Synthesis and fluorescence properties of lanthanide(III) complexes of a novel bis(pyrazolyl-carboxyl)pyridine-based ligand

Article abstract of DOI:10.1016/j.saa.2008.09.019

A novel bis-pyrazolyl-carboxyl ligand, 2,6-bis(5-methyl-3-carboxypyrazol-1-ylmethyl)pyridine (L), was designed and synthesized and its several lanthanide(III) complexes Eu(III), Tb(III), Sm(III) and Gd(III) were successfully prepared and characterized in detail based on elemental analysis, infrared, mass, proton nuclear magnetic resonance spectroscopy and TG-DTA studies. Analysis of the IR spectra suggested that each of the lanthanide metal ions coordinated to the ligand via the carbonyl oxygen atoms and the nitrogen atom of the pyridine ring and pyrazole rings. The fluorescence spectra exhibits that the Tb(III) complex and the Eu(III) complex display characteristic metal-centered fluorescence in solid state while ligand fluorescence is completely quenched. However, the Tb(III) complex displays more effective fluorescence than the other complexes, which is attributed to especial effectivity in transferring energy from the lowest triplet energy level of the ligand (L) onto the excited state (⁵D₄) of Tb(III).

Full text of DOI:10.1016/j.saa.2008.09.019

Spectrochimica Acta Part A 72 (2009) 198–203
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and fluorescence properties of lanthanide(III) complexes
of a novel bis(pyrazolyl-carboxyl)pyridine-based ligand
Xiao-Ming Shi, Rui-Ren Tang∗, Guo-Liang Gu, Ke-long Huang
School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel bis-pyrazolyl-carboxyl ligand, 2,6-bis(5-methyl-3-carboxypyrazol-1-ylmethyl)pyridine (L), was
designed and synthesized and its several lanthanide(III) complexes Eu(III), Tb(III), Sm(III) and Gd(III)
were successfully prepared and characterized in detail based on elemental analysis, infrared, mass,
proton nuclear magnetic resonance spectroscopy and TG–DTA studies. Analysis of the IR spectra sug-
gested that each of the lanthanide metal ions coordinated to the ligand via the carbonyl oxygen atoms
and the nitrogen atom of the pyridine ring and pyrazole rings. The fluorescence spectra exhibits that
the Tb(III) complex and the Eu(III) complex display characteristic metal-centered fluorescence in solid
state while ligand fluorescence is completely quenched. However, the Tb(III) complex displays more
effective fluorescence than the other complexes, which is attributed to especial effectivity in trans-
ferring energy from the lowest triplet energy level of the ligand (L) onto the excited state (5D4) of
Tb(III).
Received 18 February 2008
Received in revised form
16 September 2008
Accepted 26 September 2008
Keywords:
Synthesis
Fluorescence
Lanthanide(III) complexes
Pyrazolyl-carboxyl antenna
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
electrons can efficiently serve as the light absorbing chromophores.
If the excited state of the antenna organic ligands is energeti-
In recent years there has been an increasing interest in lumines-
cent lanthanide (Ln) complexes because of their attractive emission
properties such as long lifetime, large stokes shift, and line-like
emission [1–3]. The f–f transitions in lanthanide, Ln, ions lead to
interesting light-emission properties. These ions have been widely
studied for a number of applications such as luminescent materials
[4,5], chemosensors [6,7], fluorescence labels [8], and as photolu-
minescence devices[9,10] for their photophysical properties arising
from f–f transitions. However, since the f–f transitions are forbid-
den by spin and parity selection rules, the emission occurs through
sensitization with coordinated ligands, which function as anten-
nas [11]. The ligands range from simple aromatic acids, such as
the widely utilized dipicolinic acid, to more complicated multi-
dentate ligand architectures capable of selectively binding different
lanthanide ions [12].
cally high enough, and the gathered energy is transferred to the
lanthanide metal ions held nearby, the characteristic fluorescence
would be emitted. For effective excitation of Ln ions, organic chro-
mophore, so-called “antenna”, is usually attached to Ln complex
[14–16]. In these systems, the photoexcited antenna successfully
transfers its energy to the adjacent Ln ion (energy transfer (ET)
process), forming excited-state Ln. For practical application of lumi-
nescent Ln ions for analytical purpose, these must be encapsulated
within suitable ligand groups that can stabilize the ions kinetically
and thermodynamically [17].
In the present paper we report the synthesis and fluores-
cence study of lanthanide ions [Eu(III), Tb(III), Sm(III) and Gd(III)]
complexes of a novel ligand 2,6-bis(5-methyl-3-carboxypyrazol-
1-ylmethyl)pyridine (L, Scheme 1). The ligand contain amine
functionalities which known for its capability to bind lanthanide
ions via interaction with C O oxygen atoms and amine nitrogen
atoms [18].
Our interest in the properties of metal complexes, and especially
in a good aqueous stability, led us to investigate the preparation of
structures containing both subunits, pyrazolyl-carboxyl. Besides,
the geometry of the five-membered pyrazole ring was expected to
provide interesting spectral properties to its metal complexes. To
this end the both subunits pyrazolyl-carboxyl of the ligand were
exploited as organic antennae.
It is known that, although free lanthanide metal ions are inef-
ficient at absorbing and emitting light, many lanthanide complex
compounds containing organic ligands are highly capable of emit-
ting strong and characteristic fluorescence [13]. In the lanthanide
complexes, the organic ligands that have lonepair and/or ␲-
∗
Corresponding author. Tel.: +86 731 8836961; fax: +86 731 8879616.
E-mail address: trr@mail.csu.edu.cn (R.-R. Tang).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.09.019

X.-M. Shi et al. / Spectrochimica Acta Part A 72 (2009) 198–203
199
84.4%): mp 77–79 ◦C, 1H NMR (400 MHz, CDCl3): ıH 2.37 (s, 3H,
CH3), 3.89 (s, 3H, CH3O), 6.69 (s, 1H, Pz–H). EI MS: m/z 140 (M+),
109 (M+−OMe) and 81 (M+−OCOMe). Anal. calcd. for C6H8N2O2: C,
51.42; H, 5.75; N, 19.99; found: C, 51.36; H, 5.88; N, 20.08, respec-
tively.
2. Experimental
2.1. Reagents and instruments
Methyl ␤-acetylpyruvate 1 [19] and 2,6-bis(methoxycarbo-
nyl)pyridine 3 [20] was prepared according to the literature meth-
ods. Pyridine-2,6-dicarboxylic acid and other regents used were
purchased and used as analytical grade. Rare earth chlorides were
prepared according to the literature method [21].
2.2.2. Synthesis of pyridine-2,6-dimethanol (4)
To pyridine-2,6-dicarboxylic acid dimethyl ester
3 5.00 g
(25.6 mmol) in THF (50 mL) was slowly added NaBH4 4.21 g
(110.5 mmol), and the solution was then stirred at room temper-
ature for 12 h. After evaporation of the solvent, the residue was
dissolved in 30 mL water, adjusted to pH 3 with 2 M HCl, and then
adjusted to pH 9 with saturated aqueous Na2CO3 solution. Evapora-
tion of the solvent, the residue was extracted with CHCl3 (300 mL)
by continuous liquid–liquid extraction for 10 h. After evaporation of
the solvent, the white product was washed with Et2O and dried in
vacuo to give compound 4 (3.1 g, 85.2% yield): mp 111–113 ◦C (Ref.
[22] 112–114 ◦C). 1H NMR (400 MHz, CDCl3): ıH 3.64 (bs, 2H), 4.34
(s, 4H), 6.94 (d, 2H, J = 7.8 Hz) and 7.35 (t, 1H, J = 7.8 Hz). Anal. calcd.
for C7H9NO2: C, 60.42; H, 6.52; N, 10.07; found: C, 60.33; H, 6.58;
N, 10.14.
Contents of carbon, hydrogen and nitrogen were deter-
mined using an Elementar vario EL elemental analyzer. Content
of Ln(III) was determined by EDTA titration. Infrared spectra
(4000–400 cm−1) were recorded with samples as KBr discs using
a
Nicolet NEXUS 670 FTIR spectrophotometer. 1H-NMR was
measured with a Bruker-400 MHz nuclear magnetic resonance
spectrometer with CDCl3 or DMSO as solvent and TMS as internal
reference. Mass spectrum was measured using a ZAB-HS analyzer.
Fluorescence measurements were made on a Hitich F-4500 spec-
trometer.
2.2. Synthesis of the ligand
2.2.1. Synthesis of methyl 5-methyl-1H-pyrazole-3-carboxylate
(2)
2.2.3. Synthesis of 2,6-bis(bromomethyl)pyridine (5)
To a mixture of bromhydric and acetic acid (150 mL), 1.20 g
(8.6 mmol) of pyridine-2,6-dimethanol 4 were added and the solu-
tion refluxed for 48 h. Upon cooling at 0 ◦C, a white precipitate
formed, which was recovered by filtration, dissolved in CH2Cl2
(40 mL) and washed with an aqueous solution (100 mL) of NaHCO3
(2.4 g). The crude product was then extracted from the aqueous
phase with 5 mL× 20 mL portions of CH2Cl2. The organic phases
were combined, dried over MgSO4, and evaporated to dryness in
vacuo, affording 0.98 g of analytically pure compound in 43% yield.
To 50 mL of anhydrous MeOH was added 5.00 g of methyl
␤-acetylpyruvate 1 (34.7 mmol), after stirring for 10 min, 2.39 g
of NH2NH2·H2O (80%, 38.2 mmol) in anhydrous MeOH (15 mL)
at 0 ◦C was treated dropwise within 30 min. The mixture was
stirred at room temperature for a further 30 min and then concen-
trated under reduced pressure. The residue was dissolved in EtOAc,
washed with water, dried (Na2SO4), evaporated and the crude prod-
ucts was purified by recrystallization from EtOAc, to give 2 (4.11 g,
Scheme 1. Synthesis of the ligand (L).

200
X.-M. Shi et al. / Spectrochimica Acta Part A 72 (2009) 198–203
Table 1
Elemental analytical data for the complexes.
Complex
M (%) found (calc.)
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Na2TbLCl3·9H2O
Na2EuLCl3·7H2O
Na2SmLCl3·6H2O
Na2GdLCl3·7H2O
19.21(19.22)
19.42(19.39)
19.65(19.68)
19.91(19.93)
24.73(24.70)
26.09(26.05)
26.76(26.72)
25.92(25.88)
3.98(4.02)
3.76(3.73)
3.51(3.56)
3.67(3.70)
8.45(8.47)
8.91(8.94)
9.15(9.17)
2.85(8.88)
Table 2
Characteristic IR bands (cm−1) of the ligand and its complexes.
L
Na2TbLCl3·9H2O
Na2EuLCl3·7H2O
Na2SmLCl3·6H2O
Na2GdLCl3·7H2O
ꢀ(OH)
3397
3390
3393
3408
ꢀ(C O)
1727
ꢀas(COO−
)
1597
1381
1506
1314
849
644
570
454
1596
1379
1509
1316
844
645
575
464
1596
1381
1506
1314
845
643
570
453
1597
1380
1505
1314
850
647
573
453
ꢀs(COO−
)
ꢀpy(C N)
ꢀpz(C N)
ꢁꢂ (H2O)
ꢁω(H2O)
ı(Ln–N)
ı(Ln–O)
1578
1390
The product is light sensitive and was stored in a dark brown bottle.
1H NMR (400 MHz, CDCl3): ıH 4.50 (s, 4H, CH2), 7.40 (d, 2H, ArH),
7.70 (t, 1H, ArH). EI MS: m/z 266 (M+H+). Anal. calcd. for C7H7NBr2:
C, 31.73; H, 2.66; N, 5.29; found: C, 32.03; H, 2.57; N, 5.23.
to adjust the pH to 1, and the solution was stirred for 3 h at room
temperature. The precipitate was collected by filtration and washed
with 1% aqueous HCl. After drying, the product was added to 50 mL
of acetonitrile, and the mixture was refluxed 1 h with stirring. The
precipitate was filtered and dried to give compound L (0.75 g, 80.9%
yield): mp 294 ◦C (decomp.). 1H NMR (400 MHz, DMSO-d6): ıH 2.18
(s, 6H, CH3), 5.41 (s, 4H, CH2), 6.51 (s, 4H, Pz (4) H), 7.03 (d, 2H, ArH),
7.77 (t, 1H, ArH) and 12.54 (s, 2H, COOH). EI MS: m/z 355 (M+). Anal.
calcd. for C17 H17 N5O4: C, 57.46; H, 4.82; N, 19.71; found: C, 57.41;
H, 4.91; N, 19.76.
2.2.4. Synthesis of
2,6-bis(5-methyl-3-methoxycarbonyl-pyrazol-1-ylmethyl)
pyridine (6)
A
mixture of 2,6-bis(bromomethyl)pyridine
5
1.00 g
(3.79 mmol) and 5-methyl-1H-pyrazole-3-carboxylate 2 (1.06 g,
7.58 mmol) in acetone (40 mL), anhydrous K2CO3 0.63 g
(4.49 mmol) was refluxed for 24 h. The organic layer was then
separated, and evaporated in vacuo. The crude product was washed
with water (40 mL) and chromatographed on silica gel, eluting
with EtOAc/CH2Cl2 (1:1), followed by crystallization from EtOAc,
to give 6 (849 mg, 58.5% yield): mp 136–138 ◦C. 1H NMR (400 MHz,
CDCl3): 2.14 (s, 6H, CH3) 3.84 (s, 6H, CH3O), 5.37 (s, 4H, CH2), 6.56
(s, 2H, Pz (4) H), 6.76 (d, 2H, ArH) and 7.51 (t, 1H, ArH). EI MS: m/z
383 (M+). Anal. calcd. for C19 H21N5O4: C, 59.52; H, 5.52; N, 18.27;
found: C, 59.45; H, 5.65; N, 18.38.
2.3. Synthesis of the complexes
An solution of LnCl3·6H2O (Ln = Eu, Tb, Sm and Gd) (0.2 mmol)
in ethanol (5 mL)was added dropwise to a solution of the ligand
(0.6 mmol) in ethanol and the mixture stirred at 60 ◦C for 3 h. The
resulting precipitate was collected by filtration, washed three times
each with ethanol, chloroform and dried in vacuo to give a flake
solid (typically about 80% yield).
3. Result and discussion
2.2.5. Synthesis of
2,6-bis(5-methyl-3-carboxypyrazol-1-ylmethyl)pyridine (L)
A mixture of 50 mL ethanol, 0.8 g KOH, 2 mL H2O, and 1.00 g 2,6-
bis(5-methyl-3-methoxycarbonyl-pyrazol-1-ylmethyl) pyridine 6
(2.61 mmol) was refluxed with stirring for 5 h. After the solvent
was evaporated, the residue was dissolved in 30 mL water, and the
solution was filtered. To the solution was added dropwise 3 M HCl
3.1. Properties of the complexes
The results of elemental analysis (see Table 1) indicated that
the composition of the complexes conforms to be Na2TbLCl3·9H2O,
Na2EuLCl3·7H2O, Na2SmLCl3·6H2O and Na2GdLCl3·7H2O. All com-
plexes were found to be soluble in H2O, DMF, DMSO, slightly soluble
in ethanol and acetone, and insoluble in benzene, diethyl ether and
tetrahydrofuran.
Table 3
Fluorescence data for the complexes.
3.2. IR spectra of the complexes
Complex
ꢃex (nm)
ꢃem (nm)
RFIa
Assignment
Na2TbLCl3·9H2O
259
490
543
583
619
590
616
648
694
372
443
3553
9574
786
5D4–7F6
5D4–7F5
5D4–7F4
5D4–7F3
5D0–7F1
5D0–7F2
5D0–7F3
5D0–7F4
The IR spectra of all four complexes are similar, indicating that
they are structurally alike. Table 2 summarises the characteris-
tic bands observed for the ligand and its metal complexes. The
IR spectrum of the free ligand shows bands at 1727, 1578 and
1390 cm−1, which can be assigned as ꢀ(C O), ꢀpy (C N) of pyridine
ring and ꢀpz (C N) of pyrazole ring respectively. In the complexes,
these bands are shifted upfield by 69–73 cm−1 for ꢀpy (C N) and
upfield by 74–76 cm−1 for ꢀpz (C N). The bands (C O) in free lig-
ands disappear and the new bands appear at 1596–1597 cm−1 and
1379–1381 cm−1 assignable to [ꢀas (COO−) +ꢀs (COO−)]. In each
287
Na2EuLCl3·7H2O
256
3416
9776
171
241
9220
9739
Na2SmLCl3·6H2O
Na2GdLCl3·7H2O
298
363
a
The width of emission slit and excitation slit of the Tb(III) complex was 2.5 nm
and other complexes were 5.0 nm, the voltage of photomultiplier tube was 700 V.

X.-M. Shi et al. / Spectrochimica Acta Part A 72 (2009) 198–203
201
Fig. 1. The excitation and emission spectrum of the Tb(III) complex. The excitation
Fig. 3. The excitation and emission spectrum of the Sm(III) complex. The excitation
and emission slit widths were 2.5 nm in solid state and the drive voltage was 700 V.
and emission slit widths were 5.0 nm in solid state and the drive voltage was 700 V.
case, the shifts suggested that the relevant oxygen and nitrogen
atoms of the ligand were involved in coordination to the metal
centre. The bands centered at about 3400 cm−1 and character-
istic of ꢀ(OH) vibrations are always relatively intense and can
be attributed to crystal water and combined water molecules.
The absorption bands assigned to the Ln–O and Ln–N stretch-
ing frequencies of the complexes were observed at 453–464 and
470–475 cm−1, respectively. The appearance of the ꢁꢂ (H2O) and
ꢁω(H2O) bands in the spectra of the complexes at approximately
850 and 645 cm−1, respectively, indicates the presence of coordi-
nated water [23].
very strong from the Tb(III) complex (Fig. 1) under the same exper-
imental conditions. It appears that the energy-transfer from the
organic ligand (L) to the central Tb(III) ions is much more effective
compared to other Ln(III) ions we have studied. It is thought that
the strong emission observed is due to the 5D4–7F5 transitions of
the 4f electrons of the Tb(III) ions [24].
The emission spectrum of solid Tb(III) complex consists of
7
7
four main lines at 490 nm (5D4→ F6), 543 nm (5D4→ F5), 583 nm
(5D4→ F4) and 619 nm (5D4→ F3) (Fig. 1). Due to the presence of
7
7
7
a scattering signal at 490 nm, and the 5D4→ F5 emission is a mag-
netic dipole transition, which is less affected by the ligand field,
so the peak height at 543 nm for terbium was used to measure
the fluorescence intensities. We can see that the emission band
3.3. Fluorescence studies
(5D4→ F5) is obviously stronger than the other emission bands
7
(5D4→ F6, 5D4→ F4, 5D4→ F3) from Fig. 1. The typical narrow
emission bands of Tb(III) ions can be detected upon excitation of
the ligand-centered absorption band, indicating that the ligand is
a comparative good organic chelator to absorb energy and transfer
them to Tb(III) ion.
7
7
7
The fluorescence data for each of the complexes in the solid state
was listed in Table 3. The maximum excitation wavelengths (ꢃex
)
of the Tb(III), Eu(III), Sm(III) and Gd(III) complexes were 259, 256,
298 and 363 nm, respectively (Table 3).
Fluorescent spectra for the Eu(III), Sm(III) and Gd(III) complexes
were measured at two different slit widths, 2.5 nm (Fig. 1) and
5.0 nm (Figs. 2–4). At 2.5 nm, the fluorescent intensity observed
from Eu(III), Sm(III) and Gd(III) complexes were very weak, while
The fluorescence spectrum of solid Eu(III) complex exhibits four
sharp characteristic emission peaks at 590 nm (5D0→ F1), 616 nm
7
7
7
7
(5D0→ F2), 648 nm (5D0→ F3) and 694 nm (5D0→ F4) (Fig. 2)
Fig. 2. The excitation and emission spectrum of the Eu(III) complex. The excitation
Fig. 4. The excitation and emission spectrum of the Gd(III) complex. The excitation
and emission slit widths were 5.0 nm in solid state and the drive voltage was 700 V.
and emission slit widths were 5.0 nm in solid state and the drive voltage was 700 V.

202
X.-M. Shi et al. / Spectrochimica Acta Part A 72 (2009) 198–203
Table 4
TG–DTA date of the ligand and the complexes.
Complex
Tendo
(
◦C)
H2O loss (calcd.) (%)
Texo
(
◦C)
Total weight loss (calcd.) (%)
t1
t2
t1
t2
Na2TbLCl3·9H2O
Na2EuLCl3·7H2O
Na2SmLCl3·6H2O
Na2GdLCl3·7H2O
48
50
49
52
261
274
265
270
20.05(20.23)
15.14(14.89)
18.77(18.56)
17.11(17.25)
356
352
358
354
599
601
603
605
59.28(59.03)
71.04(70.25)
66.98(65.55)
62.39(61.47)
corresponding to the 5D0–7FJ (J = 1–4) transition of Eu(III) ion,
respectively. The hypersensitive 5D0–7F2 transition of Eu(III) com-
plex is observed at 616 nm as the most prominently single peak
without splitting which indicates that the energy collected by the
“antenna” effect of the ligand can be effectively transferred to the
central Eu(III) ion.
The excitation and emission maximum wavelengths are 298
and 372 nm for Sm(III) complex is shown in Fig. 3. A broad exci-
tation band (280–360 nm) centered at 298 nm was observed on
the excitation spectrum for solid Sm(III) complex, and the emission
spectrum for the solid Sm(III) complex showed a wide fluorescence
emission band-centered at 372 nm at room temperature, which was
attributed to the free ligand emission.
The excitation and emission maximum wavelengths are 363 and
443 nm for Gd(III) complex is shown in Fig. 4. A broad excitation
band (300–420 nm) centered at 363 nm was observed on the excita-
tion spectrum for solid Gd(III) complex, and the emission spectrum
for the solid Gd(III) complex showed a wide fluorescence emis-
sion band-centered at 443 nm at room temperature, which was
attributed to the free ligand emission.
Fig. 6. TG and DTA curves for original Eu(III) complex.
The fluorescence of Ln(III) complexes is related to the efficiency
of the intra-molecular energy transfer between the triplet energy
states of the ligand and the emitting energy states of the metal
ions. One factor that can contribute to the observed fluorescence
intensity of the Tb(III) complex at 545 nm was much stronger than
that of the Eu(III), Sm(III) and Gd(III) complexes, it can be inferred
that the energy difference between the ligand triplet states and the
emitting energy state of Tb(III) is more favorable for energy transfer
than those of the other three rare earth ions.
3.4. Thermal analysis
In order to examine the thermal stability of the complexes, ther-
mal gravimetric (TG) and differential thermal analyses (DTA) were
carried out between 30 and 750 ◦C in the static atmosphere of air
(Figs. 5–8). The heating rate is 10 ◦C/min and the TG–DTA curves
Fig. 7. TG and DTA curves for original Sm(III) complex.
Fig. 8. TG and DTA curves for original Gd(III) complex.
Fig. 5. TG and DTA curves for original Tb(III) complex.

X.-M. Shi et al. / Spectrochimica Acta Part A 72 (2009) 198–203
203
showing the weight loses gradually as the increase of temperature.
Acknowledgement
Some data of TG–DTA spectra are listed in Table 4. The ther-
mal behavior of La(III) ion complexes is interesting. The TG–DTA
curves show two mass loss, each coinciding with one kind of water
molecule which shows clear the presence of crystal and coordi-
nated water; this is also confirmed by IR spectroscopy. The TG
curves of Tb(III), Eu(III), Sm(III) and Gd(III) ion complexes show
the first mass loss between 48–147, 50–142, 49–140 and 52–150 ◦C,
respectively, which corresponds to the release of crystal water con-
tent. The relatively high temperature of mass loss between 162–261,
166–274, 156–265 and 164–270 ◦C shows that this is coordinated
water hold. The third mass loss stage of the La(III) ion complexes
in region of 350–600 ◦C is attributed to elimination and/or decom-
position of free ligand. The initial temperature of decomposition
is over 350 ◦C, which indicates that the thermal stability of the
complexes is higher than that of the free ligand (294 ◦C decomp.)
showing that there may be large conjugation in the chelate ring in
the complexes [25].
This project was supported by The Postdoctoral Science Founda-
tion of Central South University and the China postdoctoral Science
Foundation.
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