Investigations of luminescent behavior and intramolecular energy transfer mechanism of europium(III) complexes with fluorinated β-ketoester ligand

Article abstract of DOI:10.1016/j.jfluchem.2015.11.004

A fluorinated β-ketoester ligand, ethyl-(2-fluorobenzoyl) acetate (EFBA) has been employed to synthesized new europium binary and ternary complexes with ancillary ligands neocuproine (neo), bathophenanthroline (batho), 1,10-phenanthroline (phen) and 2,2-bipyridyl (bipy) in high yield by the solution precipitation method. The ligand (EFBA) and the synthesized complexes Eu(EFBA)₃·(H₂O)₂ (C1), Eu(EFBA)₃·neo (C2), Eu(EFBA)₃·batho (C3), Eu(EFBA)₃·phen (C4), Eu(EFBA)₃·bipy (C5) were characterized by elemental analysis, ¹H NMR, IR, UV-vis, thermal analysis (TG/DTG) and luminescence spectroscopy. The thermal analysis results show that the complex is stable up to 231 °C temperature which is sufficient for the normal working of OLEDs. The luminescence measurements indicate that π electron rich organic ligand ethyl-(2-fluorobenzoyl) acetate possessing high absorption coefficient acts as a suitable chromophoric moiety for the transfer of energy to the central Eu³⁺ ion by the process of sensitization, this results in strong emissions band in red spectral region attributed to electric dipole ⁵D₀ → ⁷F₂ transition of europium ion. The luminescence intensity further enhanced by the introduction of N-containing ancillary ligands neocuproine, bathophenanthroline, 1,10-phenanthroline, and 2,2-bipyridyl in the ternary complexes C2-C5 because these ligands replace the solvent molecule in the coordination sphere and satisfy the coordination number around central europium ion. The sensitization process in europium complexes is investigated through the proposed energy transfer mechanism. The decay curves observations show the similar chemical environment around the europium ion in all the complexes.

Full text of DOI:10.1016/j.jfluchem.2015.11.004

Journal of Fluorine Chemistry 181 (2016) 36–44
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Investigations of luminescent behavior and intramolecular energy
transfer mechanism of europium(III) complexes with fluorinated
b
-ketoester ligand
*
Rekha Devi, Manju Bala, S.P. Khatkar, V.B. Taxak, Priti Boora
Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India
A R T I C L E I N F O
A B S T R A C T
A fluorinated
Article history:
b
-ketoester ligand, ethyl-(2-fluorobenzoyl) acetate (EFBA) has been employed to
Received 8 August 2015
synthesized new europium binary and ternary complexes with ancillary ligands neocuproine (neo),
bathophenanthroline (batho), 1,10-phenanthroline (phen) and 2,2-bipyridyl (bipy) in high yield by the
solution precipitation method. The ligand (EFBA) and the synthesized complexes Eu(EFBA)3ꢀ(H2O)2 (C1),
Eu(EFBA)3ꢀneo (C2), Eu(EFBA)3ꢀbatho (C3), Eu(EFBA)3ꢀphen (C4), Eu(EFBA)3ꢀbipy (C5) were characterized
by elemental analysis, 1H NMR, IR, UV–vis, thermal analysis (TG/DTG) and luminescence spectroscopy.
The thermal analysis results show that the complex is stable up to 231 8C temperature which is sufficient
for the normal working of OLEDs. The luminescence measurements indicate that
ligand ethyl-(2-fluorobenzoyl) acetate possessing high absorption coefficient acts as
chromophoric moiety for the transfer of energy to the central Eu3+ ion by the process of sensitization, this
results in strong emissions band in red spectral region attributed to electric dipole 5D0 7F2 transition
Received in revised form 15 October 2015
Accepted 10 November 2015
Available online 14 November 2015
Keywords:
Luminescence
Eu(III) complexes
Sensitization
p
electron rich organic
a
suitable
b
-Ketoester
!
of europium ion. The luminescence intensity further enhanced by the introduction of N-containing
ancillary ligands neocuproine, bathophenanthroline, 1,10-phenanthroline, and 2,2-bipyridyl in the
ternary complexes C2–C5 because these ligands replace the solvent molecule in the coordination sphere
and satisfy the coordination number around central europium ion. The sensitization process in europium
complexes is investigated through the proposed energy transfer mechanism. The decay curves
observations show the similar chemical environment around the europium ion in all the complexes.
ß 2015 Elsevier B.V. All rights reserved.
1. Introduction
as a chromophore to indirectly sensitize the metal ion by the
transfer of energy from triplet excited state of the ligand to the
In last decade, the lanthanide ion based complexes have
received intensive and greater attention owing to long lumines-
cence life time, large stoke shift and narrow emission bands of
lanthanide ion which makes them strong candidates in many
applications like luminescent solar concentrators [1], pH sensing
[2], MRI contrast agents [3], UV dosimeters [4] and organic light
emitting devices (OLEDs) [5–7]. In lanthanide ions the 4f orbitals
are well shielded by 5s and 5p orbitals which results in narrow
emission bands and long radiative lifetime arising mainly due to
the electronic transitions between specific 4fn energy levels [8]. As
4f–4f transitions are laporte forbidden transitions therefore
possess low molar absorption coefficient of lanthanide ion
resulting in weak luminescence intensity. Hence this mandates
the use of organic ligand having high molar absorption coefficient
metal ion by the process of sensitization [9–11]. In the sensitiza-
tion process the ligand get excited by absorbing the energy in its
singlet level which further populates the triplet energy level of
ligand by the intersystem crossing (ISC) and then transfer the
energy to the emitting level of central metal ion, which finally
relaxes through radiative and non-radiative transitions. The
efficient transfer of energy requires a good overlap between
the triplet excited level of ligand and the emitting level of
lanthanide ion. As
extended -conjugation fulfils the above conditions and makes
them desirable applicant in the field of luminescent materials
[12]. However, the complexes with -ketoester ligand are vaguely
a consequence, b-ketoester ligand with
p
b
investigated in the literature because such ligands contain high
energy C–H and O–H oscillators which non-radiatively quench the
emitting level of lanthanide ion and lead to diminished lumines-
cence. To overcome this problem, we have selected fluorinated
b-
ketoester as main sensitizer as it possesses several potential
advantages such as C–F bond has low vibrational frequency which
*
Corresponding author. Tel.: +91 9416175000.
E-mail address: pritiboora@gmail.com (P. Boora).
http://dx.doi.org/10.1016/j.jfluchem.2015.11.004
0022-1139/ß 2015 Elsevier B.V. All rights reserved.

R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
37
can decrease the rate of nonradiative deactivation and also reduces
the self quenching phenomenon. Moreover, the C–F bond in ligand
can change the molecular packing and it also increases the electron
mobility of the complex [13–15]. It is further noticed that the
fluorine substitution induces heavy atom effect thereby accelerat-
ing the spin orbit coupling, which promotes the ISC (intersystem
crossing) phenomenon during energy transfer process, resulting
into enhanced luminescence [16–18]. Among lanthanide com-
plexes, the complexes containing Eu(III) ion have gained valuable
significance for full color flat panel displays due to their peculiar
with calculated values on the basis of proposed molecular formula.
The results clearly reveal that the composition of complexes
confirms the formulas Eu(EFBA)3ꢀ(H2O)2 (C1), Eu(EFBA)3ꢀneo (C2),
Eu(EFBA)3ꢀbatho (C3), Eu(EFBA)3ꢀphen (C4), Eu(EFBA)3ꢀbipy (C5),
respectively. All the europium (III) complexes are stable in air, non
hygroscopic, white color and are highly soluble in CHCl3 and
ethanol solvent.
2.2. 1H NMR spectra
photophysical properties and intense red emission at 613 nm
The characteristic 1H NMR spectral data of free ligand and its
europium complexes were recorded in CDCl3 solvent. The spectral
data of europium complexes as compared to the free ligand, exhibit
apparent changes due to the paramagnetism of the europium(III)
7
attributed to 5D0 ! F2 transition. For this purpose,
a new
fluorinated
b-ketoester ligand ethyl-(2-fluorobenzoyl) acetate
(EFBA) was employed to synthesize new binary and ternary
europium complexes. The ternary complexes contain ancillary
ion. The spectra of free ligand displays a singlet of enolic proton at
d
ligands beside
b
-ketoester ligand (EFBA) due to liability of rare
12.69 ppm, methine proton at 5.86 ppm and keto CH2 proton at d
d
earth ion to extend its coordination sphere. An ancillary ligand
substitutes the water molecules in its coordination sphere and
reduces the non-radiative loss which in turn enhances the
luminescence properties of metal complexes [19].
4.39 ppm. The purity of the product was confirmed by the absence
of specific peaks of keto CH2 proton and enolic proton in the spectra
of complexes C1–C5 [21,22]. In the complexes C1–C5, the peak
corresponding to methine proton undergoes slightly up-field as
compared to free ligand. The chemical shift for aryl protons in
ligands (EFBA, ancillary ligand) shows noticeable changes in the
NMR spectra of europium complexes.
In the present communication, we have synthesized five new
Eu(III) complexes with fluorine functionalised
b-ketoester as
primary ligand and neocuproine, bathophenanthroline, 1,10-
phenanthroline and 2,2-bipyridyl as N-containing ancillary
ligands. The synthesized Eu(III) complexes (C1–C5) were
characterized by elemental analysis, 1H NMR, IR, UV–vis,
thermogravimetric analysis and luminescence spectroscopy.
The result clearly indicates that these ternary complexes possess
higher thermodynamic stability, long luminescence life time and
strong emission intensity. The energy transfer mechanism was
discussed by calculating the singlet and triplet energy levels of
ligand which suggest that the ancillary ligands enhance the
process of sensitization by the efficient transfer of energy to the
europium ion.
2.3. IR spectra
The characteristic IR spectral data of ligand EFBA and its
europium complexes C1–C5 are tabulated in Table 2.
The IR spectra displays a broad band at 3471 cmꢁ1 and
3417 cmꢁ1 attributed to enolic O–H stretching vibration of organic
ligand EFBA and coordinated water molecule in the complex C1
respectively. The broad band in the spectra of complexes C2–C5
disappears due to coordination of enolic O–H with the europium
ion. The spectra of ligand displays a strong bands at 1625 cmꢁ1 and
1538 cmꢁ1 accredited to >C55O and >C55C which undergoes red
shift in the complexes due to the enlarged
p-conjugated system of
2. Results and discussion
europium complexes [23]. As compared to IR spectra of free ligand
and complex C1, the complexes C2–C5 shows a strong stretching
vibration band at 1579–1562 cmꢁ1 ascribed to C55N bonds of N-
containing ancillary ligands like neocuproine, bathophenanthro-
line, 1,10-phenanthroline and 2,2-bipyridyl. The absorption peaks
in the range of 450–490 and 535–542 cmꢁ1 in the spectra of
europium complexes correspond to new bonds of Eu–O and Eu–N
which are absent in the spectra of the ligand [24].
2.1. Properties of the complexes
The analytical data of (C, H and N) and the europium ion content
of C1–C5 europium(III) complexes are listed in Table 1. The
europium ion content of complexes is estimated by the com-
plexometric titration with the help of EDTA using Eriochrome
Black T as an indicator. The results obtained are in good agreement
2.4. Thermogravimetric analysis
Table 1
Elemental composition data of europium complexes C1-C5.
The thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis of europium complexes were conducted at a
heating rate of 10 8C minꢁ1 from room temperature to 1050 8C
temperature under N-atmosphere. The europium complexes C1–
C5 show a similar pattern of thermal analysis curve (TG/DTG)
hence the thermal analysis curve of complex C3 is taken as
analogue for the thermal decomposition of other europium
complexes and represented in Fig. 1. The TG curve shows the
Complexes
C (%) found
(cal.)
H (%) found
(cal.)
N (%) found
(cal.)
Eu (%) found
(cal.)
C1
C2
C3
C4
C5
48.46 (48.82)
56.87 (57.36)
61.39 (61.76)
56.35 (56.32)
55.39 (55.41)
3.96 (4.19)
4.21 (4.27)
4.05 (4.15)
3.92 (3.97)
3.98 (4.08)
–
18.21 (18.24)
15.44 (15.05)
13.28 (13.36)
15.44 (15.49)
15.84 (15.89)
2.76 (2.84)
2.45 (2.52)
2.90 (2.93)
3.04 (3.01)
Table 2
The characteristic IR data of the ligand EFBA and complexes.
Complex
n(O–H)
n
(C5O)
n(C5N)
n(C5C)
n(C–F)
n(Eu–N)
n(Eu–O)
EFBA
C1
3471 (b)
1625 (s)
1622 (s)
1623 (s)
1619 (s)
1622 (s)
1622 (s)
–
–
1538 (s)
1515 (s)
1514 (s)
1514 (s)
1505 (s)
1515 (s)
1256 (s)
1250 (s)
1249 (s)
1247 (s)
1248 (s)
1250 (s)
–
–
–
3417 (b)
490 (m)
491 (m)
449 (m)
459 (m)
490 (m)
C2
–
–
–
–
1579 (s)
1562 (s)
1579 (s)
1578 (s)
537 (s)
542 (m)
535 (m)
538 (s)
C3
C4
C5
b =broad, s = strong, m =medium.

38
R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
Fig. 1. The TG/DTG curve of complex C3 measured from room temperature to 1050 8C under nitrogen atmosphere.
starting weight loss of approximately 2% from temperature 100–
155 8C due to the loss of adsorbed and coordinated water
molecules. The first decomposition step occurs from 231 8C to
473 8C accompanied with weight loss of 66.04% due to the
elimination of two EFBA molecules and one bathophenanthroline
molecule from the europium complex. The second step involves
the decomposition of one EFBA molecule started at 473 8C with a
weight loss of 17.22% ending at 946 8C. Concordant to these weight
losses, the DTG curve of complex exhibit three obvious peaks at
281 8C, 370 8C and 931 8C, respectively. The remaining product
approximately 15% corresponds to the formation of europium
oxides. The thermal analysis result shows that the europium
complexes decomposes at temperature above 150 8C and are
thermally stable for the fabrication of light emitting devices.
solution (c = 1 ꢂ 10ꢁ5 M) at room temperature. The spectral profile
of Eu(III) complexes are similar to that of free ligand, which
demonstrate that the introduction of ligand facilitate the absorp-
tion in complexes because Eu(III) ion displays very weak
absorption in the selected range due to laporte forbidden
transitions [25]. Fig. 2 shows the maximum absorption peak for
the ligand EFBA and europium complexes C1–C5 at 310 nm,
317 nm, 319 nm, 315 nm, 318 nm, 316 nm and their correspond-
ing values of extinction coefficient are in range 2.24–
2.46 ꢂ 105 Lmolꢁ1 cmꢁ1, respectively [26]. The bathochromic shift
in the absorption value of europium complexes arise mainly due to
prominent
p
!
p* transition of conjugated chromophoric moiety
which efficiently transfer the energy to the europium ion [27].
2.5.2. Luminescent spectra
2.5. Luminescent behavior
The excitation and emission spectra of binary and ternary
Eu(III) complexes in solid state at room temperature are shown in
Fig. 3(a)–(e). The excitation spectra of all complexes (C1–C5)
monitored at ^em 613 nm illustrates the broad excitation band at
357 nm except for the complex C4 which shows the excitation
2.5.1. UV–vis absorption spectra
UV–vis absorption spectra of the free ligand (EFBA) and the
corresponding europium complexes were recorded in chloroform
Fig. 2. UV–vis absorption spectra of the ligand EFBA and europium complexes C1–C5 in chloroform solution (1 ꢂ 10ꢁ5 mol/L).

R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
39
Fig. 3. (a–e) Excitation and emission spectra of complexes C1–C5 in solid state at room temperature.
band at 370 nm. This broad excitation band in the europium
complexes ascribed to * transition of EFBA ligand. The
coordination environment around Eu(III) ion and responsible
for the extreme red emission of europium complexes [28].
Further, stark splitting sublevels caused by the external field are
p
!
p
emission spectra of Eu(III) complexes (C1–C5) comprised of many
sharp lines in the range 400–700 nm rooting ^ex 357 nm (for C4 ^ex
370) with 400 PMT voltage set up and 2.5 nm excitation and
emission slit width as illustrated in Table 3. The spectra depicts
intense emission bands at 580 nm, 590 nm, 613 nm and 652 nm
7
shown in 5D0 ! F2 transition which depends on the local
symmetry around the luminescent center. The intensity ratio
7
7
(I1/I2 = 5D0 ! F2/5D0 ! F1) of electric to magnetic dipole transi-
tion lies in range 5–10 and this high intensity ratio reveals that the
electric dipole transition has highly asymmetric chemical envi-
7
7
7
7
attributed to 5D0 ! F0, 5D0 ! F1, 5D0 ! F2 and 5D0 ! F3
7
transitions of Eu(III) ion, respectively. Among these transitions
ronment around Eu(III) ion [29,30]. The spectra display 5D0 ! F0
the 5D0 ! F1 is magnetic dipole transition insensitive to the
forbidden transition with very low intensity induced by the J-
mixing effect [31]. This effect arises due to coupling between the
even components of the ligand field Hamiltonian. The intensity
7
local coordination symmetry while 5D0 ! F2 transition is
7
electric dipole allowed which is extremely sensitive to the

40
R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
Table 3
Photoluminescence data, color coordinates and decay time of C1–C5 complexes.
Complexes
C1
?
(cmꢁ1) in nm
?
(cmꢁ1) in nm
em
Energy transition
Color coordinates (x,y)
Lifetime (ms)
0.603
ex
7
357 (28,011.21)
580 (17,241.38)
590 (16,949.15)
613 (16,313.21)
652 (15,337.42)
580 (17,241.38)
5D0! F0
(0.5625, 0.4281)
7
5D0! F1
7
5D0! F2
7
5D0! F3
7
5D0! F0
7
C2
357 (28,011.21)
590 (16,949.15)
613 (16,313.21)
652 (15,337.42)
580 (17,241.38)
5D0! F1
(0.6077, 0.3893)
0.615
7
5D0! F2
7
5D0! F3
7
5D0! F0
7
C3
C4
357 (28,011.21)
370 (27,027.03)
590 (16,949.15)
613 (16,313.21)
652 (15,337.42)
580 (17,241.38)
590 (16,949.15)
613 (16,313.21)
652 (15,337.42)
580 (17,241.38)
5D0! F1
(0.6210, 0.3766)
(0.6352, 0.3627)
0.655
0.668
7
5D0! F2
7
5D0! F3
7
5D0! F0
7
5D0! F1
7
5D0! F2
7
5D0! F3
7
5D0! F0
7
C5
357 (28,011.21)
590 (16,949.15)
613 (16,313.21)
652 (15,337.42)
5D0! F1
(0.6373, 0.3606)
0.865
7
5D0! F2
7
5D0! F3
7
7
ratio between the 5D0 ! F0 and 5D0 ! F2 transition gives the
information about the J-mixing effect and points the interaction
between the Eu(III) ion and chemical environment.
prove that these complexes can be used as a red component in
white OLEDs.
Fig. 6 displays the fluorescence decay profile of 5D0 level of
europium(III) complexes (C1–C5) measured by monitoring the
The emission of ternary Eu(III) complex containing ancillary
ligand is noticed to be highly intense as compared to the binary
complex C1 (Fig. 4) due to the quenching caused by the O–H
oscillators in the solvent molecule. The result obtained reveals that
the ancillary ligand acts as a good sensitizer to effectively transfer
the energy to Eu(III) ion [32].
7
emission band of 5D0 ! F2 transition at 612 nm in solid state. The
decay time values for the complexes C1–C5 are calculated by the
software FL solution for F-7000 spectrophotometer and listed in
Table 3. The decay curve obey single exponential function
I = I0exp(ꢁt/
t) where t is the decay time for radiative emission,
Generally, the color of complexes reported in terms of CIE
(Commission Internationale de I’ Eclairage) chromaticity coordi-
nates (x, y). The color coordinates of europium complexes
calculated from respective PL spectra are shown in Table 3 and
Fig. 5 which depicts that with the introduction of ancillary ligand
the coordinates of complexes C2–C5 moves toward optimal red
region which can be assigned to reshuffling of emission intensity as
compared to complex C1. The CIE color coordinates of the
complexes are quiet close to National Television System commit-
tee (NTSC, 1987) primary color (0.63, 0.34). These observations
I0 and I are the luminescence intensities at time 0 and t
respectively. The decay curve clearly reveals the presence of
single chemical environment around the central europium ion. The
decay time value of all the complexes are comparable (C1;
0.603 ms, C2; 0.615 ms, C3; 0.655 ms, C4; 0.668 ms, C5; 0.865 ms),
however slightly longer lifetime for complexes C2–C5 are due to
the negligible nonradiative decay and efficient energy transfer
from the ancillary ligand to metal ion [33].
The relative quantum efficiency (
is calculated by using Rhodamine 6G as a standard compound
std = 0.5) in MATLAB software by applying following equation
[34]:
h) of Eu(III) complexes C1–C5
(
h
I
Astd n2
h
¼
hstd
:
:
:
(1)
n2std
Istd
A
hstd is the quantum efficiency of the standard compound. Astd
(0.029) is the absorbance of the standard at the excitation
wavelength of 488 nm and A is the absorbance of the complexes
with the excitation wavelength of 357 nm for complexes C1–C5
except for C4 (370 nm). Their absorbance values are found to be
0.2877, 0.3405, 0.0897, 0.1103 and 0.75 for complex C1–C5,
respectively. I and Istd are the integrated intensities of corrected
emission spectra of complex and standard compound, respectively.
n and nstd are the refractive index of the solvent for the complex
and standard compound respectively. The europium(III) com-
plexes C1–C5 show the relative quantum efficiency in range 9.06–
111.70. This may be the consequence of efficient energy transfer by
the ligand and ancillary ligands to the europium(III) ion.
2.5.3. Intramolecular energy transfer mechanism
In order to demonstrate the sensitization pathway in Eu(III)
complexes, the proposed mechanism of energy transfer in
Fig. 4. The emission spectra for complexes C1–C5 in solid state at room temperature
excited at 357 nm (for C4 ?ex 370), respectively.

R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
41
Fig. 5. CIE chromaticity diagram of europium (C1, C5) complexes with ancillary ligand.
europium complexes of distinct skeleton in terms of ancillary
ligand (C3–C5) are shown in Figs. S1, S2 and Fig. 7, respectively. The
excited singlet and triplet energy levels of ligand EFBA were
calculated from the absorbance edge wavelength of UV–vis
absorption spectra (Fig. 1) and the shortest wavelength phospho-
rescence band of the corresponding Gd complex C6 (Fig. S3),
respectively [35]. The Gd complexes were selected as a reference
complex to determine the energy of triplet level of organic ligand
The intersystem crossing (ISC) occurs effectively only with a
significant value of energy difference between the singlet and
triplet excited levels [
by S1 and T1 values from the Table 4 that
phen and bipy is 8104 cmꢁ1 8000 cmꢁ1
D
E (S1 ꢁ T1)] of ligand. It is clearly revealed
E for ligand EFBA, batho,
8900 cmꢁ1 and
D
,
,
7000 cmꢁ1, respectively which illustrate the efficient intersystem
crossing in complexes C3–C5. Latva’s empirical rule suggest that
the energy gap
D
E (T1 ꢁ M3+) should be in range of 2000–
as the energy of lowest emitting level of Gd3+ (6P7/2 ! S7/2) is
5000 cmꢁ1 for the effective transfer of energy from triplet excited
level of ligand to emitting level of lanthanide ion [36]. The energy
gap between triplet energy level of ligand (EFBA, batho, phen and
8
much higher (32051 cmꢁ1) as compared to the energy of triplet
level of EFBA (24154 cmꢁ1) which clearly indicate that Gd3+ ion
cannot be sensitized by the ligand and tabulated in Table 4, which
also involve the energy value of excited singlet (S1) and triplet (T1)
levels of ancillary ligands batho, phen and bipy as recorded from
literature [33].
bipy) and emitting level of europium ion is 6913 cmꢁ1, 3759 cmꢁ1
,
4859 cmꢁ1, 5659 cmꢁ1, respectively, which indicate that the
energy gap between ligand EFBA and Eu3+ is not appropriate for
the optimal transfer of energy from ligand to Eu3+ ion. For the
efficient energy transfer, the energy absorbed by EFBA ligand is
transferred to the triplet energy level of ancillary ligands first and
finally to emitting level of Eu(III) ion which results in higher
luminescence intensity of ternary complexes as compared to that
of binary complex. This intramolecular energy transfer in Eu3+
complexes were supported by the Dexter electron exchange theory
and the thermal de-excitation theory. According to Dexter electron
exchange theory the process of sensitization is favoured by the
appropriate energy difference between the triplet energy level of
ligand and emitting level of Eu3+ ion which results in higher
Table 4
The excitation energy of EFBA, phen, bipy and batho ligands.
Ligands
Excitation energy in cmꢁ1(eV)
Singlet level (S1)
Triplet level (T1)
EFBA
Phen
Bipy
32,258 (3.99)
31,000 (3.84)
29,900 (3.71)
29,000 (3.58)
24,154 (2.98)
22,100 (2.74)
22,900 (2.83)
21,000 (2.59)
Fig. 6. The luminescence decay curve of europium complexes (C1–C5) at room
Batho
temperature.

42
R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
Fig. 7. The schematic energy level diagram and the energy transfer mechanism in Eu(EFBA)3ꢀbipy (C5).
luminescence of complexes [37]. The probability of energy transfer
is given as
have significant effect on the luminescent decay time, chromaticity
coordinates and thermal stability of these complexes which makes
them as potent material for organic light emitting devices (OLEDs)
The energy transfer mechanism also furnishes the utilisation of
these europium complexes in the field of development of highly
luminescent materials.
!
Z
2p
Z2
h
Ps ¼
FsðEÞꢀ
j
sðEÞꢀdE
(2)
where Ps represents the transfer probability constant, Fs(E) is
luminescence spectrum of energy donor (ligand), s(E) is absorp-
tion spectrum of energy acceptor (Eu3+) and 2 Z2/h is constant
relating to mutual distance between Eu3+ and coordinating atom.
The thermal de-excitation theory involves the inverse energy
transfer from emitting level of Eu(III) ion to the triplet excited level
of ligand [38]. The rate of inverse energy transfer is
j
4. Experimental
p
4.1. Starting material
All reagent grade chemicals and solvent employed were
commercially available and used as received without further
purification. Eu(NO3)3ꢀ5H2O and Gd(NO3)3ꢀ5H2O with high purity
(99.9%) were purchased from Sigma Aldrich.
D
E=RT
KðTÞ ¼ Aꢀeꢁ
(3)
where
DE is the energy difference between the triplet excited level
4.2. Physical measurements
of ligand to emitting level of Eu(III) ion. The lower energy
difference between the T1 of ligand to emitting level of Eu(III) ion
facilitate the inverse energy transfer process. Apparently it appears
that the energy gap between bipy and Eu(III) ion is much higher as
compared to energy gap between batho to Eu(III) ion and phen to
Eu(III) ion, therefore it diminished the inverse energy transfer
process in complex C5 resulting in high luminescence intensity as
compared to complex C3 or C4 in which there is efficient back
energy transfer may take place. The above energy transfer
mechanism consistent with the luminescent measurements
indicating the higher luminescent intensity of ternary complexes
C2–C5 as compared to binary complex C1.
All the measurements were carried out at room temperature.
The composition of complexes was recorded on Perkin Elmer
2400 CHN elemental analyser. IR spectra were obtained on Perkin
Elmer Spectrum 400 spectrometer in dried KBr pellets in the range
of 4000–400 cmꢁ1 1H NMR spectra was obtained in CDCl3
.
solution on Bruker Avance II 400 NMR spectrometer with
tetramethylsilane (TMS) as an internal reference. Luminescence
excitation and emission spectra were obtained on a Hitachi F-
7000 fluorescence spectrophotometer furnished with xenon
lamp. The luminescence decay time was recorded by software
of the spectrophotometer. Thermal stability of the complexes was
measured with SDT Q600 with a heating rate 10 8C/min under
nitrogen atmosphere upto 1050 8C. UV–vis absorption spectra of
the complexes were recorded with Shimadzu-2450 UV–vis
spectrophotometer.
3. Conclusion
In summary, the five novel Eu(III) complexes C1–C5 have been
synthesized by using a fluorinated
b-ketoester as organic ligand
and characterized by elemental analysis, IR, 1H NMR, TGA, UV–vis
and luminescence spectroscopy. The strong characteristic red
emission of Eu3+ ion indicates that the ethyl-(2-fluorobenzoyl)
acetate (EFBA) ligand acts as a good protector of europium ion from
the non radiative losses by the solvent molecules and provides a
good sensitization effect for the efficient transfer of energy from
ligand to europium ion. The result of emission spectra clearly
reveals that the N-containing ancillary ligands neocuproine,
bathophenanthroline, 1,10-phenanthroline, and 2,2-bipyridyl
4.3. Synthesis of
(EFBA)
b-ketoester ligand ethyl-(2-fluorobenzoyl) acetate
The synthesis of ligand EFBA was completed in two steps as
given below and illustrated in Scheme 1:
Step 1: 2,2-dimethyl-1,3-dioxane-4,6-dione was prepared by
adding a solution of acetone in malonic acid in presence of acid
catalyst (H2SO4). Then excess of crushed ice was added and the

R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
43
Scheme 1. The synthetic route of the ligand EFBA.
reaction mixture was left for 2 h. To the above reaction mixture
excess of acetic anhydride was added leading to the formation of
product. The product formed was recrystalized with ethanol and
used as such for the next step.
Step 2: A mixture is prepared by dissolving 2-fluorobenzoyl
chloride and 2,2-dimethyl-1,3-dioxane-4,6-dione in dry methy-
lene chloride (CH2Cl2). 4-(Dimethylamino) pyridine was added
into the reaction mixture at about 0 8C and stirred for 1 h at room
temperature. The resulting mixture was washed with water and
then with 1 N HCl and further dried by using anhydrous
Na2SO4. The product formed was mixed with ethanol in toluene
and heated at 90 8C by using reflux condenser for approximately
24 h. The excess ethanol and solvents were removed and progress
of reaction was checked by TLC [20]. The final product EFBA was
obtained with 52% yield. IR(KBr): cmꢁ1 3471 (b), 3358 (b), 2984 (s),
1743 (s), 1625 (s), 1611 (s), 1538 (s), 1454 (s), 1413 (s), 1328 (s),
1256 (s), 1103 (s), 1028 (s), 812 (s), 765 (m), 594 (m); 1H NMR
(CDCl3, 400 MHz):
d 12.69 (s, 1H, enolic OH), 7.36 (m, 4H, C6H4),
5.86 (s,1H, C = CH), 4.39 (s, 2H keto CH2), 4.03 (q, 2H, OCH2CH3),
1.29 (t, 3H, OCH2CH3) ppm.
Scheme 2. Synthetic route of the europium complexes C1–C5.

44
R. Devi et al. / Journal of Fluorine Chemistry 181 (2016) 36–44
4.4. Synthesis of europium complexes
1184 (m), 1030 (s), 916 (s), 847 (s), 795 (s), 764 (s), 742 (s), 671 (m),
622 (m), 491 (m); 1H NMR (CDCl3, 400 MHz):
7.40 (m, 12H, C6H4),
d
The synthetic route of binary and ternary europium complexes
C1–C5 is demonstrated in Scheme 2. A solution of EFBA (3 mmol) in
ethanol was added dropwise to the aqueous solution of europium
nitrate (1 mmol) with constant stirring on magnetic stirrer at room
temperature. pH of the resulting mixture was set between 6–7 by
using dilute NaOH (0.05 M) and white color precipitates were
obtained. The resulting precipitates were vaccum filtered along
with washing of water and ethanol to remove the unreacted ligand
and then dried in vaccum desiccator to obtain the white powder of
Eu(EFBA)3.(H2O)2 (C1) complex.
2.19 (s, 3H, C = CH), 4.66 (q, 6H, OCH2CH3), 1.30 (t, 9H, OCH2CH3)
ppm; Anal. Cal. For GdC33H34O11F3: C, 48.70; H, 4.18; Gd, 18.45;
Found: C, 47.88; H, 4.15; Gd, 18.40.
Acknowledgement
One of the authors, Rekha Devi gratefully acknowledge the
financial support in the form of junior research fellowship (JRF)
from University Grants Commission (UGC), New Delhi, India
(Award No: 2121210101).
Eu(EFBA)3ꢀ(H2O)2 (C1): White powder, yield 79%; IR(KBr): cmꢁ1
3417 (b), 2980 (s), 1740 (s), 1622 (w) 1515 (s), 1406 (s), 1250 (s),
1188 (m), 1028 (s), 917 (s), 848 (s), 797 (s), 766 (s), 747 (s), 673 (m),
Appendix A. Supplementary data
624 (m), 490 (m); 1H NMR (CDCl3, 400 MHz):
d 7.42 (m, 12H, C6H4),
2.21 (s, 3H, C = CH), 4.65 (q, 6H, OCH2CH3), 1.32 (t, 9H, OCH2CH3)
ppm; Anal. Cal. For EuC33H34O11F3: C, 48.82; H, 4.19; Eu, 18.24;
Found: C, 48.46; H, 3.96; Eu, 18.21.
Supplementary material related to this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jfluchem.2015.
11.004.
Complexes C2–C5 were prepared by the same procedure as
adopted for the synthesis of complex C1 but in addition to the
reaction mixture of EFBA (3 mmol) and europium nitrate (1 mmol)
there is extra addition of ethanolic solution of neo (1 mmol), batho
(1 mmol), phen (1 mmol), and bipy (1 mmol) for the synthesis of
ternary europium complexes C2–C5, respectively.
Eu(EFBA)3ꢀneo (C2): White powder, yield 81%; IR(KBr): cmꢁ1
3072 (w), 2979 (s), 1739 (s), 1623 (s), 1579 (s), 1515 (s), 1456 (s),
1362 (m), 1249 (s), 1092 (s), 1029 (s), 917 (s), 849 (m), 795 (m), 766
(m), 674 (m), 563 (s), 537 (s), 491 (m), 443 (m); 1H NMR
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(m), 627 (m), 567 (s), 542 (s), 449 (m); 1H NMR (CDCl3, 400 MHz):
d
8.42 (d, 2H, batho-H), 8.24 (d, 2H, batho-H), 8.09 (d, 2H, batho-H)),
7.93 (dd, 4H, batho-H), 7.40 (m, 12H, C6H4), 6.03 (dd, 4H, batho-H),
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(t, 9H, OCH2CH3) ppm; Anal. Cal. For EuC57H46N2O9F3: C, 61.76; H,
4.15; N, 2.52; Eu, 13.36; Found: C, 61.08; H, 3.94; N, 2.74; Eu, 13.28.
Eu(EFBA)3ꢀphen (C4): White powder, yield 82%; IR(KBr): cmꢁ1
3072 (w), 2979 (s), 1735 (s), 1622 (s), 1579 (s), 1505 (s), 1425 (s),
1364 (m), 1248 (s), 1092 (s), 1029 (s), 915 (s), 844 (m), 794 (m), 767
(m), 677 (m), 562 (s), 535 (s), 459 (m), 413 (m);1H NMR (CDCl3,
400 MHz):
d 8.02 (d, 2H, phen-H), 7.91 (s, 2H, phen-H), 7.46 (m,
12H, C6H4), 7.29 (d, 2H, phen-H), 7.10 (dd, 2H, phen-H) 4.21 (q, 6H,
OCH2CH3),2.29 (s, 3H, C = CH), 1.60 (t, 9H, OCH2CH3) ppm; Anal.
Cal. For EuC45H38N2O9F3: C, 56.32; H, 3.97; N, 2.93; Eu, 15.49;
Found: C, 56.35; H, 3.92; N, 2.90; Eu, 15.44.
Eu(EFBA)3ꢀbipy (C5): White powder, yield 81%; IR(KBr): cmꢁ1
2980 (s), 1738 (s), 1622 (s), 1578 (s), 1515 (s), 1456 (s), 1363 (m),
1276 (s), 1250 (s), 1093 (s), 1029 (s), 918 (s), 848 (m), 796 (m), 766
(m), 675 (m), 564 (s), 538 (s), 490 (m), 445 (m); 1H NMR (CDCl3,
400 MHz):
d 8.68 (d, 2H, bipy-H), 8.39 (dd, 2H, bipy-H), 7.49 (m,
12H, C6H4), 7.28 (d, 2H, bipy-H), 7.16 (dd, 2H, bipy-H), 4.49 (q, 6H,
OCH2CH3), 2.41 (s, 3H, C = CH), 1.49 (t, 9H, OCH2CH3) ppm; Anal.
Cal. For EuC43H38N2O9F3: C, 55.41; H, 4.08; N, 3.01; Eu, 15.89;
Found: C, 55.39; H, 3.98; N, 3.04; Eu, 15.84.
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Gadolinium(III) binary complex Gd (EFBA)3.(H2O)2 was pre-
pared in order to calculate the triplet energy level of ligand EFBA by
adopting the similar procedure as for the europium(III) complexes.
Gd(EFBA)3ꢀ(H2O)2 (C6): White powder, yield 59%; IR(KBr): cmꢁ1
3415 (b), 2982 (s), 1736 (s), 1620 (w) 1517 (s), 1411 (s), 1252 (s),