Versatile Luminescent Europium(III)-β-Diketonate-imidazo-bipyridyl Complexes Intended for White LEDs: A Detailed Photophysical and Theoretical Study

Article abstract of DOI:10.1021/acs.inorgchem.7b01565

Three ancillary ligands based on imidazo-bipyridyl with phenyl (Ph), naphthyl (Np), and triphenylamine (TPA) substitution were synthesized and secondhand to formulate the consistent europium(III) ternary complexes using thenoyltrifluoroacetone as an anionic ligand. The complete investigation of spectroscopic, photophysical, and electrochemical properties was carried out. The attained results for all the ancillary ligands and their corresponding Eu complexes were compared with one another. All the Eu complexes reveal a broad excitation band ranging from the near-UV to blue region, along with high intense emission and apposite color purity. To further understand the ligand-to-metal energy transfer (ET) process, the geometry of the ligand was optimized and the energy level location (singlet and triplet) was calculated by using DFT and TD-DFT calculations. On the basis of the theoretical calculation, the ET mechanism was proposed. From PL emission spectra in the solid state, complete ET occurs from Ph, Np based ancillary ligands to the Eu³⁺ ion, which yields a pure red emission, whereas the TPA functionalized based Eu complex shows incomplete ET. Fortunately, white emission was observed in the TPA based Eu complex in the solid state. The white LED was fabricated by using a white emitting complex integrated with 395 nm emitted LED (InGaN) chips under 20 mA forward-bias current. The excitation source from LED was fully observed by the complex shown for 3Eu and showed yellowish emission in different concentrations (the similar observation also reflected in solid). However, in the case of 1Eu and 2Eu complexes, they showed close to white emission. The Commission International de I'Eclairage (CIE) chromaticity coordinates are close to the National Television Standard Committee standard value for white emission, and in addition, the complex 3Eu coated with the blue LED chip (460 nm) by PMMA (1:10) showed bright white emission with CIE x, y values of 0.30, 0.33, respectively.

Full text of DOI:10.1021/acs.inorgchem.7b01565

Article
pubs.acs.org/IC
Versatile Luminescent Europium(III)−β-Diketonate-imidazo-bipyridyl
Complexes Intended for White LEDs: A Detailed Photophysical and
Theoretical Study
Kasturi Singh, Rajamouli Boddula, and Sivakumar Vaidyanathan*
Department of Chemistry, National Institute of Technology Rourkela, Rourkela-769 008, Odisha, India
*
S Supporting Information
ABSTRACT: Three ancillary ligands based on imidazo-bipyridyl with phenyl
Ph), naphthyl (Np), and triphenylamine (TPA) substitution were synthesized and
(
secondhand to formulate the consistent europium(III) ternary complexes using
thenoyltrifluoroacetone as an anionic ligand. The complete investigation of
spectroscopic, photophysical, and electrochemical properties was carried out. The
attained results for all the ancillary ligands and their corresponding Eu complexes
were compared with one another. All the Eu complexes reveal a broad excitation
band ranging from the near-UV to blue region, along with high intense emission
and apposite color purity. To further understand the ligand-to-metal energy
transfer (ET) process, the geometry of the ligand was optimized and the energy
level location (singlet and triplet) was calculated by using DFT and TD-DFT
calculations. On the basis of the theoretical calculation, the ET mechanism was
proposed. From PL emission spectra in the solid state, complete ET occurs from
Ph, Np based ancillary ligands to the Eu3+ ion, which yields a pure red emission,
whereas the TPA functionalized based Eu complex shows incomplete ET. Fortunately, white emission was observed in the TPA
based Eu complex in the solid state. The white LED was fabricated by using a white emitting complex integrated with 395 nm
emitted LED (InGaN) chips under 20 mA forward-bias current. The excitation source from LED was fully observed by the
complex shown for 3Eu and showed yellowish emission in different concentrations (the similar observation also reflected in
solid). However, in the case of 1Eu and 2Eu complexes, they showed close to white emission. The Commission International de
I’Eclairage (CIE) chromaticity coordinates are close to the National Television Standard Committee standard value for white
emission, and in addition, the complex 3Eu coated with the blue LED chip (460 nm) by PMMA (1:10) showed bright white
emission with CIE x, y values of 0.30, 0.33, respectively.
INTRODUCTION
can efficiently transfer their excited energy to the lanthanide ions
is a stimulating research task. It also plays a vital role in tailoring
the photophysical properties of the lanthanide molecular
complexes for specific applications. β-Diketones based ligands
were considered as an important class of energy harvesters for the
■
Lanthanide based molecular complexes have received a great deal
of attention due to their brilliant applications in different
provinces, such as biomedical diagnostics, biological labeling,
photonic devices, and solar energy conversion.
ions are known to emit light in the visible or else near-infrared
IR) spectral region under UV or IR excitation by either a down-
1
−4
Lanthanide
3+
5−8
Ln ions,
as well as they give very volatile, thermodynami-
cally stable, and highly luminescent complexes, which are
(
essential conditions for optoelectronic or lighting applica-
or up-conversion process. Lanthanides have unique spectral
properties, atomic-like or line-like emission bands, and long-lived
excited states (microseconds to milliseconds). The 4f electrons
can be arranged liberally in any other seven 4f orbitals, resulting
in them being able to absorb or emit various wavelengths ranging
from the ultraviolet and visible to IR spectral region in the
electromagnetic spectrum, excluding for when the 4f sublevels of
9
,10
tions.
Tris-β-diketonate lanthanide complexes are coordina-
tively unsaturated and usually remain solvated. The solvent
molecules contain high energy O−H/C−H oscillators, and these
oscillators lead to nonradiative deactivation which has a
1
1
detrimental impact on Ln luminescence. The energy
deactivation via a nonradiative process could be prohibited by
introducing neutral ancillary ligands, such as phenanthroline
3
+
3+
3+
Ln are completely empty or full (La and Lu ). By choosing
the organic ligand, one can obtain highly luminescent lanthanide
complexes by means of the “antenna effect” and populate the
1
2
(
(
phen), 2,2′-bipyridine (bpy), bis(pyrazolyl)methane
13 14 15
bpm), and pyrazole, or fully fluorinated β-diketones.
3
+
These bidentate ancillary ligands replace the solvated water
emitting levels of the Ln ions, as the 4f−4f electronic transition
is a parity forbidden one. In other words, accumulating the
ligating moieties could shield the central metal ion against the
solvent in order to avoid nonradiative deactivation processes.
The molecular design of the apt organic antennas/ligands which
3+
16
molecules and lead to a coordinately saturated Ln complex. It
Received: June 20, 2017
Published: July 26, 2017
©
2017 American Chemical Society
9376
DOI: 10.1021/acs.inorgchem.7b01565
Inorg. Chem. 2017, 56, 9376−9390

Inorganic Chemistry
Article
is worth to note that the functionalization of ligands (either
anionic or ancillary) is apparently based on the problems that
need to be addressed. The water insoluble (soluble in polar
solvents) molecular complexes are necessary for optoelectronic
applications, whereas, in biomedical diagnostics, the molecular
complexes should be soluble in aqueous medium and that can be
achieved by introducing a polar functional group in the ligand
Ln(III) ions and the ligands, the entity of energy transfer is less.
Use of an additional sensitizer always facilitates the energy
transfer pathway to the triplet state of the ligands through an
31
intersystem crossing. Hence, a perfect design of ligand that is
suitable with the complex not only helps to emit blue light but
also acts as an operative sensitizer is crucial for succeeding high-
efficiency white light emission. Keeping all of these in mind,
1
7,18
3+
structure.
For application, specific molecular design of
efforts have been made to design and synthesize three Eu
lanthanide complexes is a striking research task.
complexes with TTA and imidazo-bipyridyl that are new to the
organic electronics and have been reported for the first time. The
introduction of various functional groups in an ancillary ligand
has given an elegant approach to optimize the lanthanide
luminescence with a view to develop high performance
luminescent materials for lighting or optoelectronic applications.
The enhancement in the hole transporting properties may be
anticipated by the attached TPA group and may increase the
solubility in common organic solvents and thermal stability of the
The cumulative global energy crisis and climate change are
demanding high energy efficient lighting systems (solid state
lighting (SSL)) that can help to conserve energy and reduce
overall lighting costs. Visible light (near-UV to blue) excited Eu
complexes are extremely desirable for the high-efficiency warm
white LEDs as a red-emitting phosphor. Design and synthesis of
highly efficient europium molecular complexes are expand-
1
9−21
ing.
Here, we report photophysical characteristics of three
32
newly synthesized ternary europium complexes [Eu(tta) Ph, Ph
complex. The photophysical and electrochemical studies of the
bipolar ligand and its corresponding Eu complexes were studied
in detail. In order to further understand the energy transfer
process between the ancillary ligand to the central metal ion, we
have optimized the ligand structure and calculated the exact
location of the singlet and triplet levels by using DFT and TD-
DFT. Electrochemical studies have also been carried out for both
ligands as well as complexes to find the redox potential (HOMO
and LUMO energy gap). The CIE color coordinates were
calculated for the complex as well as ligands, and it showed
NTSC (National Television Standards Committee) standard
values. Among all the complexes, we found that the TPA
integrated complex showed white emission in the solid state. We
have taken benefit of this characteristic and fabricated a white
LED based on an InGaN LED chip (460 nm). In addition, on the
basis of the PL emission spectra, we also tried to fabricate the
LED by using currently synthesized Eu(III) complexes. The
Eu(III) complexes integrated with a near blue LED chip (395
nm) showed nearly white emission for 1Eu and 2Eu and yellow
emission for the 3Eu complex. The synthesized bipolar ligand
and its corresponding Eu(III) complex structures are repre-
sented in Figure 1.
3
=
3-phenyl-1-(pyridin-2-yl)H-imidazo[1,5-a]pyridine] (1Eu),
[
[
Eu(tta) Np, Np = N-phenyl-N-(4-(1-(pyridin-2-yl)H-imidazo-
1,5-a]pyridin-3-yl)phenyl)benzenamine] (2Eu), and [Eu(tta)3-
3
TPA, TPA = 3-(naphthalen-3-yl)-1-(pyridin-2-yl)H-imidazo-
1,5-a]pyridine] (3Eu) with TTA and a bipyridyl (heterocyclic
[
amine) with phenyl (Ph, L-Ph), naphthyl (Np, L-Np), and
triphenylamine (TPA, L-TPA) substitution in solutions and in
the solid state. The anionic ligand TTA (primary sensitizer) was
preferred because of its versatile nature and stability (under
ambient conditions) as well as strong absorption in the UV
region and could harvest the energy to the central Eu3 ion.
Imidazo-bipyridyl as the ancillary ligand is chosen because it is an
efficient chromophore (can be easily functionalized), which has
the triplet state at the optimum energy level (18 974 cm ) and
stabilizes the molecular structure and reduces the nonradiative
channels (in the absence of imidazo-bipyridyl, water molecules
would remain associated with the complex). Thus, the existence
of bipyridyl in the complex aids a 2-fold purpose, namely,
increase thermal stability and red emission enhancement. It is
well familiar that the Ln(III) complexes suffer through low
thermal stability, limited photostability, and poor mechanical
+
−1
22,23
properties in their original state.
In this context, the polymers
provide a sequence of compensations for the development of
molecular materials, for instance, thermal and chemical stability,
flexibility, versatility, biocompatibility, hydrophobic−hydrophilic
balance, and the characteristic improved luminescence of
EXPERIMENTAL SECTION
■
General Information for Synthesis. All reactions were performed
under an inert (nitrogen (N )/argon (Ar)) atmosphere. Solvents
2
(
(
tetrahydrofuran (THF), ethanol (EtOH), dimethyl sulfoxide
DMSO), hexane, etc.) were carefully dried and distilled from
appropriate drying agents prior to use. Commercially available reagents
2
4
lanthanide ions. Hence, the use of a polymethyl methacrylate
PMMA) matrix is quite common, which will certainly enhance
the luminescence properties, versatility, and thermal stability of
(
25
the newly synthesized complexes. To understand the optical
properties in the thin film of Eu complexes, we have investigated
the optical properties by incorporating in the PMMA matrix with
different concentrations. White light generation from a single
molecule or component always helps to simplify the device
26
structure and fabrication procedure. The generation of white
light from a single molecule is rarely observed in Ln containing
complexes. Here, the white light is generated by combining the
blue emission that is generated from the ligand or an organic
molecule with the unique luminescent properties of Ln
2
7−30
ions.
The use of ternary complexes with Eu(TTA) is
3
very much common in white light generation procedures. The
choosing of the ligand always plays an important role as the
ligands not only play a role as sensitizers for the Ln(III) ions via
an “antenna effect” but also act as bluish-yellow-emitting sources
in the complexes. Due to mismatch in the energy levels of the
Figure 1. Structure of the europium complexes.
9
377
DOI: 10.1021/acs.inorgchem.7b01565
Inorg. Chem. 2017, 56, 9376−9390

Inorganic Chemistry
Article
are used without further purification unless otherwise stated.
Benzaldehyde, β-naphthyl, 2,2′-dipyridyl ketone, triphenylamine
ammonium acetate (1.04 g, 13.58 mmol). Then, the resultant mixture
was stirred for 12 h at 110 °C. The progress of the reaction was
(
TPA), 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (TTA),
monitored by TLC (MeOH in chloroform 1:9, R - 0.3). The RM was
f
europium(III) chloride hexahydrate (99.99%), and tetrabutyl-
poured into a minimum amount of water and then ammonium
ammonium perchlorate (nBu NClO ) electrolyte were purchased
hydroxide (NH OH) solution was added. The obtained solid was
4
4
4
from Sigma-Aldrich chemicals company. Thin-layer chromatography
filtered and dissolved in dichloromethane, followed by dried with
anhydrous sodium sulfate. The solvent was evaporated to get 0.600 g of
crude compound. This was purified with column chromatography by
using silica gel (100−200 mesh), eluent with 5% methanol in
chloroform. The obtained product was dissolved in a minimum amount
of THF solution and an excess amount of THF solvent was added to the
same; the pale yellow color solid that was formed is 450 mg (60.0%).
The spectra are displayed in Figures S1−S3.
(
TLC) was used to monitor all the reactions (silicagel 60 F254 aluminum
plates, Merk). Column chromatography was carried out using silica gel
from Aldrich (70−230 mesh, 60 Å).
Measurements. H and 13C NMR spectra were recorded using an
AV 400 Avance-III 400 MHz Fourier transform nuclear magnetic
resonance (FT-NMR) spectrometer (Bruker Biospin International,
Switzerland), in deuterated chloroform/dimethyl sulfoxide solution.
Chemical shifts were quoted relative to tetramethylsilane (TMS,
standard reference). Fourier transform infrared (FT-IR) spectra were
recorded on a PerkinElmer, USA/RX-I FTIR, spectrophotometer, and
elemental analysis was measured by an Elementar Analysen Systeme,
Germany/Vario EL spectrometer. Powder X-ray diffraction of ligands
and the Eu(III) complex were measured by a powder X-ray
diffractometer (XRD) using Cu−Kα1 radiation (Rigaku, ULTIMA
IV). The absorption spectra in solution, solid (diffuse reflectance
spectroscopy, DRS), and thin film were measured by using a UV−visible
spectrometer (Shimadzu Corporation, Japan or UV-2450 PerkinElmer,
USA/Lamda 25 and Lamda PerkinElmer). The photoluminescence
1
-Phenyl-1-(pyridin-2-yl)H-imidazo[1,5-a]pyridine (L-Ph). 1H
3
NMR data (400 MHz, DMSO-d ): δ 8.68−8.62 (m, 2H), 8.50 (d,
6
1
H, J = 7.2 Hz), 8.15 (dd, 1H, J = 0.4, 1.6 Hz), 7.90 (dd, 2H, J = 1.2, 3.2
Hz), 7.83 (t, 1H, J = 7.6 Hz), 7.60 (t, 2H, J = 7.2 Hz), 7.54−7.50 (m,
H), 7.22−7.19 (m, 1H), 7.19 (t, 1H, J = 6.4 Hz), 6.85 (t, 1H, J = 7.2
1
13
Hz). C NMR data (100 MHz, DMSO-d ): δ 159.6, 154.2, 142.5, 141.7,
6
1
1
1
1
6
34.8, 134.6, 134.4, 134.1, 133.9, 133.1, 127.6, 127.1, 125.9, 125.7, 124.2,
−1
19.4. FT-IR data: υ (in cm ) = 3059, 2927, 1663.6, 1590.3, 1534,
507, 1477, 1443, 1425, 1401, 1351, 1305, 1275, 1248, 1195, 1138,
122, 1088, 1073, 1031, 1008, 978, 951, 920, 886, 829, 787, 772, 737,
92, 620, 593, 505. ESI-Mass (m/z): 274.3
(
PL) emission spectra in solution phase and thin film were recorded by a
3
-(Naphthalen-3-yl)-1-(pyridin-2-yl)H-imidazo[1,5-a]pyridine (L-
Horiba Jobin Yvon, USA/Fluoromax 4P spectrophotometer. The
relative photoluminescence quantum yield was measured with a Horiba
Jobin Yvon, HR 320, and the absolute fluorescence quantum yield was
measured with an Edinburgh Instruments, spectrofluorometer, FS5,
Integrating Sphere SC-30. The CIE color coordinates were calculated by
using PL emission data (MATLAB software). The electrochemical
properties of the complex and ligand were measured by using cyclic
voltammetry (CV), AUTOLAB 302N Modular potentiostat, at RT in
dichloromethane. The working (glass-carbon rod), auxiliary (counter,
Pt wire), and reference (Ag/Agcl wire) electrodes were used for CV
analysis. The dimethylformamide (DMF) which contains 0.1 M
Bu NClO was used as the supporting electrolyte, and the scan rate
1
Np). H NMR data (400 MHz, DMSO-d ): δ 8.62−8.78 (m, 3H),
6
8
.54 (S, 1H), 8.20 (d, 1H, J = 7.2 Hz), 8.14−8.01 (m, 4H), 7.85 (td, 1H, J
=
=
1.6 Hz, 9.6 Hz), 7.62−7.60 (m, 2H), 7.24−7.11 (m, 2H), 6.92 (t, 1H, J
6.4 Hz). 13C NMR data (100 MHz, DMSO-d ): δ 154.9, 148.9, 138.0,
6
136.2, 133.3, 133.2, 130.7, 130.3, 128.8, 128.2, 127.8, 127.4, 127.2, 126.7,
126.6, 125.8, 121.8, 129.6, 121.1, 120.4, 119.4, 119.9, 114.0. FT-IR data:
−1
υ (in cm ) = 3381, 1683, 1601, 1533, 1456, 1408, 1360, 1302, 1186,
1137, 1060, 935, 857, 780, 722, 683, 645, 577, 519. ESI-Mass (m/z):
322.5
N-Phenyl-N-(4-(1-(pyridin-2-yl)H-imidazo[1,5-a]pyridin-3-yl)-
1
phenyl)benzenamine (L-TPA). H NMR data (400 MHz, DMSO-d
): δ
4
4
6
−1
was maintained as 100 mV s . The ligand structure (geometry) was
8.61 (d, 2H, J = 5.2 Hz), 8.49 (d, 1H, J = 7.2 Hz), 8.12 (d, 1H, J = 8 Hz),
7.84−7.78 (m, 2H), 7.72 (d, 1H, J = 8.4 Hz), 7.44−7.35 (m, 4H), 7.25−
33
optimized within the density functional theory (DFT) framework
using the B3LYP/6-31G(d, p) level of theory. After confirming the
ground state geometry for the ligand, we have vertically excited the
molecule to get the UV−visible spectra and first excited state using TD-
13
7.04 (m, 10H), 6.89−6.82 (m, 1H). C NMR data (100 MHz, DMSO-
d ): δ 149.5, 149.3, 137.7, 137.5, 137.0, 133.3, 133.1, 130.3, 130.0, 129.8,
6
129.0, 128.8, 128.7, 128.0, 127.3, 127.3, 127.1, 126.9, 126.3, 126.1, 123.3,
34
−1
DFT. To understand PL spectra of the Eu(III) complex, the triplet
excited state of the ligand is also obtained by using the same
methodology mentioned above. The optimization was carried out in
the G09W suit of programs. Lifetimes of the Eu(III) complex as well as
ligand were measured at 298 K with an Edinburgh Instruments FLS 980
based on the time correlated single photon counting technology upon
the excitation at 380 nm for all complexes and ligands. A pulsed xenon
lamp was used as the excitation source, and the signals were detected
with a photomultiplier. The triplet energy of the synthesized ligand was
estimated by measuring the phosphorescence spectra of the Gd complex
of the ligand at 77 K by using an Edinburgh FLS 920 spectrofluorimeter,
UK, system that consists of a 450 W xenon lamp (Xe 900) mounted in
an air-cooled lamp housing with a built-in de-ozonizer filter. The ozone-
free Xe arc lamp emits a continuous radiation from 190 to 2600 nm.
Thermal analysis of the complexes was carried out using thermo-
gravimetric/differential thermal analysis (TG/DTA), TMA (Shimad-
zu).
1
1
8
22.6, 121.2, 121.1, 119.6, 114.9. FT-IR data: υ (in cm ) = 3045, 1585,
531, 1511, 1487, 1403, 1354, 1334, 1285, 1191, 1176, 1136, 1008, 954,
40, 801, 757, 742, 698, 629, 515. ESI-Mass (m/z): 438.5
Synthesis of Eu(TTA) R. A 50 mL two-neck round-bottom flask was
3
taken with a N containing balloon adaptor and followed by pouring
2
Eu(TTA) ·2H O (0.300 g, 0.352 mmol, 1 equiv) dissolved in dry THF
3
2
(
(
15 mL). To the above solution was added a mixture of ligand (R)
0.095 g, 0.352 mmol, 1 equiv) in THF (10 mL), and the reaction
mixture (RM) was stirred for 12 h at 60 °C. The resulting product was
dissolved in a minimum amount of THF and an excess of hexane was
added to get a solid product that is a pale yellow color with 285 mg
(72.0%).
Eu(TTA) Ph. Elemental analysis: Anal. Calc. for C H EuF N O S :
3
42 25
9
3
6 3
C, 46.42; H, 2.32; N, 3.87; S, 8.85; Found: C, 46.68; H, 2.23; N, 3.95; S,
−1
8
1
7
.69%. FT-IR data: υ (in cm ) = 3657, 3110, 2957, 1685, 1606, 1539,
506, 1451, 1488, 1356, 1305, 1246, 1191, 1138, 1060, 933, 859, 785,
18, 641, 581, 520. ESI-Mass (m/z): 1087.2.
Materials. The commercially available reactants are used except for
diphenylamino)benzaldehyde (TPA-CHO). The compound TPA-
Eu(TTA) Np. Elemental analysis: Anal. Calc. for C H EuF N O S :
(
3
46 27
9
3
6 3
C, 48.60; H, 2.39; N, 3.70; S, 8.46; Found: C, 48.80; H, 2.17; N, 3.82; S,
CHO was synthesized by the well-known Vilsmeier−Haack reaction,
and the metal compound tris(thenoyltrifluoroacetone)europium(III)
−
1
8
1
.32%. FT-IR data: υ (in cm ) = 3350, 1679, 1604, 1537, 1453, 1415,
349, 1301, 1188, 1141, 1056, 983, 858, 782, 716, 679, 641, 575, 518.
(
Eu(TTA) ·2H O) was synthesized by using a well-recognized
3 2
35,36
ESI-Mass (m/z): 1136.8.
Eu(TTA) TPA. Elemental analysis: Anal. Calc. for C54
C, 51.72; H, 2.73; N, 4.47; S, 7.67; Found: C, 52.02; H, 2.53; N, 4.57; S,
method.
General Synthesis of Ligand. 2,2′-Dipyridyl ketone (0.5 g, 2.717
mmol) was taken in a round-bottom flask, and to this was added glacial
acetic acid (15 mL) at room temperature (RT). To this reaction mixture
were subsequently added the aldehyde compound (benzaldehyde/β-
naphthyl/4-(diphenylamino)benzaldehyde) (0.576 g, 5.434 mmol) and
3
H34EuF N O S :
9 4 6 3
−1
7.47%. FT-IR data: υ (in cm ) = 3106, 3656, 1683, 1594, 1540, 1486,
1361, 1301, 1200, 1139, 1056, 936, 841, 799, 763, 721, 694, 643, 584,
512. ESI-Mass (m/z): 1255.6.
9
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Article
Scheme 1. Synthesis Route of the Ligand and Corresponding Eu(III) Complex
Figure 2. UV−vis absorption spectra of ligands (left) and respective complexes (right) in chloroform solution.
Table 1. PL Emission Intensity Ratios as Well as Their Respective CIE Coordinates for 1Eu, 2Eu, and 3Eu in Diverse Polarity of
Solvents
1
Eu
2Eu
3Eu
CIE
CIE
CIE
solvent
asym. ratio
x
y
asym. ratio
x
y
asym. ratio
x
y
acetone
DMSO
DMF
8.0366
6.6740
8.8023
3.0356
8.6639
7.8347
6.2596
9.0502
11.2483
10.584
0.6611
0.6624
0.6635
0.6493
0.6627
0.6621
0.6576
0.6650
0.6623
0.6652
0.3385
0.3372
0.3362
0.3503
0.3369
0.3376
0.3420
0.3347
0.3374
0.3344
7.4428
11.2561
14.8450
13.7521
12.0440
7.5295
0.6610
0.6613
0.6651
0.6657
0.6670
0.6641
0.6654
0.6662
0.3333
0.6643
0.3386
0.3383
0.3346
0.3340
0.3327
0.3355
0.3342
0.3334
0.6663
0.3353
1.5554
2.7005
5.3477
1.5675
1.4387
1.4058
2.4239
2.3467
3.9763
8.5966
0.6365
0.6383
0.6573
0.6400
0.6412
0.6371
0.6442
0.6444
0.6500
0.6622
0.3631
0.3613
0.3423
0.3596
0.3583
0.3624
0.3553
0.3551
0.3496
0.3374
MeOH
EtOAc
CHCl3
DCM
13.4092
13.2930
15.0378
19.1317
THF
ACN
toluene
RESULTS AND DISCUSSION
The PXRD study analyses are measured for all the complexes.
These results show the amorphous nature of complexes (Figure
S4), which are suitable for device fabrication and further analysis.
The PXRD data of 1Eu, 2Eu, and 3Eu complexes are also
provided in Tables ST1−ST3. FT-Infrared spectroscopy: The
FT-IR spectra of ligands and their respective Eu(III) complexes
■
Characterization of the Complex. The synthesis direction
method of Eu(III) complexes is summarized in Scheme 1. The
1
13
compounds are characterized by H and C NMR, FT-IR, and
elemental analysis. All the measurements were completed at RT.
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Table 2. UV-Absorption and PL Emission Data of Synthesized Ligands and Corresponding Eu(III) Complexes
a
ac
λem , (nm)
S. no.
compound
λmax (abs) (nm) solution ,
ab
λex (nm)
275
I /I ratio
fwhm
2
1
1
2
3
4
5
6
Eu(TTA) Ph
271, 340
580, 592, 612, 652, 702
580, 592, 612, 652, 703
580, 592, 612, 652, 703
465
11.7
10.9
10.5
8.2
10.8
11.2
3
Eu(TTA) Np
270, 340
275, 389, 356
275, 389
365
3
Eu(TTA) TPA
304, 340
3
L-Ph
243, 323, 385
246, 290, 330, 366
L-Np
L-TPA
365
465
242, 303, 343
330
440
a
b
c
Measured in chloroform solution at 298 K. Absorption peaks from the UV−vis absorption spectra. Emission peaks from photoluminescence
emission spectra.
−
1
were measured in the range of 500−4000 cm (Figures S5 and
S6). The FT-IR spectra of the Eu(TTA) ·2H O complex showed
manipulating the excited state of europium(III) complex energy
levels is major task. The synthesized ligands, l-L, 2-L, and 3-L
showed emission at 465, 465, and 440 nm under excitation
wavelengths 365, 365, and 330 nm (Figure 3) in solution,
3
2
−1
a strong carbonyl (CO) stretching frequency at 1610 cm
−1
and the complexes showed around 1600 cm . The change in the
stretching frequency of the carbonyl group is an indirect evidence
to that the β-diketonate (Eu(TTA) ·2H O) compound
3
2
3
7,38
coordinated with the neutral ligand.
In addition, to the
same, the imidazole moiety peak at around 1600 cm− in the
ligand was shifted toward the higher wavenumber. All the above
strategies are suggesting that the Eu metal ion was coordinated
with the ligand molecule.
1
Photophysical Properties. UV−Visible Absorption and
DRS Studies. The UV−visible absorption spectra of the ligands
and the respective Eu(III) complexes were carried out in solution
−
5
−1
(
(
chloroform solution conc. 1.0 × 10 mol L ) as well as in solid
Figure 2). The absorption spectra of ligands show absorption
ranging from 240 to 450 nm with λmax values ∼ 242, 330, and 366
nm, respectively. These bands are attributed to the π → π*
transitions of the ligands. The absorption studies of Eu(III)
complexes and Eu(TTA) ·2H O show the peak maxima at ∼340,
3
2
2
70 nm and 340, 275 nm, respectively. Similar spectral features
were observed for the Eu(III) complexes as for the ligand as well
as Eu(TTA) . However, a noticeable change was observed for the
3
peaks maxima of the complexes compared to those of ligands. It
is suggesting the presence of the ligand in the corresponding
Eu(III) complexes, and the detailed study is listed in Table 2. The
theoretically calculated UV absorption of the ligands is
interpreted in Figure S7. The absorption maxima of the L-Ph,
L-Np, and L-TPA are shown at 305, 348, and 350 nm,
respectively. The observed peak maxima from the theoretical
calculations are similar to those of the experimental analysis.
The relative study of absorption spectra of the europium(III)
complexes in solution and solid shows that the absorption
maximum shifts toward the longer spectra range for solid phase
as compared to that of complex in solution. The experiential
bathochromic shift in the absorption spectra is due to the
Figure 3. PL excitation and emission spectra of the ligands in
chloroform.
respectively. The corresponding solid state showed emission at
4
76, 465, and 553 nm for L-Ph, L-Np, and L-TPA, respectively
(
Figure 4). Among all, L-TPA show distinguished emission,
39
aggregation of the complex in the solid phase. The band gaps
for all the ligands and respective complexes were calculated from
the DRS with the help of the Kubelka−Munk function (Figures
4
0−42
S8 and S9).
The attained data from the DRS is adapted to
the Kubelka−Munk function, and α is substituted with F(R ) (α
∞
=
F(R )).
∞
n
αhν = A(hν − Eg)
(1)
Here, h: Planck’s constant, α: absorption coefficient, ν: frequency
of vibration, A: proportional constant, E : band gap. Here, n
g
values are given, for example, in the case of exponent n = 1/2, 3/2
is for direct transition allowed/forbidden and n = 2, 3 for
allowed/forbidden indirect transition, respectively.
Photoluminescence Studies. In the luminescence process of
Eu(III) complexes, populating the ligand triplet state as well as
Figure 4. PL emission spectra of the Eu(III) complexes in solid form.
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which consisted of the broad emission which covers the region of
primary spectral colors. Obviously, one can expect white light
emission from the complex (cf. CIE color coordinates, Figure
ST4). The observed emission of the ligands (365, 365, and 330 to
475, 465, and 553 nm) showed wavelength moving toward the
red spectral region (red shift) in the solution state with increasing
the solvent polarity. This might be illuminated as the dipole of
the excited state is high in polar solvents and leads to peak shift.
The solvatochromism analysis of the Eu(III) complexes is shown
in Figure 6, and their respective CIE color coordinates are listed
in Table 1. The PL excitation and emission in different solvents
with 2D spectra are displayed in Figures S17 and S18. The
intensity of the ligand emission in the Eu(III) complexes is high
in acetone solution, whereas it is less intense in toluene. The
Eu(III) complex in less polar solvent is expected to be have
energy levels that are high compared to those of the Eu(III)
complex in polar solvent, which leads to intense high emission
peak of the ligand along with europium emission (complex
emission), observed in polar solvents like acetone, DMF, etc.
The CIE color coordinates of the TPA based Eu(III) complex
showed nearly white emission and showed close CIE white color
coordinates in chloroform solution. This observation are
expected as seen from the emission spectrum, where the ligand
broad emission covered from blue to green emission in the
complex as well as Eu(III) metal ion gives red emission.
The complexes in solution showed red emission at 612 nm
along with additional emission peaks at around 420−580 nm
regions. It belongs to ligand emission from the complex. It leads
to deviating from the red emission of the Eu complexes.
However, no emission from the ligand was observed in the solid
state for 1Eu and 2Eu complexes (Figure 7) and showed red
emission (digital images are shown in the inset of Figure 7). In
the case of the TPA conjugated ligand based Eu complex (3Eu),
the observation is weird; emissions from both ligand and Eu(III)
metal ion were observed. These uncommon emissions were
observed from the complex in solution as well as solid (3Eu); this
1
2). The peak maxima of ligands in solid were shifted to higher
wavelength compared to that of the peak maxima in solution.
The red shift is expected due to aggregation of the molecule in
the solid state. The ligands L-Ph, L-Np, and L-TPA showed blue
emission in solution. The emission spectra of the ligands in
different solvents were measured and are given in Figure S10. In
the case of solid L-Ph and L-Np, they showed blue emission;
however, L-TPA showed greenish-yellow emission. The
complexes in ∼360 nm excitation wavelength showed intense
5
7
emission peaks at 612 nm attributed to the D → F transition,
0
2
due to electrical dipole (ED) transition in solution as well as
solid. The excitation spectra of the ligands as well as complexes in
solid form are displayed in Figures S11 and S12.
The Eu(III) complexes characteristic emission peaks are
5
7
attributed to the intraconfigurational D → F (J = 0−4)
0
J
transitions. The emission peaks appear at 579, 592, 612, 652, and
02 nm, and these emission lines correspond to 4f−4f transitions
of D → F , D → F , D → F , D → F , and D → F ,
7
5
7
5
7
5
7
5
7
5
7
0
0
0
1
0
2
0
3
0
4
respectively. These results are indirectly indicating that there is
5
7
energy transfer from ligand (L) to metal (M) ion. The D → F
0
0
transition (579 nm) in the emission spectra indicates that all the
3+
Eu ions occupy a site with the same symmetry. The magnetic
5
7
character of the D → F transition is independent of the
0
1
3
+
coordinate environment of the Eu ion. The observed transition
D → F at 612 nm is the most intense induced ED transition,
5
7
0
2
and it is clearly indicating that the Eu(III) occupies the site
without an inversion center in the complex (Figure 5). The
might be due to less energy difference between the triplet state of
5
the ligand and the excited state ( D ) of the Eu(III) metal ion (cf.
0
computational study). The presence of ligand emission in the Eu
complex leads to greenish-yellow emission shown under UV
light.
To understand the above optical characteristics of the
complexes as well as the ligand-to-metal ion energy transfer
mechanism, we extended our study to computational. The
energy level locations (ligand singlet and triplet) were calculated
by using DFT and TD-DFT analysis. The ligands, L-Ph, L-Np,
and L-TPA, singlet energy levels were shown at 27 183 (3.37 eV),
−
1
2
(
68 421 (3.32 eV), and 25 591 (3.17 eV) cm , respectively
Figure 8). The corresponding triplet energy levels were shown
at 18 974 (2.35 eV), 18 904 (2.34 eV), and 18 582 (2.30 eV)
−1
cm , respectively. Further, it was confirmed experimentally by
the help of phosphorescence emission spectra at 77 K and found
to be the triplet energy level, 2.55, 2.53, 2.36 eV for 1Eu, 2Eu,
Figure 5. PL excitation and emission spectra of the Eu(III) complexes in
solution form.
3
Eu, respectively (Figure S19). It is well-known that TTA energy
43
levels are located at 25 164 (singlet) and 18 954 (triplet). The
excited state of the D energy level of the Eu(III) metal ion is
located at the 17.500 cm . For effective energy transfer from
ligand E(T ) to Eu excited level E( D ), according to the Letva et
al. investigation, the energy difference ΔE should be in the range
2500−4000 cm . The present investigation indicates that the
complexes 1Eu and 2Eu are satisfying the specified strategy,
whereas 3Eu showed less energy difference between singlet and
triplet levels as well as the D level of Eu ion. It is clearly
5
0
−1
44
calculated narrow fwhm’s for 1Eu, 2Eu, and 3Eu in solution are
5
1
1
1.7, 10.9, and 10.5 nm, respectively. In the solid state, fwhm’s for
Eu, 2Eu, and 3Eu are 7.9, 8.1, and 12.8 nm, respectively. The
1
0
−1
45
fwhm of the 1Eu showed a better value for solid than the
remaining complexes. The highest value of the 3Eu complex is
due to expressed dominating character of the ligand emission
from the complex.
5
0
The solvatochromism effect on ligands as well as Eu(III)
complexes was studied, and their Stokes shift, linear fitting was
discussed and shown in 2D, 3D spectra (Figures S11−S14; Table
indicating that the complexes, 1Eu and 2Eu, have shown efficient
energy transfer; however, the energy mismatch in the 3Eu
indicates the inefficient energy transfer between ligand excited
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Figure 6. PL emission of the complexes in different solvents (polarity variations).
Figure 8. Energy transfer process from the ligands to center metal
3
+
(
Eu ) ion.
Figure 7. PL emission spectra of the Eu(III) complexes in solid phase,
and digital images under UV light are shown in the inset.
L (λ) − L (λ)
0
i
φ =
L0(λ)
E (λ) − (1 − φ)E (λ)
i
0
5
III
η =
levels to D Eu ion. In addition, the overlapping of the
0
E0(λ)φ
absorption spectra of TTA (acceptor) and emission spectra of
the ligands indicates that the energy can also transfer from the
TTA to the ligand triplet energy and reach to Eu(III) metal ion
Here, L (λ) = integrated excitation profile when the sample is
0
directly excited by the incident beam, E (λ) and E (λ) =
0
i
(
interaction in the range of 400−440 nm) (Figure S20).
Photoluminescence Quantum Efficiency. By using the
integrated luminescence of powder caused by direct excitation
and by indirect illumination from the sphere, respectively. L (λ) =
i
following formulas, the quantum efficiency (φ) can be obtained
for the presently investigated complexes.
integrated excitation profile obtained from the empty integrated
sphere (empty holder). PLQYs of the complexes were measured
46
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Table 3. Observed Luminescence Lifetimes τobs and Intensity Parameters of Synthesized Ligand and Corresponding Eu(III)
Complex
intensity parameters
−
20
2
a
(
10 cm )
τ (ms)
S. no.
compound
Ω2
Ω4
solid
acetone
CHCl3
DMF
THF
toluene
1
2
3
4
5
6
Eu(TTA) Ph
6.33
4.78
5.84
2.16
0.79
0.13
0.092
0.084
0.31
0.32
0.24
0.20
0.44
0.43
0.31
0.30
0.36
0.37
3
Eu(TTA) Np
3
b
b
b
b
b
b
Eu(TTA) TPA
0.070
1.64
3.14
2.21
0.005
0.005
0.005
2.76
0.005
4.34
0.006
3
L-ph (10− ms)
L-Np (10− ms)
L-TPA (10− ms)
3
1.91
2.17
2.15
3
b
b
b
b
b
3.63
3.16
4.18
2.16
2.72
3
b
b
2.06
2.75
2.67
2.49
2.67
a
b
The decay curves for the β-diketonate europium(III) complexes and its ligands. The decay curves were found to be biexponential for the β-
diketonate europium(III) complex and its ligand.
Figure 9. Lifetime of the Eu(III) complexes (1, 2, and 3 complexes) in different solvents and in the form of solid.
in chloroform solution as well as solid state. The PLQYs of the
−t
exp
, where A is the scalar quantity obtained from the
1
(
)
τ
1
1
Eu, 2Eu, and 3Eu are shown as 6.3, 6.6 and 3.8% in solution and
.4, 0.42 and 0.1% in solid. However, in the case of the solid, QY
curve fitting, t is the time in ms, I = 0 is the offset value, and τ is
the decay time value for the exponential component. However,
among them, some of the complexes are best fitted with the
biexponential curve, and details are interpreted in Table 3.
Luminescence decay curves of the D excited state were
measured at 298 K and under 360 nm excitation (Figure 9). To
0
was reduced and it can be believed that nonradiative channels are
expected to be more in solid. In addition, the PLQY of the
complexes in thin film with PMMA doping showed enhanced
observations and detailed discussions are given in PMMA film of
Eu(III) complexes.
5
0
Lifetime and Judd−Ofelt Analysis. The lifetime data of the
complexes as well as ligands were fitted with the single
understand the solvent influence on lifetime, the PL lifetime was
investigated in various solvents (acetone, CHCl , DMF, THF,
3
exponential function given by the equation, I(t) = I + A1
toluene). The luminescent lifetime values (τ) of 1Eu and 2Eu
0
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Table 4. Radiative Properties, Internal Quantum Efficiency, and Branching Ratios of the Complexes
complex
Arad (s−1)
Trad (ms)
Anr (ms)
ηLn (%)
ηsens (%)
β1
β2
β4
1
2
3
Eu
Eu
Eu
901.85
148.78
641.60
1.10
6.72
1.6
9.96
11.75
13.66
8.36
1.25
4.4
36
5.54
33.60
7.79
77.67
50.24
75.78
16.79
16.15
16.42
19.84
54.09
Figure 10. Cyclic voltammetry analysis of the ligands (left) and their corresponding complexes (right).
complexes showed high in solution, almost 0.44 ms (in DMF
solution), and their solids showed lower lifetime values,
respectively. It indicates that the nonradiative relaxation is high
in solid for 1Eu and 2Eu complexes. However, in the case of the
percentage of Eu(III) presence in the Eu complex, which leads to
deviating from red emission.
The Judd−Ofelt intensity parameters were utilized further to
calculate other vital optical properties unlike transition
probabilities (Arad, Anr), radiative lifetime (τrad), and branching
ratio (β). The total radiative transition probability can be written
as
3
Eu complex, a lower value of lifetime was observed. It could be
expected that the presence of the ligand broad emission along
with the Eu(III) metal ion emission peak in the 3Eu complex
while, in liquid systems, the quenching of lifetime is due to
47
Arad
=
A
0−J
∑
nonradiative relaxation via vibronic coupling. The complexes,
Eu, 2Eu, and 3Eu, follow the order of lifetime such as DMF >
J
(2)
1
toluene > THF ≈ acetone > CHCl > solid, DMF > toluene >
3
where Arad is the total radiative transition probabilities. The
radiative lifetime of the excited state of the Eu ion and the total
radiative transition probabilities are interrelated, which can be
expressed as
acetone > THF > CHCl > solid, solid > toluene > acetone ≈
3+
3
CHCl ≈ DMF ≈ THF, respectively. In addition, the lifetime of
3
the ligands was also calculated in solution as well as in solid. The
details of the lifetime values are interpreted and summarized in
Table 3. The lifetimes of the ligands in different solutions are
interpreted and shown in Figure S21. The J−O parameter is one
of the essential information for Eu(III) ions. The detailed
1
Arad
τrad
=
(3)
3
8
calculation of the J−O parameter is reported elsewhere.
The fluorescence branching ratio (β) for the transitions starting
5
Parameters of Ω and Ω were calculated for all Eu(III)
complexes. The calculated values are tabulated in Table 3. The
from the D level can be resulting from the following equation:
0
2
4
A(0 − J)
rad(0 − J)
observed higher Ω value clearly indicates that the Eu(III) ion
2
β = A
was located in a more covalent environment.
(4)
Further, to understand the emission behavior by observing the
lifetime with monitoring the ligand (465 nm) and Eu(III) metal
ion (612 nm) at 340 nm excitation wavelength in the Eu complex
was executed (Figure S22). The lifetimes of both monitoring
wavelength are well fitted with a monoexponential function. The
calculated lifetime values for 1Eu, 2Eu, and 3Eu are 0.243, 0.247,
and 0.087 ms at 612 nm as a monitoring wavelength, and 0.0022,
The rate of nonradiative relaxation (A ) can be expressed as
nr
1
τmes
1
Anr
=
−
τ
cal
where τmes is the measured from the instrument decay curves and
τcal is radiative lifetimes which was calculated from J−O theory,
0.0022, and 0.0021 ms at 460 nm as a monitoring wavelength.
respectively. A is the nonradiative relaxation rate which
nr
The lifetime of the Eu complexes at ∼465 nm as monitoring
wavelength (ligand emission in Eu complex) showed much lesser
values than 612 nm monitoring wavelength, and it indicates that
the less energy transfer from the ligand and more energy transfer
from the Eu(III) metal ion in the complex. These results are
clearly indicating the low percentage of ligand emission and high
comprises multiphonon relaxation, energy transfer through
cross-relaxation, and several other nonradiative processes.
Luminescence quantum efficiency (η) is defined as the ratio of
the number of photons emitted to the number of photons
absorbed. However, for the system containing lanthanide ion, it
can be referred to as the ratio of the experimental lifetime to the
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a
Table 5. Electrochemical Properties of the Ligands and Respective Eu(III) Complexes
Oxi
onset
Red
onset
b
band gap/energy gap, E og pt, (eV)c
S. no.
compound
voltage
(V) (ELUMO [eV])
voltage
(V) (EHOMO [eV])
λonset, (nm)
1
2
3
4
5
Eu(TTA) Ph
1.04 (−5.44)
1.15 (−5.55)
0.90 (−5.30)
1.23 (−5.63)
1.09 (−5.49)
1.20 (−5.60)
−0.96 (−3.43)
−1.13 (−3.26)
−1.27 (−3.12)
−1.04 (−3.35)
−1.06 (−3.33)
−1.07 (−3.32)
419
454
423
451
446
464
2.96 (2.0)
3
Eu(TTA) Np
2.73 (2.29)
2.93 (2.17)
2.75 (2.28)
2.78 (2.16)
2.67 (2.27)
3
Eu(TTA) TPA
3
L-Ph
L-Np
6
Red
onset
L-TPA
a
= the onset reduction potentials, EOxi = the onset oxidation potentials, E = band gap, E = E
b
E
− E
. Calculated from the optical
onset
g
g
LUMO
HOMO
c
absorption (DRS spectra). Calculated the onset wavelength of optical absorption (onset) in solid state film (DRS spectra). The energy differences
(
HOMO and LUMO) values were calculated by cyclic voltammogram and are displayed in parentheses.
Figure 11. HOMO−LUMO energy levels and their calculated band gap of the ligands and their corresponding Eu(III) complexes.
5
Oxi
calculated radiative lifetime of the D level and can be expressed
as
0
EHOMO = −(E
ELUMO = −(E
+ 4.4) eV
+ 4.4) eV
onset→SCE
Red
onset→SCE
τmes
ηLn(%) =
× 100
τ
cal
The ground state optimized structures of the ligand were
obtained from computational study, and their xyz coordinates for
L-Ph, L-Np, and L-TPA are listed in Figure S23. The HOMO−
LUMO of the ligands have also been calculated by DFT analysis.
The obtained results as well as their HOMO and LUMO are
listed in Table ST5. The HOMO and LUMO energy levels with
respective band gaps of the ligands as well as their singlet and
triplet energy levels are listed in Table ST6. The energy levels
from HOMO−LUMO and their calculated band gap of the
ligands and the corresponding Eu(III) complexes are shown in a
pictorial way in Figure 11.
CIE Chromaticity Coordinates. CIE chromaticity coor-
dinates were calculated for ligands and respective Eu(III)
complexes from emission spectral data. The CIE color
coordinates for 1Eu, 2Eu, and 3Eu are shown to be x = 0.36, y
= 0.30; x = 0.42, y = 0.29; and x = 0.38, y = 0.29, respectively, in
The sensitized quantum efficiency (η ) can be determined from
the equation given below
sen
ηoverall
ηsen
=
ηLn
where ηoverall is the total quantum efficiency of the system
measured through the standard measurement procedure. The
corresponding radiative properties along with the branching
ratios were calculated for the complexes, and the same has been
tabulated in Table 4.
Electrochemical Properties. The HOMO and LUMO of
the complexes are measured by cyclic voltammetry analysis. It
gives significant redox properties that can be used to understand
the energy levels of the Eu(III) complexes as well as ligand
molecules. The observed 1Eu, 2Eu and 3Eu(III) complexes onset
potentials are 1.04, 1.15, and 0.90 V; their ligand potentials are
CHCl solution. In the solid state, the CIEs are shown to be x =
3
0.64, y = 0.35; x = 0.60, y = 0.35; and x = 0.32, y = 0.44 for 1Eu,
2Eu, and 3Eu, respectively. In addition, the ligand CIEs were also
measured in chloroform as well as solid phase. The CIE
coordinates in solutions are shown for L-Ph, L-Np, and L-TPA, x
= 0.17, y = 0.211; x = 0.18, y = 0.17; and x = 0.18, y = 0.24,
respectively. In the case of solid of L-Ph, L-Np, and L-TPA, x =
0.24, y = 0.33; x = 0.38, y = 0.49; and x = 0.20, y = 0.30,
1.23, 1.09, and 1.20 V, respectively (Figure 10). The reduction
waves with onset potentials data as well as their obtained band
gaps are listed in Table 5. The HOMO and LUMO energy levels
were calculated by using the well-reported equation by de Leeuw
4
8
et al. The lowest energy gap is observed for 1Eu in cyclic
voltammetry analysis, and 2Eu showed more energy gap.
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respectively (Figure 12). In the case of solution, the ligands L-Ph,
L-Np, and L-TPA showed CIE toward blue emission to pale blue
(nearly white). However, in the case of respective complexes,
they showed white emission in the CIE color coordinates. 1Eu
and 2Eu showed characteristic red emission. However, the 3Eu
complex showed greenish-yellow emission. Comparing with
solution and solid of the ligands, CIE color coordinates are
showing shifting toward nearly white location in the CIE diagram
from solution to solid. The same is indicating that the
conjugation of the ligands increases with their color tuning
toward white emission. It is clearly indirectly indicating that the
ligand modification can lead to pure white emission as a single
molecular complex. The CIE color coordinates (x, y) for the
ligands L-Ph, L-Np, and L-TPA in different solvents are tabulated
in Table ST7.
PMMA FILM OF EU(III) COMPLEXES
■
In order to understand the optical properties in the thin film, we
have investigated the optical properties of Eu complexes by
incorporating in PMMA matrix with different Eu complex
concentrations. All the complexes showed red color PL emission
except the TPA based complex. The TPA based complex
fortunately showed white light emission at 1% doping (Figure
1
3). The intensity ratio (asymmetric ratio) as well as
corresponding CIE color coordinates is interpreted in Table 6.
The PLQYs of the complexes were also measured to understand
the variance between the emission in different forms like
solution, solid, and thin film. PLQYs of the complexes, 1Eu, 2Eu,
and 3Eu, are shown in the case of pure thin film to be 1.44, 0.42,
and 0.35%, respectively. The extreme enhanced PLQYs of the
Figure 12. CIE color coordinates for the 1Eu, 2Eu, and 3Eu complexes
and respective ligands in solution as well as solid.
Figure 13. PMMA doped thin films emission spectra for the 1Eu, 2Eu, and 3Eu complexes in different dopant concentrations and their respective CIE
color coordinates (right side, down).
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Table 6. Intensity Ratios as Well as CIE Color Coordinates for
the Eu(III) Complexes in Different Concentration Ratio
Doped PMMA
Fabrication of LED with Eu Complexes. Light-emitting
diodes were fabricated by using a combination of a 395 nm
emitting InGaN chip with the complexes in different ratios like
1
:10 and 1:50 (Eu(III) complex:PMMA). The complexes were
characterization
I2/I1
1Eu
2Eu
3Eu
coated by matching the above ratio and dried for 6 h at beyond
ambient temperature (40 °C). Figure 15 shows emission of the
original naked InGaN LED with a 395 nm chip and emission
from the complexes under 20 mA forward-bias current. The
excitation source from the LED was fully observed by the
complex shown for 3Eu. The 3Eu complex showed yellowish
emission at 20 mA forward-bias in both concentrations. The
same was reflected in the case of solid also. However, in the case
of 1Eu and 2Eu complexes, the observed emission is close to
white emission. The obtained results were confirmed by CIE
color coordinates and shown for 1Eu are x = 0.28, y = 0.35
12.97 (pure)
11.46 (pure)
16.79 (0.1)
16.63 (0.5)
15.55 (1.0)
0.498 (pure)
0.297 (0.1)
0.315 (0.5)
0.355 (1.0)
0.416 (pure)
0.240 (0.1)
0.251 (0.5)
0.301 (1.0)
6.04 (pure)
7.67 (0.1)
3
+
(
% of Eu )
1
1
1
7.06 (0.1)
6.07 (0.5)
7.00 (1.0)
8.08 (0.5)
7.55 (1.0)
CIE
x
y
0.647 (pure)
0.530 (pure)
0.549 (0.1)
0.555 (0.5)
0.580 (1.0)
0.312 (pure)
0.295 (0.1)
0.297 (0.5)
0.304 (1.0)
0
0
0
.608 (0.1)
.583 (0.5)
.589 (1.0)
0.335 (pure)
0
0
0
.312 (0.1)
.304 (0.5)
.305 (1.0)
(1:10); x = 0.26, y = 0.33 (1:50); 2Eu are x = 0.26, y = 0.34
(1:10); x = 0.27, y = 0.39 (1:50); and 3Eu are x = 0.41, y = 0.52
(1:10); x = 0.46, y = 0.50 (1:50), respectively. In addition, the
complexes were observed in the Eu(III) complex doped PMMA
matrix. The Eu(III) complex doped with 0.5% in PMMA showed
a high value than the remaining percentage of complexes, which
are 30.1, 23.8, and 24.8% for 1Eu, 2Eu, and 3Eu, respectively.
Temperature Dependent Emission of the Eu Complex.
There have been reports on the use of coordinated Eu(III) β-
diketone complexes in temperature sensing as well as in laser
CIE of the obtained results are interpreted in Figure S24.
The above analysis include discussion about white emission
obtained by integrating the 395 nm emitting InGaN chip.
However, yellow emission was only obtained in the case of 3Eu
instead of pure white or red emission, due to the broad ligand
emission. Complex 3Eu has greenish yellow as well as red
spectral region range. Due to lack of blue emission in the range of
4
9,50
technologies.
To use the above synthesized dual character-
4
00−450 nm, there is no white emission observed. On the basis
of this criteria, we also coated with the blue LED with PMMA
1:10), and it showed white emission with CIE color coordinates
istic emissive Eu complex as a temperature sensing material, the
temperature dependent PL spectra of the complexes have been
monitored and the same is depicted in Figure 14. For the
explanation of temperature sensing behavior of the Eu(III)−
TPA complex, we have monitored the PL intensity at 612 nm
(
of x = 0.29, y = 0.37 (1:10) (Figure S25). The presence of red
emission from Eu(III) ion is very weak, and to improve the same,
the percentage of Eu complex (3Eu) was increased. The 3:10
ratio was made to achieve the bright white emission with
appropriate CIE color coordinates x = 0.30, y = 0.33 (Figure 16).
In addition, to achieve the white emission from the UV-
emitted LED, the ligand and Eu complex were conjugated in
different ratios. The L-Np and 3Eu were taken in 1:1, 3:6, 6:3,
1:9, and 1:15 ratios in the PMMA. Increasing with concentration
of the ligand in the PMMA, emission in the region of bluish green
increment was observed and CIE values are shifting toward the
blue region. To avoid the same, Eu complex concentration was
5
7
3+
(
i.e., the D → F electric dipole transition of the Eu ion)
0 2
versus various temperatures (from 20 to 90 °C) at an interval of
0 °C. The PL emission spectra reveal that the luminescence
1
intensity linearly decreases with increasing temperature; the
ligand emission also follows the same behavior and was found to
decrease constantly. In addition, the CIE color coordinates for
each of the emissions show a color transition from greenish blue
toward blue region (CIE diagram in Figure 14, right). These
obtained results are indicating that the currently synthesized
TPA-Eu complex can serve as a temperature sensor.
Figure 14. Temperature dependent emission spectra of Eu(TTA) TPA and their corresponding CIE color coordinates.
3
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Inorganic Chemistry
Article
Figure 15. Spectra A, B, and C are Eu(III) complexes coated on the 395 nm emitted LED chip. In the inset a is the original 365 nm emitted LED chip; aa
is under forward bias. b and c are coated with Eu(III) complexes, and bb and cc are with forward bias, respectively.
Figure 16. Emission spectra of Eu(III) complex with coated on the blue (450 nm) emitted LED chip. In inset a is the original 450 nm emitted blue LED
chip; aa is under forward bias. b is coated with Eu(III) complex, and bb is with forward bias, respectively (left side). The respective CIE of the 3Eu
complex (right side).
increased up to 1−15 ratio and the CIE was found to be x = 0.28,
y = 0.34 (Figure S26). Remaining CIE values are x = 0.26, y =
513 °C (second stage), and >515 °C (third stage). The major
decomposition of the Eu complexes is responsible for the
ancillary ligands present in the complex. The second thermal
decomposition is due to TTA molecules present in the Eu
complex. The minor thermal decomposition that has also been
observed in the region of 85−110 °C belongs to the loss of water
traces present in the complex. The final decomposition of the
complexes is due to the presence of a pyridyl moiety and
aromatic phenyl groups. As compared with 2Eu and 3Eu, 1Eu
showed that the end of thermal decomposition was found to be
0.51 (1:1); x = 0.19, y = 0.30 (3:6); x = 0.26, y = 0.42 (6:3); and x
=
0.22, y = 0.35 (1:9). Further, increment of the Eu complex
concentration may lead to bright white emission.
Thermal Study. The thermogravimetric analysis of the
complexes, 1Eu, 2Eu, and 3Eu, is measured from ambient
temperature to 800 °C under a nitrogen atmosphere (Figure 17).
The thermal decomposition of the complexes are shown in three
stages in the region of ∼134−294 °C (first stage, major), ∼300−
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Inorganic Chemistry
Article
1
13
The H and C NMR spectra of the ligands, powder X-ray
diffraction patterns of the complexes and the ligands,
detailed solvatochromism studies of the complexes,
emission spectra of the ligands L-Ph, L-Np, and L-TPA
in different solvents, lifetime of the Eu complexes as well as
ligands at different monitoring wavelengths, and HOMO,
LUMO energy levels of the ligands and their correspond-
ing complexes (PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: vsiva@nitrkl.ac.in. Tel: +91-661-2462654.
■
*
ORCID
Sivakumar Vaidyanathan: 0000-0002-2104-2627
Notes
The authors declare no competing financial interest.
Figure 17. Thermal decomposition analysis of the Eu complexes; the
inset shows the end point of the thermal decomposition.
ACKNOWLEDGMENTS
■
The authors gratefully thank the Board of Research in Nuclear
Sciences (BRNS), Department of Atomic Energy (DAE),
INDIA, the Novel Research Board (NRB), and the Defence
Research and Development Organisation (DRDO) for the
financial support. We are also thankful to the Fund for
Improvement of S&T Infrastructure (FIST), Department of
Science & Technology (DST), for support of the mass
instrumentation.
higher, which is shown in Figure 17, inset. However, the thermal
decomposition of the Eu complexes is shown to be more than
1
85 °C. These results indicate that the current synthesized Eu
complexes are potential candidates.
CONCLUSIONS
In the summary, the mononuclear Eu(III) metal ion with tris-β-
diketones and bipyridyl ancillary based ligands (1L-Ph, 2L-Np,
■
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