Synthesis, structures, and luminescent properties of uranyl terpyridine aromatic carboxylate coordination polymers

Article abstract of DOI:10.1021/ic3024698

Six novel uranyl terpyridine aromatic carboxylate coordination polymers, [UO₂(C₆H₂O₄S)(C₁₅H ₁₁N₃)] (1), [UO₂(C₆H ₂O₄S)(C₁₅H₁₀N₃Cl)] ·H₂O (2), [UO₂(C₈H₄O ₄)(C₁₅H₁₁N₃)] (3), [UO ₂(C₈H₄O₄)(C₁₅H ₁₀N₃Cl)] (4), [UO₂(C₁₂H ₆O₄)(C₁₅H₁₁N₃)] (5), and [UO₂(C₁₂H₆O₄)(C₁₅H ₁₀N₃Cl)] (6), were synthesized under solvothermal conditions and characterized by single-crystal and powder X-ray diffraction and luminescence and UV-vis spectroscopy. Compounds 1, 2, and 5 crystallize as molecular uranyl dimers, whereas compounds 3, 4, and 6 contain ladder motifs of uranyl centers. Fluorescence spectra of 1-4 show characteristic UO ₂²⁺ emission, wherein bathochromic and hypsochromic shifts are noted as a function of organic species. In contrast, uranyl emission from 5 and 6 is quenched by the naphthalene dicarboxylic acid linker molecules.

Full text of DOI:10.1021/ic3024698

Article
pubs.acs.org/IC
Synthesis, Structures, and Luminescent Properties of Uranyl
Terpyridine Aromatic Carboxylate Coordination Polymers
Sonia G. Thangavelu,† Michael B. Andrews,† Simon J. A. Pope,‡ and Christopher L. Cahill*,†
†Department of Chemistry, The George Washington University, 725 21st Street, NW, Washington, D.C. 20052, United States
‡School of Chemistry, Main Building, Cardiff University, Cardiff, Cymru/Wales, CF10 3AT U.K.
S
* Supporting Information
ABSTRACT: Six novel uranyl terpyridine aromatic carbox-
ylate coordination polymers, [UO2(C6H2O4S)(C15H11N3)]
(1), [UO2 (C6 H2 O4 S)(C1 5 H1 0 N3 Cl)]·H2 O (2),
[UO2(C8H4O4)(C15H11N3)] (3), [UO2(C8H4O4)-
(C15H10N3Cl)] (4), [UO2(C12H6O4)(C15H11N3)] (5), and
[UO2(C12H6O4)(C15H10N3Cl)] (6), were synthesized under
solvothermal conditions and characterized by single-crystal and powder X-ray diffraction and luminescence and UV−vis
spectroscopy. Compounds 1, 2, and 5 crystallize as molecular uranyl dimers, whereas compounds 3, 4, and 6 contain ladder
2+
motifs of uranyl centers. Fluorescence spectra of 1−4 show characteristic UO2 emission, wherein bathochromic and
hypsochromic shifts are noted as a function of organic species. In contrast, uranyl emission from 5 and 6 is quenched by the
naphthalene dicarboxylic acid linker molecules.
side of a metal “node” using multitopic linker molecules.
Indeed, a number of uranyl terpyridine complexes have been
reported and confirm either uranyl monomers12,13 or, at most,
dimers.14,15
INTRODUCTION
■
Uranyl coordination polymers (CPs) synthesized using hydro-
(solvo)thermal techniques exhibit tremendous structural
diversity owing, in part, to the tendency of the UO2 cation
2+
Beyond the potential selectivity (and, indeed, structural
rigidity) offered by TPYs, these ligands and other extended π
systems may also contribute to the overall electronic properties
of a material. For example, unique photophysical behavior may
result as a consequence of π−π* transitions within the linkers
or from energy transfer between metal centers and coordinated
(or neighboring) organic species.16 CPs containing lanthanide-
(III) ions are exemplary in this regard in that energy transfer
from excited linker molecules or noncoordinated “guests” to
emissive states with the lanthanide has been well docu-
mented.16−19 Such are examples of sensitized emission, also
known as the antenna effect, wherein nonradiative energy
transfer commonly takes place from an excited ligand triplet
state to excited state(s) on the lanthanide.20,21 Requirements
for promoting this energy transfer include a reasonable physical
proximity between the organic chromophore and the emissive
ion, as well as an appropriately matched energy level of the
excited states on each. As such, thiophene-2,5-dicarboxylate
(TDC),22,23 benzene-1,4-dicarboxylate (BDC),24,25 and naph-
thalene-1,4-dicarboxylate (NDC)26 are known to be efficient
sensitizing ligands for different lanthanide ions, and examples of
uranyl CPs containing these and related ligands along with their
luminescence have been reported.27−33 TPYs also exhibit
interesting luminescent properties and are useful for forming
building blocks for the supramolecular assemblies of emissive
lanthanide materials.34,35
to hydrolyze under such conditions. The resulting oligomeric
species (or secondary building units) contribute to the
structural diversity of uranyl bearing hybrid architectures.
Indeed uranyl CPs consisting of monomers through octamers
(as well as infinite sheets) have been well documented and
continue to be a rich area of inquiry owing not only to this
diverse speciation but also to the nearly limitless combinations
of organic linkers.1−9
An inherent challenge to these syntheses, however, is
directing speciation to a specific uranyl species of interest.
One approach to simplifying aqueous reaction systems and
effectively thwarting hydrolysis is to use high halide media
wherein speciation is restricted to the [UO2X4]2− (X = Cl, Br)
units almost exclusively.1 The route to the formation of hybrid
materials then becomes one of assembly of the tetrahalide
anions using (for example) various organic cations.10,11 This
approach has been rather successful in producing a number of
materials assembled through hydrogen-bonding and halogen−
halogen interactions.
Another approach to offsetting the otherwise complex
aqueous speciation of the uranyl cation is to use ligands that
may show an affinity for a given building unit and effectively
“select” a single species. As we have explored herein, chelating
ligands such as terpyridines (TPYs) may prefer to coordinate to
a single uranyl species and, consequently, impart a specific
geometric arrangement to uranyl centers. From a crystal
engineering perspective, this provides for coordination wherein
TPY ligands may “lock” the metal center into place and allow
for subsequent coordination at open binding sites on the other
Received: November 12, 2012
Published: January 31, 2013
© 2013 American Chemical Society
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Table 1. Crystallographic Data for Compounds 1−6
1
2
3
4
5
6
formula
[UO2(C6H2O4S)
(C15H11N3)]
[UO2(C6H2O4S)
(C15H10N3Cl)]·H2O
[UO2(C8H4O4)
(C15H11N3)]
[UO2(C8H4O4)
(C15H10N3Cl)]
[UO2(C12H6O4)
(C15H11N3)]
[UO2(C12H6O4)
(C15H10N3Cl)]
fw
673.44
100
725.91
100
667.41
293
701.86
100
717.47
100
751.92
300
temperature (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/c
0.71073
monoclinic
C2/c
0.71073
monoclinic
C2/c
0.71073
triclinic
0.71073
monoclinic
P21/c
P1
̅
9.5824(3)
13.5216(5)
16.2164(5)
11.0786(16)
12.1200(18)
16.164(2)
13.011(5)
15.901(5)
11.739(5)
12.486(3)
14.736(3)
12.268(3)
8.9758(7)
11.3079(8)
12.2348(9)
78.4550(10)
81.2450(10)
67.1190(10)
1117.13(14)
2
10.6869(13)
8.3588(9)
26.656(4)
b (Å)
c (Å)
α (deg)
β (deg)
102.0320(10)
97.958(2)
118.035(5)
113.494(2)
92.075(4)
γ (deg)
volume (Å3)
2054.99(12)
2149.4(5)
4
2143.7(14)
2070.1(8)
4
2379.6(5)
4
Z
4
4
density (calcd)
(Mg/m3)
2.170
2.184
2.068
2.265
2.133
2.054
abs coeff (mm−1
reflns collected
indep reflns
)
8.046
7.818
7.618
6665
8.023
8545
7.318
6.983
40347
34042
21319
14646
5938 [R(int) =
0.0810]
4707 [R(int) = 0.0886]
1887 [R(int) =
0.0384]
2207 [R(int) =
0.0437]
6015 [R(int) =
0.0302]
4176 [R(int) =
0.0989]
final R indices [I > R1 = 0.0359, wR2 = R1 = 0.0405, wR2 =
2σ(I)] 0.0889 0.1012
R1 = 0.0208, wR2 = R1 = 0.0251, wR2 = R1 = 0.0351, wR2 = R1 = 0.0421, wR2 =
0.0509 0.0499 0.0822 0.1025
TPY (1.5 equiv, 0.377 mmol), and 25 μL of 6 M NaOH was dissolved
in 2.5 mL of a H2O−2-propanol (1:1.5) mixture. All reagents were
placed in a 23 mL Teflon-lined stainless steel Parr bomb, which was
sealed and heated statically for 5 days at 120 °C. The Parr bomb was
then allowed to cool slowly to 25 °C overnight. Solids were collected,
washed with H2O and 2-propanol, and allowed to air-dry. Single
crystals were then isolated and characterized by single-crystal X-ray
diffraction (XRD). Elemental analyses were performed on all products
by Galbraith Laboratories, Knoxville, TN. Syntheses of 1−6 were also
performed in deionized H2O (2.5 mL) yet yielded impure reaction
products, as evidenced by powder X-ray diffraction (PXRD; see
Figures S18−S23 in the Supporting Information).
The emissive properties of the uranyl cation have been
known for some time and, consequently, studied extensively
from a fundamental perspective as well as from a more
analytical and applied approach as relevant to (for example)
environmental investigations of uranyl speciation.36−38 Despite
this interest, studies of uranyl emission from within hybrid
materials have been a bit less frequent and more qualitative in
nature, even considering the potential for these compounds to
serve as photocatalysts.8,39,40 An inherent challenge to studying
energy transfer between linker and uranyl centers is the overlap
of the π−π* absorption of the organic linkers in UV−vis and
ligand-to-metal charge-transfer (LMCT) absorption within the
[UO2(C6H2O4S)(C15H11N3)] (1). Yellow block crystals using TDC
and TPY. Yield: 116.0 mg, 69% based on uranium. Elem anal. Obsd
(calcd): C, 37.32 (37.42); H, 1.95 (1.95); N, 5.50 (6.24).
2+
UO2 itself. As a consequence, one cannot completely resolve
the contribution from the linker(s) or UO22+ without thorough
quantitative analysis and modeling efforts. Indeed some of our
own earlier studies were highly qualitative and speculative with
respect to UO22+ sensitization.41,42 The continued development
of hybrid materials particularly using the approaches described
herein may contribute to a platform for accessing materials that
allow for a more systematic study of the photophysical
properties. As such, this contribution reports the synthesis of
six uranyl terpyridine dicarboxylate CPs that have been
characterized by single-crystal and powder X-ray diffraction,
luminescence, and UV−vis spectroscopy in which we
demonstrate that uranyl emission may be influenced as a
function of coordinated aromatic carboxylate and TPY ligands.
[UO2(C6H2O4S)(C15H10N3Cl)]·H2O (2). Yellow block crystals using
TDC and Cl-TPY. Yield: 131.6 mg, 72% based on uranium. Elem anal.
Obsd (calcd): C, 34.74 (35.68); H, 1.94 (1.71); N, 5.79 (5.94).
[UO2(C8H4O4)(C15H11N3)] (3). Yellow block crystals using BDC and
TPY. Yield: 143.6 mg, 86% based on uranium. Elem anal. Obsd
(calcd): C, 41.76 (41.39); H, 2.42 (2.27); N, 5.60 (6.30).
[UO2(C8H4O4)(C15H10N3Cl)] (4). Yellow block crystals using BDC
and Cl-TPY. Yield: 91.7 mg, 52% based on uranium. Elem anal. Obsd
(calcd): C, 39.09 (39.32); H, 2.11 (2.01); N, 5.81 (5.98).
[UO2(C12H6O4)(C15H11N3)] (5). Yellow block crystals using NDC and
TPY. Yield: 158.2 mg, 87% based on uranium. Elem anal. Obsd
(calcd): C, 45.16 (44.95); H, 2.42 (2.37); N, 5.70 (5.85).
[UO2(C12H6O4)(C15H10N3Cl)] (6). Yellow block crystals using NDC
and Cl-TPY. Yield: 86.2 mg, 47% based on uranium. Elem anal. Obsd
(calcd): C, 45.38 (43.09); H, 2.13 (2.19); N, 5.71 (5.59).
PXRD. Diffraction patterns of compounds 1−6 were obtained on a
Rigaku MiniFlex II Desktop powder X-ray diffractometer (Cu Kα, 3−
60°) and analyzed using the JADE software package. The phase purity
of bulk samples 1−6 was assessed by a comparison of the observed
and calculated PXRD patterns. These patterns can be found in the
Supporting Information (Figures S1−S6). Compound 6 required
washing of the bulk sample with chloroform, in addition to H2O and
2-propanol, to achieve phase purity.
EXPERIMENTAL SECTION
General Synthesis of Compounds 1−6. All starting materials
were purchased from VWR except TDC (Sigma Aldrich) and used as
received without further purification.
■
Caution! Uranyl acetate, UO2(CH3COO)2·2H2O, contains depleted
uranium. Standard operating procedures for the handling of radioactive
and toxic substances should be followed.
All compounds were synthesized according to the following general
procedure with the same molar ratio of reactants. A mixture of uranyl
acetate (1.0 equiv, 0.251 mmol), organic acid (1.5 equiv, 0.377 mmol),
UV−Vis and Fluorescence Measurements. Solid-state UV−vis
diffuse-reflectance spectra were recorded on a Jasco V-570 UV−vis−
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Figure 1. Molecular dimer found in 1. Yellow polyhedra are uranyl centers, whereas spheres represent nitrogen (blue), oxygen (red), and sulfur
(yellow).
Figure 2. Packing diagram of 1 showing π−π interactions of the thiophene rings (A) and outer pyridine rings of TPY (B) viewed along [001].
near-IR spectrophotometer equipped with an ISN-470 integrating
sphere reflectance accessory at 298 K. These spectra can be found in
the Supporting Information (Figure S13). Solid-state emission spectra
of 1−6 and uranyl acetate were recorded on a Shimadzu RF-5301
spectrofluorophotometer at 298 K. All fluorescence spectra were
collected with a 1.5 nm slit width for both excitation and emission
monochromators, respectively, with the excitation wavelength fixed to
365 nm and are uncorrected.
spectra can be found in the Supporting Information (Figures S14−
S17).
Crystal Structure Determination. Single crystals isolated from
each bulk sample were mounted on MiTeGen micromounts. For
structure determinations of 1, 2, and 5, reflections were collected using
0.5° φ and ω scans on a Bruker SMART diffractometer equipped with
an APEX II CCD detector using Mo Kα radiation. For structure
determinations of 3 and 6, reflections were collected using 0.5 φ scans
on a Bruker SMART X2S benchtop diffractometer using Mo Kα
radiation. For structure determination of 4, reflections were collected
on a Siemens SMART 1000 CCD three-circle X-ray diffractometer
equipped with a LT-2 low-temperature apparatus using Mo Kα
radiation. All data was integrated using the SAINT software package,
and an absorption correction was applied using SADABS. All structures
were solved using direct methods (SHELXS-97) and refined using
SHELXL-97 within the WinGX software package,43,44 in which all of
the non-hydrogen atoms were refined anisotropically with satisfactory
refinements. Tests for additional symmetry were done using PLATON.
A summary of the crystallographic data for 1−6 can be found in Table
1.
Phosphorescence Measurements. To determine the triplet
states of TDC and NDC, the following experiments were performed
using Gd3+ to enhance the triplet yield of the ligand. A mixture of
Gd(NO3)3·6H2O (34 mg, 0.077 mmol, 1 equiv) and NDC (50 mg,
0.231 mmol, 3 equiv) or TDC (39.8 mg, 0.231 mmol, 3 equiv) was
dissolved in 5 mL of a 1:9 MeOH−EtOH mixture in a 25 mL
scintillation vial. The solution was gently heated to 50 °C to ensure
complete dissolution of all materials. Control experiments for NDC
and TDC were also prepared using the same molar ratios and solvents
without Gd3+. An aliquot of all prepared solutions was used as the glass
matrix (i.e., 1:9 MeOH−EtOH) for spectral measurements at low
temperature. Phosphorescence measurements were taken on a Horiba
JobinYvon Fluorolog-3 spectrophotometer equipped with a liquid-
nitrogen dewar assembly and a pulsed-UV xenon flash lamp at 77 K.
Spectra were recorded under the right angle setting using a time delay
of 5 ms to remove any residual fluorescence with the following
parameters: the total cycle time per flash, sample window, and number
of pulsed flashes were fixed to 61 ms, 0.2 ms, and 100, respectively. All
phosphorescence spectra were collected with a slit width of 5 nm for
both excitation and emission monochromators, respectively, with the
excitation wavelength fixed to 320 nm, and are uncorrected. These
RESULTS AND DISCUSSION
■
Structural Description. Crystal Structure of 1. The crystal
structures of 1−6 each exhibit a uranium(VI) center bound to
two axial oxygen atoms at bond lengths between 1.763(3) and
1.784(4) Å and a O−U−O bond angle that ranges from
175.25(18)° to 178.26(17)°. These values are consistent with
2+
the characteristic bond distances and bond angles of the UO2
cation.45 The uranyl center of 1 (Figure 1) is bound
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Figure 3. Molecular dimer found in 2. The green spheres are chlorine atoms. H2O is not shown for clarity.
1
Figure 4. Extended structure of 3. The subscript i represents the symmetry transformation −x + 1, y, −z + /2.
equatorially to one TPY and two distinct monodentate TDC
ligands to form an overall distorted pentagonal-bipyramidal
geometry. Each TDC linker is bound to two uranyl centers
through carboxylate oxygen atoms O3 and O5 at distances of
2.278(3) and 2.286(3) Å, respectively. The nonbonded
carboxylate oxygen atoms O4 and O6 at distances of
1.225(6) and 1.223(5) Å, respectively, were identified as
carbonyl oxygen atoms, which is in agreement to a C−O
double bond distance of 1.20−1.25 Å.46 For TPY, each
nitrogen atom N1, N2, and N3 is bound to uranium at an
average distance of 2.583(3) Å. The N1−U−N2 and O3−U−
O5 bond angles formed by either TPY or each TDC linker
were found to be 63.19(10)° and 80.03(10)°, respectively. The
uranium metal centers are linked together by two bridging
thiophenedicarboxylates to form discrete molecular dimers
oriented along [010] (Figure 2), where the outer pyridines π-
stack above and below each dimer at a ring centroid−centroid
distance of 3.919(3) Å (A) as determined in PLATON. The
bridging thiophene linkers also show π−π interactions at a ring
centroid−centroid distance of 3.661(2) Å (B). These values are
in agreement with typical π-stacking distances of 3.3−3.8 Å for
metal−aromatic complexes.47
Crystal Structure of 2. The uranyl center of 2 (Figure 3) is
bound equatorially to Cl-TPY and two distinct TDC ligands to
form an overall pentagonal-bipyramidal geometry. As in 1, each
TDC linker is bound to two uranyl centers through carboxylate
oxygen atoms O3 and O5 at bond distances of 2.256(4) and
2.303(4) Å, respectively. Tridentate coordination by the
nitrogen atoms N1, N2, and N3 in Cl-TPY to uranium
shows an average bond distance of 2.590(5) Å. The nonbonded
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Figure 5. Portion of the extended structure of 4.
Figure 6. Molecular dimer found in 5.
oxygen atoms O4 and O6 at distances of 1.213(7) and 1.225(8)
Å, respectively, were identified as carbonyl oxygen atoms. The
N1−U−N2 and O3−U−O5 equatorial bond angles formed by
Cl-TPY and TDC was found to be 62.79(16)° and 79.75(15)°,
respectively. Like 1, each uranium atom is linked together by
bridging thiophenecarboxylates in a monodentate fashion to
form an overall molecular dimer (Figure 3) with an analogous
packing arrangement. The bridging thiophene linkers, like those
in 1, show π−π interactions at a ring centroid−centroid
distance of 3.684(6) Å, yet the outer pyridine distance of Cl-
TPY is 8.279(7) Å.
Crystal Structure of 3. Two crystallographically distinct
uranyl centers, U1 and U2, are bound by two BDC linkers and
one TPY, which have pentagonal-bipyramidal geometries
(Figure 4). The BDC linkers are bound to uranyl by
monodentate coordination through the carboxylate oxygen
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Figure 7. Packing diagram of 5 showing π−π interactions of the naphthalene (A) and outer pyridine rings (B) of TPY and between the outer rings of
TPY and naphthalene (C).
Figure 8. Extended structure of 6.
atoms O2 and O2i with a bond length of 2.292(3) Å.
Tridentate coordination through nitrogen atoms N1 and N2 of
TPY to the uranyl center illustrates an average bond distance of
2.593(4) Å. Nonbonded oxygen atoms O3 and O3i at a bond
length of 1.223(5) Å were identified as carbonyl oxygen atoms.
Bond angles of N1−U−N2 and O2−U−O2i formed by TPY
and BDC were found to be 63.01(7)° and 80.60(13)°,
respectively, which are similar to the bond angles of 1 at
63.19(10)° and 80.03(10)°, respectively. Unlike the dimeric
uranyl species found in 1 and 2, the overall structure of 3 shows
a “ladder motif” that propagates along [001] wherein uranyl
terpyridine monomers are connected through BDC groups.
Crystal Structure of 4. The local geometry of the uranyl
center U1 in 4 is a pentagonal bipyramid with equatorially
bound Cl-TPY and BDC linkers. Like 3, each BDC ligand is
linked to two crystallographically distinct uranyl centers U1 and
U2 (Figure 5). The BDC linker is coordinated to the uranium
through a carboxylate oxygen atom O2 in a monodentate
fashion at a bond distance of 2.303(3) Å. For Cl-TPY, the
nitrogen atoms N1 and N2 are coordinated to the uranyl center
with an average bond distance of 2.605(4) Å. The nonbonded
oxygen atoms O3 and O3i at a bond length of 1.227(5) Å were
identified as carbonyl oxygen atoms. The N1−U−N2 and O2−
U−O2i bond angles formed by TPY and BDC are 62.87(8)°
and 81.74(14)°, respectively. In addition, the N1−U−N2 and
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Figure 9. Solid-state emission spectra of 1−4 excited at 365 nm at room temperature: (a) comparison of 1 and 2; (b) comparison of 1 and 3; (c)
comparison of 3 and 4; (d) comparison of 2 and 4.
O2−U−O2i bond angles of 4 are similar to 3 at 63.01(7)° and
80.60(13)°, respectively. Like 3, the overall structure of 4
contains a ladder motif that propagates along [001].
direction to form the overall extended structure (Figure 7).
Three types of π-stacking arrangements are observed in the
molecular packing. First, two bridging naphthalene rings show
π−π interaction with each other at a ring centroid C−H
distance of 3.618(5) Å (A). This is consistent with the reported
π stacking of naphthalene rings for uranyl NDC CPs at
distances of 3.3−3.45 Å,5,25 suggesting that π−π interactions for
5 are comparable and within the acceptable range of π-stacking
distances.47 Another π−π interaction is observed between the
outer pyridine rings, which stack above and below each dimer
at a ring centroid−centroid distance of 3.651(5) Å (B). Lastly,
π stacking is also present between the naphthalene and outer
pyridine rings of adjacent dimers with a ring centroid−centroid
distance of 3.580(5) Å (C).
Crystal Structure of 5. The crystal structure of 5 shows an
overall pentagonal-bipyramidal geometry of the uranium metal
center bound by two distinct NDC linkers and one TPY
(Figure 6). Like 1 and 2, the oxygen atoms O3 and O5 of NDC
are coordinated to the metal in a monodentate fashion,
exhibiting bond distances of 2.303(6) and 2.280(6) Å,
respectively. For TPY, coordination through the nitrogen
atoms N1, N2, and N3 shows an average bond distance of
2.589(7) Å. The nonbonded oxygen atoms O4 and O6 at bond
lengths of 1.223(10) and 1.211(10) Å, respectively, were
identified as carbonyl oxygen atoms. The N1−U−N2 and O3−
U−O5 bond angles formed by NDC and TPY are 63.15(2)°
and 80.61(2)°, similar to the bond angles of 1−4. The uranium
metal centers are linked together by bridging NDC carboxylate
groups to form a molecular dimer of 5, as seen previously for 1
and 2. These dimers are then oriented along the [110]
Crystal Structure of 6. The uranyl center of 6 is bound
equatorially to two distinct NDC linkers and one Cl-TPY to
form an overall pentagonal-bipyramidal geometry. Similarly to
3 and 4, each NDC linker is bound to two distinct uranyl
centers (Figure 8). The oxygen atoms O3 and O5 of NDC are
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2+
Figure 10. Diagram of the energy levels of ligand triplet states (77 K) and the highest excited state of UO2 (298 K).
coordinated to the uranyl center in a monodentate fashion at
distances of 2.257(7) and 2.312(7) Å, respectively. For Cl-TPY,
the nitrogen atoms N1, N2, and N3 are coordinated to the
uranyl center at an average distance of 2.585(8) Å. The
nonbonded carboxylate oxygen atoms O4 and O6 at distances
of 1.222(12) and 1.229(12) Å, respectively, were identified as
carbonyl oxygen atoms. The N1−U−N2 and O3−U−O5 bond
angles formed by either Cl-TPY or each NDC linker were
found to be 62.72(3)° and 83.61(3)°. Like 3 and 4, the uranyl
centers in 6 are linked together by NDC carboxylate groups as
a ladder motif propagating roughly along [101].
bond.49,50 The solid-state emission spectra of 1−4 obtained at
an excitation wavelength of 365 nm show uranyl emission
2+
(Figure 9), and additional spectroscopic features of UO2 can
be found in its UV−vis diffuse-reflectance spectrum (see Figure
S13 in the Supporting Information). The UV component
corresponds to the LMCT transitions from the HOMO U−O
bond to the LUMO nonbonding uranium(VI) orbitals, whereas
the visible component corresponds to the LMCT band
between coordinated equatorial ligands and the nonbonding
29
2+
or antibonding molecular orbitals of the UO2 cation.
2+
We observe that UO2 emission can be systematically
influenced when different aromatic carboxylate and TPY
ligands are used (Figure 9a−d). This is consistent with many
other studies including that by Severance and co-workers, who
noted shifts in uranyl emission as a function of pyridyl and
pyrazine carboxylates.51 To explain these shifts, we measured
the peak positions of 1−4 and calculated the difference
between the peak 1 energy values to each superimposed
spectra. These values can be found in the Supporting
Information (Tables S1 and S2). When TPY and Cl-TPY are
kept constant, with changes between the TDC and BDC linkers
(Figure 9a,d), an overall blue shift of −250 to −120 cm−1 of 1,
3 and 2, 4 is observed. Interestingly, the blue shift for 1 and 3
(Figure 9b) is more pronounced than that for 2 and 4 (Figure
9d) by −120 cm−1, suggesting that the presence of chlorine has
an influence on the observed spectra. On the other hand, when
TPY and Cl-TPY are changed, keeping TDC and BDC
constant (Figure 9a,c), the emission shifts are varied. A
comparison of 3 and 4 (Figure 9c) using BDC shows a blue
shift of −40 cm−1, which seems to indicate that only chlorine
has an influence on the observed shift. However, when 1 and 2
are compared using TDC (Figure 9a), a red shift of +80 cm−1 is
shown. We speculate that this may be due to the presence of
sulfur, which seems to counteract the effects of chlorine, with
sulfur having a larger influence on the observed shifts. From our
observations, we note that the presence of TDC and perhaps
the chlorine of Cl-TPY influence the spectral shifts of uranyl
TPY and Cl-TPY compounds. Like the emission spectra, the
UV−vis diffuse-reflectance spectra (see Figure S13 in the
Supporting Information) also show blue and red shifts for 1−4
compared to each other and uranyl acetate. It is unclear how
the heteroatoms in these ligands affect the energy levels of the
uranyl molecular orbitals. As such, a theoretical investigation to
explain these phenomena is currently underway.
Synthesis. The syntheses of 1−6 were carried out under
solvothermal reaction conditions in a H2O−2-propanol mixture
at 120 °C for 5 days. The initial pH was adjusted between 5.2
and 5.8 using 6 M NaOH, which resulted in crystals suitable for
single-crystal XRD. Different structures were observed as either
uranyl dimers (1, 2, and 5) or ladder motifs (3, 4, and 6) of the
uranyl centers. It is worth noting that the uranyl cation exists as
a monomer in 1−6 considering the tendency of this moiety to
hydrolyze in this pH range.2,38 A possible explanation of why
we do not observe oligomers in our system may be attributed
to the role of sterics and choice of ligands, which may influence
uranyl hydrolysis. One may speculate that the presence of TPY
ligands prevents uranyl hydrolysis because the steric contribu-
tion of the aromatic groups may reduce the number of available
open coordination sites necessary to promote oligomerization.
The reasons for the different structural types for 1−6 can be
explained as a function of the aromatic linker and substituent
on the TPY ligand. When TDC linkers are used, dimers are
observed (1 and 2), whereas the use of BDC results in ladder
motifs (3 and 4). This may be due to the size of the aromatic
ring (i.e., five-membered thiophene versus six-membered
benzene), which results in different structural motifs. For the
NDC ligand, however, the steric argument is not a clear
indication of why we observe both dimer and ladder motifs (5
and 6) and may point to more complex packing considerations
between TPY and Cl-TPY.
2+
Fluorescence Studies. The origin of UO2 emission is
caused by electronic transitions between the lowest unoccupied
molecular orbital (LUMO) 5fδ and 5fφ nonbonding uranium-
(VI) orbitals and the highest occupied molecular orbital
(HOMO) 6d−5f−2p U−O 3σu bond.48 Within these spectra,
six distinct peaks between 470 and 590 nm are observed, which
represent the vibronic structure of the uranyl cation caused by
symmetric and antisymmetric oscillations within the U−O
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Inorganic Chemistry
Article
Figure 11. Solid-state emission spectra of 5 and 6 excited at 365 nm at room temperature.
2+
To explore the potential for energy transfer between UO2
and the ligands used in this study, the triplet energy levels for
TPY, BDC, TDC, and NDC were obtained from measured
values of gadolinium (Gd3+) complexes described in the
literature or determined via low-temperature phosphorescence
spectra (see Figures S14 and S16 in the Supporting
Information).34,52 Phosphorescence measurements from Gd3+
complexes are a useful method to determine the triplet states of
organic linkers because of the large energy gap (32000 cm−1)
between the 8S7/2 ground state and the 6P7/2 excited state of the
Gd3+ ion.53 As a result, the excited state of the Gd3+ ion cannot
accept energy from the organic ligand, leading to a loss of
energy by nonradiative decay and thereby allowing the ligand
triplet energy to be measured. The energy levels of TDC, BDC,
NDC, and TPY and the emissive uranyl state are compared
(Figure 10).
influence of π stacking on the fluorescence of uranyl CPs has
been discussed54 and may imply an additional contribution to
the luminescence of 1−6 beyond the energy-transfer
mechanisms presented thus far. That said, no obvious
relationship between π stacking and luminescence is apparent
in these compounds.
In summary, we have synthesized and characterized six novel
uranyl terpyridine aromatic carboxylates using single-crystal
XRD and PXRD and UV−vis and fluorescence spectroscopy.
The coordination environment of the uranyl center with
aromatic linkers and TPY ligands produces monomeric units
that crystallize into molecular dimers (1, 2, and 5) or ladder-
like chains (3, 4, and 6) under solvothermal conditions.
Depending on the TPY ligands and coordinated aromatic
carboxylates, we note that uranyl emission can be tuned, in
which bathochromic and hypsochromic shifts are observed. At
present, it is unclear what influence π stacking may have on
uranyl emission, but future work to systematically explore such
phenomena is currently in progress.
In general, energy transfer requires two criteria that must be
fulfilled. First, the position of the ligand triplet energy level
2+
should be slightly above the energy level of the emissive UO2
ion (>1500 cm−1 is generally accepted to preclude efficient,
thermally mediated back energy transfer). Second, the ligand
itself should be at an appropriate transfer distance to the metal.
Although the energy levels and distances of the ligands are
indeed appropriate, the UV−vis diffuse-reflectance spectra (see
Figure S13 in the Supporting Information) show that π → π*
absorption of the TPY ligands and aromatic carboxylates occurs
in the same region as uranyl absorption, suggesting that we
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, PXRD patterns,
ORTEP diagrams, and UV−vis spectra for 1−6, tables of
emission peak positions and spectral shifts, and phosphor-
escence spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
2+
cannot indicate explicitly whether UO2 sensitization has
occurred.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cahill@gwu.edu. Phone: (202) 994-6959.
Notes
■
Compared to the emission spectra of 1−4, compounds 5 and
6 do not exhibit characteristic uranyl fluorescence (Figure 11),
the absence of which suggests back energy transfer to NDC. In
fact, of the ligands studied, the energy level of the NDC triplet
is lowest compared to that of the excited UO22+ and may allow
for a nonradiative decay from the uranyl to the ligand. In
contrast, the triplet energy levels of TDC, BDC, and TPY are
above the energy level of an excited UO22+, which would, in
turn, explain the observed uranyl emission in 1−4. Lastly, the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge funding from the Materials Science
of Actinides, an Energy Frontier Research Center, supported by
the U.S. Department of Energy Office of Basic Energy Science
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