Electroluminescent properties of lanthanide pentafluorophenolates

Article abstract of DOI:10.1039/c3tc32054e

Lanthanide pentafluorophenolates Ln(OC₆F₅) ₃(L)_x (Ln = Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb; L = 1,10-phenanthroline, 2,2′-bipyridine; x = 1 and 2) are used as emissive layers in organic light emitting devices (OLEDs). Single-layer ITO/Ln(OC ₆F₅)₃(L)_x/Yb devices reveal no electroluminescence (EL) with the exception of Tb-derivative-based OLEDs. Bilayer ITO/TPD/Ln(OC₆F₅)₃(L)_x/Yb samples exhibit a broad band emission peaked at 580 nm assigned to an electroplex at the TPD/complex interface. Besides, the spectra of the devices based on Pr, Nd, Sm, Eu, Er, Tb and Yb derivatives contain the characteristic narrow bands of f-f transitions. Terbium-based bilayer OLEDs exhibit unusual changes in the EL spectra with increasing the applied voltage. The emission color of the devices tunes from orange towards green. The possible nature of the phenomenon is discussed.

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PAPER
Electroluminescent properties of lanthanide pentafluorophenolates
Anatoly P. Pushkarev,a Vasily A. Ilichev,a Alexander A. Maleev,a Anatoly A. Fagin,a Alexey N. Konev,a
Alexander F. Shestakov,b Roman V. Rumyantzev,a Georgy K. Fukina and Mikhail N. Bochkarev*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Lanthanide pentafluorophenolates Ln(OC6F5)3(L)x (Ln = Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb; L = 1,10ꢀ
phenanthroline, 2,2’ꢀbipyridine; x = 1, 2) are used as emissive layers in organic light emitting devices
(OLEDs). Singleꢀlayer ITO/Ln(OC6F5)3(L)x/Yb devices reveal no electroluminescence (EL) with the
exception of Tbꢀderivativesꢀbased OLEDs. Bilayer ITO/TPD/ Ln(OC6F5)3(L)x/Yb samples exhibit a
10 broad band emission peaked at 580 nm assigned to electroplex at the TPD/complex interface. Besides, the
spectra of the devices based on Pr, Nd, Sm, Eu, Er, Tb and Yb derivatives contain the characteristic
narrow bands of fꢀf transitions. Terbiumꢀbased bilayer OLEDs exhibit unusual changes in the EL spectra
with increasing the applied voltage. The emission color of the devices tunes from orange towards green.
The possible nature of the phenomenon is discussed.
45 ligand and the low LnꢀS or LnꢀSe phonon energies are thought to
minimize competitive vibrational relaxation pathways, and in turn
result in the formation of exceptionally emissive molecules. As
far as we know, the EL properties of the complexes with
exclusively nonꢀchelating anionic ligands have never been
50 studied. To close this gap, we synthesized a series of lanthanide
pentafluorophenolates and tested them as emissive materials in
singleꢀ and bilayer OLED devices.
15 1. Introduction
It is known that the luminescence properties of organic
complexes of nonꢀtransition, dꢀtransition and lanthanide metals in
a great extent depend on the nature of the ligands. Analysis of
literature data reveals that virtually all of metalꢀorganic
20 luminophores contain biꢀ or polydenatate ligands which provide
an extended conjugated chains (necessary for luminescence) and
sufficient stability of the material.1ꢀ5 In the case of
organolanthanide emitters the ligands play a role of sensitizers,
excitation energy from which is transferred to emissive f
25 sublevels of the metal ion increasing their luminescence.6ꢀ8
Fluorescent complexes with monodentate anionic ligands are
rare. The group of luminophores containing one monodentate
ligand is limited by 8ꢀoxiquinolinolateꢀaryloxides of aluminum
(or gallium) of the type Al(q)2(OR),9ꢀ14 and similar N,Nꢀbis(3,5ꢀ
2. Results and discussions
2.1. Synthesis
55 Among the lanthanide complexes with pentafluorophenolate
ligands the promising as emitters in the OLED devices could be
simplest tris(phenolates) Ln(OC6F5)3 because they do not contain
the ancillary groups with luminescence quenching CꢀH, NꢀH or
OꢀH fragments. However, low thermal stability and high
60 hygroscopicity of these compounds21 prevent the preparation of
thin films by vacuum sublimation or spinꢀcoating methods. In the
case of vacuum deposition the complexes explode on the
evaporator, while the films prepared from solutions by spinꢀ
coating are quickly destroyed because of water absorption. Both
65 these drawbacks of the tris(phenolates) are caused by open
coordination sphere of the metal ion (vide infra). To solve the
problem we synthesized the phenolates containing 1,10ꢀ
phenanthroline (Ln(OC6F5)3(phen) – 1(Ln), Tb(OC6F5)3(phen)2 –
2) or 2,2’ꢀbipyridine (Tb(OC6F5)3(bpy) – 3, Tb(OC6F5)3(bpy)2 –
70 4) as neutral ancillary ligands. As expected, the introduction of
additional groups in the coordination sphere of the lanthanides
significantly increased thermal and hygroscopic stability of the
complexes.21
30 diꢀtertꢀbutylꢀsalycilidene)ꢀethylenediamine
derivatives
(tBu)4Al(OC6H4ꢀpꢀR).15 Photoꢀ or electroexcitation of the
complexes results in blue or greenꢀblue emission of moderate
efficiency. The luminescent properties of the complexes with two
monodentate ligands were not studied. A set of metalꢀorganic
35 luminophores with three nonꢀchelating anionic OC6F5 and SC6F5
ligands has been prepared and studied by Brennan, Riman and
coauthors.16ꢀ20 The complexes (DME)2Ln(OC6F5)3 (Ln = Nd, Er,
Tm)20 and (DME)2Ln(SC6F5)3 (Ln = Nd19, Er17, Tm18) showed
significantly greater metalꢀcentered NIR PL efficiency than that
40 of previously reported nonꢀfluorinated compounds. High
quantum yield of metalꢀcentered PL was observed for the clusters
containing
thiophenolate
and
selenophenolate
ligands
(THF)8Nd8O2Se2(SePh)1619, (THF)6Tm4Se9(SC6F5)2, (DME)4(ꢀ2ꢀ
SPh)8Tm4(SPh)418. Both, the absence of CꢀH bonds in the anionic
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3.560 Å) indicate an existence of some intermolecular πꢀπ
interactions.
Our attempts to prepare other phenolates in the crystalline form
suitable for Xꢀray analysis failed. The structures some of the
35 complexes were determined using Quantum chemical modeling.
According to these data the complex 1(Tb) exsists in the form of
dimer with two C6F5O bridges (phen)Tb(OC6F5)2(ꢀꢀ
OC6F5)2Tb(OC6F5)2(phen) (Fig. S1). Energy of this molecule is
19 kcal/mol lower than that of system of two monomers
40 Tb(OC6F5)3(phen) (Fig. S2(a)) and 34 kcal/mol lower than that of
the dimer with bridging phen ligands (Fig. S3). The phenolates
1(Pr, Nd, Sm, Yb) are isostructural to 1(Tb) (Table S1).
The DFT calculations revealed as well that in three coordinated
phenolates Ln(OC6F5)3 there are weak intermolecular FꢀLn
45 interactions in axial directions. These interactions persist in dimer
form with four C6F5O bridges (Fig. S4) which is formed with
energy gain ca. 40 kcal/mol. Presumably, such an interaction
causes low thermal stability of the perfluorinated phenolates free
of ancillary neutral ligands because facilitates the formation of
50 strong LnꢀF bonds. Coordination to the metal center of 1,10ꢀ
phenanthroline or 2,2’ꢀbipyridine molecules in mononuclear
complexes Ln(OC6F5)3 leads to large energy release about 45 and
25 kcal/mol for the first and the second neutral ligand,
respectively, and suppresses the dimerization. Since the
55 coordination sphere of the metal ion turned out filled the
coordination FꢀLn or H2OꢀLn is hindered resulting in enhance
the thermal and hygroscopic stability.
a
2.3. Electroluminescence
The
complexes
Ln(OC6F5)3(phen),
Ln(OC6F5)3(phen)2,
b
60 Ln(OC6F5)3(bpy), Ln(OC6F5)3(bpy)2 can be sublimed in vacuum
or spin casted from solutions to give amorphous stable in air stout
entire films. That feature allowed to use them as emissive layers
in OLEDs and investigate their EL properties. We have found
that the EL properties of all the obtained compounds are similar
65 except the Tb derivatives. In the singleꢀlayer devices the tested
complexes showed no EL and relatively low chargeꢀtransporting
properties judging by currentꢀvoltage curves (Fig. S8). In
contrast, the devices based on Tbꢀcomplexes displayed intense
Fig. 1 Molecular structure of Tb(OC6F5)3(phen)2 (2). Selected distances
[Å] and angles [°]: Tb(1)ꢀO(3) 2.1934(17), Tb(1)ꢀO(1) 2.2248(17), Tb(1)ꢀ
O(2) 2.2354(16), Tb(1)ꢀN(2) 2.5310(19), Tb(1)ꢀN(3) 2.542(2), Tb(1)ꢀ
N(1) 2.564(2), Tb(1)ꢀN(4) 2.571(2), N(2)ꢀTb(1)ꢀN(1) 64.30(6), N(3)ꢀ
Tb(1)ꢀN(4) 64.22(7), C(1)ꢀO(1)ꢀTb(1) 138.63(16), C(7)ꢀO(2)ꢀTb(1)
143.55(14), C(13)ꢀO(3)ꢀTb(1) 143.14(16).
5
2.2. Structure
10 According to Xꢀray analysis the Tb(1) atom in 2 is bounded by
three pentafluorophenolate ligands and two 1,10ꢀphenanthroline
molecules (Fig. 1a). In an asymmetric unit there are also two
solvate molecules of benzene. Atom Tb(1) in 2 has a distorted
pentagonalꢀbipyramidal environment. The TbꢀO(OC6F5) bond
15 lengths range from 2.1934(17) to 2.2354(16) Å in 2. Interesting
to note that the maximal Tb(1)ꢀO(1, 2) distances are observed for
pentafluorophenolate ligands disposed in transꢀposition relative
to each other (2.2248(17), 2.2354(16) Å). The same situation
takes places in related (THF)3Yb(OC6F5)3 and (THF)3Ln(EC6F5)3
20 (Ln=Er, Yb; E=S, Se)20,22 complexes containing THF molecules
as the neutral ligands. The TbꢀN distances are 2.5310(19) ꢀ
2.571(2) Å. The shortest Tb…F contact in the molecule is at
3.312(3) Å which excludes possible dative TbꢀF interaction.
The dihedral angles between pentafluorophenolate and 1,10ꢀ
25 phenanthroline ligands (Fig. 1b) are 20.0º (A and B) and 18.8º (C
and D). At the same time the distances between the centers of
rings A and B, C and D are equal 3.621 and 3.527 Å that satisfy
the conditions of intramolecular πꢀπ interactions.23 Besides,
relative positions of OC6F5 and 1,10ꢀphenanthroline ligands in
30 adjacent molecules in a crystal (centroidꢀcentroid distance is
5
bands of D4ꢀ7Fj (j=6ꢀ3) transitions characteristic of Tb3+ ion
70 (Fig. S9).
It is known that the insertion of the holeꢀtransporting layer (HTL)
between the anode and the emissive layer as well as the holeꢀ
blocking layer (HBL) between the emissive layer and cathode as
a rule significantly increases efficiency of the OLEDs. In the case
75 of the devices based on lanthanide pentafluorophenolates
insertion of HTL also leads to improvement their performance
but to a different extent depending on the metal. In OLEDs based
on 1(Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb) insertion of TPD layer
between ITO and phenolate layer resulted in appearance of broad
80 band peaked at 580 nm (Table 1, Fig. S10), which can be
assigned to emission of the electroplex formed at TPD/complex
interfaces. This assumption is confirmed by photoluminescece
(PL) spectra of double layer 1(Ln)/TPD and blend samples
1(Ln):TPD containing the broad band of TPD at about 420 nm
85 and in the cases of Sm, Dy and Tb derivatives the narrow bands
of fꢀf transitions characteristic of the respective Ln3+ cation (Fig.
S11). The intensity of the band at 580 nm in the EL spectra varies
slightly and the maxima remains virtually the same on going from
2
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DOI: 10.1039/C3TC32054E
Table 1 Performance characteristics of the devices of ITO/TPD/Lnꢀ
complex/Yb; (L = OC6F5)
1,0
0,5
b)
Complex
V0
Lmax
EQEc)
[%]
c)
λ max
ηcc)
ηp
[V]a)
[cd/m2,
*ꢁW/cm2]
9 (13)
[nm]
[cd/A] [lm/W,
**mW/W]
Pr(L)3(phen)
5
580
602
0.09
0.03
0.09
0.14
0,0
20
Nd(L)3(phen) 7.5 580
5 (20)
0.01
ꢀ
<0.01
18
808
900
18* (20)
0.06**
16
1064
14
12
Sm(L)3(phen)
5
566
600
646
40 (12)
<1 (19)
0.35
0.1
1.1
10
400
500
600
700
800
900
Eu(L)3(phen) 10 580
617
<0.01
<0.01
<0.01
Wavelength (nm)
Dy(L)3(phen)
Ho(L)3(phen)
Er(L)3(phen)
Yb(L)3(phen)
5
7
6
6
580
580
580
580
979
25 (17.5)
25 (18.5)
8 (15)
29 (18.5)
90* (18.5)
0.26
0.3
0.07
0.31
ꢀ
0.07
0.08
0.02
0.08
0.8**
0.21
0.24
0.06
1.2
a
a)Voltage at 0.1 mA/cm2; b)values in parentheses are the voltage at which
the luminance was obtained; c)value at 12 V.
5
one lanthanide to another. In the EL spectrum of device
ITO/TPD/1(Sm)/Yb the band of the electroplex at 580 nm is
overlopped with the band of metalꢀcentered emission at 600 nm
(4G5/2ꢀ6H5/2 transition) and manifests as a shoulder. Unlike the
enhanced
PL
intensity of the
similar
lanthanide
10 pentafluorophenolates reported by Brennan and coworkers20, the
EL of the bilayer OLEDs based on the obtained compounds
turned out comparable or even lower than those of devices with
b
most
conventional
nonꢀfluorinated
organoꢀlanthanide
luminophores.6ꢀ8 Extremally poor performance characteristics
15 were found for the diode based on 1(Eu) which exhibited very
weak bands at 580 nm and 615 nm (5D0ꢀ7F2 transition). In
contrast, the device with 1(Yb) showed high luminescence
efficiency. Weak EL activity of Eu complexes and enhanced
characteristics of Yb analogs we observed earlier with substituted
20 phenolates24 and naphtholates25 of these metals. It was assumed
that great difference in the EL properties of Eu and Yb
derivatives is caused by the difference stability of Ln2+ ions and
energy levels of excited [Ln3+]* ions.25 It is possible that the same
reasons are responsible for the difference in EL efficiency of Eu
25 and Yb pentafluorophenolates. Insertion of HBL (Bath, 4,7ꢀ
biphenylꢀ1,10ꢀphenanthroline, 20 nm) in the ITO/TPD/1(Tb)/Yb
device between 1(Tb) and Ybꢀcathode did not enhance the
brightness but a little increased the operating voltage due to the
increased resistance of the sample.
Fig. 2 EL spectra (a) and CIE coordinates (b) of ITO/TPD (20 nm)/1(Tb)
45
(40 nm)/Yb device recorded at bias voltage from 10 to 20 V.
level of Tb3+ ion (20457 cmꢀ1) providing complete excitation
energy transfer from the ligands to the metal. The second, the EL
spectra of bilayer devices based on Tbꢀcomplexes with TPD or
NPD as a HTL vary greatly with voltage increasing. The
50 luminance appeared at 4V as a single broad band centered at 580
nm. Its intensity increased with increasing voltage up to 16 V but
then gradually reduced and virtually vanished at 20 V.
Simultaneously, at 14 V four characteristic narrow bands arising
from Tb3+ ion at 491, 546, 586 and 622 nm (5D4ꢀ7Fj, j=6ꢀ3)
55 appeared on a background of the first band. The intensity of these
bands run up with voltage increasing and reached maximum at 20
V as it is shown in Fig. 2a for device ITO/TPD/1(Tb)/Yb. Change
of EL spectra is accompanied by change of the luminance color
from orange (CIE, x 0.53; y 0.43) at 10 V to green (CIE, x 0.34; y
60 0.46) at 20 V (Fig. 2b). The same dependence was found when
HTL was N,N’ꢀbiphenylꢀN,N’ꢀbis(1ꢀnaphthyl)ꢀ1,1’ꢀbiphenylꢀ
4,4’ꢀdiamine (NPD) (Table 2). However, the phenomenon was
not observed when 4,4’ꢀdicarbazolylꢀ1,1’ꢀbiphenyl (CBP) was
used as HTL.
65 The bilayer OLEDs with phenolates of other lanthanides also
revealed dependence of EL spectra on applied voltage but to a
much lesser degree. No strong enhancement of fꢀf emission at
high voltages was observed but electroplex lumination still tends
to decay. Since the ligand composition, structure and energy
70 levels of frontier orbitals, determined from the data of cyclic
voltamperometry and DFT calculations (Table S2), are the same
or very similar the only explanation for the phenomenon
30 2.4. Electroluminescence of devices based on Tb-complexes
Dramatic difference in EL behavior was found for Tb derivatives.
First, in contrast to the phenolates of other lanthanides, the
compounds 1(Tb), 2, 3 and 4 exhibit the intense metalꢀcentered
EL not only in the devices containing holeꢀtransporting layer but
35 also in the singleꢀlayer OLEDs. Shape of EL spectra of singleꢀ
layer devices was virtually the same as that of bilayer samples at
high voltage while the luminance intensity (50 cd/m2) and other
characteristics (current efficiency 1.12 cd/A, power efficiency 0.3
lm/W, EQE 1.8%) were even higher (Fig. S2). Note, that triplet
40 level of the ligand (20408 cmꢀ1, determined from low temperature
PL spectra of 1(Gd)) matches well with the hypersensitive 5D4
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DOI: 10.1039/C3TC32054E
Table 2 Performance characteristics of the devices of ITO/HTL/Tbꢀ
complex/Yb; (L = OC6F5)
b)
EQEc)
[%]
c)
ηcc)
ηp
Complex
HTL Luminance Lmax
[cd/m2]
[cd/A] [lm/W]
Tb(L)3(phen) TPD electroplex 6 (14)
0.31
0.34
0.07
0.06
0.8
0.59
fꢀf
28.5
(18.5)
NPD electroplex 2 (19)
0.05 <0.01
0.13
0.25
ꢀ
0.5
0.9
0.45
0.15
0.6
1.0
0.6
fꢀf
CBP electroplex
fꢀf
30 (27) 0.13 0.02
ꢀ
ꢀ
ꢀ
31 (23) 0.33
0.35
45 (19) 0.26
0.05
0.09
0.04
0.02
0.05
0.1
Tb(L)3(phen)2 TPD electroplex 4 (13)
fꢀf
TPD electroplex 1 (13)
fꢀf 40(19)
Tb(L)3(bpy)2 TPD electroplex 5 (12.5)
fꢀf
Tb(L)3(bpy)
0.09
0.34
0.4
40 (20) 0.35
0.05
a)Voltage at 0.1 mA/cm2; b)values in parentheses are the voltage at which
the luminance was obtained; c)value at maximum luminance.
Chart 1 Scheme of electroplex formation in the TPD/complex interface.
transporting properties of the complexes (Chart 1). At low
voltage the holes are not injected into Tbꢀcomplex layer but the
50 electrons easy pass through it reaching the organic interface and
providing the electronꢀhole recombination at the interfacial area.
As indicated above, the emission from the electroplex is observed
as a broad band at 580 nm. As the bias increases, the holes begin
to inject into the Tbꢀcomplex layer and as a result the
55 recombination zone shifts to this layer leading to decrease of the
electroplex emission and the appearance of emission of Tb3+
cations. In favor of such assumption says absence of a band at
580 nm in the spectra of singleꢀlayer devices. Substitution of
TPD in the device ITO/TPD/1Tb/Yb with NPD resulted in a
60 decrease of dependence of EL spectrum on voltage. When CBP
was used as HTL the phenomenon was not observed at all
although intensity of the metalꢀcentered emission remained high
in both cases (Table 1). The reason of the difference in
luminescence behavior of the dvices with TPD (NPD) and CBP
65 layiers probably is due to difference in the energy levels of
frontier orbitals of these materials (TPD (NPD): HOMO 5.5 eV,
LUMO 2.4 eV; CBP: 6.1 and 2.7 eV, respectively36). Change of
the number of neutral ligands (compounds 1(Tb)/2; 3/4) and the
type of these ligands (complexes 1(Tb)/3; 2/4) does not affect
70 essentially on the EL properties of the OLEDs. These data
suggest that the formation of electroplex is the result of a donorꢀ
acceptor interaction of the HTL molecules and OC6F5 groups but
not the phen or bpy ligands in the lanthanide complexes.
5
observed in the diodes with terbium complexes is a felicitous
correlation energies of the triplet and the resonance levels of Tb3+
ion.
The change of EL spectra with increasing of operating voltage is
typical for many OLEDs of various compositions.26 However,
10 such dramatic alterations as in the devices with Tb
pentafluorophenolates to the best of our knowledge were
observed for the first time. In most cases with bilayer and
multilayer nonꢀdoped devices change of EL spectra upon
increasing of applied voltage is explained by the offset of the
15 electronꢀhole recombination zone from one emissive layers to
another due to differences of the energy barriers for the injection
of electrons and holes into these layers.27ꢀ29 In the devices with
the doped emissive layers or with blend luminophores the change
of voltageꢀcontrolled luminance color is caused by shift of the
20 emission from matrix to dopant or from one component of a
blend to another.30 The shift of the luminance color from greenꢀ
yellow to almost white upon increasing the bias voltage in the
device based on Pt(acac)(ppyꢀR) (acacꢀacetylacetone, ppyꢀR –
substituted phenylpyridyl) complex was explained by the change
25 the relative tripletꢀsinglet intensity ratio.31,32 The dependence of
EL spectra on bias voltage for the device with Sm(TTA)3(TPPO)2
emissive layer (TTA
ꢀ thenoyltrifluoroacetonate, TPPO ꢀ
triphenylphosphine oxide) Reyes and coworkers explained by
different intensity of the metalꢀcentered and ligandꢀcentered
30 emission at different voltages.33 The change of visible EL spectra
of
bilayer
device
TPD/Ho(DBM)3(Bath)
(DBM
ꢀ
3. Experimental Section
dibenzoylmethanato, Bath – bathophenanthroline) at various
voltages the authors attributed to luminescence of interfacial
exciplex.34 The emission intensity of the exciplex showed a
35 tendency to saturation at high driving voltage, while the
emissions of the Ho3+ ions kept increasing resulting in change of
the luminance color. Varying of emission colour of bilayer device
75 3.1. General procedure
The syntheses and manipulations of the compounds described
below were realized with exclusion of air and water using
Schlenk techniques. Reagentꢀgrade chemicals were used in all
experiments, and solvents were purified using standard
80 procedures and collected in a reaction vessel by condensation in
vacuum. Pentafluorophenole, 2,2’ꢀbipyridine and 1,10ꢀ
phenanthroline (purchased from Aldrich) were purified and dried
by sublimation under reduced pressure. Silylamides
Ln[N(SiMe3)2]3 were prepared according to the published
85 procedures.37 Content of metal in the products was determined by
complexometric titration. The lanthanide pentafluorophenolates
Ln(OC6F5)3(phen) (1(Pr, Nd, Er)), Tb(OC6F5)3(phen)2 (2),
NPD/Zn(4ꢀTfmBTZ)2 (4ꢀTfmBTZ
ꢀ
2ꢀ(4ꢀtrifluoromethylꢀ2ꢀ
hydroxyphenyl)benzothiazole) the authors explained by
40 electroplex formation in the interface.35 The intensity of the
electroplex emission was found to decrease progressively with
applied voltage.
We suppose that peculiarity of EL of devices based on Tb
pentafluorophenolates is also caused by the formation of
45 electroplex at TPD/Tbꢀcomplex interface and relatively low holeꢀ
4
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DOI: 10.1039/C3TC32054E
Tb(OC6F5)3(bpy)2 (4) were synthesized by the reactions of
amides Ln[N(SiMe3)2]3 with pentafluorophenol in benzene or
toluene in the presence of 1,10ꢀphenanthroline or 2,2’ꢀbipyridine
as described in the paper.21 Preparation and characterisation of
resistive heaters for organic and metal layers.42 The bilayer
devices ITO/HTL(20 nm)/complex(40 nm)/Yb(150 nm) (HTL =
TPD, NPD, CBP) were prepared similarly. A commercial ITO on
a glass substrate with 5 ꢃ/□. was used as the anode material
5
Ln(OC6F5)3(phen) (1(Sm, Eu, Dy, Ho, Tm, Yb)) and 60 (Luminescence Technology Corp.) and commercial Yb, 99.9%
Tb(OC6F5)3(bpy) (3) are described in the Supplymnetary
information.
trace metals basis (SigmaꢀAldrich) as the cathode material. The
deposition rate for the TPD and metalloꢀcomplexes was 1 nm/s.
The active area of the devices was 4×4 mm. The EL spectra in
visible region and currentꢀvoltageꢀluminance characteristics were
65 measured with Ocean Optics USB2000 fluorimeter calibrated
with Ocean Optics LSꢀ1 CAL lamp, the computer controlled GW
Instek PPEꢀ3323 power supply and GW Instek GDMꢀ8246 digital
multimeter. The spectra and power efficiency in the NIR region
was determined by Ocean Optics NIRꢀ512 spectrometer. The
70 device characteristics measurements were carried out under
ambient conditions.
3.2. Photophysical measurements
IR spectra were recorded on a Specord Mꢀ75 spectrometer from
10 4000 to 450 cm–1. The samples were prepared as Nujol mulls and
as films between KBr plates. UVꢀvis spectra were recorded with
a Perkin Elmer Lambda 25 spectrometer in a region from 200 to
1100 nm. The photoluminescence spectra were recorded from
400 to 800 nm on a spectrometer Perkin Elmer Fluorescence LSꢀ
15 55 (spectral resolution 1 nm) with excitation at 270 nm. The
samples were prepared as the films on optical quartz.
4. Conclusions
3.3. Cyclic voltammetry
The EL properties of lanthanide pentafluorophenolates were
studied for the first time. In the singleꢀlayer devices the
75 complexes revealed no EL but insertion of HTL resulted in
appearance of the metalꢀcentered emission in the cases of Pr, Nd,
Sm, Eu and Yb complexes along with the luminescence of
electroplex formed at HTL/complex interface which is observed
also for the OLEDs based on the Ho and Dy complexes.
80 Efficiency of the metalꢀcentered luminescence is comparable with
that of the devices based on conventional nonꢀfluorinated organoꢀ
lanthanide luminophores unlike the enhanced PL of
perfluorinated lanthanide phenolates. Unusually strong change in
the EL spectra and luminance color with increasing the applied
85 voltage was found for the bilayer devices based on Tbꢀ
complexes. The observed phenomenon is explained by shifting of
recombination zone and respectively the emission from the
interfacial HTL/complex electroplex to the layer of Tbꢀcompound
that causes metalꢀcentered luminescence. Due to these features,
90 the systems may be used for the fabrication of voltageꢀcolor
tunable organic LEDs. Besides, the observed tendency of
lanthanide pentafluorophenolates to form electroplex with donor
compounds suggests that these complexes may be suitable
acceptor materials in design of efficient molecular photovoltaic
95 cells. Further investigations in this direction are in progress.
Cyclic voltammetry of the complexes was performed in a typical
threeꢀelectrode cell with a platinum sheet working electrode, a
20 platinum sheet counter electrode (bigger surface), and
a
silver/silver nitrate (Ag/Ag+) pseudoreference electrode.38 All
electrochemical experiments were carried out under an argon
atmosphere at room temperature in an electrolyte solution of 0.2
M tetrabutylammonium tetrafluoroborate in THF at a sweep rate
25 of 100 mV×sꢀ1. Concentration of investigated compounds
was 0.01 M.
3.4. X-ray Crystallography
The Xꢀray data of 2 was collected on an Agilent Xcalibur E
diffractometer (graphiteꢀ monochromator, MoKaꢀradiation, ωꢀ
30 scan technique, λ = 0.71073 Å, T = 100(2) K). ABSPACK
(CrysAlis Pro)39 was used to perform areaꢀdetector scaling and
absorption corrections. Data Collection, Reduction and
Correction Program, CrysAlis Pro – Software Package, Agilent
Technologies (2012)40 were used to integration of experimental
35 data. The structure was solved by direct methods and was refined
on F2 using SHELXTL41 package. All hydrogen atoms were
placed in calculated positions and were refined in the riding
model.
The crystal data for 2 (C54H28F15N4O3Tb): monoclinic crystal
40 system, space group C2/c, unit cell dimensions a = 23.7548(2) Å,
b = 13.8996(1) Å, c = 28.8695(2) Å, β = 100.8412(8)°, V =
9362.06(13) Å3, Z = 8, dcalc.. = 1.738 g/cm3, ꢁ = 1.622 mmꢀ1,
F(000) = 4832, R1 = 0.0309 (I>2σ(I)), wR2 = 0.0636 (all data),
Acknowledgements
This work was partially supported by the Russian Foundation for
Basic Research (Grants No. No. 13ꢀ03ꢀ00097, 13ꢀ03ꢀ00891, 12ꢀ
03ꢀ31273, 12ꢀ03ꢀ31474, 13ꢀ03ꢀ97046, 13ꢀ03ꢀ00891) and the
100 Ministry of Education and Science of the Russian Federation
(agreement 8637). The authors thank I. D. Grishin for
voltammetric measurements.
S(F2) = 1.147, largest diff. peak and hole 2.035 and ꢀ1.502 eꢂ Å ꢀ
3
45
.
CCDC 947374 (2) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html
(or
from
the
Cambridge Crystallographic Data Centre, 12 Union Road,
50 Cambridge CB21EZ, UK; fax: (+44) 1223ꢀ336ꢀ033; or
deposit@ccdc.cam.ac.uk).
Notes and references
a G. A. Razuvaev Institute of Organometallic Chemistry of Russian
105 Academy of Sciences, Tropinina 49, 603950 Nizhny Novgorod, Russian
Federation. Fax: +7 (831) 4627497; Tel: +7 (831) 4627709; Eꢂmail:
mboch@iomc.ras.ru
3.5. Device fabrication and measurements
The simple singleꢀlayer devices ITO/complex(50 nm)/Yb(150
b Institute of Problems of Chemical Physics of Russian Academy of
Sciences, Academician Semenov avenue 1, 142432 Chernogolovka,
nm), containing complexes 1(Ln), 2, 3 and 4 as an emissive layer,
55 were fabricated in a vacuum chamber (10ꢀ6 mbar) with different 110 Russian Federation. Fax: +7(49652) 23507; Tel: +7(49652) 2ꢂ44ꢂ76
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C3TC32054E
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