From a Tb3+ chelated compound to a hybrid material: Selective emission responses to anions

Article abstract of DOI:10.2478/s11696-011-0123-1

A novel terbium 2-hydroxymethyl-benzoimidazole-6-carboxylic acid complex has been designed and unique emission changes to fluoride anions in comparison with HSO^?₄ , AcO^?, Cl^?, Br ^?, and I^? were observed. Then, the complex was encapsulated into an inorganic matrix. The novel hybrid material, with strong green emission was successfully synthesized as an anions receptor in water. More importantly, this hybrid material not only gave luminescence response to F ^?, but also to HSO^?₄ . Spectroscopic studies demonstrated that the recognition process for fluoride ions can be mainly ascribed to its hydrogen bonding interactions with hydrogen bond donor units (NH and OH). In case of hydrogen sulfate, the sensing effects can be probably attributed to its acidity instead of hydrogen bonding interactions.

Full text of DOI:10.2478/s11696-011-0123-1

Chemical Papers 66 (3) 221–225 (2012)
DOI: 10.2478/s11696-011-0123-1
ORIGINAL PAPER
From a Tb3+ chelated compound to a hybrid material:
selective emission responses to anions
Zheng-Yang Zhang, Qian-Ming Wang*, Jian-Zhong Wu, Wei-Sheng Cai, Yi-Fei Yu
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China
Received 19 August 2011; Revised 10 November 2011; Accepted 12 November 2011
A novel terbium 2-hydroxymethyl-benzoimidazole-6-carboxylic acid complex has been designed
and unique emission changes to fluoride anions in comparison with HSO−4 , AcO−, Cl−, Br−, and
I− were observed. Then, the complex was encapsulated into an inorganic matrix. The novel hybrid
material, with strong green emission was successfully synthesized as an anions receptor in water.
More importantly, this hybrid material not only gave luminescence response to F−, but also to
HSO−4 . Spectroscopic studies demonstrated that the recognition process for fluoride ions can be
mainly ascribed to its hydrogen bonding interactions with hydrogen bond donor units (NH and
OH). In case of hydrogen sulfate, the sensing effects can be probably attributed to its acidity
instead of hydrogen bonding interactions.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: sensor, luminescent, terbium, fluoride, hydrogen sulfate
Introduction
have been widely investigated over the last decades
(Dong et al., 2000). We have discovered that the in-
corporation of lanthanide complexes can enable their
emission in aqueous media (Tan et al., 2010). More
significantly, some of them have displayed highly
selective and sensitive interactions toward anions
through hydrogen bond donor moieties. In this re-
search, we focused on the differentiation between
the complex and hybrid material during the an-
ion sensing process. Accordingly, a novel terbium 2-
hydroxymethyl-benzoimidazole-6-carboxylic acid (a)
complex (Tb(a)3) has been prepared and a novel hy-
brid material (Ha) was obtained by encapsulating it
into an inorganic matrix (tetraethoxysilane, TEOS)
(Fig. 1). Both of them, as sensing species, have intense
green luminescence and their sensing abilities toward
anions were investigated in solutions.
During the past two decades there has been consid-
erable interest in the recognition of specific anions due
to their essential role in both biological systems and
environmental chemistry (Gale et al., 2008; Kavallier-
atos et al., 1999). Especially, the luminescent sensor
is an attractive application in the detection processes,
achieving fast responses and effects easy to be detected
by naked eyes (Sambrook et al., 2005; Santacroce et
al., 2006).
Although a variety of methods have been employed
to design fluorescent materials endowed with anion
recognition properties, lanthanide luminescent com-
plexes have recently received increased attention in
anion sensing due to their special photophysical per-
formances such as long lifetime, sharp emission peaks,
and large Stokes shifts (Bu¨nzli & Piguet, 2005; Parker
& Yu, 2005; Gunnlaugsson et al., 2006; Wang et al.,
2010; Zhang et al., 2011). However, instability of lumi-
nescent complexes in aqueous media seriously restricts
their practical use in real life.
A point worth emphasizing is that the hybrid ma-
terial still retains striking luminescence even in pure
water. It is interesting to find that luminescence of
the complex can be quenched by fluoride ions. For the
hybrid material, both fluoride and hydrogen sulfate
anions can induce smart emission changes. Spectro-
Lanthanide luminescent-based hybrid materials
*Corresponding author, e-mail: qmwang@scnu.edu.cn

222
Z. Y. Zhang et al./Chemical Papers 66 (3) 221–225 (2012)
Fig. 1. Scheme of predicted structure of terbium complex (Tb(a)3) and hybrid material (Ha).
scopic studies indicated that for fluoride ions, hydro-
gen bonding interactions with the imidazole-NH (or
hydroxyl OH) resulted in quenching effects. Unlike the
fluoride ions, we believe that the recognition process
was caused by the acidity of hydrogen sulfate. In ad-
dition, the hybrid material presented higher stability
than its complex.
desai et al., 2007). A mixture of 3,4-diaminobenzoic
acid (456 mg, 3 mmol) and glycolic acid (228 mg,
3 mmol) was stirred at 120◦C for 8 h in hydrochloric
acid (4 M, 5 mL). The mixed solution was cooled to
room temperature and the precipitate was filtered off,
washed with water followed by acetonitrile and ether
to yield the product as a brown solid. Yield: (486 mg,
1
71 %). H NMR (DMSO-d6) δ: 8.27 (1H, s, H1), 8.07
Experimental
(1H, d, J = 8.4 Hz, H2), 7.84 (1H, d, J = 8.0 Hz,
H3), 5.02 (2H, s, H4); MS (LC-MS) found: m/z 191.3
[MH]−.
All starting materials were obtained from commer-
cial suppliers and used as received. 1H NMR spec-
tra were recorded at 293 K using a Varian 400 (400
MHz) with TMS as an internal standard (Varian,
Palo Alto, CA, USA). LC-MS was measured by a
Thermo Finnigan LCQ Deca XP Max (Thermo Finni-
gan, San Jose, CA, USA) Fluorescence spectra were
measured using a Hitachi F-4500 spectrophotometer
with a 150 W xenon lamp as the light source. The
scan speed was fixed at 300 nm min−1. Both excita-
tion and emission slit widths were 2.5 nm (Hitachi,
Tokyo, Japan). Thermogravimetric analysis was car-
ried out using a STA409PC under air at the rate
of 10◦C min−1 (Netzsch, Selb, Germany). IR spectra
were measured by a Varian 660-IR (Varian, Palo Alto,
CA, USA). The fluorescence images were taken using
a Nikon Eclipse TS100 inverted fluorescence micro-
scope system equipped with a 50 W mercury lamp
source (Nikon, Japan).
Preparation of Tb(a)3 was described as follows
(Tan et al., 2010): 2-hydroxymethyl-benzoimida-zole-
6-carboxylic acid (115.2 mg, 0.6 mmol) was dissolved
in 5 mL of ethanol in a 50 mL round bottom flask.
Tb(NO3)3 · 6H2O (90.6 mg, 0.2 mmol) dissolved in
5 mL of ethanol and ammonia (1 mL) was added
into the flask. The mixture was stirred under 90◦C
for 1 h. Crude product was washed with ethanol and
water then dried in vacuo overnight and the titled
complex (169 mg) was obtained as a brown powder
Tb(a)3 ·2H2O. Elemental analysis (EA), wi/mass %
(found), revealed: C, 42.27; H, 3.38; N, 10.88; (calc)
for C27H25N6O11Tb: C, 42.20; H, 3.28; N, 10.94.
Preparation of Ha was realized as follows:
2-hydroxymethyl-benzoimidazole-6-carboxylic
acid
(115.2 mg, 0.6 mmol) was dissolved in 5 mL of wa-
ter in a 50 mL beaker. TEOS (416 mg, 2 mmol) and
Tb(NO3)3 · 6H2O (90.6 mg, 0.2 mmol) dissolved in
10 mL water were added into the beaker, ammonia
(5 mL) was added into the solution. The mixture was
2-Hydroxymethyl-benzoimidazole-6-carboxylic
acid (a) was prepared according to literature (Thakur-

Z. Y. Zhang et al./Chemical Papers 66 (3) 221–225 (2012)
223
Fig. 3. 1H NMR spectra measured by titration of a DMSO-
d6 solution of pure a (1 mM) with two equivalents of
[Bu4N]F.
Fig. 2. Emission spectra of Tb(a)3 (10−5 M in acetonitrile,
λex = 284 nm) upon addition of 0–10 equivalents of
ranging from 220 nm to 300 nm (insert in Fig. 2). Nar-
row emission bands with maxima at 488 nm, 544 nm,
583 nm, and 621 nm were recorded. These bands were
[Bu4N]F. Insert: excitation spectra of Tb(a)3 (10−5
in acetonitrile, λem = 544 nm).
M
7
7
7
ascribed to the 5D4 → F6, 5D4 → F5, 5D4 → F4, and
7
5D4 → F3 transitions of Tb3+ ions, respectively. How-
stirred under room temperature for around 5 h, dried
at 80◦C for 24 h, and the resulting product (364 mg)
was obtained as a brown powder.
ever, upon addition of fluoride ions from 10−5 mol L−1
to 10−4 mol L−1, the emission of terbium ions gradu-
ally decreased and totally disappeared. Therefore, sig-
nificant changes can be observed by naked eye under
the excitation by ultraviolet light (insert in Fig. S3).
Similar fluorescence experiments were done by titra-
tion of ten equivalents of HSO−4 , AcO−, Cl−, Br−, and
I− corresponding [Bu4N]+ salts (Fig. S4). All these an-
ions caused only minor changes to the luminescence of
Tb(a)3.
Results and discussion
IR spectra of ligand a (a), terbium complex Tb(a)3
(b), and hybrid material Ha (c) are shown in Fig. S1.
The band at 3384 cm−1 is due to the –NH stretching
vibrations. The broad band located around 2814 cm−1
can be ascribed to the intra-molecular hydrogen bond-
ing and molecular packing (Fig. S1a). In Fig. S1b, the
coordination interaction between the carboxylate and
the terbium ion is supported by the bands located at
1562 cm−1 and 1386 cm−1, which are induced by the
asymmetric ( as) and symmetric ( s) stretching vibra-
tions of carboxylate. Moreover, spectra of the hybrid
material (Fig. S1c) show the formation of an Si—O—
Si framework, which is evidenced by the broad bands
In order to determine the interaction between the
complex and anions, the same species were also char-
acterized by UV-VIS spectrum. Tb(a)3 in acetoni-
trile (1 × 10−5 M) gave a broad band ranging from
205 nm to 240 nm (Fig. S3). Its absorption signif-
icantly increased when the concentration of fluoride
anions increased from two to ten equivalents. Con-
tinuous addition of fluoride caused no changes in the
spectrum. Hydrogen bonds between the fluoride an-
ions and the ligand were assumed to induce the ab-
sorption changes. In an analogous fashion, the bind-
ing behavior of HSO−4 , AcO−, Cl−, Br−, and I− an-
ions were also detected. Almost no changes could be
observed, showing that there is not enough binding to
generate changes in the absorption region (Fig. S3).
To study the interaction between anions and ligand
a more clearly, 1H NMR spectroscopy was used to in-
vestigate the binding affinity of anions to a (Fig. 3).
The addition of two equivalents of fluoride ions in
DMSO-d6 into a generated an upfield shift (from 8.27,
8.07, 7.84, and 5.02 to 8.08, 7.76, 7.45, and 4.70) of
the protons of a (H1, H2, H3, and H4, respectively).
Changes of these proton signals were caused by the
co-contribution of hydrogen bonding interactions with
located at 1102 cm−1
( as, Si—O), indicating the suc-
cess of hydrolysis and polycondenzation (Dong et al.,
2000).
As shown in Fig. S2, terbium complex Tb(a)3 with
strong green luminescence shows characteristic ter-
bium ion emission. Its excitation spectrum is rep-
resented by a broad band ranging from 230 nm to
370 nm with the maximum at 245 nm (Fig. S2). The
overall quantum yield was measured as 4.2 % in pure
DMSO.
Tb(a)3 was dissolved in acetonitrile (10−5 M) and
its sensing abilities towards anions were studied by
fluorescence spectroscopy. The excitation spectrum of
Tb(a)3 was obtained by fixing the emission wave-
length of Tb3+ at 544 nm; it exhibited a broad band

224
Z. Y. Zhang et al./Chemical Papers 66 (3) 221–225 (2012)
Fig. 5. Thermogravimetric analysis of Tb(a)3 and Ha.
Fig. 4. Emission spectra of Ha (0.5mg mL−1 in water, λex
=
5×10−5 mol L−1. The recognition process in complex
was not observed probably because HSO−4 was not
hydrolyzed in the organic solvent (CH3CN), showing
no acidity in CH3CN. The quenching effect can be
also observed upon the titration of fluoride ions from
5×10−5 mol L−1 to 10−4 mol L−1 (Fig. S6). Hence,
significant changes can be observed by naked eye un-
der the excitation by ultraviolet light (Fig. S7).
In analogous experiments, AcO−, Cl−, Br−, or
I− and 10−3 mol L−1 titration of corresponding
[Bu4N]+ salts were measured by emission spec-
troscopy (Fig. S6). Almost no changes could be ob-
served in their luminescence. Accordingly, it is pro-
posed that the target material can be applied as a po-
tential luminescent receptor for recognition of HSO4−
as well as F− in water. This hybrid material is a little
more sensitive to hydrogen sulfate anions than to the
fluoride ones.
We believe that there are hydrogen bonding inter-
action between the imidazole ring NH/hydroxyl OH
and fluoride anions as evidenced by the NMR and UV
spectra above. In terms of fluoride ions, their hydro-
gen bonding interactions with the ligand resulted in
the disappearance of the luminescence of the complex
as well as that of the hybrid material (Santacroce et
al., 2006). However, for HSO−4 , we are convinced that
the coordination interaction between the ligand and
terbium were disrupted by its acidity; also, the re-
leased protons were combined by carboxylate (COOH
formed again), which terminated the energy transfer
between them. In this way, recognition effects observ-
able by naked eye occurred.
290 nm) upon addition of 0–5×10−5 M [Bu4N]HSO4.
Inset: Excitation spectra of Ha (0.5 mg mL−1 in water,
λem = 544 nm).
fluoride between the imidazole ring NH and the hy-
droxyl (OH) proton of a−−. An−alogous NMR experi-
ments using HSO−4 , AcO , Cl , Br−, and I− were
done but a did not give signal changes with each pro-
ton (data not shown).
However, to dissolve Tb(a)3 in an aqueous solution
was attempted and it was discovered that the emis-
sion of the terbium ion is quenched in the presence
of water. It was found that the introduction of a lan-
thanide complex into a silica matrix enables emissions
even in pure water. Consequently, Tb(a)3 was encap-
sulated into an inorganic host (TEOS) and a novel
hybrid material (Ha) with intense green emission was
developed as a luminescent receptor in water. Its ex-
citation spectrum exhibits a sharp peak at 248 nm
and its emission spectra show characteristic terbium
emission, which can be interpreted as follows: excited
7
5D4 → FJ transitions demonstrate four main compo-
nents for J = 6, 5, 4, and 3, respectively (Fig. S5).
As described in Fig. 4, the ability of Ha to recognize
anions was also studied using emission spectroscopy
in water. Ha was dispersed in water in the concentra-
tion of 0.5 mg mL−1. When the solution was irradi-
ated with light of 290 nm, narrow emission peaks were
observed and assigned to the transitions between the
excited 5D4 level and various J-levels of the ground
7
state 7FJ (J = 6, 5, 4, and 3) at 490 nm (5D4 → F6),
Moreover, thermal stability of Tb(a)3 and Ha
were studied by thermogravimetric analysis (TGA).
At temperatures lower than 250◦C, Ha has relatively
higher thermal stability in comparison with the ter-
bium complex (Fig. 5). Therefore, incorporation of
the terbium complex into the silica matrix can sig-
nificantly enhance its thermal stability and increase
its decomposition temperature.
7
7
544 nm (5D4 → F5), 584 nm (5D4 → F4), and 620
nm (5D4 → F3). For the excitation spectra, a broad
7
band ranging from 220 nm to 300 nm was observed
when the emission wavelength was fixed at 544 nm
(insert in Fig. 4). Luminescence of Ha decreased grad-
ually and was almost quenched at last upon the ad-
dition of hydrogen sulfate ions from 10−5 mol L−1 to

Z. Y. Zhang et al./Chemical Papers 66 (3) 221–225 (2012)
225
From the fluorescence microscopy image (Fig. S8)
can be clearly seen that the prepared hybrid mate-
rial is homogeneously dispersed on the glass surface
and displays strong green emission with the diameter
about of 10∼30 µm. Besides, formation of large aggre-
gates with the diameter of about 50 µm was clearly
observed.
Gale, P. A., García-Garrido, S. E., & Garric, J. (2008). An-
ion receptors based on organic frameworks: highlights from
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10.1039/b715825d.
Gunnlaugsson, T., Glynn, M., Tocci, G. M., Kruger, P. E., &
Pfeffer, F. M. (2006). Anion recognition and sensing in or-
ganic and aqueous media using luminescent and colorimetric
sensors. Coordination Chemistry Reviews, 250, 3094–3117.
DOI: 10.1016/j.ccr.2006.08.017.
Kavallieratos, K., Bertao, C. M., & Crabtree, R. H. (1999). Hy-
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Conclusions
In summary, a novel terbium complex and its
hybrid material were prepared as sensing materials.
Both of them can quench luminescence of fluoride
ions through hydrogen bonding interactions. However,
emission of the hybrid material can also be terminated
by an addition of hydrogen sulfate anions. Addition-
ally, the complex enables recognition in organic sol-
vents whereas the hybrid material can be used in pure
water. More importantly, the hybrid material exhibits
higher thermal stability than the complex. These fun-
damental results show great potential of the hybrid
material for future tailored recognition of target ions
in practical aqueous systems.
Parker, D.,
& Yu, J. H. (2005). A pH-insensitive, ratio-
metric chemosensor for citrate using europium lumines-
cence. Chemical Communications, 2005, 3141–3143. DOI:
10.1039/b502553b.
Sambrook, M. R., Beer, P. D., Wisner, J. A., Rowena, L., Cow-
ley, A. R., Szemes, F., & Drew, M. G. B. (2005). Anion-
templated assembly of pseudorotaxanes: Importance of anion
template, strength of ion-pair thread association, and macro-
cycle ring size. Journal of the American Chemical Society,
127, 2292–2302. DOI: 10.1021/ja046278z.
Santacroce, P. V., Okunola, O. A., Zavalij, P. Y., & Jeffery,
T. D. (2006). A transmembrane anion transporter selective
for nitrate over chloride. Chemical Communications, 2006,
3246–3248. DOI: 10.1039/b607221f.
Tan, C. L., Wang, Q. M., & Ma, L. J. (2010). Fluorescent-
based solid sensor for HSO−4 in water. Photochemistry
and Photobiology, 86, 1191–1196. DOI: 10.1111/j.1751-
1097.2010.00795.x.
Acknowledgements. Qian-Ming Wang appreciates support
of the National Natural Science Foundation of China
(21002035) and the Start Fund of Guangdong Talents (C10208).
Thakurdesai, P. A., Wadodkar, S. G., & Chopade, C. T.
(2007). Synthesis and anti-inflammatory activity of some
benzimidazole-2-carboxylicacids. Pharmacologyonline, 1,
314–329.
Supplementary data
Supplementary data associated with this article can
be found in the online version of this paper (DOI:
10.2478/s11696-011-0123-1).
Wang, Q. M., Tan, C. L., Chen, H. Y.,
& Tamiaki, H.
(2010). A new fluoride luminescence quencher based on a
nanostructured covalently bonded terbium hybrid material.
Journal of Physical Chemistry C, 114, 13879–13883. DOI:
10.1021/jp105035v.
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