A highly fluorescent metallosalalen-based chiral cage for enantioselective recognition and sensing

Article abstract of DOI:10.1002/chem.201304606

A highly fluorescent coordination cage [Zn₈L₄I ₈] has been constructed by treating enantiopure pyridyl- functionalized metallosalalen units (L) with zinc(II) iodide and characterized by a variety of techniques including microanalysis, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, and single-crystal and powder X-ray diffraction. Strong intermolecular π-π, CH-π, and CH-I interactions direct packing of the cage molecules to generate a 3D polycage network interconnected by pentahedral cages formed by adjacent pentamers. The cage has an amphiphilic helical cavity decorated with chiral NH functionalities capable of interactions with guest species such as saccharides. The fluorescence of the cage was greatly enhanced by five enantiomeric saccharides in solution, with enantioselectivity factors of 2.480-4.943, and by five enantiomeric amines in the solid state, with enantioselective fluorescence enhancement ratios of 1.30-3.60. This remarkable chiral sensing of both saccharides and amines with impressive enantioselectivity may result from the steric confinement of the cavity as well as its conformational rigidity. It holds great promise for the development of novel chiral cage materials for sensing applications. Cage-based chiral sensor: A highly fluorescent coordination cage [Zn₈L ₄I₈] can be prepared from enantiopure pyridyl- functionalized metallosalalen units (L). The cage has an amphiphilic helical cavity decorated with chiral NH functionalities and supramolecular interactions generate a 3D polycage network interconnected by pentahedral cages formed by adjacent pentamers (see graphic). The fluorescence of the cage is greatly enhanced either in solution or in the solid state in the presence of enantiomeric saccharides or amines, respectively, with significant enantioselectivity factors.

Full text of DOI:10.1002/chem.201304606

DOI: 10.1002/chem.201304606
Full Paper
&
Sensors
A Highly Fluorescent Metallosalalen-Based Chiral Cage for
Enantioselective Recognition and Sensing
Jinqiao Dong, Yanfang Zhou, Fangwei Zhang, and Yong Cui*[a]
Abstract: A highly fluorescent coordination cage [Zn8L4I8]
has been constructed by treating enantiopure pyridyl-func-
tionalized metallosalalen units (L) with zinc(II) iodide and
characterized by a variety of techniques including microanal-
ysis, thermogravimetric analysis (TGA), circular dichroism
(CD) spectroscopy, and single-crystal and powder X-ray dif-
fraction. Strong intermolecular p–p, CH···p, and CH···I interac-
tions direct packing of the cage molecules to generate a 3D
polycage network interconnected by pentahedral cages
formed by adjacent pentamers. The cage has an amphiphilic
helical cavity decorated with chiral NH functionalities capa-
ble of interactions with guest species such as saccharides.
The fluorescence of the cage was greatly enhanced by five
enantiomeric saccharides in solution, with enantioselectivity
factors of 2.480–4.943, and by five enantiomeric amines in
the solid state, with enantioselective fluorescence enhance-
ment ratios of 1.30–3.60. This remarkable chiral sensing of
both saccharides and amines with impressive enantioselec-
tivity may result from the steric confinement of the cavity as
well as its conformational rigidity. It holds great promise for
the development of novel chiral cage materials for sensing
applications.
icate cage [Zn8L0 Cl8] (2) built from a Zn(salan) complex (H2L0 =
Introduction
4
N,N’-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-diaminocyclo-
hexane). This salan-based cage displayed enantioselective fluo-
rescence upon interaction with amino acids,[5g] but failed to
detect saccharides. The salan ligand is saturated at the nitro-
gen centers and could be partially or completely oxidized to
salalen and salen, respectively.[11] We envisaged that oxidation
of cage 2 to a salalen-based analogue might improve its fluo-
rescent discriminating behavior, such that it might be applica-
ble for recognizing saccharides.[10] We report here the assembly
of a chiral helicate cage from the salalen ligand H2L and ZnI2
(Scheme 1) and the first observation of enantioselective lumi-
nescence sensing of saccharides in solution and amines in the
solid state by a coordination cage.
Metal–organic containers have been extensively pursued
owing to their relative ease of synthesis and potential applica-
tions in host–guest chemistry, catalysis, and sensing.[1,2] Coordi-
nation cages with specified stereochemistry may, in principle,
provide chirotopic inner phases for stereoselective guest rec-
ognition, sensing, and asymmetric transformation, as a result
of their unique chemical environments and the confinement
effect exerted by their well-defined central cavities.[3] While
many chiral cages of both metal–organic and organic types
have already been described, systems possessing well-defined
chiral functional sites that have the ability to induce enantiose-
lectivity remain limited.[4–6] Reports on enantioselective reac-
tions, chiral recognition, and sensing with chiral cages are
even more limited.[5,6]
The potential application of enantioselective fluorescent sen-
sors in rapid chiral assay has attracted significant research ac-
tivity in this area.[7] Although many enantioselective fluorescent
hosts have been developed for chiral substrates such as car-
boxylic acids, amino acids, amines, and amino alcohols,[8,9] rela-
tively few have the ability to detect biologically important sub-
strates such as saccharides.[10] We recently reported a chiral hel-
[a] J. Dong, Y. Zhou, F. Zhang, Prof. Y. Cui
School of Chemistry and Chemical Technology and
State Key Laboratory of Metal Matrix Composites
Shanghai Jiao Tong University
Shanghai 200240 (P.R. China)
Fax: (+86)21-5474-1297
E-mail: yongcui@sjtu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304606.
Scheme 1. The assembly of cage 1.
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two IÀ ions forming a tetrahedral geometry. The cavity of the
cage has inner dimensions of around 13.7ꢂ6.4 ꢁ. The helical
structure of 1 is similar to that of the un-oxidized cage 2.
Strong intermolecular p–p, CH···p, and CH···I interactions
direct the packing of the cage molecules into a porous 3D
structure (Figure 1b). Specifically, distorted pentahedral cages
Results and Discussion
Synthetic considerations and spectroscopic characterization
The ligand H2L was prepared in an overall 68.2% yield by the
Schiff-base condensation of 5-tert-butyl-3-(4-vinylpyridyl)sali-
cylaldehyde and enantiopure 1,2-cyclohexanediamine mono-
(hydrogen chloride), followed by reduction with NaBH4 and
further condensation with 5-tert-butyl-3-(4-vinylpyridyl)salicylal-
dehyde. Heating H2L and ZnI2 (1:2 molar ratio) in a mixed sol-
vent of acetone and MeOH afforded [Zn8L4I8]·4MeOH·4H2O (1)
in good yield. The formulation was supported by the results of
1
microanalysis, H NMR, ESI-MS, and thermogravimetry (TGA).
1
The H NMR spectrum of (R)-1 displays one complete set of
proton resonances for the ligand H2L, as evidenced by two
sets of signals for the tert-butyl groups at d=1.27 and
1.25 ppm. The peaks attributable to the CH2-N groups of the
coordinated ligand appear at d=3.79–4.07 ppm and the CH=N
peak appears at d=8.40 ppm, confirming the partial oxidation
of the imine bonds. In comparison with those of H2L, the
proton signals due to the cyclohexane units show downfield
shifts, while those of the pyridyl and phenyl groups show up-
field shifts, suggesting unsymmetrical coordination behavior of
the ligand. All of the proton signals are very sharp, consistent
with the formation of discrete metal complexes rather
than oligomeric species. The formation of the cage structure
was also supported by ESI-MS, which showed fragment
ions
[Zn8L4I8+5THF+2H]2+
,
[Zn8L4I8+5THF+H+NH4]2+
,
[Zn8L4I8+5THF+Na+NH4]2+, and [Zn4L4+NH4]+ at m/z=2225.8,
2232.1, 2244.9, and 1425.4, respectively. Assembly of the cage
was accompanied by amplification of the molar optical rota-
tion. The molar optical rotation (f) values of (R)-1 and (R)-H2L
were
determined
as
27500.08 cm3 dmÀ1 molÀ1
and
996.78 cm3 dmÀ1 molÀ1, respectively. The cage has an optical ro-
tation per mole of 27.6 times (ca. 7 times per ligand unit) that
of the ligand H2L.
Figure 1. a) View of the molecular structure of (R)-1; b) the 3D supramolec-
ular structure of (R)-1 showing helicate cages interconnected by pentahedral
cages. The cavities are shown with large spheres.
Structural description
The formation of a cage with wide apertures was revealed by
single-crystal X-ray diffraction analysis.[12] (R)-1 crystallizes in
the chiral tetragonal P41212 space group, with the asymmetric
unit containing half a formula unit. The basic building unit is
a Zn2L2 dimer, in which two trigonal-bipyramidal Zn centers
are enclosed in the N2O2 pockets and linked by two phenolato
oxygen atoms generating a coplanar Zn2O2 unit. The bond
lengths and angles of the structures are typical of the salan
and salen structural motifs. In particular, the Zn-Nimine distance
[2.061(10) ꢁ] is considerably shorter than the Zn-Namine distance
[2.151(10) ꢁ]. The Zn2L2 dimer adopts a skewed U-shape, with
the four pyridyl groups oriented towards the same face of the
Zn2O2 plane and pointing away from the adjacent units (maxi-
mum distance between respective N atoms of around 11.0 ꢁ).
A pair of Zn2L2 dimers is connected through the peripheral pyr-
idyl groups by binding to four Zn centers to generate an octa-
nuclear cage that contains four P-configured quadruple-strand-
ed helices. Each of the four Zn2+ ions is further coordinated by
with a maximum inner width of around 8 ꢁ are formed by the
stacking of adjacent pentamers. The chiral coordinated amine
groups of L are oriented toward the outside of the cages and
are exposed to the interstitial pores that are accessible to
guest molecules, which offers great potential for enantioselec-
tive host–guest interactions. Notably, the supramolecular engi-
neering of porous solids from intrinsically porous assemblies is
of significance but remains challenging.
Solid-state CD spectra of 1 (Figure 2) prepared from the (R)-
and (S)-enantiomers of L are mirror images of one another, in-
dicative of their enantiomeric nature. PLATON calculations indi-
cated the presence of 27.8% void space in 1.[13] The phase
purity of the bulk samples was established by comparison of
their observed and simulated powder X-ray diffraction (PXRD)
patterns. TGA revealed that the solvent molecules were re-
moved from 1 in the range 90–1608C. PXRD experiments sug-
gested that the sample retained its structural integrity and
crystallinity upon removal of the guest molecules. The N2 ad-
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Figure 2. Solid-state CD spectra of (R)/(S)-1.
sorption isotherm at 77 K revealed that 1 showed type I sorp-
tion behavior, with a BET surface area of 25.5 m2 gÀ1
.
Fluorescence-based sensor
The fluorescence of 1 shows a strong emission at l=530 nm
in THF. Compared with that of the salan-based cage analogue
2, the emission position of 1 is little changed (530 vs 534 nm),
but the intensity shows a twofold increase. Moreover, the
quantum yield increased from 5.60% to 7.29% and the fluores-
cence lifetime from 1.615 ns to 3.622 ns. We then proceeded
to investigate the interaction of the two chiral forms of 1 bear-
ing chiral NH groups with a variety of sugars (Scheme 2). The
Figure 3. a, b) Fluorescence spectra of (R)- and (S)-1 (1.0ꢂ10À5 m in THF)
upon titration with d-sorbitol (1ꢂ10À3 m); c) Benesi–Hildebrand plots of (R)-
and (S)-1 titration with d-sorbitol.
Scheme 2. Sugars used in the study.
fluorescence lifetime of 3.533 ns remained almost unchanged.
Control experiments showed that no obvious enantioselectivity
was observed for the ligand H2L, suggesting a better-defined
chiral environment conferred by the cage structure. In sharp
contrast, under identical experimental conditions, the addition
of d-sorbitol to a solution of (R)- or (S)-2 in THF caused only
a slight enhancement in the fluorescence intensity with negli-
gible enantioselectivity (Figure S19).
results for titrations of (R)- and (S)-1 with d-sorbitol are shown
in Figure 3. When 1 was treated with d-sorbitol, the emission
at 530 nm was shifted to 520 nm and enhanced by the sugar,
but the rate of change with (S)-1 was faster than that with (R)-
1, implying enantioselectivity in the fluorescent recognition. In
accordance with the linear Benesi–Hildebrand equation, the
measured absorbance [I0/(IÀI0)] at 530 nm varied as a function
of 1/[m] in a linear relationship (R>0.9900), indicating 1:1 stoi-
chiometry of the interaction between the sugar and cage 1.
The association constants Kbh were calculated as 4424.8mÀ1
with (S)-1 and 895.1mÀ1 with (R)-1, giving an enantioselectivity
factor kbh(S-1)/kbh(R-1) of 4.943Æ0.241:1. After the titration, the
quantum yield of 1 increased from 7.29% to 16.34%, while the
The increase in fluorescence may be due to static enhance-
ment upon formation of a cage–saccharide adduct. Changes in
the structure of the emitting species, such as rigidification of
the conformation, may be induced, along with excimer forma-
tion.[14] The weak interactions of 1 with a sugar generate differ-
ent diastereomeric adducts, thereby leading to a distinct fluo-
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rescence enhancement. The static nature of the complexation
is indicated by consistent fluorescence lifetimes of the cage
before and after titration with d-sorbitol (3.622 vs 3.533 ns).
After the titration, the peak position and shape of the UV/Vis
spectrum of (R)-1 remained unchanged, indicative of the stabil-
ity of the host structure. The complexation of 1 and d-sorbitol
1
was also reflected in the H NMR spectra (the proton signals of
cyclohexane and the CH2-N units of the cage were broadened
after the addition of d-sorbitol).
This cage system is enantioselective towards a range of sac-
charides, including d-glucose, d-fructose, d-galactose, and d-
maltose. The rates of fluorescence increase for (S)-1 caused by
d-glucose and d-fructose were slower than those for (R)-1,
whereas the rates caused by d-galactose and d-maltose were
faster for (S)-1 than those for (R)-1. Their enantioselectivity fac-
tors kbh(S-1)/kbh(R-1) were determined as 1:3.493Æ0.101, 1:4.380Æ
0.196, 2.480Æ0.144:1, and 4.176Æ0.225:1, respectively. The
enantio-differentiation reversal may originate from different
binding affinities between the hosts and guests induced by
specific intermolecular interactions, resembling the stereocon-
trol of an enzyme pocket in enzymatic recognition.[15,16] The
opposite trend in enantioselectivity was observed for the fluo-
rescence enhancement of (R)- and (S)-1 by l-glucose with an
enantioselectivity factor kbh(S-1)/kbh(R-1) of 3.961Æ0.172:1, which
further confirmed a chirality-based luminescence-enhancing
selectivity. The values of the enantioselectivity factors for the
examined sugar alcohols are of the same order of magnitude
as those obtained for sugar acids with binol–bisboronic-acid-
based fluorescence sensors, although the latter exhibit lower
selectivity for sugar alcohols.[10] To the best of our knowledge,
no metal–organic assemblies, such as helicates, macrocycles,
or cages, have previously been reported to enantioselectively
recognize saccharides.[5,6,17]
Cage 1 is also highly fluorescent in the solid state, with an
emission maximum at 610 nm and a short emission lifetime
(<3 ns). Solid 1 was found to exhibit fluorescence enantiose-
lectivity towards chiral amines. The fluorescence spectra of
thin-layer samples were monitored upon exposure to the
vapors of chiral molecules for various durations. When
(R)-1 was exposed to 1-cyclohexylethylamine (1-CEA) vapor,
the emission at 610 nm was shifted to 565 nm and enhanced
by both the (R)- and (S)-enantiomers, but the rate of increase
caused by (R)-1-CEA was faster than that caused by (S)-1-CEA
(Figure 4). The enhancement percentage (%) was estimated
using the formula (IÀI0)/I0 ꢂ100%, where I0 is the maximum
fluorescence intensity of cage 1 before exposure to the ana-
lyte, and I is the maximum intensity after exposure to the ana-
lyte. The enhancement percentages of (R)- and (S)-1-CEA were
determined as 482% and 134% after 450 s, respectively. The
fluorescence intensity ratio (I/I0) of (R)-1 was maximally in-
creased to 5.72 and 2.31 times that of the original value by (R)-
and (S)-1-CEA, respectively, after 450 s, with an enantioselective
fluorescence enhancement ratio [EF=(IR/I0À1)/(IS/I0À1)] of 3.60.
Control experiments verified that solid H2L showed no obvious
fluorescence change in the presence of 1-CEA and no enantio-
selectivity. Notably, under otherwise identical conditions, the
I/I0 ratio of the salan-based cage (R)-2 was maximally increased
Figure 4. a, b) Fluorescence enhancement upon exposure of thin films of
(R)-1 to vapors of (R)- and (S)-1-CEA; c) time-dependent fluorescence en-
hancement of (R)-1 by (R)- and (S)-1-CEA.
to 1.86 and 1.72 by (R)- and (S)-1-CEA, respectively, with an EF
of 1.19, much smaller than that observed for (R)-1 (Figure S29).
Significant enantioselective fluorescence enhancements of
(R)-1 with other chiral amines, including 1-phenylethylamine
(1-PEA), 1-phenylpropylamine (1-PPA), 1-(4-methylphenyl)ethyl-
amine (4-Me-1-PEA), and 1-(4-chlorophenyl)ethylamine (4-Cl-1-
PEA), were also observed. In all cases, the rate of increase
caused by the (R)-enantiomer was much faster than that
caused by the (S)-enantiomer (Figure 5). The EF values were
determined as 2.87, 1.32, 1.30, and 1.89 for 1-PEA, 1-PPA, 4-
Me-1-PEA, and 4-Cl-1-PEA, respectively. The enantioselectivity
decreases in the order 1-CEA >1-PEA > 4-Cl-1-PEA > 1-PPA ꢀ
4-Me-1-PEA.
Association of the analyte with the salalen ligand in the
ground state through hydrogen bonding followed by the for-
mation of an excellent emissive proton-transfer-assisted
charge-transfer excited state may be responsible for the fluo-
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molecular interactions to generate a 3D polycage network in-
terconnected by pentahedral cages formed by adjacent pen-
tamers. Owing to the steric confinement of the amphiphilic
helical cavity together with the conformational rigidity, the
chiral cage is capable of enantioselective recognition of the
enantiomers of saccharides and amines through fluorescence
enhancement in solution and the solid state, respectively. The
amplified chiral discrimination of analytes afforded by the rigid
cage structure may be utilized to design assembled materials
with practical enantioselectivity for sensing applications.
Experimental Section
Figure 5. Maximum enhancement percentages of (R)-1 after exposure to
chiral amines.
Chemicals, reagents, and analyses
All chemicals were obtained commercially and were used without
further purification. Elemental analyses of C, H, and N were per-
formed with an EA1110 CHNS-0 CE elemental analyzer. IR spectra
were recorded from samples in KBr pellets in the region 400–
4000 cmÀ1 on a Nicolet Magna 750 FTIR spectrometer. Solid-state
CD spectra were recorded on a J-800 spectropolarimeter (JASCO,
Japan). Thermogravimetric analyses (TGA) were carried out in an
air atmosphere at a heating rate of 208CminÀ1 on an STA449C in-
tegration thermal analyzer. Powder X-ray diffraction (PXRD) data
were collected on a DMAX2500 diffractometer using CuKa radiation.
Calculated PXRD patterns were produced using the SHELXTL-
XPOW program and single-crystal reflection data. All UV/Vis ab-
sorption spectra were recorded on a Lambda 20 UV/Vis spectro-
photometer (Perkin–Elmer, Inc., USA). Fluorescence spectra were
recorded on an LS 50B fluorescence spectrometer (Perkin–Elmer,
Inc., USA). Fluorescence lifetimes were measured with an Edin-
burgh Instruments lifespec-red F900 fluorescence lifetime spec-
trometer. 1H and 13C NMR experiments were carried out on a Mercu-
ry plus 400 spectrometer operating at resonance frequencies of
400 MHz and 100 MHz, respectively. Electrospray ionization mass
spectra (ESI-MS) were recorded on a Finnigan LCQ mass spectrom-
eter using dichloromethane/methanol as the mobile phase. Analyt-
ical high-performance liquid chromatography (HPLC) was per-
formed on a YL-9100 HPLC with UV detection at 254 nm. Analytical
Chiralcel OD-H columns (4.6 mm ꢂ 25 cm) from Daicel were used.
rescence enhancement.[14,18] Time-resolved fluorescence meas-
urements of 1 after addition of the amine showed only a slight
decrease in the decay rate, consistent with the above fluores-
cence-enhancing mechanism. We propose that the enantiose-
lective enhancement is due to the formation of different dia-
stereomeric host–guest complexes by selective adsorption. To
test this, we soaked evacuated crystals of (R)-1 in solutions of
racemic 1-CEA, 1-PEA, 4-Cl-1-PEA, 1-PPA, and 4-Me-1-PEA in di-
ethyl ether. Chiral HPLC analysis of the desorbed guests from
(R)-1 yielded 38.5%, 17.1%, 15.7%, 12.6%, and 10.8% ee, re-
spectively, with the (R)-enantiomers being in excess. This result
is consistent with the order of decreasing selectivity from 1-
CEA to 4-Me-1-PEA in fluorescence enhancement. The steric
confinement of the cage creates a more discriminating chiral
environment for L, leading to enhanced enantioselectivity. The
conformational rigidity of the coordinated salalen ligand may
also contribute to the enantioselectivity, which would reduce
the non-enantioselective entropic term of the analyte associa-
tion process and allow more stereospecific generation of host–
guest adducts. Further study on this selective behavior and un-
derstanding the process is still in progress.
After each measurement, (R)-1 could be regenerated by
heating at 1008C in vacuo for about 30 min and directly
reused for a second cycle of sensing 1-CEA without significant
loss of enantioselectivity (EF=3.06, 3.01, 2.92 for runs 1–3, re-
spectively). PXRD analysis showed that the sample of (R)-1 re-
mained highly crystalline after three cycles. The decrease in
fluorescence intensities of (R)-1 in the recycling process is be-
lieved to be due to incomplete removal of the adsorbed
amines during re-activation. An alternative approach for recy-
cling 1 is to recrystallize the sample from THF. Although
metal–organic solids for sensory applications have been re-
ported,[19] to the best of our knowledge there has only been
one report on chiral sensing (amino alcohols).[20]
X-ray crystallography
Single-crystal XRD data for (R)-1 were collected on a Bruker SMART
Apex II CCD-based X-ray diffractometer with CuKa radiation (l=
1.54178 ꢁ) at 123 K. An empirical absorption correction was ap-
plied by using the SADABS program (G. M. Sheldrick, SADABS, Pro-
gram for empirical absorption correction of area detector data;
University of Gçttingen, Germany, 1996). All structures were solved
by direct methods and refined by full-matrix least-squares on F2
(G. M. Sheldrick, SHELXTL97, Program for crystal structure refine-
ment, University of Gçttingen, Germany, 1997). In all compounds,
the guest molecules and H atoms were refined isotropically, while
all other atoms were refined anisotropically. All phenyl rings were
constrained to ideal six-membered rings. Contributions to scatter-
ing due to the highly disordered solvent molecules in (R)-1 were
removed using the SQUEEZE routine of PLATON; structures were
then refined once more using the data generated. Crystal data and
details of the data collection are given in Table S1. Selected bond
distances and angles are presented in Table S2.
Conclusion
We have presented the assembly of a chiral helicate cage from
metallosalalen that could be packed through strong supra-
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Chiralcel OD-H column: 258C; hexane/iPrOH (95:5); flow rate
1.0 mLminÀ1; tmajor =10.850 min, tmin =20.583 min; ee=38.5%.
Synthesis of 1
A mixture of ZnI2 (31.9 mg, 0.1 mmol), (R)-H2L or (S)-H2L (32.1 mg,
0.05 mmol), acetone (5 mL), and MeOH (5 mL) was sealed in
a 10 mL vial with a screw-cap and heated at 658C for 2 days. The
mixture was then cooled to room temperature, whereupon light-
yellow block-shaped crystals were collected, washed with diethyl
ether, and dried in air. Yield: 38.9 mg, 76.0%. Elemental analysis
calcd (%) for [Zn8L4I8]·4MeOH·4H2O (C172H220I8N16O16Zn8): C 47.98, H
5.11, N 5.21; found: C 47.51, H 5.08, N 5.14; IR (KBr): n˜ =3443 (m),
2950 (m), 2861 (w), 1604 (s), 1546 (w), 1501 (w), 1447 (s), 1463 (m),
1362 (w), 1285 (w), 1251 (w), 1202 (m), 1024 (m), 1229 (s), 977 (w),
874 (w), 829 (w), 774 (w), 731 (w), 539 cmÀ1 (w); 1H NMR
([D6]DMSO) d=8.40–8.36 (m, 3H; pyridyl-H+ArH), 8.07–8.02 (m,
3H; pyridyl-H+ArH), 7.69–7.52 (m, 5H; vinyl-H+ArH), 7.30–7.03
(m, 6H; vinyl-H+pyridyl-H), 4.07–3.79 (m, 2H; methylene-H), 3.53–
3.51 (m, 1H; cyclohexadecyl-H), 3.18–3.15 (m, 1H; cyclohexadecyl-
H), 2.47–2.45 (m, 1H; cyclohexadecyl-H), 2.36–2.34 (m, 1H; cyclo-
hexadecyl-H), 1.99–1.78 (m, 4H; cyclohexadecyl-H), 1.27–1.25 (d,
18H; 2CMe3), 1.03–1.01 ppm (m, 2H; cyclohexadecyl-H). The guest
molecules could be readily removed to generate apohost (R)-1 by
heating the pristine sample at 1008C for 4 h. The 1H NMR spectrum
was recorded from the apohost sample.
The enantiomeric excess of 1-phenylethylamine was determined
by benzoylation followed by HPLC analysis. Conditions: Chiralcel
OD-H column: 258C; hexane/iPrOH (90:10); flow rate 1.0 mLminÀ1
;
t
major =12.833 min, tmin =19.183 min; ee=17.1%.
The enantiomeric excess of 1-(4-chlorophenyl)ethylamine was de-
termined by benzoylation followed by HPLC analysis. Conditions:
Chiralcel OD-H column: 258C; hexane/iPrOH (90:10); flow rate
1.0 mLminÀ1; tmajor =12.250 min, tmin =21.267 min; ee=15.7%.
The enantiomeric excess of 1-phenylpropylamine was determined
by benzoylation followed by HPLC analysis. Conditions: Chiralcel
OD-H column: 258C; hexane/iPrOH (95:5); flow rate 1.0 mLminÀ1
;
t
major =16.967 min, tmin =30.483 min; ee=12.6%.
The enantiomeric excess of 1-(4-methylphenyl)ethylamine was de-
termined by benzoylation followed by HPLC analysis. Conditions:
Chiralcel OD-H column: 258C; hexane/iPrOH (95:5); flow rate
1.0 mLminÀ1; tmajor =17.017 min, tmin =32.017 min; ee=10.8%.
Acknowledgements
This work was supported by the NSFC through grants
21025103 and 21371119, the “973” program (2014CB932102
and 2012CB8217), and the Shanghai Science and Technology
Committee through grant 10J1400100.
General procedures for fluorescence sensor experiments
Titration experiments were carried out by adding aliquots (20 mL)
of substrate solution (1.0ꢂ10À3 molLÀ1) to a solution of (R)- or (S)-
1 (1.0ꢂ10À5 molLÀ1) in THF (2 mL) at intervals of 5 min. Fluores-
cence spectra were recorded after addition of the substrate. The
excitation wavelength was 350 nm and the slit width was set at
4 nmꢂ4 nm.
Keywords: chirality · coordination cage · fluorescence sensor ·
metal–organic frameworks · metallosalalen · saccharides
For solid-state experiments, apohost cage 1 was ground to
a powder and sprinkled evenly and firmly onto the surface of
double-sided tape and then applied to the lower half of a quartz
slide (on either side). In this way, thin-layer samples of quartz slides
bearing cage 1 were obtained, which were placed in quartz cuv-
ettes saturated with analyte vapor and the temporal evolution of
the fluorescence spectra was monitored.[19b] The excitation wave-
length was 380 nm and the slit width was set at 4 nmꢂ4 nm.
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HPLC analysis of resolved substrates by (R)-1
Racemic 1-cyclohexylethylamine (25.4 mg, 0.2 mmol), 1-phenyle-
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0.2 mmol), 1-(4-methylphenyl)ethylamine (27.0 mg, 0.2 mmol), and
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Chem. Eur. J. 2014, 20, 6455 – 6461
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6460
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: November 25, 2013
Published online on April 7, 2014
Chem. Eur. J. 2014, 20, 6455 – 6461
www.chemeurj.org
6461
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim