A silver-induced metal-organic gel based on biscarboxyl-functionalised benzimidazole derivative: Stimuli responsive and dye sorption

Article abstract of DOI:10.1080/10610278.2013.822968

A multi-responsiveness supramolecular metal-organic gel (MOG) has been prepared by using silver (I) nitrate and a simple ligand (L17) based on a biscarboxyl-functionalised benzimidazole derivative. The MOG that displays the formation of well-developed nanofibrillar networks composed of intertwined fibres was characterised by using field emission scanning electron microscopy (FESEM), Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD) techniques. The FT-IR spectra and XRD show that supramolecular MOG was formed through an effective coordination of the ligand and Ag ⁺. This MOG reveals outstanding reversible sol-gel transitions induced by changes of temperature and pH, as well as by addition of NH4OH, ethylene diamine tetraacetic acid and Na2S. In addition, the MOG also shows a good ability in dye sorption. These results demonstrate that this multichannel responsive smart MOG has the potential to be widely applied in materials science. 2013

Full text of DOI:10.1080/10610278.2013.822968

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A silver-induced metal-organic gel based on
biscarboxyl-functionalised benzimidazole derivative:
stimuli responsive and dye sorption
a
a
a
a
a
a
You-Ming Zhang , Xing-Mei You , Hong Yao , Ying Guo , Peng Zhang , Bing-Bing Shi , Jun
a
a
a
Liu , Qi Lin & Tai-Bao Wei
a
Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education,
Gansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, 730070, China
Published online: 21 Aug 2013.
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To cite this article: You-Ming Zhang, Xing-Mei You, Hong Yao, Ying Guo, Peng Zhang, Bing-Bing Shi, Jun Liu, Qi Lin & Tai-
Bao Wei (2014) A silver-induced metal-organic gel based on biscarboxyl-functionalised benzimidazole derivative: stimuli
responsive and dye sorption, Supramolecular Chemistry, 26:1, 39-47, DOI: 10.1080/10610278.2013.822968
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Supramolecular Chemistry, 2 01 4
Vol . 26 , N o. 1, 39 –4 7 , h t t p: / / d x .d o i . org / 10 . 1 08 0 / 1 06 1 02 7 8 . 2 01 3 . 8 2 2 9 68
A silver-induced metal-organic gel based on biscarboxyl-functionalised benzimidazole
derivative: stimuli responsive and dye sorption
You-Ming Zhang, Xing-Mei You, Hong Yao, Ying Guo, Peng Zhang, Bing-Bing Shi, Jun Liu, Qi Lin and Tai-Bao Wei*
Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Gansu Key Laboratory of Polymer Materials,
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
(
Received 12 January 2013; final version received 25 June 2013)
A multi-responsiveness supramolecular metal-organic gel (MOG) has been prepared by using silver (I) nitrate and a simple
ligand (L17) based on a biscarboxyl-functionalised benzimidazole derivative. The MOG that displays the formation of well-
developed nanofibrillar networks composed of intertwined fibres was characterised by using field emission scanning
electron microscopy (FESEM), Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD)
techniques. The FT-IR spectra and XRD show that supramolecular MOG was formed through an effective coordination of
þ
the ligand and Ag . This MOG reveals outstanding reversible sol–gel transitions induced by changes of temperature and
pH, as well as by addition of NH OH, ethylene diamine tetraacetic acid and Na S. In addition, the MOG also shows a good
4
2
ability in dye sorption. These results demonstrate that this multichannel responsive smart MOG has the potential to be
widely applied in materials science.
Keywords: supramolecular metal-organic gel; benzimidazole derivative; reversible change; stimuli responsive;
dye sorption
1
.
Introduction
( 24, 25) and l ig h t (26, 27); however, onl y a few r ep or t s
cur rent l y exis t on con tr ol la bl e gel at i on s ys t em s b as ed o n
Low molecular weight organogels (LMWGs) represent an
important type of soft materials, in which the organic or
aqueous phase is immobilised in a nanofibrillar network
formed from self-assembled nanostructures of gelator
molecules (1, 2). Th e no n- coval ent in te ra ct io ns , s uch as
h y d ro g e n b o n d i ng , p– p s ta cki ng , hy dr o p ho bi c i nt er-
ac ti o n s a n d va n de r Waa ls in te ra ct io ns , p la y an i m port ant
r o l e in s e l f - a s s e m b l i n g LM W Gs t o f o r m e n t a n g l e d
na n o s tr u c tu r es (3, 4) . Re ce nt ly, LM WG s ha ve rec eived
mo r e a tt e n t i o n t h an th ei r po ly m er ic an al og ue s f or a l arge
am o u n t o f s ci e n ti fic ap pl ica tio n s , s uc h as f oo d ( 5), t is s u e
en g i n e eri n g ( 6), p ol lu tan t ca pt ur e an d r em oval ( 7), opt i cal
dev i ce s , d ru g d e live ry ve hi cle s ( 8, 9) an d s o on . I n s pi t e of
s up r a m o l ec u la r o rga no ge ls or hy dr o g el s u s in g mo le cu l es
co n t a in i n g d iv e rse f un ct io na l gr o up s , w hic h have bee n
w el l re s ea rc h e d ( 2– 4, 10, 11) , m et al- or g ani c ge ls (M OG s )
or m et a l l o g el s b a s ed on m et al co or d in at ion bo n ds have
al s o b e en c o nc e rn ed by Row an et al . ( 12, 13) . I n gen er al ,
m e t a l – l i g a n d c o o r d i n a t i o n i s u s e d t o e n h a n c e t h e
i nt e rm o l e cu l a r i nt er act io ns . I n ad di ti on , li ga nd mo le cu l es
t ha t i n i ti a l ly c o ul d n ot f or m ge ls m us t be ge la te d by t he
ad d i t i o n o f m et al io ns ( 14) . N ow ad ay s , th er e have bee n
r ep o r te d o rg a n o g els an d m et all og el s th at ar e r es p ons ive t o
ul t ra s o u n d (15– 18) , m ec han ic al s ti m ul ati on (19), s oun d
rever s ib le cha nge s i n m et al l oge ls i nduc ed by pH, Na 2 S
and et hyl ene di am i n e t et ra acet i c aci d (ED TA) (28) .
Recently, it has been explored that the molecules
including pyridyl bis (urea), terpyridyl or bis (benzimida-
zole). Show good properties in gelating with metal ions
(
29– 31). For i ns t ance, t he t erpy ri dyl -b as ed l ig an d s h ave
bee n effec tively s hown t o fo rm gel s wi t h pl at i num sa l t s
and t hei r pho to phy s ica l pr ope rt i es wer e i nves t i gat e d. T he
pyr i dyl bi s (u rea) - bas e d l ig and has bee n s hown t o f or m a
s h e a r-i n d u c e d M O G wi t h Cu ( I I ) b ro m i d e s a lt ( 24) .
R ecent l y, t he cat al yt i c act ivi t y of i m id azol e-b as ed g el s
has al s o bee n res e arc hed (32). In ver y few ca se s, d ye
s or pti on ( 13, 33) phe nom ena have al s o bee n s t ud i ed wi t h
t he gel m at eri al s . Am o ng al l t he abo ve-m e nt i one d MO Gs ,
t he N- cont ai ni ng uni t of t he gel at or m ol ecul e p la y s a n
i m port an t ro le i n t he fo rm at i on of gel . To d at e, a f ew
exa m pl es of bi s c arb oxy l -fu nct i ona l i s e d ben zi mi d a zo l e
der ivat ives as gel at or s have bee n rep or t ed ( 34, 35) .
In this paper, we designed and synthesised a biscarboxyl-
functionalised benzimidazole derivative with a simple
structure (Scheme 1) on the basis of our previous work
(34– 36). As a resul t , t h e exp eri m ent s show ed t hat l igand L17
a s a n LM W g ela tor c oul d b e i n du ce d to fo rm MO G wi th
selec tivit y f o r A g (I ). T he L17 M OG also r eve aled a
(
20, 21), m et a l l i c ca t io ns (22) , te m per at ur e (23), ani on s
*Corresponding author. Email: weitaibao@126.com
q 2013 Taylor & Francis

4
0
Y.-M. Zhang et al .
Scheme 1. Synthesis of Gelator L17.
rever sibl e sol – ge l tr an si ti on i nd uc ed b y t he ch ang es of
t em pe rat u re , p H, NH O H, E D TA a n d N a S . Mo r eove r, it ca n
diffractometer (Japan Rigaku, http://www.rigaku.com).
XRD patterns were recorded at a scanning rate of 58 per
minute in the 2u ran ge of 2 – 50 8 wi t h C u Ka rad iat i on . T he
m or phol ogi es and s i zes of t he xer ogel s wer e cha r a ct e ri s ed
u s i n g fi e l d e m i s s i o n s c a n n i n g e l e c t r o n m i c r o s c o p y
(F ES EM, J S M- 670 1F ) at an acce l erat i ng volt age o f 8 k V.
Al l rea g ent s of ana ly ti ca l gr ade wer e pur chas ed f ro m Al f a
Aes a r.
4
2
a ls o b e u se d i n dy e s or p t i on ( met h y l o ra n g e). We b el i eve tha t
t he mul t i c ha nne l re spons ive s ma r t s il ve r (I ) n i t ra t e M OG of
L 17 h as th e p oten tial t o b e wide ly a pp lied in m a t e ria ls
scie nce.
2
.
Experimental
2
.1 General experimental section
2
.2 Synthesis of compound L17
Melting points were measured on an X-4 digital melting
1
point apparatus (uncorrected). H NMR spectra were
The synthesis of 2-heptadecylbenzimidazole (A17)
was accomplished on the basis of previous work (37) .
2- H e pt ad ecy lb e nzi m i d azo l e (6 . 8 8 g , 20 m m ol ) , me t h yl
acr yl ate (2 .07 g, 24 m m o l ) and a cat al yt i c am o u nt o f
K P O (1 . 33 g , 5 m m ol ) wer e com bi ne d i n a bs ol u te
recorded with a Mercury-400BB spectrometer (VARIAN,
USA, http://www.varian.com) at 400 MHz, with tetra-
methylsilane as an internal standard. NMR spectra were
referenced to the solvent. The infrared spectra were
recorded on a Digilab FTS-3000 Fourier transform
infrared (FT-IR) spectrophotometer (Digilab Inc, USA,
http://210028.us.all.biz). Mass spectra were recorded on
an esquire 6000 MS instrument (Bruker Daltonics Inc,
http://www.bruker.com) equipped with an electrospray
3
4
acet on i t ri le (6 0 m l ). The s ol ut i on was s t ir red at 8 0 8C f or
10 h. Af t er coo l i ng t o ro om t em per a t ure, t he wh i te
pr eci pit a t e was fi l t ere d and t he fi l tr ate was eva po ra t ed
(u s ing a ro tar y evapo ra t or) t o obt a i n a br own o il y l i qu i d
B 17 (4 .64 g, 54% ) . A m i xt u re of com p ound B 17 ( 4. 3 0 g ,
10 m m ol ) and br om oac et at e (2 .00 g, 12 m m ol ) i n d ry
abs o lut e acet on i t ri le (6 0 m l ) was s t ir r ed at 80 8C f or 2 4 h .
Af te r coo li ng t o ro om t em per at ur e, t he s ol ut i on wa s
evapo ra t ed (u s ing a ro tar y evapo rat or ) t o obt a i n a d am p-
dr y wh it e s ol i d, and t he wh it e s ol i d was evapo ra te d u nd er
red uced p res s u re and was h ed wi t h abs o lut e a ce t on e
s ever al t im es , aft er s t ovi ng t o o bt ai n a dr y wh it e p owd er y
pr oduc t C 17 (3 .05 g, 50% ). Af te r fu rt her pur i fic at i o n ,
(
ESI) ion source with version 3.4 of Bruker Daltonics Data
Analysis (Bruker Daltonics Inc, http://www.gongchang.
com/Bruker_Daltonics_Inc-company) as the data collec-
tion system. The 1:1 xerogel of MOG was coated on a
glass plate and the solvent was slowly evaporated. The
glass plates with dry gels were then fixed on a sample
holder and subjected to X-ray diffraction (XRD) analysis
at room temperature on a Japan Rigaku D/MAX-2400/PC

Supramolecular Chemistry
41
Table 1. Gelation properties of MOG.
2
1
Solvent
MeOH
Gelation behavior
Gelating time
MGC/mg ml
Tg e l / 8C
G
G
S
G
S
15 min
10 min
–
2 h
5.00
1.51
–
2.67
–
18
19
–
28
–
MeOH–H O (V/V ¼ 1:1)
2
EtOH
EtOH–H
nPrOH
2
O (V/V ¼ 1:1)
–
nPrOH–H O (V/V ¼ 1:1)
S
G
G
S
SP
SP
SP
SP
SP
G
SP
G
S
–
19 min
14 min
–
–
–
–
–
2
iPrOH
4.17
1.57
–
–
–
–
–
–
3.64
–
3.33
–
9.52
4.76
–
33
25
–
–
–
–
–
–
16
–
18
–
19
16
–
iPrOH–H
nBuOH
tBuOH
2
O (V/V ¼ 1:1)
iPeOH
Cyclohexanol
Benzene
Acetone
–
–
–
6 min
Acetone–H
Acetonitrile
2
O (V/V ¼ 1:1)
Acetonitrile–H O (V/V ¼ 1:1)
14 min
2
DMSO
DMF
–
18 min
ca.1 day
G
DMF–H O (V/V ¼ 1:1)
WG
SP
SP
SP
2
Dichloromethane
Chloroform
Carbon tetrachloride
–
–
–
–
–
–
–
Note: G, gel; WG, weak gel; S, solution; SP, solution precipitate.
c o m p o u n d C 1 7 (6 .09 g, 10 m m ol ) w as di s s ol ved i n 50 m l
2
3.2 Gelation behaviour of MOG in selected solvents
1
o f h o t HC l (4 mo l l ) , an d th e s ol u ti on w as st i rr ed at
1 0 8C fo r 1 2 h . Th e s ol u ti on w as evap or at ed un de r red uced
pr es s u re , a n d t h e r es id ue w as r ecr ys t al li se d f ro m Et OH –
The gelation abilities of MOG were researched in a variety
of polar, non-polar organic and component solvents
1
(
organic solvents–H O (V/V ¼ 1:1)), respectively. Gela-
2
H O (1 : 1 0 V/ V) to give th e pr o du c t L 1 7 ( 2.7 7 g, 57% ).
2
tion experiments with 1 wt% MOG in a selection of
solvents were summarised in Table 1. As a result, MOG
forms thermoreversible opaque gel within a few minutes in
some polar solvents, such as DMF, MeOH and iPrOH.
Firm gels were obtained not only from some pure
solutions, such as MeOH, iPrOH and DMF, but also from
1
M p : 2 0 3 – 2 0 5 8C . H N MR ( 40 0 MH z, D M S O d ) : d (p pm )
6
0
.8 5 – 0 .8 6 (t , 3 H), 1. 15 – 1. 2 3 ( m , 26 H ), 1. 43 ( t, 2 H), 1. 65
(
t, 2 H), 2 .8 8 – 2 .9 0 ( t, 2H ) , 3. 25 – 3. 27 ( t, 2H ) , 4.6 8 – 4. 71
t, 2 H) , 4 .9 1 (s, 2 H) , 7.57 – 7 .5 8 ( m, 2H) , 7.82 – 7.8 4 (d, 1H),
(
21
8
3
.0 1 – 8 .03 (m, 1 H) , 12.13 (s, 1H) . I R ( KBr, c m ): v 3421,
05 5, 2 92 1, 2 85 1, 1 726, 1671, 1526, 1470, 121 1, 7 55. ESI-
some component solvents, such as MeOH–H O
2
þ
MS: cal cu late d for ( L17 þ H ) 487 a nd f ound 487.6.
(
(
V/V ¼ 1:1), EtOH–H O (V/V ¼ 1:1), iPrOH–H O
2
2
V/V ¼ 1:1), acetone–H O (V/V ¼ 1:1), acetonitrile–
2
H O (V/V ¼ 1:1) and DMF–H O (V/V ¼ 1:1). It may be
2
2
3
.
Results and discussion
suggested that van der Waals interactions between the
alkyl chains of gelator and solvent molecules were crucial
for the self-assembly process required for gelation (38,
3
.1 The forming process of the gel
A simple LMW gelator L17 was designed and synthesised,
which included two carboxyl functional groups. Accord-
ing to the literature, carboxyl is very easy to coordinate
with metal ions (34, 35) ; th us , th e ge la tio n abi l i t y was
t es t e d b y m i x in g li ga nd L1 7 an d var i ou s m et al s al t s wi t h a
39). However, gel at i n can not be obs e r ved i n n on - po l ar
s ol vent s s uch as di chl or om et hane , chl or of or m an d c ar b o n
t et rachl or id e. Thes e res u l ts i ndi cat e t hat t he pol a r i ty o f t he
s ol vent s has a s i gn i fi cant effec t on t he fo rm at i on of t he g el .
In order to investigate the gelating abilities of MOG,
minimum gelator concentration (MGC) values (Table 1),
which is defined as the minimum amount of gelator
required to gelate a given volume of solvent, have been
determined for MOG. The thermal stability of the MOG
was researched by measuring the gel–sol transition
1
: 1 m o l ar ra t i o i n th e co m po ne nt s ol ve nts s uc h as MeO H –
H O ( V / V ¼ 1 : 1 ) , he at ing to di s s ol ve an d th en coo l i ng t o
2
r oo m t em p e r a tu r e. L1 7 w as f ou nd to f or m MO G a t 1 wt %
a ft er 5 m i n i n t he pr es en ce of A g( I) . Th e ge la ti on was
c o nfi rm e d b y re s is t anc e t o flow up on inve rs i on of t he
s cr ew- ca p p e d v i a ls . Mo r e over, th e in divi du al L1 7 was not
a b le t o g e la t e i n an y s ol ve nt.
temperature (T ). It was det er m i ned by heat i n g t he
g e l

4
2
Y.-M. Zhang et al .
Figure 1. FESEM micrographs of the MOG. (a) MOG; (b)
Xerogel of MOG on the glass sheet.
s am p l e v i al i n t h e te m per at ur e- co nt r ol led w ate r b at h u nt i l
t u b e i nv e rs i o n s h ow ed th at th e ge l h ad m el t ed . Th e hi ghe r
t h er m a l s t ab i l i ty o f th e MO G in di ff er ent s ol ve nts i s s hown
b y t h e h i g h Tg e l val ues ( Tab le 1 ) an d ca n be at tri bu ted t o
t h e s t ro n g e r h y d r og en bo n di ng in te ra cti on am o n g gel at or
m o l ec u l es (40). Th e ti m e of ge l f or m at io n w as al s o t es t ed
Figure 2. FTIR spectra of (a) L17 and (b) xerogel.
(
Ta b le 1 ).
Morphological features of the MOG have been studied
using a FESEM. Figure 1 shows the FESEM image of the
2
1
increase in AgNO content, the peak at 1670.67 cm
3
corresponding to the carbonyl groups of free L17
increased significantly in magnitude (Figure 2(b)).
Besides, the CvO band shifted to higher wavenumbers,
MOG in the presence of AgNO . It can be clearly seen that
3
MOG forms nanofibrillar network structures from the
FESEM, which might be responsible for encapsulating a
large volume of solutions to form gels (Figure 1(a)).
However, when the gel was dried onto the glass sheet, the
FESEM image showed a discrete nanofibrillar network
structures (Figure 1(b)). We proposed that the reason was
2
1
from 1670.67 to 1678.04 cm . These changes suggest
that silver ions had coordination with the carbonyl oxygen
atom of protonated carboxyl functional groups and oxygen
anion of deprotonated carboxyl functional groups of L17,
respectively. These IR data suggest that Ag(I) has a very
different effect on the L17. It also suggests that silver ion
coordinated with the carboxyl groups of L17 induced the
gel formation.
þ
the coordination ability of L17 and Ag became weaker
when the gel was dried. As a result, L17 was self-
assembled in the presence of selective valence metal ion
þ
Ag to form a nanofibrillar network structure that enables
In order to establish the formation mechanism of
xerogel, the XRD of L17 and MOG was determined
the trapping of a large volume of molecule containing
solutions to form MOG. It may be suggested that metal ion
chelation, intermolecular hydrogen bonding and p–p
stacking play a significant role in promoting the self-
organisation and self-association of the L17 in the gel
state.
(
Figure 3). Compared with the L17, a sharp diffraction
peak was obtained in the big angle region at 2u ¼ 3 9. 7 8,
˚
wh ich cor r es po nd s t o a d-s p aci n g of 2. 2 7 A, i ndi ca ti n g t he
bon d l eng t h of Ag ( I) and car b ony l oxy gen a to m o f
To confirm whether cross-linking of gel fibres via
metal coordination could have a significant strengthening
effect, we compared the FT-IR spectra of the free L17 with
its xerogel (Figure 2). The FT-IR spectra for xerogel of
L17 showed distinct and interesting changes relative to the
free L17. The strong absorption peaks at 1726.29 and
2
1
1
CZO stretching vibrations, indicating the presence of
211.29 cm can be assigned to combined CvO and
protonated carboxylic acids. A strong absorption peak at
2
1
1
670.67 cm , which is the characteristic of deprotonated
carboxylic acids, was observed for free L17 (Figure 2(a)).
2
1
The peak at 1726.29 cm corresponding to the carbonyl
groups of free L17 decreased significantly in magnitude
after an increase in AgNO3 content (Figure 2(b)). In
addition, the CvO band shifted to lower wavenumbers
when carboxyl groups coordinate with silver, from
2
1
1
726.29 to 1719.14 cm . On the other hand, after an
Figure 3. XRD diagram of L17 and xerogel.

Supramolecular Chemistry
43
Figure 4. Schematic representation of the possible arrangement of molecules during MOG.
p ro t o n at e d c ar b o xy l f un cti on al gr o up s , an d 2 u ¼ 37. 9 8
addition of a few drops of NH OH to MOG resulted in
4
˚
w h ic h c o rre s p o n ds to a d- s pac ing of 2. 38 A s howi ng t he
clear colourless solutions (Figure 5). This transformation
could be the result of the formation of the metal ammonia
complex breaking the metal–ligand (L) coordination,
which resulted in the clear solutions. The above result
indicated that we should study the effect of the metal
trapping reagent to trap the metal atoms in gel matrixes.
In view of that purpose, we chose a well-known chelating
agent EDTA. The same equivalents of EDTA were added
to the gel as the equivalents of the metal salt in the gel. The
vial was sealed and placed at room temperature. For MOG,
which contains Ag(I) metal, the solid EDTA was found to
penetrate slowly into the gel and thereby gradually transfer
the gel into sol from top to bottom. About two days later,
the whole gel turns into a clear transparent solution with a
white coloured precipitate at the bottom of the vial. Our
investigations present that the white coloured precipitate is
an EDTA and L complex of Ag(I), while the clear solution
bo n d l en g t h o f A g( I) an d ox yg en an io n of de pr ot ona t ed
ca rb o x y l fu n c ti o na l gr o up s ( 41, 42) . Th e 2 u ¼ 32. 1 8
˚
co rr es p o n d s t o a d- s pa ci ng of 2. 79 A , s ig n if yin g t he
hy d r o g e n b o n d i ng be tw een un pr o ton a te d ca rb oxy l fu nc-
t io n a l g ro u p s a n d ot he rs ; w e al s o ob ta in ed a d- s p aci n g of
˚
3
. 9 2 A b y 2 u ¼ 2 2. 7 8, w hic h s ug ge s te d th at it w as a p– p
s ta ck i n g b e tw ee n ar om a t ic nu cl eus ( 38) . Be s ide s , t he
˚
2
u ¼ 1 9 . 5 8 c o r r e s p o n d s t o a d- s p a c i n g o f 4 . 5 6 A ,
e x pr es s i n g t h e va n de r Waa ls in te ra ct io ns be tw een t he
l o n g a lk y l c h ai n s .
Thus, on the basis of the above spectroscopy,
microscopic studies as well as the XRD results, it can be
concluded that in the gelation process the MOG possibly
forms repeating bilayers in which the molecules are
connected by intermolecular hydrogen bonding and p–p
stacking (43). Th e po s s ib l e in te rd ig it ate d bi la yer pac kin g
o f t h e M OG i s d es c r ib ed in F ig ur e 4 . It ha s be e n s hown
t h a t 1 : 1 c o m p l ex es o f L1 7 a n d s i l ve r io n c a n b e
p r e p a r e d . T h e r e s u l t s o f E S I - M S ( c a l c u l a t e d f o r
þ
(
ex p e ri m en t s a ls o s up po rt th is hy po t he s is .
L 1 7 þ A g þ 4 H O ) 6 6 5 . 8 a n d f o u n d 6 6 5 . 6 )
2
3.3 Sol–gel transformations of MOG by chemical
stimuli
The transformation of gel–sol–gel has been observed by
the addition of reasonable chemicals. For instance, the
Figure 5. Illustration for gel–sol transformation of MOG by the
addition of external chemical stimuli (ammonia).

4
4
Y.-M. Zhang et al .
transparent sol, and the sol could transform into a gel
again if we added silver ion to the solution after the black
precipitate has been filtered off. The change from a gel to a
sol could be evidently confirmed by SEM measurement.
In the beginning stage of the complex gel being exposed
to the Na S, we could see that the long and entangled belt-
2
like fibres rapidly break into short and untangled fibrils
(
Figure 8(a)). Because the short belt-like fibres cannot
Figure 6. Illustration for reversible gel–sol transformation of
MOG by the addition of EDTA and AgNO , respectively. (a) The
keep the 3D network structure, which is crucial for the gel,
the gel–sol transformation takes place. Substantially, the
gel–sol transformation is evidently derived from the
3
1
:1 gel formed at 0.4 wt%; (b) immediately after addition of solid
EDTA (1 equiv.); (c) after 1 h; (d) after 5 h; (e) after 12 h; (f)
immediately after addition of AgNO (1 equiv.) to the above
2
2
3
reaction of the S
with silver ion, which relieves
obtained solution; (g) after 30 min; (h) the ‘stable-to-inversion
test’ succeeds after ultrasound.
the coordination of silver ion with L17 and thus destroys
the gel structure. This was confirmed by the dark
precipitate of silver sulphide after a longer time of
exposure. If we add excess silver ion to the fluid solution
again after the precipitate has been filtrated off, the gel
re-forms (Figure 8(b)).
The pH responsiveness of the MOG was also
investigated. This MOG showed a quick gel–sol transition
upon addition of either base or acid (Figure 9). Added
HClO4 (1 mol/l) to the 1:1 gel (1 wt%) at ambient
temperature resulted in a gradual decomposition of the
gelatinous state in about 10 min. Finally, we obtained a
colourless, transparent solution (Figure 9(a)). Probably,
this was due to carboxyl group protonation, causing
breakage of the L17–Ag complexation and physical cross-
linking. After stoichiometric addition of NaOH (1 mol/l) to
Figure 7. Illustration for reversible gel–sol transformation of
MOG by the addition of Na
:1 gel formed at 0.4 wt%; (b) immediately after addition of solid
Na S (1 equiv.); (c) after 4 min; (d) after 30 min; (e) after 12 h; (f)
2 3
S and AgNO , respectively. (a) The
1
2
2
Ag S precipitate from (e); (g) immediately after addition of
3
AgNO (1 equiv.) to the above obtained solution; (g) after 2 h; (h)
after 6 h; (i) the ‘stable-to-inversion test’ succeeds.
the solution obtained from the HClO disintegrated MOG,
4
is the acetonitrile–H O (V/V ¼ 1:1) solution of the
the stable sol state was completely converted into a gel
after sonication for a few seconds. However, by adding
NaOH (1 mol/l) to the 1:1 gel, small amount of
white colour precipitate was observed in the sol state
(Figure 9(c)). We attributed this to the formation of silver
hydroxide, resulting in a transformation from the gel to
2
remaining ligand. Moreover, if we added AgNO into
3
the clear solution again, it could be turned into a white
coloured thick gel, similar to the beginning. It confirms
that the colourless solution is the solution of the
L (Figure 6). The same result could be observed if the
same equivalents of Na S were added to the gel as
2
sol. After stoichiometric addition of HClO (1 mol/l) to
4
the equivalents of the metal salt in the gel (Figure 7).
Observably, the MOG responded to Na S rapidly. The gel
this solution followed by sonication for a few seconds, the
solution turned back to gel rapidly. These pH-dependent
gel–sol transformations came from the reversible proto-
nation reactions of the carboxyl groups. The pH cycling
experiments showed that the process was repeated at least
2
surface immediately changed from white to colourless
when it was exposed to Na S. After several hours of
exposure, the MOG completely transformed into a
2
Figure 8. SEM images corresponding to the Ag S precipitate in (a) and the reform gel (b), respectively.
2

Supramolecular Chemistry
45
two times. Besides, when the temperature reached a limit
of 808C, the gelation of a 1:1 gel was observed in all
heating–cooling cycles, suggesting the thermoreversibil-
ity of the MOG (Figure 9(b)).
3.4 Dye sorption by MOG
The elimination of toxic dye substances from wastewater
has been a major concern. There are several types of smart
materials, such as clays, dendritic polymers and supramo-
lecular gels, which were shown to act as efficient dye
sorbent in the literature (44, 45). Un iver s al ly, t he d ye
s or bent s hou ld i ncl ude t wo dom a ins : hyd rop hi l ic d om ai n
t o i nt era ct wi t h wat er and hyd roph ob i c dom a in t o a bs or b
t he dye . The l ig and L17 pos s e s s es bot h fea tu re s, so t he
MOG of L1 7 m ay have a p ot ent i al t o act as a dye so r be n t.
Fo r t hi s pur pos e, xer ogel of t he MOG was te st e d f or
m et hyl or ang e dye s or pti on. A t ot al of 6 m g of me t h yl
or ang e dye was di s s ol ved i n 500 m l of di s t i l le d wa t er t o
pr epare t he s t oc k s ol ut i on of dye . In a t ypi cal r ea ct i o n,
Figure 9. Illustration for
thermoreversibility of MOG.
pH
reversibility
and
5
m g of xerog el was i m m e rs ed i n 7 m l o f t he a bo ve
s ol ut i on i n a vi al and l eft und is t ur bed fo r 3 h . Aft e r 3 h, i t
was s hown t hat al m os t t he wh ole dye was t rans f er re d f ro m
aqu eou s s ol u t io n t o xerog el , res u lt i ng i n an al m o st c le a r
wat er s ol ut i on i n t he vi al . At vari ous t im e i nt erva ls d ur i ng
3 h, t he s or pti on of dye fr om aqu eous s ol ut i on by x er og e l
was det ect ed by UV – vi s i bl e s pec tr a (F i gur e 10 ).
In order to understand the sorption mechanism
clearly, the sorption capacity of sorbent that was affected
by the pH change in the solution has been investigated
(
Figure 11(a)). It is well known that methyl orange can
exist in both anionic form at basic pH and cationic form
at acidic pH (Figure 11(b)), and the benzimidazolium in
MOG is positively charged, so the initial pH of the dye
solution plays a significant role in the sorption process.
Lower sorption at pH , 4 is due to the interionic
repulsion between the positively charged dye molecule
and the sorbent. At pH . 4, the methyl orange molecules
were deprotonated, which results in an electrostatic
attraction between negatively charged dye molecule,
Figure 10. UV–vis spectra of dye solution (black line) after the
addition of MOG xerogel at various time intervals (colored
lines).
Figure 11. (a) Dye adsorption onto MOG as a function of pH. (b) Structure of methyl orange at different pH.

4
6
Y.-M. Zhang et al .
positively charged sorbent and silver ions to the
maximum removal. As pH increased from 11 to 12, the
formation of silver hydroxide resulted in the collapse of
the 3D network. It indicates that the electrostatic
mechanism is not the sole mechanism for dye sorption,
in that the 3D networks are also responsible for sorption.
We had also researched the sorption of gel for methyl
orange dye, the result indicated that gel has weak
sorption capacity as compared with xerogel in a same
time. It must be that there are a large number of solution
molecules in the nanofibrillar network structures, so it
will decrease the sorption capacity of gel for methyl
orange.
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4
.
Conclusions
The gelating properties of L17 ligand in the presence of
transition metal salt (AgNO ) have been thoroughly
3
investigated. It was suggested that the coordination of
metal to ligand plays a key role in the formation of MOG.
Compared with previous literature about stimuli respon-
sive of MOG, it was found to be a multi-responsiveness
supramolecular MOG and showed good stimuli respon-
siveness towards the changes in pH, EDTA, Na S and NH
(19) Hamilton, T.D.; Bur c´ ar, D.; Baltrusaitis, J.; Flanagan, D.R.;
Li, Y.J.; Ghorai, S.; Tivanski, A.V.; MacGillivray, L.R.
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(
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21) Cravotto, G.; Cintas, P. Chem. Soc. Rev. 2009, 38,
6 8 4 – 2 6 9 7.
2
3
9
solutions. These results proved the importance of the
coordination bond in the gelation process. We can also use
the xerogel for the methyl orange dye sorption and then
obtain a desired result. Finally, the application of the gel in
sewage purification, which contains some dye molecules,
becomes possible.
(
2
(22) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc.
2005, 127, 1 7 9 – 1 8 3 .
(
(
(
(
(
(
23) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.;
Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2 0 16 – 2 0 2 1 .
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010, 6, 3 5 4 1 – 3 5 4 7.
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3 3 – 5 3 8 .
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Chem. Soc. 2008, 130, 1 6 1 66 – 1 6 1 67 .
27) Zhou, Y.F.; Xu, M.; Yi, T.; Xiao, S.Z.; Zhou, Z.G.; Li, F.Y.;
Huang, C.H. Langmuir. 2007, 23, 2 0 2 – 2 0 8 .
2
5
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos 21064006, 21262032 and 21161018),
the Program for Changjiang Scholars and Innovative Research
Team in University of Ministry of Education of China (No.
IRT1177), the Natural Science Foundation of Gansu Province
28) Komatsu, H.; Matsumoto, S.; Tamaru, S.; Kaneko, K.;
Ikeda, M.; Hamachi, I. J. Am. Chem. Soc. 2009, 131,
(
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No. 1010RJZA018), the Youth Foundation of Gansu Province
No. 2011GS04735) and NWNU-LKQN-11-32.
5 5 8 0 – 5 5 8 5.
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