Synthesis, NMR, photoluminescence studies and intramolecular energy transfer process of europium(III) complexes

Article abstract of DOI:10.1016/j.jfluchem.2016.07.005

The studies related to the synthesis and luminescence properties of five europium(III) complexes of the type [Eu(OFHD)L] where OFHD is deprotonated 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dione and L is H₂O (C1), 2,2′-biquinoline (C2), neocuproine

Full text of DOI:10.1016/j.jfluchem.2016.07.005

Journal of Fluorine Chemistry 188 (2016) 177–184
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis, NMR, photoluminescence studies and intramolecular energy
transfer process of europium(III) complexes
*
Manju Bala, Satish Kumar, Rekha Devi, V.B. Taxak, Priti Boora, S.P. Khatkar
Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, Haryana, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The studies related to the synthesis and luminescence properties of five europium(III) complexes of the
type [Eu(OFHD)L] where OFHD is deprotonated 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dione and L is H2O
(C1), 2,20-biquinoline (C2), neocuproine (C3), 1,10-phenanthroline (C4) and 2,20- bipyridyl (C5) are
reported in the present communication. The synthesized complexes were investigated by means of
elemental analysis, IR, 1H NMR, UV–vis spectroscopy and TGA techniques. The high fluorination in the
complexes makes them best candidates of NMR shift reagents to resolve the spectra as revealed in 1H
NMR study. The excitation and emission spectra of complexes in solid state were investigated in detail.
Received 18 May 2016
Received in revised form 13 July 2016
Accepted 15 July 2016
Available online 16 July 2016
Keywords:
Luminescence properties
NMR shift reagents
Emission spectra
Further, on the basis of emission spectra, the CIE color coordinates (x and y), luminescence decay time (
t),
radiative decay rate (Arad), nonradiative rate (Anrad), quantum efficiency ( ) and Judd-Ofelt intensity
h
Judd-Ofelt intensity parameter
parameter (V2) were calculated and analyzed. The interaction of lowest triplet levels of ligands with the
emitting levels of europium(III) ion is represented by proposed intramolecular energy transfer
mechanism. The beauty of these complexes is that they can be used as NMR shift reagents and are of
particular interest in the field of light emitting materials.
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
These organic moieties have high absorption coefficient over a
wide range of wavelength, which is advantageous in strong
The investigations of lanthanide (Ln) ion complexes functional-
lized with fluorinated -diketone ligands have mesmerised the
absorption of energy and subsequent transfer of the absorbed
energy to Ln(III) ion, thus acting as antenna. There are number of
b
researchers, making it the most fascinating field of study. These
complexes possess unique properties by virtue of which they find
applications in the field of organic light emitting devices (OLEDs),
optical fibers, lasers [1–3], chemical and biological sensors [4,5].
The most potential chemical property of fluorinated tris Ln(III)
complexes i.e. Lewis acidity is due to presence of more fluorine
atom which permits them best candidature as NMR shift reagent in
spectral simplification [6–8]. Since the 4f electrons of Ln ion are
shielded from ligand environment by 5s and 5p electrons, the
narrow emission peaks are noticed in luminescence spectra. The
most significant aspect of luminescence of Ln(III) complexes is
associated with Laporte-forbidden f-f transitions having weak
dipole strength, which are accountable for low molar extinction
coefficient, resulting into impracticable direct excitation of metal
ion. To solve the aforementioned difficulty, the Ln(III) ion is
encapsulated with light harvesting chromophoric organic moeity.
chromophoric organic moieties, however the fluorinated b-dike-
tone are of peculiar interest because of fluorine substituents, which
facilitate spin-orbit coupling via heavy atom effect and promote
intersystem crossing of sensitization process [9–13]. As we know
that the fluorine atom is heavier than hydrogen atom so, the
vibration energy of the C-F bond is lower than that of CꢀꢀH bond.
These lower energy frequency C-F oscillators reduce the energy
loss during non-radiative decay process. Additionally, it also
increases the acidity of the central Ln(III) ion and creates an easily
available complexing site for the N,N-donor ancillary ligands. In
this way, Ln(III) ion saturates their coordinative sites and prevents
the inner coordination sphere from OꢀꢀH, CꢀꢀH and NꢀꢀH higher
energy oscillators of solvent molecules that are responsible for
quenching of Ln(III) excited state by non-radiative decay process
[14]. The coordination of ancillary ligands also provides rigidity
and stability to the coordination sphere and diminishes the non-
radiative decay [15]. We have chosen four N,N-donor neutral
ancillary ligands comprising of two different basic frameworks i.e.
one similar to bipy while the other is similar to phen. All the
* Corresponding Author.
E-mail address: s_khatkar@rediffmail.com (S.P. Khatkar).
characteristic features of fluorinated
b-diketone and ancillary
http://dx.doi.org/10.1016/j.jfluchem.2016.07.005
0022-1139/ã 2016 Elsevier B.V. All rights reserved.

178
M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
Table 1
The elemental analytical data of europium(III) complexes C1–C5.
Complexes
C(%) found (cal.)
H(%) found (cal.)
N(%) found (cal.)
Eu(%) found (cal.)
HOFHD
C1
C2
C3
C4
27.82 (27.90)
22.45 (22.52)
36.55 (36.64)
33.87 (33.95)
32.58 (32.63)
31.08 (31.13)
0.75 (0.77)
0.70 (0.72)
1.22 (1.27)
1.28 (1.32)
0.95 (0.99)
1.00 (1.01)
–
–
–
15.68 (15.84)
12.78 (12.89)
13.35 (13.43)
13.72 (13.78)
14.02 (14.08)
2.25 (2.37)
2.40 (2.47)
2.49 (2.53)
2.48 (2.59)
C5
ligands greatly enhance the photoluminescence of Ln(III) metal
complexes.
downfield as well as upfield shifts in spectra of complexes were
observed due to large magnetic anisotropy of europium. The
spectrum of ligand showed the signal of enolic ꢀꢀOH proton at
15.30 ppm. This signal disappeared in spectra of complexes,
manifesting the coordination of ligand to the metal ion through
In the present communication, we have successfully synthe-
sized five new europium(III) complexes and characterized with
various techniques. A detailed study of photophysical properties
has been done in order to explore the excitation and emission
spectra, CIE coordinates, luminescence decay curves, radiative
enolic form. The singlet of methine (¼CꢀꢀH) proton in spectrum of
ligand shifted to upfield in the spectra of complexes as the OFHD
binds with Eu3+ ion ligated by bipy or phen. This shift also occurred
due to the high polarization of Eu3+ ion after withdrawing the
electron density from ancillary ligand, hence, some electron
density shifted from Eu3+ ion to the OFHD ligand which can be
easily accomodated by highly electronegtive fluorine atoms
present in the OFHD. Consequently, the aromatic signals of
ancillary ligands exhibited remarkable downfield shift in spectra
of complexes. The NMR spectra of HOFHD ligand and complexes of
phen and bipy ancillary ligands of different basic frameworks i.e.
C4 and C5 complexes are depicted in Figs. 2 and 3 [17,18].
decay rate (Arad), nonradiative rate (Anrad), quantum efficiency (
h)
and Judd-Ofelt intensity parameter V2). Furthermore, the
(
intramolecular energy transfer process in complexes has been
examined by the proposed energy transfer mechanism.
2. Results and discussion
The results of elemental analyses and complexometric titration
of europium ion with EDTA are summarized in Table 1. Table 1
clearly shows the conformity between theoretical values and
experimental values which indicates successful synthesis of ligand
and complexes. The complexes were stable in normal atmospheric
conditions and amorphous in nature.
The peak broadening effect could be observed due to presence
of protons in close proximity of europium(III) ion and absence of
spin-spin splitting. Among the complexes, only C5 complex
showed broad signals due to coordination of bipy which leads
to disappearing of spin-spin splitting. This demonstrated that both
ends of bipy firmly coordinated to the europium(III) ion [17]. Other
complexes showed no signal broadening, therefore these were
more applicable in state of presence of spin-spin splitting in
complexes, hence used as NMR shift reagents to solve the spectra.
2.1. UV–vis spectra
The UV–vis absorption spectra of ligand and their complexes
are depicted in Fig. 1. The maximum absorption of both ligand and
complexes was located around 325 nm, showing that singlet
energy of the ligand was not affected by coordination of europium
ion. These absorption bands were attributed to
p-p
* transition of
2.3. IR spectra
ligand. Fig.1 shows that ligand exhibits absorption over wide range
of wavelengths, as compared to complexes and transfer the
absorbed energy to the europium ion in an effective way. The
spectra were slightly blurred out due to the solvent effect [16].
Fig. 4 depicts the IR spectra of ligand and complexes, the
important IR bands are tabulated in Table 2. The complexes
showed almost similar IR spectra, suggesting similar modes of
binding of ligand in all the complexes. These spectra differ from the
spectrum of free ligand. As we know that the stretching vibration
2.2. NMR spectra
of >C
of ligand exhibited the absorption bands at 1718 cmꢀ1 and
1606 cmꢀ1, ascribed to the stretching vibrations of >C
O and
C respectively. These two bands shift to 1659–1648 cmꢀ1 and
1546–1500 cmꢀ1 respectively in the spectra of complexes which
make it clear that the >C O group was involved in complexation.
This is further confirmed by the bands of Eu-O vibrations in region
471–462 cmꢀ1 which came into existence in the spectra of
complexes only. Two new bands in the region 1597–1592 cmꢀ1
and 536–534 cmꢀ1 appeared due to coordination of ancillary
¼
O group is highly sensitive to complexation. The IR spectrum
The 1H NMR spectra of paramagnetic complexes showed some
interesting changes as compared to spectra of ligands. Some
¼
C¼
¼
ligands in C2–C5 complexes corresponding to C N and Eu-N
¼
stretching vibrations respectively. The spectrum of ligand dis-
played a broad band of enolic ꢀꢀOH at 3270cmꢀ1. On the other
hand, a broad band of ꢀꢀOH vibration of water molecules at
3429 cmꢀ1 was observed in spectra of C1 complex only which
disappeared in C2–C5, suggesting the successful substitution of
solvent molecule by ancillary ligands. The strong absorption band
of CꢀꢀF stretching vibrations in the region 1203–1196 cmꢀ1 were
common to all complexes as well as ligand and exhibited no change
in the values which showed that C-F bond did not participate in
coordination process.
Fig. 1. The UV–vis absorption spectra of HOFHD ligand and C1–C5 europium(III)
complexes in DMSO solvent (1 ꢁ10ꢀ5 mol/L).

M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
179
Fig. 2. The 1H NMR spectra of HOFHD ligand. *Impurity and # signal for ꢀꢀCH2 protons of keto form.
2.4. Thermal analysis
displayed in Figs. 6 and 7. The excitation spectra of complexes were
recorded by monitoring the emission maxima of 5D0-7F2 transition.
A broad band centred at 351 nm in the range of 200–500 nm
The thermal analysis of the complexes by their TG-DTG
thermograms were carried out under dinitrogen atmosphere at
a heating rate of 20 ꢂC/min. All the complexes showed almost
similar thermal behavior and were stable at about 190 ꢂC
temperature [18]. The C5 complex was selected as representative
of all complexe as depicted in Fig. 5 to illustrate the decomposition
pattern. The first mass loss of 2.2% upto 118 ꢂC was observed due to
moisture present in complex. The second large step of mass loss of
85.7% between 190 and 356 ꢂC temperatures corresponds to the
removal of three OFHD and one bipy molecules. At last, europium
oxide remained as final product.
prevailed in the spectra of complexes, ascribed to
p-p* transition
of ligand. It also indicated that the ligands efficiently sensitized the
europium ion by transferring the energy.
Under the UV irradiation of 351 nm, the emission spectra of
complexes exhibited characteristic peaks of Eu3+ ion originating
5
7
from electronic transition from D0,1 to F0,1,2 levels. No ligand
based emission was observed in emission spectra of complexes,
suggesting the complete transfer of absorbed energy of ligand to
the metal ion. The most intense 5D0-7F2 transition at 616 nm was an
induced electric dipole transition which was highly sensitive to its
coordination environment and amenable for red emission of the
complexes. The 5D0-7F1 transition around 591–592 nm was
magnetic dipole transition which was insensitive to its coordina-
tion environment. On the contrary, the 5D0-7F0 transition at
580 nm was forbidden both in electric as well as magnetic dipole.
The values of intensity ration (I02/I01) of 5D0-7F2 to 5D0-7F1
2.5. Luminescence features
In order to investigate the photoluminescence properties of C1–
C5 europium(III) ion complexes, the excitation and emission
spectra were measured in solid state at room temperature as
Fig. 3. The 1H NMR spectra of C4 (a) and C5 (b) complexes.

180
M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
Fig. 4. IR absorption spectra of HOFHD ligand and its C1–C5 complexes.
Table 2
The IR characteristics bands (cmꢀ1) of ligand and its europium complexes.
Complexes
n
(OꢀꢀH)
n
(C¼O)
n
(C¼
N)
n(C
¼
C)
n
(CꢀꢀF)
n
(EuꢀꢀN)
n
(EuꢀꢀO)
HOFHD
C1
C2
C3
C4
3270 (b)
3429 (b)
–
–
–
–
1718 (s)
1651 (s)
1648 (s)
1651 (s)
1659 (s)
1656 (s)
–
–
1606 (s)
1546 (s)
1541 (s)
1540 (s)
1520 (s)
1500 (s)
1200 (s)
1202 (s)
1196 (s)
1198 (s)
1199 (s)
1203 (s)
–
–
–
463 (m)
471 (m)
462 (w)
463 (m)
462 (m)
1595 (s)
1597 (s)
1592 (s)
1595 (s)
536 (s)
535 (m)
534 (s)
535 (s)
C5
b = broad, s = strong, m = medium, w = weak.
transition are given in Table 3, suggesting that the europium ion
was engaged in non-centrosymmetric position in complexes
[19,20]. Only one peak for 5D0-7F0 and 5D0-7F1 transitions was
noticed in spectra pointing towards the presence of single
chemical environment around the europium(III) ion. The analysis
of emission spectra clearly revealed that the introduction of
ancillary ligands in C2–C5 complexes enhanced the luminescence
intensity, assigned to the synergistic effect of these ancillary
ligands. It can be seen that the ancillary ligands could sensitize the
luminescence intensity of europium ion. Hence we obtained the
higher luminescence intensity of C2–C5 complexes as compared to
C1 complex. On the other hand, the OꢀꢀH oscillators of water
molecule in C1 complex quenched the luminescence intensity by
increase the non-radiative decay. The nature of the ancillary
ligands controlled the order of luminescence emission intensity in
the C2–C5 complexes. As we have chosen four ancillary ligands
which belong to two different basic framework groups i.e one
framework group have bipy and biq while other have phen and
neo. The steric effect in case of biq and neo ligands makes them less
intense in their respective groups. The luminescence intensity
order of bipy and phen can be explained on the basis of energy gap
D
E (T1-M3+) value. This gap was found to be the most appropriate
match in bipy and hence has highest emission intensity, also
explained elaborately in energy transfer mechanism.
The CIE (Commission Internationale de Eclairage) color
coordinates (x and y) of complexes were estimated from emission
spectra and their values are listed in Table. 3. CIE coordinates of
complexes fell under the red region of chromaticity diagram as
shown in Fig. 8 and are close to the standard red color values of
SMPTE (Society of Motion Picture and Television, x = 0.63, y = 0.34)
and EBU (European Broadcasting Union, x = 0.64, y = 0.33).
2.6. Luminescence decay time (t), quantum efficiency (h) and Judd-
Ofelt analysis
The luminescence decay curves of 5D0 excited state of Eu(III) ion
in complexes were examined by monitoring the emission of
Fig. 5. TG-DTG curves of C5 complex under dinitrogen atmosphere.

M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
181
5D0-7F2 transition at 616 nm excited at 351 nm as shown in Fig. 9.
These curves fitted with a single exponential function, suggesting
that europium(III) ion was located at the same parity site in the
complexes. The luminescence decay time values were calculated
from fitting of decay curves and are summarized in Table. 3. It is
seen that the decay time of C1 complex was lower than that of C2–
C5 complexes because the former complex had OꢀꢀH quencher of
water molecule while in latter complexes water molecules are
substituted by ancillary ligands. Similar results were also obtained
in case of quantum efficiency (h).
The quantum efficiency of the 5D0 excited state was determined
from emission spectra and decay time of the complexes. It
expressed the competition between radiative and nonradiative
process as given below [21,22]:
h
¼ Arad=Arad þ Anrad
ð1Þ
The radiative and nonradiative processes are represented by the
rate constant Arad and Anrad respectively. It was considered that
only these processes participate in the depopulation of 5D0 excited
state and are related with luminescence decay time by the Eq. (2).
¼ ðArad þ AnradÞꢀ1
ð2Þ
Fig. 6. Solid state excitation spectra of C1–C5 complexes monitored at 616 nm at
room temperature.
t
4
X
X
I0J
v01
Arad ¼ A01
¼
jA0J
ð3Þ
I01 j¼0 v0J
Here, Arad was obtained by summation of all radiative rates A0J
7
for each characteristics transitions (5D0 ! F0-J) of europium ion as
shown in Eq. (3). A0J was calculated according to Eq. (4) [23,24].
The transitions which were not detected in emission spectra such
7
as 5D0 ! F4-6, can be neglected as they were not involved in
7
depopulation process. Since 5D0 ! F1 transition was magnetic
dipole transition and insensitive to chemical environment around
europium ion, it can be used as an internal reference for whole
spectrum.
ꢀ
ꢁꢀ
ꢁ
A0J ¼ A01 I0J=I01 n01
=
n0J
ð4Þ
Here, A01 is the Einstein’s coefficient of spontaneous emission
between the 5D0 and 7F1 energy states. The value of A01 has to be
taken 14.65 sꢀ1 in vacuum while 50 sꢀ1 approximately (A01 = n3
A01vac) in air, when an average index of refraction (n) is considered
to be 1.506 [25]. The nn01 and n0J are energy barycenter of
Fig. 7. Solid state emission spectra of C1–C5 complexes excited at 351 nm.
5D0 ! F1 and 5D0 ! FJ transition respectively with I01 and I0J as
integrated intensities respectively. The quantum efficiency of all
the complexes was calculated using above equations and the data
7
7
are tabulated in Table 3. It is concluded that value of
mainly on decay time and ratio of I02/I01, higher the value of decay
h depends
Table 3
The luminescence data of C1–C5 complexes.a
time and I02/I01 ratio, higher will be the quantum efficiency. The
Systems
C1
C2
C3
C4
C5
lower value of
h in C1 as compared to C2–C5 complexes may be
x and y color
coordinates
0.5264,
0.2913
17241
16920
16233
30
100
350
3.5
14.72
50.0
182.40
0.36
247.12
2530.65
8.89
0.5343,
0.3142
17241
16891
16233
25
75
520
6.93
16.32
50.0
360.71
0.44
427.03
1845.69
18.78
8.08
0.5928,
0.3183
17241
16891
16233
25
96
616
7.68
12.75
50.0
400.0
0.75
462.75
870.58
34.70
8.96
0.6212,
0.3322
17241
16891
16233
75
208
1530
7.35
17.66
50.0
382.70
0.87
450.36
699.06
39.18
8.56
0.6471,
0.3524
17241
16891
16233
60
250
1856
7.42
11.75
50.0
386.24
0.95
448.0
604.63
42.56
8.65
explained on the basis of existance of more nonradiative process
which comes into existance due to ꢀꢀOH vibronic coupling of water
molecules in former complex.
The Judd-Ofelt theory is implemented to analyze the possible
structural changes around europium ion and line strength of an
electrical dipole transition in the form of Judd-Ofelt intensity
parameters V2 and V4. Out of these parameters, V2 parameter is
highly sensitive to the symmetry and sequence of ligand field. The
large value of V2 parameter gives rise to non-symmetric
n00 (cmꢀ1
)
)
)
n01 (cmꢀ1
n02 (cmꢀ1
I00
I01
I02
I
02/I01
A00 (sꢀ1
A01 (sꢀ1
A02 (sꢀ1
(ms)
Arad (sꢀ1
Anrad (sꢀ1
)
)
)
environment around europium ion and faster radiation rate (Arad
)
t
[26].
)
)
As discussed earlier, the emission transitions such as
7
5D0 ! F0,3,5 were forbidden in both electric and magnetic dipole
h
(%)
V2 (x10ꢀ20
)
4.08
7
transition while the 5D0 ! F1 transition was magnetic dipole
7
transition. The 5D0 ! F2,4 transitions are contributing towards
a
The intensity of 5D0-7F3 transition is very low so their corresponding n03 and I03
are neglected.

182
M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
Fig. 10. The proposed energy transfer mechanism in C5 complex.
occured between triplet energy level (T1) of ligand to the nearby
emitting level (M3+
of europium ion. The triplet level of
fluorinated -diketone was more favourable for efficient energy
)
Fig. 8. The CIE color coordinates of C1–C5 complexes.
b
transfer to the metal ion because the fluorine atom can lower the
triplet energy level which make it suitable match with the emitting
level of metal ion. The singlet (S1; 27397 cmꢀ1) and triplet
(T1; 22371 cmꢀ1) levels of OFHD ligand were calculated from the
UV–vis absorption edge wavelength of spectra (Fig. 1) and shortest
phosphorescence emission wavelength of its corresponding
gadolinium complex (G1) as depicted in Fig. S1. In the literature,
it can be observed that the triplet levels of ancillary ligands are
nearly close to one another, therefore, the lowest singlet and triplet
levels of bipy (S1; 29900 cmꢀ1, T1; 22900 cmꢀ1) and phen (S1;
31000 cmꢀ1, T1; 22100 cmꢀ1) ligands were estimated, which
matched with the literature [29,30]. But, the energy gap (T1-M3
+) should be appropriate for efficient energy transfer, it should be
neither too large nor too small. Latva et al. initially stated that this
electric dipole so, the V2 and V4 parameters are estimated
according to Eq. (5) as expressed below [27].
l
Vl ¼ 3ћc3A0j=4e2v3x < 5D jUð Þj7F
>
j
ð5Þ
2
0
Here, £ is taken as Planck’s constant over 2
p, c is velocity of
light, e is the electronic charge,
v
is angular frequency of electronic
2
transition,
x
is Lorenz local field correction ½n2ðn2 þ 2Þ =9ꢃin which
1.5 average refractive index is used and < 5D jUð Þj7F >2 is taken as
l
0
j
0.0032, representing the square reduced matrix elements [28]. The
V2 intensity parameter for all the complexes is shown in Table 3
7
and V4 parameter was not calculated because 5D0 ! F4 transition
could not be experimentally detected. It was noticed that the C5
complex had relatively higher value of V2 as a consequence of
energy gap
D
E(T1-M3+) should be in 2000–5000 cmꢀ1 range for
E)
7
hypersensitive behavior of 5D0 ! F2 transition, reflecting the most
efficient energy transfer process [31]. The value of energy gap (
D
polarizable chemical environment around Eu3+ in complex.
for HOFHD, bipy and phen were found to be 3750 cmꢀ1, 4279 cmꢀ1
and 3479 cmꢀ1 respectively. It can be seen that the value of energy
gap of phen was lower than that of bipy energy gap value which
was responsible for small thermally activated M3+ to T1 inverse
energy transfer. Hence, the luminescence emission intensity of
phen complex was noticed to be lower than that of bipy complex.
Furthermore, the prime factor governing the transfer of energy
from triplet level of ligand to the emitting levels of the metal ion
2.7. Energy transfer mechanism
The proposed channel for the intramolecular energy transfer
process is shown in Fig. 9. The energy transfer predominantly
was energy gap (DE), which was found to be most appropriate in
case of C5 complex. Hence, this complex showed maximum
luminescence emission intensity as illustrated in the proposed
energy transfer mechanism of complexes C4 and C5 and
respectively represented in Figs. S2 and 10 .
3. Conclusion
In this report, a series of five europium complexes with
fluorinated b-diketone and ancillary ligands have been succesfully
synthesized and well characterized. The 1H NMR study shows that
these complexes can be used as NMR shift reagent to simplify the
spectra. The solid state emission spectra excited at 351 nm display
characteristic peaks of Eu3+ ion and intensity increases as the
solvent molecules are substituted by ancillary ligands. The data of
quantum efficiency, V2 intensity parameter and decay curves
indicate that Eu3+ ion is situated in most polarizable chemical
environment and acts as one luminescent site. The triplet level of
OFHD ligand matches well with emitting level of Eu3+ ion, which
Fig. 9. Luminescence decay curves of C1–C5 complexes.

M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
183
leads to the efficient energy transfer. The CIE coordinates of
complexes fall in deep red region. The excellent results of emission
spectra, CIE, decay curves, quantum efficiency and V2 intensity
parameter make these complexes best candidates as light emitting
materials and also widely applicable as red component in display
devices (Scheme 1).
Sodium methoxide (0.2 mol) was added in 150 ml of absolute
diethyl ether followed by dropwise slow addition of methylpenta-
fluoropropionate (0.2 mol) in the stirred mixture. Then ethereal
solution of 1,1,1-trifluoropropane-2-one (0.2 mol) was added
slowly, which resulted into basic solution. The mixture was stirred
for about 2 h and allowed to cool for 15 h. The resulting mixture
was acidified with sulphuric acid (100 ml, 2 M) with constant
stirring. The organic and aqueous phase was formed and latter was
separated and washed with fresh diethyl ether. Further, these
washings of aqueous layer were added to the organic layer and
finally the solution was evaporated. Then resulting solution was
vacuum distilled to get the final product [32,33].
4. Experimental details
4.1. Materials
The lanthanide nitrates i.e. Eu(NO3)3
ꢄ
5H2O (99.9%); Gd
(NO3)3 5H2O (99.9) of high purity and ancillary ligands such as
ꢄ
2,20-bipyridyl, 1,10-phenanthroline, 2,20-biquinoline and neocu-
proine were purchased from Sigma Aldrich and were used as
received. The solvents and reagents like sodium methoxide,
methylpentafluoropropionate, 1,1,1-trifluoropropane-2-one used
in this study were of analytical grade.
4.3. Synthesis of complexes
All five Eu(III) complexes were synthesized adopting identical
procedure only differing in addition of ancillary ligands in C2–C5
complexes. For C1, 3.2 mmol of HOFHD in 15 ml solution of
methanol was mixed with 10 ml solution of distilled water
containing 1.0 mmol of europium nitrate. The resulting mixture
was stirred on magnetic stirrer set at temperature around 60 ꢂC.
Also the pH of the mixture was adjusted to 6.5 with 0.05 M NaOH
solution. The off white precipitates formed were filtered, washed
with water followed by methanol and then dried in hot air oven.
Similarlly, the Gd(OFHD)3(H2O)2 (G1) was also prepared to
estimate the lowest excited triplet state of OFHD ligand. To
synthesize the C2–C5 complexes, the 1.0 mmol of biq for C2, neo for
4.2. Synthesis of ligand
By using Claisen condensation reaction, the fluorinated
b
-diketonic ligand 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dione
(HOFHD) was synthesized. The synthesis and isolation of ligand
demands extra care and attention because high fluorination
decreases the intermolecular van der waals forces and H-bonding
which lead to high volatility of ligands.
Scheme 1. Pathway for the synthesis of the ligand and its corresponding europium(III) ion complexes.

184
M. Bala et al. / Journal of Fluorine Chemistry 188 (2016) 177–184
C3, phen for C4 and bipy for C5 in 10 ml methanol was mixed with
resulting mixture as prepared for C1. Then followed the
aforementioned method.
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This work is supported by CSIR (Council of Science and
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jfluchem.2016.07.005.