3-Phenyl-4-acyl-5-isoxazolonate complex of Tb3+ doped into poly-β-hydroxybutyrate matrix as a promising light-conversion molecular device

Article abstract of DOI:10.1039/b905304b

A novel class of efficient antenna complexes of Tb³⁺ based on the use of 3-phenyl-4-acyl-5-isoxazolone ligands has been designed, synthesized, characterized and their photophysical properties evaluated. The new heterocyclic β-diketonate complex

Full text of DOI:10.1039/b905304b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
3-Phenyl-4-acyl-5-isoxazolonate complex of Tb3+ doped into poly-b-
hydroxybutyrate matrix as a promising light-conversion molecular device†
a
a
Silvanose Biju, M. L. P. Reddy, Alan H. Cowleyb and Kalyan V. Vasudevanb
*
Received 16th March 2009, Accepted 18th May 2009
First published as an Advance Article on the web 10th June 2009
DOI: 10.1039/b905304b
A novel class of efficient antenna complexes of Tb3+ based on the use of 3-phenyl-4-acyl-5-isoxazolone
ligands has been designed, synthesized, characterized and their photophysical properties evaluated .
The new heterocyclic b-diketonate complexes of Tb3+ exhibit high green luminescence efficiency in the
solid state with quantum yields between 59–72%. Furthermore in this work, the synthesis,
characterization and luminescent properties of poly-b-hydroxybutyrate (PHB) polymer films doped
with Tb3+-3-phenyl-4-acyl-5-isoxazolonate complexes at 5, 10, 15 and 20% (mass) are reported. The fact
that the luminescent efficiency of doped films is enhanced (quantum yields between 74–86%) compared
with precursor samples revealed that the polymer matrix acts as a co-sensitizer for Tb3+centers. The
luminescence intensity decreases, however, with increasing precursor concentration in the doped PHB
to greater than 15% where a saturation effect is observed, indicating that changes in the polymer matrix
improve the absorption properties of the film, consequently quenching the luminescent effect.
Synthesized luminescent polymers containing Tb3+-hetrocyclic b-diketonate complexes showed
promising photoluminescence efficiency for applications to polymer light-emitting diodes and active
polymer optical fibers.
as sensitizers for Eu3+ and Tb3+. In a previous report,5b it was
Introduction
shown that 3-phenyl-4-aroyl-5-isoxazolones sensitize Tb3+
poorly due to possible back transfer of energy from the Tb3+
excited state to the 3p–p* triplet state of the 3-phenyl-4-
benzoyl-5-isoxazolone (HPBI) and 3-phenyl-4-(4-toluoyl)-5-
isoxazolone (HTPI) ligands. In turn, this was attributed to the
small energy gap between the 5D4 state of the Tb3+ cation
(20 400 cmꢀ1) and the 3p–p* state of the supporting ligands
[DE(3pp*–5D4) ¼ 1820 cmꢀ1 for HPBI and 2220 cmꢀ1 for
HTPI].5b With the twin objectives of increasing this energy gap
and developing a robust sensitizer for Tb3+, we have synthesized
two new 3-phenyl-4-acyl-5-isoxazolones by introducing
aliphatic substituents into the 4-position of the isoxazolone
ring, namely 3-phenyl-4-propionyl-5-isoxazolone (HPPI) and
4-isobutyryl-3-phenyl-5-isoxazolone (HIBPI) (Fig. S1 in ESI†).
We report (i) the first use of 3-phenyl-4-acyl-5-isoxazolones as
sensitizers for Tb3+, and (ii) the isolation and structural
authentication of the highly luminescent complexes
Tb(PPI)3(H2O)2 (1) and Tb(IBPI)3(C2H5OH) (H2O) (2). The
use of the 3-phenyl-4-acyl-5-isoxazolones as sensitizers for Tb3+
results in high quantum yields in the solid state (59 ꢁ 6% and 72
ꢁ 7% for HPPI and HIBPI, respectively). Eu3+ complexes
[Eu(PPI)3(H2O)2 (3) and Eu(IBPI)3(C2H5OH) (H2O) (4)] have
also been synthesised and their photophysical properties were
compared with Tb3+ analogues for better understanding of the
photosensitization of Ln3+ ions.
The fascinating optical properties of lanthanide ions have
promoted the use of their complexes in an increasing number of
technological applications ranging from biomedical analysis to
materials science.1,2 Unfortunately, due to the Laporte forbidden
character and intraconfigurational nature of the 4f transitions, the
molar absorption coefficients of lanthanide transitions are typi-
cally very small (less than 10 Mꢀ1 cmꢀ1). To obviate this problem,
organic ligands with large molar absorption coefficients can be
coordinated to the lanthanide ion, resulting in sensitized emission
by means of the so-called ‘‘antenna effect’’. The b-diketone ligand
class is emerging as one of the important ‘‘antennas’’ in terms of
high harvest emissions due to the effectiveness of the energy
transfer from this ligand type to the Ln3+ cation. In part, this is due
to the fact that the p–p* transition for b-diketones is intense and
occurs over a significant range of wavelengths that is appropriate
for sensitization of Ln3+ cation luminescence.3 An additional and
practical advantage of the use of b-diketone ligands is that they
form stable complexes with Ln3+ cations.
It is also worth noting that heterocyclic b-diketones such as
1-phenyl-3-methyl-4-acyl-5-pyrazolones4
and
3-phenyl-4-
benzoyl-5-isoxazolones5 have also been successfully employed
aChemical Sciences and Technology Division, National Institute for
Interdisciplinary Science & Technology (NIIST), Thiruvananthapuram,
695 019, India. E-mail: mlpreddy55@gmail.com
It has always been a challenging task to synthesize lanthanide
complexes with high quantum yields that will also be thermo-
dynamically stable and compatible with advanced microelec-
tronic technologies for thin film production. The growing
importance of polymer films in integrated optic technology
makes it interesting to investigate the incorporation of lantha-
nide complexes in a polymer matrix.6 Although there have been
bDepartment of Chemistry and Biochemistry, The University of Texas at
Austin, 1 University Station A5300, Austin, Texas, 78712, USA
† Electronic supplementary information (ESI) available: Synthetic
protocols and characterisation, experimental details and additional
data for photophysical experiments. CCDC reference numbers 703223,
702914 and 703224 for the ligand HIBPI and complexes 1–2. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b905304b
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several reports on photoluminescence properties of polymer
systems doped with Ln3+ complexes,7 studies on the incorpora-
tion of Tb3+-3-phenyl-4-acyl-5-isoxazolonate complexes into
a polymer matrix are lacking. Hence in the present study, an
attempt has also been made to incorporate the highly lumines-
cent Tb3+complexes 1–2 into poly-b-hydroxybutyrate (PHB), by
replacing the solvent molecules present in the first coordination
sphere with the C]O group of PHB. Furthermore, PHB is an
environmentally benign material owing to its biodegradable
properties which make it an advantageous material as the host
structure in luminescent devices.
significant proportions of O-acylated products.10 The ligands
HPPI and HIBPI were synthesized according to Scheme 1 and
the detailed procedure is given in the experimental section. The
new ligands were identified by elemental analyses, FT IR, FAB-
MS, H NMR and 13C NMR spectral analyses (Fig. S2–S3 in
1
ESI†). The chemical shift of H-4 in the H NMR spectrum was
1
diagnostic: H-4 protons in N-acylated compounds are generally
detected around d 5.4, and H-4 in O-acylated derivatives around
d 6.1. Additionally, the infrared spectrum of an N-acylated
compound would be expected to show a lactone stretching
frequency around 1760 cmꢀ1 10
. None of the ligands in the present
study shows either an IR stretching band at 1760cmꢀ1 or an
NMR peak in the region d 5–7. The existence of the ligands in the
enolic form was confirmed by single-crystal X-ray diffraction
analysis (Fig. S1c in ESI†).
Results and discussion
Synthesis and characterization of the ligands
The ligands HPPI and HIBPI were synthesized in 80 and 76%
yields, respectively, by treatment of 3-phenylisoxazol-5(4H)-one
with the appropriate acid anhydride using a modification of the
literature procedure.8 The substrate 3-phenylisoxazol-5(4H)-one
has the possibility of undergoing C-acylation at C-4, however
such reactions have generally been reported only when acid
orthoesters are used.9 However, Korte and Storiko8 claimed that
they have achieved a C-4 acylated product for 3-phenylisoxazol-
5(4H)-one by boiling acetic anhydride in the presence of sodium
acetate without offering any spectral evidence.10 Our efforts to
synthesize the C-4 acylated products in the above mentioned
procedures were not fruitful due to the formation of N-acyl
derivatives as reported by Prager and co-workers.10 A detailed
study on the relative proportions of acylation products has been
investigated and reported that aliphatic acid anhydrides and acid
chlorides generally react at nitrogen and aroyl halides give
Synthesis and characterization of the complexes 1–6
The procedure employed for the syntheses of Ln3+ complexes 1–6
is shown in Scheme 2. The elemental analyses and FAB-MS
studies of complexes 1–6 reveal that each Ln3+ ion has reacted
with the b-diketones in a metal-to-ligand mole ratio of 3:1 and
that two solvent molecules are incorporated in the coordination
sphere. Powder X-ray diffraction studies (Fig. S4 in ESI†)
showed that complexes 1–6 are isostructural. The carbonyl
stretching frequency of HPPI and HIBPI (1704 cmꢀ1) is shifted to
lower wave numbers in complexes 1–6 (1648 cmꢀ1 in 1; 1642 cmꢀ1
in 2; 1648 cmꢀ1 in 3; 1645 cmꢀ1 in 4; 1650 cmꢀ1 in 5; 1658 cmꢀ1 in
6), indicating the involvement of carbonyl oxygen in the complex
formation with the Ln3+ ion. The appearance of a broad band
(3000–3500 cmꢀ1) in the IR spectra of complexes 1–6 reveals the
presence of coordinated solvent molecules in the corresponding
complexes. The presence of solvent molecules in these complexes
has been further confirmed by the fact that all of these complexes
exhibit mass loss at around 150 ꢂC corresponding to the removal
of solvent molecules during thermogravimetric analysis (Fig. S5
in ESI†). Subsequent thermal decomposition ofꢂ1–6 takes place
in two steps in the temperature region 240–600 C. The residue
for each complex is approximately 20% of the initial mass which
corresponds to the respective lanthanide oxide.
Structural insights of the complexes 1–2 were provided by
single-crystal X-ray studies. Crystal data, collection, and struc-
ture refinement parameters for the complexes 1–2 are presented
in Table 1 and selected bond lengths and angles in Table 2. Both
Scheme 1 Synthesis procedure for the ligands.
Scheme 2 Synthesis procedure for complexes 1–6.
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Table 1 Crystal data, collection, and structure refinement parameters
for the ligand HIBPI and complexes 1–2
Parameters
HIBPI
1
2
Empirical formula C13H13NO3
fw
C36H34N3O11Tb C41H44N3O11Tb
231.24
843.58
Orthorhombic Monoclinic
913.71
Monoclinic
P21/c
Crystal system
Space group
Cryst size (mm3)
Pbca
P21/c
0.3 ꢃ 0.2
ꢃ 0.2
293(2)
16.244(5)
8.813(5)
16.888(5)
90
0.13 ꢃ 0.12
ꢃ 0.10
153(2)
22.803(5)
5.9919(12)
26.097(5
90
0.3 ꢃ 0.2
ꢃ 0.2
Temperature (K)
˚
293(2)
8.756(5)
38.268(5)
12.162(5)
90
a/A
˚
b/A
˚
c/A
a (deg)
b (deg)
g (deg)
90
90
101.27(3)
90
96.327(5)
90
3
˚
V/A
Z
2417.7(17)
8
3496.9(12)
4
1.602
4050.4(3)
4
1.498
rcalcd/g cmꢀ3
m/mmꢀ1
F(000)
1.271
0.091
976
Fig. 1 Asymmetric unit of complexes 1, thermal ellipsoids drawn with
2.088
1.809
30% probability and hydrogen atoms omitted for clarity.
1696
1856
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
wR2 (all data)
GOF
0.0427
0.1112
0.0864
0.1269
0.965
0.0466
0.0999
0.0886
0.1282
1.052
0.0512
0.1125
0.0816
0.1271
1.197
ꢂ
˚
Table 2 Selected bond lengths [A] and angles [ ] for complexes 1 and 2
1
2
Tb–O(1)
Tb–O(2)
Tb–O(3)
Tb–O(4)
Tb–O(5)
Tb–O(6)
Tb–O(10)
Tb–O(11)
2.358(4)
2.369(4)
2.363(4)
2.399(4)
2.358(4)
2.386(4)
2.415(4)
2.373(4)
Tb–O(1)
Tb–O(2)
Tb–O(4)
Tb–O(5)
Tb–O(7)
Tb–O(8)
Tb–O(10)
Tb–O(11)
2.367(2)
2.348(3)
2.371(2)
2.319(2)
2.329(2)
2.337(3)
2.419(3)
2.463(3)
Fig. 2 Asymmetric unit of complexes 2, thermal ellipsoids drawn with
30% probability and hydrogen atoms omitted for clarity.
O(1)–Tb–O(2)
O(3)–Tb–O(4)
O(5)–Tb–O(6)
O(10)–Tb–O(11)
73.07(13)
72.81(12)
74.19(13)
144.64(13)
O(1)–Tb–O(2)
O(4)–Tb–O(5)
O(7)–Tb–O(8)
O(10)–Tb–O(11)
72.32(8)
74.66(8)
73.66(9)
75.05(10)
Steady-state photoluminescence of 1–2
It is clear both from the excitation spectra of complexes 1 and 2
and also from the absorption spectra of the ligands (Fig. S6 in
ESI†) that overlap exists between the excitation band of each
Tb3+ complex and the absorption bands of the ligands. Such an
observation is typically diagnostic of ligand sensitization of
a lanthanide cation11 and therefore confirms that the HPPI and
HIBPI ligands are coordinated to the Tb3+ cation in 1 and 2,
respectively.12 The excitation spectra (Fig. 3) for both 1 and 2
exhibit a broad band between 250 and 350 nm which is attrib-
utable to the p–p* transition of the heterocyclic b-diketone
ligand. A series of sharp lines that are assignable to transitions
complexes crystallize in the monoclinic space group P21/c with Z
¼ 4. There are no unusually short intermolecular contacts in
either structure. Each complex is eight-coordinate and the
coordination sphere features three bidentate PPI or IBPI ligands
and two solvent molecules (Fig. 1 and 2). In the case of 1, these
are both water molecules, while in the case of 2 there is one water
and one ethanol molecule. The coordination geometries of both
complexes are best described as bicapped trigonal prismatic. The
average bond distance between the Tb3+ cation and the iso-
between the lanthanide F5 and 5G6, 5L10 and 5L9 levels are also
7
˚
xazolone oxygen atoms is some what shorter for 2 (2.344(8) A)
˚
than for 1 (2.372(4) A) which suggests that the HIBPI ligand
observed in the excitation spectra of both complexes. The latter
transitions are less intense than the absorptions due to the
organic ligands and are overlapped by a broad excitation band.
In turn, this proves that luminescence sensitization via excitation
of the ligand is considerably more efficient than the direct exci-
tation of the Tb3+ absorption level.
binds slightly more strongly to Tb3+ than the HPPI analogue. For
both 1 and 2, the metal-oxygen distances are shorter for the
isoxazolone ligands than for the solvent molecules. The latter
observation could be attributed to the presence of a formal
negative charge on the isoxazolone oxygen atoms which could
enhance the binding to the Tb3+ cation due to electrostatic
effects.5
The room-temperature emission spectra of complexes 1 and 2
(Fig. 3) exhibit the characteristic emission bands of the Tb3+
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where AMD,0 (14.65 sꢀ1) is the spontaneous emission probability
of the 5D0 / 7F1 transition in vacuo, ITOT/IMD is the ratio of the
total area of the corrected Eu3+ emission spectrum to the area of
the 5D0 / 7F1 band and n is the refractive index of the medium.
An average index of refraction equal to 1.5 was considered.5a The
intrinsic quantum yield of Tb3+ (FTb) can be estimated through
Equation (4) by using the assumption that the decay process at 77
K in deuterated solvent is purely radiative.11a,15
sobs ð298KÞ
sRAD ð77KÞ
FTb
¼
(4)
Quantum yields (Table 3) of the complexes in the solid state
were determined according to the absolute method of Wrighton.16
The quantum yields of the Tb3+complexes were found to be
Fig. 3 Solid state excitation and emission spectra for complexes 1 and 2
at 298 K (lex 320 nm and emission monitored at 545 nm).
unexpectedly high (59 ꢁ 6% for 1 and 72 ꢁ 7% for 2; lexc
¼
320 nm), which is surprising in view of the presence of C2H5OH
and H2O molecules in the first coordination sphere, as they are
usually both essential vibrational deactivators of the excited states
of Ln3+ ions. However, similar high quantum yields (88%) have
been observed previously for Tb3+-p-aminobenzoates in the
presence of H2O molecules.17 The energy gap between the lumi-
nescent state and the ground state manifold is approximately
12000 cmꢀ1 for Eu3+ and 14800 cmꢀ1 for Tb3+.18 Relatively efficient
coupling of the Eu3+ excited states occurs to the third vibrational
overtone of proximate OH oscillators (nOH ꢄ 3300–3500 cmꢀ1),
and to the fourth harmonic in the case of Tb3+, consistent with the
observed less efficient quenching for Tb3+, where the Franck–
Condon overlap factor is less favourable.18 The obtained
quantum yield values of Tb3+-3-phenyl-4-acyl-isoxazolonates are
found to be significantly higher than those reported for Tb3+
hexafluoroacetylacetonate and dipivaloylmethanato complexes
(Foverall ¼ 27 and 40%, respectively).19 On the other hand, poor
quantum yields for the Eu3+ complexes have been noticed.
cation (lex 320 nm) centered at 488, 545, 585, and 620 nm which
5
result from deactivation of the D4 excited state to the corre-
sponding FJ ground state of the Tb3+ cation (J ¼ 6,5,4,3). The
7
most intense emission is centered at 545 nm and corresponds to
the 5D4 / 7F5 transition.4c,5b Fig. S7 in ESI† illustrates the room-
temperature excitation and emission spectra of the Eu3+
complexes 3–4. It is clear from the excitation spectra of 3–4 that
Eu3+ absorption peaks are dominating the ligand centered exci-
tation band. Thus it can be concluded that HPPI and HIBPI
ligands are not efficient sensitizers for Eu3+.
The overall quantum yield (Foverall) for a lanthanide complex
treats the system as a ‘‘black box’’ in which the internal process is
not considered explicitly. Given that the complex absorbs
a photon (i.e. the antenna is excited), the overall quantum yield
can be defined as follows:13
Foverall ¼ FsenFLn
(1)
The 5D4 lifetime of the green emission of Tb3+ complexes was
determined by single-exponential fitting of the decay curve
(Fig. S8–S11 in ESI†) and is consistent with the presence of one
major luminescent species. The pertinent lifetime values are 785
ꢁ 2 and 920 ꢁ 3 ms for 1 and 2, respectively. Again it is surprising
Here, Fsen represents the efficiency of energy transfer from the
ligand to the Ln3+ ion and FLn represents the intrinsic quantum
yield of the Ln3+ ion, which can be calculated as
ꢀ
ꢁ
ARAD
sobs
5
to note that the magnitude of the D4 lifetime values of these
FLn
¼
¼
(2)
ARAD þ ANR
sRAD
complexes is not very high as compared to some of the recently
reported highly luminescent complexes of Tb3+ (Foverall ¼ 56%;
sobs ¼ 2.63 ms).20 However, there are reports with Foverall ¼ 40%
and sobs ¼ 0.46 ms for Tb3+-dipivaloylmethanato complexes.19
The lifetime is the inverse of the total de-activation rate, itself the
sum of the radiative and non-radiative rates. Therefore
a complex may be highly luminescent and have a short lifetime,
there is no contradiction. It simply means that the radiative rate
For the Eu3+ complexes, the radiative lifetime (sRAD) can be
calculated using Equation (3),14,3c assuming that the energy of the
5D0
/
7F1 transition (MD) and its oscillator strength are
constant
ꢀ
ꢁ
1
ITOT
ARAD
¼
¼ AMD;0n3
(3)
sRAD
IMD
Table 3 Photophysical data for Tb3+ and Eu3+ complexes
Foverall [%]a
5
Complex
DE (3pp* ꢀ DJ) [cmꢀ1
]
sRAD [ms]
sobs [ms]
FLn [%]
Fsen [%]
1
2
3
4
3240
2750
6390
5300
807
961
1700
1650
785 ꢁ 2
920 ꢁ 3
259 ꢁ 1
261 ꢁ 1
97
96
15
16
61
75
2
59 ꢁ 6
72 ꢁ 7
0.3 ꢁ 0.03
0.5 ꢁ 0.05
3
a
Average value of three measurements performed under the same experimental conditions.
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is fast as observed in the present Tb3+complexes. Due to its
electronic structure, Tb3+ has many levels which can mix with
ligand wave functions, including a relatively low-lying 4f 5d state,
which may explain why the lifetime is relatively short (i.e. it
means that the phosphorescence character of the transition is
therefore supports the observation of stronger sensitization of
the Tb3+ complexes (Fsen ¼ 61–75%) than the Eu3+ complexes
(Fsen ¼ 2–3%) because of the smaller overlap between the ligand
triplet and Eu3+ ion excited states than that of Tb3+ (Fig. S14 in
ESI†). A schematic diagram representing all of the possible
energy transfer pathways for complexes 1–4 is depicted in
Fig. S15 (ESI†).
partly lost). The relatively shorter D0 lifetime values of Eu3+
5
analogues (Fig. S12–S13 in ESI†) observed may be due to the
dominant non-radiative decay channels associated with vibronic
coupling because of the presence of solvent molecules, also
documented in many of the hydrated europium b-diketonate
complexes.3c,5a The radiative lifetimes (sRAD), intrinsic quantum
yields (FLn), and sensitization efficiency (Fsen) values of the
ligands in Ln3+ complexes 1–4 were calculated and are given in
Table 3.
Photophysical properties of Tb3+ complexes doped PHB polymer
films
Inspired by the excellent luminescent efficiency of the Tb3+com-
plexes 1–2, the authors incorporated these complexes into
a polymer matrix, namely poly-b-hydroxybutyrate (PHB), by
replacing the solvent molecules present in the first coordination
sphere with the C]O group of PHB. A schematic representation
of the Tb3+ complex coordinated to the PHB polymer film
backbone is shown in Fig. 4. The PHB polymer was doped with
the Tb3+ complexes 1–2 in the proportions of 5, 10, 15 and 20%
(w/w). The broad absorption band assigned to the H2O vibra-
tional modes in the FTIR spectra of the precursor Tb3+
complexes identified in the region (3000–3500 cmꢀ1) is absent for
the doped PHB polymer films confirming that the polymer films
In order to elucidate the energy transfer process in 1 and 2, it
was necessary to determine the energy levels of the relevant
electronic states of the ligands. The singlet (1pp*) and triplet
(3pp*) energy levels of the HPPI and HIBPI ligands were esti-
mated by reference to the wavelengths of the UV-Vis absorption
edges of 1 an 2 (320 nm: 31250 cmꢀ1 and 321 nm: 31150 cmꢀ1 for
HPPI and HIBPI, respectively: Fig. S6 in ESI†) and the lower
wavelength emission edges (423 nm: 23640 cmꢀ1 and 432 nm:
23150 cmꢀ1 for HPPI and HIBPI, respectively) for the corre-
sponding phosphorescence spectra of the gadolinium complexes
Gd(PPI)3(H2O)2 5 and Gd(IBPI)3(H2O)2 6 (Fig. S14 in ESI†). It
can be seen from Fig. S14 in ESI† that there exists a large area of
overlap between the room-temperature emission spectra of the
ligands and the low-temperature phosphorescence spectra of
complexes 5–6. Furthermore, it is also evident that there is an
overlap between the low-temperature phosphorescence spectra
of complexes 5–6 and the emission spectra of the Ln3+ complexes.
The above spectral overlap clearly indicates the sensitization
pathway in luminescent Ln3+ complexes consists of excitation of
the coordinated ligands into their excited states, subsequent
intersystem crossing of the ligands to their triplet states and
5
energy transfer from the triplet state of the ligand to the DJ
manifold of the Ln3+ ion. This is followed by internal conversion
to the emitting state of the Ln3+, and finally the Ln3+ ion emits
radiation. Therefore, the energy-level match between the triplet
states of the ligands to the 5DJ state of the Ln3+ cation is one of
the key factors that govern the luminescence properties of Ln3+
complexes. According to Reinhoudt’s empirical rule,21 the
intersystem crossing process will be effective when
DE(1pp*–3pp*) is approximately 5000 cmꢀ1. It has been
observed that the DE(1pp*–3pp*) energy gaps for HPPI and
HIBPI are 7610 and 8000 cmꢀ1, respectively and hence the
intersystem crossing processes are effective for both the ligands.
Latva’s empirical rule22 states that an optimal ligand-to-metal
energy transfer process for Ln3+ needs DE (3pp*–5DJ) ¼ 2500–
4000 cmꢀ1 for Eu3+and 2500–4500 cmꢀ1 for Tb3+. On this basis it
can be concluded that the energy transfer to the Tb3+ cation will
be effective for the HPPI and HIBPI ligands since DE(3pp*–5D4)
for 1 and 2 are 3240 and 2750 cmꢀ1, respectively. Complex 2
exhibits a higher luminescence efficiency than complex 1 due to
a superior match of the triplet energy level with that of the Tb3+
emitting level. On the other hand, poor sensitization for Eu3+ has
been noted for the above chromophores mainly due to the larger
energy gap between the ligand triplet state and 5D0 level of Eu3+
DE(3pp*–5D0) ¼ 6390 cmꢀ1 for 3 and 5900 cmꢀ1 for 4. This
Fig. 4 A schematic representation of the structure of Tb3+ complex
doped in PHB polymer.
Fig. 5 Excitation and emission spectra of PHB: 5–20% Tb(PPI)3 poly-
mer film systems recorded at 298 K (excited at 320 nm and emission
monitored at 545 nm).
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are anhydrous. This is again in good agreement with the TG
analyses of doped polymer films, where no mass loss was noted in
the temperature region 50–200 ꢂC (Fig. S16 in ESI†). The typical
thermogravimetric analyses of the PHB polymer film and Tb3+
complexes doped with PHB polymer at a ratio of 15% (w/w)
were recorded under inert atmosphere of nitrogen and the results
are illustrated in Fig. S16 (ESI†). It is clear from the
thermogravimetric curve that the un-doped polymer film
decomposes in a one step event and its decomposition starts at
260 ꢂC. Similarly, the Tb3+ complex doped polymer films also
decompose in a single step event under an inert atmosphere.
However, the doped polymer films exhibit decreasing onset
temperatures of decomposition of 230 and 231 ꢂC for 1–2,
respectively.
The excitation spectra of the PHB polymer films doped with
the Tb3+ complexes (1–2) at different doping concentrations (5,
10, 15 and 20% w/w) recorded at 298K in the spectral range 200–
400 nm, by monitoring the emission at 545 nm, are given in Fig. 5
and 6. These excitation spectra exhibit an intense broad band in
the spectral region 250–350 nm, which can be assigned to both
PHB polymer and organic chromophore absorptions. Further-
more, the typical intraconfigurational transitions pertaining to
Tb3+ ion noted in the precursor samples are absent in these
excitation spectra, owing to the efficient energy transfer from
both organic moiety and polymer matrix to the central metal ion.
These facts suggest that polymer matrices serve as efficient co-
sensitizers for the Tb3+ ions.
The emission spectra of the un-doped PHB polymer and
PHB:15% Tb(IBPI)3 films recorded at 298 K in the range from
330–650 nm under excitation at 320 nm are depicted in Fig. 7.
The absence of a broad polymer emission band in the doped film
indicates highly efficient energy transfer via the polymer matrix
to the Tb3+ ion. As shown in Fig. 5 and 6, the luminescent
intensity of the Tb3+ emission at 545 nm increases with increasing
Tb3+ content and reaches a maximum at the Tb3+ content of 15%.
A further increase of the Tb3+ content decreases the luminescent
intensity of the Tb3+emission. The energy transfer between the
lanthanide ions themselves is a nonradiative process, which
would account for the decrease of the Tb3+ emission, especially at
high Tb3+ content.5d,23
Fig. 6 Excitation and emission spectra of PHB: 5–20% Tb(IBPI)3
polymer film systems recorded at 298 K (excited at 320 nm and emission
monitored at 545 nm).
The luminescent peaks of the Tb3+ complexes with and without
the presence of PHB do not display noticeable shifts in the
positions. However, some transitions show variations in intensity
that depends strongly on the coordinative environment, as
shown in Fig. 5 and 6. It is well documented that for Tb3+com-
5
plexes the magnetic dipole transition D4 /7F5 is largely inde-
pendent of the ligand field and therefore can be used as an
internal standard to account for the ligand differences.19,24 The
5
7
electric dipole transition D4 / F6, a so-called hypersensitive
transition,19,24 is sensitive to the symmetry of the coordination
sphere. The intensity ratio of the electric dipole transition to the
magnetic dipole transition in the lanthanide complex measures
the symmetry of the coordination sphere25,26 and this ratio
increases with the number and mass of the ligand coordinated by
Fig. 7 Emission spectra recorded at 298 K, under excitation monitored
at 320 nm: (a) undoped PHB film and (b) PHB:15% Tb(IBPI)3 film.
5
7
5
7
Table 4 Integral corrected intensities of the D4 / FJ relative to the D4 / F5 transitions, Foverall and sobs for Tb3+ complex 1, and PHB:5–20%
Tb(PPI)3 polymer film systems upon ligand excitation at 298 K
Ð
Ð
Ð
Ð
Complex
s
obs [ms]a
Foverall [%]a
4–6
4–5
4–4
4–3
1
0.35
0.43
0.40
0.40
0.39
1
1
1
1
1
0.11
0.15
0.12
0.11
0.11
0.06
0.10
0.08
0.06
0.06
785 ꢁ 2
765 ꢁ 5
795 ꢁ 4
780 ꢁ 3
745 ꢁ 2
59 ꢁ 6
68 ꢁ 7
71 ꢁ 7
74 ꢁ 7
61 ꢁ 6
PHB:5% 1
PHB:10% 1
PHB:15% 1
PHB:20% 1
a
Average value of three measurements performed under the same experimental conditions.
5184 | J. Mater. Chem., 2009, 19, 5179–5187
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5
7
5
7
Table 5 Integral corrected intensities of the D4 / FJ relative to the D4 / F5 transitions, Foverall and sobs for Tb3+ complex 2, and PHB:5–20%
Tb(IBPI)3 polymer film systems upon ligand excitation at 298 K
Ð
Ð
Ð
Ð
Complex
s
obs [ms]a
Foverall [%]a
4–6
4–5
4–4
4–3
2
0.36
0.40
0.41
0.41
0.40
1
1
1
1
1
0.11
0.12
0.10
0.11
0.11
0.06
0.07
0.06
0.06
0.06
920 ꢁ 3
931 ꢁ 2
922 ꢁ 5
929 ꢁ 3
904 ꢁ 5
72 ꢁ 7
79 ꢁ 8
83 ꢁ 8
86 ꢁ 9
75 ꢁ 8
PHB:5% 2
PHB:10% 2
PHB:15% 2
PHB:20% 2
a
Average value of three measurements performed under the same experimental conditions.
the Ln3+ ion.27 For the Tb3+ complex 1 in the absence of PHB, the
intensity ratio of the transitions of 5D4 / 7F6 to 5D4 / 7F5 was
0.35. It increases to 0.39–0.43 (Table 4) for the complex incor-
porated into the PHB matrix. Similarly, for the Tb3+ complex 2
the intensity ratio of the transitions of 5D4 / 7F6 to 5D4 / 7F5
increases from 0.36 to 0.40–0.41 (Table 5). In short, the presence
of PHB generally increases the luminescent intensity of the
hypersensitive transitions of the Tb3+ ion. For the complexes 1–2
without the capping of PHB, the surrounding environment is less
disturbed and the relative intensity of the electric dipole transi-
a saturated solution of NaHCO3. The products were precipitated
from the collected aqueous layer using 4 M HCl. The new ligands
1
were identified by elemental analyses and FT IR, FAB-MS, H
NMR and 13C NMR spectral data (Fig. S2–S3†). Single crystals
of the ligand HIBPI were obtained from a saturated solution of
the compound in ethyl acetate after storage for one month.
ꢂ
1
HPPI. Yield ¼ 80%, mp ¼ 76 ꢁ 2 C. H NMR (500 MHz,
CDCl3, TMS) ppm: 7.46–7.49 (m, 5H), 2.33–2.377 (q, 2H), 1.10–
1.13 (t, 3H). 13C NMR (500 MHz, CDCl3, TMS) ppm: 188.00,
177.06, 161.22, 130.72–128.50, 97.28, 77.25–76.75 (CDCl3),
27.07, 10.00. IR (KBr) nmax: 3052, 1704, 1608, 1364, 1200, 848,
698 cmꢀ1. m/z ¼ 218.50 (M + 1)+. CHN anal. calcd for
C12H11NO3: C, 66.35; H, 5.10; N, 6.45. Found: C, 66.12; H, 5.37;
N, 6.65%.
tion 5D4
/
7F6 is relatively weak. When incorporated into
microcavities of the PHB matrix however, the complexes exhibit
disorder of a certain magnitude. Under the influence of the
electric field of the surrounding ligands, the distortion of the
symmetry around the lanthanide ion by the capping PHB results
in the polarization of the Tb3+, which increases the probability
for the electric dipole allowed transition. The influence of PHB
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 Tb3+ doped
PHB films have been enhanced as compared to the precursor
samples (Tables 4 and 5).
HIBPI. Yield ¼ 76%, mp ¼ 137 ꢁ 2 ꢂC. 1H NMR (500 MHz,
CDCl3, TMS) ppm: 7.52–7.47 (m, 5H), 2.71–2.63 (m, 1H), 1.13–
1.11 (d, 6H). 13C NMR (500 MHz, CDCl3, TMS) ppm: 191.44,
177.31, 161.11, 130.69–128.35, 96.25, 77.26–76.75 (CDCl3),
31.79, 19.15. IR (KBr) nmax: 3050, 1704, 1610, 1366, 1202, 842,
700 cmꢀ1. m/z ¼ 232.46 (M + 1)+. CHN anal. calcd for
C13H13NO3: C, 67.52; H, 5.67; N, 6.06. Found: C, 67.74; H, 5.77;
N, 6.27%.
Experimental details
Materials
Synthesis of the Ln3+ complexes
The following chemicals were acquired commercially and used
without subsequent purification: terbium(III) nitrate hexahy-
drate, 99.9% (Acros Organics); 3-phenyl -5-isoxazolone, 97%
(Aldrich); propanoic anhydride, 97% (Aldrich); isobutric anhy-
dride, 97% (Aldrich) and poly(3-hydroxybutyric acid), natural
origin (Aldrich). All the other chemicals used were of analytical
reagent grade.
Complexes 1–6 were synthesized by means of the following
general procedure.5 1 mmol of the appropriate lanthanide nitrate
in 2 mL of ethanol was added to a solution of 3 mmol of the
ligand and 3 mmol of NaOH in 50 mL of ethanol along with 50
mL of water. The reaction mixture was stirred at room temper-
ature for 12 h. The precipitate thus obtained was filtered off,
washed with water and dried in vacuo. Single crystals of
complexes 1 and 2 suitable for X-ray diffraction experiments
were obtained from an ethanol/dichloromethane (1:1) solution
kept in a sealed flask at room temperature for two weeks. These
were used for further analyses and the photophysical studies.
Synthesis of the ligands
The ligands HPPI and HIBPI were synthesized according to
Scheme 1. 3-Phenyl-5-isoxazolone (6 mmol) was dissolved in 20
mL of dry THF and stirred for 10 min at 0 ꢂC in an ice bath. To
this solution, sodium hydride (12 mmol) was added in an inert
atmosphere, followed by the corresponding acid anhydride (12
mmol) then stirred at 0 ꢂC for 30 min,ꢂroom temperature for 30
min followed by further stirring at 70 C for 12h. The resulting
pale yellow paste was decomposed by adding 2 M HCl (50 mL),
and the solution was extracted twice with dichloromethane (70
mL). The organic layer was separated and treated with
Tb(PPI)3(H2O)2 (1). Elemental analysis (%): calcd for
C36H34TbN3O11 (843.59): C, 51.26; H, 4.06; N, 4.98. Found: C,
51.11; H, 4.06; N, 5.00. IR (KBr) lmax: 3300, 1648, 1496, 1394,
936, 764, 968 cmꢀ1; m/z ¼ 808.13 [Tb(PPI)3] + 1.
Tb(IBPI)3(C2H5OH)(H2O) (2). Elemental analysis (%): calcd
for C41H44TbN3O11 (913.73):C, 53.89; H, 4.85; N, 4.60. Found:
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C, 53.64; H, 4.95; N, 4.86. IR (KBr) lmax: 3292, 1642, 1496, 1394,
962, 770 cmꢀ1; m/z ¼ 836.16 [Tb(IBPI)3] + 1.
the excitation wavelength. Samples were prepared by drop casting
the material placed between two quartz cover slips, The quantum
yield was determined by comparing the spectral intensities of the
lamp and the sample emission as reported in the literature.28
Using this experimental setup and the integrating sphere system,
the solid-state fluorescence quantum yield of thin film of the
standard green OLED material tris-8-hydroxyquinolinolato
aluminium (Alq3) was determined to be 0.19, which is consistent
with previously reported values.29 Each sample was measured
several times under slightly different experimental conditions.
The estimated error for quantum yields is ꢁ10%.19
Eu(PPI)3(H2O)2 (3). Elemental analysis (%): calcd for
C36H34EuN3O11 (836.63): C, 51.68; H, 4.10; N, 5.02. Found: C,
51.51; H, 4.09; N, 4.96. IR (KBr) lmax: 3310, 1648, 1496, 1392,
935, 766, 969 cmꢀ1; m/z ¼ 802.12 [Eu(PPI)3] + 1.
Eu(IBPI)3(C2H5OH)(H2O) (4). Elemental analysis (%): calcd
for C41H44EuN3O11 (906.76):C, 54.31; H, 4.89; N, 4.63. Found:
C, 54.60; H, 4.95; N, 4.78 IR (KBr) lmax: 3322, 1645, 1494, 1394,
962, 772 cmꢀ1; m/z ¼ 829.66 [Eu(IBPI)3] + 1.
Crystallographic characterization
Gd(PPI)3(H2O)2 (5). Elemental analysis (%): calcd for
C36H34GdN3O11 (841.92): C, 51.36; H, 4.07; N, 4.99. Found: C,
50.99; H, 4.11; N, 4.85. IR (KBr) lmax: 3300, 1650, 1494, 1394,
936, 764, 700, cmꢀ1; m/z ¼ 806.90 [Gd(PPI)3] + 1.
The X-ray diffraction data were collected at 153 K on a Nonius
Kappa CCD diffractometer equipped with an Oxford Cryo-
stream low-temperature device and a graphite-monochromated
˚
Mo Ka radiation source (l ¼ 0.71073 A). Corrections were
applied for Lorentz and polarization effects. All of the structures
were solved by direct methods30 and refined by full-matrix least-
squares cycles on F2. All of the non-hydrogen atoms were
allowed anisotropic thermal motion, and the hydrogen atoms
were placed in fixed, calculated positions using a riding model
Gd(IBPI)3(H2O)2 (6). Elemental analysis (%): calcd for
C39H40GdN3O11 (884.00): C, 52.99; H, 4.56; N, 4.75. Found: C,
53.06; H, 4.76; N, 4.69. IR (KBr) lmax: 3428, 1658, 1390, 964,
766, 702 cmꢀ1; m/z ¼ 835.16 [Gd(IBPI)3] + 1.
˚
(C–H, 0.96 A).
Synthesis of Tb3+ complex doped PHB polymer films
The PHB polymer was doped with the Tb3+ complexes 1–2 in the
proportions 5, 10, 15 and 20% (w/w). The PHB powder was
dissolved in chloroform followed by addition of the required
amount of the Tb3+complex in chloroform/ethanol solution. The
mixed solution was heated at 40 ꢂC for 30 min. The polymer film
Conclusions
In summary, 3-phenyl-4-acyl-5-isoxazolones have proved to be
highly efficient sensitizers for Tb3+ cation luminescence as
demonstrated by the high emission quantum yields (59–72%) for
luminescence which are up to three times as high as those
reported previously for other b-diketonate ligands. This may be
due to the superior match of the triplet energy level of the ligands
with that of the Tb3+ emitting level. On the other hand, poor
sensitization for Eu3+ has been noted for the above chromo-
phores mainly due to the larger energy gap between the ligand
was obtained after the evaporation of excess solvent at 60 C.7b
ꢂ
Characterization
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 used to
record the 1H NMR and 13C NMR spectra of the new
compounds in CDCl3 solution. The mass spectra were recorded
on a JEOL JSM 600 fast atom bombardment (FAB) high reso-
lution mass spectrometer (FAB-MS) and the thermogravimetric
analyses were performed on a TGA-50H instrument (Shimadzu,
Japan). X-Ray powder diffraction (XRD) analyses were per-
formed with a Philips X’Pert Pro diffractometer. The XRD
patterns were recorded in the 2q range 10–70ꢂ using Ni filtered
Cu Ka radiation. The absorbances of the ligands were measured
in CH3CN solution on a UV-vis spectrophotometer (Shimadzu,
UV-2450) and the photoluminescence (PL) spectra were recor-
ded on a Spex-Fluorolog FL22 spectrofluorometer equipped
with a double grating 0.22m Spex 1680 monochromator and
a 450W Xe lamp as the excitation source operating in the front
face mode. The lifetime measurements were carried out at room
temperature using a Spex 1040 D phosphorimeter.
triplet state and D0 level of Eu3+. Furthermore, in the present
5
study, poly-b-hydroxybutyrate (PHB) polymer films doped with
Tb3+-3-phenyl-4-acyl-5-isoxazolonate complexes have been
successfully prepared, characterized and investigated photo-
physical properties. The absence of the broad emission band of
the PHB in doped systems indicated that the energy transfer via
the polymer matrix to the Tb3+ is highly efficient, suggesting that
the polymer matrix acts as a co-photoluminescent sensitizer. In
addition, the higher quantum yields obtained for the doped PHB
films (74–86%) as compared to hydrated Tb3+ complexes confirm
the effect of sensitization induced by the polymer matrix.
Furthermore, the influence of PHB on the coordinative envi-
ronment of the Tb3+ ions changes the energy-transfer probabili-
ties of the electric dipole transitions, accounting for the increases
in luminescent intensity. In conclusion, the poly-b-hydrox-
ybutyrate polymer films doped with Tb3+-3-phenyl-4-acyl-5-iso-
xazolonate complexes are desirable for developing many
applications in light conversion molecular devices.
The overall quantum yields (Foverall) were measured using an
integrating sphere in a SPEX Fluorolog spectrofluorimeter. The
PL quantum yields in thin films (Foverall) were determined using
a calibrated integrating sphere system. The Xe-arc lamp was used
to excite the thin-film samples placed in the sphere with 320 nm as
Acknowledgements
The authors acknowledge financial support from the Depart-
ment of Science and Technology (SR/S1/IC-36/20073), Univer-
sity Grants Commission and the Council of Scientific and
5186 | J. Mater. Chem., 2009, 19, 5179–5187
This journal is ª The Royal Society of Chemistry 2009

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authors also thank Dr Suresh Das, Head, Chemical Sciences &
Technology Division, Mr Shinto Varghese and Dr Vidhya N.,
National Institute for Interdisciplinary Science & Technology
(NIIST), Trivandrum, India, for their constant encouragement
and valuable discussions. Dr Babu Varghese, Senior Scientific
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