Lanthanide tetrafluorobenzoates as emitters for OLEDs: New approach for host selection

Article abstract of DOI:10.1016/j.orgel.2017.01.026

Brightly luminescent and highly soluble lanthanide tetrafluorobenzoates, as well as their mixed ligand complexes, were synthesized and thoroughly characterized. The low charge carrier mobility hampered their use in OLED, but this problem was overcome by a

Full text of DOI:10.1016/j.orgel.2017.01.026

Accepted Manuscript
Lanthanide tetrafluorobenzoates as emitters for OLEDs: New approach for host
selection
Valentina V. Utochnikova, Nikolay N. Solodukhin, Andrey N. Aslandukov, Lukasz
Marciniak, Ivan S. Bushmarinov, Andrey A. Vashchenko, Natalia P. Kuzmina
PII:
S1566-1199(17)30036-8
10.1016/j.orgel.2017.01.026
ORGELE 3938
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 22 December 2016
Revised Date: 17 January 2017
Accepted Date: 17 January 2017
Please cite this article as: V.V. Utochnikova, N.N. Solodukhin, A.N. Aslandukov, L. Marciniak, I.S.
Bushmarinov, A.A. Vashchenko, N.P. Kuzmina, Lanthanide tetrafluorobenzoates as emitters for OLEDs:
New approach for host selection, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.01.026.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Lanthanide Tetrafluorobenzoates as Emitters for OLEDs:
New Approach for Host Selection
Valentina V. Utochnikova*, Nikolay N. Solodukhin, Andrey N. Aslandukov, Lukasz Marciniak, Ivan
S. Bushmarinov, Andrey A. Vashchenko, and Natalia P. Kuzmina
Brightly luminescent and highly soluble lanthanide tetrafluorobenzoates, as well as their mixed ligand complexes, were
synthesized and thoroughly characterized. The low charge carrier mobility hampered their use in OLED, but this problem
was overcome by a thoughtful selection of host material. The organic molecules acted both as ligands in the complex and
as the host material, leading to zero increase in the Stokes shift.
select a host material that can provide energy transfer directly
to the lanthanide ion. As the energy transfer is a short-range
Introduction
Continued interest in the design and construction of new
luminescent materials based on lanthanide coordination
compounds is due to their luminescence features such as
quantum efficiency up to 100%1 and narrow luminescence
bands2–4. Lanthanide aromatic carboxylates Ln(carb)3 are of
particular interest because they combine remarkable
luminescent properties with excellent ultraviolet durability and
chemical stability5. Luminescent lanthanide aromatic
carboxylates attract constant attention as potential emitters
for organic light emitting diodes (OLEDs) due to their high
quantum yields as well as exceptional stability, including
stability under electric current6,7. However, most of these
materials show low solubility, which significantly hampers their
thin film deposition. Recently, fluorinated aromatic
carboxylates were proposed as luminescent materials
combining bright luminescence with high solubility in organic
solvents as well as in water, along with high thermal and
process, it is necessary to ensure the host material
coordination by the lanthanide ion, that is, the search for such
transportation of materials should be carried out among the
compounds which (i) demonstrate charge carrier mobility and
(ii) can act as ligands and transfer energy to the lanthanide ion.
In the present work, this approach to host material selection
was tested for europium 2,3,5,6-tetrafluorobenzoate
Eu(tfb)3(H2O)2. As lanthanide complexes with this ligand were
unknown, synthesis and investigation of the whole row of
Ln(tfb)3(H2O)x (Ln = Nd, Eu, Gd, Tb, Er, Yb, Lu) was carried out,
as well as their mixed-ligand complexes with o-phenanthroline
(Phen), and its derivative, bathophenanthroline (BPhen),
known for its electron transport properties14,15, as well as its
ability to coordinate europium16.
Results and discussion
photostability common for aromatic carboxylates8–11. This fact, 1. Synthesis and crystal structures
together with the opportunity to tune their properties by the
Complexes Ln(tfb)3(H2O)x were synthesized from H(tfb) and
introduction of substituents at different positions of the
benzene ring, makes them promising candidates for various
applications, including emitting films in OLEDs.
excess of freshly prepared Ln(OH)3 to ensure that the soluble
H(tfb) has fully reacted:
Ln(OH)3+3H(tfb)→Ln(tfb)3(H2O)x+(3-x)H2O (1)
However, for efficient exciton formation, the emission layers in
OLEDs must possess both high luminescence quantum yield
and transport properties, i.e. mobility toward at least one type
of charge carriers (holes or electrons). Lanthanide complexes
combining these properties may be used to fabricate efficient
host-free OLEDs7,12,13, but a proper host material is needed
otherwise. The host material must be chosen among
compounds which demonstrate good transport properties and
ensure efficient host-emitter energy transfer at the same time.
Therefore the energy of the host material excited state must
be higher than that of the emitter material which in the case of
lanthanide complexes as emitters means that the host excited
state should be higher than the excited state of the ligand
within the complex. Such a choice of the host material is
possible, but the presence of the host to ligand energy transfer
together with the ligand to the metal energy transfer in the
complex prior to luminescence will lead to extremely large
Stokes shift and, consequently, to a low energy quantum yield
of the device.
Slow crystallization in air allowed obtaining Tb(tfb)3(H2O)3 as a
single crystal, while evaporation of the clear water solution at
o
the elevated temperature (60-100 C) resulted in formation of
hydrates Ln(tfb)3(H2O)x, where x=1 for Ln = Yb, Lu and x=2
otherwise (Ln = Nd, Eu, Gd, Tb, Er).
a)
b)
Figure
1
a) Dimeric structure and b) terbium coordination
environment of Tb(tfb)3(H2O)3
Mixed ligand complexes were obtained by reaction (2)
between methanol solutions of Ln(tfb)3(H2O)x and Q = Phen
(Ln = Nd, Eu, Er, Yb) or Q = BPhen (Ln = Eu). Slow evaporation
Thus, to reduce the Stokes shift, the obvious solution is to
build a host-free OLED based on the emissive layer possessing
both luminescent and transport properties. Another way is to

of the solution resulted in the formation of Yb(tfb) (Q) (H O) different from the single crystal of Tb(tfb)3(H2O)3 (Figure 3a).
ACCEPTED MANUSCRIPT
3
2
2
as a single crystal.
Ln(tfb)3(H2O)x +2Q → Ln(ꢀb)3(Q)2(H2O)↓ + (x-1)(H2O) (2)
Its unit cell parameters (given for Tb) were determined by
indexing and refined using Pawley method as implemented in
The structures of Tb(tfb)3(H2O)3 and Yb(tfb)3(Phen)2(H2O) were TOPAS, and are as follows: a = 7.764(3), b = 11.774(3), c =
determined from single-crystal. 12.790(3) Å; α = 90.149(10), β = 94.79(2), γ = 106.939(19)°.
X-ray diffraction data (see Experimental). The structural data With the powder pattern similar to the theoretical single-
are summarized in the Electronic supporting information (ESI).
crystal one, and the unit cell volume of 1114.1(6), smaller by
40 Å3 at r.t. than the volume of Tb(tfb)3(H2O)3 at 120 K, the
phase observed in powder most likely corresponds to the loss
of one water molecule (≈20 Å3) per metal and the composition
Tb(tfb)3(H2O)2. Indeed, the single-crystal structure contained
one water molecule in the outer sphere which could exit the
crystal without significant changes to the overall crystal
packing.
At the same time, Lu(tfb)3(H2O) in powder consists of at least
two unknown phases. No H(tfb) impurities were found in the
studied powders. Formation of the non-single-phase product
for Lu, while other lanthanides behaved differently, was
observed for pentafluorobenzoates, too, and is likely due to
lutetium peculiarities due to its smallest radius in the
lanthanide row8.
Figure 2 Overview of Yb(tfb)3(Phen)2(H2O) in crystal
In crystal, Tb(tfb)3(H2O)3 consists of dimers (composition
Tb2(tfb)6(H2O)4∙2H2O), with two bridging μ:κ1,κ2, two bridging
μ:κ1,κ1 and two κ2 ligands (Figure 1). Each metal is coordinated
by two water molecules, and one more water molecule is in
the outer coordination sphere.
The powder patterns of the mixed-ligand complexes showed
no agreement with single-crystal data whatsoever (Figure 3b).
Hydrate composition of Ln(tfb)3(H2O)x in powder was
determined by TGA data (Figure 4, S1, S2, S3, S4) which shows
two-step weight loss, corresponding to the water molecules
elimination, after which the anhydrous complex is stable until
at least 300 °C. According to TGA, the powder of freshly
prepared Eu(tfb)3(H2O)3 contains three water molecules per
metal ion, the same as its single crystals. However, one water
molecule disappears over time, leading to the formation of
Ln(tfb)3(H2O)2 (Ln = Nd, Gd, Tb, Er) in accordance with the
PXRD data. Complexes of heavier metals Ln(tfb)3(H2O) (Ln = Yb,
Lu) have lower water content, which is typical for
fluorobenzoates9.
A reaction with o-phenanthroline results in the formation of
mixed-ligand complex (MLC). In crystal, Yb(tfb)3(Phen)2(H2O)
consists of monomeric units (Figure 2). The metal coordinates
four nitrogen atoms, three oxygen atoms of κ1-tfb-, and a
water molecule. As expected, Phen has stronger coordinating
ability than fluorinated benzoate anion19. These monomeric
units form chains bound by strong HOH…OOC hydrogen bonds
(O...O distance 2.639(6) Å, O-H…O angle 162(6)°), which in turn
are linked together by π-π interactions between ligands (with
the shortest C…C distance of 3.236(9) Å between neighboring
Phen fragments).
According to the PXRD data, in bulk the Ln(tfb)3(H2O)2 (Ln =
Nd, Eu, Gd, Tb, Er) complexes have a common structure
(1)
(2)
(1)
(2)
(3)
(4)
(5)
(3)
(4)
(6)
(7)
(8)
(9)
5
10
15
20
25
10
20
30
2θ,°
2θ,°
a)
b)
Figure 3 a) PXRD data of (1) H(tfb) (simulated from17), Ln(tfb)3(H2O): Ln= (2) Lu, (3) Yb and Ln(tfb)3(H2O)2: Ln = (4) Er, 6 (Gd), 7 (Eu), 8 (Nd), (5, 9)
Tb (experimental and simulated) and b) experimental PXRD data of Ln(tfb)3(Phen)2, where (1) Ln = Nd, (2) Ln = Er, (3) Ln = Yb, and (4)
simulated PXRD data of Yb(tfb)3(Phen)2(H2O)

ACCEPTED MANUSCRIPT
100
100
80
60
40
20
0
-2 Phen
(3), n=1
(2), n=2
(1), n=3
80
60
40
20
- n H2O
0
200
400
600
800
1000
200
400
600
800
1000
Temperature,°C
o
Temperature,
C
a)
b)
Figure 4 TG (―) and DTG (---) curves of a) Eu(tfb)3(H2O)3 (1), Tb(tfb)3(H2O)2 (2) and Lu(tfb)3(H2O) (3); b) Nd(tfb)3(Phen)2
TGA data of the mixed-ligand complexes confirm the presence positions of fluorescence (410 nm) and phosphorescence
of two ancillary Phen ligands that eliminate at 200-250 °C, bands (490 nm) the excited state energy value were calculated
followed by thermodestruction of the complex. Water as S1 = 23200 cm-1 and T1 = 20500 cm-1.
molecule coordinated by the lanthanide ion in the single Molar extinction coefficient ε, which together with the
crystal structure is absent in the powder product.
The 2:1 ratio of the ancillary ligand to the carboxylate was also brightness (LE=ε
confirmed by the NMR data (Figure and S7). The 6b) and compared with those of pentafluorobenzoate anion
paramagnetic shift of the Phen proton signals suggests that
pfb-). It turned out that substitution of only one fluorine atom
quantum yield (PLQY) affects the photoluminescence
⨯
PLQY), was measured in methanol (Figure
5
(
the Phen molecules do not eliminate in the highly donor with a hydrogen results in twice-higher absorption of the
DMSO-d6. Indeed, if dissociation took place, no shift would be complex.
observed, as can be seen from Figures S5 and S6 via the
1500
comparison of NMR spectra of Ln(tfb)3(H2O)x in CD3OD (no
1,0
1
dissociation) and in H2O (complete dissociation). The stability
0,8
1000
of the complex in methanol also allows measuring its
luminescent properties in alcoholic media.
300 K
77 K
0,6
0,4
0,2
0,0
500
0
2
Phen
♥
(1)
240
280
320
400
500
600
700
800
♥
λ,nm
(2)
λ,nm
a)
b)
♥
(3)
Figure 6 a) Luminescence spectra of Gd(tfb)3(H2O)2 in propanol-1
solution at 300K and 77K and b) absorption spectra of Na(tfb) (1) and
Na(pfb) (2) in MeOH (Hpfb = pentafluorobenzyl acid) (concentration
10-5 M)
♥
(4)
BPhen
♥
(5)
9,5
9,0
8,5
8,0
7,5
7,0
Luminescent characteristics of the obtained complexes are
δ,ppm
summarized in In the excitation spectrum of Eu(tfb)3(BPhen)2
the band at 350 nm (through-BPhen) is significantly more
intense than the band at 270 nm (through-tfb-), which is
associated with a significantly greater absorption of BPhen
compared to the tfb- (Figure 8c) and with efficient
BPhen→Eu3+energy transfer. In the case of Eu(tfb)3(Phen)2, the
intensity of bands at 330 nm and 270 nm is comparable
despite the essentially the same sensitization efficiency by
Phen and BPhen (Error! Not a valid bookmark self-reference.).
We attribute this to the fact that Phen exhibits two absorption
bands, one of which is at 270 nm and overlaps with the
absorption band of tfb- (Figure 8c).
Figure 5 1H NMR spectra of Eu(tfb)3(BPhen)2 (5) and Ln(tfb)3(Phen)2 in
DMSO-d6: Ln = (1) Er, (2) Nd, (3) Yb, (4) Eu (
represents the signal of
♥
the tfb proton), as well as of Phen in DMSO and BPhen in CD3CN
Composition of the Eu(tfb)3(BPhen)2 powder was also
determined using 1H NMR spectroscopy and supported the 2:3
neutral-to-anionic ligand ratio (Figure 5, S8).
So, the high acidity of H(tfb) results in low complexation ability
of its anion and leads to the introduction of two ancillary
ligands into the composition of MLC, which is possible due to
anionic ligands adopting monodentate coordination.
Photophysical properties
Due to gadolinium fluorobenzoate in powder state exhibiting
both fluorescence and phosphorescence at room temperature
(Figure. S9), ligand triplet state energy was estimated from the
luminescence spectra of Gd(tfb)3(H2O)2 in propanol-1 at 77K
and room temperature (Figure 6a).
Table 1. The complex Tb(tfb)3(H2O)2 exhibited intense ionic
luminescence, but the PLQY of 13% was too low: probably a
result of low energy transfer efficiency. NIR emitting complex
Ln(tfb)3(H2O)x (Ln = Nd, Er, Yb) did not exhibit luminescence at
all due to the low efficiency of energy transfer from the ligand
to the metal ion and quenching by coordinating water
Spectra of lutetium complexes were used to help in assigning
fluorescence and phosphorescence (Figure S10). So, from the

Figure 8 a) Luminescence and b) excitation spectra of powders of
molecules. At the same time, their MLCs with Phen exhibited
pure ionic luminescence (Figure 7).
ACCEPTED MANUSCRIPT
Eu(tfb)3(H2O)2, Eu(tfb)3(Phen)2 and Eu(tfb)3(BPhen)2. c) Absorption spectra
of Na(tfb), Phen and BPhen
Ln(tfb)3(Phen)2
Ln(tfb)3(H2O)2
1,0
0,8
0,6
0,4
0,2
0,0
When neutral ligand (Phen, BPhen) is included in the complex
composition, the PLQY values of resulting MLCs Eu(tfb)3(Phen)2
and Eu(tfb)3(BPhen)2 increase to 45% ( In the excitation
spectrum of Eu(tfb)3(BPhen)2 the band at 350 nm (through-
BPhen) is significantly more intense than the band at 270 nm
(through-tfb-), which is associated with a significantly greater
absorption of BPhen compared to the tfb- (Figure 8c) and with
Ln=Tb
Ln=Eu
Ln=Yb
Ln=Nd
Ln=Er
400
500
600
700
800
1000
1200
1400
1600
λ,nm
Figure 7 Luminescence spectra of powders of Ln(tfb)3(H2O)2 (Ln = Tb, efficient BPhen→Eu3+energy transfer. In the case of
Eu) and Ln(tfb)3(Phen)2 (Ln = Yb, Nd, Er)
Eu(tfb)3(Phen)2, the intensity of bands at 330 nm and 270 nm
is comparable despite the essentially the same sensitization
Due to their potential use in OLEDs, particular attention was
paid to europium complexes. The PLQY of Eu(tfb)3(H2O)2
turned out to be only 6%, due to the low efficiency of energy
transfer from the ligand to the metal ion (ηsens = 20%, In the
excitation spectrum of Eu(tfb)3(BPhen)2 the band at 350 nm
(through-BPhen) is significantly more intense than the band at
270 nm (through-tfb-), which is associated with a significantly
greater absorption of BPhen compared to the tfb- (Figure 8c)
and with efficient BPhen→Eu3+energy transfer. In the case of
Eu(tfb)3(Phen)2, the intensity of bands at 330 nm and 270 nm
is comparable despite the essentially the same sensitization
efficiency by Phen and BPhen (Error! Not a valid bookmark
self-reference.). We attribute this to the fact that Phen
exhibits two absorption bands, one of which is at 270 nm and
overlaps with the absorption band of tfb- (Figure 8c).
efficiency by Phen and BPhen (Error! Not a valid bookmark
self-reference.). We attribute this to the fact that Phen
exhibits two absorption bands, one of which is at 270 nm and
overlaps with the absorption band of tfb- (Figure 8c).
Table 1). The change of the shape of the luminescence spectra
(Figure 8a) shows the alteration of the coordination
environment of europium ion. The introduction of a neutral
ligand also results in the appearance of an additional excitation
band (Figure 8b). In the excitation spectrum of
Eu(tfb)3(BPhen)2 the band at 350 nm (through-BPhen) is
significantly more intense than the band at 270 nm (through-
tfb-), which is associated with a significantly greater absorption
of BPhen compared to the tfb- (Figure 8c) and with efficient
BPhen→Eu3+energy transfer. In the case of Eu(tfb)3(Phen)2, the
intensity of bands at 330 nm and 270 nm is comparable
despite the essentially the same sensitization efficiency by
Phen and BPhen (Error! Not a valid bookmark self-reference.).
We attribute this to the fact that Phen exhibits two absorption
bands, one of which is at 270 nm and overlaps with the
absorption band of tfb- (Figure 8c).
Table 1). This value was obtained from the calculated value of
internal quantum yield (QLnLn) by the common approach18: ηsens
= QLLn/QLnLn. The calculated value of QLnLn = τobs/τrad = 30%, was
compared with the experimentally measured value of
quantum yield when excited through the absorption band of
the europium 7F0→5L6 on λex = 395 nm. It was possible because
it does not overlap with the ligand excitation band (Figure.
S11). The experimentally determined value of the internal
quantum yield coincides with the theoretically calculated
value.
Table 1 Luminescent characteristics of several complexes
PLQY
QL
QLn
,
ηsens
%
,
Ln
τobs
, ms
≡
τrad, ms clcd/
Ln
(±0.05)
(±5%)
6
exp
Eu(tfb)3(H2O)2
Eu(tfb)3(Phen)2
Eu(tfb)3(BPhen)2
0.63
1.00
1.61
2.03
1.80
2.51
30/30
55/-
19
80
70
6
5
7
45
D → F
0
2
5
45
65/-
Eu(tfb)3(H2O)2
Eu(tfb)3(Phen)2
Eu(tfb)3(Bphen)2
5
7
4
D → F
0
4
3
OLED fabrication
2
Both Q = Phen and BPhen were selected as host materials for
OLED fabrication based on Eu(tfb)3:host composite layers.
Prior to OLED fabrication, thin films of Eu(tfb)3:nQ, where n
varied from 0 to 20, were deposited to estimate their
luminescent properties and the Q→ Eu(tfb)3 energy transfer
efficiency. Thin films were deposited from solutions of
corresponding composition in acetone:methanol mixture.
Luminescence studies of these solutions revealed the absence
of ligand Q luminescence, which indicated a high efficiency of
Q→Eu3+ energy transfer even at high Eu(tfb)3:nQ ratio. In
addition, the shape of the luminescence spectra of these
solutions does not depend on the Eu(tfb)3:Q ratio (n), which is
shown in Figure 9 on example of Eu(tfb)3:2BPhen and
Eu(tfb)3:10BPhen witnessing the constant europium
5
7
D → F
0
1
1
0
500
550
600
650
700
750
λ, nm
a)
20
15
10
5
25000
BPhen
Eu(tfb)3(Bphen)2
20000
15000
10000
5000
0
Eu(tfb)3(Phen)2
Eu(tfb)3(H2O)2
Phen
-
tfb
0
250
300
350
400
250
300
350
λ,nm
400
λ, nm
b)
c)

coordination environment, which coincides with the
ACCEPTED MANUSCRIPT
coordination environment of europium MLC (n=2).
2,0
1,5
Eu(tfb)3:10Bphen
1,0
0,5
Eu(tfb)3:2Bphen
0,0
a)
b)
200
300
400
500
600
700
800
Figure 11 AFM data of Eu(tfb)3:10BPhen films: a) on glass substrate
and b) above the heterostructure ITO/PEDOT:PSS/PVK
λ, nm
Figure 9 Emission spectra of Eu(tfb)3:nBPhen (n = 2, 10) solutions
For the OLED fabrication, thin films of pure Eu(tfb)3, as well as
Eu(tfb)3:10Phen and Eu(tfb)3:10BPhen were used. Initially, two
The excitation spectra of the thin films, containing Phen, were
dominated by the intense through-Phen excitation band,
witnessing efficient energy transfer (Figure 10a). The emission
spectra of films Eu(tfb)3:nBPhen contained only the bands of
Eu3+ ionic luminescence (Figure 10b), meaning that complete
energy transfer is observed even from the BPhen molecules
distant from the Eu(tfb)3. The mechanism of such an efficient
energy transfer is probably due to the BPhen→…→
devices
Table
A
and
B were fabricated (
2
) to compare electroluminescent (EL) properties of
Eu(tfb)3 in the presence and in the absence of Phen host. As
Phen possesses very high LUMO level21, TAZ was selected as an
electron-transport layer (ETL), as its LUMO energy of 2.8 eV
was the highest among those of accessible materials. As the
hole-transporting layer (HTL) the commonly used PVK was
selected, as its HOMO energy value was suitable to facilitate
hole transport from PEDOT:PSS hole-injecting layer (HIL) to
emission layer (EML) (Figure 12).
BPhen→Eu(tfb
)
3
cascade of energy transfer processes19.
Moreover, the change of intensity ratio of europium bands
should be taken into account as it is known to be related to the
change of europium coordination environment20. The band
intensity ratio is significantly different in the thin film of
Eu(tfb)3 and the thin film of Eu(tfb)3(BPhen)2, and no further
changes occur as the ratio of BPhen increases (Figure 10b), and
spectra of films Eu(tfb)3:nBPhen coincided with each other and
with spectra of Eu(tfb)3:nBPhen in solution (Figure 9). It proves
that the thin films, containing more than two BPhen molecules
per one Eu3+ ion, consist of the BPhen matrix, doped with
Eu(tfb)3(BPhen)2 MLC, and in solution undissociated
Eu(tfb)3(BPhen)2 is present.
Table 2 OLED heterostructures and switch-on voltages (Von)
Von,
Device
Heterostructure
V
A
B
C
D
ITO/PEDOT:PSS/PVK/Eu(tfb)3/TAZ/Al
n/a
7
ITO/PEDOT:PSS/PVK/Eu(tfb)3:10Phen/TAZ/Al
ITO/PEDOT:PSS/PVK/Eu(tfb)3:10BPhen/BPhen/Al
ITO/PEDOT:PSS/PVK/Phen/TAZ/Al
3.5
6
Since Phen is not a common OLED material, and as it possesses
an extremely high LUMO level, a model device was
preliminary fabricated to ensure that the proposed
heterostructure can work electrically. Device had the same
heterostructure as device , but with only Phen thin film
D
5
7
→ F
2
D
0
5
7
5
7
D
→ F
1,0
0,5
0,0
0
4
D
→
F
1:20
0
1
Eu(tfb)3:2Phen
1:10
1:5
D
B
1:2
1:1
1:0
instead of EML. The obtained device exhibited relatively high
current, and its switch-on voltage was 6.5V, corresponding to
the HOMO-LUMO gap of Phen.
500
550
600
650
700
750
300
400
λ, nm
500
λ, nm
a)
b)
Figure 10 a) Excitation spectrum of Eu(tfb)3:2Phen thin film and b)
emission spectra of Eu(tfb)3:nBPhen thin films, n = 0…20
The spectrum of Eu(tfb)3:BPhen is a superposition of the
spectra of homo-ligand complex and MLC Eu(tfb)3(BPhen)2,
which is another prove of pure 1:2 attachment of the neutral
ligand (Figure 10b).
Prior to OLED fabrication, film quality was analyzed. Thin films
were deposited on a glass substrate and on heterostructure
ITO/PEDOT:PSS/PVK which was further used in OLEDs (Table
2). According to the AFM data (Figure 11), the thickness of the
film of Eu(tfb)3:10Bphen on the glass substrate was 35 nm, and
its mean-square roughness is Sa = 5.1 nm, which is quite a high
value for this thickness. However, Sa value of the film
deposited on the heterostructure decreases down to 0.9 nm,
indicating its high enough quality for OLED fabrication.
a)
Figure 12 OLED heterostructures: a) Phen host and b) BPhen host
b)
Device
A based on pure Eu(tfb)3(H2O)2 emissive layer operated
at extremely high voltage, as we expected due to low charge
carrier mobility of Eu(tfb)3(H2O)2. It performed badly, and no
EL spectrum could be detected until its breakdown at 28V. At
the same time, Eu(tfb)3:10Phen as EML resulted in the
appearance of the europium ionic luminescence bands (Figure
13a). Such a difference in the performance of two devices,

based on the materials with similar PLQY (Eu(tfb) (H O) and
ACC3EPTED MANUSCRIPT
2
2
Experimental Section
Eu(tfb)3(Phen)2), witnesses the efficiency of the proposed
approach. So, the exciton is indeed generation on the Phen
molecule, as well as excitation transfer Phen*→Eu in the OLED
operation regime. At the same time, the presence of intense
TAZ luminescence6 witnesses more efficient exciton
generation of TAZ rather than on Phen molecules. It resulted
from the too high gap between LUMO levels of TAZ and Phen
and the coincidence of their HOMO levels, making Phen act
rather like an electron blocking layer. The high switch-on
Materials and methods
All solvents and chemicals were purchased from commercial
sources.
1H and 19F NMR spectra were recorded at 25 °C using Agilent
400 MR spectrometer with the operating frequency of 400.130
MHz, 100.613 MHz and 376.498 MHz, respectively. Chemical
shifts are reported in ppm relative to Me4Si (1H) and CFCl3(19F).
Elemental analysis was performed on the microanalytical
device Heraeus Vario Elementar. Thermal analyses were
carried out on a thermoanalyzer STA 409 PC Luxx (NETZSCH,
Germany) in the temperature range of 20–1000 ºC in O2 and
Ar atmosphere, heating rate 10 º/min.
voltage of the device
LUMO level of Phen.
B (Figure 13b) also resulted from too high
Substituting of Phen with BPhen resulted in the obtaining of
the device , where vapor-deposited BPhen layer acted as ETL
C
Single crystal X-ray diffraction study were carried out using a
Bruker APEX CCD diffractometer (ω-scans) at 120K, using Mo-
Kα radiation (λ = 0.71073 Å). All structures were solved by
direct methods and refined by full matrix least-squares on F2 22
with anisotropic thermal parameters for all non-hydrogen
atoms. All hydrogen atoms were placed in calculated positions
and refined using a riding model. The general crystallographic
data and refinement parameters were summarized in Table S1.
CCDC numbers 1518604-1518605 contain all crystallographic
information and can be obtained free of charge via
www.ccdc.cam.ac.uk.
instead of TAZ. It was due to LUMO level of BPhen being much
lower, as well as very efficient electron transfer was expected
between ETL and EML due to essentially the same
composition. Remarkably decreased HOMO-LUMO gap
resulted in the decrease of the device switch-on voltage
(Figure 13b), while the shift of the recombination zone in the
EML leading to the bright and pure europium luminescence
(Figure 13a).
90
60
30
0
12000
9000
6000
3000
0
X-ray powder diffraction (XRD) measurements were performed
C
B
on a Rigaku D/MAX 2500 diffractometer in the 2θ range 5-80
with Cu Kα radiation (λ=1,54046 Å).
°
D
C
B
0
3
6
9
12 15
400 500 600 700 800 900
Absorption spectra were recorded in the region 250–800 nm
with a Perkin–Elmer Lambda 650 spectrometer. Emission and
λ, nm
U, V
a)
b)
Figure 13 a) EL spectra (12V) and b) I-V curves of devices B, C. Inset:
photo of device C (14V)
excitation spectra were measured with
spectrofluorometer over excitation with
a
Fluorolog
3
a
xenon lamp.
Luminescence lifetime measurements were recorded and
detected on the same system. Lifetimes are averages of at
least three independent measurements. All luminescence
decays proved to be perfect single-exponential functions.
Photoluminescence quantum yields were determined with the
FLS980 Fluorescence Spectrometer from Edinburgh
Instruments equipped with 450 W Xenon lamp. Both the
excitation and emission 300 nm focal length monochromators
were in Czerny Turner configuration. Excitation arm was
supplied with holographic grating of 1800 lines/mm, blazed at
250nm. While the emission spectra was supplied with ruled
grating, 1800 lines/mm blazed at 500 nm. The spectral
resolution was 0.1 nm. The R928P side window
photomultiplier tube from Hamamatsu was used as a detector.
OLED manufacturing took place in a clean room class 10000
(Lebedev Physical Institute, Moscow, Russia) in a glovebox
with argon atmosphere. The substrates were cleaned by
ultrasonication in the following media: NaOH aqueous
solution, distilled water, acetone and 2-propanol for 16 min
each.
Thus, both Phen and BPhen act as host materials for europium
tetrafluorobenzoate, increasing europium EL intensity due to
electron transport properties as well direct energy transfer to
europium ion. Despite ten times excess of the host material,
complete energy transfer took place. Despite the fact that
PLQY of MLCs Eu(tfb)3(Phen)2 and Eu(tfb)3(BPhen)2 are the
same, better performance was observed using BPhen, because
of its lower LUMO level, and switch-on voltage was decreased
down to 3.5V, which corresponds to the theoretically possible
minimum value for BPhen (E(LUMO)-E(HOMO)=3.4 eV).
Conclusions
The new approach of the host selection for lanthanide
complexes was proposed, namely the use of electron-transfer
material able to be coordinated by the lanthanide ion. The
efficiency of the proposed approach has been demonstrated
on the example of highly luminescent and soluble europium
tetrafluorobenzoate, which was synthesized for the first time
and thoroughly characterized together with other lanthanide
homo- and mixed ligand complexes with this ligand and Phen
or BPhen. MLCs of NIR emitting lanthanides demonstrated
pure ionic luminescence.
A
40
nm-thick
thin
film
of
poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
hole- injective layer was first deposited. An aqueous solution
of PEDOT:PSS (5 ml) was poured onto the preheated (70 °C)
patterned ITO glass substrate, after which the substrate was
rotated for 60 seconds at 2000 rpm. Finally the deposited film
Use of BPhen as a host material resulted in obtaining of OLED
ITO/PEDOT:PSS/PVK/Eu(tfb)3:10BPhen/BPhen/Al with pure
europium luminescence and switch-on voltage of 3.5V.

was annealed in air at 80 °C for 60 min. As h
A
ole
C
-tr
C
ansp
E
PVK solution was spin-coated from toluene (c = 5 g/L).
P
o
T
rt
E
la
D
yerMAstir
N
re
U
d
S
at
C
sl
R
igh
I
t
P
ly
T
elevated temperature and then evaporated
to the half volume, the obtained precipitate was filtered off
The emission layer (Eu(tfb)3:10Phen or Eu(tfb)3:10BPhen) was and dried on air
spin-coated from solution (c = 5 g/L) on the heterostucture or Nd(tfb)3(H2O)2: clcd. for NdC21H7F12O8 (M = 760), %: C 33,21, H
on the glass substrate. The electron-transport/hole-blocking 0,93 found, %: C 32,97, H 0,97.
2
layer (TAZ or BPhen) was thermally evaporated (Univex-300, 1H NMR (400.130 MHz, ppm): (in D2O) δ= 7.11 (tt, 3H, JH-H
2
LeybordHeraeus) under a pressure below 10−6 mm Hg. The =10.17 Hz, JH-H =7.63 Hz, aromatic protons); (in CD3OD) δ=
thickness (~30 nm) was controlled by a quartz indicator. The 7.84 (m, 3H, aromatic protons).
contacts were attached to the electrodes, and the device was 19F NMR (376.498 MHz, ppm) δ= -140.56 to –140.68 (m, 6F),
sealed with epoxy resin (Norland Optical Adhesive).
δ= -143.69 (m, 6F) (in CD3OD)
Electroluminescence spectra were measured on a PicoQuant Eu(tfb)3(H2O)2: clcd. for EuC21H7F12O8 (M = 767), %: C 32,88, H
time-correlated single photon counting system used as a 0,92 found, %: C 32,72, H 0,86.
2
conventional spectrofluorimeter. Spectral resolution was 1H NMR (400.130 MHz, ppm) (in D2O) δ= 7.12 (tt, 3H, JH-H
2
=10.17 Hz, JH-H =7.43 Hz, aromatic protons); (in CD3OD) δ=
4 nm.
The surface morphology, roughness and thickness were 6.80 (m, 3H, aromatic protons).
characterized with a NT-MDT INTEGRA Aura AFM (Russia, 19F NMR (376.498 MHz, ppm): (in D2O) δ= -139.19 to –139.31
2005) in semi-contact mode at ambient atmosphere (25 °C). (m, 6F), δ= -144.82 to –144.91 (m, 6F); (in CD3OD) δ= -142.36
The probes used were Micromash NSC15/AlBS cantilevers. The to –142.47 (m, 6F), δ= -145.32 (m, 6F).
observed surface structures were analyzed by Nova 1.0.26 Gd(tfb)3(H2O)2: clcd. for GdC21H7F12O8 (M = 773), %: C 32,65, H
software.
Synthesis of H(tfb)
0,91 found, %: C 32,54, H 0,85.
1H NMR (400.130 MHz, ppm): (in D2O) δ= 7.12 (m, 3H,
aromatic protons); (in CD3OD) δ= 7.48 (m, 3H, aromatic
protons);
Pentafluorobenzoic acid (15 g, 71 mmol) was added to zinc
powder (18.39 g, 283 mmol) in aqueous ammonia (400 ml),
and the mixture was stirred at room temperature for 12 hours.
Unreacted zinc was separated and washed with water. The
aqueous solution was acidified with hydrochloric acid and
19F NMR (376.498 MHz, ppm): (in D2O) δ= -139.28 (m, 6F), δ= -
144.85 (m, 6F); (in CD3OD) δ= -142.66 (m, 12F);
Tb(tfb)3(H2O)2: clcd. for TbC21H7F12O8 (M = 774), %: C 32,58, H
0,91 found, %: C 32,47, H 0,87.
1H NMR (400.130 MHz, ppm): (in D2O) δ= 6.97 (m, 3H,
aromatic protons); (in CD3OD) δ= 13.27 (m, 3H, aromatic
protons).
19F NMR (376.498 MHz, ppm): (in D2O) δ= -139.43 to –139.48
(m, 6F), δ= -145.24 (m, 6F); (in CD3OD) δ= -126.82 (m, 6F), δ= -
136.22 (m, 6F).
extracted with diethyl ether (3⨯40 ml). The combined ether
extract was dried over anhydrous MgSO4. The solvent was
removed by reduce pressure to yield 12.1 g of compound
(yield 89 %).
Er(tfb)3(H2O)2: clcd. for ErC21H7F12O8 (M = 782), %: C 32,23, H
0,90 found, %: C 31,97, H 0,96.
1H NMR (400.130 MHz, ppm): (in D2O) δ= 7.19 (m, 3H,
aromatic protons); (in CD3OD) δ= 6.82 (m, 3H, aromatic
protons).
19F NMR (376.498 MHz, ppm): (in D2O) δ=-139.09 to -139.21
(m, 6F), δ=-144.87 to -144.97 (m, 6F); (in CD3OD) δ= -142.46
(m, 6F), δ= -145.11 (m, 6F).
1H NMR (400.130 MHz, ppm): (in D2O) 7.26 (tt, 1H, 2JH-H =10.03
Hz, 2JH-H =7.58 Hz, aromatic proton); (in CD3OD) 7.56 (tt, 1H, 2JH-
H =10.13 Hz, 2JH-H =7.47 Hz, aromatic proton).
19F NMR (376.498 MHz, ppm): (in D2O) δ= -138.80 to –139.93
(m, 2F), δ= -142.63 to –142.75 (m, 2F); (in CD3OD) δ= -140.43
to –140.56 (m, 2F), δ= -142.98 to –143.11 (m, 2F).
Synthesis of lanthanide complexes
Yb(tfb)3(H2O): clcd. for YbC21H5F12O7 (M = 770), %: C 32,74, H
0,65 found, %: C 32,64, H 0,58.
2
1H NMR (400.130 MHz, ppm): (in D2O) δ= 7.14 (tt, 3H, JH-H
2
=10.17 Hz, JH-H =7.43 Hz, aromatic protons); (in CD3OD) δ=
5.25 (m, 3H, aromatic protons).
19F NMR (376.498 MHz, ppm): (in D2O) δ=-139.13 to -139.25
(m, 6F), δ=-144.96 to -145.07 (m, 6F); (in CD3OD) δ= -143.69
(m, 6F), δ= -149.93 (m, 6F).
Ln(tfb)3(H2O)x (Ln = Nd, Eu, Gd, Tb, Er, Yb, Lu)
Complexes Ln(tfb)3(H2O)x (Ln = Nd, Eu, Gd, Tb, Er, Yb, Lu) were
synthesized by reaction of the 50% excess of wet freshly
prepared Ln(OH)3 (from aqueous ammonia and LnCl3∙6H2O)
and H(tfb) in acetone-methanol mixture (3:1, 20 ml). A
hydroxide excess was filtered off, and the obtained solution
was rotor evaporated (30 min, 60 °C, 10 mmHg). The solid
product was recrystallized from water and dried in air.
Lu(tfb)3(H2O): clcd. for LuC21H5F12O7 (M = 790), %: C 32,63, H
0,65 found, %: C 33,04, H 0,83.
2
1H NMR (400.130 MHz, ppm): (in D2O) δ= 7.12 (tt, 3H, JH-H
2
=10.17 Hz, JH-H =7.63 Hz, aromatic protons); (in CD3OD) δ=
7.39 (m, 3H, aromatic protons).
19F NMR (376.498 MHz, ppm): (in D2O) δ= -139.17 to –139.29
(m, 6F), δ= -144.96 to –145.07 (m, 6F); (in CD3OD) δ= -141.23
to –141.35 (m, 6F), δ= -144.15 (m, 6F).
Mixed ligand complexes were synthesized by reaction between
methanol solutions of Ln(tfb)3(H2O)x and Q = Phen (Ln = Nd,
Eu, Er, Yb) or Q = BPhen (Ln = Eu). The obtained solution was

o-phenanthroline: 1H NMR (400.130 MHz, p
A
pm
C
):
C
(in
E
D
P
M
T
S
E
O-
D
d6)MANUSCRIPT
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1
δ=9.06 (dd, 2H, 1JH-H=4.11 Hz, JH-H=1.76 Hz, aromatic protons),
5
O. V. Kotova, V. V. Utochnikova, S. V. Samoylenkov, A. D.
Rusin, L. S. Lepnev and N. P. Kuzmina, J. Mater. Chem., 2012,
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1
8.44 (dd, 2H, JH-H=8.22 Hz, JH-H=1.96 Hz, aromatic protons),
1
1
1
7.94 (s, 2H, aromatic protons), 7.72 (dd,2H, JH-H=8.22 Hz, JH-
H=4.3 Hz, aromatic protons).
6
A. S. Kalyakina, V. V. Utochnikova, E. Y. Sokolova, A. A.
Vaschenko, L. S. Lepnev, R. Van Deun, A. L. Trigub, Y. V.
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Nd(tfb)3(Phen)2: clcd. for NdC45H19F12N4O6 (M=1084), %: C
49,87, H 1,77, N 5,17 found %: C 49,72, H 1,83, N 4,95.
1H NMR (400.130 MHz, ppm): (in DMSO-d6) δ= 7.65 (m, 10H,
4H aromatic protons, 3NH2), 7.93 (br. s., 4H, aromatic
protons), 8.43 (br. s., 4H, aromatic protons), 9.06 (br. s., 4H,
aromatic protons).
7
8
9
19F NMR (376.498 MHz, ppm): (in DMSO-d6) δ=-139.7 (m, 6F),
δ=-141.19 (m, 6F).
Eu(tfb)3(Phen)2: clcd. for EuC45H19F12N4O6 (M= 1092), %: C
49,51, H 1,75, N 5,13 found, %: C 49,41, H 1,54, N 5,00.
1H NMR (400.130 MHz, ppm): (in DMSO-d6) δ= 7.44 (br. s., 4H,
aromatic protons), 7.69 (br. s., 6H, 3NH2), 7.93 (br. s., 4H,
aromatic protons), 8.51 (br. s., 4H, aromatic protons), 9.11 (br.
s., 4H, aromatic protons).
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19F NMR (376.498 MHz, ppm): (in DMSO-d6) δ=-140.13 (m, 6F),
δ=-143.76 (m, 6F).
Eu(tfb)3(BPhen)2: clcd. for EuC69H35F12N4O6 (М= 1396), %: C
59,37, H 2,53 N 4.01 found, %: C 59,12, H 2,39, N 3,91.
1H NMR (400.130 MHz, ppm): (in DMSO-d6) δ= 7.44(m, 4H), δ=
7.57(m, 12H), δ= 7.71(m, 4H), δ= 7.80-7.90(m, 7H), δ= 9.15(s,
4H).
Er(tfb)3(Phen)2: clcd. for ErC45H19F12N4O6 (М=1107), %: C
48,83, H 1,73, N 5,06 found, %: C 48,92, H 1,61, N 4,92.
1H NMR (400.130 MHz, ppm): (in DMSO-d6) δ= 7.84 (m, 12H,
aromatic protons and 3NH2), 8.48 (br. s., 4H, aromatic
protons), 9.08 (br. s., 4H, aromatic protons).
19F NMR (376.498 MHz, ppm): (in DMSO-d6) δ=-139.82 (m, 6F),
δ=-142.93 (m, 6F).
Yb(tfb)3(Phen)2: clcd. for Yb C45H19F12N4O6 (М=1113), %: C
48,57, H 1,72, N 5,04 found, %: C 48,38, H 1,64, N 5,00.
1H NMR (400.130 MHz, ppm): (in DMSO-d6) δ= 7.23 (br. s., 4H,
aromatic protons), 7.86 (m, 10H, 4H aromatic protons, 3NH2),
8.47 (br. s., 4H, aromatic protons), 9.08 (br. s., 4H, aromatic
protons).
19F NMR (376.498 MHz, ppm): (in DMSO-d6) δ=-140.14 (m, 6F),
δ=-145.34 (m, 6F).
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Acknowledgements
This research has been financed by the joint project EraNet-
Rus (project SOLUM 228) and particular by Russian Foundation
for Basic Research (Grant No. 16-53-76018). AAV thanks
Russian Science Foundation for financial support of OLED
fabrication (Grant No. 15-19-00205).
UV thanks President’s grant МК-2810.2017.3 for support of
studies of photophysics.
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ACCEPTED MANUSCRIPT
* New approach of host selection for OLED emitters based on lanthanide complexes was proposed
* Organic materials that are both ligands for lanthanide ions and sensitizers for their luminescence were
proposed to use as host materials
* Homo- and mixed-ligand lanthanide tetrafluorobenzoates were synthesized and shown to exhibit
quantum yield up to 45%
* The proposed approach for host selection was successfully tested using europium tetrafluorobenzoate
as emitter in OLED and Bphen as host material