Quantum Yields over 80% Achieved in Luminescent Europium Complexes by Employing Diphenylphosphoryl Tridentate Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b01028

Four tridentate europium(III) complexes containing a diphenylphosphoryl group are prepared with strong bonding between the ligands and centered ion, convinced by crystal structures. Compared to their parent bidentate complexes, the tridentate complexes di

Full text of DOI:10.1021/acs.inorgchem.8b01028

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Quantum Yields over 80% Achieved in Luminescent Europium
Complexes by Employing Diphenylphosphoryl Tridentate Ligands
Chen Wei, Boxun Sun, Zelun Cai, Zifeng Zhao, Yu Tan, Weibo Yan, Huibo Wei,* Zhiwei Liu,*
Zuqiang Bian, and Chunhui Huang
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
*
S Supporting Information
13
and in some situations reduced energy-transfer efficiency.
ABSTRACT: Four tridentate europium(III) complexes
containing a diphenylphosphoryl group are prepared with
strong bonding between the ligands and centered ion,
convinced by crystal structures. Compared to their parent
bidentate complexes, the tridentate complexes display
improved and exceptionally high photoluminescence
quantum yields (PLQYs) in powder (all over 80%, best
Nevertheless, the frequently used oxygen site (carboxyl/
phosphonate group) usually leads to the embedding of water
molecules in the first/second coordination sphere because of its
hydrophilic character. Consequently, the PLQYs of most
tridentate europium(III) complexes reported so far are below
14
7
0% (Table S1).
Herein, we introduce diphenylphosphoryl (DPPO), a hydro-
9
1%), as well as in a CH Cl solution and poly(methyl
2 2
phobic oxygen-containing functional group, to a series of
bidentate ligands, hence constructing tridentate ligands and
their europium(III) complexes (Chart 1), with aim of high
methacrylate) films, benefiting from compact, stable, and
saturated coordination.
Chart 1. Tridentate Europium(III) Complexes Employing
DPPO Group
uminescent lanthanide complexes have been widely used in
L
bioimaging and sensing, solid-state lighting, and light-
conversion devices because of their special photophysical
1
properties. High photoluminescence quantum yield (PLQY) is
quite essential for these applications in order to get high signal
intensity as well as high energy conversion efficiency, which
requires the specific design of the complex structure, especially
the ligands. Because the ligands serve as “antenna” to sensitize
3
+
the luminescence of a lanthanide (Ln ) ion, the proper energy
level (usually a triplet state) and tight coordination (short
3
+
distance between the ligands and Ln ion) are preferred to
2
achieve high energy-transfer efficiency. The ligands are also
expected to occupy the lanthanide coordination sites as much a3s
possible for eliminating quenching by the coordinated solvent.
Other deactivation pathways such as low-lying ligand-to-metal
charge-transfer (LMCT) states and O−H or N−H vibrations
PLQYs. Two methods were used to integrate DPPO with the
bidentate ligands. HL1 (corresponding to the free ligand in Eu1,
similarly hereinafter) and HL2 were initially synthesized by the
nucleophilic substitution reaction between potassium diphenyl-
2
should be avoided when designing new lanthanide complexes.
With these considerations, many types of ligands have been
4
15
developed, among which bidentate ligands like β-diketone, 1,10-
phosphanide and a chloro/bromo precursor, followed by
5
6
hydrogen peroxide oxidation (Scheme S1). However, potassium
diphenylphosphanide is a very sensitive and strong nucleophile;
thus, side reactions always take place. Alternatively, a modified
phenanthroline, and 2,2′-bipyridine are the most studied ones
because of their ease of fabrication. Nevertheless, bidentate
ligands, especially the neutral ones, suffer from weak chelating
16
3+
nickel-catalyzed C−P cross-coupling method was adopted for
HL1 by the reaction of a bromo precursor and diphenylphos-
phine oxide and proved to be a better route with a mild synthetic
condition and good yield (Scheme S1). Therefore, L3 and L4
were both synthesized using a catalyzed reaction (Scheme S2).
stability with the Ln ion, which may lead to ease of destruction
of the complex in solution or when heated. Introducing another
coordination site onto bidentate ligands in order to prepare
tridentate ligands is a simple way to increase the coordination
stability. However, up to now, functional groups that can be
grafted onto bidentate ligands are mostly N-coordination
The corresponding complexes Eu(L) (Eu1 and Eu2 with L = L1
3
7
8
9
and L2, respectively) were prepared by the “self-assembly” of the
moieties, such as pyridine, benzimidazole, and tetrazole,
while the groups containing coordinated oxygen sites are limited
ligand, NaOH, and EuCl ·6H O in a ratio of 3:3:1 in methanol
3
2
10
primarily to carboxyl and phosphinate/phosphonate deriva-
tives. In fact, a nitrogen coordination site bonds less tightly with
Ln ion than an oxygen site, causing less stable coordination
1
1
Received: April 17, 2018
3+
12
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Scheme S3), and [Eu(L) ](OTf) (Eu3 and Eu4 with L = L3
Communication
(
while the sharp bands at 580, 592, 615, 650, 692, and 812 nm are
3
3
III
5
and L4; OTf = CF SO ) were synthesized through 3:1
emissions of the Eu ion, originating from the transitions of D
3
3
0
7
stoichiometry of the ligand and Eu(OTf) in CH Cl . For
to F (J = 0−5). Different intensity proportions (e.g., 592/615
nm) of the four complexes are ascribed to the various
coordinating environments (symmetry) of the centered ion.
3
2
2
J
comparison, four europium(III) complexes based on the parent
bidentate ligands were also synthesized with the formulas of
2
1
Na[Eu(L) ] (Eu1′ and Eu2′, where L = L1′ and L2′) and
Similar emission spectra were obtained in solution and powder
compared to those in PMMA films for each individual complex
(Figure S2), indicating favorable coordination stability without
much dissociation in a dilute solution or PMMA film. This is in
accordance with the luminescent lifetimes, which are all single
exponential for the tridentate complexes (Table 1 and Figure
S3).
4
[
Eu(L) ](OTf) (Eu3′ and Eu4′, where L = L3′ and L4′).
4
3
By slow evaporation of the complex solution, high-quality
single crystals of Eu1, Eu2, and Eu4 were obtained. X-ray
diffraction reveals the 3:1 ligand/Eu structures of the complexes
with a coordination number of 9 (Figure 1 and Table S2). No
From the edge of the UV absorption spectra (Figure S4), we
can obtain the singlet energy (E ) of the ligands, which are listed
g
in Table 1, together with the triplet-energy information (ET)
obtained from the 0-phonon component of phosphorescence
spectra of corresponding gadolinium(III) complexes in the glass
state at 77 K (Figure S4). It is noteworthy that the triplet-energy
levels are almost unchanged (for L1 and L4) or slightly lowered
−
1
(
ΔE < 850 cm for L2 and L3) after grafting the DPPO group
T
to their parent bidentate ligands (Table 1). This phenomenon is
interpretable because the sp hybrid nature of the phosphoryl
Figure 1. Perspective view of (a) Eu1, (b) Eu2, and (c) Eu4 at the 50%
probability level, with Eu in cyan, N in blue, O in red, P in orange, and C
in gray. Hydrogen atoms, counterions, and uncoordinated solvent are
omitted for clarity.
3
group does not significantly enlarge the conjugation system,
different from the commonly used sp functional groups. This is
2
of great importance considering that the introduction of a third
coordination group usually changes the energy level of the parent
ligand as well as the luminescent efficiency of the complex. For
further illustration, two tridentate ligands were prepared by
solvent molecule is observed in the first coordination sphere,
which can substantially eliminate the quenching of luminescence
from O−H vibration. The polyhedra around the metal ion
2
introducing two kinds of sp hybridization functional groups,
(
Figure S1) are best categorized to a tricapped trigonal prism by
acetyl and pyridine groups, onto the bidentate ligand L2′
calculating the degree of distortion of the title compounds from
(
products named AcMCND and PyMCND, respectively), which
1
8
7
all possible nine-atom polyhedral structures1
using the
showed much-reduced triplet-energy level (ΔE = 1850−2770
T
continuous shape measures method (Table S3). The average
coordination bond length of the complexes (2.48 Å for Eu1 and
Eu2 and 2.54 Å for Eu4; Table S4) is shorter than that of the
−1
III
cm ) and hence quite poor behavior in the sensitizing Eu ion
(
Figure 3 and Table 1).
Absolute PLQYs of the complexes were measured using a
9
,19
complexes based on tridentate N^N^N (2.55−2.58 Å)
or
Hamamatsu C9920-02 PLQY measurement system with an
2
0
bidentate N^N (2.62−2.67 Å) ligands. This compact
coordination is beneficial for energy transfer from the ligands
to centered ion.
22
integrating sphere. Exceptionally high PLQYs were obtained
for all of the tridentate complexes (Eu1−Eu4; all over 80%, best
9
1%, in powder; Table 1), which are far better than those of the
Eu1−Eu4 showed strong red emission in solution, poly-
methyl methacrylate) (PMMA) film, and powder under UV
corresponding bidentate complexes (Eu1′−Eu4′, in the range of
(
1
0−73%) and are among the highest values of europium(III)
irradiation (Figure 2a,b). Figure 2c displays the excitation and
emission spectra of the four tridentate europium(III) complexes
in PMMA films (1%, w/w). The broad bands ranging from 230
to 410 nm are excitation profiles of the corresponding ligands,
23
complexes in the literature.
To explain the substantial increase of PLQYs, we calculated
the sensitization efficiencies (ηsens) and intrinsic quantum yields
Eu
Eu
(
Φ ) of the corresponding complexes in PMMA films,
according to eqs S1 and S2. The results in Table 1 show that
two categories can be classified: one is the (O)^N^O ionic
ligand-based complexes Eu1′ (Eu1) and Eu2′ (Eu2); the other is
the (O)^N^N neutral ligand-based complexes Eu3′ (Eu3) and
Eu4′ (Eu4). For the first category, both bidentate and tridentate
Eu
complexes display very high η
(near unity), whereas Φ
sens
Eu
dramatically increases after DPPO modification. This increased
Eu
III
Φ
is attributed to the better protection of the Eu ion by
Eu
tridentate ligands than by bidentate ones. Taking Eu2 and Eu2′,
for example, the tridentate complex Eu2 possesses a saturated
complex structure without solvent coordination, as disclosed by
the crystal structure shown above. However, the bidentate
complex Eu2′ is very likely to contain quenching solvent
molecules due to its complicated and unsaturated coordination
feature, which, although not directly proven here, could be
verified from the crystal structure of a similar bidentate complex
Figure 2. Photograph of (a) a PMMA film doped with Eu2 (5%, w/w)
and (b) luminescent logo celebrating the 120th anniversary of Peking
University employing anticounterfeiting inks containing Eu2, under 365
and 254 nm UV lamps, respectively. (c) Excitation and emission spectra
of the tridentate europium(III) complexes in PMMA (1%, w/w) films.
λem = 614 nm and λex = 270, 360, 300, and 320 nm for Eu1 to Eu4,
respectively.
24
that we reported earlier (Na[Eu(8mCND) (H O)]), possess-
4
2
ing one coordinating water molecule (Figure S6). For the second
B
DOI: 10.1021/acs.inorgchem.8b01028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. Photophysical Properties of the Europium(III) Complexes
c
PLQY (%)
Eg (cm−1
a
)
ET (cm−1
b
)
powder
solution
PMMA
τobs (ms)
d
τrad (ms)
d
Φ
Eu
Eu
f
ηsens
g
Td (°C)
h
complex
Eu1
35210
25130
29070
30670
35590
27400
29590
30580
23870
22220
27780
20750
21410
23260
27620
21460
22220
23360
19610
18690
81
91
73
88
82
92
2.44
1.65
2.74
2.49
1.52
0.56
3.09
2.03
3.53
3.71
2.24
1.31
2.25
2.82
2.41
1.82
0.79
0.81
0.78
0.67
0.68
0.43
0.46
0.16
0.24
0.26
1.0
414
450
390
304
392
434
226
105
491
378
Eu2
1.0
Eu3
91
76
91
1.0
Eu4
82
42
65
0.97
1.0
Eu1′
Eu2′
Eu3′
Eu4′
73
53
70
31
49
42
0.98
0.28
0.37
0.39
e
10
16
13
1.03
e
14
11
5.8
9.4
0.80
0.46
e
Eu(PyMCND)3
3.5
0.64
0.77
0.71
0.58
e
Eu(AcMCND)3
0.47
0.031
a
b
Singlet energy (0-phonon) of the ligand obtained from the edge of UV absorption spectra. Triplet energy deduced from the 0-phonon component
c
of the phosphorescence spectra. Absolute photoluminescence quantum yield measured by Hamamatsu C9920-02 with an integrating sphere.
d
e
f
Observed lifetime τ and calculated radiative lifetime τrad in the PMMA film. Dual exponential decay. Intrinsic quantum yields calculated from
obs
g
h
eqs S1 and S2. Sensitization efficiency calculated by the ratio of PLQYs and intrinsic quantum yields. Decomposition temperature determined by
the loss of 5% desolvated weight. Relative errors: τobs, ±2%; PLQY, ±3%; Φ , ±12%; ηsens, ±15%.
Eu
Eu
3
sp hybrid feature of the phosphoryl group, the energy level of the
sensitizing chromophore remains almost unchanged. As a result,
the title complexes displayed exceptionally high PLQYs. The
DPPO modification strategy can also be applied to other ligands
and other lanthanide ions, which are being investigated in our
laboratory.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01028.
Figure 3. Triplet-energy levels and PLQYs (in powder) of Eu2′, Eu2,
and control samples Eu(PyMCND) and Eu(AcMCND) , showing
3
3
2
much-reduced luminescent efficiency after the introduction of sp
Synthesis protocols and characterization data, photo-
physical measurements, and thermogravimetric analysis
curves (PDF)
hybrid pyridine and acetyl groups, due to the reduced energy level.
III 5
5
The dashed lines are the locations of the Eu D and D levels.
1
0
Eu
Eu
category, both ηsens and Φ are noticeably increased for O^N^N-
Accession Codes
based complexes Eu3 and Eu4 compared with bidentate N^N-
based complexes Eu3′ and Eu4′. Weak coordination and
relatively long bond lengths are considered to be the main
reason for the poor sensitizing ability of neutral bidentate N^N
ligands (ηsens = 0.28 and 0.37 for Eu3′ and Eu4′, respectively).
Therefore, adding an oxygen-containing DPPO group to such a
N^N ligand can bring about much shortened distance between
the ligands and centered ion and better suppression of possible
dissociation, leading to an increase of ηsens for the tridentate
CCDC 1508105, 1814238, and 1814264 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
25
Corresponding Authors
complexes (near unity for both Eu3 and Eu4). The improved
coordination stability can also be perceived from the enhanced
thermal stability of Eu3 and Eu4 compared with that of bidentate
Eu3′ and Eu4′, showing an increase of the decomposition
*
*
E-mail: weihuibo@163.com.
E-mail: zwliu@pku.edu.cn.
ORCID
temperature (T ) over 160 °C (Table 1 and Figure S7). One may
Zhiwei Liu: 0000-0003-3001-5310
Zuqiang Bian: 0000-0003-4788-6032
Notes
d
notice that, for Eu4, much reduced PLQY values in a PMMA film
and solution compared to that in powder are obtained, which is
probably due to the comparably weak rigidity of the bipyridyl
group, leading to nonradiative quenching in solution and PMMA
with more degrees of freedom.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
In summary, we have developed a strategy of preparing highly
emissive europium(III) complexes by introducing a hydrophobic
oxygen-containing functional DPPO group to bidentate ligands.
■
We thank Dr. Jiajia Liu and Dr. Xiaonan Yao for their help in
crystal refinement. This research is supported through grants
from the National Key R&D Program of China (Grants
2016YFB0401001 and 2017YFA0205100), National Natural
Science Foundation of China (Grant 21621061), and Beijing
Natural Science Foundation (Grant 2172017).
III
The tridentate ligands and Eu ion form mononuclear complex
structures with ligands occupying all of the coordination sites.
The DPPO group draws the ligands closer to the ion and
improves the coordination stability of the complex. Thanks to the
C
DOI: 10.1021/acs.inorgchem.8b01028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(
17) Ruiz-Martinez, A.; Casanova, D.; Alvarez, S. Polyhedral structures
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DOI: 10.1021/acs.inorgchem.8b01028
Inorg. Chem. XXXX, XXX, XXX−XXX