Controlled Energy Transfer from a Ligand to an EuIII Ion: A Unique Strategy to Obtain Bright-White-Light Emission and Its Versatile Applications

Article abstract of DOI:10.1021/acs.inorgchem.7b01255

A new diphenylamine-functionalized ancillary-ligand-coordinated europium(III) β-diketonate complex showed incomplete photoexcitation energy transfer from a ligand to a Eu^III ion. A solvatochromism study led to a balancing of the primary colors to obtain single-molecule white-light emission. Thermal-sensing analysis of the europium complex was executed. The europium complex, conjugated with a near-UV-light-emitting diode (395 nm), showed appropriate white-light-emission CIE color coordinates (x = 0.34 and y = 0.33) with a 5152 K correlated color temperature.

Full text of DOI:10.1021/acs.inorgchem.7b01255

Communication
pubs.acs.org/IC
Controlled Energy Transfer from a Ligand to an EuIII Ion: A Unique
Strategy To Obtain Bright-White-Light Emission and Its Versatile
Applications
Rajamouli Boddula, Kasturi Singh, Santanab Giri, and Sivakumar Vaidyanathan*
Department of Chemistry, National Institute of Technology, Rourkela, Odisha 769008, India
S
* Supporting Information
solution phase, and solid-state white-light emission is quite rare.
ABSTRACT: A new diphenylamine-functionalized ancil-
lary-ligand-coordinated europium(III) β-diketonate com-
plex showed incomplete photoexcitation energy transfer
from a ligand to a EuIII ion. A solvatochromism study led
to a balancing of the primary colors to obtain single-
molecule white-light emission. Thermal-sensing analysis of
the europium complex was executed. The europium
complex, conjugated with a near-UV-light-emitting diode
(395 nm), showed appropriate white-light-emission CIE
color coordinates (x = 0.34 and y = 0.33) with a 5152 K
correlated color temperature.
Very recently, we have explored triphenylamine (TPA)-
functionalized imidazole−phenanthroline based, a new bipolar
ligand for a monochromatic red-light-emitting europium(III)
complex.10 Further, efforts have been made to obtain
monochromatic red emission as well as investigate the effect of
functionalization [extended the TPA moiety with diphenylamine
(DPA)] on the luminescence properties of the complex in detail,
where the ET process from a ligand to a EuIII metal ion plays a
vital role. The DPA-decorated phenanthroline−fluorene−TPA
ligand (Phen−Fl−TPA−DPA) and its corresponding europium-
(III) β-diketonate complex [Eu(TTA)3−Phen−Fl−TPA−DPA,
where TTA = thenoyltrifluoroacetone] have been synthesized.
The fluorene moiety was used to design the europium complex
because its strong π−π* absorption will improve the
morphological properties and photostability of the complex.
The presence of DPA moieties in the ligand widen the absorption
outline and can act as light-harvesting units. TTA can act as an
antenna, and the presence of fluorine in the ligand can decrease
the vibrational quenching and increase the decay time.11 In the
presently studied europium(III) complex, ET to a central metal
ion from a ligand and an antenna, is expected (Figure 1). The
ligand can act as both sensitizer and a yellow-emitting source.
he new single-organic-molecule- or molecular-complex-
T
based white-light-emitting sources are attractive owing to
their potential applications in full-color smart displays and
lighting sources [including white-organic-light-emitting diodes
and solid-state lighting (SSL)].1 In general, white light can be
generated by mixing three primary colors, to cover the entire
visible spectrum. At present, several organic fluorophores having
the capacity of emitting individual red−blue−green (RGB) color
are known. However, white light generated by a single molecule
(a single-component approach) has several advantages over that
of simple RGB mixing (multicomponent emitters).2 The benefits
include improved stability, stable Commission International de
I’Eclairage (CIE) color coordinates, and a simple fabrication
process.3 Stable white-light photo- and electroluminescence
released in a single-molecule dyad have been documented.4
White-light creation by aggregation-induced emission in a single
organic molecule has also been reported.5 An iridium-based
molecular complex, which emits white light (from 440 to 800 nm
in the spectral window), is known.6 When the sensitizing/
energy-harvesting capability of a IrIII ion to lanthanides is used, an
iridium−europium dyad has been used to release white light
(bluish-green emission from the IrIII emissive center and red
emission from the EuIII metal center).7 However, generating
white-light emission from a single-molecular complex is still a
stimulating research manifold. The ever-increasing demand of
the cumulative global energy crisis is reducing energy sources,
and it can be overcome by highly energy-efficient lighting
systems (SSL) that can help to conserve energy and reduce the
overall lighting costs.8
Figure 1. Chemical structure of europium(III) complex.
The detailed experimental procedure for the synthesis of the
ligand and corresponding europium(III) complex is given in
Scheme S1 and Figures S1−S10, and their thermal stabilities
were investigated (Figure S12). The amorphous nature of the
complex was confirmed by X-ray diffraction (Figure S11). The
UV−visible absorption spectra of the ligand and complex were
carried out in solution (CHCl3, 1.0 × 10−4 mol L−1), thin film,
and solid state (Figure S13). The absorption spectrum of the
ligand shows absorption ranging from 240 to 450 nm with λmax
values at 306 and 280 nm (attributed to the π → π* transitions of
Recently, ligand-based incomplete/partial energy transfer
(ET) to a EuIII metal center leading to white-light generation
has become an attractive research task.9,1d However, white-light
emission from a single lanthanide complex is limited in its
Received: May 16, 2017
© XXXX American Chemical Society
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the aromatic moieties). This was verified theoretically by the
time-dependent density functional theory (TD-DFT) (Figure
S14). Similarly, the absorption studies of the europium(III)
complex and Eu(TTA)3·2H2O show peak maxima at 340 and
296 nm and at 340 and 275 nm, respectively. The europium(III)
complex shows comparable peaks, which are observed for the
ligand as well as the Eu(TTA)3 moiety (Table S6), suggesting
presence of ligand in the same.
The dual (yellow and red) emission behavior was used to
generate white light from a single molecule. If color balancing
among the ligand broad (BG) emission and the EuIII (R)
emission will be proper, white light can be generated from a
single-molecule system. In the emission spectra for a europium-
(III) complex at different concentrations (1, 10, 20, and 75 ×
10−5 mmol) using chloroform, no distinguished result was
observed, while for a red shift, an increase in the emission
intensity was observed (ligand emission) by exciting with
different excitation wavelengths for the europium complex
(Figures S24−S26). On basis of ligand emission behavior in
different solvents, emission spectroscopy of the europium(III)
complex in different solvents has also been carried out, and the
strange responses were perplexing. These perplexing outcomes
made it extremely difficult to obtain white-light emission with the
appropriate CIE [0.31, 0.32; correlated color temperature
(CCT) = 6558 K; Figure S23 and Table S3]. The color gamut
observed from toluene (Figure 2b) was due to the proper balance
between primary colors.
From solvatochromism analysis, it was found that the
presently synthesized europium(III) complex is acting as a
single white-light-emitting component with tunable emission
color with changes in the solvent. An antenna (ligand) present in
the complex is responsible for partial sensitization to obtain
white-light emission. This is due to less energy difference
between the T1 state of the ligand and EuIII ion excited state. It
was expected that the free rotation of the phenyl rings of the DPA
molecule is sufficient in a less polar solvent (less density), which
leads to a decrease in the excited-state energy level of the ligand.
In addition, under the same conditions, theoretical analysis was
performed to understand the energy levels by arresting the C−C
bond of DPA [carboxybenzyl (CBZ)]. From the obtained
theoretical results, the optimized CBZ moiety with a ligand
showed high singlet (25707 cm−1, 3.18 eV) as well as triplet
(21277 cm−1, 2.63 eV) energy levels, which was greater than the
DPA ligand.15b More polar solvents like acetonitrile (ACN),
N,N-dimethylformamide, and dimethyl sulfoxide showed almost
pure red emission, revealing the existence of a sufficient energy
gap (complete ET). This clearly indicates that the solvent plays a
major role and the aromatic ring in toluene is more responsible
for white-light emission. Morever, to generate bright-white-light
emission (CIE, x = 0.33, y = 0.33), the corresponding europium
complex was excited with different exciting wavelengths, and 380
nm found to be proper excitation wavelength (CIE; 0.34, 0.33;
CCT = 5152 K; Figures S27−S29).
The photoluminescence (PL) emission of the ligand was
extended from 400 to 700 nm (covering the entire visible spectral
window) with a maxima at 517 nm at different excitation
wavelengths (285, 305, and 365 nm) in CHCl3 (Figure S17). The
ligand emission in different solvents was monitored to explain the
broadness observed in the ligand emission spectra (Figures S33
and S34 and Table S5). Among all of the solvents, toluene
showed a distinct emission behavior. In the case of the
europium(III) complex, a characteristic red emission at 612
nm from the EuIII ion was shown; emission belonging to the
ligand was also observed from 400 to 600 nm with a peak
maximum at 517 nm. The partial ET from a ligand to a central
metal ion is responsible for the obtained orange-red emission,
instead of pure red (Figure S20). The intensity ratio (2:1) of the
red emission was higher than that of the yellow emission from the
ligand. According to Latva et al., investigations, i.e., the difference
between the ligand (3ππ*) and the europium excited state (5D0),
should be >2500 cm−1 for efficient ET.12 The singlet and triplet
energy levels of TTA located at ∼25164 (3.12 eV) and 18954
cm−1 (2.35 eV)13 and the EuIII-metal-ion radiative excited state
5D0 energy level located at 17500 cm−1 are known (Table S4).14
However, to understand the process of ET in the europium(III)
complex, it is essential to know the exact location/position of the
singlet (S1) and triplet (T1) energy levels of the ligand.15
To well understand the reason behind the complete or partial
ET, the excited-state energy levels were calculated by employing
TD-DFT methodology embedded in the G09 program,16 and
there is strong supporting evidence that the singlet and triplet
energy levels are located at 23256 (2.88 eV) and 19960 cm−1
(2.47 eV), respectively (Figure 2a and Table S4 and Figure S30).
To study solvent polarity effect on the emission performances
of white-light emission, the solvatochromism studies were
executed using a mixture of solvents [polar (ACN; pure EuIII
emission) and less polar (toluene; both ligand and EuIII
emission)] by different ratios from 90:10 to 10:90, respectively
(Figure S35a−j). The intensity of the EuIII ion, 612 nm emission
peak started to increase from 90:10 to 10:90 toluene−ACN, and
the broad emission at the 450−600 nm range started to decrease.
The intense EuIII ion red emission was higher in the pure solvent
compared to that of a mixture of solvents (CIE shown in Table
S12). The decrease of the EuIII emission intensity as well as the
increase in the ligand emission was observed by changing the
mixture from 90:10 to 10:90 toluene−ethyl acetate (EtOAc)
(Figure S36a−j), respectively (CIE shown in Table S10). The
above study indicates that the solvent polarity plays a vital role.
The characteristic dual-emission behavior of the complex extend
their applications for luminescent temperature sensors. The PL
measurements were performed in the 283−358 K temperature
Figure 2. (a) Ligand-to-EuIII-metal-ion ET process. (b) Europium(III)
complex emission in different solvents and white-light emission in
toluene with a CIE color gamut.
This was also confirmed by emission spectra of the gadolinium
complex at 77 K (Figure S46). The difference between the triplet
excited state and EuIII-metal-ion-excited 5D0 energy level was less
than 2500 cm−1 and also singlet excited level was <25000 cm−1.
The ligand singlet- and triplet-state energy level,
ΔE(1ππ*−3ππ*), difference was found to be 3296 cm−1, which
is less than 5000 cm−1. These lower energy levels of the ligand
suggest that the excited-state energy can be back-transferred
from the EuIII excited state (5D0) to the ligand.17 Hence, it is clear
that both the ligand (yellow) and EuIII ion (red) emissions were
observed in the PL spectra of the complex.
B
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Communication
range. It is well documented that the europium(III) complexes
with organic ligands also showed high-temperature sensitivity,
particularly with β-diketonates.18 In the present study with
increasing temperature, the ligand emission from the complex
was found to be enhanced, while the intensity of the 612 nm peak
decreased and a change in the emission color was observed from
orange-red (283 K) to near-white (303 K) to green (358 K)
(Figures 3A and S39). The temperature-dependent PL emission
from ligand to EuIII metal ion is efficient only up to 380 nm and
partial for 390 nm excitation. These observations reveal that
ligand emission from the europium complex supports the bright-
white-light emission. In the case of the solid, no such type
(solution) of lifetime difference observation was detected
(Figures S43 and S44). The obtained lifetime results (Table
S9) clearly support ligand and EuIII emission in solution and no
ligand emission (minute) contribution along with EuIII red
emission in the solid.
The ligand exhibits a distinct excited-state rotation con-
formation in different polarity solvents, which might be possible
for polarity-dependent fluorophores. Therefore, white-light
emission in toluene may come from a mixture of either several
excited states or even ground states. Fortunately, weak ligand
emission was observed from the complex in solid form, taken as
an advantage to fabricate the white-light-emitting diode (LED).
The white-LED was fabricated by integrating the complex with
395 nm emitting UV-LED (InGaN) chips under 20 mA forward
bias (Figure 3B). The excitation source from LED was fully
observed by the europium complex shown at different
concentrations [1:10 and 1:50 Eu3+−PMMA). The fabricated
LED showed white-light emission (PLQY 0.68%) with CIE
values of x = 0.34 and y = 0.33, which are close to the National
Television Standard Committee (NTSC) standard value.
In conclusion, the newly synthesized europium(III) complex is
acting as a single-component white-light emitter, and it is due to
the balancing of the primary colors. The combined experimental
and TD-DFT calculations support the ET mechanism. The
temperature-dependent PL emission color tuning will certainly
give directions for using the presently studied europium complex
as a temperature sensor. In addition, the complex conjugated
with LED (395 nm) showed bright-white-light with appropriate
CIE color coordinates. All of these observations have important
implications in the design of single-molecule white-light emissive
complex and its ability as a temperature sensor.
Figure 3. (A) Europium(III) complex with different temperatures.
Inset: Ligand emission from the europium(III) complex. (B) White-
light emission from the LED (395 nm): (a) LED without coating; (aa)
LED with forward bias. (b) 1:10 and (c) 1:50 europium(III)-complex-
coated LEDs; (bb and cc) with forward bias.
spectra state that the complex exhibits color tunability. The
emission profile in different wavelengths with specific temper-
atures and their respective CIE color coordinates were
investigated and are presented in Figure S38 and Table S11. A
comparison study was discussed in the Supporting Information.
It is worth noting that, without DPA, functionalization on the
Phen−Fl−TPA ligand in toluene showed emissions ranging from
380 to 530 nm with λmax = 423 nm, blue region.9 When the DPA
moiety was connected to TPA (TPA−DPA), the emission
wavelength was shifted from 300 to 630 nm with a maximum at
480 nm. However, after insertion of the EuIII metal ion into the
TPA−DPA ligand; the emission spectrum was found to cover a
broad range from 410 to 630 nm (Figure S37). However, similar
results were also observed in other solvents (tetrahydrofuran,
EtOAc, dichloromethane, trichloromethane). A primary color
balance was perceived in only toluene, which leads to white-light
emission. To the best of our knowledge, we observed white-light
emission from the europium(III) complex with an imidazole-
based neutral ligand for the first time. It is also notable that the
above-mentioned observation influences the PL quantum yield
(PLQY) of the europium(III) complex and is found to be 15.3%.
The PLQY indirectly supports the partial ET from a ligand to a
central EuIII metal ion. The experimental outcome was supported
by the electrochemical (Figure S21 and Table S2), theoretical,
diffuse-reflectance spectroscopy (Figure S15), and PL lifetime
(Figure S22) studies.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01255.
Experimental section, NMR, FT-IR, LC-MS, XRD, DSC-
TGA, UV, PL, CIE, electrochemical and DFT analysis
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vsiva@nitrkl.ac.in. Tel: +91-661-2462654.
To further understand the ET process and emission
contribution (ligand and EuIII ion), PL lifetime measurements
with different monitoring wavelengths [517/500 nm (solution/
solid) belong to ligand emission and 612 nm (solution/solid)
belongs to EuIII ion emission in the complex] were executed. In
addition, different excitation wavelengths (spectral window,
300−400 nm with an interval of 10 nm) were used for the
experimental procedure. The lifetimes of the complex in solution
with 517 and 612 nm as a monitoring wavelength and λexc = 300−
360 nm have shown similar outcomes. However, beyond (>360
nm excitation) where the lifetime values started decreasing,
found at 390 nm, there was much less contribution (λem = 612
nm, shown in Figures S41 and S42). This indicates that the ET
ORCID
Rajamouli Boddula: 0000-0003-0414-715X
Santanab Giri: 0000-0002-5155-8819
Sivakumar Vaidyanathan: 0000-0002-2104-2627
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Board of Research in Nuclear Sciences, DAE and
Naval Research Board, DRDO, India, for funding and are grateful
to the reviewers for their suggestions.
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