Highly fluorescent M2L4 molecular capsules with anthracene shells

Article abstract of DOI:10.1039/c1cc12946e

M₂L₄ molecular capsules self-assembled from M(ii) ions (where M = Zn, Ni, and Pd) and bent bidentate ligands constructed from anthracene fluorophores. The Ni(ii) and Zn(ii) capsules exhibited weak to strong blue emission unlike traditional Pd(ii) cages and capsules. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc12946e

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COMMUNICATION
Highly fluorescent M2L4 molecular capsules with anthracene shellsw
Zhiou Li,a Norifumi Kishi,a Kimiko Hasegawa,b Munetaka Akitaa and Michito Yoshizawa*a
Received 19th May 2011, Accepted 14th June 2011
DOI: 10.1039/c1cc12946e
M2L4 molecular capsules self-assembled from M(II) ions (where
M = Zn, Ni, and Pd) and bent bidentate ligands constructed
from anthracene fluorophores. The Ni(II) and Zn(II) capsules
exhibited weak to strong blue emission unlike traditional Pd(II)
cages and capsules.
emission, lmax = 438 nm and FF = 0.81 and, surprisingly,
the Ni(II) capsule was also found to be emissive, FF = 0.20.
Anthracenes are a well-known class of highly emissive
fluorophores,6,7 yet the incorporation of anthracenes or other
similar, highly emissive large aromatics into the framework of
discrete coordination cages to install desirable photophysical
properties has yet to be reported.8 We recently succeeded in
the preparation of M2L4 molecular capsule 1Pd by using Pd(II)
ions and analogous bidentate ligand 2.9 The Pd(II) capsule
contains eight anthracene fluorophores but no emission was
observed. In a manner similar to other coordination hosts,
quenching likely occurs via the prerequisite transition metal
ions.3,10 To engender potentially useful emissive properties in
M2L4 coordination capsules, Zn(II) ions were employed as the
metal hinges as their fully occupied d10 electronic configuration is
generally more tolerant of emission from nearby fluorophores.4
Zn(OTf)2 proved to be the best source of the Zn(II) hinge and
we were delighted to find that simply switching from Pd(II) to
Zn(II) provided the highly fluorescent M2L4 coordination
capsule (Fig. 1).
Zn(II)-linked molecular capsule 1Zn formed quantitatively
upon mixing ligand 211 (17.0 mmol) and Zn(OTf)2 (8.50 mmol)
in CD3CN (0.5 mL) at 80 1C for 1 h (Fig. 2). In the 1H NMR
spectrum (Fig. 2b and c), the two signals of the peripheral
pendant methoxy groups Hk and Hj were clearly observed (at
4.22 and 3.91 ppm, respectively) where only Hk displayed a
significant downfield shift (Dd = +0.34 ppm) upon the
capsule formation.12 Eleven proton signals were observed for
Fluorescence has widespread applications from fundamental
sciences to practical materials and devices. Host–guest inter-
actions within the well-defined cavities of self-assembled,
nanometre-sized coordination capsules portend practical
applications as chemosensors, biological probes, and light-
emitting diodes.1,2 However, nearly all capsules and cages
composed of reversible metal–ligand bonds are non-emissive
due to quenching by the heavy transition metal ions.3 The few
coordination capsules assembled using Cd(II), Hg(II), or Zn(II)
ions can exhibit weak or moderate emission from the typically
small, aromatic ligand frameworks.4,5 Here we describe highly
fluorescent M2L4 coordination capsule 1Zn, quantitatively self-
assembled from Zn(II) ions and bent bidentate ligand 2 with
expanded aromatic surfaces (Fig. 1). The eight anthracene
panels define and enclose a large interior cavity, B1 nm in
diameter. In sharp contrast to non-emissive traditional Pd(II)
cages and capsules,3 the Zn(II) capsule shows strong blue
0
0
anthracene (Hc,c ,d,d ,e,f), pyridine (Hg,h,i), and m-phenylene
(Ha,b) rings in the aromatic region (Fig. 2c). The large upfield
shift of Hb on the central m-phenylene ring (Dd = ꢀ1.34 ppm)
is indicative of a capsule structure and increased shielding
from the flanking anthryl moieties. Upon heating (B50 1C),
Zn(II) capsule 1Zn remained intact and the broad proton signals
became relatively sharp (Fig. S11b, ESIw). At low temperature
(0 1C), they resolved into two sharp signals, indicating that the
motion of the anthracene panels was suppressed. The diameter of
the capsule was calculated to be approximately 1.6 nm on
the basis of the single band at log D = ꢀ9.1 (D = 7.5 ꢁ
Fig. 1 Design of highly fluorescent Zn(II)-linked molecular capsule
1Zn assembled from two Zn(II) ions and four bidentate ligands 2
consisting of large anthracene panels.
a Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: yoshizawa.m.ac@m.titech.ac.jp; Fax: +81-45-924-5230;
Tel: +81-45-924-5284
10ꢀ10 m2 ꢀ1) in the 1H DOSY NMR spectrum (Fig. S12,
s
ESIw).12 The ESI-TOF MS spectrum of a CH3CN solution of
1Zn unambiguously confirmed the M2L4 composition with
prominent signals at m/z = 737.5 ([1Zn ꢀ 4ꢂTf Oꢀ]4+), 747.7
([1Zn + CH3CN ꢀ 4ꢂTf Oꢀ]4+), 1032.9 ([1Zn ꢀ 3ꢂTf Oꢀ]3+),
b Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima,
Tokyo 196-8666, Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, physical properties, and crystallographic data. CCDC
800615. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc12946e
1046.6 ([1Zn
+
CH3CN
ꢀ
3ꢂTf Oꢀ]3+), and 1623.9
([1Zn ꢀ 2ꢂTf Oꢀ]2+) (Fig. 2d). These results, when coupled
c
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Chem. Commun., 2011, 47, 8605–8607 8605

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Ni
Fig. 3 X-ray crystal structure of Ni(II)-linked molecular capsule 10
:
(a) a stick and ball representation without hydrogen atoms (Ni: green
ball), and (b) side and (c) top views using a space-filling representation.
The counterions and solvent molecules are omitted for clarity.
Fig. 2 (a) Schematic representation of the preparation of Zn(II)-
linked molecular capsule 1Zn via self-assembly of bidentate ligand 2
and Zn(OTf)2.12,13 1H NMR spectra (400 MHz, rt) of (b) ligand 2 in
CDCl3 (*: CHCl3) and (c) capsule 1Zn in CD3CN. (d) ESI-TOF MS
spectrum of a CH3CN solution of 1Zn
.
with the lack of signals indicating 1Zn fragmentation, demon-
strate the stability of the M2L4 capsule structure in high
dilution under ESI-TOF MS conditions. Further 1H NMR
titration experiments corroborated the 2 : 4 metal to ligand
ratio and revealed that no proton signals derived from inter-
mediate species were observable during the self-assembly
processes. The analogous M2L4 molecular capsules 1M where
M = Ni or Pd9 were prepared using similar procedures.12
X-ray crystallographic analysis firmly established the discrete,
three-dimensional, hollow structure of the M2L4 molecular
Fig. 4 (a) UV-vis (CH3CN, 1M: 0.056 mM, 2: 0.23 mM, rt) and
(b) fluorescence (degassed CH3CN, lex = 396 nm, rt) spectra of
capsules 1Zn (blue line), 1Pd (orange line), 1Ni (green line) and ligand
2 (gray lines). (c) Blue emission from 1Zn and no emission of 1Pd in
CH3CN under UV irradiation (lex = 365 nm) at rt.
The UV-vis spectra of molecular capsules 1M in CH3CN are
dominated by the p–p* transitions of the anthracene moieties
from 320 to 430 nm (e.g., 1Zn: lmax = 378 nm, e = 75 ꢁ
103 Mꢀ1 cmꢀ1) (Fig. 4a). In ligand 2 and capsule 1M, the
anthracene rings are held orthogonal to the pyridine rings16 so
that the absorption bands in capsules 1M exhibit only a minor
red shift (Dl r +5 nm) with respect to ligand 2.
capsules. Pale yellow, X-ray quality single crystals of the Ni(II)
Ni
were obtained by slow vapor diffusion
analogue 10
of CH3OH into a CH3CN solution of the capsule.14 The
Ni
distorted spherical structure of capsule 10
possesses
D4-symmetry and is composed of four ligands and two Ni(II)
ions (Fig. 3).14 The coordination geometry of each Ni(II) atom
is octahedral with the four pyridyl groups coordinated in the
square planar position. The apical positions of one of the Ni
atoms are occupied by one H2O and one CH3CN molecule,
whereas two H2O molecules occupy the apical positions of the
other Ni atom. Both of the distance between the two Ni(II)
atoms and that between the antipodal anthracene rings are
B13.5 A. A single counter ion (ClO4ꢀ) and several CH3OH
molecules are incarcerated within the anthracene framework
Whereas the absorptive properties were insensitive to the
identity of the metal hinge in capsules 1M (M = Zn, Ni, and
Pd), the emissive properties turned out to be dependant on
the transition metal ion. Irradiation of a degassed CH3CN
solution of Zn(II) capsule 1Zn at 396 nm resulted in blue
emission at lmax = 438 nm with an absolute quantum yield
(FF) of 0.81.12 Ni(II) capsule 1Ni also exhibited similar
emission (lmax = 431 nm, FF = 0.20) but capsule 1Pd was
essentially non-emissive (FF E 0.00) (Fig. 4b and c).
Ni
of 10 (Fig. 3b and c). The optimized structure of 1Zn closely
The emission features and the quantum yield of 1Zn
closely match that of the parent luminophore in ligand 2
(lmax = 434 nm, FF = 0.79) and indicate that each of the
eight, closely assembled, anthracene rings of the capsule
Ni
resembles the crystal structure of 10 and, combined with the
NMR and MS data, substantiates a spherical M2L4 capsule
structure for 1Zn (Fig. S33, ESIw).12,15
c
8606 Chem. Commun., 2011, 47, 8605–8607
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behaves as independent luminophores.17 Switching from
Zn(II) to Ni(II) results in slight quenching of the emission
due to the presence of the paramagnetic metal ions, yet the
emission maximum and shape remain almost unchanged.
Emission from the Zn(II) capsule displayed greater oxygen
sensitivity than free ligand 2: in aerated solutions, emission
from 1Zn was quenched by 46% whereas that from 2 was only
quenched by 19%.18
5 There are a few exceptions where Pt(II)-linked coordination cages show
strong emission from the large aromatic subunits: (a) M. Busi,
M. Laurenti, G. G. Condorelli, A. Motta, M. Favazza,
I. L. Fragala, M. Montalti, L. Prodi and E. Dalcanale, Chem.–Eur.
J., 2007, 13, 6891–6898; (b) S. Ghosh and P. S. Mukherjee, Organo-
metallics, 2008, 27, 316–319.
6 (a) M. Kessinger and J. Michl, Excited States and Photochemistry
of Organic Molecules, VCH, New York, 1995; (b) N. J. Turro,
Modern Molecular Photochemistry, Benjamin, Menlo Park, 1978.
7 (a) S. Yamaguchi, S. Akiyama and K. Tamao, J. Am. Chem. Soc.,
2001, 123, 11372–11375; (b) K. Suzuki, A. Seno, H. Tanabe and
K. Ueno, Synth. Met., 2004, 143, 89–96; (c) A. Petitjean,
R. G. Khoury, N. Kyritsakas and J.-M. Lehn, J. Am. Chem.
Soc., 2004, 126, 6637–6647; (d) W. J. Yang, D. Y. Kim,
M.-Y. Jeong, H. M. Kim, Y. K. Lee, X. Fang, S.-J. Jeon and
B. R. Cho, Chem.–Eur. J., 2005, 11, 4191–4198; (e) Y. H. Kim,
H. C. Jeong, S. H. Kim, K. Yang and S. K. Kwon, Adv. Funct.
Mater., 2005, 15, 1799–1805.
8 (a) M. Scherer, D. L. Caulder, D. W. Johnson and
K. N. Raymond, Angew. Chem., Int. Ed., 1999, 38, 1588–1592;
(b) M. R. Johnston, M. J. Latter and R. N. Warrener, Org. Lett.,
2002, 4, 2165–2168; (c) W. Meng, B. Breiner, K. Rissanen,
J. D. Thoburn, J. K. Clegg and J. R. Nitschke, Angew. Chem.,
Int. Ed., 2011, 50, 3479–3483.
9 N. Kishi, Z. Li, K. Yoza, M. Akita and M. Yoshizawa, submitted.
10 M2L4 coordination cages with Pd(II) ions: (a) D. A. McMorran
and P. J. Steel, Angew. Chem., Int. Ed., 1998, 37, 3295–3297;
(b) D. K. Chand, K. Biradha and M. Fujita, Chem. Commun.,
2001, 1652–1653; (c) N. L. S. Yue, D. J. Eisler, M. C. Jennings and
R. J. Puddephatt, Inorg. Chem., 2004, 43, 7671–7681;
(d) G. H. Clever, S. Tashiro and M. Shionoya, Angew. Chem.,
Int. Ed., 2009, 48, 7010–7012; (e) P. Liao, B. W. Langloss,
A. M. Johnson, E. R. Knudsen, F. S. Tham, R. R. Julian and
R. J. Hooley, Chem. Commun., 2010, 46, 4932–4934.
In summary, we have prepared metal-linked M2L4 molecular
capsules with B1 nm cavities fully enshrouded by the eight
large panels of anthracene fluorophores. In sharp contrast to
the majority of previous coordination cages and analogous
Pd(II) capsules, the Zn(II) and Ni(II) capsules are emissive with
the high and moderate quantum yields, respectively. The
emissive properties of these new molecular capsules bearing
fluorescent shells promise novel applications where supra-
molecular host–guest interactions can be used to develop
new fluorescent functional materials, sensors, and devices.
These novel fluorescent capsules are particularly attractive as
zinc is a ‘‘green’’ transition metal and the exterior functional
groups can be readily modified and elaborated.
This work was supported by the Japanese Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
via Grants-in-Aid for Scientific Research on Innovative Areas
‘‘Coordination Programming’’ (Area 2107, No. 21108011) and
the global COE program (GCOE) ‘‘Education and Research
Center for Emergence of New Molecular Chemistry’’, and
the Japan Society for the Promotion of Science (JSPS) via
‘‘Funding Program for Next Generation World-Leading
Researchers’’. The authors would like to thank Dr. J. K.
Klosterman for his helpful discussions.
11 Ligand 2 was synthesized in six steps from 1,3-dimethoxybenzene
using Negishi and Suzuki–Miyaura palladium catalyzed cross-
coupling procedures (48% total yield).12 The pyridyl methoxy
groups of 2 serve as useful NMR tags to monitor the capsule
formation, as they are quite sensitive to metal coordination.
12 See ESI.w The Ni(II) capsule exhibited paramagnetic behavior as is
evident from its 1H NMR spectra. Instead, the M2L4 composition
of the capsule was determined by ESI-TOF MS analysis.
Notes and references
13 Axial ligands (L0) coordinated to the Zn(II) center (L0 = CD3CN,
TfOꢀ, and/or H2O) are omitted for clarity.
1 (a) B. Valeur, Molecular Fluorescence: Principles and Applications,
Wiley-VCH, Weinheim, 2002; (b) V. Balzani, A. Credi and
M. Venturi, Molecular Devices and Machines: Concepts and
Perspectives for the Nanoworld, Wiley-VCH, Weinheim, 2nd edn,
2008.
2 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice,
Chem. Rev., 1997, 97, 1515–1566; (b) L. Prodi, F. Bolletta,
M. Montalti and N. Zaccheroni, Coord. Chem. Rev., 2000, 205,
59–83.
3 (a) B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001,
40, 2022–2043; (b) S. R. Seidel and P. J. Stang, Acc. Chem. Res.,
2002, 35, 972–983; (c) F. Hof, S. L. Craig, C. Nuckolls and
J. Rebek, Jr., Angew. Chem., Int. Ed., 2002, 41, 1488–1508;
(d) D. Fiedler, D. H. Leung, R. G. Bergman and
K. N. Raymond, Acc. Chem. Res., 2005, 38, 349–358;
(e) M. Fujita, M. Tominaga, A. Hori and B. Therrien, Acc. Chem.
Res., 2005, 38, 369–378; (f) J. Rebek, Jr., Angew. Chem., Int. Ed.,
2005, 44, 2068–2078; (g) J. K. Klosterman, Y. Yamauchi and
M. Fujita, Chem. Soc. Rev., 2009, 38, 1714–1725;
(h) M. Yoshizawa, J. K. Klosterman and M. Fujita, Angew. Chem.,
Int. Ed., 2009, 48, 3418–3438.
4 (a) N. K. AI-Rasbi, C. Sabatini, F. Barigelletti and M. D. Ward,
Dalton Trans., 2006, 4769–4772; (b) K. Harano, S. Hiraoka and
M. Shionoya, J. Am. Chem. Soc., 2007, 129, 5300–5301;
(c) I. S. Tidmarsh, T. B. Faust, H. Adams, L. P. Harding,
L. Russo, W. Clegg and M. D. Ward, J. Am. Chem. Soc., 2008,
130, 15167–15175.
14 The methoxy groups of 1Ni were replaced with methoxyethoxy
Ni
groups in 10 to facilitate crystallization. X-Ray crystal data of
10Ni: C234H249Cl4N9Ni2O59, Mr = 4390.78, crystal dimensions
0.15 ꢁ 0.11 ꢁ 0.10 mm3, tetragonal, P4nc, a = b = 19.4133(4) A,
c = 29.968(2) A, V = 11294.4(9) A3, Z = 2, rcalcd = 1.291 g cmꢀ3
,
F(000) = 4624, radiation, l(CuKa) = 1.54187 A, T = 93(2) K,
reflections collected/unique 124 070/10 352 (Rint = 0.0448). The
structure was solved by the direct methods (SIR2004) and refined
by full-matrix least-squares methods on F2 with 745 parameters.
R1 = 0.0573 (I 4 2s(I)), wR2 = 0.1606, GOF 0.981; max/min
residual density 0.592/ꢀ0.603 eAꢀ3. CCDC 800615.
15 Octahedral coordination geometry of Zn(II)-complexes:
(a) P. Losier and M. J. Zaworotko, J. Chem. Crystallogr., 1996,
26, 277–280; (b) D. R. Turner, M. Henry, C. Wilkinson,
G. J. McIntyre, S. A. Mason, A. E. Goeta and J. W. Steed,
J. Am. Chem. Soc., 2005, 127, 11063–11074; (c) T. D. Hamilton,
D.-K. Bucar, M. B. J. Atkinson, G. S. Papaefstathiou and
L. R. MacGillivray, J. Mol. Struct., 2006, 796, 58–62.
16 J. Iwasa, K. Ono, M. Fujita, M. Akita and M. Yoshizawa, Chem.
Commun., 2009, 5746–5748.
Zn
17 The emission capability of capsule 10 bearing methoxyethoxy
groups was similar to that of 1Zn
.
18 The emission lifetime (t) of 1Zn (12 ns) is longer than that of 2
(10 ns). In the solid state, an emission spectrum (lex = 396 nm) of
capsule 1Zn showed a broadened band (lmax = 460 nm) with an
absolute quantum yield (FF) of 0.06.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8605–8607 8607