Relative Study of Luminescent Properties with Judd-Ofelt Characterization in Trivalent Europium Complexes Comprising ethyl-(4-fluorobenzoyl) Acetate

Article abstract of DOI:10.1007/s10895-017-2069-3

Five new europium(III) complexes Eu(p-EFBA)₃.(H₂O)₂ (C1), Eu(p-EFBA)₃.neo (C2), Eu(p-EFBA)₃.batho (C3), Eu(p-EFBA)₃.phen (C4), Eu(p-EFBA)₃.bipy (C5) have been synthesized by using ethyl-(4-fluorobenzoyl) acetate (p-EFBA) as β-ketoester ligand and neocuproine (neo), bathophenanthroline (batho), 1,10-phenanthroline (phen) and 2,2-bipyridyl (bipy) as ancillary ligands. The synthesized complexes C1-C5 were characterized by elemental analysis, nuclear magnetic resonance spectroscopy (¹H-NMR), infrared (IR) spectroscopy, thermogravimetric analysis (TG/DTG), UV-visible and photoluminescence (PL) spectroscopy. The relative study of luminescence spectra of complexes with the previously reported complexes of isomeric ligand (ortho and meta substituted ligand) indicate the higher luminescence properties of complexes as an effect of fluorine position on β-ketoester ligand. The para substituted ligand shows a remarkable effect on quantum efficiencies and Judd-Ofelt intensity parameters (Ω_2, Ω₄) of the complexes. The higher value of intensity parameter Ω₂ associated with hypersensitive ⁵D₀?→?⁷F₂ transition of europium(III) ion revealing highly polarizable ligand field. The purposed energy transfer mechanism of complexes indicates the efficient energy transfer in complexes.

Full text of DOI:10.1007/s10895-017-2069-3

J Fluoresc
DOI 10.1007/s10895-017-2069-3
ORIGINAL ARTICLE
Relative Study of Luminescent Properties with Judd-Ofelt
Characterization in Trivalent Europium Complexes Comprising
ethyl-(4-fluorobenzoyl) Acetate
Rekha Devi1 & Sangeeta Chahar1 & S. P. Khatkar1 & V. B. Taxak 1 & Priti Boora1
Received: 3 December 2016 /Accepted: 2 March 2017
#
Springer Science+Business Media New York 2017
Abstract Five new europium(III) complexes Eu(p-
EFBA)3.(H2O)2 (C1), Eu(p-EFBA)3.neo (C2), Eu(p-
EFBA)3.batho (C3), Eu(p-EFBA)3.phen (C4), Eu(p-
EFBA)3.bipy (C5) have been synthesized by using ethyl-(4-
fluorobenzoyl) acetate (p-EFBA) as β-ketoester ligand and
neocuproine (neo), bathophenanthroline (batho), 1,10-
phenanthroline (phen) and 2,2-bipyridyl (bipy) as ancil-
lary ligands. The synthesized complexes C1-C5 were
characterized by elemental analysis, nuclear magnetic
resonance spectroscopy (1H-NMR), infrared (IR) spec-
troscopy, thermogravimetric analysis (TG/DTG), UV-
visible and photoluminescence (PL) spectroscopy. The
relative study of luminescence spectra of complexes
with the previously reported complexes of isomeric ligand
(ortho and meta substituted ligand) indicate the higher lumi-
nescence properties of complexes as an effect of fluorine po-
sition on β-ketoester ligand. The para substituted ligand
shows a remarkable effect on quantum efficiencies and
Judd-Ofelt intensity parameters (Ω2, Ω4) of the complexes.
The higher value of intensity parameter Ω2 associated with
Introduction
The interesting luminescent behaviour of organic rare earth
complexes initiate the utilization of these complexes in mag-
netism [1], sensors [2], fluoroimmunoassays [3], chelates for
bioanalysis and imaging [4, 5], catalyst [6], luminescent de-
vices and displays [7]. The synthesis of these metal complexes
particularly involves the coordination between organic ligand
and trivalent lanthanide ion, owing to smaller molar absorp-
tion coefficient inherent with 4f-4f transition of lanthanide ion
which is not allowed according to Laporte’s rule [8–10]. The
organic ligand inserted into the coordination environment of
lanthanide ion should be able to harvest energy and effectively
transfer it to the resonating level of metal ion. Following this
pioneer investigation by Weissman and co-worker [11]; a
wide variety of complexes involving organic ligand and
Eu(III) and Tb(III) ions as main luminescent centers have been
synthesized. Based on the different choices of chromophoric
organic ligands, functionalised β-ketoester ligands are found
to be best candidate for the synthesis of the complexes.
Particularly the fluorination of ligand with low energy C-F
oscillators reduces the non-radiative energy losses occur
due to vibrational energy of C-H oscillators, resulting
higher luminescence of metal complexes [12, 13]. In
addition, the fluorination of ligand possesses significant
effect on the properties of materials mainly contributed
by its small size and highest electronegativity. The ther-
mal stability is also affected by the substitution of fluo-
rine on the ligand resulting longer luminescence life
time [14]. Thus the fluorinated ligand provide a fascinating
prospect in the synthesis of organic metal complexes which
prompt us to synthesize a series of complexes based on organ-
ic ligand having different position of substituent fluorine and
to study the effect of substituent position on the luminescent
properties of these complexes.
7
hypersensitive 5D0 → F2 transition of europium(III) ion re-
vealing highly polarizable ligand field. The purposed energy
transfer mechanism of complexes indicates the efficient ener-
gy transfer in complexes.
.
.
Keywords Luminescence Infrared spectroscopy Quantum
.
.
efficiency Judd-Ofelt intensity parameters Energy transfer
* Priti Boora
pritiboora@gmail.com
1
Department of Chemistry, Maharshi Dayanand University,
Rohtak 124001, India

J Fluoresc
The present communication deals with the synthesis of five
new europium(III) complexes (C1-C5) by using ethyl-(4-
fluorobenzoyl) acetate (p-EFBA) as fluorinated β-ketoester
ligand and neocuproine, bathophenanthroline, 1,10-
phenanthroline and 2,2-bipyridyl as ancillary ligands. All the
synthesized complexes were characterized by elemental anal-
resulting mixture was stirred at room temperature for one
hour. The mixture was washed with water and 1 N HCl and
then dried by employing anhydrous sodium sulphate. The
resulting product was dissolved with ethanol in toluene and
heated on reflux condenser at 90 °C for 24 h. Subsequently
solvents were allowed to evaporate at room temperature and
product formed was recrystallized with ethanol [15]. Ligand
ethyl-(4-fluorobenzoyl) acetate (p-EFBA) was obtained with
56% yield. IR(KBr): cm−1 3670 (b), 2984 (s), 1740 (s), 1692
(s), 1648 (s), 1587 (s), 1443 (s), 1412 (s), 1326 (w), 1275 (s),
1
ysis, H–NMR, IR, thermogravimetric analysis, UV-visible
and photoluminescence spectroscopy. The investigation is
made on the basis of fluorine substituent position (ortho, meta
and para) on the ligand and its effect on the luminescent prop-
erties is studied. The luminescence properties of complexes
are compared and effect of different position of fluorine is
studied. The calculated quantum efficiencies and intensity pa-
rameters of complexes further confirms the obtained results.
1
1217 (s), 1148 (m), 1030 (s), 876 (s), 591 (m); H NMR
(CDCl3, 400 MHz): δ 12.54 (s, 1H, enolic OH), 7.46 (m,
4H, Ar-H), 5.64 (s, 1H, C = CH), 3.96 (s, 2H keto CH2),
3.67 (q, 2H, OCH2CH3), 1.32 (t, 3H, OCH2CH3) ppm.
Synthesis of europium(III) Complexes C1-C5
Experimental
The synthesis of ternary complexes C2-C5 follow the similar
procedure as used for the synthesis of binary complex C1 but
differed only in addition of 1 mmol ethanolic solution of ancil-
lary ligand (neo batho, phen and bipy) respectively. For the
synthesis of complex C1, an ethanolic solution of ligand p-
EFBA (3 mmol) was added dropwise to the aqueous solution
of europium nitrate (1 mmol) and stirred the mixture on mag-
netic stirrer. The pH of solution was adjusted (6.5–7) with
0.05 M sodium hydroxide solution, leading to white precipi-
tates in mixture which were collected by filtration. The washing
of precipitates with water and ethanol makes complete removal
of free ligand from mixture. The white powder of complex C1
was obtained by drying solid residue in vacuum desiccator.
Eu(p-EFBA)3.(H2O)2 (C1): White powder, yield 79%;
IR(KBr): cm−1 3612 (b), 2984 (s), 1733 (s), 1630 (s) 1517
(s), 1480 (s), 1385 (s), 1283 (s), 1215 (s), 1094 (m), 1036
Materials and Methods
The chemicals and solvent used were of reagent grade and
used without further purification. Lanthanide nitrates and an-
cillary ligands used were purchased from commercially avail-
able sources. Elemental analyses (C, H and N) of complexes
were performed on Perkin Elmer 2400 CHN elemental
analyser. The concentration of Eu(III) ion determined by
complexometric titration with ethylenediaminetetraacetate
(EDTA). Nuclear magnetic resonance spectra (1H–NMR)
were obtained on a Bruker Avance II 400 NMR spectrometer
in deuterated chloroform solution by using tetramethylsilane
(TMS) as standard. The Infrared spectra (IR) were carried out
with Perkin Elmer Spectrum 400 spectrometer in the range of
4000–400 cm−1 using dried KBr disks. Shimadzu-2450
UV-Visible Spectrophotometer was employed to measure
the UV-visible absorption spectra of the complexes.
Thermogravimetric analyses (TG/DTG) were performed at
heating rate of 10 °C/min on SDT Q600. Luminescent (PL)
measurements were recorded with Hitachi F-7000 fluo-
rescence spectrophotometer equipped with a xenon lamp
as excitation source. The decay time of complexes were
calculated by using software (FL solution of F-7000) of
the spectrophotometer.
1
(s), 943 (s), 875 (s), 732 (m), 705 (m), 423 (m); H NMR
(CDCl3, 400 MHz): δ 7.24 (m, 12H, Ar-H), 4.34 (q, 6H,
OCH2CH3), 2.69 (s, 3H, C = CH), 1.36 (t, 9H, OCH2CH3)
ppm; Anal. Cal. For EuC33H34O11F3: C, 48.82; H, 4.19; Eu,
18.24; Found: C, 48.54; H, 4.13; Eu, 18.19.
Eu(p-EFBA)3.neo (C2): White powder, yield 77%;
IR(KBr): cm−1 2984 (s), 2360 (m), 1741 (s), 1628 (s), 1582
(s), 1520 (s), 1478 (s), 1283 (s), 1211 (s), 1171 (s), 1096 (s),
1036 (s), 946 (s), 876 (m), 737 (m), 709 (m), 530 (s), 420 (m);
1H NMR (CDCl3,400 MHz): δ 8.31 (d, 2H, Ar-H), 7.87 (m,
4H, Ar-H), 7.49 (m, 12H, Ar-H), 4.51 (q, 6H, OCH2CH3),
3.49 (s, 6H, Ar-H), 2.61 (s, 3H, C = CH), 1.34 (t, 9H,
OCH2CH3) ppm; Anal. Cal. For EuC47H42N2O9F3: C,
57.36; H, 4.27; N, 2.84; Eu, 15.05; Found: C, 57.21; H,
4.24; N, 2.71; Eu, 14.96.
Synthesis
The synthetic route of ligand (p-EFBA) and corresponding
europium(III) complexes C1-C5 shown in Scheme 1.
Synthesis of β-ketoester Ligand
Eu(p-EFBA)3.batho (C3): White powder, yield 82%;
IR(KBr): cm−1 2981 (s), 1738 (s), 1617 (s), 1582 (s), 1518
(s), 1479 (s), 1370 (s), 1279 (s), 1212 (s), 1165 (s), 1096 (s),
1040 (s), 875 (s), 735 (m), 702 (m), 541 (s), 422 (m); 1H NMR
(CDCl3, 400 MHz): δ 10.31 (d, 2H, Ar-H), 9.11 (d, 2H, Ar-
To a solution of 4-fluorobenzoyl chloride and 2, 2-dimethyl-
1,3-dioxane-4,6-dione in dry methylene chloride, addition of
4-(dimethylamino) pyridine was done at about 0 °C and the

J Fluoresc
Scheme 1 Synthetic route of the ligand and europium(III) complexes C1-C5
H), 8.31 (m, 12H, Ar-H), 7.40 (d, 2H, Ar-H)), 6.76 (s, 10H,
Ar-H), 4.20 (q, 6H, OCH2CH3), 2.52 (s, 3H, C = CH), 1.39 (t,
9H, OCH2CH3) ppm; Anal. Cal. For EuC57H46N2O9F3: C,
61.76; H, 4.15; N, 2.52; Eu, 13.36; Found: C, 61.57; H,
4.04; N, 2.38; Eu, 13.22.
Eu(p-EFBA)3.phen (C4): White powder, yield 81%;
IR(KBr): cm−1 2982 (s), 1733 (s), 1624 (s), 1576 (s), 1511
(s), 1481 (s), 1423 (s), 1382 (s), 1311 (s), 1223 (s), 1182 (s),
1092 (s), 1033 (s), 941 (m), 871 (s), 743 (m), 704 (m), 538 (s),
419 (m); 1H NMR (CDCl3, 400 MHz): δ 9.41 (d, 2H, Ar-H),
8.42 (m, 4H, Ar-H), 7.65 (d, 2H, Ar-H), 7.36 (m, 12H, Ar-H),
6.94 (d, 2H, Ar-H) 4.42 (q, 6H, OCH2CH3), 2.65 (s, 3H,
C = CH), 1.38 (t, 9H, OCH2CH3) ppm; Anal. Cal. For
EuC45H38N2O9F3: C, 56.32; H, 3.97; N, 2.93; Eu, 15.49;
Found: C, 56.13; H, 3.76; N, 2.82; Eu, 15.38.
(s), 1483 (s), 1369 (s), 1272 (s), 1215 (s), 1161 (s), 1094 (s),
1036 (s), 942 (s), 876 (s), 732 (m), 708 (m), 529 (s), 428 (m);
1H NMR (CDCl3, 400 MHz): δ 8.55 (d,2H, Ar-H), 7.98 (d,
2H, Ar-H), 7.56 (m, 4H, Ar-H),7.30 (m, 12H, Ar-H), 4.31 (q,
6H, OCH2CH3), 2.66 (s,1H, C = CH), 1.29 (t, 9H, OCH2CH3)
ppm; Anal. Cal. For EuC43H38N2O9F3: C, 55.41; H, 4.08; N,
3.01; Eu, 15.89; Found: C, 55.29; H, 3.94; N, 2.91; Eu, 15.71.
Results and Discussion
Composition and Spectroscopic Characterization
of Complexes
The elemental analysis data of C, H, N and europium(III) ion
of complexes are represented in Table 1. The obtained results
are compared with the theoretically calculated values which
Eu(p-EFBA)3.bipy (C5): White powder, yield 79%;
IR(KBr): cm−1 2979 (s), 1742 (s), 1622 (s), 1583 (s), 1514

J Fluoresc
Table 1 Elemental analysis data
of europium(III) complexes
C1-C5
Complexes
C (%) found (cal.)
H (%) found (cal.)
N (%) found (cal.)
Eu (%) found (cal.)
C1
C2
C3
C4
C5
48.54 (48.82)
57.21 (57.36)
61.57 (61.76)
56.13 (56.32)
55.29 (55.41)
4.13 (4.19)
4.24 (4.27)
4.04 (4.15)
3.76 (3.97)
3.94 (4.08)
-
18.19 (18.24)
14.96 (15.05)
13.22 (13.36)
15.38 (15.49)
15.71 (15.89)
2.71 (2.84)
2.38 (2.52)
2.82 (2.93)
2.91 (3.01)
confirms the proposed formulae Eu(p-EFBA)3.(H2O)2, Eu(p-
EFBA)3.neo, Eu(p-EFBA)3.batho, Eu(p-EFBA)3.phen,
Eu(p-EFBA)3.bipy for complexes C1-C5 respectively.
The infrared spectral data of ligand p-EFBA and corre-
sponding europium(III) complexes C1-C5 are listed in
Table 2. In comparing with complexes C2-C5, ligand and
complex C1 shows a broad band at 3670 cm−1 and
3612 cm−1 accredited with vibrational frequency of enolic
O-H and absorbed or coordinated water molecule respectively.
The absence of band in C2-C5 hinting the coordination of
enolic O-H with the europium(III) ion. A shift in vibrational
frequencies of > C = O and >C = C bands in complexes occurs
due to extension of π-conjugation in complexes [16]. A
strong band at 1576–1583 cm−1 in complexes C2-C5
assigned to C = N bands of coordinating ancillary li-
gands which are supported by the presence of Eu-N
bands in complexes. The binding mode of ligand with
europium ion further approved by the appearance of
new absorption bands Eu-O at 419–428 cm−1 in the
spectra of complexes C1-C5 [17, 18]. The observation
indicates that europium ion coordinated through enolic
O-H and > C = O group of ligand in complexes.
1H–NMR spectral analysis of ligand and complexes in
the deuterated chloroform solution confirms the bonding
mode of ligand with europium(III) ion. In the spectrum
of ligand, there is a singlet at 12.54 ppm and 3.96 ppm
attributed to enolic O-H and ketonic -CH2 peak respec-
tively [19]. The coordination of ligand with europium
ion is confirmed by the absence of specific proton peak
(enolic and ketonic proton) in complexes. Due to para-
magnetic behavior of europium ion, a singlet at
5.64 ppm in spectrum of ligand ascribed to methine
proton move toward higher field value in spectra of
complexes [20].
Thermal Behaviour (TG/DTG)
The thermal analysis (TG/DTG) of europium(III) complexes
are carried out from ambient temperature to 1070 °C under
the environment of nitrogen gas alongwith a heating rate of
10 °C min−1. Figure 1 depict the decomposition processes of
complex Eu(p-EFBA)3.bipy, chosen as representative for all
the complexes as an effect of similar pattern followed by them.
The TG curve demonstrate the start of decomposition around
235 °C and continue upto 507 °C with a loss of weight 66.94%
(theoretical weight loss 67.34%) indicating the loss of three
organic ligand coordinating to europium ion in the complex.
The second stage of decomposition occurs in the temperature
range of 507 °C–959 °C with a loss of 16.65% weight (theo-
retical weight loss 16.75%) estimated with the removal of 2,2-
bipyridine ligand from complex. The DTG curve of complex
also support the decomposition pattern by the presence of peaks
at 271 °C and 951 °C corresponds with the weight losses.
Finally remaining product of approximately 14.58% associated
with the oxides of europium. The analysis indicate the higher
thermal stability of europium complexes.
UV-Visible Absorption Spectra
The UV-visible absorption spectra of ligand (p-EFBA) and its
corresponding binary and ternary Eu(III) complexes are
shown in Fig. 2. The measurements were made in chloroform
solution at room temperature in the range 200–400 nm. The
free ligand shows strong absorption at 318 nm. Although the
spectral profile of europium(III) complexes were similar to
that of ligand but chelating nature of ligand around central
metal ion extends the conjugated system as well as electron
density around ion which causing the absorption bands of
complexes to shift toward higher wavelength [21].
Table 2 The characteristic IR
spectral bands of the ligand
p-EFBA and complexes
Complex
ν(O-H)
ν(C = O)
ν(C = N)
ν(C = C)
ν(C-F)
ν(Eu-N)
ν(Eu-O)
p-EFBA
C1
3670 (b)
1692 (s)
1630 (s)
1628 (s)
1617 (s)
1624 (s)
1622 (s)
-
1587 (s)
1517 (s)
1520 (s)
1518 (s)
1511 (s)
1514 (s)
1217 (s)
1215 (s)
1211 (s)
1212 (s)
1223 (s)
1215 (s)
-
-
3612 (b)
-
423 (m)
420 (m)
422 (m)
419 (m)
428 (m)
C2
-
-
-
-
1582 (s)
1582 (s)
1576 (s)
1583 (s)
530 (s)
541 (m)
538 (m)
529 (s)
C3
C4
C5

J Fluoresc
Fig. 1 The TG/DTG curve of complex Eu(p-EFBA)3.bipy recorded
under nitrogen atmosphere
Fig. 3 Excitation spectra of complexes C1-C5 in solid state at
room temperature
Photoluminescence Spectra
5D0
→ → → →
7F0, 5D0 7F1, 5D0 7F2, 5D0 7F3 and
5D0 → F4 of europium ion at 580 nm, 590 nm, 613 nm,
652 nm and 686 nm arise as a result of intraconfigurational
transitions from the emitting state (5D0) to the low lying multi-
plet 7FJ (J = 0,1,2,3,4) of central metal ion [23]. The spectra of
complexes are devoid of peaks related to free ligand pointing
out the efficient sensitization mechanism in complexes. In ac-
cordance with the selection rule followed by the forced electric
and magnetic dipole transition, the intense emission peak at
612 nm associated with the electric dipole transition and weaker
intensity band at 590 nm belongs to magnetic dipole transition
[24]. The intensity ratio of these characteristic bands imparts
useful information about the coordination environment of cen-
tral metal ion. The complexes shows a centrosymmetric behav-
iour around the europium ion with an intensity ratio lower than
0.7 while the value higher than 8 reflects the asymmetric envi-
ronment possessed by luminescent centre [25]. Thus the higher
intensity ratio of studied complexes directs the asymmetric
7
Figures 3 and 4 represents the excitation and emission spectra
of complexes obtained at room temperature by indexing the
intense emission band 5D0 → F2 and broad excitation band
7
7F0 → 5D4 respectively. The excitation spectra of complexes
consist of a broad band at 366–369 nm assigned to π → π*
transition of ligand, alongwith a narrow peak at 463 nm (C1-
C3, C5) and 466 nm (C4) associated with intraconfigurational
4f-4f transition 7F0 → 5D2 of europium ion [22]. The higher
intensity of broad band clearly suggests the role of ligand in
energy transfer processes to metal ion.
The emission spectra of europium(III) complexes C1-C5
shows magnificent luminescence behaviour on excitation with
UV light. The spectra were measured in 500–700 nm range
with slit width of 2.5 nm and voltage of 400PMT in the solid
state. Figure 4 displays the characteristic emission peaks
Fig. 2 UV-visible absorption spectra of the ligand p-EFBA and
Fig. 4 Emission spectra of complexes C1-C5 in solid state at
europium(III) complexes C1-C5
room temperature

J Fluoresc
environment around Eu(III) ion which is also pointed by the
presence of stark splitting in emissions bands of complexes.
The emission spectra also indicate a weak emission band at
Luminescence Quantum Efficiency (η) and Judd-Ofelt
Parameters
5
7
535 nm attributed to D1 → F0 transition of europium ion.
As a result of weak interaction between the ligand field
Hemiltonian and central europium ion there is appearance of
The luminescence quantum efficiency (η) is an important pa-
rameter to study the sensitization effect of ligand field on the
central metal ion. Hence to calculate quantum efficiencies, an
investigation based on luminescence decay time and emission
intensity of complexes has been made [29]. The luminescence
decay curve of europium(III) complexes C1-C5 are recorded
by monitoring the intense emission band 5D0 → 7F2 at 613 nm
and displayed in Fig. 7. Decay curve of all complexes show
monoexponential behaviour which is directed by the equation
I = I0 exp ( − t/ τ) where τ is the life time for radiative
emission, I0 and I are the luminescence intensities at
time 0 and t respectively, suggesting presence of similar
ligand field around europium(III) ion [30]. The obtained
results are listed in Table 3. By inversing this lumines-
cence lifetime, total transition rates of radiative (Arad) and
nonradiative (Anrad) processes can be calculated according to
equation [31]
5
7
electric and magnetic forbidden transition D0 → F0 in the
spectra [26]. Additionally, the spectra also shows the presence
of primarily forbidden 5D0 → 7F3 transition of Eu(III) ion. The
complexes shows similar behaviour of emission spectra with an
increasing intensity parameter from C1-C5 due to the presence
of ancillary ligands in complexes (C2-C5) which restrict the O-
H vibrations and thermal quenching resulting enhanced lumi-
nescence of complexes. The substitution of ancillary ligands
also diminished the non radiative losses of energy and effec-
tively increases the luminescence [27]. Figure 5 clearly displays
the higher luminescence properties of complexes from the pre-
viously reported complexes of isomeric ligands on the basis of
combined effects of electron withdrawing and resonance phe-
nomenon attributed to position of substituent fluorine in the
ligand [28]. The photoluminescence data of complexes C1-C5
tabulated in Table 3.
Atotal ¼ 1=τ ¼ Arad þ Anrad
ð1Þ
The CIE (Commission Internationale de I′ Eclairage) color
coordinate diagram are applied to observe the luminescence
intensity of organic metal complexes in terms of x and y
values. The coordinates (x, y) recorded by employing the
emission spectra of complexes and given in Table 3. The co-
ordinates of complexes fall in red region of diagram but due to
close proximity between values only C1 and C5 values shown
in Fig. 6. The calculated values are comparable with the
Society of Motion Picture and Television (SMPTE, x = 0.63,
y = 0.34), National Television System committee (NTSC
1987, illuminant red 0.63, 0.34) and Europian Broadcasting
Union (EBU illuminant red 0.64, 0.33) reference color sys-
tems. The observed values mark the complexes as worthy
candidates in regime of luminescent materials.
The radiative rate Arad can be calculated by carrying out the
sum of individual radiative rate A0J for each emission transi-
7
tion (5D0 → FJ) of europium ion. The individual transition
rates can be estimated by the given equation [32]
A0J ¼ A01ðI0J =I01Þðν01=ν0J
Þ
ð2Þ
where A01 is the spontaneous emission coefficient for
5D0 → 7F1 transition having a value found to be 50 s−1 [33].
Depending on the independent coordination environment
around europium(III) ion for magnetic dipole transition, A01
is used as standard to calculate the A0J value for each
transition (J = 0,2,3,4) of europium ion.
is the ener-
0J
gy barycentre and I0J is the integrated emission intensity
7
for different transitions (5D0 → FJ) respectively.
The radiative and nonradiative processes affect the
quantum efficiency of complexes as expressed by fol-
lowing equation [34]
η ¼ Arad=Arad þ Anrad
ð3Þ
The results for efficiency parameters are tabulated in
Table 3. The higher quantum efficiency of complexes indi-
cates that ligand acts as good sensitizer for europium ion.
The Judd-Ofelt intensity parameters Ωλ (λ = 2, 4) of
europium(III) complexes C1-C5 were estimated by mak-
ing utilisation of calculated radiative transition rate
(Arad) and Judd-Ofelt theory which was developed by
Judd and Ofelt for the analysis of laporte forbidden
4f-4f electronic transitions [35]. The intensity parameters
Fig. 5 Relative luminescence of ortho, meta and para derivative
complexes of β-ketoester ligand

J Fluoresc
Table 3 Photoluminescence data of complexes C1-C5 in solid state
Complexes
CIE Coordinates (x, y)
τ (ms)
Atotal (s−1
)
Arad (s−1
)
Anrad (s−1
)
Ω2 (10−20 cm2)
Ω4 (10−20 cm2)
η (%)
C1
(0.6301, 0.3691)
(0.6309, 0.3684)
(0.6388, 0.3605)
(0.6318, 0.3674)
(0.6436, 0.3558)
(0.6077, 0.3893)
(0.6431, 0.3563)
0.466
0.570
0.594
0.655
1.181
0.668
0.760
2145
1754
1683
1526
846
369
449
531
537
597
473
576
1776
1305
1152
989
6.41
7.49
9.31
9.43
10.7
8.31
10.11
0.18
0.25
0.26
0.24
0.35
−
17.2
25.6
31.6
35.2
69.4
40.9
44.1
C2
C3
C4
C5
297
Eu(o-EFBA)3.bipy
Eu(m-EFBA)3.bipy
1156
1305
683
729
0.25
Ωλ for the emissions 5D0 to 7FJ (J = 2, 4) can be calculated by
following equation [36]:
Table 3 which clearly reveals the effect of fluorine position
on the luminescent properties and confirms that para position
of fluorine is most significant for the development of highly
luminescent complexes [39, 40].
ꢀ
ꢁ
2
Ωλ ¼ 3 ћc3A0j=4e2ω3χ 5D │UðλÞ│7F
ð4Þ
0
j
Energy Transfer Mechanism
Where χ = n2(n2 + 2)2/9 is Lorentz local field correction, n
is the refractive index (1.5) and e is the electronic charge
The purposed intramolecular energy transfer mechanism
for highly intense complex C5 is shown in Fig. 8 to
describe the sensitization process in complexes. In the
sensitization process, first of all ligand gets excited to
its singlet state by absorbing energy and then from sin-
glet excited state to triplet excited state by the process
of intersystem crossing (ISC). The triplet excited state
of ligand transfers energy to the emitting level of euro-
pium ion [41]. Hence singlet and triplet excited states
(S1; 27,472 cm−1, T1; 22,271 cm−1) of ligand play a
significant role in energy transfer and calculated from
UV-visible absorbance edge and phosphorescence spec-
tra of corresponding gadolinium complex respectively
[42]. The phosphorescence spectrum of complex is
displayed in Fig. 9. The singlet and triplet excited states of
À
Á
(4.80 × 10−10 esu) used. 5D │UðλÞ│7F
are the square
2
0
J
reduced matrix elements 0.0032 (J = 2) and 0.0023 (J = 4)
respectively. The Judd-Ofelt intensity parameters for
complxes C1-C5 are represented in Table 3.
The intensity parameter for transition 5D0 → F2 (Ω2) re-
7
lated with the hypersensitive and covalence character of
chemical environment around europium(III) ion while Ω4 as-
sociated with the interionic bond distances [37]. The intense
5
7
emission of transition D0 → F2 leads to stronger radiative
rates and resulting much higher value of Ω2 as compared to Ω4
in all the complexes [38]. A comparative analysis of efficiency
and intensity parameters of highly intense complex Eu(p-
EFBA)3.bipy (C5) with the previously reported complexes
Eu(o-EFBA)3.bipy and Eu(m-EFBA)3.bipy shown in
Fig. 7 The luminescence decay curve of europium(III) complexes
Fig. 6 CIE color coordinate diagram of europium(III) complexes C1and C5
(C1-C5) at room temperature

J Fluoresc
Fig. 8 The energy transfer
mechanism in complex
Eu(p-EFBA)3.bipy (C5)
ancillary ligand bipy (29,900 cm−1, 22,900 cm−1), phen
energy transmitted to the 5D1 level of europium ion. This also
suggests the role of ancillary ligand in the enhancement of
luminescence for complexes. The T1 levels of phen and batho
ligand are lower and may be associated with back energy
transfer processes resulting weaker luminescence complexes
(C3, C4) compared to complex C5. The Dexter theory [45]
and thermal de-excitation theory [46] helps to conclude the
energy transfer processes in complexes.
(31,000 cm−1, 22,100 cm−1) and batho (29,000 cm−1
,
21,000 cm−1) are calculated and found to be well matched
with the literature [12].
The effective energy transfer in complexes attributed as a
result of suitable energy difference between triplet state of
ligand to the resonating level of europium(III) ion according
to Latva’s empirical rule suggesting an optimal energy gap ΔE
(T1-M3+) of the range 2000–5000 cm−1 [43]. The triplet level
5
of ligand tailored to maintain the upper emitting level D1
(18,691 cm−1) and then vibrationally relaxes to the lower
Conclusion
5
emitting level D0 (17,241 cm−1). From this emitting level
5D0 releases energy to ground 7Fj levels of europium ion and
gives the luminescence [44]. Figure 8 clearly depicts that T1
state of ancillary ligand first absorb energy and then transfer
this energy to the triplet level of p-EFBA, from this level the
In this study, a series of europium(III) complexes C1-C5 have
been synthesized and characterized by elemental analysis, IR,
1H–NMR, UV-visible, TG/DTG and PL spectroscopy. The
effect of position of substituent fluorine on the luminescence
properties of complexes Eu(p-EFBA)3.(H2O)2 (C1), Eu(p-
EFBA)3.neo (C2), Eu(p-EFBA)3.batho (C3), Eu(p-
EFBA)3.phen (C4), Eu(p-EFBA)3.bipy (C5) was investigated
successfully. The enhanced luminescence of complexes from
previous reported complexes shows the remarkable effect of
substituent position on luminescence properties. The effect of
position studied as a combined effect of resonance phenome-
non and electron withdrawing nature of substituent attached.
The studied complexes show higher intensity and efficiency
parameters which pointing the utilisation of complexes in de-
velopment of excellent luminescent materials.
Acknowledgements This work was financially supported in the form
of senior research fellowship (SRF) from UGC, New Delhi, India (Award
No: 2121210101).
Compliance with Ethical Standards
Conflict of Interest The authors indicated no potential conflict
Fig. 9 The phosphorescence spectra of complex Gd(p-EFBA)3.(H2O)2
of interest.

J Fluoresc
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