Near-infrared emitting ytterbium metal-organic frameworks with tunable excitation properties

Article abstract of DOI:10.1039/b909658b

The design of metal-organic frameworks (MOFs) incorporating near-infrared emitting ytterbium cations and organic sensitizers allows for the preparation of new materials with tunable and enhanced photophysical properties. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b909658b

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Near-infrared emitting ytterbium metal–organic frameworks with tunable
excitation propertiesw
Kiley A. White,a Demetra A. Chengelis,a Matthias Zeller,b Steven J. Geib,a Jessica Szakos,a
a
a
´
Stephane Petoud* and Nathaniel L. Rosi*
Received (in Austin, TX, USA) 15th May 2009, Accepted 11th June 2009
First published as an Advance Article on the web 6th July 2009
DOI: 10.1039/b909658b
The design of metal–organic frameworks (MOFs) incorporating
near-infrared emitting ytterbium cations and organic sensitizers
allows for the preparation of new materials with tunable and
enhanced photophysical properties.
Yb3+ MOFs. MOFs are extensively studied for their gas
storage, catalytic, and molecular sieving properties; however,
their regular and tailorable structures render them potentially
useful materials for numerous other applications. Several
lanthanide-containing MOFs emitting in the visible have been
prepared and characterized spectroscopically5 but, to the best
of our knowledge, no MOF has been demonstrated to sensitize
NIR emitting lanthanides such as Yb3+, which offer a new
variety of applications for Ln-MOF materials. It has also not
been shown that one can modify the structural parameters of
MOFs to tune the photophysical properties of NIR emitting
lanthanide–MOF compounds. The strategies presented
herein will be useful for creating, optimizing, and tailoring
lanthanide-based materials for specific NIR applications.
We identified a ligand that could both sensitize NIR-emitting
Yb3+ and direct its assembly into an extended porous network.
4,40-[(2,5-Dimethoxy-1,4-phenylene)di-2,1-ethenediyl]bis-benzoic
acid (H2-PVDC) was chosen because it absorbs strongly in the
visible range, it could promote the formation of extended
MOF structures, and our preliminary studies (vide infra)
demonstrated that it was capable of sensitizing NIR-emitting
Yb3+. Reacting Yb(NO3)3ꢀ5H2O with H2-PVDC yielded
yellow needles of Yb-PVDC-1, [Yb2(C26H20O6)3(H2O)2]ꢀ
(DMF)6(H2O)8.5.z The material maintains its crystallinity in
a variety of solvents, as confirmed by powder X-ray diffraction
studies of solvent exchanged samples. Single crystal X-ray
diffraction analysis revealed that Yb-PVDC-1 crystallizes in
the high symmetry Fddd space group and is composed of
infinite Yb-carboxylate chains aligned along the a crystallo-
graphic direction (Fig. 1a–c).6 The chains consist of alternating
octa- and hexa-coordinated Yb3+, bridged in a di-monodentate
fashion via the carboxylates of three different PVDC linkers
(Fig. 1a and b). Two water molecules terminally coordinate to
the octa-coordinate Yb3+. The chains are connected along
[110] via the phenylene vinylene portion of the ligand resulting
in the formation of large 24 ꢁ 40 A channels (Fig. 1c).
Near-infrared (NIR) luminescent lanthanides exhibit useful
photophysical properties that make them crucial components
for applications such as photonic materials and optical
telecommunication devices, as well as bioanalytical and
biological imaging probes and sensors.1 To take advantage
of lanthanide luminescence, several requirements must be
fulfilled to obtain a sufficient number of photons for an
adequate detection sensitivity. Lanthanides need to be sensitized
by a suitable chromophoric moiety since the weak absorbtivity
of free lanthanide cations limits their luminescence intensity.1
The sensitization (called ‘‘antenna effect’’)2,3 involves placing
lanthanide cations in proximity to chromophoric molecules
having high absorptivity (‘‘the antenna’’) that efficiently
transfer energy to lanthanide accepting levels to trigger their
emission. Secondly, the photophysical properties of lanthanide
cations can be affected by their environment; for example,
–OH, –NH, and –CH vibrational overtones quench luminescence
intensity.1 Thus it is crucial to control the environment around
these cations; however, lanthanides have low stereochemical
requirements. Their coordination must be tridimensionally
manipulated by the design of the overall molecular complex
in which the antennae are arranged to provide control of the
photophysical properties and protection of the metal cations.
In addition, for practical applications, it is advantageous to
have control over the excitation wavelength.
In this communication, we address these antenna require-
ments by using a novel metal–organic framework (MOF)
approach4 to achieve control of lanthanide luminescence
properties. Our results demonstrate for the first time that
(i) within a MOF, efficient sensitization is achieved of a NIR
emitting lanthanide, Yb3+, and (ii) modification of our MOF
structure, without changing the chromophore linker, allows
for tuning of the luminescence properties of the resulting
a Department of Chemistry, University of Pittsburgh,
219 Parkman Avenue, Pittsburgh, PA 15260, USA.
E-mail: nrosi@pitt.edu, spetoud@pitt.edu; Fax: +1-412-624-8611;
Tel: +1-412-624-3987
b Department of Chemistry, Youngstown State University,
One University Plaza, Youngstown, OH 44555, USA
w Electronic supplementary information (ESI) available: Full experi-
mental information, data, and crystallographic details for all
compounds. CCDC 698331 and 698332. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b909658b
The UV-Vis absorption, emission, and excitation spectra
were measured for Yb-PVDC-1 and compared to corresponding
spectra recorded for the ligand H2-PVDC and a 1 : 1 Yb-PVDC
molecular complex (see ESIw) to determine how the MOF
ꢂc
This journal is The Royal Society of Chemistry 2009
4506 | Chem. Commun., 2009, 4506–4508

View Online
Fig. 2 Structure and spectral data for Yb-PVDC-2. Ball and stick
depiction of infinite SBU (a) and SBU with Yb3+ represented as
polyhedra (b); projection view of the framework viewed along the a
crystallographic direction (c); ligand stacking motifs (d) (C, grey; O,
red; Yb3+, dark green), and (e) luminescence data comparing the
excitation profiles of Yb-PVDC-1 (red; lem = 980 nm) to Yb-PVDC-2
Fig. 1 Structure and spectral data for Yb-PVDC-1. (a) Ball and stick
depiction of infinite SBU and (b) SBU with Yb3+ represented as
polyhedra (C, grey; O, red; Yb3+, dark green); (c) projection view of
the framework viewed along the a crystallographic direction; (d) ligand
stacking motif along [110], and (e) luminescence data comparing the
(blue; lem = 980 nm) and displaying the Yb3+ emission (green; lex
500 nm) upon excitation of Yb-PVDC-2.
=
excitation profiles of the Yb–PVDC molecular complex (red; lem
=
980 nm) to Yb-PVDC-1 (blue; lem = 980 nm) and displaying the YbIII
emission (green; lex = 470 nm) upon excitation of Yb-PVDC-1.
structure impacts the luminescence properties of the system.
The absorption spectrum of H2-PVDC displays two bands
with apparent maxima centered at 340 and 415 nm. Excitation
through either of these bands produces a fluorescence band
centered at 485 nm. The excitation spectrum recorded upon
this fluorescence band shows a profile similar to the absorption
spectrum (Fig. S2, ESIw). The Yb-PVDC molecular complex
displays the typical Yb3+ emission band in the NIR range
with an apparent maximum at 980 nm (Fig. S3, ESIw). The
excitation spectrum for this complex collected upon monitoring
Yb3+ emission (Fig. 1e and Fig. S3 (ESIw)) also contains two
bands centered at 340 and 415 nm that adopt the same profile
as the absorption of H2-PVDC, indicating that PVDC
sensitizes Yb3+ via the antenna effect. Luminescence analysis
of Yb-PVDC-1 (chloroform-exchanged material) displays
Yb3+ luminescence in the NIR (Fig. 1e). The MOF excitation
spectrum is notably red-shifted, displaying bands with maxima
at 370 and 470 nm (Fig. 1e). The apparent maximum of
the excitation band shifts significantly from 415 nm for the
Yb-PVDC complex to 470 nm for Yb-PVDC-1. Although the
Yb-PVDC complex experiments were performed in DMSO due
to solubility constraints, this observed shift of over 50 nm cannot
solely be attributed to a solvatochromic effect. We tentatively
attribute a significant component of this shift to organizational
constraints that the MOF architecture imparts on the phenylene
vinylene linkers. In Yb-PVDC-1, the ligands are arranged in
parallel along [110], which may allow for weak interactions
between neighboring chromophoric ligands (Fig. 1d). These
interactions are hypothesized to affect the electronic structure
of the chromophore, resulting in decreased excitation energy.
To evaluate the extent to which ligand–ligand interactions
impact the excitation and emission properties of Yb-PVDC
infinite Yb-carboxylate SBUs. However, the connectivity
within the SBU differs from that of Yb-PVDC-1. The SBU
is composed of alternating octa- and hexa-coordinated Yb3+
.
The Yb3+ are bridged by two carboxylates in a di-monodentate
fashion and by a third carboxylate that chelates the octa-
coordinate Yb3+ and coordinates in a monodentate fashion to
the hexa-coordinate Yb3+ (Fig. 2a and b). These coordination
modes result in a chain of corner-sharing polyhedral Yb3+
.
Each chain is linked to six other chains via the phenylene
vinylene portion of the PVDC linkers (Fig. 2c). The linkers
connecting the chains along the [001] stack in parallel, while
those that connect the chains in the [011] form criss-crossing
pairs with close p–p interactions7 (perpendicular distance
between planes: 3.6 A) between the central phenyl rings of
the PVDC linkers (Fig. 2d). Because each infinite SBU is
connected to six other SBUs, the resulting triangular channels
are smaller than those observed for Yb-PVDC-1, measuring
B13–14 A from corner to edge (Fig. 2c).
The luminescent properties of Yb-PVDC-2 (chloroform-
exchanged material) were examined to determine the impact
the structural changes have on the photophysical properties
of this system. The excitation spectrum collected upon
monitoring the emission intensity of Yb3+ luminescence at
980 nm displayed apparent band maxima at 370 and 500 nm
(Fig. 2e). The emission spectra collected in the NIR range
upon excitation at these wavelengths produce characteristic
Yb3+ emission. Interestingly, the lowest energy excitation
band of Yb-PVDC-2 is further red-shifted to 500 nm from
470 nm in Yb-PVDC-1. We tentatively propose that the close
p–p interactions between the PVDC linkers decrease the
energy of the p - p* transition, resulting in a lowered
excitation energy.
systems, we prepared
a
second MOF, Yb-PVDC-2,
To determine whether the MOF architecture provides
efficient protection for the lanthanide cations from solvent
quenching and to quantify the intramolecular energy transfer
[Yb2(C26H20O6)3]ꢀ(DMF)12(H2O)10.z Yb-PVDC-2 crystallizes
in the orthorhombic Pnna space group and also exhibits
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4506–4508 | 4507

View Online
Table 1 Absolute emission quantum yields (F)a and luminescent lifetimes (tx, ms)b of Yb3+ centered emission at 980 nm for the MOFsc
d
d
FYb
t1
t2
t3
t4
Yb-PVDC-1
Yb-PVDC-2
3.3 (ꢃ0.5) ꢁ 10ꢄ3
29 (ꢃ2)
22 (ꢃ4)
10 (ꢃ1)
1.5 (ꢃ0.5)
1.7 (ꢃ0.3)
0.34 (ꢃ0.06)
0.61 (ꢃ0.17)
1.8 (ꢃ0.2) ꢁ 10ꢄ2
5.6 (ꢃ1.5)
a
b
c
d
lex = 490 nm. lex = 354 nm. MOFs as crystalline solids under chloroform. Error included in parentheses.
of the systems, we measured quantum yield values using
an integration sphere (Table 1). The quantum yield of
Yb-PVDC-2 is five times higher than Yb-PVDC-1 when
excited through the lower energy band (490 nm). The quantum
yield of Yb-PVDC-2 is among the highest values reported for
ytterbium systems under solvent.8 These quantum yields are
global: the excitation is performed through the sensitizer and
the emission is observed through the Yb3+ cations that have
different coordination environments and levels of protection in
both MOFs. In Yb-PVDC-1, the octa-coordinate Yb3+
coordinate two water molecules which quench ytterbium
emission and lower the global quantum yield.
DBI0352346, SP) for funding this work. We thank Professor
J.-C.G. Bunzli and F. Gumy for the help with the construction
¨
of an integration sphere.
Notes and references
z Crystal data. Yb-PVDC-1. C39H30O10Yb, Mw = 831.67, ortho-
rhombic, a = 16.247(6), b = 48.939(19), c = 80.84(3) A, V =
64 280(43) A3, T = 253 K, space group Fddd, Z = 32, 95 070
reflections collected, 11 506 unique (Rint = 0.1522) which were used
in all calculations. The final wR(F2) was 0.1484 (all data). Yb-PVDC-2.
C78H60O18Yb2, Mw = 1631.34, orthorhombic, a = 16.0798(14),
b = 22.7096(19), c = 38.484(3) A, V = 14 053(2) A3, T = 298 K,
space group Pnna, Z = 4, 123 644 reflections collected, 17 435 unique
(Rint = 0.0580) which were used in all calculations. The final wR(F2)
was 0.2528 (all data).
We monitored ytterbium centered luminescence lifetimes in
order to further determine the effectiveness of the MOFs in
protecting the lanthanide cations from non-radiative deactiva-
tion. Both MOFs displayed multi-exponential decay patterns
and were best fit with four components (Table 1), which are
attributed to four different lanthanide environments: the hexa-
coordinate and octa-coordinate Yb3+ sites within the core of
the MOF structures and those along the terminating edges of
the crystals, where the lanthanide cations are more exposed to
sources of non-radiative deactivation. The long component
values are up to two times longer than the longest lifetimes
reported for Yb3+ molecular species in solution.2,9 These
luminescence lifetimes demonstrate that MOFs can provide
coordination environments with improved protection from
quenching than molecular complexes.
1 J.-C. G. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048–1077.
¨
2 J. Zhang, P. D. Badger, S. J. Geib and S. Petoud, Angew. Chem.,
Int. Ed., 2005, 44, 2508–2512.
3 (a) V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, M. Gorka and
F. Voegtle, J. Am. Chem. Soc., 2002, 124, 6461–6468; (b) R. Van
Deun, P. Fias, P. Nockemann, K. Van Hecke, L. Van Meervelt and
K. Binnemans, Inorg. Chem., 2006, 45, 10416–10418;
(c) T. K. Ronson, T. Lazarides, H. Adams, S. J. A. Pope,
D. Sykes, S. Faulkner, S. J. Coles, M. B. Hursthouse, W. Clegg,
R. W. Harrington and M. D. Ward, Chem.–Eur. J., 2006, 12,
9299–9313; (d) T. Gunnlaugsson and F. Stomeo, Org. Biomol.
Chem., 2007, 5, 1999–2009; (e) S. Comby, D. Imbert,
A.-S. Chauvin and J.-C. G. Bunzli, Inorg. Chem., 2006, 45,
¨
732–743; (f) A. Beeby, R. S. Dickins, S. Faulkner, D. Parker and
J. A. G. Williams, Chem. Commun., 1997, 1401–1402.
4 (a) J. L. C. Rowsell and O. M. Yaghi, Microporous Mesoporous
Mater., 2004, 73, 3–14; (b) B. Moulton and M. J. Zaworotko, Chem.
Rev., 2001, 101, 1629–1658; (c) D. Maspoch, D. Ruiz-Molina and
J. Veciana, Chem. Soc. Rev., 2007, 36, 770–818; (d) S. Kitagawa,
R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43,
2334–2375; (e) G. Ferey, Chem. Soc. Rev., 2008, 37, 191–214;
(f) J. A. Brant, Y. L. Liu, D. F. Sava, D. Beauchamp and
M. Eddaoudi, THEOCHEM, 2006, 796, 160–164.
We have illustrated the validity of a MOF-based approach
to sensitize NIR emitting Yb3+, which results in materials
with enhanced and controlled luminescence properties.
Specifically, we have shown that a chromophoric antenna
molecule and NIR emitting Yb3+ can be assembled into rigid
MOF structures that effectively control the coordination
environments around the lanthanide cations and the arrange-
ment of chromophoric antennae. Using this strategy, we
obtained a lower energy excitation wavelength by modifying
the three-dimensional MOF structure to allow for close p–p
interactions between the chromophores. The possibility to
have different excitation ranges without altering the structure
of the sensitizer provides a new route for controlling the
photophysical properties of lanthanide complexes, in contrast
to more traditional approaches which involve changing the
antenna’s structure. The structures of the MOFs also provide
protection of the lanthanide cations from solvent vibrations.
The ability to design MOFs and control their structural
features makes them ideal materials for carefully controlling
and optimizing the photophysical properties of lanthanide
cations.
5 (a) W. J. Rieter, K. M. L. Taylor and W. B. Lin, J. Am. Chem. Soc.,
2007, 129, 9852–9853; (b) F. Pelle, S. Surble, C. Serre, F. Millange
´ ´
and G. Ferey, J. Lumin., 2007, 122, 492–495; (c) D. T. de Lill, A. de
´
Bettencourt-Dias and C. L. Cahill, Inorg. Chem., 2007, 46,
3960–3965; (d) B. L. Chen, Y. Yang, F. Zapata, G. N. Lin,
G. D. Qian and E. B. Lobkovsky, Adv. Mater., 2007, 19,
1693–1696; (e) T. M. Reineke, M. Eddaoudi, M. O’Keeffe and
O. M. Yaghi, Angew. Chem., Int. Ed., 1999, 38, 2590–2594.
6 N. L. Rosi, J. Kim, M. Eddaoudi, B. L. Chen, M. O’Keeffe and
O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 1504–1518.
7 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112,
5525–5534.
8 (a) N. V. Rusakova, Y. V. Korovin, Z. I. Zhilina, S. V. Vodzinskii
and Y. V. Ishkov, J. Appl. Spectrosc., 2004, 71, 506–511;
(b) G. M. Davies, R. J. Aarons, G. R. Motson, J. C. Jeffery,
H. Adams, S. Faulkner and M. D. Ward, Dalton Trans., 2004,
1136–1144; (c) G. A. Hebbink, L. Grave, L. A. Woldering,
D. N. Reinhoudt and F. C. J. M. van Veggel, J. Phys. Chem. A,
2003, 107, 2483–2491.
9 (a) T. J. Foley, B. S. Harrison, A. S. Knefely, K. A. Abboud,
J. R. Reynolds, K. S. Schanze and J. M. Boncella, Inorg. Chem.,
2003, 42, 5023–5032; (b) F. R. Goncalves e Silva, O. L. Malta,
C. Reinhard, H.-U. Guedel, C. Piguet, J. E. Moser and
The authors thank the University of Pittsburgh (NLR and
SP), the ACS Petroleum Research Fund (PRF 47601-G10
NLR), and the National Science Foundation (NSF
J.-C. G. Bunzli, J. Phys. Chem. A, 2002, 106, 1670–1677.
¨
ꢂc
This journal is The Royal Society of Chemistry 2009
4508 | Chem. Commun., 2009, 4506–4508