Synthesis, crystal structure, and photoluminescence of homodinuclear lanthanide 4-(dibenzylamino)benzoate complexes

Article abstract of DOI:10.1021/ic902257u

Three new binuclear lanthanide complexes of general formula [Ln ₂(L)₆(H₂O)₄] (Ln = Tb (1), Eu (2), and Gd (3)) supported by the novel aromatic carboxylate ligand 4-(dibenzylamino)benzoic acid (HL) have been synt

Full text of DOI:10.1021/ic902257u

Inorg. Chem. 2010, 49, 2407–2415 2407
DOI: 10.1021/ic902257u
Synthesis, Crystal Structure, and Photoluminescence of Homodinuclear
Lanthanide 4-(Dibenzylamino)benzoate Complexes
†
,†
‡
‡
A. R. Ramya, M. L. P. Reddy,* Alan H. Cowley, and Kalyan V. Vasudevan
†
Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science & Technology
‡
(
NIIST), CSIR, Thiruvananthapuram-695 019, India, and Department of Chemistry and Biochemistry,
The University of Texas at Austin, 1 University Station A5300, Austin, Texas 78712
Received November 15, 2009
Three new binuclear lanthanide complexes of general formula [Ln (L) (H O) ] (Ln = Tb (1), Eu (2), and Gd (3))
2
6
2
4
supported by the novel aromatic carboxylate ligand 4-(dibenzylamino)benzoic acid (HL) have been synthesized.
Complexes1 and 2 werestructurally characterized bysingle-crystal X-ray diffraction. Both1 and 2 crystallize inthetriclinic
space group P , and their molecular structures consist of homodinuclear species that are bridged by two oxygen atoms
1
from two carboxylate ligands via different coordination modes. The discrete bridged dimer of 1 is centrosymmetric and
features 8-coordinate terbium atoms, each of which adopts a distorted square-antiprismatic geometry. Both coordination
2
1
1
spheres comprise two η -chelating benzoates, two μ-η :η -carboxylate interactions from the bridging benzoates, and
two water molecules. By contrast, in complex 2, the Eu ion coordination environment is best described as a distorted
3þ
2
tricapped-trigonal prism, each europium ion being coordinated to three η -chelating benzoate ligands and two water
2
molecules. One of the η -carboxylate ligands is involved in a further interaction with an adjacent metal, thus rendering the
2
1
overall binding mode bridging tridentate, μ-η :η . Scrutiny ofthe packing diagrams for 1 and 2 revealed the existence ofa
one-dimensional molecular array that is held together by intermolecular hydrogen-bonding interactions. The Tb
complex 1 exhibits high green luminescence efficiency in the solid state with a quantum yield of 82%. On the other hand,
3þ
3þ
poor luminescence efficiency has been noted for the Eu -4-(dibenzylamino)benzoate complex.
6
Introduction
displays and organic light-emitting diodes (OLEDs). How-
ever, since f-f transitions are parity forbidden, unligated
luminescent lanthanide cations have extremely low molar
extinction coefficients; hence, direct lanthanide excitation
The unique electronic structures of lanthanide cations
ligated with conjugated organic ligands continue to stimulate
an ever increasing number of important technological appli-
cations in fields as diverse as biomedicine and materials
7
results only in modest luminescence intensities. Therefore,
1-5
over the past few years, efforts have been made to augment
the absorption coefficients and thereby obtain significantly
more intense lanthanide ion emissions. Fortunately, this
objective can be accomplished by prudent selection and
synthesis of organic ligands with conjugated motifs. Aro-
science.
Moreover, the long excited-state lifetimes and
the high chromaticities of the lanthanides are also pertinent
to applications in the domain of solid-state photonic materi-
als. For instance, Tb , Eu , and Tm cations are used as
green, red, and blue emitters, respectively, in multicolor
3þ
3þ
3þ
8
9
matic carboxylic acids and β-diketones are particularly
*
To whom correspondence should be addressed. E-mail: mlpreddy55@
(
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Dalton Trans. 2007, 22, 2229–2241. (d) Kuriki, K.; Koike, Y.; Okamoto, Y.
Chem. Rev. 2002, 102, 2347–2356.
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Fields, P. R.; Wybourne, B. G. J. Chem. Phys. 1965, 42, 3797. (d) Kim, Y. H.;
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(
(
1
r 2010 American Chemical Society
Published on Web 02/02/2010
pubs.acs.org/IC

2
408 Inorganic Chemistry, Vol. 49, No. 5, 2010
Ramya et al.
valuable in this context because such ligands can absorb
ultraviolet light and transfer the absorbed energy to the
central lanthanide ions in an appropriately effective manner
by replacing the hydrogens of the -NH group with benzyl
2
groups. The highly conjugated benzoic acid functionality has a
significant influence on the distribution of π-electron density
within the ligand system. Accordingly, the effective charges on
10
(the so-called antenna effect). In particular, when aromatic
3þ
carboxylic acids are employed as the antenna ligands, the
coordinated lanthanide ions exhibit higher luminescent
the atoms coordinated to the Ln ions can be changed, and the
interaction of the ligand with metal ion can be modified. As a
consequence, the energies of the ligand-metal charge transfer
states (LMCTs) and the position of the triplet level are changed,
which in turn has profound effects on the luminescent properties
11
stabilities than those ligated with other organic ligands.
This enhanced stability is of obvious practical importance in
terms of device performance and stability.
3þ
A number of lanthanide benzoate coordination complexes
of various Ln ions. In the present work, a new derivative
of benzoic acid has been designed, characterized, and utilized
12
with unique photophysical properties and intriguing struc-
13
3þ
3þ
3þ
tural features have been disclosed recently. The benzoate
ligands were selected on the basis of the fact that carboxylate
groups interact strongly with the oxophilic lanthanoids and
the fact that the delocalized π-electron system provides a
for the synthesis of the desired Tb , Eu , and Gd com-
plexes. Two of the new lanthanide 4-(dibenzylamino)benzoates
have been structurally characterized by single-crystal X-ray
diffraction, and the photophysical properties of all three
of the new lanthanide benzoate complexes have been investi-
gated and correlated with the triplet energy levels of the designed
ligand.
12,14
strongly absorbing chromophore.
Prior results suggested
that derivitization of the benzoic acid analogues with thio-
phene had a beneficial effect in terms of tuning the triplet state
of the antenna. In particular, the enhanced emission quantum
yield originated from a better match between the pertinent
Experimental Section
15
ligand orbitals and lanthanide ion excited states. The intense
fluorescent emissions of homodinuclear lanthanide com-
Materials and Instrumentation. The following chemicals were
procured commercially and used without further purification:
terbium(III) nitrate hexahydrate, 99.9% (Acros Organics);
europium(III) nitrate hexahydrate, 99.9% (Acros Organics);
gadolinium(III) nitrate hexahydrate (Treibacher); methyl 4-
aminobenzoate, 98% (Aldrich); and benzyl bromide, 99.9%
(Aldrich). All of the other chemicals used were of analytical
reagent grade.
Elemental analyses were performed with a Perkin-Elmer
Series 2 Elemental Analyzer 2400. A Perkin-Elmer Spectrum
One FT-IR spectrometer was used to obtain the IR spectral data
3þ
plexes of 4-cyanobenzoic acid imply that the ligand-to-Ln
energy transfer is efficient and that coordinated water mole-
cules do not quench the luminescence by nonradiative dis-
16
sipation of energy. Very high quantum yields (88%) have
1
7
been reported with terbium-aminobenzoate complexes,
which is somewhat surprising in view of the coordination of
both -NH and OH ligands, since such moieties usually
2
2
18
function as vibrational deactivators of the excited state.
Given the important potential applications of lanthanide
carboxylates and the fascinating properties of benzoate ligands,
we were prompted to prepare a new series of lanthanide
complexes featuring the 4-(dibenzylamino)benzoic acid ligand
(
used to record the H NMR and C NMR (125 MHz) spectra of
3
the ligands in CDCl solution. The mass spectra were recorded
on a JEOL JSM 600 fast atom bombardment (FAB) high
resolution mass spectrometer (FABMS), and the thermogravi-
metric analyses were performed on a TG/DTA-6200 (SII Nano
Technology Inc., Japan). The absorbances of the ligands were
neat KBr), and a Bruker 500 MHz NMR spectrometer was
1
13
(
9) (a) de Sa, G. F.; Malta, O. L.; de Mello Donega, C.; Simas, A. M.;
Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr. Coord. Chem. Rev. 2000,
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3
1
2
96, 165–195. (b) Binnemans, K.; Gorller-Walrand, C. Chem. Rev. 2002, 102,
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(
Shimadzu, UV-2450), and the photoluminescence (PL) spectra
Chem. 2006, 45, 10651–10660. (d) Zheng, Y.; Cardinali, F.; Armaroli, N.;
Accorsi, G. Eur. J. Inorg. Chem. 2008, 28, 2075–2080. (e) Fratini, A.; Richards,
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were recorded on a Spex-Fluorolog FL22 spectrofluorimeter
equipped with a double grating 0.22 m Spex 1680 monochro-
mator 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 1040D phosphorimeter.
The overall quantum yields (Φoverall) were measured using an
integrating sphere in a SPEX Fluorolog spectrofluorimeter. The
PL quantum yields of thin films (Φoverall) were determined using
a calibrated integrating sphere system. A Xe-arc lamp was used
to excite the thin film samples that were placed in the sphere. All
samples were prepared by drop casting the material placed
between two quartz coverslips. The quantum yields were deter-
mined by comparing the spectral intensities of the lamp and the
(
10) (a) Lehn, J. M. Angew. Chem., Int. Ed. 1990, 29, 1304–1319. (b) Kido,
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(
Inorg. Chem. 2007, 46, 11025–11030. (b) Raphael, S.; Reddy, M. L. P.; Cowley,
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12) (a) Hilder, M.; Junk, P. C.; Kynast, U. H.; Lezhnina, M. M. J.
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K.; Zolin, V.; Gawryszewska, P.; Legendziewicz, P.; Kudryashova, V.; Pekareva,
I. J. Photochem. Photobiol. A: Chem. 2006, 177, 314–323.
1
9
sample emission as reported in the literature. Using this
experimental setup and the integrating sphere system, the solid
state fluorescence quantum yield of a thin film of the standard
green OLED material tris-8-hydroxyquinolinolato aluminum
(
13) (a) Busskamp, H.; Deacon, G. B.; Hilder, M.; Junk, P. C.; Kynast, U.
H.; Lee, W. W.; Turner, D. R. Cryst. Eng. Comm. 2007, 9, 394–411. (b)
Deacon, G. B.; Hein, S.; Junk, P. C.; Justel, T.; Lee, W.; Turner, D. R. Cryst. Eng.
Comm. 2007, 9, 1110–1123. (c) Zhong, R.; Zou, R.; Du, M.; Jiang, L.; Yamada,
T.; Maruta, G.; Takeda, S.; Xu, Q. Cryst. Eng. Comm. 2008, 10, 605–613.
(
Alq
previously reported values. Each sample was measured several
3
) was determined to be 0.19, which is consistent with
20
(
14) de Bettencourt-Dias, A.; Viswanathan, S. Dalton Trans. 2006, 34,
093–4103.
15) Viswanathan, S.; de Bettencourt-Dias, A. Inorg. Chem. 2006, 45,
0138–10146.
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Huang, J. S. Inorg. Chem. 2006, 45, 6308–6316.
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Warzala, M. Eur. J. Inorg. Chem. 2007, 2, 291–301.
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23, 201–228.
4
1
(
(
19) (a) De Mello, C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997, 9,
ꢀ
(
230–232. (b) Palsson, L.-O.; Monkman, A. P. Adv. Mater. 2002, 14, 757–758. (c)
Shah, B. K.; Neckers, D. C.; Shi, J.; Forsythe, E. W.; Morton, D. Chem. Mater.
(
2006, 18, 603–608.
(20) (a) Colle, M.; Gmeiner, J.; Milius, W.; Hillebrecht, H.; Br u€ tting, W.
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(
1

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2409
times under slightly different experimental conditions. The
Table 1. Crystallographic and Refinement Data for 1 and 2
2
1
estimated error for the quantum yields is (10%. For measur-
ing the quantum yield by the relative method, the diffuse
reflectance spectra of the new lanthanide complexes and the
standard phosphor were recorded on a Shimadzu UV-2450
1
2
formula
fw
C
130
H
136
N
6
O
22
S
2
Tb
2
130 128 6 18 2 2
C H N O S Eu
2516.41
triclinic
2430.42
triclinic
P
1
UV-vis spectrophotometer using BaSO
4
as a reference. The
overall quantum yields (Φoverall), were measured at room tem-
perature using the technique for powdered samples described by
cryst syst
space group
cryst size
temp/K
P
1
0.25 ꢁ 0.22 ꢁ 0.15
0.24 ꢁ 0.20 ꢁ 0.13
2
Bril and De Jager-Veenis, along with the following expression:
2
153(2)
153(2)
˚
a (A)
9.829(5)
17.180(5)
17.816 (5)
84.917 (5)
82.794 (5)
87.271(5)
2971.2 (19)
1
1.406
1.288
1296
0.0396
9.879(5)
17.259(5)
17.896 (5)
85.264(5)
83.206 (5)
88.609 (5)
3019(2)
1
1.337
1.131
1252
0.0521
˚
b (A)
1
1
-rst
-rx
Ax
˚
c (A)
φoverall
¼
ꢁ
ꢁ φst
ð1Þ
R (deg)
β (deg)
γ (deg)
Ast
x
where r and rst represent the diffuse reflectance of the com-
3
˚
V/A
Z
D
plexes and the standard phosphor, respectively (with respect to a
fixed wavelength), and Φ is the quantum yield of the standard
st
-1
calcd, g cm
phosphor. The terms A
x
and Ast represent the areas under the
complex and the standard emission spectra, respectively. To
-1
μ(Mo,KR), mm
F(000)
R1 [I > 2σ(I)]
acquire absolute intensity values, BaSO was used as a reflecting
4
standard. The standard phosphor used was pyrene (Aldrich),
the emission spectrum of which comprises a large broad band
peaking around 471 nm, with a constant Φ value (Φst = 61%,
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
GOF
0.0872
0.0532
0.0915
1.093
0.1146
0.0996
0.1246
1.023
23
λex = 313 nm). Three measurements were carried out for each
sample, and the reported Φoverall value corresponds to the
arithmetic mean value of the three values. The errors in the
quantum yield values associated with this technique were esti-
chromatography using hexane/ethyl acetate, thereby affording the
desired product as a white solid. Yield, 0.2 g (36.4%). H NMR
1
2
4
mated to be within (10%.
(
(
4
500 MHz, CDCl ): δ (ppm) 7.85 (d, 2H, J = 9 Hz, Ar-H), 7.34
3
The X-ray diffraction data were collected at 153 K on
a Nonius Kappa CCD diffractometer equipped with an
Oxford Cryostream low-temperature device and a graphite-
t, 4H, J = 7 Hz, Ar-H), 7.27 (t, 2H, J = 7.5 Hz, Ar-H), 7.21 (d,
H, J = 7 Hz, Ar-H), 6.72 (d, 2H, J = 9 Hz, Ar-H), 4.71 (S, 4H,
13
-
2 3 3
NCH ), 3.83 (S, 3H, -OCH ). C NMR (125 MHz, CDCl ):
˚
monochromated Mo KR radiation source (λ = 0.71073 A).
Corrections were applied for Lorentz and polarization effects.
δ (ppm) 167.26, 152.54, 137.37, 131.48, 128.83, 127.24, 126.45,
2
5
þ
1
17.93, 111.26, 53.98, 51.53. MS (FAB) m/z = 332.53 (M þ 1) .
Both structures were solved by direct methods and refined by
full-matrix least-squares cycles on F . All of the nonhydrogen
s
FT-IR (KBr) νmax: 1690 (νas(CdO)), 1432 (ν (CdO)), 1600, 1530,
2
-
1
313, 1280, 1174, 1113, 834, 768 cm .
1
atoms were allowed anisotropic thermal motion, and the hydro-
gen atoms were placed in fixed, calculated positions using a
riding model (C-H, 0.96 A). Selected crystal data and data
Synthesis of 4-(Dibenzylamino)benzoic Acid (HL). Methyl
-(dibenzylamino)benzoate (0.3 g, 0.902 mmol) was refluxed
4
˚
for 24 h in a solution of KOH (0.15 g, 2.67 mmol) in 50 mL of
ethanol. The reaction mixture was poured into ice cold water
and acidified with dilute HCl, and the resulting precipitate was
filtered, washed, dried, and recrystallized from CH Cl . Yield,
collection and refinement parameters are listed in Table 1.
X-ray crystallographic information files can be obtained free
of charge via www.ccdc.cam.ac.uk/consts/retrieving.html (or
from CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.;
fax, þ44 1223 336033; e-mail, deposit@ccdc.cam.ac.uk). The
CCDC numbers are 743231 and 746200 for 1 and 2, respectively.
A molecule of DMSO has been added to the molecular formulas
of 1 and 2 since in each case it was removed via “Squeeze” due to
the extreme disorder which could not be solved. Furthermore,
the water protons were not located, and the hydroxyl protons
have been placed in the positions that were calculated on the
basis of optimum hydrogen bonding.
2
2
1
0
3
.25 g (87%): H NMR (500 MHz, CDCl ): δ (ppm) 7.92 (d, 2H,
J = 9.5 Hz, Ar-H), 7.36 (t, 4H, J = 7 Hz, Ar-H), 7.29 (t, 2H,
J = 7 Hz, Ar-H), 7.23 (d, 4H, J = 7 Hz, Ar-H), 6.74 (d, 2H,
13
2
J = 9 Hz, Ar-H), 4.74 (s, 4H, -NCH ). C NMR (125 MHz,
3
CDCl ): δ (ppm) 172.06, 153.17, 137.21, 132.26, 128.87, 127.29,
1
26.43, 116.92, 111.27, 53.99. MS (FAB) m/z = 318.57
þ
(
(
(
M þ 1) . Elemental analysis (%) calcd (found) for C H O N
21
19 2
317.381): C, 79.47 (79.00); H, 6.03 (6.27); N, 4.41 (4.36). FT-IR
KBr), νmax: 1663 (νas(CdO)), 1450 (ν (CdO)), 1599, 1557, 1529,
s
-
1
450, 1413, 1363, 1288, 723, 694 cm .
Synthesis of Methyl 4-(Dibenzylamino)benzoate. Potassium
carbonate (0.68 g, 4.92 mmol) was added to a solution of methyl
1
Syntheses of Lanthanide Complexes. In a typical procedure, an
ethanolic solution of Ln(NO3)3 3
4
(
-amino benzoate (0.25 g, 1.65 mmol) in freshly distilled DMF
50 mL). The resulting reaction mixture was refluxed for 30 min,
6H O (0.5 mmol) (Ln = Eu, Tb,
2
or Gd) was added to a solution of 4-(dibenzylamino)benzoic acid
1.5 mmol) in ethanol in the presence of NaOH (1.5 mmol).
following which benzyl bromide (0.56 g, 3.29 mmol) was added,
and the solution was refluxed at 78 °C for an additional 48 h.
The reaction mixture was then poured into water, and the
resulting precipitate was filtered off, washed with water, and
dried. The resulting residue was purified by silica gel column
(
Precipitation took place immediately, and the reaction mixture
was stirred subsequently for 10 h at room temperature. The crude
product was filtered, washed with ethanol, and dried. The resulting
complexes were then purified by recrystallization from a dichloro-
methane/methanol solvent mixture. Single crystals of the terbium
and europium complexes suitable for X-ray study were obtained
from a dimethylsulfoxide/ethanol/dichloromethane solvent mix-
ture after storage for 3 weeks at ambient temperature.
(
21) Eliseeva, S. V.; Kotova, O. V.; Gumy, F.; Semenov, S. N.; Kessler, V.
G.; Lepnev, L. S.; B u€ nzli, J.-C. G.; Kuzmina, N. P. J. Phys. Chem. A. 2008,
1
12, 3614–3626.
22) Bril, A; De Jager-Veenis, A. W. J. Electrochem. Soc. 1976, 123, 396–
98.
(
2 6 2 4
Tb (L) (H O) (1). Elemental analysis (%) calcd (found) for
3
C126H116N O Tb (2288.15): C, 66.14 (66.38); H, 5.11 (4.75); N,
6
16
2
(
23) Melhuish, W. H. J. Opt. Soc. Am. 1964, 54, 183–186.
24) (a) Mello Donega, C. D.; Junior, S. A.; de Sa, G. F. Chem. Commun.
3
1
1
.67 (3.89). FT-IR (KBr), νmax: 3429 (ν(O-H)), 1603 (νas(CdO)),
572 (νas(CdO)), 1402 (ν (CdO)), 1358 (ν (CdO)), 1503, 1221,
199, 1072, 783 cm . MS (FAB) m/z = 1109.06 (Tb(L)
) þ 1.
Eu (L) (H O) (2). Elemental analysis (%) calcd (found)
for C126H116N O Eu2 (2274.23): C, 66.54 (66.82); H, 5.14
(
s
s
1
996, 11, 1199–1200. (b) Carlos, L. D.; Mello Donega, C. D.; Albuquerque, R. Q.;
-1
3
Junior, S. A.; Menezes, J. F. S.; Malta, O. L. Mol. Phys. 2003, 101, 1037–1045.
25) Sheldrick, G. M. SHELL-PC, version 5; Siemens Analytical X-ray
Instruments. Inc.: Madison, WI, 1994.
(
2
6
2
4
6
16

2
410 Inorganic Chemistry, Vol. 49, No. 5, 2010
Ramya et al.
-
1
Scheme 1. Synthesis of the Ligand, HL
3400 cm which is characteristic of an O-H stretching
vibration, thereby indicating the presence of water mole-
cules in the coordination sphere of each complex.
It is clear from the thermogravimetric analysis data for
1
and 2 (Figure S2 in the Supporting Information) that
each complex undergoes a mass loss of approximately 3%
calcd, ∼3.14%) in the first step (120-240 °C), which
corresponds to elimination of the coordinated water
(
2
7
molecules. Subsequent thermal decomposition of 1
and 2 takes place in two steps in the temperature region
2
40-900 °C. The quantity of residue for each complex
represents approximately 16% of the initial mass and
corresponds to formation of the respective lanthanide
oxide.
(
1
(
4.84); N, 3.70 (3.91). FT-IR (KBr), νmax: 3413 (ν(O-H)),
604 (νas(CdO)), 1518 (νas(CdO)), 1403 (ν (CdO)), 1358
(CdO)), 1495, 1225, 1121, 783 cm . MS (FAB) m/z =
s
-
1
ν
s
1
124.7 (Eu(L) þ Na) þ 1.
X-Ray Crystal Structures. The solid state structures of
Tb (L) (H O) (1) and Eu (L) (H O) (2) were deter-
2 6 2 4 2 6 2 4
3
Gd (L) (H O) (3). Elemental analysis (%) calcd (found)
4
2
6
2
for C126H116N O Gd (2284.79): C, 66.24 (66.60); H, 5.12
6
mined by single-crystal X-ray diffraction. Figures 1
and 2 depict the molecular structures of complexes 1
and 2, respectively. The pertinent data collection para-
meters and a listing of significant bond distances and
bond angles for the metal coordination environments are
presented in Tables 1 and 2, respectively. Compounds 1
16
2
(
1
4.85); N, 3.68 (3.84). FT-IR (KBr), νmax: 3414 (ν(O-H)),
603 (νas(CdO)), 1574 (νas(CdO)), 1395 (ν (CdO)), 1356
νs(CdO)), 1513, 1226, 1199, 1078, 946 cm . MS (FAB)
s
-
1
(
m/z = 1130.49 (Gd(L)
3
þ Na) þ 1.
Results and Discussion
Synthesis and Characterization of Ligand and Ln
and 2 crystallize in the triclinic space group P , and
1
3
þ
their molecular structures consist of homodinuclear
species that are bridged by two oxygen atoms from two
carboxylate ligands in different coordination modes. It is
interesting to note that the dimeric structures of the
Complexes 1-3. The ligand 4-(dibenzylamino)benzoic
acid (HL) was synthesized in 87% yield according to the
method described in Scheme 1. The ligand was character-
1
13
3þ
ized by FT-IR, H NMR, and C NMR spectroscopy
Figure S1 in the Supporting Information) as well as by
mass spectroscopy (FAB-MS) and elemental analysis.
Ln -4-(dibenzylamino)benzoate complexes possess an
(
inversion center of symmetry, thus indicating that the
Ln(1) and Ln(2) centers reside in equivalent chemical
environments. The distance between the Tb(1) and Tb(2)
3
þ
The synthetic procedures for the Ln complexes 1-3
are described in the Experimental Section. The micro-
˚
cations in compound 1 is equal to 4.347 A
corresponding Eu-Eu distance in 2 is 4.309 A
distances fall within the range of 3.785-4.532 A that has
, while the
3
þ
analyses of complexes 1-3 revealed that each Ln ion
had reacted with the HL ligand in a metal-to-ligand molar
ratio of 1:3. The FT-IR spectra of the coordinated ligand
HL exhibits two intense bands at approximately 1450 and
˚
. These
˚
3
þ
been observed for other Ln carboxylate complexes that
feature both bidentate and tridentate bridging coordina-
tion modes. In the case of complex 1, each terbium atom
is coordinated to two η -bidentate chelating benzoates,
two η -carboxylate interactions from the bridging benzo-
-
1
28
1
663 cm , which are attributable to the symmetric
2
ν (CdO) and anti-symmetric ν (CdO) vibration modes,
s
as
1
respectively. In each case, coordination of the ligand HL
to the respective lanthanide ion was confirmed by
the absence of the ν(COOH) absorption bands of the
ates (the coordination modes can be viewed in Figure 3),
which is in good agreement with the IR data. The
coordination polyhedra can be described best as distorted
square antiprisms, in which six oxygen atoms are
furnished by the four benzoate moieties and two oxygen
atoms are provided by the two water molecules. The
longest Tb-O bonds involve the oxygen atoms of the
-
1
ligand at ∼1660 cm . Moreover, the asymmetric and
symmetric stretching vibrational modes of the carboxylic
acid in complexes 1-3 are further split into two peaks
-1
-
1
[
ν (CdO): 1402, 1358 cm ; ν (CdO): 1603, 1572 cm
s as
1
-
in 1; ν (CdO): 1403, 1358 cm ; ν (CdO): 1604,
s
-
as
1
-1
1
518 cm in 2; ν (CdO): 1395, 1356 cm ; ν (CdO):
bidentate chelating ligands [Tb(1)-O(1), 2.458 A
Tb(1)-O(2), 2.420 A; Tb(1)-O(4), 2.560 A; Tb(1)-O(3),
2.358 A], and the shortest such bonds are associated with
the bridging carboxylate ligand [Tb(1)-O(7), 2.330 A
Tb(1)-O(5), 2.332 A]. On the other hand, the Tb-O
bond distances for the coordinated water molecules
[Tb(1)-(O8), 2.421 A and Tb(1)-O(6), 2.377 A, re-
spectively] are shorter than those for the bidentate chelat-
˚
;
s
as
-
1
603, 1574 cm in 3]. The difference between the asym-
1
metric and symmetric stretching vibration modes
˚
˚
˚
(
Δν(CdO) = ν - ν ) falls in the ranges 245-247 and
˚
;
as
s
-
1
1
groups are coordinated to the Tb or Gd ions in two
70-179 cm , which in turn implies that the carboxylate
˚
3
þ
3þ
different ways, namely, via chelating and bidentate brid-
˚
˚
2
6
ging. On the other hand, the difference between the
asymmetric and symmetric stretching vibration modes is
2
9
ing benzoates. Scrutiny of the packing diagram for 1
revealed the presence of a 1D molecular array that is
oriented along the c axis. This feature is illustrated by the
-
1
46 and 115 cm in the case of complex 2, indicating the
2
coordination of Eu with carboxylate groups in chelat-
3
þ
2
ing and tridentate bridging modes. Furthermore, the IR
6
spectra of 1-3 exhibit a broad band at approximately
(
27) Mahata, P.; Ramya, K. V.; Natarajan, S. Chem.;Eur. J. 2008, 14,
839–5850.
28) Cambridge Structural Database, version 5.27. http//www.ccdc.ac.uk
5
(
(
26) (a) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227–
50. (b) Teotonio, E. E. S.; Brito, H. F.; Felinto, M. C. F. C.; Thompson, L. C.;
Young, V. G.; Malta, O. L. J. Mol. Struct. 2005, 751, 85–94.
(accessed Jan 2010).
(29) Sun, H.-L.; Ye, C.-H.; Wang, X.-Y.; Li, J.-R.; Gao, S.; Yu, K.-B. J.
Mol. Struct. 2004, 702, 77–83.
2

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2411
Figure 1. Asymmetric unit of complex 1 with ball and stick model shown at the 30% probability level and all hydrogen atoms omitted for clarity.
Figure 3. Different types of binding modes for the HL ligand observed
in Tb and Eu complexes 1 and 2.
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1 and 2
1
2
Tb1-Tb2
Tb1-O1
Tb1-O2
Tb1-O3
Tb1-O4
Tb1-O5
Tb1-O6
Tb1-O7
Tb1-O8
4.347
Eu1-Eu2
4.309
Figure 2. Asymmetric unit of complex 2 with ball and stick model
shown at the 30% probability level and all hydrogen atoms omitted for
clarity.
2.458(2)
2.420(2)
2.358(2)
2.560(2)
2.332(3)
2.377(2)
2.330(2)
2.421(3)
Eu1-O1
2.483(3)
2.435(3)
2.390(3)
2.577(3)
2.362(3)
2.410(3)
2.830(3)
2.455(3)
2.369(3)
53.25(10)
52.23(10)
78.21(10)
49.29(9)
68.34(11)
75.24(10)
92.56(10)
Eu1-O2
Eu1-O3
Eu1-O4
intermolecular hydrogen bonding interaction between
C28 and O8 through H28 with a H O distance of
Eu1-O5
3
3 3
Eu1-O6
˚
2
the Supporting Information).
.650 A and a C-H O angle of 154.97° (Figure S3 in
3 3
Eu1-O7
3
3
0
Eu1-O8
Eu1-O7#
O1-Eu1-O2
O3-Eu1-O4
O8-Eu1-O6
O5-Eu1-O7
O7-Eu1-O7#
O7-Eu1-O6
O7-Eu1-O8
In the case of complex 2, the 4-(dibenzylamino)benzoate
ligands exhibit two different coordination modes to the
O1-Tb1-O2
O3-Tb1-O4
O8-Tb1-O6
O5-Tb1-O7
O2-Tb1-O5
O7-Tb1-O6
O7-Tb1-O8
53.75(8)
52.85(7)
77.51(8)
116.08(9)
79.53(8)
74.56(8)
92.69(9)
(
30) (a) Roesky, H. W.; Andruh, M. Coord. Chem. Rev. 2003, 236, 91–119.
b) Remya, P. N.; Biju, S.; Reddy, M. L. P.; Cowley, A. H.; Findlater, M. Inorg.
Chem. 2008, 47, 7396–7404.
(

2
412 Inorganic Chemistry, Vol. 49, No. 5, 2010
Ramya et al.
Figure 5. Room-temperature excitation and emission spectra for com-
plex 1 (λex = 334 nm) with emission monitored at approximately 545 nm.
Figure 4. UV-visible absorption spectra of the HL ligand and com-
-6
plexes 1, 2, and 3 in CHCl
3
solution (2 ꢁ 10 M).
3
þ
2
induced by the metal coordination. The molar absorption
coefficient values for complexes 1-3 at 309 nm of 1.8 ꢁ
Eu ions, namely, bidentate chelating (η -chelating
benzoates) and bidentate chelating with an oxygen
atom bridging two metal ions and another oxygen atom
coordinating to one of the ions (triply coordinated;
5
5
5
-1
-1
1
0 , 1.78 ꢁ 10 , and 1.79 ꢁ 10 L mol cm , respec-
tively, are approximately six times higher than that of the
4
2
1
HL ligand (3ꢁ 10 at 311 nm), which is consistent with the
η -η -chelating benzoates), as illustrated in Figure 2,
3
þ
presence of six carboxylate ligands in each complex. Note
also that the large molar absorption coefficient for the HL
ligand indicates that it has a strong ability to absorb light.
Steady-State Photoluminescence. In order to under-
thus corroborating the IR data. The Eu ions are
nine coordinate and feature two bidentate chelating
carboxylates and one tridentate bridging carboxylate
ligand. The coordination sphere is completed by the
presence of two water molecules. The coordination
sphere can be described best as a tricapped trigonal
prism in which seven oxygen atoms are provided by
four 4-(dibenzylamino)benzoate ligands, and two oxy-
gen atoms are furnished by two water molecules. The
Eu-O bond lengths range from 2.362 to 2.830 A and
thus fall within the range anticipated for this type of
complex.
oxygen atoms of one of the triply coordinated ligands
3þ
stand the energy transfer processes in the new Ln com-
plexes, it was necessary to determine the singlet and triplet
energy levels of the ligand, 4-(dibenzylamino)benzoic acid
1
(
HL). The singlet ( ππ*) energy level of this ligand was
estimated by reference to the wavelength of the UV-vis
absorptionedgeoftheGd complex 3. The pertinent value
3þ
˚
-
1
was found to be 331 nm (30221 cm ) in the case of the HL
3
1
1a,15
ligand. The triplet energy level ( ππ*) of this ligand was
calculated by reference to the lower wavelength emission
The longest Eu-O bonds involve the
-
1
edge (423 nm: 23 640 cm ) from the low-temperature
˚
[
2
Eu(1)-O(7): 2.830 A]. Finally, the packing diagram for
reveals the presence of a one-dimensional array that is
3þ
phosphorescence spectra of the Gd complex of the
-(dibenzylamino)benzoic acid (Figure S6 in Supporting
Information). According to Reinhoudt’s empirical rule, the
4
aligned along the c axis and which involves an inter-
molecular hydrogen bonding interaction between the
C(28)-H(28) bond of a phenyl ring of one of the
carboxylate ligands and the O(8) of a water molecule
intersystem crossing process becomes effective when ΔE
1
1
3
-1 31
(
(
ππ* - ππ*) is at least 5000 cm . The energy gap ΔE
3 -1
ππ* - ππ*) for the ligand HL is 6581 cm ; hence, our
˚
with a separation of 2.684 A [H(28) O(8)] and an
angle of 154.44° [C(28)-H(28)-O(8) (Figure S4 in the
Supporting Information)].
3
3 3
new complexes amply satisfy this condition. As a conse-
quence, the intersystem crossing process is effective for this
ligand. The triplet energy level of HL appears at appreci-
UV-Vis Spectra. The UV-vis absorption spectrum of
the free ligand HL and those of the corresponding com-
5
3þ
5
3þ
ably higher energy than D for Tb or D for Eu , thus
0
4
indicating that the designed new ligand can act as an
antenna for the photosensitization of trivalent Ln ions.
plexes 1-3 were measured in CHCl solution (c = 2 ꢁ
3
3þ
-
6
1 M) and are displayed in Figure 4. The reflectance
spectra of complexes 1 and 2 are depicted in Figure S5 (in
0
The solid-state excitation and emission spectra of
complex 1 recorded at room temperature are displayed
in Figure 5. The excitation spectrum monitored at the
the Supporting Information). The absorption maxima for
1
-3 (309 nm), which are attributable to singlet-singlet
3
þ
1
characteristic emission of the Tb ion in the solid state
overlaps with the absorption spectrum in the 250-360 nm
region (Figure 4), which indicates that energy transfer
from the ligand to the metal ion is operative. The excita-
tion spectrum of 1 exhibits a broad band between 250 and
380 nm which is attributable to the π-π* transition of the
π-π* absorptions of the aromatic rings, are slightly
blue-shifted with respect to that of the free ligand HL
(
λmax = 311 nm). No significant changes are apparent in
the shapes of the absorption bands upon formation of the
lanthanide complexes, therefore suggesting that the co-
ordination of the Ln ion does not have a significant
3
þ
1
influence on the π-π* transition. However, a small blue
shift that is discernible in the absorption maximum of
all three complexes is attributable to the perturbation
(31) Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; Vander Tol, E. B.;
Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408–9414.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2413
9
c,33,34
“hypersensitive” transition is intense. The intensity
of the D f F transition (electric dipole) is greater than
5
7
0
2
5
that of the D f F transition (magnetic dipole), which
7
0
1
3
þ
indicates that the coordination environment of the Eu
ion is devoid of an inversion center. Furthermore, an
intense broad band in the region 400-500 nm due to a
π-π* transition of the ligand was evident in the emission
spectrum of 2. Such an observation is typically diagnostic
3
þ
of poor sensitization of the ligand toward the Eu ion.
3
þ
The luminescent lifetimes of Ln complexes 1 and 2
were measured at both ambient (298 K) and low tem-
peratures (77 K) on the basis of the respective luminescent
decay profiles by fitting them with monoexponential
decay curves (Figures S7 and S8 in the Supporting
Information). Collectively, these data imply the existence
3
þ
of a single chemical environment around the Ln ion
in each case. The pertinent values are summarized in
5
Table 3. A longer D lifetime value (τ
= 1.02 ms)
obs
4
3
þ
Figure 6. Room-temperature excitation (inset) and emission spectra
for complex 2 (λex = 308 nm) with emission monitored at approximately
was observed in the case of the Tb complex 1 despite the
presence of solvent molecules in the first coordination
sphere, since such molecules are typically vibrational
612 nm.
3
þ
aromatic carboxylate ligand. The absence of any absorp-
deactivators of the excited states of Ln ions. It is
life-
4
3
þ
5
somewhat surprising that the magnitude of the D
tion bands due to the f-f transitions of the Tb cation
proves that luminescence sensitization via excitation of
the ligand is effective. The room-temperature emission
spectrum of complex 1 exhibits the characteristic emission
time for complex 1 is not very high compared with some
recently reported data for highly luminescent Tb com-
3
þ
3
5
plexes (Φoverall = 56%; τobs = 2.63 ms). On the other
hand, there is a recent report of a Φoverall value of 40%
3
þ
bands of the Tb cation (λex = 334 nm) centered at 488,
45, 585, and 620 nm which result from deactivation of
the D excited state to the corresponding F ground
3
þ
5
and τobs = 0.46 ms for Tb -dipivaloylmethanato com-
plexes. Furthermore, the present authors also observed
5
7
21
4
J
3
þ
11,30b,32
3þ
state of the Tb cation (J = 6, 5, 4, 3).
Interest-
a Φoverall value of 72% and τobs = 0.92 ms for a Tb
4-isobutyryl-3-phenyl-5-isoxazolonate complex.
-
The
3
2a
ingly, no emission bands from the organic ligand were
observed, which leads to the conclusion that energy
lifetime is the inverse of the total deactivation rate, which
in turn is the sum of the radiative and nonradiative rates.
Therefore, the fact that a lanthanide complex is highly
luminescent yet possesses a short lifetime does not repre-
sent a contradiction. It simply means that the radiative
3
þ
center is very
transfer from the ligand to the Tb
efficient.
The solid-state excitation and emission spectra for
3
þ
the Eu complex 2 at room temperature are shown in
3
þ
3þ
Figure 6. The excitation spectrum of the Eu complex of
HL has negligible contributions from the ligand and
exhibits a series of sharp lines that are characteristic of
rate is rapid, as observed in the case of Tb complex 1.
Moreover, on account of its electronic structure, the Tb
3
þ
cation has many energy levels that can mix with appro-
priate ligand wave functions, including a relatively low-
lying 4f5d state, which may explain why the lifetime is
relatively short (i.e., it implies that the phosphorescence
3
þ
the Eu energy-level structure and can therefore be
assigned to transitions between the F and L and
7
5
0
,1
6
5
9c,31,33
D2,1 levels. Accordingly, luminescence sensitiza-
tion of the Eu complex via ligand excitation is not
efficient in this case. The ambient-temperature emission
spectrum of the Eu complex is characteristic of the
metal in the 550-700 nm region and exhibits well-
resolved peaks that are attributable to transitions from
3
þ
character of the transition is partly lost). The somewhat
lifetime value (τobs = 0.33 ms) that was
5
shorter D
observed for the Eu
0
3
þ
3þ
analogue may be due to the
dominant nonradiative decay channels associated with
vibronic coupling due to the presence of solvent mole-
cules. Similar observations have been made for several
5
7
the metal-centered D excited state to the F ground-
0
J
9
c,36
state multiplet. Maximum peak intensities at 580, 593,
17, 652, and 694 nm were observed for the J = 0, 1, 2, 3,
and 4 transitions, respectively, and the J = 2 so-called
europium complexes.
6
In order to gain a better understanding of the lumines-
cence efficiencies of the new 4-(dibenzylamino)benzoate
complexes 1 and 2, it was appropriate to calculate the
overall quantum yields. The overall quantum yield
(
32) (a) Biju, S.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan, K. V. J.
Mater. Chem. 2009, 19, 5179–5187. (b) Xia, J.; Zhao, B.; Wang, H.-S.; Shi, W.;
Ma, Y.; Song, H.-B.; Cheng, P.; Liao, D.-Z.; Yan, S.-P. Inorg. Chem. 2007, 46,
(34) (a) Ambili Raj, D. B.; Biju, S.; Reddy, M. L. P. Dalton Trans. 2009,
36, 7519–7528. (b) Fu, L.; Sa Ferreira, R. A.; Silva, N. J. O.; Fernandes, A. J.;
Ribeiro-Carlo, P.; Gonalves, I. S.; de Zea Bermudez, V.; Carlos, L. D. J. Mater.
Chem. 2005, 15, 3117–3125. (c) Pavithran, R.; Saleesh Kumar, N. S.; Biju, S.;
Reddy, M. L. P.; Alves, S., Jr.; Freire, R. O. Inorg. Chem. 2006, 45, 2184–2192.
3
450–3458. (c) Carnall, W. T. In Handbook on the Physics and Chemistry of
Rare Earths; Gschneidner, K. A., Eyring, L., Eds.; Elsevier: Amsterdam, The
Netherlands, 1987; Vol. 3, Chapter 24, pp 171-208. (d) Dieke, G. H. Spectra and
Energy levels of Rare Earth Ions in Crystals; Interscience: New York, 1968.
(
33) (a) Pavithran, R.; Reddy, M. L. P.; Junior, S. A.; Freire, R. O.;
(d) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. Phys. Chem. Chem. Phys.
Rocha, G. B.; Lima, P. P. Eur. J. Inorg. Chem. 2005, 20, 4129–4137. (b)
Zucchi, G.; Olivier, M.; Pierre, T.; Gumy, F.; B €u nzli, J.-C. G.; Michel, E. Chem.;
Eur. J. 2009, 15, 9686–9696. (c) Kajiwara, T.; Hasegawa, M.; Ishii, A.; Katagiri,
K.; Baatar, M.; Takaishi, S.; Iki, N.; Yamashita, M. Eur. J. Inorg. Chem. 2008, 36,
2
002, 4, 1542–1548.
(35) Samuel, A. P. S.; Moore, E. G.; Melchior, M.; Xu, J.; Raymond,
K. N. Inorg. Chem. 2008, 47, 7535–7544.
(36) Ambili Raj, D. B.; Biju, S.; Reddy, M. L. P. Inorg. Chem. 2008, 47,
8091–8100.
5
565–5568.

2
414 Inorganic Chemistry, Vol. 49, No. 5, 2010
Ramya et al.
5
5
Table 3. Radiative (ARAD) and Nonradiative (ANR) Decay Rates, D
0
/ D
4
Lifetimes (τobs), Radiative Lifetimes (τRAD), Intrinsic Quantum Yields (ΦLn), Energy Transfer
Efficiencies (Φsen), and Overall Quantum Yields (Φoverall) for Complexes 1 and 2
-1
-1
NR/s
complex
A
RAD/s
A
τ
obs/μs
τRAD/μs
ΦLn (%)
Φsen(%)
Φoverall (%)
a
1
1020 ( 0.8
330 ( 1
1040 ( 0.8
4720 ( 1
98
84
8
82 ( 8
85 ( 8
<0.01
b
7
0.14
a
2
212
2812
7.0
a
b
Absolute quantum yield. Relative quantum yield.
(
“
Φoverall) for a lanthanide complex treats the system as a
black box” in which the internal process is not consid-
ered explicitly. Given that the complex absorbs a photon
amino)benzoate complex 1. This large value is particu-
larly surprising in view of the presence of four H O
2
molecules in the first coordination sphere, since such
solvent molecules typically serve as vibrational deactiva-
(i.e., the antenna is excited), the overall quantum yield can
be defined as follows:
3
7
3þ
3þ
tors of the excited states of Ln ions. Only a few Tb
complexes have been reported to exhibit higher quantum
yields in the solid state.
1
7,33b,33c
Φoverall ¼ ΦsenΦln
Here, Φsen represents the efficiency of the energy
ð2Þ
Recently, the present
authors reported another case of a high quantum yield
(72%) for a 4-isobutyryl-3-phenyl-5-isoxazolonate com-
plex of Tb that also features solvent molecules in the
3
þ
3
þ
transfer from the ligand to the Ln ion and Φ repre-
Ln
3
sents the intrinsic quantum yield of the Ln ion, which
þ
3
2a
primary coordination sphere. The energy gap between
the luminescent state and the ground state manifold is
approximately 12 000 cm for Eu and 14 800 cm for
can be calculated from the following equation:
-
1
3þ
-1
ꢀ
In the case of the Eu complex 2, the radiative lifetime
ꢁ
3
þ
3þ
ARAD
τobs
Tb . Relatively efficient coupling of the Eu excited
states occurs to the third vibrational overtone of proxi-
φln
¼
¼
ð3Þ
ARAD þ ANR
τRAD
-
1
mate OH oscillators (νOH ∼ 3300-3500 cm ), and to
3
þ
3þ
the fourth harmonic in the case of Tb , which is con-
sistent with the observation of less efficient quenching
1
5,7d
(
energy of the D f F transition (MD) and its oscillator
strength are constant.
τRAD) can be calculated using eq 4,
assuming that the
5
7
3
þ
0
1
for Tb , when the Franck-Condon overlap factor is less
40
favorable.
It is well recognized that the energy-level match bet-
ween the triplet states of the ligands to the D state of the
ꢀ
ꢁ
5
1
ITOT
IMD
3
J
ARAD
¼
¼ AMD,0
n
ð4Þ
3þ
Ln cation is one of the key factors that governs the
τRAD
3
þ
41
luminescence efficiency of Ln
empirical rule states that an optimal ligand-to-metal
complexes. Latva’s
-
1
Hence, AMD,0 (14.65 s ) represents the spontaneous
5
7
emission probability of the D f F transition in vacuo,
I
Eu emission spectrum to the area of the D f F
band, and n is the refractive index of the medium. An
average index of refraction equal to 1.5 was employed in
3þ
3þ
3
0
1
energy transfer process for Ln requires ΔE ( ππ* -
5
-1
-1
/IMD is the ratio of the total area of the corrected
D ) = 2500-4500 cm for Tb and 2500-4000 cm
TOT
3
4
þ
5
7
3þ 42
0
1
for Eu
energy transfer to the Tb ion will be more effective
.
On this basis, it can be concluded that the
3
þ
for the ligand 4-(dibenzylamino)benzoic acid, since ΔE
3
4c
3þ
3
5
-1
the calculation. The intrinsic quantum yield for Tb
Φ ) was estimated by means of eq 5 with the assumption
that the decay process at 77 K in a deuterated solvent is
( ππ* - D ) for 1 is 3140 cm . The very high intrinsic
4
(
quantum yield (Φ ) and energy transfer efficiency (Φsen)
Tb
Ln
for complex 1 further confirms the view that 4-(dibenzyl-
amino)benzoic acid is an efficient sensitizer for the Tb
1
8,32a,38
3þ
purely radiative.
ion. Poor luminescence efficiency (Φoverall < 0.01) is also
apparent in complex 2 and may be due to the weak
sensitization efficiency (Φsen = 0.14%) of 4-(dibenzyl-
τ
obsð298 KÞ
φTb ¼ τ
ð5Þ
RADð77 KÞ
3
þ
amino)benzoic acid with respect to the Eu ion. The
latter observation can be explained on the basis of the
large energy gap between the triplet state of the ligand
The overall quantum yields (Φoverall), radiative (ARAD
)
and nonradiative (ANR) decay rates, intrinsic quantum
-
1
5
3þ
(6340 cm ) and the D emitting level of the Eu ion
0
(
yields (Φ ), and energy transfer efficiencies (Φsen) for
Ln
-
1
complexes 1 and 2 are presented in Table 3. In the solid
state, the overall quantum yields for these complexes
were determined according to the absolute method of
A remarkably high quantum yield
value of 82% has been observed for the Tb-4-(dibenzyl-
17 300 cm ).
Conclusions
3
9,19a
Wrighton et al.
3þ
A novel efficient antenna complex of Tb supported by
the 4-(dibenzylamino)benzoic acid ligand has been designed,
(
37) (a) Xiao, M.; Selvin, P. R. J. Am. Chem. Soc. 2001, 123, 7067–7073.
(40) (a) Dossing, A. Eur. J. Inorg. Chem. 2005, 8, 1425–1434. (b) Beeby, A.;
Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.; Royle, L.; de Sousa, A. S.;
Williams, J. A. G.; Woods, M. J. Chem. Soc., Perkin Trans. 2 1999, 3, 493–503.
(41) (a) Shi, M.; Li, F.; Yi, T.; Zhang, D.; Hu, H.; Huang, C. Inorg. Chem.
2005, 44, 8929–8936. (b) Xin, H.; Shi, M.; Gao, X. C.; Huang, Y. Y.; Gong, Z. L.;
Nie, D. B.; Cao, H.; Bian, Z. Q.; Li, F.; Huang, C. H. J. Phys. Chem. B. 2004, 108,
10796–10800.
(
b) Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.; Armaroli, N.; Ventura, B.;
Barigelletti, F. Inorg. Chem. 2005, 44, 529–537. (c) Comby, S.; Imbert, D.; Anne-
Sophie, C.; B €u nzli, J.-C. G.; Charbonniere, L. J.; Ziessel, R. F. Inorg. Chem.
2
1
2
004, 43, 7369–7379.
38) Nasso, I.; Bedel, S.; Galaup, C.; Picard, C. Eur. J. Inorg. Chem. 2008,
2, 2064–2074.
39) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974, 78,
229–2233.
(
(
(42) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; Rodriguez-
Ubis, J. C.; Kanakare, J. J. Lumin. 1997, 75, 149–169.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2415
synthesized, and characterized and its photophysical pro-
perties evaluated. The new aromatic carboxylate complex of
Tb exhibits bright green luminescence efficiency in the solid
state with a quantum yield of 82%, thus rendering it an
excellent candidate for use in various photonic applications.
(SR/S1/IC- 36/2007) and the Council of Scientific and
Industrial Research, New Delhi (NWP0010). A.R.R.
thanks CSIR, New Delhi for the award of Junior
Research Fellowship. A.H.C. thanks the Robert A.
Welch Foundation (F-0003) for financial support.
3þ
The high quantum yield is attributable to the superior match
5
of the triplet energy level of the ligand with that of the D
4
Supporting Information Available: X-ray crystallographic
3þ
emitting level of Tb . By contrast, the poor sensitization
1
13
data of complexes 1 and 2 in CIF format, H and C NMR
spectra for ligand HL, thermogravimetric data, view of com-
plexes 1 and 2 showing the intermolecular hydrogen bonding
interactions, reflectance spectra of complexes 1 and 2, phos-
phorescence spectrum of gadolinium complex 3 at 77 K, and
luminescence decay profiles. This material is available free of
charge via the Internet at http://pubs.acs.org.
3þ
observed for the corresponding Eu complex is mainly due
to the larger energy gap between the ligand triplet state and
5
3þ
.
D level of Eu
0
Acknowledgment. The authors acknowledge financial
support from the Department of Science and Technology