these
7
5
1 1
F - D
2 3
Fig. 2 Normalized photoluminescence spectra of the complexes [EuL (hfac) ]
(
L = Ph
2 2 2 3
P(O)Py (1), Ph P(O)Pym (3), Ph P(O)Pyr (5), Ph PO) in the solid state at transitions are weaker than the absorption of the organic
3
00 K (lex = 350 nm).
ligands and are overlapped by a broad excitation band, which
proves that luminescence sensitization via excitation of the
ligand is much more efficient than the direct excitation of the
3
+
narrow split emission peaks centered at 579, 594, 615, 655, Eu absorption levels.
5
7
3+
and 700 nm arising from the intra-configurational D
0
- F
J = 0–4) transitions of the Eu ion, respectively. The ligand- solid state measured under ligand excitation increase in the
centered emission is not observed, indicating an efficient series Ph P(O)Py E Ph P(O)Pyr (Table 1). In any
J
The absolute quantum yields for the Eu complexes in the
3
+
(
2
2
P(O)Pym o Ph
2
ligand-to-metal energy transfer process. The luminescence case, compounds 1, 3, and 5 show significant improvement
spectra of the Eu(III) complexes are dominated by the hyper- over FTOT for [Eu(hfac) (H O) ].
sensitive electronic dipole D
3
2
2
5
7
3+
0
- F
2
transition, the intensity of
The luminescence decay times for the Eu complexes 1, 3,
which represents 78–81% of the total integrated emission. The and 5 were measured at room temperature using an excitation
5
7
5
7
high relative integrated intensity of the D
0
- F
2
transition wavelength of 350 nm and monitored by D
0 2
- F transition
5
7
with respect to that of the magnetic dipole D
R) suggests that the Eu ions are coordinated in a site without single exponentials, indicating that all Eu ions occupy the
0
- F
1
transition (Fig. 3). The lifetime profiles for all complexes are fitted with
3
+
3+
(
an inversion center (Table 1), which is consistent with the X-ray same coordination environment. The luminescence lifetimes
5
data for 1, 3, and 5. The presence of only one sharp peak in the (tobs) of the D level lie in the region of 718–831 ms, which is
0
5
7
59–61
region of D - F transition at 579 nm suggests the existence typical for europium b-diketonates (Table 1).
The photo-
0
0
3
+
56,57
of a single chemical environment around the Eu ion.
Notably, the different coordination modes of the Ph P(O)Pym, that of [Eu(hfac)
P(O)Py, and Ph P(O)Pyr ligands in the complexes obtained are 3, and 5 are well shielded from nonradiative deactivations.
reflected in their luminescence properties, especially in the PL
spectra. So, the emission spectra of [EuL (hfac) ] (L = Ph P(O)Py which explains the strong emissive properties of these compounds
1) and Ph P(O)Pyr (5)) with O-coordinated phosphine oxides are despite the relatively short observed lifetime (Table 1). The intrinsic
very similar in shape and show almost the same relative quantum yields for 1, 3, and 5 lie in the region 68–72%, which is
luminescence lifetimes of the complexes are much longer than
3
+
2
3 2 2
(H O) ], which suggests that the Eu ions in 1,
Ph
2
2
3
+
Radiative lifetimes for the Eu complexes are rather short,
2
3
2
(
2
3
+
intensities of the different Eu transitions. The similarity B3 fold larger compared to the parent [Eu(hfac) (H O) ]. The
3
2
2
comes probably from the fact that the phosphine oxides in sensitization efficiencies (Zsens) increase 5–6 fold upon substitution
3
+
3 2 2
these complexes are coordinated to the Eu ion to form very of water molecules in [Eu(hfac) (H O) ] by phosphine oxides.
similar surroundings and distortion of the ion environment The large radiative rate constants are due to the asymmetric
symmetry. At the same time, the emission spectrum of 3, having coordination geometry around Eu(III) ions. The presence
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Table 1 Photophysical parameters for complexes 1–6, [Ln(hfac)
3
(H
2
O)
2
] and [Ln(Ph
3
PO)
2
(hfac)
3
] (Ln = Eu, Tb) in the solid state at 300 K (lex = 350 nm)
a
b
c
d
e
f
g
À1
h
À1
Compound
t
obs (ms)
t
rad (ms)
F
Ln (%)
F
TOT (%)
Z
sens
R
k
r
(s
)
k
nr (s )
[
[
[
[
[
[
[
[
[
[
Eu{Ph
Tb{Ph
Eu{Ph
Tb{Ph
Eu{Ph
Tb{Ph
2
2
2
2
2
2
P(O)Py}
P(O)Py}
P(O)Pym}
P(O)Pym}
P(O)Pyr}
P(O)Pyr}
2
(hfac)
(hfac)
3
] (1)
] (2)
763
418
831
358
718
443
741
96
1065
n.a.
1205
n.a.
993
n.a.
928
n.a.
1389
n.a.
72
n.a.
68
n.a.
72
n.a.
80
n.a.
24
n.a.
43
31
44
29
56
39
54
10
3
0.60
n.a.
0.65
n.a.
0.78
n.a.
0.68
n.a.
0.13
n.a.
15.4
n.a.
13.2
n.a.
16.3
n.a.
16.8
n.a.
11.3
n.a.
939
n.a.
930
n.a.
1007
n.a.
1078
n.a.
720
n.a.
372
n.a.
373
n.a.
386
n.a.
272
n.a.
2348
n.a.
2
3
2
(hfac)
(hfac)
3
] (3)
] (4)
2
3
2
(hfac)
(hfac)
3
] (5)
] (6)
2
3
Eu(Ph
Tb(Ph
3
PO)
PO)
2
(hfac)
(hfac)
3
]
]
3
2
3
Eu(hfac)
Tb(hfac)
3
(H
(H
2
O)
O)
2
]
]
326
536
3
2
2
50
a
b
3+
3
Lifetimes measured in the solid state. Radiative lifetimes for Eu complexes calculated as trad = 1/n AMD,0 Â IMD/ITOT, where AMD,0 is the
5
7
À1
spontaneous emission probability for D
refraction equal to 1.5 was used).
state. Energy transfer efficiencies between the ligand and the Eu ion calculated as Zsens = FTOT/FLn
I( D
0
- F
1
transition in vacuo (14.65 s ), and n is the refractive index of the medium (an average index of
58 c
d
Intrinsic quantum yields calculated as FLn = tobs/trad
.
Total absolute quantum yields measured in the solid
Ratios R calculated from the formula
The non-radiative rate constants calculated as 1/tobs À 1/trad
e
3+
f
.
5
7
5
7
g
h
0
- F
2
)/I( D
0
- F
1
). The radiative rate constants calculated as 1/trad
.
.
the strongest (Fig. 4). The Stark splitting shapes at 544 nm
5
7
47
(
D - F ) for 2, 6 and the known complex [Tb(Ph PO) (hfac) ]
4 5 3 2 3
are similar, which indicates the same coordination geometry in
these complexes. A broad ligand-centered band in the excitation
spectra of the Tb(III) complexes 2, 4, and 6 confirms energy transfer
from ligands to metal (Fig. S9, ESI†). The direct excitation peak of
7
5
low intensity corresponding to the F
6 4
- D transition could also
be observed at 485 nm.
The quantum yields of the terbium complexes are lower
than the quantum yields of the europium complexes (Table 1).
The emission decays of 2, 4, and 6 showed microsecond-scale
emission lifetimes by estimating single-exponential fittings. In
contrast to the analogous isostructural Eu(III) complexes with
similar photophysical parameters, the quantum yield and the
luminescence lifetime of 6 are much greater than those of
[Tb(Ph
3 2 3
PO) (hfac) ], which is due to the fact that these Tb(III)
complexes have different crystal structures.
3.3. Triboluminescence
We observed the triboluminescence (TL) of all prepared Eu(III)
and Tb(III) complexes in the dark at 300 K in air. The complexes
1–6 exhibit TL despite their nonchiral space groups, suggesting
that intermolecular structure is the dominant factor for TL
activity. The FÁ Á ÁF contacts (Fig. S2–S4, ESI†) are also related to
the TL activity of the complexes since they could prevent the
formation of strong hydrogen bonds between the molecules.
Among complexes 1–6, the TL activity was qualitatively higher
in 1, 5, and 6 (ESI,† Video). For all complexes, the hydrogen
Fig. 3 Kinetics of photoluminescence decay of the complexes [LnL
Ln = Eu, Tb; L = Ph P(O)Py, Ph P(O)Pym, Ph P(O)Pyr) in the solid state at
00 K.
2 3
(hfac) ]
(
2
2
2
3
À
atoms of aromatic cycles and fluorine atoms of the hfac ligands
of multiple intra- and intermolecular CH. . .F contacts in the form CHÁÁ ÁF contacts (Fig. S2–S4, ESI†). Since the number of
europium complexes leads to low nonradiative rate constants intermolecular CHÁÁ ÁF contacts for [Ln{Ph P(O)Pym} (hfac)
nr) by forming low-vibrational coordination structures. It (7 contacts) is greater than for [LnL (hfac) ], where L = Ph P(O)Py
should be noted that the photophysical parameters of and Ph P(O)Pyr, (4 contacts), the mechanical stability of these
2
2
3
]
(k
2
3
2
2
[
Eu(Ph PO) (hfac) ] are close to those of 5, which is apparently compounds was assumed to be [Ln{Ph P(O)Pym} (hfac) ] 4
3
2
3
2
2
3
due to the fact that these complexes are isostructural.
[Ln{Ph P(O)Py} (hfac) ] E [Ln{Ph P(O)Pyr} (hfac) ]. Thus, the
2 2 3 2 2 3
The emission spectra of the Tb(III) complexes 2, 4, and 6 observed TL activity is inversely proportional to the mechanical
show characteristic peaks at 485–496, 544, 580–590, 615–624, stability. The similar dependence of triboluminescent activity on
and 647–657 nm, which are assigned to the transitions from mechanical stability is demonstrated in ref. 18. The weak TL of 2
5
7
4 J
D to F ( J = 6–2), with the emission at 544 nm being can be attributed to its relatively low quantum yield (Table 1).
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Conflicts of interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the Multi-Access Chemical Research Centre
SB RAS for spectral and analytical measurements and XRD
Facility of NIIC SB RAS for the data collection. The work was
supported by the Russian Foundation for Basic Research (pro-
ject no. 19-73-00158).
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