Photoluminescent report on red light emitting europium(III) complexes with heterocyclic acid

Article abstract of DOI:10.1080/00387010.2020.1736099

Five luminescent europium (III) carboxylate complexes have been synthesized by using ligand 3-isopropylpyrazole-5-carboxylic acid as primary ligand and 4,4’-dimethyl-2,2’-bipyridyl, 2,2’-bipyridyl, 5,6-dimethyl-1,10-phenanthroline and 1,10-phenanthroline as secondary ligands and characterized through various techniques. These complexes exhibit excellent thermal stability and characteristic europium centered photoemission spectra under the excitation of ultraviolet light. Luminescence decay curves, Judd–Ofelt analysis, internal quantum efficiency and energy transfer mechanism have also been discussed. The color coordinates and color purity are calculated to investigate red emission of the complexes. The study results reveal that these complexes can be potentially used as red light emitting materials in various optoelectronic devices.

Full text of DOI:10.1080/00387010.2020.1736099

Spectroscopy Letters
An International Journal for Rapid Communication
ISSN: 0038-7010 (Print) 1532-2289 (Online) Journal homepage: https://www.tandfonline.com/loi/lstl20
Photoluminescent report on red light emitting
europium(III) complexes with heterocyclic acid
Priyanka Dhankhar, Manisha Bedi, Jyoti Khanagwal, Vinod B. Taxak,
Satyender P. Khatkar & Priti Boora Doon
To cite this article: Priyanka Dhankhar, Manisha Bedi, Jyoti Khanagwal, Vinod B. Taxak,
Satyender P. Khatkar & Priti Boora Doon (2020): Photoluminescent report on red light
emitting europium(III) complexes with heterocyclic acid, Spectroscopy Letters, DOI:
10.1080/00387010.2020.1736099
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SPECTROSCOPY LETTERS
https://doi.org/10.1080/00387010.2020.1736099
Photoluminescent report on red light emitting europium(III) complexes with
heterocyclic acid
Priyanka Dhankhar, Manisha Bedi, Jyoti Khanagwal, Vinod B. Taxak, Satyender P. Khatkar and
Priti Boora Doon
Department of Chemistry, Maharshi Dayanand University, Rohtak, India
ABSTRACT
ARTICLE HISTORY
Received 24 October 2019
Accepted 25 February 2020
Five luminescent europium (III) carboxylate complexes have been synthesized by using ligand
3-isopropylpyrazole-5-carboxylic acid as primary ligand and 4,4’-dimethyl-2,2’-bipyridyl, 2,2’-bipyr-
idyl, 5,6-dimethyl-1,10-phenanthroline and 1,10-phenanthroline as secondary ligands and charac-
terized through various techniques. These complexes exhibit excellent thermal stability and
characteristic europium centered photoemission spectra under the excitation of ultraviolet light.
Luminescence decay curves, Judd–Ofelt analysis, internal quantum efficiency and energy transfer
mechanism have also been discussed. The color coordinates and color purity are calculated to
investigate red emission of the complexes. The study results reveal that these complexes can be
potentially used as red light emitting materials in various optoelectronic devices.
KEYWORDS
Energy transfer process;
Judd–Ofelt analysis;
luminescence; ther-
mal stability
Introduction
almost independent of chemical environment,
attributed to deeply shielded nature of 4f subshell
by overlying 5s and 5p subshells.[11,12] Hence, each
lanthanide ion exhibits its characteristics emission
independent of nature of surrounding ligands. But,
these ions are not able to show intense lumines-
cence themselves due to spin and parity forbidden
nature of 4f–4f electronic transitions.[13]
Therefore, intense luminescence of practical utility
in lanthanide complexes is made possible by so
called Antenna effect.[14] The organic ligand
absorbs UV-Vis light photons, then the energy is
transferred from excited levels of ligand to resonat-
ing levels of lanthanide ion, which subsequently
decay by emitting characteristics light. The
aromatic carboxylic acids and b-diketones as main
ligand have been employed for construction of
luminescent rare earth complexes. The rare earth
complexes with b-diketones exhibit excellent
photoluminescent properties; however, they suffer
from the problem of poor thermal and luminescent
stability. Nowadays, carboxylate ligands are pre-
ferred for synthesis of photoluminescent rare earth
complexes due to their strong bonding ability,
The rare earth elements have remained as puzzle
for many decades. However, with advancement in
research tools and technologies, the inherent prop-
erties of rare earth elements and related com-
pounds have been explored up to large extent.
Now, they are emerged as enablers of modern day
technology. Research work related to coordination
complexes of lanthanide ions with organic ligands
have attracted considerable attention, partially due
to their easy synthesis and mainly to their promis-
ing utility in wide range of applications such as
photoluminescent devices, laser technology, bio-
logical and chemical sensing, light emitting diodes
as well as fluoroimmunoassays.[1–7] Luminescent
lanthanide complexes act as effective emitter in
these technologies, owing to peculiar photophysi-
cal properties of lanthanide ions like spectra purity,
ligand centered stoke shift, long luminescence life-
time and theoretically high internal quantum effi-
ciency.[8–10] The reason for these rare
characteristics is their atomic structure. The elec-
tronic transitions occurring in 4f subshell are
CONTACT Priti Boora Doon
pritiboora@gmail.com
Department of Chemistry, Maharshi Dayanand University, Rohtak, Haryana 124001, India.
Supplemental data for this article can be accessed here.
ß 2020 Taylor & Francis Group, LLC

2
P. DHANKHAR ET AL.
diverse coordination modes and potential sensitiza-
tion ability for rare earth ions. The rare earth carb-
from commercial sources were used in synthesis of
ligand and complexes. The content of carbon,
hydrogen and nitrogen elements were determined
using Perkin Elmer 2400 elemental analyzer. The
percentage of europium ion in complexes was cal-
culated by EDTA (ethylenediaminetetraacetic
acid) titration. Fourier transform infrared spectra
(FT-IR) of all samples were recorded on Perkin
Elmer Spectrum 400 spectrometer, in spectral
range of 400–4000 cmꢀ1 with KBr discs.
Morphological features of the gold-coated samples
were determined with Jeol JSM-6510 scanning
electron microscope (SEM) and EDAX spectrum
was recorded on Ametek EDAX using Apex soft-
ware. 1H-NMR spectra of samples in dimethylsulf-
oxide (DMSO) solution were obtained on Bruker
Avance II 400 NMR spectrometer. A Shimadzu
UV-3600 plus spectrophotometer was used to
record absorption and diffuse reflectance spectra.
The thermogravimetric analysis of complexes was
carried out under nitrogen atmosphere with SDT
Q600 at temperature rate of 20 ꢁC/min up to
850 ꢁC. The luminescence studies of all complexes
were measured with Hitachi F-7000 fluorescence
spectrophotometer whose slit width for excitation
and emission was 2.5 nm.
oxylate
complexes
display
excellent
photoluminescent feature as well as high thermal
stability. Trivalent europium, terbium and thulium
ions emit visible light in red, green and blue region,
respectively.[15] These three colors are primary
requirement for full color display devices.[16]
In this paper, an aromatic heterocyclic acid lig-
and 3-isopropylpyrazole-5-carboxylic acid (HIPA)
is synthesized. The adjacent –NH and carboxylate
groups of ligand 3-isopropylpyrazole-5-carboxylate
(IPA) can strongly coordinate to metal ion via
formation of five membered ring (chelation
effect). Five complexes Eu(IPA)3.2H2O (C1),
Eu(IPA)3.dmbipy (C2), Eu(IPA)3.bipy (C3),
Eu(IPA)3.dmph (C4) and Eu(IPA)3.phen (C5) of
3-isopropylpyrazole-5-carboxylic acid (HIPA) and
secondary ligands 4,40-dimethyl-2,20-bipyridyl
(dmbipy), 2,20-bipyridyl (bipy), 5,6-dimethyl-1,10-
phenanthroline (dmph) and 1,10-phenanthroline
(phen) with europium metal ion have been synthe-
sized using solution precipitation method. The
synthetic procedure of ligand (HIPA) and euro-
pium carboxylates complexes (C1–C5) have been
reported. The complexes are characterized by
elemental analysis, proton nuclear magnetic reson-
ance (1H-NMR), energy-dispersive X-ray spectros-
copy (EDAX), scanning electron microscope
(SEM), Fourier transform infrared (FT-IR), ultra-
violet-visible absorption, diffuse reflectance,
photoluminescent (PL) spectroscopy and thermog-
ravimetric analysis. The chromaticity coordinates
and color purity of complexes have also been cal-
culated to investigate the characteristic red emis-
sion of europium (III) complexes. Judd–Ofelt
parameters and internal quantum efficiency of the
complexes have been determined using emission
spectral data. The energy transfer mechanism of
complexes has been proposed for better under-
standing of sensitization phenomenon.
Synthesis of 3-isopropylpyrazole-5-carboxylic acid
ligand (HIPA)
The ligand 3-isopropylpyrazole-5-carboxylic acid
(HIPA) was prepared via Claisen condensation of
diethyl oxalate with isopropylmethylketone to
give ethyl-5-methyl-2,4-dioxohexanoate (1), then
subsequent cyclization of (1) with hydrazine to
give 3-isopropylpyrazole-5-carboxylate.[17,18] The
complete procedure is illustrated below:
Isopropylmethylketone (20 mmol) was added
slowly to solution of potassium tert-butoxide
(20 mmol) in ethanol, followed by dropwise add-
ition of ethanolic solution of diethyloxalate
(15 mmol) to the mixture and the resulting mix-
ture was agitated on magnetic stirrer for 3.5 hr at
room temperature. The reaction mixture was
acidified with 2 M HCl solution and then cooled
the mixture on ice bath for 1 hr. The precipitates
were separated by vacuum filtration, washed
with chilled water and dried to give ethyl
5-methyl-2,4-dioxohexanoate (1). Then, 1:1 molar
Materials and methods
Experimental information
Highly pure europium and gadolinium nitrates
were purchased from Merck and used as received.
All other reagents and solvents of analytic grade

SPECTROSCOPY LETTERS
3
ratio of hydrazine and (1) in acetic acid was
stirred for 2 hr at room temperature and resulting
mixture was treated with hot 2 M NaOH solution
to give precipitates of ligand (HIPA) which were
filtered, recrystallized with ethanol and finally
dried in vacuum desiccator. The progress of reac-
tion was checked with thin-layer chromatography
(TLC). Yield 59%; IR: cmꢀ1 3283 (s), 3146 (s),
2371 (w), 1714 (s), 1490 (m), 1344 (m), 1241 (s),
1121 (m), 1019 (m), 943 (m), 847 (m), 788 (m),
2H, NH, –COOH), 6.48 (s, 1H, CH ring), 2.97
(m, 1H, CH isopropyl), 1.20 (s, 6H, –CH3), ppm.
Synthesis of complexes
The aqueous solution of Eu(NO3)3.6H2O
(1 mmol) was treated with threefold excess of
ethanolic solution of HIPA (3 mmol) and the
resulting mixture was thoroughly mixed on mag-
netic stirrer. The aqueous solution of NaOH was
added drop wise to adjust the pH of mixture by
1
531 (w), H-NMR (DMSO, 400 MHz): d 12.92 (s,
Figure 1. Synthetic routes and chemical structures of ligand 3-isopropylpyrazole-5-carboxylic acid (HIPA) and europium
(III) carboxylate complexes. The ligand HIPA coordinates to europium metal ion after losing proton at neutral pH.

4
P. DHANKHAR ET AL.
6–7. After constant stirring of 3 hr, off white pre-
cipitates of C1 binary complex were appeared
which were isolated by vacuum filtration, washed
with ethanol and water respectively, and then
dried in desiccators.[19] Similarly, ternary com-
plexes C2–C5 were synthesized by adding etha-
nolic solution of dmbipy, dipy, dmph and phen
(1 mmol) respectively, to the reaction mixture of
europium(III) nitrate (1 mmol) and ligand HIPA
(3 mmol) prior to adjustment of pH of the solu-
tion. Binary complex of gadolinium ion with lig-
and HIPA [Gd(IPA)3.2H2O] is synthesized by
similar method as of C1 to calculate the triplet
state of ligand. The synthetic routes of ligand
HIPA and europium (III) complexes C1-C5 are
shown in Fig. 1.
(s), 1568 (m), 1507 (w), 1421 (m), 1301 (s), 1250
(s), 1020 (m), 846 (m), 805 (m), 770 (m), 660
(w), 565 (w), 496 (w), 445 (w).
Results and discussion
Structural analysis
The outcome of EDAX spectroscopy confirms
presence of carbon, nitrogen, oxygen and euro-
pium elements in the complexes (Fig. S1). The
results of CHN analysis and Eu(III) ion titration
are consistent with theoretical calculated values,
signify that the composition of ligand and com-
plexes are according to their speculated structural
formulas. The elemental analytical data of ligand
HIPA and its europium complexes are enlisted in
Table 1. SEM micrographs of the complexes
points toward homogenous distribution of par-
ticles (Fig. S2). All the complexes are insoluble in
water, ethanol, and chloroform.
Eu(IPA)3.2H2O (C1): light pink powder, yield
71%; IR: cmꢀ1 3450 (s), 3180 (s), 3090 (s), 2961
(s), 2371 (w), 1644 (s), 1507 (w), 1411 (m), 1301
(s), 1250 (s), 1019 (m), 840 (m), 814 (m), 780
(m), 651 (w), 566 (w), 497 (w), 428 (w).
The main FT-IR data of free ligand and its
complexes are given in Table 2. The spectra of
complexes show characteristic ligand centered
bands with appropriate changes and analogical to
each other, points toward similar structure of the
complexes. The spectrum of ligand HIPA exhibits
two peaks at 3283 cmꢀ1 and 3146 cmꢀ1 attributed
to stretching vibrations of N–H and O–H bonds,
respectively. The stretching mode corresponding
to vibrations of C¼O bond of carboxylic group
appears at 1714 cmꢀ1. The peaks at 1490 cmꢀ1
and 1340 cmꢀ1 are associated with stretching
vibration of C¼N and C–N bonds in pyrazole
ring respectively. The band at 1121 cmꢀ1 is
assigned to N–N stretching vibrations in pyrazole
ring. In FT-IR spectra of complexes, the band at
3283 cmꢀ1 is shifted to 3175–3180 cmꢀ1 while
Eu(IPA)3.dmbipy (C2): light pink powder,
yield 73%; 3175 (s), 3099 (s), 2970 (s), 2362 (w),
1644 (s), 1558 (m), 1498 (w), 1413 (m), 1295 (s),
1250 (s), 1019 (m), 840 (m), 814 (m), 788 (m),
651 (w), 566 (w), 498 (w), 446 (w).
Eu(IPA)3.bipy (C3): light pink powder, yield
75%; 3176 (s), 3099 (s), 2970 (s), 2362 (w), 1644
(s), 1558 (m), 1507 (w), 1421 (m), 1310 (s), 1250
(s), 1027 (m), 856 (m), 822 (m), 771 (m), 660
(w), 567 (w), 497 (w), 437 (w).
Eu(IPA)3.dmph (C4): Cream powder, yield
70%; 3175 (s), 3090 (s), 2970 (s), 2371 (w), 1644
(s), 1568 (m), 1507 (w), 1421 (m), 1301 (s), 1250
(s), 1019 (m), 847 (m), 805 (m), 771 (m), 660
(w), 566 (w), 497 (w), 446 (w).
Eu(IPA)3.phen (C5): Cream powder, yield
74%; 3177 (s), 3091 (s), 2970 (s), 2371 (w), 1644
Table 1. Elemental analytical data of ligand 3-isopropylpyrazole-5-carboxylic acid (HIPA), binary complex (C1) of europium ion
with ligand 3-isopropylpyrazole-5-carboxylate (IPA) and ternary complexes (C2-C5) with primary ligand IPA and secondary ligand
4,4’-dimethyl-2,2’-bipyridyl (dmbipy), 2,2’-bipyridyl (bipy), 5,6-dimethyl-1,10-phenanthroline (dmph) and 1,10-phenanthroline
(phen), respectively (weight %).
C(%) found
(cal.)
H(%) found
(cal.)
N(%) found
(cal.)
Eu(%) found
(cal.)
Compound
HIPA
54.52 (54.54)
38.94 (38.95)
49.83 (49.81)
48.50 (48.50)
51.25 (51.28)
50.03 (50.06)
6.48 (6.49)
4.77 (4.79)
4.88 (4.90)
4.55 (4.56)
4.77 (4.76)
4.41 (4.42)
18.20 (18.18)
12.99 (12.98)
14.10 (14.08)
14.64 (14.60)
13.68 (13.67)
14.17 (14.15)
–
C1 [Eu(IPA)3.2H2O]
C2 [Eu(IPA)3.dmbipy]
C3 [Eu(IPA)3.bipy]
C4 [Eu(IPA)3.dmph]
C5 [Eu(IPA)3.phen]
23.50 (23.48)
19.08 (19.11)
19.79 (19.81)
18.57 (18.55)
19.22 (19.21)
C: carbon; H: hydrogen; N: nitrogen; Eu: europium.
The experimental elemental composition of complexes is well matched with calculated (cal.) composition.

SPECTROSCOPY LETTERS
5
Table 2. Characteristics infrared spectral bands (cmꢀ1) of ligand 3-isopropylpyrazole-5-carboxylic acid (HIPA), binary complex (C1)
of europium ion with 3-isopropylpyrazole-5-carboxylate (IPA) and ternary complexes (C2-C5) with primary ligand IPA and second-
ary ligand 4,40-dimethyl-2,20-bipyridyl (dmbipy), 2,20-bipyridyl (bipy), 5,6-dimethyl-1,10-phenanthroline (dmph) and 1,10-phenan-
throline (phen), respectively.
ꢀ
ꢀ
Compound
ꢀN–H
ꢀOH
ꢀC¼O
ꢀas(COO
ꢀs(COO
ꢀC¼N
ꢀEu–N
ꢀEu–O
)
)
HIPA
3283 (s)
3180 (s)
3175 (s)
3176 (s)
3175 (s)
3175 (s)
3146 (s)
3450 (w)
1714
–
–
1490 (m)
1507 (m)
1558 (m)
1558 (m)
1568 (m)
1568 (m)
–
–
C1[Eu(IPA)3.2H2O]
C2 [Eu(IPA)3.dmbipy]
C3[Eu(IPA)3.bipy]
C4[Eu(IPA)3.dmph]
C5 [Eu(IPA)3.phen]
–
–
–
–
–
1644 (s)
1644 (s)
1644 (s)
1644 (s)
1644 (s)
1421 (m)
1413 (m)
1421 (m)
1421 (m)
1421 (m)
566 (w)
566 (w)
567 (w)
566 (w)
565 (w)
428 (w)
446 (w)
437 (w)
446 (w)
445 (w)
–
–
–
–
s: strong; m: medium; w: weak; ꢀ: frequency (cmꢀ1); s: symmetric stretch; as: asymmetric stretch; C: carbon; H: hydrogen; O: oxygen; N: nitrogen;
Eu: europium; ¼: double bond; –: single bond.
band at 3146 cmꢀ1 get vanished, pointing that
NH and carboxylic groups of ligand are involved
in coordination with europium ion. The band at
1714 cmꢀ1 is disappeared while two new bands at
1644 cmꢀ1 and 1413–1421 cmꢀ1 are appeared,
corresponding to ꢀasym(COOꢀ) and ꢀsym(COOꢀ)
stretching vibrations of carboxylate group,
respectively. The energy gap of more than
200 cmꢀ1 between ꢀasym(COOꢀ) and ꢀsym(COOꢀ)
[D(ꢀasym(COOꢀ) ꢀ ꢀsym(COOꢀ) ¼ 213-231 cmꢀ1]
signifies the monodentate chelating binding
mode of carboxylate group.[20] The appearance of
peaks at 1558–1567 cmꢀ1 in C2–C5 complexes is
due to C¼N stretching vibrations of secondary
ligands. Further, the presence of water molecules
in the coordination sphere of C1 complex results
in an additional weak and broad band, assigned
to stretching vibration of O–H bonds at
3450 cmꢀ1. The bands at 428–446 cmꢀ1 and
566–567 cmꢀ1 in spectra of complexes are
assigned to Eu-O and Eu-N stretching vibrations
respectively, which clearly indicate the bonding
of ligands with europium ion. The FT-IR spectra
of ligand HIPA and complex C3 are displayed in
Fig. 2(a,b).
1H-NMR spectrum of free ligand shows a sig-
nal at d 12.92 ppm due to proton of –NH and
carboxylic group. The signal due to ring proton
is noticed at d 6.47 ppm and peak attributed to
isopropyl group protons come at 2.97 and
1.20 ppm, respectively. The proton NMR spectra
of the complexes are not well resolved due to
paramagnetic effect of trivalent europium ion.
However, basic phenomenon of deprotonation of
ligand HIPA during complexation is indicated by
the absence of peak at d 12.26 ppm. It may be
attributed to loss of proton of carboxylic group,
which results in net shielding of NH proton
Figure 2. Infrared spectra of uncoordinated ligand 3-isopropyl-
pyrazole-5-carboxylic acid (HIPA) (a) and complex C3
[Eu(IPA)3.bipy] of europium ion with primary ligand 3-isopro-
pylpyrazole-5-carboxylate (IPA) and secondary ligand 2,2’-bipyr-
idyl (bipy) (b). The spectral profiles reveal coordination of
deprotonated ligand to europium ion on the basis of differ-
ence in peak pattern of spectra.
during complexation. The signal due to –NH
protons in spectra of complexes is found at d
9.1 ppm. The introduction of secondary ligands

6
P. DHANKHAR ET AL.
in coordination sphere of ternary complexes is
confirmed by presence of additional peaks in
range d 8.5–7.5 ppm. The proton NMR spectra of
ligand HIPA and C5 complex as representative of
all complexes are shown in Fig. S3(a,b).
complex. The curve becomes horizontal beyond
610 ꢁC that corresponds to formation of euro-
pium oxide as final residue. The thermal studies
of the complexes demonstrate that these com-
plexes have excellent thermal stability (400 ꢁC)
which is high enough for fabrication of various
optoelectronic devices.
Thermal stability
High thermal stability of europium(III) com-
plexes is very crucial as decomposition of com-
plex during technological applications is one of
the key factors for lowering the performance of
various devices.[21] A lot of lanthanide complexes
reported in literature have been excluded from
practical applications owing to their poor thermal
stability.[22] Thermogravimetric and differential
thermogravimetric curves (TG/DTG) of complex
C5 are displayed as depictive of all five complexes
since decomposition pattern of these complexes
are analogous (Fig. 3). The correlation between
decomposition steps of the curve with corre-
sponding weight losses are described in terms of
proposed formulae of the complex. The figure
clearly shows that there is no weight loss up to
400 ꢁC, thereafter, decomposition of complex
started and continued to 610 ꢁC with a weight
loss of 79% (theoretical weight loss 79.80%)
which is attributed to removal of three primary
ligand molecules IPA and one secondary ligand
phen coordinated to europium ion in the
Ultraviolet-visible spectroscopy
The solid state absorption spectra of ligand HIPA
and its complexes C1–C5 in range of 200–600 nm
are shown in Fig. 4. The absorption spectrum of
ligand exhibits two bands, one strong band from
200 to 315 nm and another weak band from 315
ꢂ
ꢂ
to 450 nm assigned to p!p and n!p transi-
tions, respectively. The absorption spectra of
complexes resemble spectrum of ligand HIPA,
although some perturbations are noticed, which
are completely attributable to complexation of
ligand HIPA and secondary ligands (C2–C5) to
europium ion. The absorption maxima of com-
plexes are red shifted by 10–15 nm as well as
hyperchromic shifted due to extended conjuga-
tion of coordination as compared to ligand.[23,24]
Figure 4. Solid state absorption spectra of ligand 3-isopropyl-
pyrazole-5-craboxylic acid (HIPA), binary complex C1
[Eu(IPA)3.2H2O] of europium ion with ligand 3-isopropylpyra-
zole-5-craboxylate (IPA) and ternary complexes C2
[Eu(IPA)3.dmbipy], C3 [Eu(IPA)3.bipy], C4 [Eu(IPA)3.dmph] and
C5 [Eu(IPA)3.phen] with primary ligand IPA and secondary lig-
and 4,4’-dimethyl-2,2’-bipyridyl (dmbipy), 2,2’-bipyridyl (bipy),
5,6-dimethyl-1,10-phenanthroline (dmph) and 1,10-phenanthro-
line (phen), respectively.
Figure 3. Thermogravimetric (TG) and differential thermogravi-
metric (DTG) curves of complex C5 [Eu(IPA)3.phen] of europium
ion with primary ligand 3-isopropylpyrazole-5-carboxylate (IPA)
and secondary ligand 1,10-phenanthroline (phen) recorded
under inert atmosphere of dinitrogen gas. The curves depict
that the complex is thermally stable up to 400 ꢁC.

SPECTROSCOPY LETTERS
7
Photoluminescent properties
The excitation spectra of synthesized complexes
in range of 200–500 nm are represented by
Fig. 5a. The spectra of complexes are composed
of a band ranging from 200 to 350 nm with max-
ima at around 310 nm ascribed to ligand transi-
tions and various sharp peaks ahead of 350 nm
corresponding to f-f transitions of europium ion.
The weak peaks at 360, 375, 383, 416 and 467 nm
5
5
5
are referred to 7F0! D4, 7F0! G2, 7F0! G3,
7F0! D3 and F0! D2 transitions respectively,
5
7
5
while intense peak centered at 395 nm is specified
to 7F0! L6 transition of Eu3þ ion.[25–28] The
5
7
5
band at 467 nm corresponding to F0! D2 tran-
5
sition is used to calculate the position of D2
level. The broad band centered at about 310 nm
is slightly intense as compared to the peak at
395 nm in C1 and C2 complexes while for C3
and C4 complexes, the peak at 395 nm is some-
what intense than the band at 310 nm. In C5
complex, band centered at 310 nm and peak at
395 nm are equally intense.
Fig. 5b represents the full fleshed emission
spectra of C1–C5 complexes on rooting wave-
length corresponding to maxima of respective
excitation spectra. The photoemission spectra
covering the spectral range from 400 to 700 nm
Figure 5. Solid-state excitation (a) and emission spectra (b) of
binary complex C1 [Eu(IPA)3.2H2O] of europium ion with ligand
3-isopropylpyrazole-5-craboxylate (IPA) and ternary complexes
C2 [Eu(IPA)3.dmbipy], C3 [Eu(IPA)3.bipy], C4 [Eu(IPA)3.dmph]
and C5 [Eu(IPA)3.phen] with primary ligand IPA and secondary
ligand 4,4’-dimethyl-2,2’-bipyridyl (dmbipy), 2,2’-bipyridyl
(bipy), 5,6-dimethyl-1,10-phenanthroline (dmph) and 1,10-phe-
nanthroline (phen), respectively. The excitation wavelength for
C1, C2 and C5 complexes is 310 nm whereas excitation wave-
length for C3 and C4 complexes is 395 nm.
5
consist of transitions from D0 excited state to
7
7
7
7
7F0, F1, F2, F3 and F4 levels of europium ion
centered at about 580, 592, 619, 650 and 696 nm,
7
respectively.[29] The transitions 5D0! F0 and
7
5D0! F3 are restricted in both magnetic dipole
5
7
and electric dipole scheme.[30] The D0! F2 and
7
5D0! F4 transitions are forced electric dipole
7
transitions while the 5D0! F1 transition is
In ternary complexes (C2–C5), secondary ligands
replace water molecules from coordination
sphere, which results in enhancement of photolu-
minescent signal by preventing the depopulation
of excited states via non-radiative decay process
as well as by populating higher excited states via
synergistic effect.[32,33]
allowed by magnetic dipole scheme.[31] The single
5
7
line due to D0! F0 transition suggests the pres-
ence of single luminescent Eu(III) species. The
5
7
5
7
peaks attributed to D0! F1 and D0! F2 transi-
tions have comparable integrated area, suggested
that europium(III) ion probably possess an inver-
sion center.[1] The emission signal of C1 complex
is weaker than ternary C2–C5 complexes due to
the presence of water molecules in the coordin-
ation sphere. The water molecules possess
high-energy O–H oscillators which quench lumi-
nescence signal by relaxing higher excited states
of europium ion via nonradiative decay process.
5
The luminescence decay profile of D0 excited
state of Eu3þ complexes C1–C5 are shown in
Fig. 6 and S4. The lifetime value (s) of these
complexes are determined by fitting the decay
profile into monoexponential decay law (Table 3)
and their lifetime values found to be quite high
as compared to majority of europium complexes

8
P. DHANKHAR ET AL.
reported in literature.[1] The long luminescence
lifetime of these complexes make them ideal
luminescent probe for biomedical assays because
long lived lanthanide centered luminescence can
be easily differentiated from short lived back-
ground noise using time-gated techniques.[5] The
single exponential nature of decay curve affirms
the presence of single luminescent species which
is also indicated by single emission line referred
parameters Xk (k ¼ 2, 4) and internal quantum
efficiency g of the complexes have been deter-
mined by calculating radiative rate A0J (J ¼ 0, 1,
2, 3, 4) for every transition of europium ion. A0J
can be obtained by referencing magnetic dipole
5
7
transition D0! F1 as internal standard, owing
to its almost autonomous nature from surround-
ing crystal field, according to the following equa-
tion:
to D0! F0 transition in emission spectra.[34,35]
The longer lifetime value of ternary C2–C5 com-
plexes than binary C1 complex are attributed to
additional energy transfer from secondary ligands
to europium ion and absence of nonradiative
decay process associated with water molecules in
ternary complexes.
Judd–Ofelt theory has been applied on emis-
sion spectra of the complexes to further investi-
gate their spectral properties. Judd–Ofelt intensity
5
7
ꢀ
ꢁ
ꢁ
ꢀ
ꢀ01
ꢀ0J
I0J
A0J ¼ A01
(1)
I01
In which ꢀ0J and ꢀ01 are peak positions of emis-
sion bands (cmꢀ1) while I0J and I01 are integrated
emission areas of the peaks corresponding to
7
7
5D0! FJ and 5D0! F1 electronic transitions,
respectively.[36] A01 is the Einstein’s coefficient of
5
7
spontaneous emission for D0 to F1 energy levels,
whose value is reckoned to be 50 sꢀ1 in the air.
The comprehensive radiative rate (Arad) is equal to
summation of radiative rate A0J associated with
each 5D0! FJ transition of europium ion:
7
X
Arad
¼
j ¼ 0, 1, 2, 3, 4 A0J
¼ A00 þ A01 þ A02 þ A03 þ A04
(2)
Internal Quantum efficiency g is directly pro-
portional to total radiative rate constant Arad and
inversely proportional to overall rate constant
Atotal which can be correlated with luminescence
decay time value as below[37]
:
Arad
Arad
Arad þ Anr
g ¼
¼
¼ Arads
(3)
Atotal
The internal quantum efficiency g of C1–C5
complexes are calculated accordingly and summar-
ized in Table 3. The complexes (C1–C5) exhibit
internal quantum efficiency in range ꢃ31–43%,
which are quite better as compared to numerous
photoluminescent organic complexes reported in
literature.[22,38] Higher quantum efficiency of
Figure 6. Luminescence decay curve of binary complex C1
[Eu(IPA)3.2H2O] of europium ion (C1) with ligand 3-isopropyl-
pyrazole-5-craboxylate (IPA) and ternary complex C5
[Eu(IPA)3.phen] with primary ligand IPA and secondary ligand
1,10-phenahtroline (phen) in powder form. The decay curves
5
7
are measured by monitoring D0! F2 electronic transition of
europium ion.
Table 3. Photophysical data of binary complex (C1) of europium ion with ligand 3-isopropylpyrazole-5-carboxylate (IPA) and tern-
ary complexes (C2-C5) with primary ligand IPA and secondary ligand 4,40-dimethyl-2,2’-bipyridyl (dmbipy), 2,20-bipyridyl (bipy),
5,6-dimethyl-1,10-phenanthroline (dmph) and 1,10-phenanthroline (phen), respectively.
Complexes
Eg (eV)
s (ms)
Arad (sꢀ1
)
Atotal (sꢀ1
)
g (%)
X2 (10ꢀ20 cm2)
X4 (10ꢀ20 cm2)
C1 [Eu(IPA)3.2H2O]
C2 [Eu(IPA)3.dmbipy]
C3 [Eu(IPA)3.bipy]
C4 [Eu(IPA)3.dmph]
C5 [Eu(IPA)3.phen]
4.51
4.53
4.54
4.51
4.55
2.627
2.680
2.743
2.747
3.311
118.2
122.5
124.9
129.7
131.1
380.6
373.1
364.5
364.0
302.0
31.05
32.83
34.26
35.63
43.41
1.26
1.34
1.42
1.53
1.55
0.15
0.18
0.16
0.15
0.17
Eg: optical band gap value; s: luminescence lifetime; Arad: radiative rate constant; Atot: overall rate constant; g: internal quantum efficiency; X2 and X2:
Judd–Ofelt parameters.

SPECTROSCOPY LETTERS
9
ternary complexes (C2–C5) than binary complex
(C1) highlights toward the synergistic effect of sec-
ondary ligands and absence of non radiative decay
process mediated by water molecules.
due to decrease in energy gap between highest
occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO), which
can be attributed to extended system of conjuga-
tion (Table 3). Fig. 7(a,b) represents the Tauc
plot of the ligand HIPA and C5 complex as indi-
cative of all other complexes and insets show
their respective reflectance spectra.
On the basis of CIE 1931 (Commission
Internationale de I’Eclairage) color matching
functions, the color coordinates of complexes are
determined using respective photoemission spec-
tra. Table 4 displays color coordinates of C1–C5
complexes and corresponding chromaticity dia-
gram is depicted Fig. 8. The chromaticity coordi-
nates (x, y) of the complexes with increment in
Judd–Ofelt intensity parameters of the com-
5
7
plexes corresponding to the D0! FJ transitions
(J ¼ 2, 4) are estimated by equation:
3:h:A0J
64:p4e2ῡ3v < 05D jUkjJ F >2
Xk ¼
(4)
7
Where, <50D jUkjJ F >2 is doubly reduced matrix
7
5
7
element being equal to 0.0032 for D0! F2 and
0.0023 for 5D0! F4 transition, respectively.[38]
7
v is the Lorentz local field correction term, repre-
2
sented by nðn2 þ 2Þ =9, h is the Planck’s con-
stant, e is the electronic charge and ῡ is the
frequency of transition (cmꢀ1). X2 is sensitive to
local environment created by bonding atoms of
surrounding ligands in first coordination sphere
of emitter ion, and presence of europium ion in
symmetry site with inversion center tends to
approach its value close to one.[1] The value of
X2 for C1–C5 complexes are come out in range
1.26–1.55 ꢄ 10ꢀ20 cm2, indicating the fact that
emitter ion is positioned in symmetry site close
to center of inversion. X4 intensity parameter is
affected by bonding atoms-emitter ion distances
and associated with rigidity of the system.[39] The
smaller value of X4 for C1–C5 complexes points
toward high rigidity of the complexes (Table 3).
Kubelka–Munk function F(R ) of the ligand
1
(HIPA) and complexes C1–C5 is calculated by
browsing their respective diffuse reflectance data
(R ) into the following equation[40]
:
1
2
ð1 ꢀ R Þ
K
S
1
1
FðR Þ ¼
¼
(5)
1
2R
where K/S is the ratio of absorption to scattering
coefficient. F(R ) is related to optical band gap
1
value through an equation involving energy of
incident radiation (ꢀ), Planck’s constant (h) and
exponent n which is taken as 2 for direct allowed
transitions:
n
Figure 7. Tauc plot of free ligand 3-isopropylpyrazole-5-cra-
boxylic acid (HIPA) (a), complex C5 [Eu(IPA)3.phen] of europium
ion with primary ligand 3-isopropylpyrazole-5-craboxylate (IPA)
and secondary ligand 1,10-phenanthroline (phen) (b) and
insets show their respective diffuse reflectance spectra. a:
Kubelka–Munk function; h: Planck’s constant; ꢀ: frequency of
incident radiations; Eg: optical band gap value.
½FðR Þ hvꢅ ¼ Cðhv ꢀ EgÞ
(6)
1
The optical band value is calculated by extrap-
olating the curve [F(R )hꢀ]2 versus hꢀ, known
1
as Tauc plot, to zero. Eg value of the complexes
is found to be lower than the ligand (4.64 eV)

10
P. DHANKHAR ET AL.
1
2
"
#
Table 4. Color coordinates (x, y) and color purity (CP) of light
emitted by binary complex (C1) of europium ion with ligand
3-isopropylpyrazole-5-carboxylate (IPA) and ternary complexes
(C2-C5) with primary ligand IPA and secondary ligand 4,40-
dimethyl-2,20-bipyridyl (dmbipy), 2,2-bipyridyl (bipy), 5,6-
dimethyl-1,10-phenanthroline (dmph) and 1,10-phenanthroline
(phen), respectively.
2
2
x ꢀ x þ y ꢀ yiÞ
ð
iÞ
ð
CP ¼
(7)
2
2
x ꢀ x þ y ꢀ yiÞ
ð
iÞ
d
ð d
The color purity of C1–C5 complexes is found
to be 54.56–94.43% and higher color purity of tern-
ary complexes is attributed to strengthening of
luminescence signal on introduction of secondary
ligands in their coordination sphere (Table 4). The
high color purity demonstrates that these com-
plexes can be used as red light emitting component
for fabrication of organic white light emitting
diodes via trichromatic approach.
Complexes
Color coordinates (x, y)
Color purity (%)
C1 [Eu(IPA)3.2H2O]
C2 [Eu(IPA)3.dmbipy]
C3 [Eu(IPA)3.bipy]
C4 [Eu(IPA)3.dmph]
C5 [Eu(IPA)3.phen]
x ¼ 0.494, y ¼ 0.357
x ¼ 0.556, y ¼ 0.360
x ¼ 0.576, y ¼ 0.362
x ¼ 0.587, y ¼ 0.360
x ¼ 0.607, y ¼ 0.361
54.56
76.10
82.89
85.62
94.43
Energy transfer mechanism
In order to thoroughly explain the photolumines-
cent phenomenon in europium complexes,
energy transfer mechanism of C3 and C5 com-
plexes having different framework of secondary
ligands is depicted in Fig. 9 and Fig. S5. The
excited singlet state of ligand HIPA is determined
from edge wavelength of UV-vis spectrum of lig-
and (Fig. 4) and triplet state is calculated by the
lower emission edge wavelength of phosphores-
cence spectrum of G1 complex (Fig. S6) and
found to be 31,645 and 24,630 respectively.[42,43]
The calculated value of excited singlet and triplet
state of secondary ligands phen and bipy are
quite close to values already reported in litera-
ture. The value of singlet and triplet state of
Figure 8. Chromaticity diagram for binary complex C1
[Eu(IPA)3.2H2O] of europium ion with ligand 3-isopropylpyra-
zole-5-craboxylate
(IPA)
and
ternary
complex
C5
[Eu(IPA)3.phen] with primary ligand IPA and secondary ligand
1,10-phenanthroline (phen). The color coordinates (x, y) of the
complexes (C1–C5) shift from reddish orange to deep red
region of color gamut space.
luminosity shift toward deep red region of color
gamut space. The color purity (CP) is also an
important factor for estimation of performance of
the complexes. The color purity of the complexes
is calculated to determine the saturation of emit-
ted color toward red light. The color purity of a
particular complex is defined as weighted average
of color coordinates of light emitted by the com-
plex (x, y) and dominant wavelength (xd, yd) rela-
tive to illumination source of white light (CIE E,
Figure 9. Scheme of energy transfer mechanism of complex
C5 [Eu(IPA)3.phen] of europium ion with primary ligand 3-iso-
propylpyrazole-5-carboxylate (IPA) and secondary ligand 1,10-
phenanthroline (phen). The figure illustrates various energy
transfer pathways from primary ligand IPA and secondary lig-
and phen to europium ion. The dotted and solid vertical lines
represent non-radiative transitions and radiative transitions,
respectively.
xi ¼ 0.333 and yi ¼ 0.333)[41]
:

SPECTROSCOPY LETTERS
11
Table 5. Excitation energy parameters of ligands 3-isopropyl-
ligands with europium(III) ion are prepared and
the complexation mode of ligands to metal
ion is investigated by elemental analysis,
pyrazole-5-carboxylic acid (HIPA), 1,10-phenanthroline (phen)
and 2,2’bipyridyl (bipy) in cmꢀ1
.
5
5
Ligands
(S1)
(T1)
DE(S1 ꢀ T1) DE1(T1 ꢀ D1) DE0(T1 ꢀ D0)
1
EDAX, H-NMR, FT-IR spectroscopy and ther-
HIPA
Phen
Bipy
31645 24630
31000 22100
29900 22900
7015
8900
7000
5600
3070
3870
7389
4859
5659
mogravimetric analysis. The binding interactions
between europium ion and ligands in these com-
plexes occur through nitrogen and oxygen atoms.
SEM images of the complexes exhibit homogen-
ous distribution of particles. The thermogravi-
metric analysis also reveals thermal stability of
the complexes up to 400 ꢁC and single-step
decomposition of organic framework. All the
complexes exhibit characteristic europium cen-
tered emission spectra under the excitation of
ultraviolet light. Furthermore, superior photolu-
minescent features of ternary complexes than
binary complex arise from synergistic effect of
secondary ligands as well as suppression of non-
radiative decay process mediated by water mole-
cules. Judd–Ofelt analysis presents remarkable
quantum efficiency of the complexes (31–43%)
and small values of Judd–Ofelt parameters, cor-
roborates with fact that europium ion is posi-
tioned in symmetry site close to inversion center.
The longer lifetime values and excellent photolu-
minescent features of complexes point toward
effective intramolecular energy transfer process
which is also demonstrated by energy transfer
mechanism. The optical band gap value of the
complexes C1–C5 is lower than the ligand. The
color coordinates of complexes fall in red region
of color gamut space and close to NTSC system
of color matching. Also, the complexes feature
high red color purity. The study results reveal
that these complexes can be potentially used as
red light emitting materials in various optoelec-
tronic devices and biomedical assays.
The energy parameters help to understand efficacy of sensitization
phenomenon in europium (III) complexes.
S1: lowest excited singlet state; T1: lowest excited triplet state; DE: energy
5
gap; 5D1 and D0: excited energy levels of europium ion.
ligand HIPA and secondary ligands are shown in
Table 5. Usually, efficiency of sensitization phe-
nomenon depends on two aspects; first efficiency
of intersystem crossing between singlet and trip-
let state of ligand (S1 ꢀ T1) and second, efficiency
of energy transfer event from triplet state of lig-
and to emitting level of metal ion (T1 ꢀ Euþ3).[44]
The empirical rule is that energy gap DE
(S1 ꢀ T1) of about 5000 cmꢀ1 should be needed
for effective intersystem crossing. It is clear from
Table 5 that energy gap of 7015 cmꢀ1 between S1
and T1 states is suitable for intersystem crossing.
According to empirical rule of Latva, energy gap
of 2000–5000 cmꢀ1 is optimal for energy transfer
between T1 state of ligand to resonance levels of
europium ion.[45] The energy gap between T1
5
5
5
state of ligand HIPA to D0, D1 and D2 reson-
ance levels of europium ion are 7389, 5600, and
4389 cmꢀ1 respectively, owing to transfer of
energy from ligand to emitting levels of euro-
pium ion. The secondary ligands support the
main ligand during sensitization phenomenon; as
a result, the luminescence spectra of ternary com-
plexes C2–C5 are more intense than C1 binary
complex. The photoemission spectrum of com-
plex C5 is more intense than C3 because the
energy gap of 4859 cmꢀ1 between triplet level of
5
phen and resonance D0 level of europium ion is
within Latva defined range while that of bipy is
5659 cmꢀ1. The emission signal of C4 and C2
complexes is weaker than respective C5 and C3
complexes having similar framework of secondary
ligands, may be due to vibronic quenching of
additional C-H bonds of methyl groups in dmph
and dmbipy.[11,46]
Funding
One of authors, Priyanka is financially supported in form of
junior research fellowship (JRF) from UGC, New Delhi,
India [Sr.No. 2061410096].
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