Self-assembly of cerium-based metal–organic tetrahedrons

for size-selectively luminescent sensing natural saccharidesw

Yang Liu, Xiao Wu, Cheng He,* Yang Jiao and Chunying Duan

Received (in Cambridge, UK) 29th July 2009, Accepted 20th October 2009

First published as an Advance Article on the web 9th November 2009

DOI: 10.1039/b915358f

New Ce-based Werner type tetrahedrons were achieved for size-

selectively luminescent detection of natural carbohydrates

through incorporating amide groups as both the multiple

hydrogen bonding triggers and binding-signalling transductor.

The design of artificial carbohydrate sensors operating

through non-covalent interactions is a subject of intensive

current research, due to their broad utility in wide-ranging

applications from the food and cosmetic industries to medicinal

and academic arenas.1 Because of the subtle variation in the

sugar structures and the three-dimensional arrangement of

their functionalities, the frameworks of carbohydrate receptors

must be large enough to be able to fully encapsulate an

oligosaccharide nucleus. And the receptors should have

various patterns of preorganized, inward-directed H-bond

donor and/or acceptor functionality.2 In this regard, self-

assembled metal–organic molecular polyhedrons, appealing

as synthetic hosts, are efficient receptors for mimicking biological

carbohydrate recognition processes,3 especially when amide

groups, as multiple hydrogen bonding triggers used in nature

protein–carbohydrate complexes4 were incorporated.

On the other hand, difficulties in developing saccharide

sensors also arise from the fact that saccharides just contain

one kind of recognition unit (the hydroxyl functional group)

and lack a spectroscopic handle, such as a chromophore or

fluorophore, whose modulation could be harnessed in a sensing

scheme. Since fluorescent molecular sensing, which translates

molecular recognition into tangible fluorescence signals,5 pro-

vides an efficient tool for quantitatively detecting carbo-

hydrates with high precision in both solution and complex

media.6 The incorporation of luminescent active lanthanide

ions within the metal–organic polyhedrons represents a

promising approach in constructing Werner type cage-like

molecular capsules for luminescent detection of saccharides

in solution and/or in biological media.7 However, lanthanide

ions usually exhibit low stereochemical preferences and high

coordination numbers, the rational concepts of related Werner

type molecular polyhedrons are quite rare8 and require the use

of highly predisposed and spatially restricted ligands.9

In order to control the coordination of lanthanide ions and

obtain highly ordered architectures, highly predisposed NO2

tridentate chelators with amide groups were introduced into

linear shape molecules H2L1 and H2L

2. Here, two new

luminescence-active lanthanide tetrahedrons (TE1 and TE2)

were synthesized for the size-selective sensing of saccharides

(Scheme 1). The specific electronic structure of Ce3+ possibly

allows the better control of the assembly of highly ordered

architecture, through influencing the directional 5d orbitals

in the coordination modes.10 Taking into account the

environmentally sensitive character of these parity-allowed

electric-dipole 4f–5d transitions to the electronic conformation

of the ligands,11 the formation of hydrogen bonds with the

amide groups has the potential to affect the electron transitions

associated with the Ce3+ ions, leading to significant changes in

the optical properties.

Ligands H2L1 and H2L

2 were obtained by reacting salicyl-

aldehyde with 2,6-dicarbohydrazide naphthalene and 1,10-

dicarbohydrazide 4,40-biphenyl, respectively. Evaporating a

CH3OH–DMF solution of these ligands with Ce(NO3)3�6H2O in air for several days led to the formation of crystalline

solids of compounds TE1 and TE2, in a high yield (65% and

60%), respectively. EA and powder X-ray analysis proved the

pure phase of the bulky sample. ESI-MS spectrum of TE1

exhibited two intense peaks at m/z = 1084.84 and 1097.45

with the isotopic distribution patterns separated by 0.33 � 0.01,

demonstrating the presence of negatively charged species

[Ce4L16–3H]3� and [Ce4L

16–3H * (H2O)2]

3�, respectively.

Similarly, the two peaks at m/z = 852.20 and 1137.19 in the

ESI-MS spectrum of TE2, could be assignaed to negatively

charged species [Ce4L26-4H]4� and [Ce4L

26–3H]3�,

respectively. These results indicated the successful assembly

of Ce-based molecular tetrahedrons.

Single crystal X-ray structural analysisz confirmed the

formation of the tetrahedron in TE1, [Ce4(C26H18N4 O4)6]�4C3H7NO�6H2O�2CH3OH. The Ce4L

16 tetrahedron comprised

Scheme 1 The constitute/constructional fragments of the functional

Ce-based tetrahedron Ce4L16 and Ce4L

26 showing the cavities, the

windows (drawn in orange) and the positions of functionality groups.

State Key Laboratory of Fine Chemicals, Dalian University ofTechnology, Dalian, 116012, China. E-mail: hecheng@dlut.edu.cnw Electronic supplementary information (ESI) available: Crystal datain CIF, experimental details, magnetic and additional spectroscopicdata. CCDC 705880. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/b915358f

7554 | Chem. Commun., 2009, 7554–7556 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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of four vertical metal centers that each coordinated to three

tridentate chelating groups in a coronary triangular prism

coordination geometry (Fig. 1). Each ligand positioned on

one of the six edges of the tetrahedron defined by four metal

ions and bridged two metal centers. The separation between

two metal ions was about 13.80 A and the inner volume was

estimated as 300 A3. The triangle face had an area of about

150 A2, potentially acting as a window for the guest molecules

to pass through. At room temperature, the wmT value of TE1

was 3.0 emu K mol�1. This value was globally in agreement

with the presence of four trivalent lanthanide ions as their

expected wmT value is 3.2 emu K mol�1.12 The decrease of the

wmT value was likely due to the thermal depopulation of the

ground-state sublevels (ESI Fig. S7w).13

Luminescence spectrum of TE1 (50 mM) exhibited a ligand

based broad emission band extended from 450 to 600 nm, and

an intense band at about 525 nm as well as a weak band at

about 468 nm that overlapped on the broad band, when the

solution was excited at 360 nm. The energy difference between

the two narrow-shaped bands (2200 cm�1) was close to

2000 cm�1, in good agreement with the characteristic splitting

of the two Ce3+ ground state levels 2F5/2 and the upper2F7/2 components, induced by the spin-orbital interactions.14

Therefore, the intense emission band could be attributed to the

5d - 4f transition of Ce3+ from the lowest excited state 2D3/2

to the ground state 2F5/2 and the upper 2F7/2 components.

Upon the addition of hexoses, mannose or glucose, the

wavelength of the emission maximum (525 nm) of TE1 did

not change but the luminescence intensity enhanced gradually

with the increasing concentration of the guest. Upon the

addition of 20 mole equivalent hexoses, the fluorescence

intensity of TE1 increased by 60% for mannose and 40%

for glucose, respectively (Fig. 2). The Hill-plot profile15 of the

fluorescence titration curves at 525 nm demonstrated the 1 : 1

stoichiometric host–guest complexation behavior with the

association constants (log Kass) being 3.76 � 0.33 and

3.73 � 0.41 for that of mannose and glucose, respectively.

In the meantime, the addition of pentoses, ribose or xylose,

just resulted in weak spectroscopic variations (fluorescence

intensity increased about 10%). The association constants

(log Kass) were calculated as 2.95 and 2.87 for ribose and

xylose, respectively, indicating the smaller affinities of TE1 for

the smaller size pentoses referring to those of hexoses. The

addition of excess disaccharides including the sucrose, maltose

and trelahose did not cause any obvious spectroscopic

changes, suggesting the possible size-selective recognition of

the TE1 toward the hexoses over the smaller pentose and

larger disaccharides.

From a mechanistic viewpoint, the formation of donor-type

hydrogen bonds between the amide groups and the guest

molecules could favor electronic delocalization of the electro-

nic donors and lower the highest occupied molecular orbital

(HOMO) energy of the electronic donors,16 which would lead

to a further blocking of the PET processes that take place

between the amide and the Ce3+-based chelating units and to

a significant enhancement of the metal-based luminescence

signal. The insensitive nature of the UV-vis absorptions of

TE1 upon the addition of saccharides might also be an

indicator for such a PET mechanism.17

According to the crystal structure of complex TE1 and some

related compounds,18 the possible separation between Ce ions

in the tetrahedral Ce4L26 cage of TE2 might be about 17 A

with an approximated inner volume of about 550 A3. It can be

expected that the Ce4L26 tetrahedron might hold the potential

to be selective to larger carbohydrates rather than the hexoses

with its larger window sizes and inner volume, given that the

recognition occurred within the cavity of the polyhedrons. The

luminescence titration of TE2 upon the addition of saccharides

showed that the affinities of Ce4L26 for disaccharides were

larger than those for the smaller size monosaccharides. As

shown in Fig. 3, the addition of excess monosaccharides

including the hexoses and pentoses (1 mM) caused very little

spectral variations (fluorescence intensity enhancement lower

than 5%), whereas the addition of disaccharides caused a

significant luminescence enhancement (about 18% in average).

The Hill-plot profile of the luminescence at 480 nm also

Fig. 1 Molecular structure of TE1. One of the disordered parts of the

fragments, hydrogen atoms and solvent molecules are omitted for

clarity. The metal, oxygen, nitrogen and carbon atoms are drawn in

green, red, blue, and grey, respectively. Bond distances: Ce–O (phenol)

2.24 A, Ce–O (amide), 2.43 A and Ce–N 2.67 A on average.Fig. 2 Fluorescence spectra of TE1 (50 mM) in DMF–acetonitrile

solution (15 : 85, v/v), upon the addition of a standard solution of

various saccharides, excited at 360 nm. Insert: Linear fitting for

log[(F � F0)/FL � F)] vs. log[G] for the corresponding titrations,

where F0, F and FL were the emission intensities at 525 nm of the free

TE1, of TE1 in the presence of hexoses having concentration of [G],

and of Ce4L16 in the presence of excess saccharides, respectively, and

[G] was the concentration of saccharides added.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 7554–7556 | 7555

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demonstrated the occurrence of 1 : 1 stoichiometric complexation

behavior with the association constants (log Kass) being

calculated as 4.05 for sucrose, maltose and trehalose on

average. Although it could not be proven beyond a shadow

of a doubt that the recognition of the saccharides occurred in

the cavity of the cage, the size-dependent affinities of Ce4L26

and Ce4L26 to different saccharides, as well as the stability of

corresponding host–guest species in solution all supported this

hypothesis.

ESI-MS spectra of TE1 in the presence of hexoses exhibited

two intense peaks at m/z = 1145.34 and 1085.63, respectively

(Fig. 4). The comparison of the peak at m/z = 1145.34 with

the simulation on the basis of natural isotopic abundances

revealed the presence of 1 : 1 stoichiometric host–guest species

[Ce4L16–3H * (C6H12O6)]

3�. The addition of smaller

pentoses, xylose or ribose, or larger disaccharides did not

arouse any obvious peaks corresponding to the host–guest

species. In the spectra of TE2 with disaccharides including

sucrose, maltose and trehalose, the presence of peak at 1879.67

assignable to [Ce4L26–2H * (C12H22O11)]

2� demonstrated the

1 : 1 stoichiometric complexation behavior. The addition of all

the above mentioned mono-disaccharides did not cause any

obvious peaks corresponding to the host–guest species.

This work was supported by the National Natural Science

foundation of China (20801008 and 20871025) and the

Start-up Fund of The Dalian University of Technology.

Notes and references

z Crystal data of TE1: C170H156Ce4N28O36,M= 3727.71, monoclinic,space group P21/n, black block, a = 24.009 (1), b = 37.450 (1),c = 24.065(1) A, b = 92.650(2)1, V = 21614(1) A3, Z = 4, Dc =1.146 g cm�3, m(Mo-Ka) = 0.891 mm�1, T= 180(2) K. 31 778 uniquereflections [Rint = 0.1333]. Final R1 [with I > 2s(I)] = 0.0849,wR2 (all data) = 0.1991 for 2y = 471. CCDC number 705880.

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Fig. 3 Fluorescence responses of TE1 (red bars) and TE2 (blue bars)

for saccharides mentioned. Emission intensity was recorded at 525 nm

for TE1 (excited at 360 nm) or at 480 nm for TE2 (20 mM in

DMF/acetone solution’ 5 : 95, v/v, excited at 320 nm), respectively.

Fig. 4 ESI-MS (a) of TE1 (0.1 mM) and (b) TE2 (0.1mM) in

DMF–methanol solution (containing 0.3 mM KOH) in the presence

of (a) Mannose (0.5 mM) and (b) maltose (0.5 mM), respectively. The

inserts exhibit the measured and simulated isotopic patterns (a) at

1145.34, and (b) at 1879.68.

7556 | Chem. Commun., 2009, 7554–7556 This journal is �c The Royal Society of Chemistry 2009

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