Novel β-diketonate complexes of Eu3+ bearing pyrazole moiety for bright photo- and electroluminescence

Article abstract of DOI:10.1016/j.dyepig.2018.12.006

Four novel 1,3-diketonate Eu³⁺ complexes bearing pyrazole moieties as main ligands and a 4,7-Diphenyl-1,10-phenanthroline (bathophenanthroline) ancillary ligand were synthesized. The number of fluorine atoms in the main ligand was varied from 3

Full text of DOI:10.1016/j.dyepig.2018.12.006

DyesandPigments163(2019)291–299
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel β-diketonate complexes of Eu3+ bearing pyrazole moiety for bright
photo- and electroluminescence
V.M. Korshunova,c,∗, S.A. Ambrozevicha,b,c, I.V. Taydakova,b,d, A.A. Vashchenkoa,b
,
D.O. Goriachiya, A.S. Selyukova,b, A.O. Dmitrienkod
a P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Leninskiy Prospect 53, 119991, Moscow, Russia
b Moscow Institute of Physics and Technology (State University), Institutskiy Per.9 Institutskiy per., Dolgoprudny, Moscow Region, 141701, Russian Federation
c Bauman Moscow State Technical University, 2-ya Baumanskaya Str. 5/1, 105005, Moscow, Russia
d A. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova St. 28, 119991, GSP-1 V-334, Moscow, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four novel 1,3-diketonate Eu3+ complexes bearing pyrazole moieties as main ligands and a 4,7-Diphenyl-1,10-
phenanthroline (bathophenanthroline) ancillary ligand were synthesized. The number of fluorine atoms in the
main ligand was varied from 3 to 13. From spectroscopic studies of the synthesized complexes we conclude that
increasing the degree of fluorination results in a significant rise in the quantum yield and sensitization efficiency.
With further extension of the fluorinated chain length, saturation of these characteristics was observed. The
maximum obtained values of the quantum yield and sensitization efficiency were 56% and 99%, respectively. In
order to reveal the potential of the complexes for OLED applications, electroluminescent devices based on these
compounds were fabricated and their performance was studied. It was found that the quantum efficiencies of the
OLEDs and the sensitization efficiencies exhibit similar behaviour depending on the degree of fluorination of the
ligands in the complexes.
Eu3+
Coordination compounds
β-diketonates
OLED
Electroluminescence
Fluorination
Sensitization efficiency
1. Introduction
As demonstrated for Eu3+ and Tb3+ ions, it is possible to partially
suppress multiphonon quenching in the ligands by introduction of low-
The search for efficient and stable luminescent materials to be used
in displays and lighting is an extremely important and challenging task.
Lanthanide complexes with organic ligands [1,2] as well as other me-
talorganic compounds [3,4] appear to be promising in this respect. The
enduring interest of the scientific community is drawn by a number of
essential effects (particularly, the “antenna” effect) specific to these
compounds, as well as by their highly efficient luminescence. In these
compounds the sensitization efficiency of the ion emission can be as
high as 100% [5]. Moreover, attractive optical properties of the lan-
thanide complexes such as narrow emission bands and high quantum
yields are combined with relative simplicity of their synthesis.
The physics behind the luminescence of lanthanide complexes re-
sides in the violation of the selection rules for the transitions between 4f
levels of the ion. The luminescence efficiency is governed by the pro-
cesses of energy transfer from the ligand environment to the ion, as well
as by the excitation quenching [6]. The latter process can lead to de-
creased quantum yields due to vibrational relaxation via OH, CH, and
NH oscillations in the complexes [6,7].
frequency oscillators, e.g. by replacing the CH bonds with CF ones [8,9].
It is also important to remove the solvent molecules from the inner co-
ordination sphere of the ion, since they can cause additional quenching.
This is achieved by the employment of ancillary ligands [10].
The development of the synthesis protocols and design procedures
promoting suppression of quenching and enhancement of the energy
transfer from the ligand environment to the ion would facilitate suc-
cessful implementation of the lanthanide complexes with β-diketonate
ligand environment in the OLED technology, providing the devices with
superior operational characteristics [11–14]. For example, in Ref. [15]
we obtained the turn-on voltage as low as 3.5 V for the OLED based on a
Eu3+ complex.
In the present paper we have investigated four Eu3+ complexes with
β-diketonate main ligands and a 4,7-Diphenyl-1,10-phenanthroline
(bathophenanthroline) ancillary ligand. The effect of increasing the
degree of fluorination on photo- and electroluminescent properties of
the synthesized compounds was studied.
∗ Corresponding author. P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Leninskiy Prospect 53, 119991, Moscow, Russia.
E-mail addresses: vladkorshunov@sci.lebedev.ru (V.M. Korshunov), s.ambrozevich@mail.ru (S.A. Ambrozevich), taidakov@mail.ru (I.V. Taydakov),
andrewx@mail.ru (A.A. Vashchenko), gor-dog1993@mail.ru (D.O. Goriachiy), selyukov@sci.lebedev.ru (A.S. Selyukov), dmitrienka@gmail.com (A.O. Dmitrienko).
https://doi.org/10.1016/j.dyepig.2018.12.006
Received 9 October 2018; Received in revised form 8 November 2018; Accepted 5 December 2018
Availableonline10December2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
2. Experimental
Hamamatsu R928 photomultiplier. A diffusing screen was mounted
inside the sphere to avoid direct irradiation of the detector. The setup
provided a means to investigate powder samples emitting within
450–800 nm. The measurements were carried out at ambient tem-
perature. The samples in quartz cells were placed near the centre of the
sphere. An emission-standard 45 W quartz tungsten-halogen lamp
(Oriel) was employed to measure the instrument response function.
2.1. Materials and methods
The ligands were obtained by condensation of the corresponding
pyrazolic ketones and ethyl esters in the presence of NaH as a base,
according to the previously published procedure [16]. Europium (III)
oxide (99.99%) and all other reagents and solvents were purchased
from Aldrich and used without additional purification.
2.2. Synthesis
Elemental analysis was performed on an Elementar Vario
MicroCube CHNO(S) analyzer. The metal content was determined by
complexometric titration with a Trilon B solution in the presence of
Xylenol Orange as an indicator. Before the analysis, the complexes were
decomposed by heating with concentrated HNO3. Nuclear magnetic
resonance (NMR) spectra were recorded for CDCl3 solutions of the
complexes at 298 K on a Bruker AC-300 instrument operated at 300.13
and 282.40 MHz for 1H and 19F nuclei, respectively. TMS ( = 0.00
ppm) was used as a standard for 1H measurements and CFCl3 ( = 0.00
ppm) was taken as an external standard for 19F measurements. Infrared
spectra were measured in KBr pellets on a Bruker Tensor 27 FTIR
Stock 0.5 M solution of EuCl3 was prepared by the treatment of Eu2O3
(4.400 g, 12.50 mmol) by minimum amount of concentrated HCl in a
quartz crucible. The resulting solution was evaporated to dryness at 90 °C
and the residue was dissolved in a minimum amount of distilled water.
After that, the solution was transferred quantitatively to a volumetric flask
and the volume was adjusted to 50 mL. This solution was then kept in a
polypropylene flask. To a stirred warm (40 °C) solution of the ligand
(3 mmol) and (1 mmol) bathophenanthroline (0.33 g) in 30 mL of ethanol,
2 mL of an 0.5 M aqueous solution of EuCl3 (1 mmol) were added drop-
wise, followed by careful addition of 3 mL (3 mmol) of an 1.0 M NaOH
solution in water until the pH of the mixture reached 6–7. The mixture was
heated at 50 °C during 4 h in a closed flask and cooled. Further operations
depended upon the properties of the reaction products.
(Fourier-transform
infrared
spectroscopy)
instrument.
Thermogravimetric analysis and differential scanning calorimetry
(TGA/DSC) was performed on a TA Instruments SDT Q600 analyzer in
air (100 mL/min).
For X-ray powder diffraction data collection, the samples were
placed between two polyimide films and mounted into a Bruker AXS D8
Advance Vario X-ray powder diffractometer equipped with a primary
2.2.1. Complex A: (1-(1,3-Dimethyl-1H-pyrazol-4-yl)-4,4,4-trifluoro-
butane-1,3-dionato)(4,7-diphenyl-1,10-phenanthroline) europium (III)
The reaction mixture was evaporated to dryness and extracted by
hot 2-propanol (30 mL). The resulting solution was filtered, con-
centrated to a small volume (2 mL), and left for 4 weeks in a closed
container. The crystals were separated, washed with a small amount of
monochromator (Cu
K
1, λ = 1.54056 Å) and a LynxEye PSD. The
data were collected in the transmission mode at ambient temperature
for 2θ = 2–90° with a step size of 0.01°. The diffraction patterns were
indexed using the SVD (singular value decomposition) index algorithm
[17] as implemented in the Bruker TOPAS 5.0 software [18], the space
group was determined using the analysis of systematic absences, and
the cell parameters were refined using the Pawley method.
2
cold MeOH, and dried to constant weight at 10 Torr and 40 °C.
The yield was 0.45 g (38%) of a light yellow powder. Anal. Calcd.
for C51H40EuF9N8O6 (1183.86): C, 51.74; H, 3.41; N, 9.47, Eu, 12.84%.
Found: C, 51.99; H, 3.63; N, 9.81, Eu 12.93%. FTIR (KBr, cm−1): 1615,
1597, 1571 ( C=O); 1331 ( CeN); 1306, 1181 ( CF). 1H NMR
Optical absorption spectra were recorded at ambient temperature
with the use of a Specord M40 spectrophotometer operating within
200–800 nm. The experiments were carried out for the complexes and
corresponding neat ligand bathophenanthroline in the solutions poured
into 1-cm-pathlength quartz optical cells. The samples were dissolved in
acetonitrile (HPLC-grade super gradient, Panreac, Spain) with con-
s
s
s
(300 MHz, CDCl3): δ 9.74 (br. s, 2H), 8.91 (br. s, 4H), 8.57 (m, 4H), 8.26
(m, 5H), 8.03 (br. s, 4H), 6.09 (br. s, 3H, CH=CO), 4.94 (br. s, 9H,
N
CH3), 3.83 (br. s, 9H, CH3). 19F NMR (282.5 MHz, CDCl3): −81.31
(s, 9H, CF3).
6
centrations of about 5 × 10 mol/l.
2.2.2. Complex B: (4,4,5,5-Tetrafluoro-1-(1-methyl-1H-pyrazol-4-yl)pen-
tane-1,3-dionato)(4,7-diphenyl-1,10-phenanthroline) europium (III)
The reaction mixture was evaporated to dryness and extracted by
hot CH2Cl2 (20 mL). The solution was evaporated to dryness and the
residue was dissolved in 5 mL of hot 2-propanol. The solution was kept
at room temperature overnight. Spontaneous crystallization of the
complex occurred upon standing. The precipitate was separated, wa-
shed successively with a small amount of cold 2-propanol and hexane,
Photoluminescence (PL) spectra were obtained at ambient tem-
perature with an Ocean Optics Maya 2000 Pro CCD spectrometer sen-
sitive within 200–1100 nm and
a Perkin-Elmer LS-45 spectro-
fluorimeter equipped with
a Hamamatsu R928 photomultiplier
sensitive within 200–900 nm. In the photoluminescence experiments
involving the CCD spectrometer, a 365 nm CW laser was employed as
the excitation source. To resolve the fine structure of the lanthanide ion
luminescence (if needed), additional PL measurements were carried out
at 77 K. The excitation spectra were obtained using an experimental
setup based on SDL-1 and MDR-23 monochromators installed in the
registration and excitation channels of the system, respectively. A 2 kW
xenon lamp was used for excitation. The detector was a Hamamatsu
H8259-01 photomultiplier. Luminescence decays were measured using
the SDL-1 monochromator and a Lotis TII LS-2134 Nd:YAG pulsed laser
emitting at 355 nm with 5 ns pulse duration, 12 Hz repetition rate, and
40 mJ average pulse energy. PL spectra and PL decays were measured
for polycrystalline samples sandwiched between two quartz slides.
Excitation spectra were obtained for polycrystalline samples and solu-
tions of the complexes in acetonitrile.
2
and finally dried to constant weight at 10 Torr and 40 °C.
The yield was 0.76 g (61%) of a tan powder. Anal. Calcd. for
C51H37EuF12N8O6 (1237.83): C, 49.49; H, 3.01; N, 9.05, Eu 12.2%.
Found: C, 49.67; H, 2.86; N, 10.14, Eu 12.41%. FTIR (KBr, cm−1):
1637, 1561 ( C=O); 1353 ( CeN); 1255, 1159 ( CF). 1H NMR
s
s
s
(300 MHz, CDCl3): δ 9.46 (br. s, 2H), 9.15 (br. s, 2H), 8.77 (m, 4H), 8.19
(br. s, 7H), 7.97 (m, 4H), 7.47 (br. s, 3H), 7.13 (br. s, 3H, CH=CO), 5.09
(t, 1H
,
J = 53.6 Hz, CHF2
)
4.01 (br. s, 9H,
N
CH3). 19F NMR
(282.5 MHz, CDCl3): −132.23 (br. s, 6F, CF2), −140.95 (d, 6F
,
J = 47.3 Hz, CHF2).
2.2.3. Complex C: (4,4,5,5,6,6,6-Heptafluoro-1-(1-methyl-1H-pyrazol-4-
yl)hexane-1,3-dionato)(4,7-diphenyl-1,10-phenanthroline) europium (III)
The reaction mixture was evaporated to dryness and extracted by
hot CH2Cl2 (20 mL). The resulting solution was filtered, concentrated to
To measure the absolute luminescence quantum yield (QY) we used
a Horiba Jobin-Yvon Fluorolog FL3-22 spectrofluorimeter equipped
®
with a G8 Spectralon -covered sphere (GMP SA, Switzerland) and a
292

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
a small volume (3 mL), and hexane was carefully added in small por-
tions with stirring. The precipitated oil solidified upon standing. The
solid complex was separated, washed with hexane, and dried to con-
transferred to an argon-filled glovebox equipped with a Leybold-
Heraeus Univex 300 vacuum deposition chamber. The layers of α-NPD,
TPBi, LiF, and Al were deposited by thermal evaporation in vacuum
2
3
stant weight at 10 Torr and 40 °C.
(residual pressure below 10 Pa). OLED active layers were deposited by
The yield was 0.81 g (56%) of a light yellow powder. Anal. Calcd.
for C54H34EuF21N8O6 (1441.83): C, 44.98; H, 2.38; N, 7.77, Eu 10.54%.
Found: C, 45.11; H, 2.28; N, 7.96, Eu 10.68%. FTIR (KBr, cm−1): 1623,
thermal coevaporation of CBP and the corresponding Eu complexes
(10:1 wt ratio). The deposition thickness was controlled by a Leybold
Inficon IC-6000 deposition controller. The controller was calibrated by
deposition of reference films with the subsequent measurement of their
thicknesses with the use of an NT-MDT Integra atomic force micro-
scope.
1601, 1591 ( C=O); 1339 ( CeN); 1277, 1211 ( CF). 1H NMR
s
(300 MHz, CDCl3): δ 10.79 (br.ss, 2H), 9.46 (br. s, 2Hs), 8.81 (s, 4H),
8.48 (br. s, 2H), 8.22 (br. s, 4H), 7.99 (br. s, 4H), 7.75 (br. s, 4H), 7.28
(br. s, 3H, CH=CO), 4.10 (br. s, 9H,
N
CH3). 19F NMR (282.5 MHz,
The luminance of the OLED samples was measured by a TKA-PKM
luminance meter produced by TKA Scientific Instruments. Current-
voltage characteristics were obtained using an automated setup invol-
ving a Keitley 6485 picoammeter, Agilent 34401A voltmeter, and a
Motech 2019 power supply. OLED optical power was determined using
a Coherent FieldMaxII Laser Power Meter with a calibrated photodiode
head. Electroluminescnece (EL) spectra were obtained with an Ocean
Optics Maya 2000 Pro CCD spectrometer.
CDCl3): −81.21 (br. s, 9F, CF3), −125.80 (br. s, 6F, CF2), −128.41 (br.
s, 6F, CF2).
2.2.4. Complex D: (4,4,5,5,6,6,7,7,8,8,9,9,9-Tridecafluoro-1-(1-methyl-
1H-pyrazol-4-yl)nonane-1,3-dionato)(4,7-diphenyl-1,10-phenanthroline)
europium (III)
The reaction mixture was evaporated to dryness and the crystalline
solid was boiled in heptane (20 mL). Absolute EtOH was added drop-
wise to this suspension until all tarry impurities were dissolved. The
suspension was cooled to room temperature, the precipitate was sepa-
rated and successively washed with hexane, small amount of MeOH and
water on a filter. The solid material was then dried to constant weight
3. Results and discussion
3.1. Thermogravimetric analysis
2
at 10 Torr and 40 °C.
The thermograms of the complexes are similar in shape and show one
main step of weight loss, which is consistent with decomposition and
combustion of the organic ligands (see Fig. 9 in the Supplementary ma-
terials). Thermal stability of the complexes strongly depends upon the li-
gand structures. Complex A starts to decompose in air at temperatures of
about 130 °C, while complexes C and D are rather stable up to 300 °C.
Complex B is more stable than complex A and its degradation starts at
248 °C. The fist step of decomposition of all complexes is associated with
evaporization of the ligands. Small endothermic effects observed for
complexes B, C, and D (at 120, 182, and 162 °C, respectively) correspond
to melting. It is worth mentioning that a small endothermic peak at 125 °C
seen for complex D can be assigned to formation of a mesophase; this
phenomenon is known for the complexes, bearing long perfluorinated
chains [19]. The main decomposition step starts between 460 and 517 °C
depending on the nature of the complex. Decomposition is accomplished
with a strong exothermic effect associated with vigorous oxidation of the
organic matter. The weight of the residue after decomposition of all four
complexes corresponds to formation of EuF3 rather than Eu2O3. The small
exothermic effect observed near 730 °C can be attributed to recrystalliza-
tion of EuF3. Thermal stability of the complexes increases with increasing
the degree of fluorination of the alkyl chain and the complexes with the
substitutients of a moderate length (C, D) are preferable for deposition of
the OLED active layers by thermal evaporation.
The yield was 1.47 g (78%) of a fine white powder. Anal. Calcd. for
C63H34EuF39N8O6 (1891.90): C, 40.00; H, 1.81; N, 5.91, Eu 8.03%.
Found: C, 40.51; H, 1.97; N, 6.24, Eu 8.37%. FTIR (KBr, cm−1): 1621,
1553 ( C=O); 1366 ( CeN); 1209, 1154 ( CF). 1H NMR (300 MHz,
s
s
CDCl3):sδ 9.30 (br. s, 2H), 8.81 (br. s, 2H), 8.48 (s, 4H), 8.22 (br. s, 2H),
8.22 (br. s, 4H), 7.99 (br. s, 4H), 7.71 (br. s, 4H), 6.68 (br. s, 3H
,
CH=CO), 4.01 (br. s, 9H, NCH3). 19F NMR (282.5 MHz, CDCl3):
−81.51 (br. s, 9F, CF3), −121.38 (br. s, 6F, CF2), −122.12 (br. s, 6F
,
CF2), −123.21 (br. s, 6F, CF2), −123.52 (br. s, 6F, CF2), −126.88 (br.
s, 6F, CF2).
Structural formulae of the synthesized complexes are given in Fig. 1.
2.3. Device fabrication and characterization
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic
acid)
(PEDOT:PSS) hole injection material, 2,2′-Dimethyl-N,N′-di-[(1-naph-
thyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) hole transport
material, 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) host layer mate-
rial, 1,3,5-tris[N-(phenyl)benzimidazole]-benzene (TPBi) electron
transport material as well as LiF and Al cathode materials were pur-
chased from Luminescence Technology Corp. and used without further
purification.
ITO-coated glass substrates were washed in an alcohol solution of
KOH followed by the ultrasonic treatment in bidistilled water for
20 min. The subatrates were dried in a flow of nitrogen. After that a 50-
nm-thick PEDOT:PSS layer was deposited by spin-coating with further
annealing in air at 120 °C for 20 min. The substrates were then
3.2. Analysis of IR spectra
IR spectra of the complexes are similar, exhibiting C=O and C=C
1
stretching vibrations at 1659 and 1517 cm , respectively, which are
Fig. 1. Structural formulae of investigated complexes A–D.
293

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Table 1
symmetry. Two weak absorption maxima at 220 and 240 nm are as-
Unit cell parameters for complexes A–D.
sociated with the absorption of the bath ligand.
Photoluminescence spectra of the neat ligands and complexes in
solid phase are shown in Fig. 3. The PL spectrum of the neat bath ligand
(curve 1 in panels A–D) has a complex structure with a maximum at
Crystal System
complex A
Monoclinic
P21c
complex B
Monoclinic
Cc
complex C
Monoclinic
P21c
complex D
Triclinic
410 nm and
a shoulder ranging up to 600 nm. Broad emission
Space group
P
1
(400–700 nm) of the neat ligands A, B, and D has a single maximum
located at 425, 410, and 525 nm, respectively, while the PL of ligand C
exhibits multiple peaks within 470–480 nm (curve 2 in panels A–D). For
all ligands the broad PL peakssss are extended with a shoulder in the
red spectral region. Successive increase in the degree of fluorination in
ligands A–D leads to progressive shift of their emission to the long-
wavelength region. Bonding bath and one of A–D ligands to Eu3+ results
in an ion-centered luminescence of the complexes with complete sup-
pression of the ligand emission (curve 3 in panels A–D). The observed
narrow emission bands with maxima at 583 nm, 588 nm, 615 nm,
a, Å
11.8710 (6)
42.077 (3)
10.4848 (6)
90.
24.835 (7)
20.1756 (18)
11.1576 (17)
90.
14.7222 (9)
39.351 (3)
11.2680 (4)
90.
14.596 (19)
16.51 (2)
21.45 (2)
98.20 (8)
117.32 (8)
99.33 (9)
4392 (11)
b, Å
c, Å
α, °
β, °
89.632 (5)
90.
107.83 (4)
90.
63.922 (3)
90.
γ, °
Volume, Å3
5236 (6)
5322 (2)
5863.3 (6)
typical of lanthanide chelate complexes containing fluorinated β-dike-
655 nm, and 680 nm are assigned to the 5D0
7F0, 5D0
7F1,
tone ligands. The bands for the C=C and C=N ring vibrations appear in
7F2, 5D0
7F3, and 5D0
7F4 transitions, respectively.
1
5D0
the region of 1650–1400 cm . The strong absorption band appearing
1
between 1257 and 1144 cm is assigned to the CF3 stretching mode.
1
The absence of absorption within 3600–3200 cm confirms that the
3.5. Optical excitation and absorption
complexes are free from coordinated or absorbed water molecules. This
statement is supported by the TGA data, since no significant weigh
losses were detected up to 105 °C.
Excitation spectra obtained for acetonitrile solutions of complexes A
and C qualitatively resemble the corresponding optical absorption
spectra (see curves 1 and 2 in panels A and C of Fig. 4). This means that
complexes with spatial group P21 may have similar optical absorption
and excitation spectra. Consequently, the quantum yield of these
complexes does not depend on the excitation wavelength. For com-
plexes B and D the maxima of the excitation spectra are shifted towards
longer wavelengths by 40 nm with respect to the absorption edge.
These maxima coincide with those observed in the excitation spectra for
the solid samples. The differences between the excitation spectra re-
corded for the complexes in solid phase and the corresponding solutions
arise from the concentration effects specific to solid samples.
3.3. Powder X-ray diffraction characterization
The PXRD analysis shows that complexes A, B, C, and D are all pure
and single phase. Refined cell parameters are summarized in Table 1;
experimental, simulated (using the Pawley method), and difference
powder patterns are presented in Fig. 10 in the Supplementary mate-
rials. The diffraction peaks for complex D are broadened and the peaks
with d < 1.75 are unobservable; the microcrystalline structure of com-
plex D is less ordered than that of complexes A, B, and C.
3.4. Absorption and photoluminescence spectra
3.6. Photoluminescence decays
Optical absorption spectra of the neat bath ligand and the complexes
with the bath ancillary ligand and A–D main ligands are presented in
Fig. 2. The absorption spectra of the complexes exhibit two pronounced
bands peaked at 280 and 320 nm that are characteristic of A–D ligands.
It can be seen from the absorption spectra that increasing the degree of
fluorination leads to the growth of the extinction coefficient. However,
for complexes A and C relatively low extinction is observed due to their
Photoluminescence decays obtained for Eu3+ complexes (A–D) in
solid phase are presented in Fig. 5. The measurements were carried out
for the emission band corresponding to the 5D0
7F2 transition
(
615 ± 2 nm).
The experimental decays can be fitted with a single-exponential
model
t/
Ith (t) = Ae
,
(1)
where and Ai are the decay times and amplitudes, respectively. The
i
measured luminescence decay is determined by
Iexp (t) =
Iirf (t )I (t t )dt .
th
(2)
0
Here, the measured instrument response function
I
irf (t) (IRF) ap-
pears as a single exponent with the corresponding decay time of
= 1
irf
µs
:
t/
irf
I
irf (t) = Ae
(3)
In order to eliminate the contribution from the instrument response
function, the Levenberg-Marquardt deconvolution technique was em-
ployed. The resulting decays can also be presented as single-exponential
curves. The corresponding characteristic times are presented in Table 2.
From the obtained results we conclude that increasing the degree of
fluorination in the ligand environment of the complexes does not
greatly affect their PL decay times.
Fig. 2. Optical absorption spectra for investigated complexes and neat bath li-
gand dissolved in acetonitrile. Optical absorption of pure acetonitrile is pre-
sented by dashed curve.
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V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Eu3+
Fig. 3. Photoluminescence spectra of neat A–D ligands (1) and complexes (2) of
ion, bath, and respective A–D ligands for solid samples excited by 365 nm CW
radiation. Fine structures of PL spectra associated with 5D0
complex A.
7F2 transition in Eu3+ are shown in insets. Inset in panel A provides PL spectrum recorded at 77 K for
3.7. Quantum yields and sensitization efficiencies
From the data presented in Table 2 we conclude that increased
degree of fluorination of the ligands in the complexes provides the
growth of the photoluminescence quantum yields as the CF bonds have
The rate of the 5D0
7F1 magnetic dipole transition in Eu3+ corre-
sponding to the PL peak at 590 nm does not depend on the electric field
induced by the ligand environment. Therefore, it is possible to evaluate
the internal quantum yield from the normalized emission spectra by
dividing the integrated intensities of the corresponding spectral bands
by the integrated intensity of the magnetic transition [5,6,20]. The
calculated internal quantum yields for complexes A–D are presented in
Table 2.
lower vibrational energy, thus, reducing the effect of quenching.
Moreover, the extension of the fluorinated chain length results in a
remarkable decrease in the rates of the nonradiative processes (see
Table 2). However, saturation of the quantum yields observed for
complexes C and D indicates that further increase in the degree of
fluorination is unlikely to improve the quantum efficiency. This beha-
viour was recently reported for the complexes based on analogous Eu
(III) 1,3-diketonates [5]. Replacing the ancillary ligand mentioned in
Refs. [5,23] with bath increases the external quantum yield from 10.6%
to 56% for complex C and from 11.4% to 55% for complex C, respec-
tively.
The data on the measured radiative decay rates and external
quantum yields are also provided in Table 2 together with the calcu-
lated internal quantum efficiencies, nonradiative decay rates, and sen-
sitization efficiencies. The radiative decay rates Arad for Eu3+ were
calculated using the following relation [21,22]:
ITOT
IMD
Arad = AMD n3
,
3.8. Electroluminescence
(4)
where AMD = 14.65 s−1 is the 5D0
7F1 magnetic dipole transition rate
In order to investigate the potential of the studied Eu3+ complexes
for OLED applications, we developed and examined ITO/PEDOT:PPSS/
α-NPD/Eu complex:CBP/TPBi/LiF/Al OLED structures for each of the
synthesized compounds. The average thickness of the obtained com-
for Eu3+ ion, ITOT/IMD is the ratio of the total integrated emission to the
integrated emission associated with the magnetic dipole transition. The
refractive index n was taken to be 1.5. The nonradiative decay rate Anrad
was estimated using a simple relationship involving the calculated ra-
posite Eu complex:CBP active layers did not exceed
40
nm, whereas
diative decay rate Arad and the observed decay time
:
the root-mean-square roughness of their surface was of about 8 nm (see
results of atomic force microscopy (AFM) in Fig. 11 of Supplementary
materials).
obs
1
Anrad = Arad
.
(5)
obs
295

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Fig. 4. Absorption spectra for complexes A–D dissolved in acetonitrile (1) and their excitation spectra for corresponding acetonitile solutions (2) and solid samples
(3) with emission recorded at 615 nm.
Table 2
Radiative (Arad) and non-radiative (Anrad) decay rates, observed decay times
(
obs) and their relative errors, internal and external quantum yields, and sen-
sitization efficiencies.
complex A
complex B
complex C
complex D
1
689
803
785
450
716
600
915
367
Arad
Anrad
obs, μs
,
s
,
1
s
670
1
810
1
710
1
780
2
Relative error,%
Internal QY,%
46
27
59
64
39
60
57
56
99
71
55
77
External QY,%
Sensitization efficiency, %
electric field. A broad EL band ranging from 350 to 450 nm results from
the electroluminescence of the CBP transport material (see Fig. 6, curve
4). A subtle peak detected at =∼540 nm probably corresponds to the
5D0
7F1 transition in Eu3+ ion. Notably, increasing the bias leads to
Fig. 5. Luminescence decays measured within 615 ± 2 nm emission band of
investigated complexes under 355 nm pulsed laser excitation. Experiments were
carried out for solid-state samples. Corresponding complexes are denoted with
letters. Inset shows instrument response function (IRF).
the growth of the contribution from the CBP material. This effect most
likely occurs due to the disbalance between the electron and hole
currents, which entails the displacement of the recombination region in
the OLED structure.
The EL spectra of the devices exhibit pronounced peaks corre-
However, for the OLED based on complex A the EL band within
430–550 nm cannot be considered in the framework of the reasoning
given above. We suggest that the regarded EL band is most likely as-
sociated with the appearance of exciplexes in the OLED structure [24].
To clarify the origin of this feature, we fabricated test films by thermal
sponding to the 5D0
7F1 transition in Eu3+ ion (see Fig. 6). The other
transitions present in the photoluminescence spectra are also observed
to a greater or lesser extent. The variation of the amplitudes of the low-
intensity transitions in Eu3+ can be attributed to the influence of the
296

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Fig. 6. Electroluminescence spectra at 12 V (1) and 8 V (2) obtained for ITO/PEDOT:PPS/α-NPD/Eu complex:CBP/TPBi/LiF/Al organic light-emitting diodes; PL
spectra for Eu3+ complexes A–D (3) and CBP transport material (4) measured under 365 nm excitation. In order to illustrate formation of exciplexes for OLED based
on complex A, additional spectrum (5) measured at 10 V bias is provided.
coevaporation of complex A and CBP. In the PL experiments the spec-
luminance-voltage characteristics of the devices (see Fig. 7, panel 2) are
similar except for the device B, which has the largest operating voltage
of 10 V indicating the presence of a large potential barrier in the OLED
structure. The highest obtained luminance was 70 cd/m2 (device C). As
expected, the OLED sample based on complex A reveals the poorest
performance in terms of the current efficiency and external quantum
efficiency due to rather high current densities within the device at low
bias (see Fig. 7, panels 3 and 4). This fact indicates that the potential
barrier in the active layer is small, which, in turn, gives rise to the
charge carrier drift through the active layer, suppressing its electro-
luminescence. The highest achieved external quantum efficiency was
1% (device C), which was slightly decreased at high voltages.
trum of the film exactly matched the superimposed PL spectra of
complex
A and CBP with no emission bands observed within
430–550 nm. Thus, we conclude that the EL band at 430–550 nm can be
attributed to formation of electroplexes [25,26].
The additional EL bands within 350–550 nm lead to a noticeable
variation in the chromaticity of the devices (see Fig. 8). Therefore, it is
potentially possible to fine-tune the device structure to achieve white
emission with appropriate colour characteristics.
3.9. Electrophysical characteristics and photometric performance
By comparing the results given in Tables 2 and 3 we conclude that
the behaviour of the sensitization efficiencies qualitatively resembles
that of the OLED quantum efficiencies depending on the degree of
fluorination in the ligand environment of the complexes.
All devices based on the studied complexes A–D exhibit character-
istic non-linear behaviour of the J-V curves (see Fig. 7, panel 1) gov-
erned by two charge transport regimes (
J
Vn, corresponding values of
parameter n are given in Table 4 of Supplementary materials). In par-
ticular, the space-charge limited regime (n 2) observed for the de-
vices A and C at low applied voltages is followed by the trap-limited
4. Conclusion
regime (n > 2) above
6
V. Alternatively, at low voltages ohmic con-
duction is the most probable charge transport mechanism in the devices
Thus, the influence of the degree of fluorination on the emissive
properties of Eu3+ complexes with different β-diketonate main ligands
and a 4,7-Diphenyl-1,10-phenanthroline (bathophenanthroline) ancil-
lary ligand was investigated. The organic light-emitting diodes based on
the investigated complexes were studied as well. It was demonstrated
that the increase in the degree of fluorination of the main ligand in the
B and D since n < 1, whereas above
8
V it is changed to the trap-
limited regime. Visible electroluminescence appears at 4–6 V. These
values of threshold voltages are comparable to the ones reported for
analogous OLEDs based on metalorganic compounds [27], as well as for
the hybrid OLEDs based on semiconductor nanoparticles [28,29]. The
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V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Fig. 7. Electrophysical and photometric performance for ITO/PEDOT:PPSS/α-NPD/Eu complex:CBP/TPBi/LiF/Al OLED structures based on Eu3+ complexes: 1 –
current density-voltage plot; 2 – luminance versus voltage; 3 – current efficiencies and 4 – external quantum efficiencies as functions of OLED luminance.
Table 3
Characteristics of OLEDs.
OLED A
OLED B
OLED C
OLED D
Turn-on voltage, V
5.5
6
4.5
5
Max. luminance, cd/m2
53 at 17 V
10 at 18 V
72 at 18 V
28 at 14 V
4
2
1
2
Max. quantum efficiency, %
4 × 10
9 × 10
7 × 10
3 × 10
2
2
1
2
Max. power efficiency, %
Operating voltage, V
2 × 10
3 × 10
2 × 10
1 × 10
8
10
8
10
OLEDs and the sensitization efficiencies reveal similar behaviour de-
pending on the degree of fluorination of the ligands in the complexes.
The maximum sensitization efficiency of 99% was achieved for complex
C with the quantum efficiency of the corresponding OLED being 0.7%.
These results can be employed in the design of the lanthanide com-
plexes with highly efficient luminescence.
Acknowledgements
Fig. 8. CIE 1931 xyY color space coordinates evaluated from EL spectra of ITO/
PEDOT:PPSS/α-NPD/Eu complex:CBP/TPBi/LiF/Al OLED structures based on
investigated Eu3+ complexes A–D. (For interpretation of the references to colour
in this figure legend, the reader is referred to the Web version of this article.)
Synthetic and analytical parts of the work were supported by the
Russian Science Foundation under project # 17-72-20088. Spectral
measurements were funded by the Russian Foundation for Basic
Research under projects # 16-02-00594 a, 18-02-00653 a, 16-29-11805
ofi_m. The X-ray diffraction data were obtained using the equipment at
the Centre for molecules composition studies of INEOS RAS. A. A.
Vashchenko, D. O. Goriachiy, and A. S. Selyukov also acknowledge the
financial support from the Russian Foundation for Basic Research under
project # 17-32-80050 mol_ev_a.
Eu3+ complexes leads to the growth of their external quantum yields.
For complexes C and D saturation of quantum yields was observed with
maximum achieved QY values of 56%. Therefore, further fluorination
of the ligands is unlikely to improve the emission efficiency of the
complexes. It was shown that the evaluated quantum efficiencies of the
298

V.M. Korshunov et al.
DyesandPigments163(2019)291–299
Appendix A. Supplementary data
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