Luminescence properties of 2-benzoyl-1,3-indandione based Eu3+ ternary and tetrakis complexes and their polymer films

Article abstract of DOI:10.1016/j.dyepig.2018.07.003

Six new Europium (III) complexes with ternary and tetrakis structures - Eu(BID)₃(PHEN), Eu(MBID)₃(PHEN), [Eu(BID)₄]^?N⁺(Et)₄, [Eu(MBID)₄]^?N⁺(Et)₄,

Full text of DOI:10.1016/j.dyepig.2018.07.003

Accepted Manuscript
3+
Luminescence properties of 2-benzoyl-1,3-indandione based EU ternary and
tetrakis complexes and their polymer films
Ilze Malina, Valdis Kampars, Sergey Belyakov
PII:
S0143-7208(18)31203-8
10.1016/j.dyepig.2018.07.003
DYPI 6857
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 May 2018
Revised Date: 30 June 2018
Accepted Date: 2 July 2018
Please cite this article as: Malina I, Kampars V, Belyakov S, Luminescence properties of 2-benzoyl-1,3-
3+
indandione based EU ternary and tetrakis complexes and their polymer films, Dyes and Pigments
(2018), doi: 10.1016/j.dyepig.2018.07.003.
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LUMINESCENCE PROPERTIES OF 2-
BENZOYL-1,3-INDANDIONE BASED EU3+
TERNARY AND TETRAKIS COMPLEXES
AND THEIR POLYMER FILMS
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Ilze Malinaa,*, Valdis Kamparsa, Sergey Belyakova,b
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Riga Technical University, Institute of Applied Chemistry, Paula Valdena Str. 3, Riga, LV-1048,
Latvia
bLatvian Institute of Organic Synthesis, Aizkraukles Str. 21, Riga, LV-1006, Latvia
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Six new Europium(III) complexes with ternary and tetrakis structures -
Eu(BID)3(PHEN), Eu(MBID)3(PHEN), [Eu(BID)4]-N+(Et)4, [Eu(MBID)4]-N+(Et)4,
[Eu(BID)4]-N+(Bu)4, and [Eu(MBID)4]-N+(Bu)4 (BID – 2-benzoyl-1,3-indandionate,
MBID - 2-(4-methylbenzoyl)-1,3-indandionate and PHEN – 1,10-phenantroline) are
synthesized, characterized, and incorporated into poly-N-vinylcarbazole (PVK) and
poly methyl methacrylate (PMMA) matrices. Complex structure shows significant
effect on thermal properties and emission properties of complexes in solid-state. Used
countercations (N+(Et)4 or N+(Bu)4) greatly affects complex solubility in solvents,
absolute photoluminescence quantum yields and photoluminescence lifetimes in
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solid-state. Complexes exhibit red-light emission attributed to D0→7FJ (J=0-4)
transitions of Eu3+ ion with moderate to high quantum yields (0.06-0.60), bi-
exponential lifetimes and pure red-light CIE chromaticity coordinates (x=0.670;
y=0.330) in solid-state. Incorporation of synthesized complexes in PVK matrices
leads to significant emission intensity and quantum yield decrease.
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*corresponding author
E-mail adress: malina.ilze1@gmail.com or Ilze.Malina_1@rtu.lv (I.Malina)
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However, doped PMMA films with synthesized complexes exhibit moderate PLQY
(0.09-0.14) and longer lifetime values than in solid-state and could show potential
application as polymer optical fibers or in OLED’s and other devices.
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Keywords: Europium; Tetrakis complex; Ternary complex; Luminescent polymer
films.
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1.
INTRODUCTION
From all luminescent organic metal complexes with lanthanide ions (Ln3+)
trivalent Europium Eu3+ compounds stand out due to bright red-light emission with
high quantum yields and long lifetimes. Popularity of Eu3+ complexes is so
remarkable, that they have been investigated in wide range of fields and different type
of materials, including nanoparticles for biomedical applications [1], mesoporous thin
films for UV sensors and bio-sensing [2], as well as up-conversion luminescent thin
films for wavelength-transfer imaging [3] and metal-organic framework microrods for
colored optical waveguides [4] have been designed. Furthermore, due to noteworthy
narrow emission line bandwidth, which defines the high red-light emission color
purity, these compounds have been extensively investigated as emissive layers for
organic/polymer light emitting diodes (OLEDs/PLEDs) [5].
To succeed with development of highly luminescent Eu3+ complexes one must
deliberate the design of attached organic ligands. For effective excited energy transfer
from ligands to Eu3+, compatibility between energy levels of ligands and Eu3+ must be
present in the complex. The main resonance level for Eu3+ is 17 250 cm-1 [6] and in
order for complex to emit light, it is necessary that the triplet energy level of ligand is
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close or even higher than this resonance level. Other just as important requirements
for organic ligands are intense UV/Vis absorption, high chemical and thermal stability
and simple, inexpensive synthesis. Till now the most popular class of organic
compounds for Eu3+ chelating agents is β-diketones, which usually inherent above
mentioned properties. However, structure of complex affects emission spectra profile
and emission efficiency of Eu3+ ion as much as utilized ligands. Different structure of
complexes can be obtained using different ratio of ligands to Eu3+ ion and utilization
of secondary ligands. By using equimolar ratio of ligands to Eu3+ ion 3:1 the
equimolar tris-complexes (Eu(β-diketone)3) can be obtained, but usually, due to fact
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that coordination number of Eu3+ is 8 [7], complexes with general structure of Eu(β-
diketone)3(Solvent)2 containing additional two solvent molecules are obtained instead.
The coordination of these undesirable solvent molecules to Eu3+ ion can divert using
secondary ligands
- N- or O-donor containing compounds, such as 1,10-
phenantroline (PHEN), therefore obtaining ternary complexes with general structure
of Eu(β-diketone)3(PHEN). It is possible to divert solvent molecules using fourth β-
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diketone ligand and obtain anionic tetrakis complexes [Eu(β-diketone)4]-, which can
be stabilized with countercations, such as quaternary ammonium ions and alkali metal
ions.
Till now literature data about Eu3+ organic complexes contains great amount
of investigations about different ligand utilization for this lanthanide ion and their
influence on emission properties. On the other hand, the data about comparison of
different Eu3+ complex structure – tris, ternary and tetrakis effect on complex optical
properties are documented poorly. Likewise, information about comparison of
different countercation utilization in Eu3+ tetrakis complexes and their influence on
the complex properties are limited [8, 9]. Therefore, for the first time comparison of
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ternary and tetrakis complexes bearing the same β-diketone ligands are presented and
full investigation about structure effect on thermal, absorption and emission properties
are reported. Furthermore, two structurally similar counterions - tetraethylammonium
and tetrabutylammonium cation are utilized in tetrakis complexes, therefore impact of
counterion size on complex properties also are discussed. Lightly investigated rigid β-
diketones
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2-benzoyl-1,3-indandione (HBID) and 2-(4-methylbenzoyl)-1,3-
indandione (HMBID) with one carbonyl group in 1,3-indandione cycle are selected as
ligands for Eu3+ complexes. Only few similar rigid binding ligands such as indone
[10, 11] and 1,3-indandione [12, 13] derivatives have been reported so far as excellent
chelating compounds for Eu3+ organic complexes. These type of compounds can
magnify Eu3+ complex luminescence efficiency due to restriction of thermal vibration
by rigid ligand, which leads to reduction of non-radiative deactivation of whole
complex. Selected ligands exhibit not only easy synthesis and excellent solubility in
commonly used solvents, but also high molar absorptivity. The synthesis of HBID and
HMBID, and six new Eu3+ complexes with structures Eu(BID)3(PHEN),
Eu(MBID)3(PHEN), [Eu(BID)4]-N+(Et)4, [Eu(MBID)4]-N+(Et)4, [Eu(BID)4]-N+(Bu)4
and [Eu(MBID)4]-N+(Bu)4 reported in this article are shown in Scheme 1.
Characterization of complexes by 1H-NMR and mass spectroscopy, FT-IR spectra, as
well as X-Ray diffraction analysis are reported and optical properties in solutions and
solid-state are investigated. Furthermore, different polymer thin films doped with
complexes are obtained and their optical properties fully examined for practical
evaluation of synthesized complexes in OLEDs.
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O
N
N
Eu
O
O
O
O
(b)
O
O
+
R
3
O
R
1
C1: R=H; Eu(BID)3(PHEN)
C2: R=CH3; Eu(MBID)3(PHEN)
2: R=H;
3: R=CH3
ꢀ
(a)
O
(c)
Eu
N+
O
R
O
O
OH
4
O
C3: R=H; [Eu(BID)4]-N+(Et)4
C4: R=CH3; [Eu(MBID)4]-N+(Et)4
R
4: R=H; HBID
5: R=CH3; HMBID
ꢀ
O
(d)
Eu
O
R
N+
O
4
C5: R=H; [Eu(BID)4]-N+(Bu)4
C6: R=CH3; [Eu(BID)4]-N+(Bu)4
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Scheme 1. Synthesis of complexes C1-C6; (a) – 1) NaOCH3, toluene, 80 °C; 2) H2O, 10% HCl; (b) –
10% NaOH/H2O, 1,10-phenantroline, EuCl3 6H2O; (c) – 10% NaOH/H2O, N(Et)4Br, EuCl3 6H2O; (d)
– 10% NaOH/H2O, N(Bu)4Br, EuCl3 6H2O.
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2. EXPERIMENTAL SECTION
2.1. Materials and instrumentation
EuCl3 6H2O (99.99%) and tetrabutylammonium bromide (98%), was
purchased from Acros Organics, 1,10-phenantroline (99%), tetraethylammonium
bromide (98%), acetophenone (96%) and 4-methylacetophenone (96%) were
purchased from Alfa Aesar. Poly-N-vinylcarbazole (PVK) was purchased from ABCR
(GmbH&Co), but poly(methyl methacrylate) (PMMA) from Sigma Aldrich. All
substances were used without further purification. CHN elemental analyses were
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performed on Euro Vector EA 3000 analyzer. H-NMR spectra were recorded in
CDCl3 or CD3CN solutions (7 mg/ml) on a Brucker Avance 300 MHz spectrometer at
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300 MHz for H nuclei. Chemical shifts were expressed in parts per million (ppm)
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relative to solvent signal. The FT-IR spectra (4000 to 650 cm-1) were recorded on a
Perkin-Elmer Spectrum 100 FTIR spectrometer using KBr pellets. Thermal properties
were determined on a Perkin Elmer STA 6000 instrument. Each sample was heated
from 30-890 °C with heating rate 10 °C/min in nitrogen atmosphere. The purity of
synthesized ligands was established on Waters 2695 HPLC using Waters 2996
Photodiode array UV-Vis detector. The chromatographic analyses were performed
using Xterra MS C18 (5µm 2.1×100 mm) column, 50% Acetonitrile/0,1% formic
acid solution as mobile phase and flow rate 0.2 ml/min. Low resolution mass spectra
were acquired on a Waters EMD 1000MS mass detector (ESI+ mode, voltage 30 V).
The UV/Vis absorption spectra were acquired using Perkin-Elmer 35 UV/Vis
spectrometer, emission and excitation spectra were measured on QuantaMaster 40
steady state spectrofluorometer (Photon Technology International, Inc.). Absolute
photoluminescence quantum yields were determined using QuantaMaster 40 steady
state spectrofluorometer (Photon Technology International, Inc.) equipped with 6 inch
integrating sphere by LabSphere. The lifetime measurements were carried out in room
temperature using QuantaMaster 40 steady state spectrofluorometer equipped with
high power Xenon flash lamp as excitation source. Polymer films were obtained with
Laurell WS-400B-NPP/LITE spin coater. Diffraction data was collected on a Bruker-
Nonius KappaCCD diffractometer using graphite monochromated Mo-Kα radiation
(λ=0.71073 Ǻ). The crystal structure was solved by direct methods [14] and refined
by full-matrix least squares [15] using maXus complex of programs [16]. The crystal
data, details of data collection and refinement are given in Table 1.
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2.2. Fabrication of PVK and PMMA films
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PVK or PMMA (20 mg) and complexes C1-C6 (1.6 mg) were dissolved in 1
ml THF and the resulting mixture was heated at 40 °C for 0.5 h. Afterwards polymer
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film was spin-coated on a glass substrate using following parameters: speed 800 rpm,
acceleration 800 rpm/sec for 1 min. Then obtained films were dried in 60 °C for 2 h.
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Table 1
Crystallographic data for complex C4.
Parameters
Formula
[Eu(MBID)4]-N+(Et)4
C76H64EuNO12
1335.25
Formula weight
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c
Unit cell dimensions
a = 27.2496(4) Å
b = 16.0957(4) Å
c = 20.2099(5) Å
90 deg
α
β
γ
132.2274(10) deg
90 deg
Volume
6563.7(2) Å3
4
Z
Density (calculated)
Absorption coefficient
F(000)
1.351 g/cm3
1.019 mm-1
2744
1.861
Goodness-of-fit on F2
Data / restraints / parameters
Final R indices [I>2sigma(I)]
R indices (all data)
7778 / 0 / 407
R1 = 0.1097, wR2 = 0.2783
R1 = 0.1301, wR2 =0.2859
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2.3. Synthesis
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The synthetic routes of compounds HBID and HMBID (4, 5) and complexes
C1-C6 are shown in Scheme 1. 2-Benzoyl-1,3-indandione (4) and 2-(4-
methylbenzoyl)-1,3-indandione (5) were synthesized as described in reference [13].
2.3.1. Synthesis of ternary complexes C1-C2
C1 Eu(BID)3(PHEN): To a solution of 2-benzoyl-1,3-indandione (4) (1.08 mmol, 3
eq.) in 10 ml distilled water, which has been neutralized with 10% NaOH solution, an
ethanol solution (5 ml) containing 1,10-phenantroline (0.36 mmol, 1 eq.) was added.
The mixture was heated to 50 °C until complete dissolution of the compounds.
Afterwards, obtained solution was added drop-wise to a 10 ml distilled water
containing EuCl3 6H2O (0.36 mmol, 1 eq.). Instantly yellow precipitate was formed,
which was allowed to stir for 4 hours in room temperature. Then, precipitate was
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separated by suction filtration and crystalized from methanol solution and dried in
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vacuum at 50 °C for 24 hours. Yellow powder, Yield 41%; H-NMR (300 MHz,
CDCl3, ppm): 10.03 (2H, d, H-PHEN), 9.41 (2H, br s, H-PHEN), 7.86-7.77 (4H, m,
H-PHEN), 7.60-7.47 (6H, br m, H-1,3-Indand.), 6.83-6.80 (6H, br m, H-1,3-Indand.),
6.55 (6H, br s, H-Ph), 6.24 (3H, br s, H-Ph), 5.29 (6H, s, H-Ph); Anal. Calcd. For
EuC60H35N2O9: C, 66.73; H, 3.24; N, 2.59; found C, 67.14; H, 3.55; N, 2.59; FT-IR
(KBr, cm−1): 3054 (νCsp2H); 1690, 1615 (νC=O); 1586, 1566, 1519(νC=C); 1447 (νC=N);
ESI(+)-MS: (m/z) 181.2 [PHEN+H]+;
899.6 [151Eu(2BID)3+H]+; 901.4
[153Eu(2BID)3+H]+.
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C2 Eu(MBID)3(PHEN): Complex was synthesized by the same procedure as for C1,
except 2-(4-methylbenzoyl)-1,3-indandione (5) was used instead of compound 4.
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Yellow powder, Yield 31%; H-NMR (300 MHz, CDCl3, ppm): 10.02 (2H, d, H-
PHEN), 9.43 (2H, br s, H-PHEN), 7.98-7.87 (4H, m, H-PHEN), 7.57-7.45 (6H, br m,
H-1,3-Indand.), 6.84 (3H, br s, H-1,3-Indand.), 6.41-4.31 (9H, m, H-1,3-Indand., H-
Ph), 5.27 (6H, br s, H-Ph), 2.24 (9H, s, Ph-CH3); Anal. Calcd. For EuC63H41N2O9: C,
67.44; H, 3.66; N, 2.50; found C, 67.07; H, 3.82; N, 2.58; FT-IR (KBr, cm−1): 3056,
3029 (νCsp2H); 2919, 2864 (νCsp3H); 1689, 1615 (νC=O); 1584, 1563, 1512 (νC=C); 1456
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(νC=N); ESI(+)-MS: (m/z) 181.1 [PHEN+H]+; 941.6 [151Eu(MBID)3+H]+; 943.4
[153Eu(MBID)3+H]+.
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2.3.2. Synthesis of tetrakis complexes C3-C6
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C3 [Eu(BID)4]-N+(Et)4: To a solution of 2-benzoyl-1,3-indandione (4) (1.08 mmol, 3
eq.) in 10 ml distilled water, which has been neutralized with 10% NaOH solution,
distilled water solution (5 ml) containing tetraethylammonium bromide (0.54 mmol,
1.5 eq.) was added. The mixture was heated to 50 °C until complete dissolution of the
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compounds. Afterwards, obtained solution was added drop-wise to a 10 ml distilled
water containing EuCl3 6H2O (0.36 mmol, 1 eq.). Instantly yellow precipitate was
formed, which was allowed to stir for 4 hours in room temperature. Then, precipitate
was separated by suction filtration and crystalized from acetonitrile solution and dried
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in vacuum at 50 °C for 24 hours. Yellow powder, Yield 55%; H-NMR (300 MHz,
CDCl3, ppm): 8.59 (4H, br s, H-1,3-Indand.), 7.70 (4H, br s, H-1,3-Indand.), 7.10
(8H, d, H-Ph), 6.78 (8H, m, H-1,3-Indand.), 6.46 (4H, br s, H-Ph), 5.96 (8H, br s, H-
Ph), 4.27 (8H, br d, (CH3-CH2)4N+), 1.71 (12H, br s, (CH3-CH2)4N+). Anal. Calcd.
For EuC72H56NO12: C, 67.61; H, 4.38; N, 1.10; found C, 67.44; H, 4.41; N, 1.30; FT-
IR (KBr, cm−1): 3083, 3054, 3018 (νCsp2H); 2997, 2977, 2948 (νCsp3H); 1681, 1619
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(νC=O); 1589, 1574, 1451 (νC=C); 1266, 1212 (νC-N); ESI(+)-MS: (m/z) 130.2
[N(Et)4]+; 899.4 [151Eu(BID)3+H]+; 901.4 [153Eu(BID)3+H]+.
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C4 [Eu(MBID)4]-N+(Et)4: Complex was synthesized by the same procedure as for C3,
except 2-(4-methylbenzoyl)-1,3-indandione (5) was used instead of compound 4.
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Yellow crystals, Yield 44%; H-NMR (300 MHz, CD3CN, ppm): 8.31 (4H, H-1,3-
Indand., d, J=9 Hz), 7.66 (4H, H-1,3-Indand., t, J=9 Hz), 7.11 (4H, H-1,3-Indand., t,
J=9 Hz), 6.72 (8H, H-Ph, d, J=6 Hz), 6.35 (4H, H-1,3-Indand., d, J=9 Hz), 6.15 (8H,
H-Ph, d, J=6 Hz), 3.17 (8H, q, (CH3-CH2)4N+), 2.30 (12H, s, Ph-CH3), 1.23 (12H, br
t, (CH3-CH2)4N+). Anal. Calcd. For EuC76H64NO12: C, 68.37; H, 4.80; N, 1.05; found
C, 67.53; H, 4.88; N, 1.09; FT-IR (KBr, cm−1): 3019 (νCsp2H); 2924, 2802 (νCsp3H);
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1679, 1615 (νC=O); 1585, 1563, 1492, 1432 (νC=C); 1284(νC-N); ESI(+)-MS: (m/z)
130.2 [N(Et)4]+; 941.6 [151Eu(MBID)3+H]+; 943.4 [153Eu(MBID)3+H]+.
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C5 [Eu(BID)4]-N+(Bu)4: Complex was synthesized by the same procedure as for C3,
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bromide. Yellow powder, Yield 50%; H-NMR (300 MHz, CDCl3, ppm): 7.02-6.87
(36H, m, H-1,3-Indand., H-Ph), 3.28 (8H, s, (CH3-CH2-CH2-CH2)4N+), 1.53 (8H, s,
(CH3-CH2-CH2-CH2)4N+), 1.41 (8H, s, (CH3-CH2-CH2-CH2)4N+), 0.97 (12H, s, (CH3-
CH2-CH2-CH2)4N+). Anal. Calcd. For EuC80H72NO12: C, 69.06; H, 5.18; N, 1.00;
found C, 68.23; H, 4.99; N, 1.07; FT-IR (KBr, cm−1): 3056 (νCsp2H); 2962, 2931, 2875
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(νCsp3H); 1767, 1688 (νC=O); 1621, 1588, 1574, 1568, 1492 (νC=C); 1260 (νC-N);
ESI(+)-MS: (m/z) 242.4 [N(Bu)4]+; 899.5 [151Eu(2BID)3+H]+; 901.5
[153Eu(2BID)3+H]+.
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C6 [Eu(MBID)4]-N+(Bu)4: Complex was synthesized by the same procedure as for
C3, except 2-(4-methylbenzoyl)-1,3-indandione (5) was used instead of compound 4
and tetrabutylammonium bromide was used instead of tetraethylammonium bromide.
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Yellow powder, Yield 42%; H-NMR (300 MHz, CDCl3, ppm): 7.70-6.50 (32H, m,
H-1,3-Indand., H-Ph), 4.02 (8H, br s, (CH3-CH2-CH2-CH2)4N+), 2.22 (12H, s, Ph-
CH3), 1.99 (8H, br s, (CH3-CH2-CH2-CH2)4N+), 1.72 (8H, br q, (CH3-CH2-CH2-
CH2)4N+), 1.12 (12H, t, (CH3-CH2-CH2-CH2)4N+); Anal. Calcd. For EuC84H80NO12:
C, 69.71; H, 5.53; N, 0.97; found C, 69.19; H, 5.29; N, 1.00; FT-IR (KBr, cm−1): 3034
(νCsp2H); 2960, 2876 (νCsp3H); 1739, 1683 (νC=O); 1616, 1584, 1563, 1511, 1427
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(νC=C); 1282 (νC-N); ESI(+)-MS: (m/z) 242.4 [N(Bu)4]+; 941.5 [151Eu(MBID)3+H]+;
943.5 [153Eu(MBID)3+H]+.
231 3. Results and Discussion
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3.1. Characterization of Eu3+ complexes with BID and MBID ligands
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To establish the structures of synthesized complexes – H-NMR, mass
spectroscopy, as well as element analysis and FT-IR spectroscopy were employed.
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Elemental analysis data were in a good agreement with proposed structures, but H-
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NMR spectroscopy was used to prove ratio of ligands in synthesized complexes. H-
NMR spectra of tetrakis complex C3 is shown in Fig. 1. Due to the presence of
paramagnetic metal ion – Eu3+ all proton signals are shifted to higher fields and shows
peak broadening. In higher fields two signals corresponding to ethylene (8 protons,
4.25 ppm) and methyl (12 protons, 1.69 ppm) group protons from quaternary
ammonium ion N+(Et)4 molecule were observed. Furthermore, at lower field six broad
signals corresponding to 36 protons from four BID ligands were observed indicating,
that in complex C3 ratio of N+(Et)4 to BID is 1:4. Similar H-NMR spectra were
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obtained for other tetrakis complexes C4-C6. Whereas in the H-NMR spectra of
ternary complexes C1 and C2 three signals at 10.00, 9.40 and 7.90 ppm
corresponding to eight protons from 1,10-phenantroline molecule were observed.
Furthermore, in higher fields four broad signals corresponding to 27 protons from
three BID ligands (C1) or five broad signals corresponding to 33 protons from three
MBID ligands (C2) were obtained, which verifies that in these complexes ratio of
PHEN:BID (or MBID) is 1:3.
Mass spectra of ternary complexes consists of three mass peaks, and for C1
they are m/z 181.2 [PHEN+H]+, 899.6 [151Eu(BID)3+H]+ and 901.4 [153Eu(BID)3+H]+
establishing, that Eu3+ and BID ratio in complex is 1:3. Unfortunately, molar masses
of ions of tetrakis complexes ([Eu(BID)4]- or [Eu(MBID)4]-) were over the mass
spectra detection limit and only tris ions ([Eu(BID)3+H]+ or [Eu(MBID)3+H]+) and
quaternary ammonium ions (m/z 242.4 [N(Bu)4]+ or 130.2 [N(Et)4]+) were detected.
In the FT-IR spectra of complexes C1-C6 absence of any broad band around
3500 cm-1 (νOH) indicates, that they are anhydrous and there are no solvent molecules
in Eu3+ coordination sphere. Furthermore, comparing complexes with BID ligands:
ternary C1 with tetrakis C3 and C5, the latter two shows vibrations of Csp3H groups
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from N+(Et)4 or N+(Bu)4 ions in the range from 2800-3000 cm-1, verifying that these
complexes are with tetrakis structure. Furthermore, compound FT-IR spectra of
HBID (4) contains sharp peak at 1713 cm-1 and one at 1644 cm-1 corresponding to
symmetric and asymmetric stretching vibrations of carbonyl group (νC=O). In the FT-
IR spectra of complexes C1, C3 and C5 both of these peaks appears on lower
frequencies (for 22-32 cm-1). Similar observations were reported for other 2-acyl-1,3-
indandione complexes [17].
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Fig. 1. 1H-NMR spectra of tetrakis complex C3 in CDCl3 at 298 K.
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Ternary complexes C1 and C2 and tetrakis complexes C5 and C6 with
N+(Bu)4 cation shows excellent solubility in CH2Cl2, CHCl3, THF, MeOH and
MeCN. However, tetrakis complexes C3 with N+(Et)4 cation exhibit slightly lower
dissolution ability in commonly used solvents and C4 is insoluble in non-polar
solvents, but soluble in MeCN. It seems that, solubility of tetrakis complexes are
mainly affected by used counterion (N+(Et)4 or N+(Bu)4) and complexes with N+(Bu)4
cation shows higher dissolution ability in polar and non-polar solvents.
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3.2. Crystal structure of complex C4
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Crystal of complex C4 was obtained by slow evaporation of MeCN solution.
X-ray structure of complex C4 is shown in Fig. 2 (A) and selected bond lengths and
angles are given in Table 2. Important disorder contribution has been encountered in
these Eu complex crystals. Tetrakis complex [Eu(MBID)4]-N+(Et)4 C4 crystallizes in
the monoclinic space group C2/c. In the crystal structure both cations and anions lie
on twofold symmetry axes. Therefore, the structure of the molecular ions consists of
two parts with identical bond length and angles. No MeCN molecules were found
trapped in crystal packing of C4.
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Table 2
Selected bond lengths (Å) and angles (°) for complex [Eu(MBID)4]-N+(Et)4 C4.
Bond lengths
Bond angles
Bond
Value, Å
Bond
Value, Å
Bond angle
Value, °
Eu(1)-O(4)
Eu(1)-O(5)
Eu(1)-O(6)
Eu(1)-O(7)
2.410(6)
2.375(6)
2.410(6)
2.378(6)
Eu(1)-O(4)
Eu(1)-O(5)
Eu(1)-O(6)
Eu(1)-O(7)
2.410(6)
2.375(6)
2.410(6)
2.379(6)
O(4)-Eu(1)-O(5)
O(4)-Eu(1)-O(5)
O(6)-Eu(1)-O(7)
O(6)-Eu(1)-O(7)
72.40(19)
72.40(19)
72.30(2)
72.30(2)
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In the structure of complex C4 each Eu3+ ion is coordinated with eight oxygen
atoms from four β-diketone moieties. The mean bond length of Eu-O is 2.390 Å,
which is a close value to other previously reported Eu-O bond lengths of Eu3+ tetrakis
complexes with β-diketone ligands [17, 18, 19]. Furthermore, mean bond angle
around central metal atom (O-Eu-O, where both oxygen atoms are from one β-
diketone molecule) is 72.35 °, which is very close to reported value (72.23 °) for other
Eu3+ complex with 2-acyl-1,3-indandionate ligands [13]. The geometrical
arrangement of oxygen atoms in the first coordination sphere of Eu3+ ion can be
described as trigondodecahedron (see Fig. 2 (B)).
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Fig. 2. (A) - X-Ray structure with 50% probability ellipsoids of crystal C4 ([Eu(MBID)4]-N+(Et)4); (B)
- Coordination polyhedron of europium in C4.
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3.3. Thermal properties of Eu3+ complexes with BID and MBID ligands
The high thermal stability is necessary for practical application of organic
luminescent materials in optoelectronic devices. Therefore, thermal stability of Eu3+
complexes was examined by thermogravimetric analysis (TGA) in the inert
atmosphere and obtained thermal decomposition temperatures Td are depicted in
Table 3. TGA shows that the starting decomposition temperatures of ternary
complexes C1 and C2 are 240 and 247 °C, respectively, and are lower than ones for
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tetrakis complexes C3-C6 (299-307 °C). It seems, that introduction of secondary
ligand PHEN in Eu3+ ion coordination sphere leads to reduction of Td and can be
explained with lower thermal stability of PHEN (start of Td~240 °C) [21] than used β-
diketonates BID and MBID. However, obtained Td values for C1 and C2 are close to
other previously reported destruction temperatures of Eu3+ ternary complexes with
rigid β-diketone ligands [11]. On the other hand, for tetrakis complexes C3-C6
thermal decomposition temperatures are higher by 52-67 °C. Moreover,
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decomposition temperatures of tetrakis complexes is not influenced by used
counterion (N+(Et)4 or N+(Bu)4) and obtained Td values differs for only 3-5 °C. It can
be concluded that synthesized tetrakis complexes exhibit ones of the highest thermal
stabilities from currently reported tetrakis Eu3+ β-diketone complexes [19, 22], which
usually do not exceed 300 °C.
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Fig. 3. TG curves of Eu3+ complexes C1-C6 obtained in the nitrogen atmosphere with heating rate of
10 °C/min.
Destruction pattern of complexes was studied through obtained termogramms
(Fig. 3). Termogramms of ternary complexes C1 and C2 consists of two rapid weight
losses in the steps around 240-452, 453-570 °C (C1) and 247-426, and 427-580 °C
(C2), which corresponds to the loss of organic ligands - one PHEN and one BID or
MBID molecule. Further destruction of complex has followed by complicated
decomposition and carbonization reactions, which lead to rather high residual weight
– 46% and 49%, respectively. Similarly, tetrakis complexes C3-C6 exhibits two rapid
weight loss stages in TGA curve, and for complex C5 they are at 299-410, 411-572
°C. The first stage can be signed to loss of counterion (N+(Bu)4) and one MBID
molecule and second stage – to one MBID molecule. Similar destruction pattern was
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reported for tetrakis Eu3+ complexes with dibenzoylmethane ligands and
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hexadecyltrimethyl ammonium or octadecyltriethyl ammonium counterions [23]. In
the end of TGA (at 890 °C) high residual weight of tetrakis complexes (40-48%) are
obtained similarly than for ternary complexes. Solid residue of all complexes was
analyzed by CHN elemental analysis, and for complex C6 it showed C, 65.31; H,
0.85; N, <0.30, which points to formation of solid residue with high fracture of
carbon. Probably in high temperatures carbonization reactions takes place with
remaining β-diketone ligands and remaining europium forms different inorganic
compounds, for example Eu2O3. All obtained complexes exhibits proper thermal
decomposition temperatures for application in the electronic devices.
3.4. UV/Vis absorption properties of Eu3+ complexes with BID and MBID ligands
UV/Vis absorption spectra of compounds HBID, HMBID, PHEN and their
Eu3+ complexes were recorded in MeCN solutions (c 1.5 10-5M), also EuCl3 6H2O
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(c 1.0 10-3M) and Na complexes with BID and MBID ligands (Na(BID), Na(MBID))
(c 1.5 10-5M) were measured in EtOH solutions. Spectra of EuCl3 6H2O, PHEN,
HBID and its Na and Eu3+ complexes are shown in Fig. 4. EuCl3 6H2O dissolved in
EtOH exhibits very weak absorption in UV light region, however secondary ligand
PHEN shows intense absorption below 300 nm, with maximum at 290 nm (ε~50881
M-1 cm-1). Compounds HBID and HMBID exhibit broad UV-light absorption between
275-400 nm with maximums at 297 and 331 for HBID and 299 and 336 nm for
HMBID. Absorption at 297 and 299 nm attributes to π→π* transitions in 1,3-
indandione, however lower energy bands corresponds to π→π* enolic transitions
from β-diketone fragment [11]. Substituent (-CH3) in benzoyl fragment has negligible
effect on absorption maximum, and for HMBID λabs max is shifted for 5 nm, compared
to HBID λabs max. Molar absorptivity ε are 25625 (HBID) and 24306 (HMBID) M-
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1 cm-1, respectively. Furthermore, solutions containing Na complexes Na(BID) and
Na(MBID) exhibits the same absorption profile as compounds HBID and HMBID
with small blue-shift of λabs max (2 nm). Similar observations, that Na complexes with
rigid β-diketones exhibit almost identical absorption spectra as free ligands were
reported previously [10, 11].
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Fig. 4. UV/Vis absorption spectra of compounds HBID, PHEN, and Eu3+ complexes C1, C3, C5 in
MeCN solutions (c 1.5 10-5M) and EuCl3 6H2O (c 1.0 10-3M) and Na complex with BID ligand
Na(BID) (c 1.5 10-5M) in EtOH solutions.
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Furthermore, coordination with Eu3+ ion leads to some changes in UV/Vis
spectra. MeCN solutions containing ternary complexes C1 and C2 exhibit absorption
at region 250-294 nm, which attributes to the absorption of secondary ligand PHEN.
However, lower energy bands location are not noticeably influenced by the presence
of lanthanide ion and λabs max lies at 332 nm for C1 and 333 nm for C2. Molar
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absorptivity for complexes C1,C2 as expected are considerably higher (ε 56385 and
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56131 M-1 cm-1) than for free ligands confirming fact, that complexes contains several
ligands in their structures. MeCN solutions containing tetrakis complexes C3-C6
shows exact absorption band shape as HBID and HMBID and since used
countercations (N+(Et)4 and N+(Bu)4) does not exhibit absorption in 250-400 nm
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range, the absorption bands for pairs C3 and C5, and C4 and C6 even overlaps with
each other. Similarly to ternary complexes, λabs max for solutions containing tetrakis
complexes differs from HBID and HMBID for 1-2 nm, but ε coefficients are in the
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range from 92370 to 94007 M-1 cm-1. It can be concluded, that absorption spectra
patterns of the solutions containing complexes C1-C6 are almost identical to the ones
observed for compounds HBID and HMBID, suggesting that coordination with Eu3+
ion shows negligible effect on π→π* transitions of the ligands. Similar conclusions
were reported for other ternary and tetrakis Eu3+ complexes with β-diketone ligands
[10, 18, 22, 24].
3.5. Photoluminescence properties of Eu3+ complexes
Photoluminescence properties of synthesized complexes were investigated in
MeCN solutions and solid-state through excitation and emission spectra. Excitation
spectra (monitored at Eu3+ emission at 611 nm) of MeCN solutions (c 1.5 10-5M)
containing complexes C1-C6 exhibits broad bands ranging from 300 to 440 nm (Fig.
5. (A)) with λexc at 350 nm, which attributes to π→π* transitions of coordinated
ligands BID and MBID. Excitation bands match very well with absorption spectra of
corresponding complexes, therefore confirming fact, that Eu3+ emission arise from
energy absorbed by coordinated BID or MBID ligands. Furthermore, excitation
spectra of ternary complexes C1 and C2 does not exhibit bands below 300 nm, where
the secondary ligand PHEN absorbs the light, meaning that direct energy transfer
from PHEN to Eu3+ ion is not present in these complexes.
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Fig. 5. (A) - Excitation spectra (λmon=611 nm) of Eu3+ complexes C1- C6 in MeCN solutions
(c 1.5 10-5M); (B) - Excitation spectra (λmon=611 nm) of complexes C1-C6 in solid-state.
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Excitation spectra of complexes in solid-state (Fig. 5. (B)) contains much
broader bands in 300-480 nm range with maximums at 408 nm for complexes
C1,C2,C5,C6, and 425 nm for tetrakis complexes C3 and C4 with N+(Et)4
counterion. Such red shift of λexc in solid-state compared to solutions spectra is
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common for Eu3+ β-diketone complexes [18] and explained with the presence of
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strong π-π interactions between ligands in the solid-state Furthermore, solid-state
.
excitation spectra of C1-C6 contains some of Eu3+ ion sharp absorption lines of f-f*
transitions at 394, 415 and 465 nm. However, these transitions are almost completely
overlapped with excitation bands of complexes, therefore confirming, that excitation
of Eu3+ ion through attached organic ligands is more efficient than direct excitation of
Eu3+ absorption bands. In conclusion, excitation window for all six complexes in
solid-state has been extended to the visible region (up to 480 nm), which is favorable
property for Eu3+ complexes, due to frequently observed low stability of β-diketones
in UV light.
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Table 3
Thermal decomposition temperatures (Td), absolute photoluminescence quantum yields (PLQY) and
CIE coordinates calculated from emission spectra of complexes C1-C6 in MeCN solutions (c 1.5 10-
5M) and in solid-state, luminescence lifetime (τ1 and τ2) of complexes C1-C6 in solid-state.
In MeCN solutions
In solid-state
Td,
°C
CIE 1931
coordinates x;y
CIE 1931
PLQY
<0.01
<0.01
0.01
PLQY
0.06
0.10
0.29
0.60
0.12
0.11
τ1, µs
τ2, µs
coordinates x;y
0.670; 0.330
121 ±4
(6%)
122±3
(5%)
233±2
(22%)
203±17
(25%)
118 ±4
(4%)
42±1
(94%)
39±1
(95%)
54±1
(78%)
118±5
(75%)
37±1
(96%)
34±1
0.669; 0.331
0.669; 0.331
0.669; 0.331
0.668; 0.331
0.669; 0.331
0.669; 0.331
240
247
302
307
299
302
C1
C2
C3
C4
C5
C6
0.670; 0.330
0.673; 0.329
0.673; 0.327
0.670; 0.330
0.670; 0.330
0.01
0.01
115±6
(2%)
0.01
(98%)
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428
Emission of complexes C1-C6 was investigated in MeCN solutions and solid-
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state, but due to fact, that emission profile is identical for complexes with similar
structure (ternary or tetrakis) only one ternary complex C2 and one tetrakis complex
C3 emission spectra are shown as examples (Fig. 6). In MeCN solutions, when
excited with 350 nm, complexes exhibit weak red-light emission with characteristic
Eu3+ ion narrow emission bands, which lies at 580, 594, 611, 652 and 701 nm and
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attributes to D0 →7FJ (J=0-4) transitions. Analogue, but more intense emission
spectra profiles were obtained for complexes in solid-state, when excited with 408 or
425 nm. Commonly reported observations in the literature about emission profile of
Eu3+ β-diketone complexes are also present for our newly synthesized complexes C1-
C6: Emission bands at 580 and 652 nm shows weak intensity due to fact, that their
corresponding transitions (5D0→7F0, 5D0→7F3) are forbidden by electric dipole and
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magnetic dipole moments. Furthermore, transition 5D0→7F1 (594 nm) is forbidden by
electric dipole moment, but allowed by magnetic dipole moment and is almost
insensitive from coordination environment around Eu3+ ion. This transition shows
splitting (Inset, Fig. 6). Moreover, band at 611 nm (5D0→7F2) is hypersensitive
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transition, which exhibit the highest intensity and is giving the red emission color for
complexes. It is electric dipole allowed transition and very sensitive to coordination
environment of central metal ion. Therefore ratio of intensities between transitions
5D0→7F2 and 5D0→7F1 shows the nature and symmetry of the first coordination sphere
of Eu3+ organic complexes. For all complexes in MeCN solutions and solid-state the
intensity ratio between these two transitions is extremely high meaning that, strong
coordination interactions are present in complexes C1-C6 and Eu3+ occupies site
without inversion symmetry. Moreover, the higher is the intensity ratio, the more
intense is the red emission from the complex [25]. CIE chromaticity coordinates for
complexes C1-C6 (Table 3) calculated from their emission spectra are situated in
explicit red region and completely corresponds to standard red color of NTSC
(x=0.67; y=0.33) therefore being a promising candidates for application in OLEDs.
Similar CIE coordinates with 100% red color purity were reported for Eu tetrakis
+
complex with butyl-methoxy-dibenzoyl-methane ligands and NH4 as counterion
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[26].
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Fig. 6. Emission spectra of Eu3+ complexes C2 and C3 in MeCN solutions (c 1.5 10-5M) (λexc=350
nm) and solid-state (λexc=425 nm). Inset shows splitting of 5D0→7F1 transition in the solid-state.
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Furthermore, Fig. 6 also shows, that emission spectra shape is slightly affected
by the complex structure. For, ternary complex C2 emission band at 611 nm are much
wider and shows shoulders at 615 and 625 nm. However, the same band of tetrakis
complex C3 exhibit one sharp line. Eu3+ ion ternary complexes with PHEN as
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secondary ligand usually display splitting of D0→7F2 transition [8, 27], which could
be explained with less ordered environment around central metal ion than in tetrakis
complexes [19, 28].
To further establish the efficiency of the emission from complexes in MeCN
solutions and in solid-state, absolute photoluminescence quantum yields (PLQY) were
determined using integrating sphere. The data are shown in Table 3. Two
relationships can be seen from PLQY:
(1) PLQY in solid state for all complexes shows higher values, than in MeCN
solutions. Lower PLQY in solutions of Eu3+ organic complexes arise from
undesirable, high frequency CH vibrational oscillators of solvent (MeCN) molecules
in the surrounding environment of the complex, which effectively lowers the emission
transition probabilities of central metal ion [29]. PLQY of complexes C1-C3,C5,C6
were also measured in THF and CHCl3 solutions and again low values ( 0.01) were
obtained. It is possible, that some dissociation processes of complexes are present in
solutions. It is known, that in non-polar solvents tetrakis complexes dissociate into
corresponding tris-complexes (for example Eu(BID)3) and a β-diketonate salt (for
example, BID-N+(Et)4) [30]. First coordination sphere of Eu3+ ion is not fulfilled in
tris-complexes, therefore, for Eu3+ to obtain coordination number 8 two solvent
molecules are attached to metal. Having high frequency CH vibrational oscillators in
first coordination sphere greatly lowers emission intensity and PLQY of Eu3+
complexes.
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(2) In MeCN and solid-state ternary complexes C1,C2 show lower PLQY than their
analogues tetrakis complexes C3-C6. In MeCN solutions differences in PLQY
between ternary and tetrakis complexes are insignificant due to low emission
intensity. However, in solid-state PLQY for tetrakis complexes C3 and C4 are five to
six times higher than for ternary complexes C1, C2 and almost three to six times
higher than for tetrakis complexes C5, C6. Tetrakis complexes usually are
characterized with higher intensity of emission compared to analogue ternary
complexes [6] due to due to larger cross section of photon absorption. Furthermore, in
tetrakis complexes Eu3+ ion is fenced by four coordinated benzoyl-1,3-indandione
ligands, which provides better shielding from surrounding environment than for
ternary complexes.
5
The luminescence decay of Eu3+ emission related to D0→7F2 (611 nm)
transition of complexes C1-C6 were determined in solid-state under pulsed laser
excitation at 408 or 425 nm. The obtained lifetime values (τ1 and τ2) are listed in
Table 3. All decay curves were fitted by bi-exponential functions and have two
lifetime components τ1 and τ2, indicating on the presence of two sites of symmetry
around the Eu3+. Usually Eu3+ complexes with two lifetime components exhibits
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broadening of transition D0→7F0 at emission spectra [27, 31], which also points to
two sites of symmetry around central metal ion. All complexes C1-C6 in solid-state
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shows slightly broad, featureless band at 580 nm, which attributes to D0→7F0
transition. Different symmetry around Eu3+ ions probably forms from different
distances between emitting Eu3+ ions in the solid-state. When distance between two
emitting Eu3+ ion sites are short, some interactions between Eu3+- Eu3+ centers take
place, which could lead to two different chemical environment formation around Eu3+
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ions. Longest decay lifetimes were obtained for tetrakis complexes with N+(Et)4
cation and are 233 and 54 µs for C3 and 203 and 118 µs for C4.
From obtained PLQY and lifetime values, we can draw a conclusion, that the
used countercation (N+(Et)4 or N+(Bu)4) affects emission properties of synthesized
complexes more than complex structure in solid-state. For ternary complexes C1, C2
and tetrakis complexes C5, C6 with N+(Bu)4 cation obtained PLQY and lifetime
values were close. However, complexes with N+(Et)4 as counteraction exhibits three
(BID ligand) or even six times (MBID) higher PLQY values than their analogues with
N+(Bu)4 cation. Such difference in emission efficiency could be as a result of different
cation incorporation in the crystal structure of complex in solid-state. Therefore, in
MeCN solutions, where these long-range order is absent and complex exist as anion
([Eu(BID)4]-; [Eu(MBID)4]-) no difference in PLQY were observed. In solid-state
counter cation size is the mayor factor, which influences the emission properties as it
was shown in the report [9]. Increase in the counteraction size usually decrease
complex emission efficiency and emission data of our newly synthesized complexes
confirms this statement.
3.6. Photoluminescence from polymer films doped with Eu3+ complexes
The mayor drawback for practical application of Eu3+ organic complexes are
their poor carrier-transporting ability and film formability. Therefore, to overcome
this problem, complexes need to be doped in host materials, which could provide
missing properties, for example, in polymers. Two polymers – N-polyvinylcarbazole
(PVK) and poly methyl methacrylate (PMMA) were chose as host materials for our
complexes. PVK owing excellent hole transporting properties as well as good match
between its triplet energy state and energy levels of most Eu3+ β-diketone complexes
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is one of the most used host materials for metal organic complexes. However, PMMA
was chosen due to its low optical absorbance (it is transparent at wavelengths >250
nm). The doping mass of polymer films was chosen 8 wt%, according to our [32] and
other authors [11, 24, 33, 34] reported investigations about relationship of doping
mass – emission efficiency of doped PVK and PMMA films with Eu3+ complexes.
Lower doping mass (0.5-2 wt%) could lead to incomplete energy transfer between
host and doping complex, however, high doping mass (>12 wt%) could lead to non-
radiative energy transfer between two metal ions (Eu3+-Eu3+), which decreases
emission intensity of doped films. PVK and PMMA polymer films doped with 8 wt%
of Eu3+ complexes were prepared by spin-coating technique from THF solutions (due
to complex C4 low solubility in commonly used solvents, PVK film with 8 wt% of
C4 wasn’t prepared, but PMMA film was obtained from MeCN solution). UV/VIS
absorption spectra of prepared films are depicted in Fig.7 (A) and Fig. 8 (A).
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Fig. 7. (A) – UV/Vis absorption spectra of pure PVK film and PVK films containing 8 wt% of Eu3+
complexes C1-C3,C5,C6; (B) - Excitation spectra (λmon PVK=435 nm) and emission spectra (λexc=346
nm) of pure PVK film and excitation spectra of PVK films containing 8 wt% of Eu3+ complexes C1-
C3,C5,C6 (λmon =611 nm).
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UV/Vis spectra of pure PVK film exhibit broad absorption in 250-350 nm
range, with λabs max at 271, 297, 327 and 348 nm, which can be assigned to π-π* and n-
π* transitions of N-polyvinylcarbazole. Nevertheless, absorption profile of doped
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PVK films exhibits the same profile as pure thin film, which appoints to absorption
overlap between host matrix and Eu3+ complexes. However, in the spectral region
from 350 to 400 nm new absorption band arises for doped PVK films and can be
attributed to the absorption of ligands (BID, MBID). This band is particular
pronounced for tetrakis complex C3, which also showed red-shift of excitation
spectra in solid-state, compared to other complexes. Completely different situation is
found for doped PMMA films (Fig. 8 (A)). These films exhibit pure complex
absorption profile with two λabs max at ~ 297 nm and ~ 337 nm (Table 4), which
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compared to λabs max in MeCN solutions are shifted to longer wavelengths.
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Excitation spectra of pure PVK film (Fig. 7 (B)) (λmon=435 nm) shows sharp
band (300-360 nm) with maximum at 346 nm and corresponding doping films
(λmon=611 nm) with complexes C1,C2,C5,C6 exhibits the same maximum, but
excitation bands shows shoulder at longer wavelengths rising from 360 to 420 nm.
Only polymer film doped with tetrakis complex C3 shows distinct excitation spectra
from pure PVK film (350-460 nm with λexc= 379 nm). Furthermore, all excitation
spectra of PVK films doped with complexes C1-C3,C5,C6 slightly overlaps with
emission spectra of PVK (355-550 nm) (Fig 7 (B)), which could also be advantageous
for polymer films due to fact, that host would provide additional excitation energy for
complex, therefore increasing emission efficiency. From these excitation spectra we
can conclude, that PVK films can be excited not only through π→π* transitions of the
ligands, but also through host matrix transitions.
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Fig. 8. (A) – UV/Vis absorption spectra of PMMA films containing 8 wt% of Eu3+ complexes C1-C6;
(B) – Excitation spectra of PMMA films containing 8 wt% of Eu3+ complexes C1-C6 (λmon =611 nm).
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Furthermore, excitation spectra of PMMA films (Fig. 8. (B)) reveal wide
bands in the range from 309 to 450 nm, with λexc ~347 nm. Excitation spectra very
well corresponds to absorption spectra, meaning that these bands can be attributed to
π→π* transitions of coordinated ligands. Slight shift of λabs and λexc in PMMA
matrices compared to ones in MeCN solution, points out to some interactions between
ligands from complexes C1-C6 and PMMA matrix.
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Table 4
Optical properties of PVK and PMMA films doped with 8 wt% of complexes C1-C6.
PVK films
PMMA films
λexc
nm
,
λabs
nm
,
λexc
nm
,
PLQYb
0.04
PLQYc
0.12
0.11
0.14
0.10
0.14
0.09
λabs, nm
τ1, µs
τ2, µs
τ1, µs
τ2, µs
47±0.5
(92%)
50±0.7
(95%)
60±1
179±3
(8%)
193±7
(5%)
199±14
(5%)
75±1
(83%)
74±1
(90%)
56±0.6
(85%)
68±1
(88%)
72±1
306±8
332, 345 344
332, 345 345
332, 345 377
337 347
341 347
337 353
339 342
336 342
339 345
C1
C2
C3
C4
C5
C6
(17%)
317±14
(10%)
204±2
(15%)
256±7
(12%)
255±6
(15%)
214±2
(12%)
0.04
0.08
(95%)
a
a
a
a
a
-
-
-
-
-
40±0.5
(94%)
42±0.8
(97%)
139±2
(6%)
154±9
(3%)
332, 345 347
0.05
(85%)
53±0.6
(88%)
332, 345 346
0.04
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a- Film wasn’t obtained due to complex C4 low solubility in THF, CHCl3;
b- PLQY determined with λexc=370 nm;
c- PLQY determined with λexc=340 nm;
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Emission spectra of doped polymer films are shown in Fig. 9. All five PVK
films shows characteristic Eu3+ ion emission lines, also no emission from matrix
(PVK) in the range from 360 to 520 nm are observed indicating, that complete energy
transfer process between host and complexes are present in these films. PLQY were
determined using two excitation wavelengths. First, doped films were excited with
350 nm (through host matrix transitions) and obtained PLQY values were low (0.03-
0.04). Second, doped PVK films were excited with 370 nm (through coordinated
ligands transitions) and obtained PLQY were higher (0.04-0.08) (Table 4) indicating,
that direct excitation through complex absorption region is more efficient, than
through host matrix absorption region. Similar to emission properties in MeCN
solutions and in solid-state, highest quantum yield was obtained for tetrakis complex
C3 with N+(Et)4 countercation, due to fact, that its excitation spectra show the largest
overlap with host matrix emission spectra, which increases the efficiency of energy
transfer process between host and doping substance. The obtained PLQY of doped
PVK films are lower than other reported doped PVK films with ternary complexes
[35].
Furthermore, all six PMMA doped films exhibit more intense red light
emission (Fig. 9 (B)) with much higher quantum yields (0.09-0.14) than for PVK
films confirming the assumption, that one of the main factors why PVK films showed
low PLQY are undesirable absorption spectra overlap between matrix and complex.
Yet again the highest PLQY were obtained for tetrakis complexes C3 and C5 – 0.14.
The obtained PLQY of doped PMMA films are close or slightly lower than other
reported doped PMMA films containing Eu complexes with rigid β-diketones [11].
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