Bright green luminescent molecular terbium plastic materials derived from 3,5-bis(perfluorobenzyloxy)benzoate

Article abstract of DOI:10.1039/c2jm30535f

Luminescent lanthanide complexes (Eu³⁺ (1) or Tb³⁺ (2)) involving a highly fluorinated aromatic carboxylate, namely, 3,5-bis(perfluorobenzyloxy)benzoic acid which acts as an antenna chromophore and sensitizes the visible emitting lanthanides, have been synthesized and characterized and their photophysical properties investigated. The results demonstrated that the replacement of high-energy C-H vibrations with fluorinated phenyl groups in the 3,5-bis(benzyloxy)benzoate effectively improves the luminescence intensity and lifetimes of lanthanide complexes. It is interesting to note that the designed fluorinated carboxylate is well suited for the sensitization of Tb³⁺ emission (Φ_sen = 52%), thanks to a favorable position of the triplet state of the ligand as investigated in the Gd³⁺ complex. On the other hand, the corresponding Eu³⁺ complex shows weak luminescence efficiency (Φ_sen = 24%) due to poor match of the triplet state of the ligand with the emissive excited states of the metal ion. In the present work, efforts have also been made to isolate luminescent molecular terbium plastic materials by combining the unique optical properties of lanthanides with the mechanical characteristics, thermal stability, flexibility and film-forming tendency of polymers (PMMA). The photoluminescence quantum yields of polymer-lanthanide hybrid materials are significantly enhanced (53-65%) as compared to that of the Tb ³⁺-3,5-bis(perfluorobenzyloxy)benzoate complex. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30535f

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PAPER
Bright green luminescent molecular terbium plastic materials derived from
3,5-bis(perfluorobenzyloxy)benzoate†
*
Sarika Sivakumar and M. L. P. Reddy
Received 28th January 2012, Accepted 23rd March 2012
DOI: 10.1039/c2jm30535f
Luminescent lanthanide complexes (Eu3+ (1) or Tb3+ (2)) involving a highly fluorinated aromatic
carboxylate, namely, 3,5-bis(perfluorobenzyloxy)benzoic acid which acts as an antenna chromophore
and sensitizes the visible emitting lanthanides, have been synthesized and characterized and their
photophysical properties investigated. The results demonstrated that the replacement of high-energy
C–H vibrations with fluorinated phenyl groups in the 3,5-bis(benzyloxy)benzoate effectively improves
the luminescence intensity and lifetimes of lanthanide complexes. It is interesting to note that the
designed fluorinated carboxylate is well suited for the sensitization of Tb3+ emission (Fsen ¼ 52%),
thanks to a favorable position of the triplet state of the ligand as investigated in the Gd3+ complex. On
the other hand, the corresponding Eu3+ complex shows weak luminescence efficiency (Fsen ¼ 24%) due
to poor match of the triplet state of the ligand with the emissive excited states of the metal ion. In the
present work, efforts have also been made to isolate luminescent molecular terbium plastic materials by
combining the unique optical properties of lanthanides with the mechanical characteristics, thermal
stability, flexibility and film-forming tendency of polymers (PMMA). The photoluminescence quantum
yields of polymer–lanthanide hybrid materials are significantly enhanced (53–65%) as compared to that
of the Tb3+–3,5-bis(perfluorobenzyloxy)benzoate complex.
of the metal.4 Accordingly, to eliminate such quenching effects,
Introduction
highly fluorinated antenna chromophores have been reported for
the sensitization of near-IR emissive metals such as Er3+, Nd3+
and Yb3+.5 However, the effect of fluorination on the lumines-
cence intensities of visible emitting lanthanide–benzoate
complexes (Eu3+ and Tb3+) has not yet been examined.
On account of their intrinsic chemical, magnetic and spectro-
scopic properties, lanthanide coordination compounds are well
recognized as promising light-emitting materials in a wide variety
of areas ranging from medicine to material science.1 Attractive
features of luminescent lanthanide compounds include their line-
like emission, large Stokes shift, very long luminescence lifetimes
and high photochemical stability. However, the forbidden nature
of the 4f–4f transition of lanthanide ions results in inefficient
direct excitation. Bright luminescence of Ln3+ ions can be
achieved by exploring the antenna effect2 allowing for indirect
population of the lanthanide emissive excited states through
ligand excitation. The efficiency of ligand-to-lanthanide energy
transfer demands compatibility between the energy levels of the
ligand excited states and accepting levels of Ln3+ ions.3 More-
over, attractive quantum yields cannot be achieved without
preventing the non-radiative deactivation by O–H, N–H or C–H
oscillators in the coordination environment or in close proximity
Herein, we have designed and synthesized a new antenna
chromophore – 3,5-bis(perfluorobenzyloxy)benzoic acid – by
replacing the hydroxyl hydrogen atoms in 3,5-dihydroxybenzoic
acid with highly fluorinated phenyl groups for the sensitization of
visible emitting Ln3+ ions. The synthesized Ln3+–fluorinated
benzoate complexes have been characterized by standard spec-
troscopic techniques and their photophysical properties
investigated.
Recent years have witnessed a growing interest in lanthanide–
polymer complexes because of the combination of the good
luminescent properties of lanthanide complexes and the excellent
mechanical processing properties of polymers.6 The poly(methyl
methacrylate) (PMMA) polymer exhibits excellent mechanical
and optical properties that favour its application as the basis for
optical devices. Furthermore, PMMA contains carbonyl groups
along with its carbon chain that can interact with Ln3+ ions and
substitute ligand water molecules. Hence, improvement in the
overall physical and chemical properties and enhancement of the
characteristics of emission quantum yield and lifetimes can be
expected with polymer films based on PMMA doped with Ln3+–
carboxylate complexes.4c,7 As an integral part of this work, the
Chemical Sciences and Technology Division, National Institute for
Interdisciplinary
Science
&
Technology
(NIIST),
CSIR,
Thiruvananthapuram-695 019, India. E-mail: mlpreddy55@gmail.com
† Electronic supplementary information (ESI) available: NMR spectra of
ligand, FT-IR spectra of ligand, complexes, PMMA and polymer films,
TG data, phosphorescence spectrum of gadolinium complex 3,
low-temperature luminescence decay profiles for complexes 1–2 and
polymer films. See DOI: 10.1039/c2jm30535f
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synthesis, characterization and luminescent properties of
PMMA polymer films doped with Tb3+–3,5-bis(per-
fluorobenzyloxy)benzoate complex are also reported.
to the Ln3+ ions in bidentate bridging and chelating modes.8 A
broad band observed around 3400 cmꢁ1 for complexes 1–3 is
attributed to the characteristic n(OH) stretching vibration
showing the presence of solvent molecules.
Complexes 1–3 were subjected to thermogravimetric analysis
(TGA) studies under a nitrogen atmosphere. TGA curves of
compounds 1–3 are similar and exhibit two steps of weight losses
(Fig. S3 in the ESI†). Compound 1 was used as an example to
interpret the data. The first step of weight loss occurs between
200 and 300 ꢂC (found: 9.3%, calcd: 9.8%), corresponding to
removal of two solvent molecules. The second step from 300 to
700 ꢂC corresponds to the thermal decomposition of the organic
ligands (found: 82.4%, calcd: 81.9%) and finally leading to the
formation of the stoichiometric amounts of Ln2O3 (found: 8.1%,
calcd: 8.4%). It can be concluded from the TGA data that the
Results and discussion
Synthesis and characterization of ligand and Ln3+ complexes 1–3
The synthetic procedure used for preparation of ligand 3,5-
bis(perfluorobenzyloxy)benzoic acid (HL) is outlined in Scheme
1. The ligand was obtained in three steps from commercially
available 3,5-dihydroxybenzoic acid. Compound 2a was
prepared via esterification reaction of 1a using H2SO4 in meth-
anol. Methyl 3,5-bis(perfluorobenzyloxy)benzoate (3a) was
obtained by refluxing 2a in the presence of K2CO3 followed by
the addition of 2,3,4,5,6-pentafluorobenzyl bromide. Full
deprotection of methyl ester was achieved with base hydrolysis
using KOH to give the desired ligand HL with an overall yield of
Gd3+ compound is thermally stable up to 319 C. On the other
ꢂ
hand, 1 and 2 are stable up to 300 ꢂC. Notably, no endothermal
peaks are observed for complexes 1–3 in DSC analysis (Fig. S4 in
the ESI†). This means that the complexes are amorphous, or at
least it is difficult for them to crystallize. This point is also
confirmed by powder XRD patterns (Fig. S5 in the ESI†), which
lack any obvious crystal diffraction peaks.
62%. The newly designed ligand was characterized by 1H and 13
C
NMR spectroscopy (Fig. S1 in the ESI†), mass spectroscopy
(FAB-MS) and elemental analysis. Reaction of 3,5-bis(per-
fluorobenzyloxy)benzoic acid (3 mmol) with lanthanide nitrates
(1 mmol) in the presence of sodium hydroxide resulted in the
formation of compounds of formula [Ln(L)3(C2H5OH)2] (Ln ¼
Eu (1), Tb (2) or Gd (3)), which were identified by elemental
analysis. The IR spectrum of the free ligand displays character-
istic absorptions of symmetric ns(C]O) and asymmetric nas(C]
O) vibrations of the carboxylate group at ꢀ1450 and ꢀ1697
cmꢁ1, respectively. Compared to the free ligand, these bands are
shifted in the FT-IR spectra of complexes 1–3 (Fig. S2 in the
ESI†), indicating that the oxygen atom of the carbonyl group
takes part in coordination to the Ln3+ ion. Further, it can also be
noted that the asymmetric and symmetric stretching vibration
modes of the carboxylate groups in complexes 1–3 are split into
two peaks. The differences between the asymmetric and
symmetric stretching vibration modes (D(C]O) ¼ nas ꢁ ns) were
calculated and these values are in the range 272–274 and 163–
164 cmꢁ1, indicating that the carboxylate groups are coordinated
The PMMA polymer was doped with Tb3+ complex of 3,5-
bis(perfluorobenzyloxy)benzoic acid in the proportions of 6, 8,
10, and 12% (w/w) and characterized by FT-IR spectroscopy.
The band at 1720 cmꢁ1 for PMMA corresponds to the (C]O)
vibration,4c,9 whereas for the Tb–PMMA films, this vibration
shifts to 1735 cmꢁ1 (Fig. S11 in the ESI†). This indicates that the
Tb3+ complex is stabilized by means of interactions with the
oxygen atoms of the carbonyl group of PMMA, due to
the donation of a pair of electrons from the carbonyl oxygen to
the lanthanide ions. The broad absorption band assigned to the
solvent vibrational modes in the FT-IR spectrum of the Tb3+
complex, which appears in the 3400 cmꢁ1 region, is absent in the
spectra of the doped PMMA polymer films thereby further
confirming that the polymer films are anhydrous. This result is in
good agreement with the TG analyses of the doped polymer film,
for which no mass loss was observed in the temperature region
ꢂ
50–230 C (Fig. S9 in the ESI†).
Electronic states of the ligand
UV-visible absorption spectra of the ligand and its correspon-
ding complexes were recorded in CH3OH solution (c ¼ 2 ꢃ 10ꢁ5
M) at room temperature and are depicted in Fig. 1. The ligand
displays an absorption band localized in the UV region with
a molar absorption coefficient (3max) of 3.53 ꢃ 103 Mꢁ1 cmꢁ1
(lmax ¼ 301 nm) that is classically observed for p–p* transitions
of benzoic acid ligands. The ligand absorption in the complexes is
significant, with the molar absorption coefficient reaching (8.90–
9.55) ꢃ 103 Mꢁ1 cmꢁ1. The absorption coefficients for the
complexes are about 3 times higher compared to the free ligand,
in line with the formation of 3 : 1 complexes. These features point
to the ligand being an adequate light-harvesting chromophore
for the sensitization of lanthanide luminescence.
According to Dexter theory,10 the intramolecular energy
transfer from the triplet state (3pp*) of a ligand to the emitting
resonance level of Ln3+ ion has significant influence on lantha-
nide luminescence. The efficiency of intramolecular energy
Scheme 1 Synthetic procedure for ligand HL.
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300 nm is attributed to the p–p* ligand transition, and the
narrow bands are assigned to the transitions of Eu3+ intra-4f6 ion,
from 7F0 to 5G6 (360 nm), 5G0–4 (375–384 nm), 5L6 (395 nm), and
5D2 (464 nm).13 Furthermore, close examination of the excitation
spectrum of 1 reveals that these narrow bands due to f–f tran-
sitions are more intense than the shoulder band observed related
to the excited states of the 3,5-bis(perfluorobenzyloxy)benzoic
acid. This proves that luminescence sensitization via excitation of
the ligand is not very promising in 1. This phenomenon is further
evident from the residual emission observed from the ligand as
can be noted from the emission spectrum of complex 1.14,3b,e The
emission spectrum of 1 was recorded under the maximum exci-
tation wavelength 300 nm and is composed of the first excited
states, 5D0, and the ground septet, 7FJ (J ¼ 0–4), of Eu3+.15
Further the broad emission peak observed in the region 400–
500 nm can be assigned to residual emission from the antenna
molecule, which indicates that the designed ligand is a weak
sensitizer for Eu3+. The transition of 5D0 / 7F0 is very weak and
5
7
is situated at 579 nm. The transition of D0 / F1 is split into
three peaks at 589, 596 and 598 nm. The hypersensitive transition
5D0 / 7F2 gives rise to the strongest band at 612 nm, resulting in
red emission. The room temperature (Fig. 3) as well as low
Fig. 1 UV–visible absorption spectra for 3,5-bis(perfluorobenzyloxy)
benzoic acid (HL), and complexes 1–3 in CH3OH solution (2 ꢃ 10ꢁ5 M).
temperature (Fig. S7†) D0 lifetime values of the Eu3+ ion were
5
monitored under excitation at 300 nm and the more intense
7
emission at 612 nm (5D0 / F2). The emission decay curve at
transfer depends mainly on two processes. One is the energy
transition from the lowest triplet state energy of the ligand to the
emissive excited state of the Ln3+ ion and the other one is the
inverse energy transition from the Ln3+ ion to the ligand by
the thermal deactivation mechanism. If the energy difference is
too small, the inverse energy transfer will take place much more
easily.3b,e According to Latva’s empirical rule, ligand-to-metal
298 K reveals a single-exponential behavior, yielding the lifetime
value of sobs ¼ 0.615 ms, as a result of the dominant non-radi-
ative decay channels associated with vibronic coupling.3b,c,e It is
interesting to note that the lifetime value (sobs) is essentially
temperature dependent, with sobs approximately 1.3 times higher
in going from 298 to 77 K (0.801 ms), thereby reflecting the
presence of thermally activated deactivation processes in the
present system. This effect has been well documented for several
other hydrated Eu3+ carboxylate complexes.3c,e
energy transfer process for Ln3+ requires DE(3pp*
ꢁ
5D4) ¼
2500–4500 cmꢁ1 for Tb3+ and 2500–4000 cmꢁ1 for Eu3+.11 It is
interesting to note that the triplet energy level of the newly
designed ligand (416 nm: 24 038 cmꢁ1) found from the phos-
phorescence spectrum of the Gd3+ complex (Fig. S6†) is situated
well above the energies of the main emitting levels of 5D0 for Eu3+
Fig. 4 shows the room temperature excitation spectrum of 2
monitored around the more intense emission line at 545 nm for
Tb3+ ion. The excitation spectrum shows an intense broad band
5
and D4 for Tb3+, confirming the suitability of this ligand as
a sensitizer for these lanthanides. The triplet energy level (3pp*)
of the ligand has been estimated by reference to the lower
wavelength emission edge (416 nm: 24 038 cmꢁ1) of the low-
temperature phosphorescence spectrum of the Gd3+ complex 3
(Fig. S6†). It is also crucial to determine the singlet energy levels
of the ligand in order to elucidate the energy migration pathways
in lanthanide complexes. The singlet (1pp*) energy level of the
ligand (326 nm: 30 675 cmꢁ1) was estimated by reference to the
wavelength of the UV-visible upper absorption edge of Gd3+
complex 3 (Fig. 1). The energy gap between the singlet and triplet
states of 3,5-bis(perfluorobenzyloxy)benzoic acid, DE(1pp*
ꢁ
3pp*), is 6637 cmꢁ1, while a value higher than 5000 cmꢁ1 is
commonly considered as optimum for efficient intersystem
crossing in these systems.12
Metal-centered luminescence
5
7
The excitation spectrum of 1 monitored at the D0 / F2 tran-
sition (612 nm) of Eu3+ ion consists of a broad band and several
narrow bands (inset in Fig. 2). A shoulder band peaking at
Fig. 2 Room-temperature emission spectrum for complex 1 (lex
¼
300 nm). (Inset shows the excitation spectrum of complex 1 with emission
monitored at approximately 612 nm.)
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Fig. 3 Luminescence decay profile for complex 1 excited at 300 nm and
monitored at ꢀ612 nm.
Fig. 5 Luminescence decay profile for complex 2 excited at 340 nm, and
monitored at ꢀ545 nm.
12 000 cmꢁ1 and 14 800 cmꢁ1 for Eu3+ and Tb3+, respectively.
Relatively efficient coupling of the Eu3+ excited states occurs to
the third vibrational overtone of the proximate OH oscillators
(nOH z 3300 to 3500 cmꢁ1), and to the fourth harmonic in the
case of Tb3+, which is consistent with the less efficient quenching
observed in the case of Tb3+ where the Franck–Condon overlap
factor is less favorable.16 The 5D4 lifetime values of Tb3+ complex
2 are found to be essentially temperature independent, with sobs
varying ꢀ1% while going from 298 to 77 K (Fig. S8†), thereby
reflecting the absence of thermally activated deactivation
processes (Table 1).3b,c
To evaluate the quantum efficiency of the lanthanide
complexes, it is necessary to determine the quantum yields
(Foverall) (measured upon ligand excitation). The overall
quantum yields (Foverall) were measured at room temperature
under the excitation wavelengths that maximize the emission of
Eu3+ and Tb3+ cations. It depends on two factors, the efficiency
with which the ligands sensitize the metal-centered luminescence,
Fsen, and the intrinsic quantum yield, FLn, which reflects the
ratio of radiative to non-radiative deactivation processes in the
coordination spheres of metal ions (eqn (1)).17 In view of
the weak absorbances of the f–f transition, FLn is difficult to
measure experimentally. However it can be estimated (eqn (2))
from the observed (sobs) and radiative (sRAD) lifetimes of Tb3+
ion with the assumption that the decay process at 77 K in
a deuterated solvent is purely radiative.16,18,2b In turn, for Eu3+,
the radiative lifetime can be simply estimated from the emission
spectrum (eqn (3)) where n represents the refractive index of the
medium.17,19 An average index of refraction equal to 1.5 was
employed in the calculation.3b AMD,0 is the spontaneous emission
probability for the 5D0 / 7F1 transition in vacuo (14.65 sꢁ1), and
Itot/IMD signifies the ratio of the total integrated intensity of the
corrected Eu3+ emission spectrum to the integrated intensity of
Fig. 4 Room-temperature excitation and emission spectra for complex 2
(lex ¼ 340) with emission monitored at approximately 545 nm.
in the region 250–400 nm ascribed to the electronic transitions of
the ligand, 3,5-bis(perfluorobenzyloxy)benzoic acid. Further it is
observed that absorption bands due to f–f transitions of the Tb3+
cation are completely absent pointing to an efficient sensitization
of Tb3+ ion via the excitation of 3,5-bis(perfluorobenzyloxy)
benzoic acid.3b,c The characteristic emission of the Tb3+ ion from
the emitting level (5D4) to the ground multiplet 7F6–3 is observed
upon excitation at 340 nm. The strongest emission is the hyper-
5
7
sensitive transition D4 / F5. The second maximum peak at
5
7
488 nm corresponds to the D4 / F6 transition. On the other
hand, weak bands at 585 and 620 nm are due to 5D4 / 7F4 and
7
5D4 / F3 transitions, respectively. Moreover, ligand-centered
emission is not detectable in the emission spectrum of 2, indi-
cating an efficient ligand-to-metal energy transfer process in
Tb3+–3,5-bis(perfluorobenzyloxy)benzoate complex.3b,c The Tb3+
ion (5D4) lifetime of complex 2 (Fig. 5) under excitation of
340 nm is measured to be sobs ¼ 1.15 ms, which is longer than
that of compound 1. The energy gaps between the luminescent
state and the ground state manifolds are approximately
5
7
the magnetic dipole D0 / F1 transition.
Foverall ¼ Fsen ꢃ FLn ¼ Fsen ꢃ (sobs/sRAD
)
(1)
(2)
FTb ¼ sobs (298K)/sobs (77K)
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5
Table 1 Radiative (ARAD) and non-radiative (ANR) decay rates, D0/5D4 lifetimes (sobs), radiative lifetimes (sRAD), intrinsic quantum yields (FLn),
energy transfer efficiencies (Fsen) and overall quantum yields (Foverall) for complexes 1 and 2
Compounds
ARAD (sꢁ1
)
ANR (sꢁ1
)
s
obs (ms)
sRAD (ms)
3540 ꢄ 1
1020 ꢄ 3
FLn (%)
16
Fsen (%)
24
Foverall (%)
1
2
283
1468
615 ꢄ 3
801 ꢄ 4c
3.8 ꢄ 1a
6.5 ꢄ 1b
52 ꢄ 2a
52 ꢄ 3b
1151 ꢄ 8
ꢀ100
52
a
Absolute quantum yield. b Relative quantum yield. c
sobs at 77 K.
1/sRAD ¼ AMD,0 ꢃ n3 ꢃ (Itot/IMD
)
(3)
ideal candidates for a wide range of technologies. In the present
study we have incorporated the newly synthesized Tb3+–3,5-
bis(perfluorobenzyloxy)benzoate into poly-(methyl methacry-
late), PMMA, a well-known, low-cost, easily prepared polymer
of excellent optical quality, and investigated the photophysical
properties. Fig. 6 shows the room temperature excitation spectra
of PMMA polymer films doped with Tb3+–3,5-bis(per-
fluorobenzyloxy)benzoate at different concentrations (6, 8, 10
and 12% (w/w)), by monitoring the emission at 545 nm. The
excitation spectra are dominated by an intense broad band in the
250–400 nm region, which can be assigned to absorptions of both
PMMA polymer and organic chromophores. Of particular note
is the observation that the excitation spectra of these polymer
films are blue shifted as compared to the solid-state spectrum of
compound 2. This can be attributed to the considerable inter-
action of the ligand with the PMMA matrix. The emission
spectra of PMMA doped with the Tb3+ compound at a variety of
concentrations (6, 8, 10 and 12% (w/w)) and excited at 320 nm
Table 1 shows the radiative (ARAD) and non-radiative (ANR
)
decay rates, Foverall, FLn, and Fsen. The solid-state overall
quantum yield value of 2 (Foverall ¼ 52%) reflects that the newly
synthesized antenna chromophore 3,5-bis(perfluorobenzyloxy)
benzoic acid is an efficient sensitizer for Tb3+ ion. The found
value is comparable with those of the recently reported Tb3+
coordination polymers assembled from 3,5-bis(benzyloxy)
benzoate (Foverall ¼ 60%)3c and Tb3+–2,5-pyridinedicarboxylate
system (Foverall ¼ 45%).20 However, the Eu3+ complex 1 shows
poor sensitization efficiency (Fsen ¼ 24%) followed by very low
quantum yield (Foverall ¼ 4%). It is noticeable from many liter-
ature reports that the emission quantum yield, Foverall, very much
3
depends upon the pp*-state energies of the ligands.3d,4c Latva
and co-workers state that the highest quantum yields are
obtained when the energy difference between the first 3pp* state
of the ligand and the emissive 5D0 level of europium and 5D4 level
of terbium is around 2500–4500 cmꢁ1 11
. According to this rule we
5
exhibit well-defined emission peaks characteristic of the D4 /
7FJ (J ¼ 6–3) transitions of Tb3+ ion in the 480–650 nm region. As
shown in Fig. 6, the luminescent intensity of the Tb3+ emission at
545 nm increases with increasing Tb3+ compound and reaches
a maximum at Tb3+ content of 8%. A further increase in the Tb3+
compound decreases the luminescent intensity. The energy
transfer between the lanthanide ions themselves is a non-radia-
tive process, which is responsible for the decrease in the Tb3+
have examined our results and concluded that 3,5-bis(per-
fluorobenzyloxy)benzoic acid can efficiently sensitize Tb3+ ion
since DE(3pp* ꢁ 5D4) for 2 is 3538 cmꢁ1. Further, it is also
observed that complex 2 exhibits good ligand-to-metal energy
transfer (Fsen ¼ 52%) due the better match of the triplet energy
level of the 3,5-bis(perfluorobenzyloxy)benzoic ligand to that of
the emissive excited state of Tb3+ ion. The low sensitization
efficiency (Fsen ¼ 24%) and quantum yield (Foverall ¼ 4%)
observed for the Eu3+ complex 1 are due to the larger energy gap
between the triplet state and 5D0 level (DE(3pp*
ꢁ
5D0) ¼
6788 cmꢁ1) of Eu3+ ion. It is also noted that the Eu3+ complex is
associated with a large value of non-radiative decay rates (ANR
¼
1468 sꢁ1), which may quench the effective radiative pathways
thereby decreasing the sensitization efficiency and overall
quantum yield values. From the current study it can be inferred
that the replacement of high-energy C–H vibrations with fluo-
rinated phenyl groups in the 3,5-bis(benzyloxy)benzoate effec-
tively improves the luminescence intensity and lifetimes of the
Eu3+ complex (Foverall ¼ 6.5; sobs ¼ 615 ms; Fsen ¼ 24%) as
compared to the Eu3+ complex of 3,5-bis(benzyloxy)benzoate
(Foverall ¼ 1.1; sobs ¼ 426 ms; Fsen ¼ 3.6%).3c
Photophysical properties of Tb3+ complex doped in PMMA
polymer films
Lanthanide complexes incorporated into polymer matrixes
represent a new class of materials that offer the characteristics of
both the antenna complexes and polymer, thus making them
Fig. 6 Excitation and emission spectra of PMMA:6–12% Tb(L)3 poly-
mer film systems recorded at 298 K (excited at 320 nm and emission
monitored at 545 nm).
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emission, especially at high metal concentrations.6a,b It is well
known that the magnetic dipole allowed transition 5D4 / 7F5 of
the Tb3+ complex is largely independent of the ligand fields and
therefore can be used as an internal standard to account for the
ligand differences.21,22 On the other hand, the electric dipole
hexahydrate, 99.9% (Acros Organics), europium(III) nitrate
hexahydrate, 99.9% (Acros Organics), gadolinium(III)
nitrate hexahydrate, 99.9% (Aldrich), 3,5-dihydroxybenzoic
acid, 98% (Alfa Aesar), 2,3,4,5,6-pentafluorobenzyl bromide,
99% (Aldrich) and poly(methyl methacrylate) (Aldrich). All
other chemicals used were of analytical reagent grade.
5
7
allowed transition D4 / F6, a so-called hypersensitive transi-
tion, is sensitive to the symmetry of the coordination sphere.21,22
The intensity ratio of the electric dipole transition to the
magnetic dipole transition in the lanthanide complex measures
the symmetry of the coordination sphere23,24 and this ratio
increases with the number and mass of the ligand coordinated by
the Ln3+ ion.25 In the absence of PMMA, the intensity ratio of the
Elemental analyses were performed with a Perkin-Elmer Series
2 Elemental Analyzer 2400. A Perkin-Elmer Spectrum One FT-
IR spectrometer using KBr (neat) was used to obtain the IR
spectral data and a Bruker 500 MHz NMR spectrometer was
1
employed to record the H NMR and 13C NMR spectra of the
ligands in CDCl3 or MeOD solution. All mass spectra were
recorded with a JEOL JSM 600 fast atom bombardment
high-resolution mass spectrometer (FAB-MS) and the ther-
mogravimetric analyses were performed with a TG/DTA-6200
instrument (SII Nano Technology Inc., Japan). Powder XRD
patterns were recorded in the 2q range of 10–70ꢂ using Cu-Ka
radiation (Philips X’pert). DSC measurements were performed
with a Perkin-Elmer Pyris 6 DSC instrument at a heating rate of
5
7
5
7
transitions of D4 / F6 to D4 / F5 of the Tb3+–3,5-bis(per-
fluorobenzyloxy)benzoate was 0.35. The intensity ratio increases
to 0.47–0.50 when complex 2 is incorporated into PMMA
matrix. For the precursor complex 2, without capping of
PMMA, the surrounding environment is less disturbed and the
5
7
relative intensity of the electric dipole transition D4 / F6 is
relatively weak. When incorporated into microcavities of
PMMA matrix, however, the complex exhibits disorder of
a certain magnitude. Under the influence of the electric fields
of the surrounding ligands, the distortion of the symmetry
around the Ln3+ ion by the capping PMMA results in the
polarization of Tb3+, which increases the probability of the
electric dipole allowed transition. The influence of PMMA on
the coordination environment of the Tb3+ ions changes the
energy transfer probabilities of the electric dipole transitions,
accounting for the increases in luminescent intensity of the
488 nm peak. Therefore the overall quantum yields of the Tb3+-
doped PMMA films have been enhanced as compared to the
precursor complex (Table 2). The overall quantum yields of the
doped polymer films were determined by absolute measurement,
and the values are in the range 53–65%. The overall quantum
yield also depends upon the intermolecular energy transfer
between the donor and acceptor. In Tb–PMMA polymer films,
the long chain of the PMMA molecule enwraps the Tb3+–3,5-
bis(perfluorobenzyloxy)benzoate complex making the distance
between donor and acceptor very close, resulting in efficient
intermolecular energy transfer thereby enhancing the overall
quantum yield of Tb3+-doped polymer films. The lifetime values
(Fig. S10†) of the doped films are found to increase with
increasing Tb3+ complex and then to decrease with more and
more doping of the metal complex (Table 2).
10 C minꢁ1 under a nitrogen atmosphere. The absorbances of
ꢂ
the ligands and complexes were measured in CH3OH solution
with a UV-visible spectrophotometer (Shimadzu, UV-2450) and
the photoluminescence (PL) spectra were recorded using 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 face mode. The
lifetime measurements were carried out at room temperature
using a Spex 1040D phosphorimeter. The overall quantum yields
(Foverall) were measured by both absolute and relative methods as
described elsewhere.3b,e,26
Synthesis of 3,5-bis(perfluorobenzyloxy)benzoic acid (HL)
(a)Synthesis of methyl 3,5-dihydroxybenzoate. A methanolic
solution of 3,5-dihydroxybenzoic acid (2.0 g, 13 mmol) was
refluxed overnight in the presence of catalytic amounts of conc.
H2SO4. The excess methanol was evaporated off and the reaction
mixture was poured into ice cold water. The resulting precipitate
was filtered off, washed with water and dried. Yield, 1.3 g (60%).
1H NMR (500 MHz, acetone-D6): d (ppm) 7.03 (s, 2H), 6.61 (s,
1H), 3.85 (s, 3H). 13C NMR (125 MHz, CDCl3): d (ppm) 167.24,
159.44, 133.07, 108.58, 107.90, 52.26. m/z ¼ 168.42 (M + H)+.
FT-IR (KBr) nmax: 3379, 1694, 1608, 1487, 1344, 1262, 1172,
1018, 766 cmꢁ1
.
Experimental materials and instrumentation
(b)Synthesis of methyl 3,5-bis(perfluorobenzyloxy)benzoate.
Methyl 3,5-dihydroxybenzoate (1.0 g, 6 mmol) was dissolved in
50 mL of freshly distilled acetonitrile and refluxed with
The following chemicals were procured commercially and
used without subsequent purification: terbium(III) nitrate
Table 2 Integral corrected intensities of the 5D4/7FJ relative to the 5D4/7F5 transitions, Foverall and sobs for Tb3+ complex 2, and PMMA:6–12% Tb(L)3
polymer film systems upon ligand excitation at 298 K
Ð
Ð
Ð
Ð
Compounds
4-6
4-5
4-4
4-3
s
obs (ms)
Foverall (%)
2
0.35
0.47
0.49
0.50
0.47
1
1
1
1
1
0.12
0.10
0.11
0.11
0.10
0.09
0.06
0.07
0.07
0.07
1.15 ꢄ 0.008
1.20 ꢄ 0.009
1.49 ꢄ 0.009
1.24 ꢄ 0.008
1.01 ꢄ 0.009
52.4 ꢄ 2
63.3 ꢄ 3
65.4 ꢄ 3
59.3 ꢄ 3
53.3 ꢄ 3
PMMA: 6% of 2 doped into PMMA
PMMA: 8% of 2 doped into PMMA
PMMA: 10% of 2 doped into PMMA
PMMA: 12% of 2 doped into PMMA
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 10852–10859 | 10857

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potassium carbonate (4.0 g, 30 mmol) for 30 min. The resulting
reaction mixture was further refluxed at 70 ꢂC for 48 h following
the addition of 2,3,4,5,6-pentafluorobenzyl bromide (3.0 g,
12 mmol). The excess acetonitrile was then evaporated off and
the residual mixture was poured into ice cold water. Methyl 3,5-
bis(pentafluorobenzyloxy)benzoate was obtained as a white
dissolved in dichloromethane followed by addition of the
required amount of the Tb3+ complex in dichloromethane solu-
tion. The resulting solution was heated at 40 ꢂC for 30 min. The
polymer film was obtained after the evaporation of excess solvent
at 60 C.27
ꢂ
1
precipitate. Yield, 0.96 g (32%). H NMR (500 MHz, CDCl3):
Conclusions
d (ppm) 7.34–7.26 (m, 2H), 6.75–6.74 (t, 1H, J ¼ 2 Hz), 5.14 (s,
4H), 3.93 (s, 3H). 13C NMR (125 MHz, CDCl3): d (ppm) 166.26,
159.01, 144.72, 138.68, 136.57, 132.50, 109.64, 108.83, 107.38,
57.70, 52.43. m/z ¼ 528.90 (M + H)+. FT-IR (KBr) nmax: 3435,
2955, 1712, 1603, 1510, 1436, 1357, 1299, 1173, 1060, 939,
A novel antenna chromophore ligand for the photosensitization
of Tb3+ has been designed based on the highly fluorinated
carboxylate ligand 3,5-bis(perfluorobenzyloxy)benzoic acid. The
newly designed Tb3+ complex exhibits impressive quantum yield
and excited state lifetime value in the solid state (Foverall ¼ 52%;
sobs ¼ 1.15 ms). Furthermore, in the present study, PMMA
polymer films doped with the Tb3+–3,5-bis(perfluorobenzyloxy)
benzoate complex have been successfully prepared and charac-
terized and the luminescence property investigated. Notably,
these terbium molecular plastic materials derived from the highly
fluorinated benzoate ligand display bright green luminescence
with high quantum yields (63–65%) and lifetime values (1.20–
1.49 ms) as compared to precursor Tb3+ compound, thus making
these polymer films interesting candidates for practical applica-
tions as polymer optical fibers or as dopants in organic light-
emitting devices.
770 cmꢁ1
.
(c)Synthesis of 3,5-bis(perfluorobenzyloxy)benzoic acid. 3,5-
Bis(perfluorobenzyloxy)benzoate (2.0 g, 4 mmol) was refluxed
for 24 h in the presence of KOH (0.6 g, 11.3 mmol) in 50 mL of
ethanol. The reaction mixture was poured into ice cold water,
acidified with dilute HCl, and the resulting precipitate was
filtered, washed, dried and recrystallized from ethanol. Yield,
1
1.20 g (62%). H NMR (500 MHz, CDCl3): d (ppm) 7.42–7.39
(m, 2H), 6.83–6.80 (m, 1H), 5.16–5.13 (d, 4H, J ¼ 12 Hz). 13C
NMR (125 MHz, MeOD): d (ppm) 168.96, 160.71, 146.21,
143.47, 141.52, 134.25, 110.16, 109.71, 107.75, 58.95. m/z ¼
514.88 (M + H)+. Elemental analysis (%): calcd for C21H8F10O4
(514): C, 50.02; H, 1.91. Found: C, 50.29; H, 2.50. FT-IR (KBr)
nmax: 2989, 1701, 1657, 1600, 1497, 1448, 1381, 1307, 1169, 1056,
Acknowledgements
938, 846, 767, 670 cmꢁ1
.
The authors acknowledge financial support from the Depart-
ment of Science and Technology (SR/S1/IC-36/2007) and the
Council of Scientific and Industrial Research (NWP0010). S. S.
K. thanks CSIR, New Delhi for the award of a Senior Research
Fellowship.
Syntheses of lanthanide complexes. In a typical procedure, an
ethanolic solution of Ln(NO3)3$6H2O (0.5 mmol) (Ln ¼ Eu, Gd
or Tb) was added to a solution of 3,5-bis(perfluorobenzyloxy)
benzoic acid (1.5 mmol) in ethanol in the presence of NaOH
(1.5 mmol). Precipitation took place immediately, and each
reaction mixture was stirred subsequently for 10 h at room
temperature. The crude products were filtered, washed with
ethanol and dried. The resulting complexes were purified subse-
quently by recrystallization from dichloromethane. However,
efforts to grow single crystals of the lanthanide–3,5-bis(per-
fluorobenzyloxy)benzoate complexes were not successful.
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