Crown-ether-like PbII-metal framework with dual- And bimodal emissive properties based on its photochromic precursor by leaching

Article abstract of DOI:10.1002/chem.200901412

Kingdom for a lead crown: A crownether-like, open Pb^II framework is generated from its phtotochromic precursor by a leaching approach. The dual and bimodal emissions are successfully realized by intercalation of the different Ln³⁺ hy

Full text of DOI:10.1002/chem.200901412

DOI: 10.1002/chem.200901412
Crown-Ether-Like PbII-Metal Framework with Dual- and Bimodal Emissive
Properties Based on Its Photochromic Precursor by Leaching
You-Yun Jiang, Shu-Kui Ren, Jian-Ping Ma, Qi-Kui Liu, and Yu-Bin Dong*[a]
Owing to their potential as high-performing photochemi-
cal and photophysical materials, numerous studies of metal–
organic frameworks (MOFs) have been reported.[1] For ac-
cessing to the dual (for example green and red emissions)
and bimodal emission (UV/Vis and NIR emissions), signifi-
cant effort has been afforded in design and synthesis of the
heterometallic complexes, especially heterolanthanide-based
complexes.[2] However, the preparation of f–f and f–d hy-
brids are very hard due to the inherent difficulty in synthesis
of the organic ligands with specific recognizing sites re-
quired by the different metal ions.[3]
networks. In such a mixed-ligand framework, the weakly co-
ordinating ligands might be selectively leached by solvent;
meanwhile the robust joints consisting of stronger coordinat-
ing ligands and metal cations are intact. In this paper, we
present the first example of such a mixed-ligand (BIPY:
4,4’-bipyridine and L:1,5-bis(m-carboxyphenoxy)-3-oxapen-
tane) PbII-framework, furthermore, the hollow oxygen-rich
framework Pb2L2 by leaching of the coligand of BIPY under
reflux in CDCl3. Consequently, the [Ln
[Ln/Ln’
(H2O)8]3+ [Pb2L2] (Ln=Tb3+; Ln’=Eu3+ or Nd3+
host–guest f–d and f–f’–d hybrids were obtained by a rever-
sible [Ln
(H2O)8]3+ encapsulation. As a result, the dual- and
ACHTUNGTRENNUNG
A
ꢀ
A
)
Motivated by our interest in tunable luminescent host–
guest complexes,[4] we have initiated a synthetic program for
preparation of the open heteroatom-rich MOFs, in which
the flexible open-chain polyether-bridged organic spacers
are chosen as the building blocks.[5] The porous metal–or-
ganic frameworks containing heteroatom-rich rungs would
incorporate electrophilic species into a host–guest system in
a crown ether manner. The synthesis of hollow MOFs based
on long, flexible, open-chain polyether-bridged ligands, how-
ever, is extremely challenging, owing to an unavoidably
large amount of framework interpenetration. Herein, we
present a new leaching approach to prepare the hollow
oxygen-rich frameworks. Our strategy for the construction
of hollow oxygen-rich MOFs, lies with the recognition that
the MOFs are designed to possess at least two different
types of ligands with different coordinating functional
groups (for example, with neutral and charged coordination
donors) can be used to synthesize mixed-ligand coordination
ACHTUNGTRENNUNG
bimodal emissions are successfully realized originated from
different Ln3+ emission colors (Scheme 1).
The flexible open-chain polyether-bridged bicarboxylate L
and rigid BIPY (Scheme 2) are our ligands of choice for
constructing oxygen-rich MOF of 1. L is more extended and
flexible than the open-chain polyether-bridged spacer previ-
ously reported by us.[5] The three bridging oxygen atoms and
the two terminal carboxylate groups on L would result in
the target oxygen-rich framework (Supporting Information).
The complex [Pb2L
crystals by treatment of PbAHCTNUGTRENNNUG
(bipy)] (1) was obtained as colorless
(OAc)2 with L/BIPY under hy-
drothermal conditions (1808C for 70 h) in high yield (93%).
The single-crystal analysis revealed that 1 crystallizes in the
monoclinic space group P21/c. The PbII center lies in the
four-coordinated {PbO4} coordination sphere consisting of
four carboxylate O-donors. In addition, the PbII center still
À
weakly coordinates to the Nbipy-donor with a long Pb N dis-
tance of 2.629(7) ꢀ (Scheme 2). As shown in Figure 1, the
PbII centers extend along the crystallographic b axis through
L to form a single-stranded helix with a period of separation
of ꢁ24 ꢀ. All the helices are further linked to each other by
the co-ligands of BIPY to generate a “double-edged” par-
quet-like net[6] in the crystallographic bc plane, in which the
right- and left-handed strands are arranged alternatively.
Complex 1 is self-intercalated and 5-fold interpenetrated.
When viewed down the crystallographic c axis, the rectangu-
lar channels are found, in which the BIPY co-ligands are lo-
cated. The opposite oxygen distances in the channel are
[a] Y.-Y. Jiang, S.-K. Ren, J.-P. Ma, Q.-K. Liu, Prof. Dr. Y.-B. Dong
College of Chemistry, Chemical Engineering and
Materials Science, Key Lab of Molecular and Nano Probes
Engineering Research Center of Pesticide and
Medicine Intermediate Clean Production
Ministry of Education, Shandong Normal University
Jinan, 250014 (P. R. China)
Fax : (+86)531-82615258
E-mail: yubindong@sdnu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901412.
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Chem. Eur. J. 2009, 15, 10742 – 10746

COMMUNICATION
tion peak at l=319 nm, while
compound 2 presents additional
two peaks at l=365 and
577 nm, which are attributed to
the purple appearance and the
radicals of 2, respectively. The
red trace in the UV/Vis spec-
Scheme 1. The synthesis of crown-ether-like, f–d hybrid host–guest system generated from flexible oxygen-rich
carboxylate ligand and rigid bipyridyl ligand. The red and green balls represent oxygen atoms and d-metal
ions, respectively.
trum indicates the solid-state
transformation occurred gradu-
ally. For confirming of 1 to 2 is
Scheme 2. Synthesis of 1 and 2.
Figure 1. Single 2D and 5-fold interpenetrating network (top) and the p–
p interaction-driven channels (bottom) in 1. The P and M helices are
highlighted in different colors.
Figure 2. The ESR spectra of a) 1 and b) 2. c) The UV/Vis spectra of the
transition between 1 and 2.
about 8–11 ꢀ. Notably, the channel-frame is significantly re-
inforced by the interlayer p–p interactions (Figure 1).
Interestingly, 1 is photochromic. Upon exposure to sun-
G
a photoinduced process instead of an oxidation process, the
crystals of 1 were kept in dark in oxygen, however, no colo-
ration occurred. On the other hand, when the crystals of 1
were kept in an inert atmosphere but exposed to light, the
coloration occurred as usual. In addition, no coloration hap-
pened by heating the crystals of 1 in either air or inert at-
mosphere without light.
To explore the origin of the radicals of 2, a parallel ex-
periment was carried out. When the crystals of 1 were re-
fluxed in CDCl3, the weakly coordinated BIPY was selec-
tively extracted to generate the hollow oxygen-rich frame-
work of 3 (Figure 3). Such selective solid–liquid extraction is
equivalent to the wet-chemical etching technique, which is
light (ꢁ2 days), 1 undergoes a radical-based single-crystal-
to-single-crystal transformation from the colorless to the
purple of 2. To our knowledge, the photochromic MOFs are
very rare.[7] Notably, the structures of 1 and 2 are identical
(Supporting Information); confirmed by the single-crystal
analysis and IR spectra. However, the variations of the
solid-state UV/Vis adsorption and ESR spectra demonstrate
the essential difference between 1 and 2. As shown in
Figure 2, the ESR spectrum of 1 shows no signal, but a
single peak radical signal appears after irradiation. In the
UV/Vis spectra, compound 1 exhibits only one p–p* absorp-
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10743

Y.-B. Dong et al.
absorption of [TbACTHNUTRGNEUNG
(H2O)8]3+ is basically saturated. The ICP
(inductively coupled plasma) measurement indicates that
the doped amount of Tb3+ is 3.24%. Interestingly, the en-
capsulated [Ln
shown in Figure 4b–c, when the obtained [Tb
[Pb2L2] was treated with excess of Eu(ClO4)3 in water for
two days, the emission spectrum indicated that the original
encapsulated [Tb
(H2O)8]3+ were partially replaced by [Eu-
(H2O)8]3+, generating a heterometallic host–guest system
(H2O)8]3+ can be reversibly displaced. As
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(H2O)8]3+
ꢀN
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[Tb/Eu
N
ꢀ
that the doped amount of Tb3+ and Eu3+ are 2.10 and
4.88%, respectively. As shown in Figure 4b, the correspond-
ing emission intensity originated from Eu3+ is stronger than
that of Tb3+. On the other hand, when the sample of (b)
was treated in water for two days with an excess of Tb3+
,
the emission intensity of Tb3+ recovered, and the emission
intensity originated from Eu3+ was correspondingly de-
pressed (Figure 4c). Thus, the emission intensities originate
from the different lanthanide species and can be tuned by
controlling the Eu/Tb displacement. The XRPD monitoring
indicates that the original structure remains intact during
the displacing course (Supporting Information). By this
gust-driven approach, two different lanthanide cations (Eu3+
and Tb3+) were successfully incorporated into a single probe
host–guest system and afford an interesting dual-emission
within the UV/Vis region upon a single excitation wave-
length. It is well known that the hydration states of the lan-
thanide complexes have a very significant effect on their
metal-centered luminescent behaviors resulted from the vi-
Figure 3. The synthesis of 3 from 2 by leaching and their corresponding
XRPD patterns and H NMR spectra.
1
widely used in preparation of the inorganic nano-materials.[8]
1
This leaching process was monitored by H NMR spectrum.
The XRPD pattern indicates that the framework is main-
tained upon leaching, which clearly benefits from the strong
interchain p–p interactions. The corresponding ESR spec-
trum of 3, however, exhibits no signal, which demonstrates
the radicals originated from the PbII-bipy electron transfer
(Supporting Information).
As shown above, the encapsulated BIPY can be selective-
ly leached to generate the hollow the oxygen-rich frame-
work of 3, which contains channels with hydrophilic inner
surfaces. The cavity size (dO···O =8–11 ꢀ) and hydrophilic
brational quenching. The intense emission of [LnACTHNUTRGNEUNG
(H2O)8]3+
À
herein reasonably indicates that the strong coupling of O H
vibrations on the coordinated water molecules is efficiently
À
reduced within the confining space by the host–guest O
H···O hydrogen-bonding interactions (Supporting Informa-
tion).[4,9]
Besides UV/Vis emitter, the encapsulated [Tb
can also be displaced by the NIR-lanthanide emitter such as
Nd3+. For example, treatment of [Tb(H2O)8]3+
[Pb2L2] with
an excess of Nd(ClO4)3 resulted in the formation of hetero-
metallic host–guest system [Tb/Nd [Pb2L2]. As
(H2O)8]3+
(H2O)8]3+
ACHTUNGTRENNUNG
A
ꢀACHTUNGTRENNUG
nature are perfect for binding [Ln
G
ACHTUNGTRENNUNG
hydrogen-bonding interactions (Supporting Information).[4]
A
ꢀACHTUNGTRENNUG
Indeed, when 3 was stirred in an aqueous solution of Tb-
ACHTUNGTRENNUNG(ClO4)3 (excess) for 3 h, the solid-state photoinduced-emis-
shown in Figure 5, the typical Nd3+ emission bands in the
NIR region increased at the expense of Tb3+ emission as
the reaction going on. The ICP analysis indicates that the
adsorbed amount of Nd3+ is up to 4.48% and corresponding
sion monitoring indicates that the maximal emission intensi-
ty of Tb3+ was obtained (Figure 4a), which reflects that the
Figure 4. Emission spectra of a) 3 stirred in aqueous solution of Tb
ACHTNUGTNERN(NUG ClO4)3 taken after 3 h (lex =303 nm); b) sample of a) stirred in aqueous solution of
EuG(ClO4)3 at 1 (black) and 2 (red) days (lex =303 nm); c) sample of b) stirred in aqueous solution of Tb
E
=
303 nm).
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Lead(II) Metal Framework
COMMUNICATION
mental analysis (%) calcd for C46H40N2O14Pb2: C 43.88, H 2.97, N 2.22;
found: C 43.73, H 2.78, N 2.35. When 1 was exposed to sunlight in air for
ꢁ2 days, compound 2 was generated. The IR spectrum of 2 is the same
as that of 1.
Synthesis of [Pb2L2] (3): Complex 2 was refluxed in CDCl3 till the BIPY
signals of the CDCl3 extract disappeared from the 1H NMR spectrum,
the resultant crystalline solids were collected and washed by water,
MeOH and CDCl3, and dried in air. IR (KBr pellet): n˜ =2919(w),
2229(w), 1681(w), 1600(w), 1538(s), 1438(m), 1391(s), 1314(w), 1281(w),
1241(m), 1119(w), 1059(w), 1024(w), 965(w), 881(w), 793(w), 763(m),
677(w), 621(w), 466 cmÀ1 (w); elemental analysis (%) calcd for
C36H32O14Pb2: C 39.16, H 2.90, N 0.00; found: C 39.08, H 2.85, N 0.07.
Figure 5. Emission spectra of [Tb
solution of NdACHTUNGTRENNUNG(ClO4)3 at 2 (black), 3 (red) and 4 (blue) days.
G
ꢀ
Typical ion-exchange reaction: G1ꢀLnIII2L4 (180 mg, G1=[Ln
ACHTUNGTRENNUNG )
(H2O)8]3+
was stirred in an aqueous solution containing excess of G2 (G2=Ln’-
residual Tb3+ is 1.62% after four days. Thus, the ion-ex-
change between Nd3+ and Tb3+ did lead to the tunable
emission between UV/Vis and NIR regions, consequently,
the bimodal-emission but excited at different wavelengths.
Again, the XRPD monitoring confirms that the original
structure remains intact during the reaction course (Sup-
porting Information). The PbII-polymeric complexes herein
are insoluble in water, so the possibility of a dissolution-re-
crystallization mechanism for explaining such ion-exchange
reaction is certainly impossible. In addition, the resulted
AHCTUNGTRENN(GNU ClO4)3·6H2O). The resulted crystalline solids were collected and washed
by water (5 mLꢂ4) and EtOH (5 mLꢂ2), Et2O (5 mLꢂ1), and dried in
air.
Crystal data for 1: C46H40N2O14Pb2, monoclinic, P21/c, Mr =1259.18, a=
12.2792(3), b=24.8799(5), c=6.9753(2) ꢀ, V=2117.26(9) ꢀ3, Z=2,
1calcd =1.975 gcmÀ3 (MoKa)=8.015 mmÀ1
, mACHTUGNTRENUNNG , FACHTUGNRETNNUG
lACHTUNGTRENNUNG
AHCTUNGTRENNUNG
=
,
m
G
,
FACHTUNGTRENNUNG
lACHTUNGTRENNUNG
AHCTUNGTRENNUNG
crystalline [Ln/Ln’
washed by water, EtOH and Et2O, so the surface adsorption
was avoided.
G
ꢀ
Crystallographic data (excluding structure factors) for the structure re-
ported in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication no. CCDC- 688537 (1)
and 688538 (2). Copies of the data can be obtained free of charge on ap-
plication to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
In summary, a crown-ether-like, open PbII-framework is
generated from its photochromic precursor by a leaching ap-
proach. Furthermore, the dual- and bimodal-emission are
successfully realized by intercalation of the different Ln3+
hydrates based on guest-driven approach. In addition, the
results herein demonstrate that the guest species within the
confining space exhibit the different behaviors from that of
unconfined state.[10] We believe that this method can be ap-
plied to other coordination frameworks and can give rise to
new hollow heteroatom-rich MOFs, which are not easy ob-
tained by the common synthetic methods.
AHCTUNGTREG(NNNU +44)1223–336–033; e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
We are grateful for financial support from NSFC (Grant Nos. 20871076
and 20671060), Shangdong Natural Science Foundation (Grant No.
JQ200803) and Ph.D. Programs Foundation of Ministry of Education of
China (Grant No. 200804450001).
Keywords: lanthanides · leaching · lead · metal–organic
frameworks · photochromism
Experimental Section
Synthesis of L: A mixture of corresponding polyether-bridged bisbenzal-
dehyde (1.256 g, 4.0 mmol) and AgNO3 (2.58 g, 15 mmol) was stirred in a
NaOH (7%) aqueous solution at 808C for 24 h. After neutralization by
HCl, the product was obtained as white solids (85%). 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=12.99 (s, 2H, -COOH), 7.53–7.50 (d,
2H, -C6H4-), 7.43–7.36 (m, 4H, -C6H4-), 7.19–7.17 (d, 2H, -C6H4-), 4.17 (s,
4H, -OCH2CH2O-), 3.84 ppm (s, 4H, -OCH2CH2O-); IR (KBr pellet):
n˜ =2956(w), 2360(w), 1743(s), 1604(s), 1488(s), 1458(s), 1391(s), 1297(s),
1223(s), 1168(m), 1138(s), 1092(m), 1039(s), 939(s), 910(m), 870(w),
832(w), 750(s), 690(s), 646(m), 525 cmÀ1 (w); elemental analysis (%)
calcd for C22H14N4O2: C 62.43, H 5.20; found: C 62.64, H 5.54.
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Synthesis of [Pb2L2ACHTUNGTRENNUNG(bipy)] (1) and (2): L (6.0 mg, 0.017 mmol), PbACHTUGNTREN(NUNG OAc)2
(9.7 mg, 0.026 mmol), 4,4’-bipyridine (5.3 mg, 0.034 mmol) and water
(2 mL) were sealed in a 5 mL glass tube. The mixture was heated at
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isolated from the tube in 93% yield. IR (KBr pellet): n˜ =2915(w),
2866(w), 1600(vs), 1538(vs), 1489(s), 1437(s), 1390(vs), 1281(s), 1239(s),
1143(m), 1120(m), 1079(m), 1060(s), 1024(w), 998(w), 964(m), 926(w),
904(w), 881(m), 864(m), 793(s), 763(vs), 676(s), 620(s), 412 cmÀ1 (w); ele-
Chem. Eur. J. 2009, 15, 10742 – 10746
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10745

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recent examples show that the encapsulated guest species in the re-
stricting space exhibits some unusual properties which are resulted
from the intriguing host–guest interactions. See a) D. H. Leung,
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˘
Dinca, K. Tamaki, J. R. Long, J. Am. Chem. Soc. 2008, 130, 5854;
[10] It is well-known that the free [Ln
G
e) C.-D. Wu, A. Hu, L. Zhang, W. Lin, J. Am. Chem. Soc. 2005, 127,
8940.
cent or weak luminescent. The [Ln
(H2O)8]3+ species within the re-
stricting space herein, however, is strong luminescent because the
À
O H oscillators in the coordinated water molecules are significantly
restricted through the host–guest H-bonding interactions. Several
Received: May 26, 2009
Published online: September 10, 2009
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Chem. Eur. J. 2009, 15, 10742 – 10746