Lanthanide-based coordination polymers assembled from derivatives of 3,5-dihydroxy benzoates: Syntheses, crystal structures, and photophysical properties

Article abstract of DOI:10.1021/ic2001249

Two new aromatic carboxylic acids, namely, 3,5-bis(benzyloxy)- benzoic acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), have been prepared by replacing the hydroxyl hydrogens of 3,5-dihydroxy benzoic acid with benzyl and pyridyl moieties, respectively. The anions derived fromHL1 and HL2 have been used for the support of a series of lanthanide coordination compounds [Eu ³⁺ = 1-2; Tb³⁺ = 3-4; Gd³⁺ = 5-6]. The new lanthanide complexes have been characterized on the basis of a variety of spectroscopic techniques in conjunction with an assessment of their photophysical properties. Lanthanide complexes 2, 4, and 6, which were synthesized from 3,5-bis(pyridine- 2-ylmethoxy)benzoic acid, were structurally authenticated by single-crystal X-ray diffraction. All three complexes were found to exist as infinite one-dimensional (1-D) coordination polymers with the general formula {[Ln(L2)₃- (H₂O)₂] xH₂O}_n Scrutiny of the packing diagrams for 2, 4, and 6 revealed the existence of interesting two-dimensional molecular arrays held together by intermolecular hydrogen-bonding interactions. Furthermore, the coordinated benzoate ligands serve as efficient light harvesting chromophores. In the cases of 1-4, the lowest energy maxima fall in the range 280-340 nm [molar absorption coefficient (ε) = (0.39-1.01) × 10⁴ M^-1 cm^-1]. Moreover, the Tb³+ complexes 3 and 4 exhibit bright green luminescence efficiencies in the solid state (φ_overall = 60% for 3; 27% for 4) and possess longer excited state lifetimes than the other complexes (t = 1.16 ms for 3; 1.38 ms for 4). In contrast to the foregoing, the Eu³⁺ complexes 1 and 2 feature poor luminescence efficiencies.

Full text of DOI:10.1021/ic2001249

ARTICLE
pubs.acs.org/IC
Lanthanide-Based Coordination Polymers Assembled from
Derivatives of 3,5-Dihydroxy Benzoates: Syntheses,
Crystal Structures, and Photophysical Properties
Sarika Sivakumar,† M. L. P. Reddy,*,† Alan H. Cowley,‡ and Rachel R. Butorac‡
†Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science & Technology (NIIST),
CSIR, Thiruvananthapuram-695 019, India
‡Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300,
Austin, Texas 78712, United States
S Supporting Information
b
ABSTRACT: Two new aromatic carboxylic acids, namely, 3,5-bis(benzyloxy)-
benzoic acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), have
been prepared by replacing the hydroxyl hydrogens of 3,5-dihydroxy benzoic
acid with benzyl and pyridyl moieties, respectively. The anions derived from HL1
and HL2 have been used for the support of a series of lanthanide coordination
compounds [Eu3þ = 1ꢀ2; Tb3þ = 3ꢀ4; Gd3þ = 5ꢀ6]. The new lanthanide
complexes have been characterized on the basis of a variety of spectroscopic
techniques in conjunction with an assessment of their photophysical properties.
Lanthanide complexes 2, 4, and 6, which were synthesized from 3,5-bis(pyridine-
2-ylmethoxy)benzoic acid, were structurally authenticated by single-crystal X-ray
diffraction. All three complexes were found to exist as infinite one-dimensional
(1-D) coordination polymers with the general formula {[Ln(L2)3-
(H2O)2] xH2O}n. Scrutiny of the packing diagrams for 2, 4, and 6 revealed
3
the existence of interesting two-dimensional molecular arrays held together by intermolecular hydrogen-bonding interactions.
Furthermore, the coordinated benzoate ligands serve as efficient light harvesting chromophores. In the cases of 1ꢀ4, the lowest
energy maxima fall in the range 280ꢀ340 nm [molar absorption coefficient (ε) = (0.39ꢀ1.01) ꢁ 104 Mꢀ1 cmꢀ1]. Moreover, the
Tb3þ complexes 3 and 4 exhibit bright green luminescence efficiencies in the solid state (Φoverall = 60% for 3; 27% for 4) and possess
longer excited state lifetimes than the other complexes (τ = 1.16 ms for 3; 1.38 ms for 4). In contrast to the foregoing, the Eu3þ
complexes 1 and 2 feature poor luminescence efficiencies.
’ INTRODUCTION
sought after. As a consequence, a number of chromophoric antenna
ligands have now been developed in an effort to achieve brighter
Ln3þ luminescence. In this regard, the β-diketonate4 and aromatic
carboxylate ligands5 have received the most attention.
Luminescent lanthanide coordination complexes display un-
ique line-like emission bands, exhibit substantial Stokes shifts,
possess very long luminescence lifetimes, and emit over the visible
and near-IR (NIR) spectral domains. Depending on the particular
lanthanide, complexes of this type offer numerous advantages in
comparison with fluorescent organic compounds and other types of
luminescent materials.1 Furthermore, the fascinating photophysical
properties of lanthanide complexes render them appropriate for a
host of applications such as display devices, solid state lighting
(including OLEDs), and sensors.2 However, the spin- and parity-
forbidden nature of the f-f transitions render direct photoexcitation
of lanthanide ions disfavored. Accordingly, it is necessary to employ
organic ligands to function as antennas by absorbing light and
transferring this energy to the excited states of the central lanthanide
ions.3 The excited lanthanide ions then undergo radiative transitions
to lower energy states which results in the characteristic multiple
narrow band emissions. Since the emission intensity (brightness)
and the color of the lanthanide emission both depend on the type of
sensitizer employed, new sensitizing chromophores are highly
Carboxylate anions are hard Lewis bases and are known to
bind strongly to Ln3þ cations which possess a pronounced hard
Lewis acid character.6 Accordingly, lanthanide benzoates and
their derivatives are stable and have attracted considerable
attention for their potential use in a wide variety of fields on
account of their novel luminescent and magnetic properties.7ꢀ9
In particular, when modified with a light-harvesting groups,
lanthanide benzoates have proved to be satisfactory lumines-
cence sensitizers. In our recent work, it was demonstrated that
replacement of the hydrogens of the ꢀNH2 moiety of amino
benzoates by benzyl groups had a significant influence on the
distribution of π-electron density within the ligand system and
resulted in the development of a novel solid state photosensitizer
Received: January 19, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Table 1. Crystallographic and Refinement Data for 2, 4, and 6
2
4
6
formula
C
57H50EuN6O14.50
C57H53TbN6O16
1236.97
monoclinic
C2/c
C57H51GdN6O15
1217.44
monoclinic
C2/c
fw
1202.99
monoclinic
C2/c
cryst sys
space group
cryst size (mm3)
temp/K
0.12 ꢁ 0.11 ꢁ 0.06
100(2)
36.843(3)
15.348(9)
9.890(6)
90
0.27ꢁ 0.07 ꢁ 0.06
293(2)
36.52(3)
15.555(14)
9.887(9)
90
0.29ꢁ 0.04 ꢁ 0.04
100(2)
a/(Å)
37.109(4)
15.3430(14)
9.8617(9)
90
b/(Å)
c/(Å)
R (deg)
β (deg)
101.133(9)
90
102.96(3)
90
99.711(4)
90
γ (deg)
V/Å3
5487(5)
4
5473(8)
4
5534.4(9)
4
Z
Dcalcd, g cmꢀ1
μ/(Mo, KR) mmꢀ1
F(000)
1.456
1.501
1.461
1.216
1.369
1.271
2452
2520
2476
R1 [I > 2σ(I)]
wR2[I > 2σ(I)]
R1(all data)
wR2(all data)
GOF
0.1022
0.2479
0.1072
0.2515
1.715
0.0991
0.0721
0.2457
0.1807
0.1050
0.0918
0.2518
0.1915
1.139
1.259
for Tb3þ with an overall quantum yield of 82%.10 It was also
shown that the presence of an electron-donating methoxy group
on position 3 of the 4-benzyloxy benzoic acid ligand resulted in a
significant improvement in the photoluminescence efficiency
(Φoverall = 33%). For example, the Tb3þ-3-methoxy-4-benzyloxy
benzoate complex has a Φoverall value of 33% while that of the
electron-withdrawing nitro-substituted Tb3þ-3-nitro-4-benzyl-
oxy benzoate complex is only 0.1%.5a It has also been well
documented that thiophenyl-substituted nitrobenzoic acid li-
benzoic acid, 98% (Alfa Aesar), 2-bromomethyl pyridine hydrobromide,
98% (Aldrich), and benzyl bromide, 99.9% (Aldrich). All 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 spectro-
meter using KBr (neat) was used to obtain the IR spectral data, and a Bruker
500 MHz NMR spectrometer was employed to record the 1H NMR and
13C NMR spectra of the ligands in DMSO-d6 or D2O solution. All mass
spectra were recorded on a JEOL JSM 600 fast atom bombardment (FAB)
high resolution mass spectrometer (FAB-MS), and the thermogravimetric
analyses were performed on a TG/DTA-6200 instrument (SII Nano
Technology Inc., Japan). X-ray diffraction (XRD) powder patterns were
recorded in the 2θ range of 10ꢀ70° using CuꢀKR radiation (Philips
X0pert). The absorbances of the ligands and complexes were measured in
CH3OH solution on a UVꢀvis spectrophotometer (Shimadzu, UV-2450),
and the photoluminescence (PL) spectra were recorded on a Spex-
Fluorolog FL22 spectrofluorimeter equipped with a double grating 0.22
m Spex 1680 monochromator and a 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 by both absolute and relative
methods as described elsewhere.10ꢀ14
The X-ray diffraction data were collected on a Rigaku AFC-12 Saturn
724þ CCD diffractometer equipped with a graphite-monochromated
Mo KR radiation source (λ = 0.71073 Å) and a Rigaku XStream low
temperature device cooled to 100 K. Corrections were applied for
Lorentz and polarization effects. The structure was solved by direct
methods and refined by full-matrix least-squares cycles on F2 using the
Siemens SHELXTL PLUS 5.0 (PC) software package15a and
PLATON.15b All non-hydrogen atoms were refined anisotropically,
and the hydrogen atoms were placed in fixed, calculated positions using
a riding model. Selected crystal data and data 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/retriev-
ing.html (or from CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.;
gands are efficient sensitizers for both Eu3þ and Tb3þ 6b
.
Inspired by the efficient sensitization of various lanthanide
benzoates, we have now designed and synthesized two new
members of this ligand class, namely, 3,5-bis(benzyloxy)benzoic
acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2).
To enhance the antenna effects of the new benzoic acid ligands, the
hydrogen atoms of 3,5-dihydroxy benzoic acid have been replaced
by benzyl and pyridyl groups, respectively, and employed subse-
quently for the support of a series of new lanthanide complexes
featuring Eu3þ, Gd3þ, or Tb3þ cations. The 3,5-bis(pyridine-2-
ylmethoxy)benzoate complexes of Eu3þ (2), Gd3þ (4), and Tb3þ
(6) were structurally authenticated by single-crystal X-ray diffrac-
tion. Unfortunately, it was not possible to grow suitable single
crystals of the corresponding 3,5-bis(benzyloxy)benzoate com-
plexes 1, 3, and 5. However, these complexes were characterized
satisfactorily on the basis of elemental analysis and FT-IR spectros-
copy. The photophysical properties of all six new lanthanide
benzoates have been explored and correlated with the electronic
states of the newly designed ligands.
’ EXPERIMENTAL SECTION
Materials and Instrumentation. The following chemicals were
procured commercially and used without subsequent purification:
terbium(III) nitrate hexahydrate, 99.9% (Across Organics), 3,5-dihydroxy
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Inorganic Chemistry
ARTICLE
Scheme 1. Synthetic Procedures for Ligands HL1 and HL2
fax þ44 1223 336033; e-mail deposit@ccdc.cam.ac.uk). The CCDC
numbers are 789677, 789676, and 781131 for 2, 4, and 6, respectively.
Two of the pyridine moieties in complexes 2, 4, and 6 were found to be
disordered by rotation about the oxygenꢀcarbon bond. As a result, a
series of constrained CꢀN and CꢀC bond lengths were employed to
model the disorder along with bond angle restraints. The site occupancy
factor of C14, C15, C16, C17, C18, C19, and N2 is refined to 63(2) % for
2, 4, and 6 and of C25, C26, C27, C28, C29, C30, and N2 is refined to
54(2) % for 2 and 6 and 62(2) % for 4.
128.45, 127.90, 127.63, 107.96, 106.50, 69.45. m/z = 335.46 (MþH)þ.
Elemental analysis (%): Calcd for C21H18O4 (334.12): C, 75.43; H, 5.43.
Found: C, 74.75; H, 5.29. FT-IR (KBr) νmax: 3038, 1693, 1596, 1446,
1422, 1378, 1301, 1167, 1030, 932, 852, 738, 695 cmꢀ1
.
Synthesis of 3,5-Bis(pyridine-2-ylmethoxy)benzoic Acid
(HL2). (a). Methyl 3,5-Bis(pyridin-2-ylmethoxy)benzoate. Methyl 3,5-
dihydroxy benzoate (1.0 g, 6 mmol) was dissolved in 50 mL of freshly
distilled acetonitrile and subsequently refluxed for 30 min with potassium
carbonate (4.1 g, 30 mmol). The resulting reaction mixture was further
refluxed at 80 °C for 48 h with 2-bromomethyl pyridine hydrobromide
(3.0 g, 12 mmol). The excess acetonitrile was evaporated off, and
the reaction mixture was poured into ice cold water. Methyl 3,5-bis-
(pyridin-2-ylmethoxy)benzoate was obtained as a brown precipitate.
Yield, 0.85 g (43%): 1H NMR (500 MHz, acetone-d6): δ (ppm)
8.58ꢀ8.59 (d, 2H, J = 5 Hz), 7.81ꢀ7.85 (m, 2H), 7.56ꢀ7.58 (d, 2H,
J = 8 Hz), 7.31ꢀ7.34 (m, 2H), 7.27ꢀ7.28 (d, 2H, J = 2.50 Hz), 7.00ꢀ7.01
(t, 1H, J = 5 Hz), 5.25 (s, 4H), 3.87 (s, 3H). 13C NMR (125 MHz,
CDCl3): δ (ppm) 166.59, 159.47, 156.59, 149.31, 136.90, 132.25, 122.79,
121.36, 108.72, 106.99, 70.86, 52.30 m/z = 350.93 (MþH)þ. FT-IR
(KBr) νmax: 3401, 1723, 1590, 1436, 1354, 1305, 1249, 1173, 1063, 840,
Synthesis of 3,5-Bis(benzyloxy)benzoic Acid (HL1). (a).
Synthesis of Methyl 3,5-Dihydroxybenzoate. A methanolic solution of
3,5-dihydroxy benzoic acid (2.0 g, 13 mmol) was refluxed overnight in the
presence of catalytic amounts of conc. H2SO4. The excess methanol was
evaporated off, and the reaction mixture was poured into ice cold water.
The resulting precipitate was filtered off, washed with water, and dried.
Yield, 1.3 g (60%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.09 (s, 2H),
6.65 (s, 1H), 3.88 (s, 3H) . 13C NMR (125 MHz, CDCl3): δ (ppm)
167.24, 159.44, 133.07, 108.58, 107.90, 52.26. m/z = 169.14 (MþH)þ.
FT-IR (KBr) νmax: 3027, 1715, 1597, 1439, 1298, 1167, 1046, 764 cmꢀ1
.
(b). Synthesis of Methyl 3,5-Bis(benzyloxy)benzoate. Methyl 3,5-
dihydroxy benzoate (2.0 g, 12 mmol) was dissolved in 50 mL of freshly
distilled acetonitrile and refluxed with potassium carbonate (8.0 g, 59
mmol) for 30 min. The resulting reaction mixture was further refluxed at
70 °C for 48 h following the addition of benzyl bromide (4.0 g,
24 mmol). The excess acetonitrile was then evaporated off, and the
residual mixture was poured into ice cold water. Methyl 3,5-bis-
(benzyloxy)benzoate was obtained as a white precipitate. Yield, 1.86 g
(45%). 1H NMR (500 MHz): δ (ppm) 7.43ꢀ7.37 (m, 8H), 7.35ꢀ7.32
(t, 2H, J = 7 Hz), 7.30ꢀ7.29 (d, 2H, J = 2 Hz), 6.81ꢀ6.80 (t, 1H, J = 2
Hz), 5.07 (s, 4H), 3.90 (s, 3H). 13C NMR (125 MHz, CDCl3): δ (ppm)
166.79, 159.81, 136.49, 132.08,128.65, 128.14, 127.59, 108.41, 107.28,
70.30, 52.29 . m/z = 349.40 (MþH)þ. FT-IR (KBr) νmax: 3406, 2947,
763 cmꢀ1
.
(b). 3,5-Bis(pyridine-2-ylmethoxy)benzoic Acid (HL2). Methyl 3,5-
bis(pyridin-2-ylmethoxy) benzoate (1.0 g, 3 mmol) was heated to reflux
for 24 h in 50 mL of a 6 M HCl solution. The solution was concentrated
to a volume of approximately 5 mL and stored at 5 °C. The resulting
brown solid was filtered, dried, and recrystallized from CH2Cl2 to yield
HL2 3HCl. Yield, 0.95 g (75%): 1H NMR (500 MHz, D2O): δ (ppm)
3
8.68ꢀ8.66 (d, 2H, J = 5.50 Hz), 8.49ꢀ8.51 (t, 2H, J = 7.75 Hz),
8.01ꢀ8.02 (d, 2H, J = 7.5 Hz), 7.91ꢀ7.94 (t, 2H, J = 6.75 Hz), 7.24
(s, 2H), 6.97 (s, 1H), 5.45 (s, 4H). 13C NMR (125 MHz, DMSO-d6):
δ (ppm) 166.42, 158.52, 152.29, 144.07, 143.58, 133.18, 125.39, 124.46,
108.69, 106.73, 66.99. m/z = 337.25 (MþH)þ Elemental analysis (%):
Calcd for C19H19Cl3N2O4 (445.72): C, 51.20; H, 4.30; N, 6.28. Found:
C, 51.57; H, 4.34; N, 6.20. FT-IR (KBr) νmax: 3376, 1705, 1613, 1593,
2857, 1714, 1597, 1439, 1355, 1298, 1166, 841, 763 cmꢀ1
.
Synthesis of 3,5-Bis(benzyloxy)benzoic Acid (HL1). 3,5-Bis-
(benzyloxy)benzoate (2.0 g, 5.74 mmol) was refluxed for 24 h in the
presence of KOH (0.97 g, 17.24 mmol) in 50 mL of ethanol. The reaction
mixture was poured into ice cold water, acidified with dilute HCl, and the
resulting precipitate was filtered, washed, dried, and recrystallized from
ethanol. Yield, 1.80 g (94%). 1H NMR (500 MHz): δ (ppm) 7.45ꢀ7.46
(d, 4H, J = 7 Hz), 7.39ꢀ7.41 (t, 4H, J = 7 Hz), 7.32ꢀ7.35 (t, 2H, J = 7
Hz), 7.16 (d, 2H, J = 2.5 Hz), 6.92ꢀ6.93 (t, 1H, J = 2.5 Hz), 5.15 (s, 4H).
13C NMR (125 MHz, DMSO): δ (ppm) 166.88, 159.35, 136.64, 132.79,
1430, 1328, 1178, 1070, 759 cmꢀ1
.
Syntheses of Lanthanide Complexes. In a typical procedure, an
ethanolic solution of Ln(NO3)3 6H2O (0.5 mmol) (Ln = Eu, Gd, or Tb)
3
was added to a solution of the appropriate carboxylic acid (1.5 mmol) in
ethanol in the presence of NaOH (1.5 mmol). Precipitation took place
immediately, and each reaction mixture was stirred subsequently for 10 h at
room temperature. The crude products were filtered, washed with
ethanol, and dried. The resulting complexes were purified subsequently
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Inorganic Chemistry
ARTICLE
by recrystallization from ethanol. Single crystals of the lanthanide-3,5-
bis(pyridine-2-ylmethoxy)benzoate complexes 2, 4, and 6 suitable for
X-ray study were obtained from a dimethylsulfoxide/methanol solvent
mixture after storage for 5 weeks at ambient temperature. However, efforts
to grow single crystals of the lanthanide-3,5-bis(benzyloxy)benzoate com-
plexes were not successful.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 2, 4, and 6
2
4
6
Eu1ꢀEu1
Eu1ꢀO6
Eu1ꢀO1
Eu1ꢀO7
Eu1ꢀO4
4.957
Tb1ꢀTb1
4.956
Gd1ꢀGd1
Gd1ꢀO1
Gd1ꢀO4
Gd1ꢀO6
Gd1ꢀO7
4.942
2.362(6) Tb1ꢀO1
2.371(7) Tb1ꢀO4
2.485(6) Tb1ꢀO6
2.492(6) Tb1ꢀO7
2.345(6)
2.505(6)
2.313(5)
2.470(6)
2.354(4)
2.498(4)
2.341(4)
2.485(4)
Eu(L1)3(C2H5OH)(H2O) (1). Elemental analysis (%): Calcd for
C65H59O14Eu (1216.12): C, 64.20; H, 4.89. Found: C, 64.65; H, 4.85.
FT-IR (KBr) νmax: 3417, 1599, 1532, 1412,1378, 1215, 1156, 1054, 785 cmꢀ1
.
[Eu(L2)3(H2O)2] 0.5H2O (2). Elemental analysis (%): Calcd for
3
C57H50N6O14.50Eu (1203): C, 56.91; H, 4.19; N, 6.99. Found: C,
O6ꢀEu1ꢀO6 144.0(3) O6ꢀTb1ꢀO6 145.6(3)
O6ꢀEu1ꢀO1 103.4(2) O6ꢀTb1ꢀO1 85.3(2)
O1ꢀEu1ꢀO1 154.1(3) O1ꢀTb1ꢀO1 154.9(3)
O6ꢀEu1ꢀO7 71.7(2) O6ꢀTb1ꢀO7 71.7(2)
O1ꢀEu1ꢀO7 79.0(2) O1ꢀTb1ꢀO7 80.0(2)
O7ꢀEu1ꢀO7 73.3(3) O7ꢀTb1ꢀO7 71.9(3)
O6ꢀEu1ꢀO4 70.5(2) O6ꢀTb1ꢀO4 70.9(2)
O1ꢀEu1ꢀO4 77.7(2) O1ꢀTb1ꢀO4 76.9(2)
O6ꢀGd1ꢀO6 144.3(2)
O6ꢀGd1ꢀO1 85.4(15)
O1ꢀGd1ꢀO1 153.1(2)
O6ꢀGd1ꢀO7 71.5(15)
O1ꢀGd1ꢀO7 80.3(16)
O7ꢀGd1ꢀO7 73.3(2)
O6ꢀGd1ꢀO4 70.6(15)
O1ꢀGd1ꢀO4 78.2(15)
56.72; H, 4.47; N, 6.81. FT-IR (KBr) νmax: 3367, 1594, 1538,
1377,1414, 1287, 1161, 1063, 787 cmꢀ1
.
Tb(L1)3(C2H5OH)(H2O) (3). Elemental analysis (%): Calcd for
C65H59O14Tb (1223.08): C, 63.83; H, 4.86. Found: C, 63.61; H, 4.78 FT-
IR (KBr) νmax: 3373, 1597, 1538, 1416, 1378, 1212, 1157, 1054, 786 cmꢀ1
.
[Tb(L2)3(H2O)2] 2H2O (4). Elemental analysis (%): Calcd for
3
C57H53N6O16Tb (1236.97): C, 55.34; H, 4.32; N, 6.79. Found: C,
55.44; H, 4.36; N, 6.67. FT-IR (KBr) νmax: 3366, 1591, 1532, 1417,
O7ꢀEu1ꢀO4 129.9(2) O7ꢀTb1ꢀO4 129.96(19) O7ꢀGd1ꢀO4 130.3(13)
1385, 1292, 1161, 990, 789 cmꢀ1
.
Gd(L1)3(C2H5OH)(H2O) (5). Elemental analysis (%): Calcd for
C65H59O14Gd (1221.41): C, 63.92; H, 4.87. Found: C, 63.69; H,
4.61. FT-IR (KBr) νmax: 3398, 1599, 1532, 1415, 1379, 1207, 1157,
1055, 842, 786 cmꢀ1
.
[Gd(L2)3(H2O)2] 2H2O (6). Elemental analysis (%): Calcd for
3
C57H51N6O15Gd (1217.44): C, 56.24; H, 4.22; N, 6.90. Found: C,
56.29; H, 3.85; N, 6.63. FT-IR (KBr) νmax: 3324, 1591, 1539, 1413,
1375, 1295, 1161, 1062, 990, 787 cmꢀ1
.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Ligands and Ln3þ Com-
plexes 1ꢀ6. The ligands 3,5-bis(benzyloxy)benzoic acid (HL1)
and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2) were readily
obtained in three steps starting from commercially available 3,5-
dihydroxy benzoic acid as summarized in Scheme 1. The overall
yields fell in the range 75ꢀ93%. The newly designed ligands were
identified on the basis of 1H and 13C NMR spectroscopy
(Supporting Information, Figures S1ꢀS2), mass spectroscopy
(FAB-MS), and elemental analysis. The protocols used for the
syntheses of the lanthanide complexes are summarized in the
Experimental Section. The elemental analysis data for the six
complexes revealed that in each case the Ln3þ ion had reacted with
the corresponding benzoic acid ligand in a metal-to-ligand mole
ratio of 1:3. The FT-IR spectra of the ligands evidenced intense
absorption bands characteristic of the carboxylate groups at ∼1440
and ∼1700 cmꢀ1, and are attributable to the symmetric νs(CdO)
and asymmetric νas(CdO) vibrations, respectively. The bands
centered at 1693 cmꢀ1 (HL1) and 1705 cmꢀ1 (HL2) in the
spectra of the free ligands, which are assigned to stretching vibrations
of the nonionized carboxylic groups, are shifted in the FT-IR spectra
of 1ꢀ6, thus confirming that the ligands are completely ionized, and
hence in accord with the X-ray crystal structures. Moreover, the
asymmetric and symmetric stretching vibration modes of the
carboxylate groups in complexes 1ꢀ6 are further split into two
peaks (Supporting Information, Table S1). The differences between
Figure 1. Capped stick model showing the coordination environment
of 2. All hydrogenatoms havebeen omitted forclarity. One ꢀ x, y, 3/2 ꢀ z
completes the coordination sphere around Eu1. The Eu centers are related
by an inversion center at 1/2, 1/2, 0. The complex extends infinitely along
the c axis.
The thermal stabilities of complexes 1ꢀ6 were explored by
means of thermogravimetric analyses (TGA) in the temperature
range 30ꢀ800 °C. The thermograms exhibit similar features and
the profiles are provided in the Supporting Information, Figure S3.
All the compounds are thermally stable up to 300 °C. The onset
of weight loss occurs between 220 and 260 °C (Found: 5.21% for
compounds 1, 3, and 5 and 5.86% for compounds 2, 4, and 6).
These weight losses correspond to the release of coordinated
water and solvent molecules (Calcd: 5.23% for compounds 1, 3,
and 5 and 5.80% for compounds 2, 4, and 6). The second weight
loss occurs between 300 and 600 °C (Found: 81.01% for 1, 3, and
5 and 80.61% for 2, 4, and 6) and is attributable to thermal
decomposition of the organic components (Calcd: 81.75% for 1,
3, and 5 and 81.57% for 2, 4, and 6) and corresponds to the
formation of the stoichiometric amounts of Ln2O3 (Calcd
∼14%; Found ∼14%). In addition to the above weight losses,
in the cases of compounds 2, 4, and 6 an initial weight loss of
∼1.5% was evident in the temperature range 105ꢀ110 °C, and is
due to the elimination of lattice water molecules. The X-ray
the asymmetric and symmetric stretching vibration modes (Δ(CdO)
=
νas ꢀ νs) fall in the ranges 206ꢀ221 and 115ꢀ126 cmꢀ1, which
indicates that the carboxylate groups are coordinated to the Ln3þ
ions in bidentate bridging and chelating modes.16 A broad band
observed above 3000 cmꢀ1 (maximum at 3400 cmꢀ1) for com-
plexes 1ꢀ6 is attributed to the characteristic ν(OH) stretching
vibration and is indicative of the presence of water molecules.
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powder diffraction patterns for complexes 1ꢀ6 are similar to
each other, thus implying that these compounds are isostructural
(Supporting Information, Figure S4).
Eu1ꢀO1: 2.371 Å].17 As evident from Table 2, the distances
between the adjacent metal centers are relatively large (∼5 Å)
which is expected for (μ-η1: η1)2 bridges that contain four Ln-O
interactions.7b Perhaps the most noteworthy structural feature of
compounds 2, 4, and 6 is the presence of free Lewis basic pyridyl
sites within the 1-D coordination polymer. In turn, this augurs
well for the potential utility of these compounds for the
recognition and sensing of metal ions.
X-ray Crystal Structures of Ln3þ-3,5-bis(pyridine-2-yl-
methoxy)benzoate Complexes 2, 4, and 6. Analysis of the
single-crystal X-ray diffraction data for compounds 2, 4, and 6
reveals that these compounds are isostructural and consist of
infinite one-dimensional (1-D) coordination polymers of the
general formula {[Ln(L2)3(H2O)2] xH2O}n. In view of the
A detailed analysis of the packing diagram of 2 reveals that the
coordination polymers are connected by an intermolecular
hydrogen bonding interaction between C25ꢀH25A of a ꢀCH2
moiety and N2 of a pyridyl group with an H25A---N2 separation
of 2.689 Å and an angle of 145.60° (Table 3) thus forming an
infinite two-dimensional layered supermolecule18 (Figure 3; see
Supporting Information, Figure S6 for 4 and Supporting Infor-
mation, Figure S8 for 6).
3
structural similarity of these complexes, detailed discussion is
limited to the geometrical features of 2. However, pertinent data
collection parameters and a listing of significant bond distances
and bond angles for all three complexes are provided in Tables 1
and 2, respectively. Compound 2 crystallizes in the monoclinic
space group C2/c. The Eu1 center of 2 is coordinated to four
carboxylate oxygen atoms of the bridging 3,5-bis(pyridine-2-
ylmethoxy)benzoate ligands, two carboxylate oxygen atoms of
the chelating benzoate ligand, and two water molecules
(Figure 1). The analogous structures of 4 and 6 are presented
in Supporting Information, Figures S5 and S7, respectively. The
Eu1 center of 2 is surrounded by eight oxygen atoms in a
distorted square antiprismatic arrangement with OꢀEu1ꢀO
bond angles ranging from 70.5 to 154°. Furthermore, the Eu1
centers of 2 are doubly bridged by the carboxylate groups of the
3,5-bis(pyridine-2-ylmethoxy)benzoate ligands thereby forming
an infinite 1-D chain (Figure 2). The carboxylate bridges also
adopt a syn-anti conformation. The longest Eu1ꢀO bonds
involve the oxygen atoms of the bidentate chelating ligands
[Eu1ꢀO4: 2.492 Å], and the shortest such bonds are associated
with the bridging carboxylate ligand [Eu1ꢀO6: 2.362 Å;
Electronic States of the Ligands. The UVꢀvisible absorp-
tion spectra of the free ligands and those of the corresponding
Ln3þ complexes 1ꢀ6 were recorded in CH3OH solution (c = 2 ꢁ
10ꢀ5 M) and are displayed in Figure 4. The trends in the
absorption spectra of these complexes are identical to those
observed for the free ligands, indicating that the singlet excited
states of the ligands are not significantly affected by complexation
to the Ln3þ ions. However, a small blue shift that is discernible in
the absorption maxima of these complexes is attributed to the
perturbation induced by metal coordination. The 3,5-bis-
(benzyloxy)benzoic acid ligand exhibits a broad band in the
1
UV corresponding to a πꢀπ* transition with a lowest energy
maximum in the range 280ꢀ340 nm (λmax = 307 nm) and a
molar absorption coefficient (ε) of 1.01 ꢁ 104 Mꢀ1 cmꢀ1. On
the other hand, replacement of the hydrogen atoms of 3,5-
dihydroxy benzoic acid by pyridyl moieties resulted in a blue shift
of λmax (302 nm), and a lower ε value (3.90 ꢁ 103 Mꢀ1 cmꢀ1) in
comparison with that of 3,5-bis(benzyloxy)benzoic acid. The
molar absorption coefficient values for complexes 1ꢀ6 were
calculated at the respective λmax values, and a summary of these
data is presented in Supporting Information, Table S2. In
general, the magnitudes of the molar absorption coefficients
for the complexes are approximately three times higher than
Table 3. Selected Hydrogen-Bond Distances (Å) and Angles
(deg) for Complexes 2, 4, and 6
H
A
D
A
D
H
A
3 3 3
3 3 3
3 3 3 3 3 3
compounds
contact type
(Å)
(Å)
(deg)
2
4
6
C25ꢀH25A N2
2.689
2.697
2.619
3.536
3.556
3.476
145.60
147.69
147.51
3 3 3
C25ꢀH25A N2
3 3 3
C25ꢀH25A N2
3 3 3
Figure 2. 1D coordination polymer chain of complex 2.
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Figure 3. View of 2 showing the intermolecular hydrogen bonding interactions between C25ꢀH25A of a CH2 groups and N2 of the pyridyl moiety,
where N2 is related by x, ꢀy, z ꢀ 1/2. Some of the ligands have been omitted for clarity.
Figure 4. UVꢀvisible absorption spectra for 3,5-bis(benzyloxy)benzoic
complexes 1ꢀ6 in CH3OH solution (2 ꢁ 10ꢀ5 M).
acid (HL1), 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), and
Figure 5. Phosphorescence spectra of gadolinium complexes 5 and 6
at 77 K.
been estimated by reference to their lower wavelength emission
edges (408 nm: 24509 cmꢀ1, 396 nm: 25253 cmꢀ1 for ligands
HL1 and HL2, respectively) of the low-temperature phosphores-
cence spectra of the Gd3þ complexes 5 and 6 (Figure 5). It is
interesting to note that the triplet energy levels of the newly
designed ligands lie well above the energies of the main emitting
levels of 5D0 for Eu3þ and 5D4 for Tb3þ, thus indicating that these
ligands can act as antennas for the photosensitization of trivalent
Ln3þ ions. To elucidate the energy migration pathways in these
Ln3þ complexes, it was also necessary to determine the singlet
energy levels of the ligands. The singlet (1ππ*) energy levels of
those for the free ligands. Such a trend is consistent with the
presence of three carboxylate ligands in each complex. It is also
noteworthy that the large molar absorption coefficients observed
for the new aromatic carboxylate ligands implies that they have a
strong ability to absorb light.
For a ligand to serve as an effective photosensitizer, the first
triplet state should be situated sufficiently above the emitting 5D0
level of Eu3þ or the 5D4 level of Tb3þ to permit efficient energy
transfer to the higher excited states of the Ln3þ ions and thereby
prevent quenching by back energy transfer processes.19 In the
present work, the triplet energy levels (3ππ*) of the ligands have
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Figure 8. Room-temperature excitation and emission spectra for com-
plexes 3 and 4 (λex = 340, 320 nm respectively) with emission monitored
at approximately 545 nm.
Figure 6. Room-temperature excitation and emission spectra for com-
plex 1 (λex = 316 nm) with emission monitored at approximately
612 nm.
the πꢀπ* transitions of the aromatic carboxylates. In addition,
the excitation spectra of 1 and 2 display another broad band in
the range 350ꢀ380 nm (centered at 360 nm) is overlapped by a
series of sharp lines characteristic of the Eu3þ energy-level
7
structure, and can be assigned to transitions between the F0,1
5
and the L6,5D3,2,1 levels.21 Moreover these f-f transitions are
more intense than the broad band observed related to the excited
states of the 3,5-bis(benzyloxy)benzoic acid, which proves that
luminescence sensitization via excitation of the ligand is not
promising in 1. On the other hand, in the excitation spectrum of
Eu3þ complex 2, the weaker signals observed in the 350ꢀ425 nm
region arise from the f-f absorption of the Eu3þcation, unequi-
vocally proving the indirect excitation of the lanthanide centers
via a typical antenna effect involving the organic ligand, bis-
(pyridine-2-ylmethoxy)benzoic acid.
Under ligand excitation, the emission spectra of the europium
complexes 1 and 2 (Figures 6 and 7) exhibit characteristic sharp
bands of the metal ion in the 580ꢀ720 nm spectral range.22 The
5
intensity of the D0f7F2 transition (electric dipole) is greater
5
than that of the D0f7F1 transition (magnetic dipole), which
Figure 7. Room-temperature excitation and emission spectra for com-
plex 2 (λex = 313 nm) with emission monitored at approximately
612 nm.
indicates that the coordination environment of the Eu3þ ions in
1 and 2 is devoid of an inversion center.23 Furthermore, the
absence of πꢀπ* transitions of the ligands were evident in the
emission spectra, which is typically diagnostic of the sensitization
of the ligand toward the europium ion. To understand more
about this phenomenon, the emission spectrum of the europium
complexes 1 and 2 were also investigated by exciting at different
f-f absorption of the Eu3þcation, and the results are summarized
in Supporting Information, Figure S9. At a 394 nm excitation
wavelength (more intense line of europium absorption), the
emission intensity of the 5D0f7F2 transition (612 nm) is
approximately 1.2 times more intense than when excited at
316 nm (ligand excitation), implying that ligand-to-europium
energy transfer is not effective in the case of europium complex 1.
Conversely in the case of complex 2, the emission intensity of the
5D0f7F2 transition (612 nm) is approximately 3.6 times higher
when excited at the ligand excitation maxima (313 nm) as
compared to direct excitation at 394 nm. This indicates that
ligand-to-europium energy transfer is effective in the case of
europium complex 2.
these ligands wereestimatedbyreference tothewavelengths of the
UVꢀvis upper absorption edges of Gd3þ complexes 5 and 6
(Figure 4). The relevant values are 327 nm (30581 cmꢀ1) and
321 nm (31152 cmꢀ1) for HL1 and HL2, respectively. It is known
that an efficient ligand-to-metal energy transfer requires a good
1ππ*f 3ππ* intersystem crossing, which is maximized when the
energy difference between these states is close to 5000 cm .
ꢀ1 20 In
3
our case, ΔE(1ππ*ꢀ ππ*) it amounts to 6072 and 5899 cmꢀ1 for
ligands HL1 and HL2, respectively, and is therefore the designed
ligands have good intersystem crossing efficiency.
Metal-Centered Luminescence. The Eu3þ and Tb3þ car-
boxylate complexes 1ꢀ4 presented in this study exhibit metal-
centered luminescence. The combined excitation and emission
spectra for the Eu3þ complexes 1 and 2 in the solid state at room
temperature are presented in Figures 6 and 7, respectively. The
excitation spectra for Eu3þ complexes 1 and 2 exhibit a broad
band in the 250ꢀ350 nm region (centered at 310 nm) because of
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Table 4. Radiative (ARAD) and Nonradiative (ANR) Decay Rates, 5D0/5D4 Lifetimes (τobs), Radiative Lifetimes (τRAD), Intrinsic
Quantum Yields (ΦLn), Energy Transfer Efficiencies (Φsen), and Overall Quantum Yields (Φoverall) for Complexes 1ꢀ4
Compounds
ARAD (sꢀ1
)
ANR (sꢀ1
)
τobs (μs)
τRAD (μs)
ΦLn (%)
Φsen (%)
Φoverall (%)
1
944
1382
426 ( 2
938 ( 2c
426 ( 1
886 ( 3c
1160 ( 7
1050 ( 2
41
3.6
1.5 ( 0.1a
1.1 ( 0.1b
1.0 ( 0.2a
0.9 ( 0.2b
60 ( 6a
2
3
4
397
1938
2518 ( 1
1070 ( 11
1232 ( 14
17
5.9
60
27
∼100
∼100
58 ( 6b
1377 ( 1
27 ( 3a
24 ( 3b
a Absolute quantum yield. b Relative quantum yield. c τobs at 77 K.
Figure 8 summarizes the room temperature excitation spectra of
complexes 3 and 4 in the solid state monitored around the more
intense 545 nm emission line of the Tb3þ ion. These excitation
spectra exhibit a large broad band in the 250ꢀ350 nm region which
is associated with the electronic transitions of the ligands. The
absence of any absorption bands due to f-f transitions of the Tb3þ
cation proves that the luminescence sensitization via excitation of
the ligand is effective.10 The room temperature emission spectra of 3
and 4 (Figure 8) excited at 340 and 320 nm, respectively, exhibit a
series of sharp lines which result from deactivation of the 5D4 excited
state to the corresponding 7FJ ground state of the Tb3þ cation (J=6,
5, 4, 3) centered at 488, 545, 585, and 620 nm.5a,10,24 Moreover, the
ligand-centered emission is not detected, thus implying the existence
of an efficient ligand-to-metal energy transfer process in these
complexes. Most importantly, the emission intensity of the
Tb3þ-3,5-bis(benzyloxy)benzoate complex (3), especially at
545 nm, is found to be significantly higher (∼2-fold) than that
of the Tb3þ-3,5-bis(pyridine-2-ylmethoxy)benzoate complex (4).
The 5D0 (Eu3þ) and 5D4 (Tb3þ) decay curves were measured
at both ambient (298 K) and low (77 K) temperatures and
FranckꢀCondon overlap factor is less favorable.25 The 5D4 lifetime
values of Tb3þ complexes 3 and 4 are also found to be essentially
temperature independent, with τobs varying by less than 1% while
going from 298 to 77 K, thereby reflecting the absence of thermally
activated deactivation processes.
To quantify the ability of the ligands designed to sensitize the
luminescence of lanthanides, and to draw conclusions concern-
ing the relationship between the structure and the properties, it
was appropriate to analyze the emission in terms of eq 1 (below)
where Φoverall and ΦLn, represent the ligand-sensitized and
intrinsic luminescence quantum yields of Ln3þ; Φsen represents
the efficiency of the ligand-to-metal energy transfer and τobs
/
τRAD are the observed and the radiative lifetimes of Ln3þ 26
.
Φoverall ¼ Φsen ꢁ ΦLn ¼ Φsen ꢁ ðτobs=τRAD
Þ
ð1Þ
The intrinsic quantum yields of Eu3þ could not be determined
experimentally upon direct f-f excitation because of very low
absorption intensity.1b Therefore, the radiative lifetimes of Eu3þ
(5D0) have been calculated from equation 226,27 where n represents
the refractive index of the medium. An average index of refraction
equal to 1.5 was employed in the calculation.10 AMD,0 is the
5
monitored within the more intense lines of the D0f7F2 and
5D4f7F5 transitions, respectively (See the Supporting Information,
Figure S10ꢀ13), and the pertinent values are summarized in
Table 4. The observed luminescence decay profiles correspond to
single exponential functions at 298 and 77 K, thus implying the
presence of only one emissive Eu3þ or Tb3þcenter. The shorter 5D0
lifetimes noted for Eu3þ complexes 1ꢀ2 (τobs = 0.426 ms) may be
due to the dominant nonradiative decay channels associated with
vibronic coupling on account of the presence of solvent molecules in
the coordination spheres of these complexes. These values are
essentially temperature dependent, with τobs (0.938 and 0.886 ms
for 1 and 2, respectively) approximately doubled in going from 298
to 77 K, thereby reflecting the presence of thermally activated
deactivation processes. This effect has been well documented for
several other hydrated Eu3þ carboxylate complexes.5a,b,10 On the
other hand, longer 5D4 lifetime values at 298 K (τobs = 1.16 for3 and
1.38 ms for 4, respectively) have been observed for the Tb3þ
complexes even when solvent molecules are present in the first
coordination sphere since they are well-known to be vibrational
deactivators of the excited states of Ln3þ ions.5a,10 The energy gaps
between the luminescent state and the ground state manifolds are
5
spontaneous emission probability for the D0f7F1 transition in
vacuo (14.65 sꢀ1), and Itot/IMD signifies the ratio of the total
integrated intensity of the corrected Eu3þ emission spectrum to the
integrated intensity of the magnetic dipole 5D0f7F1 transition:
1=τRAD ¼ AMD,0 ꢁ n3 ꢁ ðItot=IMD
Þ
ð2Þ
The intrinsic quantum yield for Tb3þ (ΦTb) was estimated using
eq 3 with the assumption that the decay process at 77 K in a
deuterated solvent is purely radiative.3b,25,28
ΦTb ¼ τobsð298 KÞ=τobsð77 KÞ
ð3Þ
Table 4 summarizes the radiative (ARAD) and nonradiative
(ANR) decay rates, Φoverall, ΦLn, and Φsen. In the case of terbium
luminescence, solid-state measurements gave a quantum yield of
60% for the Tb3þ-3,5-bis(benzyloxy)benzoate compound 3
compared with 27% for Tb3þ-3,5-bis(pyridine-2-ylmethoxy)-
benzoate compound 4. Conversely, the corresponding Eu3þ
complexes 1ꢀ2 show poor luminescence efficiency with low
quantum yields (1.5% for 1 and 1% for 2). However, the present
quantum yield, especially with Tb3þ coordination polymer
constructed from 3,5-bis(benzyloxy)benzoate is found to
be superior to that of recently reported Tb3þ coordination polymer
of 3-methoxy-4-benzyloxy benzoate system (Φoverall = 33%),5a
and comparable with that of Tb3þ-coordination polymers as-
sembled from β-diketonates and bidentate O-donor ligands
approximately 12000 cmꢀ1 and 14800 cmꢀ1 for Eu3þ and Tb3þ
,
respectively. Relatively efficient coupling of the Eu3þ excited
states occurs to the third vibrational overtone of the proximate
OH oscillators (υOH ∼ 3300ꢀ3500 cmꢀ1), and to the fourth
harmonic in the case of Tb3þ, which is consistent with the less
efficient quenching observed in the case of Tb3þ where the
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(Φoverall = 56%),4d and Tb3þ-2,5-pyridinedicarboxylate system
(Φoverall = 45%).5c To better understand these results, one must
take into account the 3ππ*-state energies of these ligands. According
to the intramolecular energy transfer mechanism reported by Dexter
and Sato et al.,29 the intramolecular energy migration efficiency
depends mainly on two energy transfer processes, namely, (i) the
energy transfer from the lowest triplet energy level of the ligand to
the resonant energy level of the lanthanide ion by Dexter’s resonant
exchange interaction; and (ii) the inverse energy transfer from Ln3þ
ion to organic ligand by a thermal deactivation mechanism. If the
energy difference is too small, the inverse energy transfer will take
place much easier. Both energy transfer processes depend on the
energy gap between the lowest triplet energy level of the organic
ligand and the resonant energy level of the lanthanide ion. Clearly,
there is an opposite influence between two energy transfer
processes. Latva et al.’s empirical rule19 states that an optimal
ligand-to-metal energy transfer process for Tb3þ needs ΔE
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data for
b
complexes 2, 4, and 6 in CIF format, 1H, 13C NMR spectra for
ligands, thermogravimetric data, powder XRD patterns for com-
plexes 1ꢀ6. Figures showing the coordination environments and
intermolecular hydrogen bonding interactions of complexes
4 and 6, and room temperature and low temperature lumines-
cence decay profiles. Excitation spectrum of complex 5. Emission
spectra of 1 and 2 excited at different wavelengths (316, 362, 378,
and 394 nm). A table for the infrared vibrational frequencies of
the ligands and complexes, and a table containing absorption
maxima and molar absorption coefficient data for the ligands and
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
5
5
(3ππ*ꢀ D4) = 2500ꢀ4500 cmꢀ1 and for Eu3þ ΔE (3ππ*ꢀ D4) =
2500ꢀ4000 cmꢀ1. On this basis it can be concluded that the
energy transfer to the Tb3þ ion will be effective for the 3,5-
bis(benzyloxy)benzoic acid and 3,5-bis(pyridine-2-ylmethoxy)-
Corresponding Author
*E-mail: mlpreddy55@gmail.com.
5
benzoic acid ligands, since ΔE (3ππ*ꢀ D4) for 3 and 4 are
’ ACKNOWLEDGMENT
4009 cmꢀ1 and 4753 cmꢀ1, respectively. Complex 3 exhibits higher
quantum yield (Φoverall = 60%) and efficient ligand-to-metal energy
transfer (Φsen = 60%) than complex 4 because of the superior
match of the triplet energy level of the 3,5-bis(benzyloxy)benzoic
ligand to that of the Tb3þ emitting level. On the other hand, poor
sensitization efficiency for Eu3þ complexes has been noted for the
above benzoate ligands mainly because of the larger energy gap
The authors acknowledge financial support from the Depart-
ment of Science and Technology (SR/S1/IC-36/2007) and the
Council of Scientific and Industrial Research (NWP0010). S.S.K.
thanks CSIR, New Delhi, for the award of Junior Research
Fellowship. A.H.C. thanks the Robert A. Welch Foundation
(F-0003) for financial support.
5
between the triplet state and 5D0 level of Eu3þ ΔE (3ππ*ꢀ D0) =
7259 cmꢀ1 for 1and 8003 cmꢀ1 for 2. Therefore, significantly lower
quantum yields have been observed in Eu3þ complexes. Thus, it can
be concluded that the derivatives of 3,5-dihydroxy benzoates are
probably not suitable sensitizers for the excitation of Eu3þ ion.
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’ CONCLUSIONS
Two new carboxylate ligands based on bis(benzyloxy)- and
bis(pyridyl)-substituted 3,5-dihydroxy benzoic acids have been
prepared from simple starting materials via a facile synthetic
route. The new ligands form luminescent coordination polymers
with trivalent lanthanides and sensitize their luminescence in the
visible region. The following statements can be made regarding
the structural and photophysical data of the new complexes:
(i) 3,5-Bis(pyridine-2-ylmethoxy)benzoic acid forms unique
luminescent coordination polymers with both Eu3þ and
Tb3þ ions. These polymers feature free Lewis basic
pyridyl sites and may therefore prove to be useful for
the sensing of heavy metal ions.
(5) (a) Sivakumar, S.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan,
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5
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