Tuning of the excitation wavelength from UV to visible region in Eu 3+-β-diketonate complexes: Comparison of theoretical and experimental photophysical properties

Article abstract of DOI:10.1039/c0dt01652g

A novel class of efficient visible light sensitized antenna complexes of Eu³⁺ based on the use of a series of highly conjugated β-diketonates, namely, 1-(1-phenyl)-3-(2-fluoryl) propanedione, 1-(2-naphthyl)-3-(2-fluoryl)propanedione, 1-(4-biphenyl)-3-(2-fluoryl) propanedione, and 2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl oxide as an ancillary ligand has been designed, synthesized, characterized and their photophysical properties investigated. The coordination geometries of the typical Eu³⁺ complexes were calculated using the Sparkle/PM3 model. Photophysical properties of europium complexes benefit from adequate protection of the metal by the rigid phosphine oxide ligand against non-radiative deactivation and efficient ligand-to-metal energy transfer exceeding 50% as compared to precursor samples. The replacement of the phenyl group with the naphthyl or biphenyl groups in the 3-position of the fluoryl based β-diketonate ligand remarkably extends the excitation window of the corresponding Eu³⁺ complexes towards the visible region (up to 500 nm). The highly conjugated β-diketonate ligands sensitize efficiently the luminescence of Eu³⁺ ions with quantum yields ranging from 19 to 43 % in the solid state, which is among the highest reported for a visible sensitized Eu³⁺complex. The theoretical quantum efficiencies from the Sparkle/PM3 structures are in good agreement with the experimental values, clearly attesting to the efficacy of the theoretical models.

Full text of DOI:10.1039/c0dt01652g

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PAPER
Tuning of the excitation wavelength from UV to visible region in
Eu3+-b-diketonate complexes: Comparison of theoretical and experimental
photophysical properties†
V. Divya,a Ricardo O. Freireb and M. L. P. Reddy*a
Received 29th November 2010, Accepted 12th January 2011
DOI: 10.1039/c0dt01652g
A novel class of efficient visible light sensitized antenna complexes of Eu3+ based on the use of a series
of highly conjugated b-diketonates, namely, 1-(1-phenyl)-3-(2-fluoryl) propanedione,
1-(2-naphthyl)-3-(2-fluoryl)propanedione, 1-(4-biphenyl)-3-(2-fluoryl) propanedione, and
2,2¢-bis(di-p-tolylphosphino)-1,1¢-binaphthyl oxide as an ancillary ligand has been designed,
synthesized, characterized and their photophysical properties investigated. The coordination
geometries of the typical Eu3+ complexes were calculated using the Sparkle/PM3 model. Photophysical
properties of europium complexes benefit from adequate protection of the metal by the rigid phosphine
oxide ligand against non-radiative deactivation and efficient ligand-to-metal energy transfer exceeding
50% as compared to precursor samples. The replacement of the phenyl group with the naphthyl or
biphenyl groups in the 3-position of the fluoryl based b-diketonate ligand remarkably extends the
excitation window of the corresponding Eu3+ complexes towards the visible region (up to 500 nm). The
highly conjugated b-diketonate ligands sensitize efficiently the luminescence of Eu3+ ions with quantum
yields ranging from 19 to 43 % in the solid state, which is among the highest reported for a visible
sensitized Eu3+complex. The theoretical quantum efficiencies from the Sparkle/PM3 structures are in
good agreement with the experimental values, clearly attesting to the efficacy of the theoretical models.
luminescent complexes emitting in the visible domain for Eu3+,
Tb3+, Sm3+ and Dy3+, or in the near infrared domain with
Introduction
Pr3+, Nd3+, Er3+, Tm3+ and Yb3+.1e,4 The commonly observed
sensitization mechanism for luminescent Eu3+complexes involves
a triplet pathway, in which the transfer of the energy absorbed by
the ligand to the Eu3+ ion takes place from the ligand-centered
triplet excited state (T1).4 In this case, the optical excitation
window for luminescent Eu3+ complexes appears to be limited
to < 385 nm owing to the energetic constraints highlighted by
Reinhoudt and co-workers.5 This can be unwanted and even
harmful for living tissue, in the case of bio-sensing applications.1
Furthermore, no cheap pump sources are available in the UV.6
There are two strategies that can be adopted to achieve visible
light excitation. One of the approaches is to introduce a 4d- or 5d-
transition metal ion such as Ir(III) or Pt(II) into the Eu3+ complex
molecule, which exhibits an efficient energy transfer to the Ln3+
ion.7 Unfortunately, photoluminescence efficiency of this kind of
Recently, a challenge in the chemistry of the lanthanide ions is
to develop luminescent Eu3+ complexes that can be sensitized
by visible light and this field has become much more important
because of the demand for less-harmful labelling reagents in the
life sciences and low-voltage-driven pure-red emitters in opto-
electronic technology.1 Due to the strongly forbidden character
of f–f transitions, direct excitation of lanthanide ions is hard to
obtain through direct means without laser sources. To circumvent
this drawback, it has been found that excitation of chromophoric
units introduced in the first coordination sphere of lanthanide
cations or in its close vicinity can result in energy transfer from
the chromophore to the lanthanide. This process, often called
the antenna effect,2 was demonstrated by Weissmann for the
excitation of Eu3+ with ligands absorbing in the UV region,3
and was subsequently extended to the sensitization of numerous
3
Eu3+ complexes based on the MMLCT (metal-metal-to-ligand
charge transfer) or 3MLCT (metal-to-ligand charge transfer) was
always very low.7,8 Another promising means of longer-wavelength
sensitization of Eu3+ emission is through the modification of the
ligand molecule with a suitable expanded p-conjugated system,
which have a smaller energy gap between the lowest singlet excited
state (S1) and the T1 state. It has been demonstrated by several
research groups that the excitation wavelength for Eu3+ complexes
aChemical Sciences and Technology Division, National Institute for In-
terdisciplinary Science & Technology (NIIST), CSIR, Thiruvananthapu-
ram, 695019, India; Tel: 91-471-2515360; Fax: 91-471-2491712. E-mail:
mlpreddy55@gmail.com
bDepartamento de Qu´ımica, Universidade Federal de Sergipe, 49.100-000,
Sa˜o Cristo´va˜o, SE, Brazil
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c0dt01652g
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Scheme 1 Synthetic Procedure for the Ligands HBFPD, HNFPD, HBPFPD.
can be extended into the visible region through the usual triplet
pathway.8,9,10,11 Some of the typical examples of such modified
ligands are Michler’s ketone,9b phenanlenone12 and tridentate
ligands.1i,4j,9a,13 However, some of these europium adducts reported
are only stable in solvents like benzene and toluene.1i,9b This limits
their applicability significantly. Furthermore, some of the above
adducts could not be obtained in the solid state but had to be made
in situ and also exhibit low quantum yields.9b,12 A highly efficient
photoluminescence quantum yield of 28% in solid state has
also been reported for visible-light excitable Eu(2-TFDBC)3phen
(where 2-TFDBC represents 2-(4¢,4¢,4¢-trifluro-1¢,3¢-dioxobutyl)-
carbazole) complex.11a
Consideration of the foregoing results motivated us to
design
Scheme 2 Synthetic Procedure for the Ligand TBNPO.
a
series of new Eu3+ antenna complexes, which
contains highly conjugated b-diketonates, namely, 1-(1-
phenyl)-3-(2-fluoryl) propanedione (HBFPD), 1-(2-naphthyl)-3-
(2-fluoryl)propanedione (HNFPD), 1-(4-biphenyl)-3-(2-fluoryl)
propanedione (HBPFPD). The coordination sphere also contains
2,2¢-bis(di-p-tolylphosphino)-1,1¢-binaphthyl oxide (TBNPO) as
an ancillary ligand. The newly synthesized Eu3+ complexes were
characterized and investigated their photophysical properties.
Photoluminescence measurements show that the ligands suc-
cessfully extend the excitation bands of the Eu3+ complexes
to the blue-light region (400 to 440 nm) and exhibit strong
red emissions. This article also reports spectroscopic properties
such as intensity parameters (Xl, l = 2, 4, and 6), energy
lutions. The chelating ligand, 2,2¢-bis(di-p-tolyl phosphino)-1,1¢-
binaphthyl oxide (TBNPO) was synthesized by simple oxidation of
2,2¢-bis(di-p-tolyl phosphino)-1,1¢-binaphthyl using H2O2. The de-
signed ligands have been characterized by the 1H NMR, 13C NMR,
31P NMR, FT-IR and mass spectroscopic (FAB-MS) methods, as
well as by elemental analysis (Figures S1–S4 in the supporting
information).† The synthesis routines for the Ln3+ binary and
ternary complexes are shown in Schemes 3 and 4, respectively.
The complexes were characterized by FT-IR, mass spectroscopy
(FAB-MS) and elemental analyses. The elemental analyses and
FAB-MS studies revealed that the central Ln3+ ion is coordinated
transfer (WET) and back-transfer (WBT) rates, radiative (Arad
)
and nonradiative (Anrad) decay rates, quantum efficiency, and
quantum yield of Eu(BPFPD)3(H2O)2 and Eu(BPFPD)3(TBNPO)
were theoretically modeled using the electronic and spectroscopic
semiempirical Sparkle/PM3 model14a and compared with that of
precursor complex Eu(BPFPD)3(H2O)2.
Results and Discussion
Synthesis of the ligands
Schemes 1 and 2 summarizes the protocols used for the syntheses
of various b-diketonates and chelate phosphine oxide ligands, re-
spectively. The ligands b-diketones (HBFPD, HNFPD, HBPFPD)
were obtained as bright yellow solids in 60–70% yields by Claisen
condensation of acetylfluorene with the corresponding methyl
carboxylate ester in the presence of a strong base (NaH). They
are present mainly as an enol form (~99%) in chloroform so-
Scheme 3 Synthetic Procedure for the Ln3+ Complexes 1–3 and 7–9 [Ln =
Eu (1, 2, 3) and Gd (7, 8, 9)].
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Scheme 4 Synthetic Procedure for the Complexes 4–6.
Table 1 Spherical atomic coordinates calculated via Sparkle/PM3 coor-
dination polyhedron of the complex 3
to three b-diketonate ligands in 1–9. In the case of complexes
4–6, one molecule of the bidentate phosphine oxide, TBNPO,
is also present in the coordination sphere. The FT-IR spectra
of complexes (1–3 and 7–9) exhibit a broad absorption in the
3000–3500 cm-1 region, there by indicating the presence of solvent
molecules in the coordination sphere of the Ln3+ ion. On the other
hand, the absence of this broad band in the 3000–3500 cm-1 region
in the case of complexes 4–6 implied that the solvent molecules
had been displaced successfully by the bidentate phosphine oxide
ligand. The carbonyl stretching frequency of HBFPD, HNFPD,
HBPFPD have been shifted to lower wave numbers in complexes
1–9 indicating the involvement of carbonyl oxygen in the complex
formation with Ln3+ ion.15 The fact that the P O stretching
frequency of TBNPO (1197 cm-1) in complexes 4–6 (to 1175 cm-1
in 4; 1173 cm-1 in 5; 1188 cm-1 in 6), show the involvement of
˚
˚
Atom
R/A
q/A
j/◦
Eu3+
0.0000
2.4445
2.4480
2.4477
2.4478
2.4460
2.4541
2.5309
2.5306
0.0000
0.0000
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (H2O)
72.8946
85.4570
131.9591
162.7256
45.4084
62.3594
59.7082
116.5014
10.5183
74.2159
164.1471
2.2786
211.8433
290.2596
136.7633
250.3729
O (H2O)
Table 2 Spherical atomic coordinates calculated via Sparkle/PM3 coor-
dination polyhedron of the complex 6
q/◦
j/◦
P
O in the complex formation with Eu3+ ion. This has been
further confirmed from the 31P NMR spectra of the complexes
4–6 (Figure S5–S8 in the supporting information) that the P
˚
Atom
R/A
Eu3+
0.0000
2.4583
2.4634
2.4650
2.4634
2.4595
2.4713
2.5017
2.4535
0.0000
0.0000
O
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (BPFPD)
O (TBNPO)
O (TBNPO)
72.4442
121.9589
135.7998
83.3610
143.6178
79.9137
89.7824
12.9621
185.0424
145.4599
43.0712
83.1887
263.8459
258.3769
326.1473
143.8172
resonances are upfield shifted as compared to the free ligand,
indicating the involvement of phosphoryl oxygen in coordination
with the Eu3+ ion.† The thermal behavior of the complexes 1–6 was
examined by means of thermo-gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) analysis under nitrogen
atmosphere. It is clear from the thermo-gravimetric analysis data
that complexes 1–3 (Figure S8 in the supporting information)
undergoes a mass loss of about 3% in the first step (30 to
140 ◦C), which corresponds to the elimination of the coordinated
water molecules.† On the other hand, complexes 4–6 are more
stable than the precursor samples and do not show any weight
loss up to 200 ◦C. Notably, no endothermal peak of melting
point is observed for complexes 1–6 in DSC analysis (Figure S9
in the supporting information). This means that complexes are
amorphous, or at least it is difficult for them to crystallize.† This
point is also confirmed by powder XRD patterns (Figure S10
in the supporting information), which lacks any obvious crystal
diffraction peaks in the XRD patterns.† The stable amorphous
states of complexes are very important for increasing the morpho-
logical stability of the emitting-layer films in their OLED applica-
tions.16
Molecular structures of Eu3+ complexes (3 and 6) by the
Sparkle/PM3 Model
The optimized molecular structures of the typical Eu3+ Complexes
(3 and 6) predicted by Sparkle/PM3 model are displayed in
Figures 1 and 2, respectively. The spherical atomic coordinates
calculated via Sparkle/PM3 coordination polyhedron of these
compounds are sumarized in Tables 1 and 2, respectively. In
complex 3, the central Eu3+ ion is coordinated with eight oxygen
atoms, six of which are from the three bidentate b-diketonate
ligands, and the other two oxygen atoms from the water molecules.
The coordination geometry of the metal center is best described as
a bicapped trigonal prism. The average bond length between the
Eu3+ ion and the b-diketonate oxygen atoms is 2.448 A, which is
˚
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complexes (1–6) in THF solutions (c = 2 ¥ 10-6 M) are shown in
Figures 3 and 4, respectively. The newly designed b-diketonate
ligand, HBFPD displays a composite broad band in the UV
corresponding to a singlet-singlet p–p* enolic transition assigned
to the b-diketonate moiety,4e,15,17 with a lowest energy maximum
at 300–410 nm (lmax = 365 nm) and a molar absorption coefficient
of 3.48 ¥104 L mol-1 cm-1. On the other hand, interestingly the
replacement of phenyl group with the naphthyl or biphenyl groups
in the 3-position of the b-diketonate ligand significantly extends
this lower energy absorption bands towards visible region (325–
425 nm; lmax = 378 nm) due to the expanded p-conjugation in the
system. Further more in these ligands, the higher energy absorp-
tion bands detected in the range 250–300 nm are attributable to
the p–p* transitions of the aromatic moiety of the b-diketonate
ligands. The determined molar absorption coefficient values of the
HNFPD and HBPFPD at 378 nm are found to be 3.89 ¥ 104 and
3.54 ¥ 104 L mol-1 cm-1, respectively, revealing that these ligands
have a strong ability of absorbing light. No significant changes are
apparent in the shapes of the absorption bands upon formation of
Fig. 1 The ground state geometry of the complex 3 calculated using the
Sparkle/PM3 model.
Fig. 3 UV–vis absorption spectra of the ligands HBFPD, HNFPD,
HBPFPD, TBNPO in THF (c = 2 ¥ 10-6 M).
Fig. 2 The ground state geometry of the complex 6 calculated using the
Sparkle/PM3 model.
˚
shorter than that of europium and water oxygen atoms (2.531 A).
This observation could be attributed to the presence of a formal
negative charge on the b-diketonate oxygen atoms which could
enhance the binding to the Eu3+ cation due to electrostatic effects.
On the other hand, the theoretical predictions of compound 6
reveals an eight-coordinate Eu3+ cation environment comprising
one chelated phosphine oxide (TBNPO) and three bidentate
conjugated b-diketonate ligands. The coordination polyhedra of
compound 6 can be described as distorted square antiprism.
Electronic states of the ligands
The UV–vis absorption spectra of the free ligands (HBFPD,
HNFPD, HBPFPD and TBNPO) and their corresponding Eu3+
Fig. 4 UV–vis absorption spectra of the complexes 1–6 in THF (c = 2 ¥
10-6 M).
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the Eu3+ complexes 1–6, therefore suggesting that the coordination
of the Eu3+ion does not have a significant influence on the 1p–p*
transition (Figure 4). The presence of the ancillary ligand TBNPO
in complexes 4–6 not only enhances the absorption intensity but
also satisfies the high coordination number of the central Eu3+ ion
and thus improves the coordination and thermal stabilities of the
complexes. The magnitudes of the molar absorption coefficient
values for 1–6 (11.0 ¥ 104, 12.0 ¥ 104, 11.5 ¥ 104, 11.3 ¥ 104,
11.5 ¥ 104, and 11.6 ¥ 104 L mol-1 cm-1 for the complexes 1–6,
respectively, calculated at the respective lmax value) were found to
be approximately three times higher than that of the b-diketonate
ligand, and this trend is consistent with the presence of three b-
diketonate ligands in each complex.
Steady-state photoluminescence
The solid-state excitation spectra for the Eu3+ complexes 1–6 at
room temperature are shown in Figures 5–7. The excitation profiles
of each complex closely mimics that of its corresponding absorp-
tion spectrum in the 300–500 nm region, thus demonstrating that
energy transfer occurs from the b-diketonate ligands to the Eu3+
ion. Further, the absence of any absorption bands due to the f-f
transitions of the Eu3+ ion proves that luminescence sensitization
via excitation of the ligand is effective. The substitution of phenyl
group with the naphthyl or biphenyl groups in the 3-position of
the b-diketonate ligand remarkably shifts the excitation window
from 275–440 nm (lmax = 400 nm for complexes 1 and 4) to
visible region 300–550 nm with an excitation maximum 430
nm (for complexes 2 and 5) and 440 nm (for complexes 3 and
6), respectively, in the corresponding Eu3+ complexes. Thus the
structure of the conjugated b-diketonate ligand with suitably
extended p-conjugation in the complex molecules dramatically
influences and shifts the excitation window of the Eu3+ complexes
towards visible region, with important applications in biomedical
analysis and lighting devices.1
Fig. 6 298 K excitation and emission spectra of complexes 2 and 5 in
solid-state (lex = 430 nm and emission monitored around 612 nm).
Fig. 7 298 K excitation and emission spectra of complexes 3 and 6 in
solid-state (lex = 440 nm and emission monitored around 612 nm).
7FJ ground state multiplet. Maximum peak intensities at 580,
592, 612, 652, and 702 nm were observed for the J = 0, 1,
2, 3, and 4 transitions, respectively, and the J = 2, so-called
“hypersensitive transition,15b–d,18 is intense. The intensity of the
7
5D0→ F2 transition (electric dipole) is greater than that of the
7
5D0→ F1 transition (magnetic dipole), which indicates that the
coordination environment of the Eu3+ ion is devoid of an inversion
center.18 It can be noted from the emission spectra that the
luminescence intensity of the complexes 4–6 greatly enhanced as
compared to the precursor complexes (1–3) by the displacement
of the solvent molecules from the complexes by the rigid chelating
phosphine oxide TBNPO. No broad emission band resulting from
organic ligand molecules in the blue region can be observed, which
indicates that the ligand transfers the absorbed energy effectively
to the emitting level of the metal ion.
The observed luminescence decays (tobs) are single exponential
functions for the Eu3+ complexes 1–6 at room temperature, thus
indicating the presence of only one emissive Eu3+ center. Typical
decay profiles for complexes 1–6 are displayed in Figures 8,
S11, S12, respectively. The somewhat shorter lifetime observed
for complexes 1–3 may be due to the dominant non-radiative
Fig. 5 298 K excitation and emission spectra of complexes 1 and 4 in
solid-state (lex = 400 nm and emission monitored around 612 nm).
The ambient-temperature emission spectra of Eu3+ complexes
(1–6) show characteristics of the metal ion emissions in the 550–
725 nm region, and exhibit well resolved peaks that are due to
5
the transitions from the metal-centered D0 excited state to the
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Table 3 The radiative (Ar) and nonradiative (Anr) decay rates, 5D0 lifetime
(tobs), intrinsic quantum Yield (QEEuu, %), energy transfer efficiency (hsens, %),
and overall quantum yield (QELu, %) for complexes 1–6 at 298 K in solid-
state
decay channels associated with vibronic coupling induced by the
presence of solvent molecules, as has been well documented for
several hydrated europium b-diketonate complexes.15b–d,19 In the
case of Eu3+, the energy gap between the luminescent state and
the ground state manifold is approximately 12,000 cm-1.17c,20 Thus,
relatively efficient coupling of the Eu3+ excited states occurs to the
Eu
Eu
Complex
t
obs/ms
Ar/s-1
Anr/s-1
QELu(%)
Q
(%)
hsens(%)
1
2
3
4
5
6
399
400
376
769
790
877
1
1
1
1
1
1
937
1060
860
906
917
807
1567
1440
1835
395
348
794
2
7
7
19
28
43
37
42
32
70
73
71
2
7
21
19
28
52
third vibrational overtone of the proximate OH oscillators (nOH
~
3300–3500 cm-1) which is consistent with the observed quenching
of Eu3+ luminescence. On the other hand, because of the absence
of non-radiative decay pathways, longer lifetime values have been
observed for complexes 4–6 in which the solvent molecules have
been replaced by the chelating phosphine oxide.
(hsens) for 1–6 are presented in Table 3. The substitution of
solvent molecules in the Eu3+-tris-b-diketonate complexes 1–3
by a chelating phosphine oxide significantly contributes to the
overall sensitization of the Eu3+-centered luminescence in 4–6
is confirmed by (i) an increase of the intrinsic quantum yields
from 32 -42 % (in 1–3) to 70–73% (in 4–6), which results from
removal of the quenching effect of the O–H vibrations, and
(ii) the significant enhancement of hsens from 2-21 % (in 1–3
) to 28–52% (in 4–6). The absolute quantum yields have been
measured in the solid state for the Eu3+ complexes 1–6. The
precursor Eu3+-tris-b-diketonate complexes 1–3 showed absolute
quantum yields average values (2 to 7%). On the other hand, Eu3+
complexes 4–6 exhibited high quantum yields (19, 28 and 43%,
respectively). In fact, to our knowledge, the biphenyl-substituted
b-diketonate complex in the presence of a chelate phosphine
oxide (6) displays one of the highest solid state quantum yield
(43%). Typical solid state quantum yields are 28% for Eu(2-
TFDBC)3phen (where 2-TFDBC represents 2-(4¢4¢4¢-trifluoro-
1¢, 3¢-dioxobutyl)-carbazole and phen is 1,10-phenanthroline),
11a
Fig. 8 5D0 decay profiles for complexes 3 and 6 (solid-state) excited at
440 nm and emission monitored around 612 nm. The straight lines are the
best fits (r2 = 0.99) considering single-exponential behaviour.
lmax = 429 nm)
and 16% for Eu(EMOCTFBD)3phen (where
EMOCTFBD represents 1-(9-ethyl-7-methoxyl-9H-carbazol-2-
yl)-4,4,4-trifluorobutane-1,3-dione, lmax = 450 nm).11b
Judd–Ofelt theory is a useful tool for analyzing f-f electronic
transitions.18,23 Interaction parameters of ligand fields are given by
the Judd–Ofelt parameters Xl (where l = 2, 4, and 6). In particular,
X2 is more sensitive to the symmetry and sequence of ligand
fields. To produce faster Eu3+ radiation rates, antisymmetrical Eu3+
complexes with larger X2 parameters need to be designed. The
experimental intensity parameters (X2 and X4) for typical Eu3+
complexes 3 and 6 were determined from the emission spectra,
To gain a better understanding of the luminescence efficiencies
of Eu3+ complexes 1–6, it was appropriate to analyze the emissions
Eu
in terms of equation 2 where QELu and Q represent the ligand-
Eu
sensitized and intrinsic luminescence quantum yields of Eu3+; hsens
represents the efficiency of the ligand-to-metal energy transfer and
tobs/ t
rad are the observed and radiative lifetimes of Eu (5D0): 15d,21
QELu = hsens ¥ QEEuu = hsens ¥ (tobs/trad
)
(1)
5
7
5
7
based on the D0→ F2 and D0→ F4 electronic transitions of
the Eu3+ ion, and they are estimated according to the following
eq 3: 24
The intrinsic quantum yields of Eu3+ could not be determined
experimentally upon direct f-f excitation because of very low
absorption intensity.22 Therefore, the radiative lifetime of Eu
(5D0) has been calculated from equation 2,18,21 where n represents
the refractive index (1.5), AMD,0 is the spontaneous emission
4e2w3A0J
Wl =
(3)
2
3ꢀc 7F U (l) 5D0
5
7
probability for the D0→ F1 transition in vacuo (14.65 s-1), and
Itot / IMD signifies the ratio of the total integrated intensity of the
corrected Eu3+ emission spectrum to the integrated intensity of the
J
where A0J are the coefficient of spontaneous emission for the
7
5D0→ FJ transition, c is the Lorentz local-field correction term
7
magnetic dipole 5D0→ F1 transition:
given by c = n(n2 + 2)2/9, n is the refractive index of the medium
5
7
(in this case n = 1.5), and ꢀ D0U(l)ꢁ FJ ꢂ|2 are the square reduced
⎛
⎜
⎜
⎞
⎟
⎟
⎟
⎟
⎠
1
ITOT
Ar =
= AMD,0
n3
(2)
matrix element whose values are 0.0032, 0.0023 and 0.0002 for
⎜
⎜
⎝
tRAD
I
MD
7
l = 2, 4, and 6 respectively.25a,25b The transition 5D0→ F6 is
The intrinsic quantum yields for Eu3+ complexes 1–6 have been
estimated from the ratio tobs/trad and the pertinent values are
listed in Table 3. The overall quantum yields (QELu ), radiative (Ar)
and nonradiative (Anr) decay rates and energy transfer efficiencies
not observed experimentally thus the experimental X6 parameter
cannot be estimated.
Table 4 presents the experimental and theoretical values for the
intensity parameters (X2, X4 and X6). Firstly, we can note the
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Table 4 Comparison of experimental and Theoretical intensity parameters (Xl, where l = 2, 4 and 6), radiative (Ar) and non-radiative (Anr) rates,
intrinsic quantum Yield (QEuEu, %) and overall quantum yield (QLEu,% ) for complexes 3 and 6
Eu
Eu
Complex
X2/10-20cm2
X4/10-20cm2
X6/10-20 cm2
Ar/s-1
Anr/s-1
Q
(%)
QELu(%)
3a
3b
3a
3b
22.50
23.60
22.01
21.20
7.10
6.20
6.18
6.60
—
1.85
—
860
852
807
786
1835
1843
794
32
32
71
68
7
9
43
48
1.30
814
a
= Experimental, b = Sparkle/PM3.
good agreement between the theoretical and experimental values
for X2 and X4 parameters. The X6 parameter was only determined
7
theoretically, since the 5D0→ F5,6 transitions are not observed
experimentally. The high values of X2 might be interpreted as
7
a consequence of the hypersensitive behavior of the 5D0→ F2
transition,25c which suggests that the dynamic coupling mechanism
is quite operative and that the chemical environment is highly
polarizable. The X4 parameter is less sensitive to the coordination
sphere than X2. However, its value reflects the chemical envi-
ronment rigidity surrounding the Eu3+ cation. The relatively low
values of the X4 parameter indicate rigidity associated with the
coordinated sphere of the complexes 3 and 6.
The experimental and theoretical values for the radiative
and non-radiative rates of spontaneous emission (Ar and Anr,
Eu
Eu
respectively), intrinsic quantum yield (Q ) and overall quantum
yield (QELu) also summarized in Table 4. We can observe the
excellent agreement between experimental and theoretical QLEu
values. Furthermore, it is possible to notice that the overall
quantum yield for complexes 6 is approximately six times higher
than that obtained for complex 3. This may be partly explained by
the fact that the Anrad value observed for complex 3 is twice that
the Anrad value observed for complex 6.
Fig. 9 UV–vis absorption spectrum at 298 K (left), and 77 K phospho-
rescence spectra (right) of the complexes 7–10 in THF (c = 2 ¥ 10-6 M).
complexes 7–10 as in Figure 9. Accordingly, the triplet energy
levels of the ligands, HBFPD, HNFPD, HBPFPD and TBNPO
are found to be 19569, 19330, 19342 and 20200 cm-1, respectively.
Thus, the replacement of phenyl moiety with the naphthyl or
biphenyl groups in the 3-position of b-diketonate ligands has very
little effect on the triplet states of these ligands. The triplet energy
levels of the ligands appears at appreciably higher energy than that
Energy transfer between ligands and Eu3+
The most commonly accepted mechanism for the sensitization
of Ln3+ luminescence by a chromophore ligand involves the
absorption of light by the singlet state followed by intersystem
crossing to the lowest triplet state, which then transfers energy
to the lanthanide-emitting resonance level.15 Many photophysical
studies on a series of analogous compounds have delineated
an empirical correlation between the efficiency of lanthanide
luminescence quantum yield and the energy of the lowest triplet
state (3pp*) with a fast and irreversible energy transfer occurring
when the triplet energy of the ligand is located at above the main
5
of the D0 state of Eu3+, thus indicating that the newly designed
b-diketonate and phosphine oxide ligands can act as a antenna
for the photosensitization of the Eu3+ ion. On the other hand, the
5D1 state of Eu3+ (18,800 cm-1) is found to be critically close to
the triplet state of these ligands, which can lead to the thermally
assisted back-energy transfer from the central core.27 It is also
1
3
noticed that the energy gap between the pp* and pp* levels
are 4702, 3925, 4022 and 3896 cm-1 for the HBFPD, HNFPD,
HBPFPD and TBNPO, respectively. According to Reinhoudt’s
empirical rule,28 that the intersystem crossing process becomes
effective when DE (1pp*–3pp*) is around 5000 cm-1 and hence
the intersystem crossing processes are effective for these ligands.
Therefore, the PL mechanism for the Eu3+ complexes is proposed
to involve a ligand sensitized luminescence process (antenna
effect).2,15 Based on the above experimental results, the typical
energy level diagram for complex 6 and the possible energy tranfer
pathways are shown in Figure 10.
3
5
luminescence state of the Eu3+ ion, i.e DE = pp* - DJ = 2500–4000
cm-1.26 Accordingly, in order to understand the PL mechanism of
the designed series of Eu3+ complexes 1–6, we have determined
the relevant electronic states of the newly synthesized ligands.
The singlet (S1) energy levels (1pp*) of the designed b-diketonate
and the chelating phosphine oxide ligands, were estimated by
referring to the upper wavelengths of the UV-vis absorption
edges of the corresponding Gd3+ complexes 7–10 (Figure 9). The
pertinent singlet energy levels of the ligands, HBFPD, HNFPD,
HBPFPD and TBNPO were found to be 24271, 23255, 23364
and 24096 cm-1, respectively. The triplet (T1) energy levels were
calculated by referring to the lower wavelength emission edges
of the corresponding phosphorescence spectra of complexes Gd3+
In Figure 11 it is proposed an energy level diagram for
complex 6. This diagram shows the most probable channels for
the intramolecular energy transfer process. The vertical full lines
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i to the Eu3+ ion. The two complexes show favorable RL values for
obtaining efficient energy transfer rates.
Table 5 Calculated values of excited state singlet and triplet energies,
RL values, intramolecular energy transfer (WET) and back-transfer (WET
rates for complexes 3 and 6
)
The energy transfer and back-transfer rates were calculated
based on some constraints: the values of the Eu3+ electronic levels
arise from the free ion at intermediate coupling; the singlet level
of the complex is resonant with the Eu3+ 5D4 (27600 cm-1), while
Parameters
3
6
singlet energy/cm-1
28784
3.94364
18353
5.41058
31226
4.75976
21898
˚
RL Singlet/A
5
5D1 and D0 are resonant with the lowest triplet. As showed in
triplet energy/cm-1
Figure 11, the ligand-to-metal energy transfer may occur through
either the singlet or triplet ligand states. According to the selection
rules, energy transfer between the singlet and the 5D4 level occurs
via the multipolar mechanism and the energy transfer between the
triplet and the 5D1 level occurs via the exchange mechanism. The
energy transfer between the triplet and the 5D0 level is forbidden
for both mechanisms, but this selection rule can be relaxed due
˚
RL Triplet/A
6.16376
5
Singlet → D4
W
W
W
W
W
W
ET1/s-1
BT1/s-1
ET2/s-1
BT2/s-1
ET3/s-1
BT3/s-1
3.34 ¥ 108
1.18 ¥ 106
7.36 ¥ 109
2.24 ¥ 1011
6.90 ¥ 109
4.57 ¥ 107
5.33 ¥ 107
1.676
5
Triplet → D1
7.13 ¥ 108
1.00 ¥ 103
2.95 ¥ 108
0.09
5
Triplet → D0
7
to the thermal population of the F1 level at room temperature
and via the mixing of the J states due to the interaction with the
ligand field. For this reason, this transition occurs mostly via the
exchange mechanism.
In Table 5 we also can observe that the triplet energy transfer
rates are ten times larger than the singlet energy transfer rates,
in other words the exchange mechanism dominates. Another
interesting point is that the energy back-transfer rates for complex
3 are much higher than for the complex 6. This fact associated with
the high difference between the Anr for the two complexes, observed
in Table 4, explain the high overall quantum yield presented by
complex 6 when compared with the complex 3 value.
Conclusions
In conclusion, a series of novel class of efficient visible light
sensitized antenna complexes of Eu3+ based on the use of
highly conjugated b-diketonates and a structurally rigid chelate
phosphine oxide ligands were developed, fully characterized and
their photophysical properties evaluated. Suitably expanded p-
conjugation in the complex molecule makes the excitation window
red shifted to the visible region (up to 550 nm). Notably, the
Eu3+-biphenyl-substituted b-diketonate complex in the presence
of a chelate phosphine oxide (6) displays a remarkable quantum
yield of 43% when excited using visible light at 440 nm, which
is among the highest observed to date for a europium complex.
The coordination geometries and theoretical energy transfer of
the typical Eu3+ complexes 3 and 6 were calculated using the
Sparkle/PM3 model. The theoretical quantum efficiencies from
the Sparkle/PM3 structures are in good agreement with the ex-
perimental values, clearly attesting to the efficacy of the theoretical
models. Thus the sensitization of europium complexes in the visible
region is of particular interest for practical applications such as
labeling, biological analysis and solid state lighting. Studies in that
direction are in progress.
Fig. 10 Schematic energy level diagram and energy transfer processes
for the complexes 6. S1 represents the first excited singlet state and T1
represents the.first excited triplet state.
concern the radiative transitions, whereas the vertical dashed lines
concern those associated with non-radiative paths. The curved
lines are related to the ligand→lanthanide energy transfer or back-
transfer. The equivalent diagram for complex 3 is presented in
Figure S13 (in supporting information).† The theoretical singlet
and triplet energies, RL parameters, energy transfer and back-
transfer rates from the singlet state (S1) to the 5D4 level and energy
transfer and back-transfer rates from the ligand triplet state (T1)
to the 5D1 and 5D0 levels are summarized in Table 5.
For the complex 3, the energies of the triplet and singlet states
calculated for the Sparkle/PM3 structure using the INDO/S-CIS
method are 18,353 cm-1 and 28,784 cm-1, respectively. The same
physical quantities obtained using the complex 6 structure are
21,898 cm-1 and 31,226 cm-1, respectively. The RL parameter is the
distance from the donor state located at the organic ligands and
the Eu3+ ion nucleus. This quantity has been calculated by:
Experimental Section
2
c R
∑
i
i
L,i
Materials and Characterization
R
=
(4)
L
2
i
c
∑
All reagents and materials are analytical grade. Solvents were
freshly distilled and dried by standard methods. The following
chemicals were procured commercially and used without sub-
sequent purification. Europium(III) nitrate hexahydrate, 99.9%
(Treibacher); Gadolinium(III) nitrate hexahydrate (Aldrich)
i
with ci being the molecular orbital coefficient of the atom i
contributing to the ligand state (triplet or singlet) involved in the
energy transfer, and RL,i corresponding to the distance from atom
3264 | Dalton Trans., 2011, 40, 3257–3268
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Fig. 11 Energy level diagram for complex 6 showing the most probable channels for the intramolecular energy transfer process.
99.9%; 2-acetylfluoren 98% (Aldrich); benzoic acid 95% (Aldrich);
2-naphthyl carboxylic acid 95% (Aldrich); 4-biphenyl carboxylic
acid 95% (Aldrich); sodium hydride 60% dispersion in mineral oil
(Aldrich); dibenzoyl methane 99% (Aldrich); 2,2¢-Bis(di-p-tolyl
phosphino)-1,1¢-binaphthyl 97% (Aldrich). Elemental analysis for
the synthesized ligands and the Eu3+ complexes was carried out
with a Perkin–Elmer Series 2 Elemental Analyzer 2400. A Perkin–
Elmer Spectrum One FT-IR spectrometer was used to obtain
the IR spectral data (neat KBr). A Bruker 500 MHz NMR
face mode. The lifetime measurements were carried out at room
temperature using a Spex 1040D phosphorimeter. The overall
quantum yields (QLEu) of the europium complexes were measured
at room temperature using the technique for powdered samples
described by Bril et al.,29 through the following expression:
1−r
1−rX Ast
Ax
QLEu
=
×
×Qst
st
(5)
where rst and rx are the diffuse reflectance of the complexes and
of the standard phosphor, respectively, and QStis the quantum
yield of the standard phosphor. The terms Ax and Ast represents
the area under the complex and the standard emission spectra,
respectively. In order to have absolute intensity values BaSO4 was
used as a reflecting standard. The standard phosphor used was
Perylene (Aldrich), whose emission spectrum is formed as a large
broad band peaking around 580 nm, with a constant QSt value
(QSt = 98%, lex = 436 nm).30 Three measurements were carried out
for each sample, so that the presented QLEuvalue corresponds to
the arithmetic mean value. The errors in the quantum yield values
associated with this technique were estimated within 10%.31
1
spectrometer was used to record the H NMR (500 MHz), 13C
NMR (125.7 MHz), and 31P NMR (202.4 MHz) spectra of the new
compounds in chloroform-d or acetone-d6 solution. The chemical
shifts are reported in parts per million relative to tetramethylsilane,
1
SiMe4 for H NMR and 13C NMR spectra and with respect to
85% phosphoric acid for 31P NMR spectra. The mass spectra
were recorded on a JEOL JSM 600 fast atom bombardment
(FAB) high resolution mass spectrometer (FAB-MS), and the
thermogravimetric analyses were performed on a TG/DTA-6200
(SII NanoTechnology Inc., Japan). DSC measurements were
performed on a DSC-Perkin Elmer Pyris 6 DSC instrument at a
heating rate of 10 ◦C/min under a nitrogen atmosphere. Typically,
2–3 mg of samples was placed in an aluminum pan, sealed properly,
and scanned from 10–280 ◦C.
Synthetic Procedures
Synthesis of methyl aromatic carboxylate. 2 g of corresponding
carboxylic acid is dissolved in 20 ml methanol and catalytic
amounts of conc. H2SO4 added, then stirred at 70 ◦C for 12 h
(Scheme 1). The reaction mixture is poured into ice cold water,
ester precipitated out, filtered, dried and used for further synthesis
step.
Powder X-ray diffraction studies
X-Ray powder diffraction using Ni-filtered Cu Ka radiation with
a Philips X¢pert Pro diffractometer. Data were collected by step
scanning from 10 to 70◦ 2q.
Methyl 4-benzyl carboxylate. Yield (90%). 1H NMR (CDC13,
500 MHz): d (ppm) 8.04 (t, 2H, J = 15 Hz), 7.56 (t, 1H, J = 18
Hz ), 7.44 (d, 2H, J = 15 Hz), 3.91 (s, 3H). 13C NMR (125 MHz,
CDCl3) d/ppm: 167.09, 132.89, 130.14, 129.55, 128.39, 128.33,
52.04. m/z = 136 (M)+.
Spectroscopic measurements
The absorbances of the ligands and their lanthanide complexes
were measured in CHCl3 solution on a UV-vis spectrophotometer
(Shimadzu, UV-2450), and the photoluminescence (PL) spectra
were recorded on a Spex-Fluorolog FL22 spectrofluorimeter
equipped with a double grating 0.22 m Spex 1680 monochromator
and a 450 W Xe lamp as the excitation source operating in the front
Methyl 2-naphthyl carboxylate. Yield (89%). 1H NMR
(CDC13, 500 MHz): d/ppm: 8.01 (s, 1H), 7.96 (d, 1H ), 7.89 (d,
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1H), 7.87 (d, 2H ), 7.59 (t, 2H ), 3.98 (s, 3H). 13C NMR (125 MHz,
CDCl3) d/ppm: 167.29, 135.52, 132.50, 131.07, 129.36, 128.24,
127.76, 127.40, 126.64, 125.23, 52.13. m/z = 187 (M+H)+.
(d, 2H, J = 15 Hz), 7.60 (d,1H), 7.46 (t, 2H, J = 15 Hz ), 7.40
(t, 2H, J = 15 Hz), 6.98 (s, 1H), 4.00 (s, 2H). 13C NMR (125
MHz, CDCl3) d/ppm): 186.10, 184.71, 146.10, 145.1, 144.36,
143.52, 140.65, 139.99 134.38, 133.94, 127.97, 128.16, 127.97–
127.08, 126.44, 125.28, 123.90, 120.79, 119.89, 93.12, 77.28–76.78
(CDCl3), 36.94 (Figure S3; in supporting information).† IR (KBr)
nmax (cm-1): 2923, 1604, 1447, 1307, 1242, 1125, 1002, 762. m/z =
389.39 (M+H)+.
Methyl 4-biphenyl carboxylate. Yield (90%). 1H NMR
(CDC13, 500 MHz): d/ppm: 8.11 (d, 2H, J = 15 Hz), 7.65 (q,
4H, J = 18 Hz ), 7.44 (m, 3H), 3.94 (s, 3H). 13C NMR (125 MHz,
CDCl3) d/ppm: 167.02, 145.65, 140.01, 130.10, 128.92, 128.14,
127.28, 127.05, 52.13. m/z = 213 (M+H)+.
Synthesis of 2,2¢-Bis(di-p-tolyl phosphino)-1,1¢-binaphthyl ox-
ide (TBNPO). 2,2¢-Bis(di-p-tolyl phosphino)-1,1¢-binaphthyl
(5 mmol) was dissolved in 10 mL of 1,4-dioxane solution, to which
1 mL of 30% H2O2 (10.5 mmol) was added drop wise with vigorous
stirring (Scheme 2). The resultant mixture was then stirred for 8
h and then 10 mL of water was added to the reaction mixture to
arrest the reaction. The mixture was extracted with 3 ¥ 30 mL
of dichloromethane. The oily phase was then washed with 2 ¥
30 mL of water to remove 1,4-dioxane. The dichloromethane layer
was dried with Na2SO4. The solvent was removed in vacuo. A
white powder was obtained with a yield of 99%. The product
was recrystallized from dichloromethane. Elemental analysis (%):
calcd for C48H40O2P2 (710.80): C 81.11, H 5.67. Found: C 81.30,
Synthesis of the ligands. The ligands 1-(1-phenyl)-3-
(2-fluoryl) propanedione (HBFPD), 1-(2-naphthyl)-3-(2-
fluoryl)propanedione (HNFPD) and 1-(4-biphenyl)-3-(2-fluoryl)
propanedione (HBPFPD) were synthesized according to Scheme
1. Acetylfluorene (1 mmol) and the corresponding ester (1 mmol)
were added to 20 mL of dry tetrahydrofuran (THF) and stirred
for 10 min at 0 ◦C in an ice bath. To this solution, sodium
hydride (2 mmol) was added in an inert atmosphere and stirred
◦
for 30 min followed by further stirring at 70 C for 12 h. To the
resulting solution, 2 M HCl (50 mL) was added, and extracted
twice with dichloromethane (2 ¥ 35 mL). The organic layer was
separated and dried over Na2SO4, and the solvent was evaporated.
The crude product thus obtained is then purified by column
chromatography on silica gel with mixture of chloroform and
hexane (1:10) as the eluent to get the product as a yellow solid.
The new ligands were identified by elemental analyses and FT IR,
1
H 5.39. H NMR (CDCl3, 500 MHz) d/ppm: 7.45 (d, 8H), 7.43
(d, 8H), 7.37(d, 2H), 7.27(d, 2H), 6.98(d, 4H), 6.87(t, 2H), 6.86(t,
2H), 2.32 (s, 12H). 31P NMR (CDCl3, 500 MHz) d/ppm: 29.25
(Figure S4; in supporting information).† IR (KBr) nmax (cm-1):
1634, 1601, 1452, 1400, 1257, 1197, 1113, 872, 775, 743, 713, 697.
m/z = 709.55 (M+).
1
FAB-MS, H NMR and 13C NMR spectral data (Fig. S1–S3; in
supporting information).†
1-(1-phenyl)-3-(2-fluoryl) propanedione (HBFPD). Yield
=
Synthesis of Eu3+ Complexes 1–3. To the solution of b-
diketonate ligand (6 mmol) in THF, added 6 mmol of NaOH
in water and stirred for 5 min. To this mixture, 2 mmol of
Ln(NO3)3·6(H2O) in 2 mL water was added drop wise and
stirred for 24 h at room temperature (Scheme 2). The reaction
mixture was extracted with CHCl3 and crude products were
obtained after solvent evaporation. The products were purified
by recrystallization from chloroform solution and used for further
analysis and photophysical studies.
72%, Elemental analysis (%): calcd for C22H16O2 (312.16): C 84.17,
1
H 5.77; Found: C 84.05, H 5.32. H NMR (CDC13, 500 MHz):
d/ppm: 17.01 (broad, enol–OH), 8.20 (s, 1H), 8.03 (t, 3H, J =
16 Hz), 7.89 (d, 2H, J = 12 Hz ), 7.86 (d, 2H, J = 15 Hz), 7.59
(d, 2H, J = 12 Hz), 7.41 (t, 2H, J = 15 Hz), 6.98 (s, 1H), 4.00 (s,
2H). 13C NMR (125 MHz, CDCl3) d/ppm: 186.16, 185.20, 146.10,
144.1, 143.36, 140.64, 135.66 133.89, 132.36, 128.69, 127.97–
127.09, 126.43, 125.28, 123.89, 120.79, 119.89, 93.15, 77.27–76.77
(CDCl3), 36.93 (Figure S1; in supporting information).† IR (KBr)
nmax (cm-1): 2924, 1604, 1447, 1307, 1243, 1125, 1002, 793. m/z =
313.62 (M+H)+.
Eu(BFPD)3(H2O)2 (1). Elemental analysis (%): calcd for
C66H49O8Eu (1121.41): C 70.90, H 4.44. Found: C 70.81, H 4.83.
IR (KBr) nmax (cm-1): 3395, 2923, 1591, 1477, 1303, 1218, 1098,
755. m/z = 775.87 [Eu(BFPD)2+1]+.
1-(2-naphthyl)-3-(2-fluoryl) propanedione (HNFPD). Yield =
59%, Elemental analysis (%): calcd for C26H18O2 (362.26): C 84.59,
1
H 5.16; Found: C 84.27, H 5.00. H NMR (CDC13, 500 MHz):
Eu(NFPD)3(H2O)2 (2). Elemental analysis (%): calcd for
C78H55O8Eu (1271.73): C 73.77, H 4.47. Found: C 73.81, H 4.53.
IR (KBr) nmax (cm-1): 3428, 2923, 1604, 1447, 1307, 1219, 1125,
1002, 758. m/z = 874.80 [Eu(NFPD)2+1]+.
d/ppm: 17.11 (broad, enol–OH), 8.58 (s, 1H), 8.24 (S, 1H), 8.20
(d, 2H, J = 15 Hz ), 7.96 (d, 2H, J = 12 Hz ), 7.93 (t, 2H, J = 15
Hz), 7.89 (d, 1H, J = 15 Hz), 7.87 (d, 1H), 7.60 (d, 2H, J = 15
Hz ), 7.46 (t, 2H, J = 15 Hz), 6.98 (s, 1H), 4.02 (s, 2H). 13C NMR
(125 MHz, CDCl3) d/ppm: 186.16, 185.20, 146.10, 144.36, 143.51,
140.64, 135.66, 133.90, 132.47, 132.35, 128.69, 127.97, 127.15,
127.08, 126.43, 125.27, 123.89, 120.79, 119.89, 93.15, 77.27–76.77
(CDCl3), 36.93 (Figure S2; in supporting information).† IR (KBr)
nmax (cm-1): 2961, 1628, 1443, 1388, 1282, 1163, 1079, 760. m/z =
363.62 (M+H)+.
Eu(BPFPD)3(H2O)2 (3). Elemental analysis (%): calcd for
C84H61O8Eu (1349.74): C 74.80, H 5.07. Found: C 74.91, H 5.48.
IR (KBr) nmax (cm-1): 3434, 2922, 1590, 1449, 1308, 1218, 1117,
1005, 758. m/z = 928.14 [Eu(BFPD)2+1]+.
Synthesis of Eu3+ Complexes 4–6
1-(4-biphenyl)-3-(2-fluoryl) propanedione (HBPFPD). Yield =
The ternary Eu3+ Complexes were prepared by stirring equimolar
solutions of their corresponding binary complexes (1–3) and
TBNPO in CHCl3 solution for 12 h at 70 ◦C (Scheme 3). The
products were isolated by solvent evaporation and purified by
recrystallization from a chloroform/hexane mixture.
60%. Elemental analysis (%): calcd for C28H20O2 (388.15): C 86.57,
1
H 5.19; Found: C 86.39, H 5.41. H NMR (CDC13, 500 MHz):
d/ppm: 17.10 (broad, enol–OH), 8.22 (s, 1H), 8.08 (t, 3H, J =
15 Hz), 7.89 (t, 2H, J = 10 Hz ), 7.35 (d, 2H, J = 15 Hz), 7.67
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Eu(BFPD)3(TBNPO) (4). Elemental analysis (%): calcd for
C114H85O8P2Eu (1795.74): C 75.95, H 5.04. Found: C 75.13, H
5.30. IR (KBr) nmax (cm-1): 2918, 1594, 1453, 1301, 1173, 1023,
749. m/z = 1482.71 [Eu(BFPD)2TBNPO +1]+. 31P NMR (CDCl3,
500MHz) d/ppm: -85.38 (Figure S5; in supporting information).†
overlap/spectroscopic (INDO/S) methodology34 implemented in
ZINDO package.35 We have used a point charge of +3e to represent
the trivalent lanthanide ion.
The Judd–Ofelt intensity parameters calculation. The coordi-
native interaction between a lanthanide cation and a ligand L can
be described by the Judd–Ofelt theory23 after which the intensity
parameters Xl (l = 2, 4 and 6) are defined by:
Eu(NFPD)3(TBNPO) (5). Elemental analysis (%): calcd for
C126H91O8P2Eu (1945.80): C 77.49, H 5.01. Found: C 77.40, H
5.31. IR (KBr) nmax (cm-1): 2921, 1604, 1464, 1398, 1183, 1025,
796. m/z = 1586.94 [Eu(NFPD)2TBNPO +1]+. 31P NMR (CDCl3,
500MHz) d/ppm: -85.38 (Figure S6; in supporting information).†
2
Bltp
(6)
W = 2l +1
(
)
l
∑
(2t +1)
t ,p
Eu(BPFPD)3(TNPO) (6). Elemental analysis (%): calcd for
C132H97O8P2Eu (2023.22): C 78.06, H 5.11. Found: C 78.16, H
5.30. IR (KBr) nmax (cm-1): 2918, 1590, 1432, 1389, 1180, 1021,
753. m/z = 1634.71 [Eu(BPFPD)2TBNPO]+. 31P NMR (CDCl3,
500MHz) d/ppm: -85.90 (Figure S7; in supporting information).†
With
2
Bltp
=
rt +1 q(t, p)g tp
DE
(7)
⎡
⎤
⎥
⎥
l +1 2l +3
(
)(
)
l
−
rt (1−sl ) f C(
f
Gtpdt ,l+1
)
⎢
⎢
2l +1
⎢
⎥
⎣
⎦
Gd(BFPD)3(H2O)2 (7). Elemental analysis (%): calcd for
C66H49O8Gd (1127.25): C 70.57, H 4.42. Found: C 70.61, H 4.50.
IR (KBr) nmax (cm-1): 3060, 1595, 1478, 1304, 1224, 1063, 754.
m/z = 780.15 [Gd(BFPD)2+1]+.
Details on the parameters of equations (6) and (7) are widely
discussed in the literature.2e,\
Energy transfer rates and quantum yield. The models used
to obtain the energy transfer rates between the ligands and the
lanthanide ion, the numerical solution of the rate equations and
the emission quantum yield are described in Refs. 24,37
36
Gd(NFPD)3(H2O)2 (8). Elemental analysis (%): calcd for
C78H55O8Gd (1277.51): C 73.47, H 4.45. Found: C 73.81, H 4.56.
IR (KBr) nmax (cm-1): 3425, 2921, 1604, 1464, 1398, 1217, 1183,
1025, 757. m/z = 880.42 [Eu(NFPD)2+1]+.
Electronic supplementary information (ESI†) available. 1H
NMR, 13C NMR and 31P NMR spectra of the ligands and
complexes, TG, DSC curves, XRD patterns and Energy level
diagram for complex 3.
Gd(BPFPD)3(H2O)2 (9). Elemental analysis (%): calcd for
C84H61O8Gd (1355.06): C 74.59, H 4.57. Found: C 75.03, H 4.63.
IR (KBr) nmax (cm-1): 3434, 2923, 1587, 1449, 1306, 1214, 1119,
1005, 758. m/z = 932.14 [Gd(BFPD)2+1]+.
Acknowledgements
Synthesis of Gd(TBNPO)(NO3)3 (10).
1
mmol of
Gd(NO3)3(H2O)6 (dissolved in 0.1 mL of water) was added
drop wise to the ethanolic solution of corresponding phosphine
oxide (1 mmol) under stirring and the mixture was refluxed for
2 h in an oil bath at 70 ◦C. The white precipitate formed from
the above reaction mixture was filtered and recrystallized from
chloroform–hexane mixture. Elemental analysis (%): calcd for
C48H40N3O11P2Gd (1053.80): C 54.70, H 3.83, N 3.99. Found: C
54.60, H 3.94, N 3.95. IR (KBr) nmax (cm-1): 1603, 1553, 1447,
1302, 1218, 1143, 1115, 1093, 873, 809, 745, 686, 647, 561. m/z =
1052.64 [Gd(TBNPO)(NO3)3]+.
The authors acknowledge financial support from the Council of
Scientific and Industrial Research (NWP 0023). One of the authors
V. D. thanks the Council of Scientific & Industrial Research
(CSIR), New Delhi for awarding her a Junior Research fellowship
and the author R. O. F appreciates the financial support from the
Brazilian agencies, institutes and networks: CNPq, FAPITEC-
SE, INAMI and thanks to Prof. A.E.A. Paixa˜o for the use of the
software Statistica.
References
1 (a) S. V. Eliseeva and J.-C. G. Bu¨nzli, Chem. Soc. Rev., 2010, 39, 189–
227; (b) L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro,
M. Cavazzini and E. Tondello, Coord. Chem. Rev., 2010, 5-6, 487–
505; (c) Y. Ma and Y. Wang, Coord. Chem. Rev., 2010, 254, 972–
990; (d) K. Binnemans, Chem. Rev., 2009, 109, 4283–4374; (e) J.-C. G.
Bu¨nzli, Chem. Rev., 2010, 110, 2729–2755; (f) T. Lazarides, D. Sykes, S.
Faulkner, A. Barbieri and M. D. Ward, Chem.–Eur. J., 2008, 14, 9380–
9389; (g) V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, M. Gorka and
F. Vo¨gtle, J. Am. Chem. Soc., 2002, 124, 6461–6468; (h) J. L. Bender, P.
S. Corbin, C. L. Fraser, D. H. Metcalf, F. S. Richardson, E. L. Thomas
and A. M. Urbas, J. Am. Chem. Soc., 2002, 124, 8526–8527; (i) C. Yang,
L.-M. Fu, Y. Wang, J.-P. Zhang, W.-T. Wong, X.-C. Ai, Y.-F. Qiao, B.-S.
Zou and L.-L. Gui, Angew. Chem., Int. Ed., 2004, 43, 5010–5013.
2 (a) N. Sabbatini, M. Guardigli and J.-M. Lehn, Coord. Chem. Rev.,
1993, 123, 201–228; (b) J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990,
29, 1304–1319; (c) C. Piguet and J.-C. G. Bu¨nzli, Chem. Soc. Rev., 1999,
28, 347–358; (d) S. Petoud, S. M. Cohen, J.-C. G. Bu¨nzli and K. N.
Raymond, J. Am. Chem. Soc., 2003, 125, 13324–13325.
Theoretical Section
The ground state geometries and excited state energies. The
ground state geometries of Ln3+ complexes 1–3 were calculated us-
ing Sparkle/PM3 model14a implemented in mopac2007 program.32
The MOPAC keywords used in all Sparkle/AM1 calculations
were: PRECISE, GEO-OK, GNORM = 0.25, SCFCRT = 1.D-10
(in order to increase the SCF convergence criterion) and XYZ (the
geometry optimizations were performed in Cartesian coordinates).
The Sparkle/PM3 model14 are instead based on a elaborated
parameterization procedure for semiempirical methodologies, al-
lowing the prediction of coordination geometries whose accuracy
is comparable to those obtained from ab-initio/ECP calculations
but at much smaller expense of computational power.33
For all calculated ground state geometries we have predicted
their singlet and triplet excited states using configuration interac-
tion single (CIS) based on the intermediate neglect of differential
3 S. I. Weissman, J. Chem. Phys., 1942, 10, 214–217.
4 (a) J. M. Stanley, X. Zhu, X. Yang and B. J. Holliday, Inorg. Chem.,
2010, 49, 2035–2037; (b) N. M. Shavaleev, S. V. Eliseeva, R. Scopelliti
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 3257–3268 | 3267
©

View Online
and J.-C. G. Bu¨nzli, Chem.–Eur. J., 2009, 15, 10790–10802; (c) A. R.
Ramya, M. L. P. Reddy, A. H. Cowley and K. V. Vasudevan, Inorg.
Chem., 2010, 49, 2407–2415; (d) S. Sarika, M. L. P. Reddy, A. H.
Cowley and K. V. Vasudevan, Dalton Trans., 2010, 39, 776–786; (e) B.
Francis, D. B. Ambili, Raj and M. L. P. Reddy, Dalton Trans., 2010,
39, 8084–8092; (f) S. Biju, M. L. P. Reddy, A. H. Cowley and K. V.
Vasudevan, J. Mater. Chem., 2009, 19, 5179–5187; (g) P. N. Remya, S.
Biju, M. L. P. Reddy, A. H. Cowley and M. Findlater, Inorg. Chem.,
2008, 47, 7396–7404; (h) S. Quici, M. Cavazzini and G. Marzanni, Inorg.
Chem., 2005, 44, 529–537; (i) J. Zhang, P. D. Badger, S. J. Geib and S.
Petoud, Angew. Chem., Int. Ed., 2005, 44, 2508–2512; (j) P. Kadjane,
L. Charbonnie`re, F. Camerel, P. P. Laine´ and R. Ziessel, J. Fluoresc.,
2008, 18, 119–129.
18 M. H. V. Werts, R. T. F. Jukes and J. W. Verhoeven, Phys. Chem. Chem.
Phys., 2002, 4, 1542–1548.
19 G. F. de Sa, O. L. Malta, C. de Mello Donega, A. M. Simas, R. L.
Longo, P. A. Santa-Cruz and E. F. da Silva Jr., Coord. Chem. Rev.,
2000, 196, 165–195.
20 (a) A. Dossing, Eur. J. Inorg. Chem., 2005, 1425–1434; (b) A. Beeby, I.
M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle, A. S. de
Sousa, J. A. G. Williams and M. Woods, J. Chem. Soc., Perkin Trans.,
1999, 2, 493–504.
21 N. M. Shavaleev, S. V. Eliseeva, R. Scopelliti and J.-C. G. Bu¨nzli, Inorg.
Chem., 2010, 49, 3927–3936.
22 J.-C. G. Bu¨nzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048–1077.
23 (a) B. R. Judd, Phys. Rev., 1962, 127, 750–761; (b) G. S. Ofelt, J. Chem.
Phys., 1962, 37, 511–520.
24 M. O. Rodrigues, F. A. Almeida Paz, R. O. Freire, G. F. de Sa’, A.
Galembeck, M. C. B. S. M. Montenegro, A. N. Araujo and S. A.
Ju´nior, J. Phys. Chem. B, 2009, 113, 12181–12188.
5 F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. VanderTol and J.
W. Verhoeven, J. Am. Chem. Soc., 1995, 117, 9408–9414.
6 K. Kuriki, Y. Koike and Y. Okamoto, Chem. Rev., 2002, 102, 2347–
2356.
7 (a) F. Chen, Z. Bian, Z. Liu, D. Nie, Z. Chen and C. Huang, Inorg.
Chem., 2008, 47, 2507–2513; (b) R. Ziessel, S. Diring, P. Kadjane, L.
Charbonnie`re, P. Retailleau and C. Philouze, Chem.–Asian J., 2007, 2,
975–982.
8 (a) H.-B. Xu, L.-X. Shi, E. Ma, L.-Y. Zhang, Q.-H. Wei and Z.-N.
Chen, Chem. Commun., 2006, 1601–1603; (b) M. Ward, Coord. Chem.
Rev., 2007, 251, 1663–1677.
9 (a) E. S. Andreiadis, R. Demadrille, D. Imbert, J. Pe`caut and M.
Mazzanti, Chem.–Eur. J., 2009, 15, 9458–9476; (b) M. H. V. Werts,
M. A. Duin, J. W. Hofstraat and J. W. Verhoeven, Chem. Commun.,
1999, 799–800; (c) A. Dadabhoy, S. Faulkner and P. G. Sammes, J.
Chem. Soc., Perkin Trans. 2, 2000, 2359–2360; (d) A. Beeby, L. M.
Bushby, D. Maffeo and J. A. G. Williams, J. Chem. Soc., Perkin Trans.
2, 2000, 1281–1283; (e) Y. Bretonniere, M. J. Cann, D. Parker and R.
Slater, Chem. Commun., 2002, 1930–1931.
10 V. Divya, S. Biju, R. Luxmi Varma and M. L. P. Reddy, J. Mater. Chem.,
2010, 20, 5220–5227.
11 (a) P. He, H. H. Wang, S. G. Liu, J. X. Shi, G. Wang and M. L. Gong,
Inorg. Chem., 2009, 48, 11382–11387; (b) P. He, H. H. Wang, H. G. Yan,
W. Hu, J. X. Shi and M. L. Gong, Dalton Trans., 2010, 39, 8919–8924.
12 R. V. Deun, P. Nockemann, P. Fias, K. V. Hecke, L. V. Meervelt and K.
Binnemans, Chem. Commun., 2005, 590–592.
13 A. D’Ale’o, A. Picot, A. Beeby, J. A. G. Williams, B. Le Guennic, C.
Andraud and O. Maury, Inorg. Chem., 2008, 47, 10258–10268.
14 (a) R. O. Freire, G. B. Rocha and A. M. Simas, J. Braz. Chem. Soc.,
2009, 20, 1638–1645; (b) R. O. Freire, G. B. Rocha and A. M. Simas,
Chem. Phys. Lett., 2006, 425, 138–141; (c) R. O. Freire, G. B. Rocha
and A. M. Simas, Chem. Phys. Lett., 2007, 441, 354–357; (d) N. B.
Costa Jr., R. O. Freire, G. B. Rocha and A. M. Simas, J. Phys. Chem. A,
2007, 111, 5015–5018; (e) A. M. Simas, R. O. Freire and G. B. Rocha,
Lect. Notes Comput. Sci., 4488, 312–318; (f) R. O. Freire, N. B. Costa,
Jr., G. B. Rocha and A. M. Simas, J. Chem. Theory Comput., 2007, 3,
1588–1596; (g) R. O. Freire and A. M. Simas, J. Chem. Theory Comput.,
2010, 6, 2019–2023; (h) A. M. Simas, R. O. Freire and G. B. Rocha, J.
Organomet. Chem., 2008, 693, 1952–1956.
15 (a) S. Biju, M. L. P. Reddy, A. H. Cowley and K. V. Vasudevan, Cryst.
Growth Des., 2009, 9, 3562–3569; (b) D. B. Ambili Raj, S. Biju and M.
L. P. Reddy, Inorg. Chem., 2008, 47, 8091–8100; (c) D. B. Ambili Raj,
S. Biju and M. L. P. Reddy, Dalton Trans., 2009, 7519–7528; (d) D. B.
Ambili Raj, B. Francis, M. L. P. Reddy, R. R. Butorac, V. M. Lynch
and A. H. Cowley, Inorg. Chem., 2010, 49, 9055–9063.
25 (a) B. R. Judd, Operator Techniques in Atomic Spectroscopy, McGraw-
Hill Book Company, USA, 1998; (b) J. B. Guan, B. Chen, Y. Y. Sun,
H. Liang and Q. J. Zhang, J. Non-Cryst. Solids, 2005, 351, 849–855;
(c) W. T. Carnall and H. M. Crosswhite, Energy Levels, Structure
and Transition Probabilities of the Trivalente Lanthanides in LaF3,
Argonne National Laboratory Illinois, 1997.
26 (a) R. Pavithran, N. S. Saleesh Kumar, S. Biju, M. L. P. Reddy, S. Alves
Jr. and R. O. Freire, Inorg. Chem., 2006, 45, 2184–2192; (b) S. Biju,
D. B. Ambili Raj, M. L. P. Reddy and B. M. Kariuki, Inorg. Chem.,
2006, 45, 10651–10660; (c) M. Shi, F. Li, T. Yi, D. Zhang, H. Hu and
C. Huang, Inorg. Chem., 2005, 44, 8929–8936.
27 N. Armaroli, G. Accorsi, F. Barigelletti, S. M. Couchman, J. S. Fleming,
N. C. Harden, J. C. Jeffery, K. L. V. Mann, J. A. McCleverty, L. H. Rees,
S. R. Starling and M. D. Ward, Inorg. Chem., 1999, 38, 5769–5776.
28 F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. VanderTol and J.
W. Verhoeven, J. Am. Chem. Soc., 1995, 117, 9408–9414.
29 A. Bril and A. W. De Jager-Veenis, J. Electrochem. Soc., 1976, 123,
396–398.
30 W. H. Melhuish, J. Opt. Soc. Am., 1964, 54, 183–186.
31 (a) C. D. Mello Donega´, S. Alves Jr. and G. F. de Sa´, Chem. Commun.,
1996, 1199–1200; (b) L. D. Carlos, C. D. Mello Donega´, R. Q.
Albuquerque, S. Alves Jr, J. F. S. Menezes and O. L. Malta, Mol. Phys.,
2003, 101, 1037–1045.
32 J. J. P. Stewart;, MOPAC2009, version 10.060W; Stewart Computa-
tional Chemistry, Colorado Springs, USA.
33 (a) R. O. Freire, G. B. Rocha and A. M. Simas, J. Mol. Model., 2006,
12, 373–389; (b) R. O. Freire, G. B. Rocha, R. Q. Albuquerque and A.
M. Simas, J. Lumin., 2005, 111, 81–87.
34 (a) J. E. Ridley and M. C. Zerner, Theor. Chim. Acta, 1976, 42, 223–
236; (b) M. C. Zerner, G. H. Loew, R. F. Kirchner and U. T. Mueller-
Westerhoff, J. Am. Chem. Soc., 1980, 102, 589–599.
35 M. C. Zerner, ZINDO manual, QTP, University of Florida, Gainesville,
FL. 32611 1990.
36 (a) O. L. Malta, S. J. L. Ribeiro, M. Faucher and P. Porcher, J. Phys.
Chem. Solids, 1991, 52, 587–593; (b) O. L. Malta, M. A. Couto, dos
Santos, L. C. Thompson and N. K. Ito, J. Lumin., 1996, 69, 77–84;
(c) O. L. Malta, H. F. Brito, J. F. S. Meneses, F. R. G. e Silva, S. Alves
Jr., F. S. Farias, Jr. and A. V. M. de Andrade, J. Lumin., 1997, 75,
255–268.
37 (a) R. O. Freire, R. Q. Albuquerque, S. A. Ju´nior, G. B. Rocha and M.
E. Mesquita, Chem. Phys. Lett., 2005, 405, 123–126; (b) O. L. Malta,
F. R. G. e Silva and R. Longo, Chem. Phys. Lett., 1999, 307, 518–526;
(c) O. L. Malta and F. R. G. e Silva, Spectrochim. Acta, Part A, 1998,
54, 1593–599.
16 H. Xu, K. Yin and W. Huang, Chem.–Eur. J., 2007, 13, 10281–10293.
17 Y. Sun, J. Gao, Z. Zheng, W. Su and Q. Zhang, Spectrochim. Acta, Part
A, 2006, 64, 977–980.
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