Ultrasound-promoted hydrogelation of terpyridine derivatives

Article abstract of DOI:10.1039/c0nj00391c

Terpyridine derivatives proved to be versatile hydrogelators when subjected to ultrasound irradiation in their bis-protonated form. The chelating ability of the terpyridine ligands allows for the capture of metal cations resulting in stable gels with tuna

Full text of DOI:10.1039/c0nj00391c

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Ultrasound-promoted hydrogelation of terpyridine derivativesw
Letizia Sambri,*a Fabio Cucinotta,b Gabriele De Paoli,b Stefano Stagnic and
Luisa De Colab
Received (in Montpellier, France) 24th May 2010, Accepted 20th July 2010
DOI: 10.1039/c0nj00391c
Terpyridine derivatives proved to be versatile hydrogelators
when subjected to ultrasound irradiation in their bis-protonated
form. The chelating ability of the terpyridine ligands allows for
the capture of metal cations resulting in stable gels with tunable
emissive properties.
Fig. 1 Reversible gelation of a solution (6.0 ꢁ 10ꢀ3 M) of 1a in water:
Low molecular weight gelators (LMWGs), which form supra-
molecular gels, have attracted much attention over the last
years1 since they represent a new class of materials with
various potential applications,1,2 such as electronic devices,3,4
smart materials,2a confined reaction media,5 light harvesting
systems,4 and sensors.6 In particular the study of hydrogelators
is an expanding area of research7 owing to their possible
applications in bio-materials,2a drug delivery8 and pollutant
capture and removal.9
(a) solution after ultrasound irradiation and (b) gel after 30 min
standing at 20 1C.
1a-aqueous solution obtained by ultrasound irradiation
(0.18 W cmꢀ2, 40 kHz) for 5 min resulted in clear gel after
standing for 30 min at room temperature (Fig. 1). A solution is
again obtained by simply subjecting the vial to ultrasounds for
a few minutes. The reversibility of the gelation process was
tested by performing six sonication cycles and a gel was again
formed by allowing the solution to stand. The critical gel
concentration (CGC) was established to be 6.0 ꢁ 10ꢀ3 M and
the soft material turned out to be transparent and highly
resistant, being stable for 6 months at least.
Since the pioneer work of Naota and Koori in 2005,14b
ultrasound promoted gelation has received much interest.13,14
A gelation process initiated by ultrasounds could be nonsensical
at first sight, sonication being generally used to disrupt inter-
molecular interactions. On the contrary, if ultrasound irradiation
provides sufficient energy to induce cavitation of the solvent, a
new set of specific interactions can dominate the system
and ultimately triggers its gelation. For example H-bonds,
p–p-stacking and Van der Waals forces were found to be
activated by sonication.2,13,14
In addition, LMWGs caught the interest of scientists since
their properties can be tuned by external stimuli,10 such as
temperature,1,2b light,11 pH,12 ultrasounds13,14 and so on.
During our studies on the synthesis of transition metal
complexes, we serendipitously found that compound 1a
(Scheme 1), when centrifuged in the presence of water, formed
a yellow gel. Since the ability of an organic molecule to gel
water is nowadays extremely interesting, we decided to investi-
gate in detail this behaviour.
When a hot solution of 1a in water was cooled to room
temperature, a precipitate was formed. On the other hand, the
It is important to underline that the ultrasonic effect is
always unpredictable, and it is only possible to speculate on
the mechanism of the gel formation. In our case, probably the
energy provided by the sonication is sufficient to dissolve the
salts and to promote the formation of H-bonding between
water and protonated terpyridine derivatives.
Various examples of gelators containing the 2,20:60,200-
terpyridine (terpy) moiety are present in the literature.15 In
particular, terpy-derivatives bearing a carboxylic group were
reported to form gels in their anionic form.16 On the contrary,
we have found that the presence of the terpyridine in its
bisprotonated form proved to be essential ( 1a, Table 1). In
fact in our experimental conditions the neutral substrate 1b did
not form any gel in water, neither under sonication nor under
heating (Table 1). Thus, the gelation occurs in a range of
pH varying from 2.2 to 4. At pH = 6 precipitation takes
place, while at pH 4 7.5 a completely soluble product was
obtained.
Scheme 1 Structures of the terpyridine derivatives 1a–l.
a Dipartimento di Chimica Organica ‘‘A. Mangini’’,
`
Universita di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy.
E-mail: letizia.sambri@unibo.it; Fax: +39 051-2093654;
Tel: +39 051-2093624
b Physikalisches Institut, Mendelstrasse 7, D-48149 Muenster,
Germany. E-mail: decola@uni-muenster.de;
Fax: +49 251 980-2834; Tel: +49 251 980-2873
c
`
Dipartimento di Chimica Fisica ed Inorganica, Universita di Bologna,
Viale Risorgimento 4, I-40136 Bologna, Italy.
E-mail: stefano.stagni@unibo.it; Fax: +39 051-2093654
w Electronic supplementary information (ESI) available: Gel tests,
SEM images and NMR spectra. See DOI: 10.1039/c0nj00391c
c
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Table 1 Ability of hydrogelation of compounds 1a–1l varying the
Ar–terpyridine substituenta
Table 2 Effect on the gelation after the addition of 1 equiv. of metal
salt to the hydrogel of 1a
Compound
State
CGCb (ꢁ 10ꢀ2 M)
Entry
Metal salt
State (colour)
1a
1b
1c
1d
1e
1f
1g
1h
1i
G(T)c
0.6
—
1.0
—
—
—
—
1.9
2.0
—
—
—
1
2
3
4
5
6
7
8
9
10
EuCl3ꢂ6H2O
CuCl2ꢂ6H2O
CeCl3ꢂ7H2O
Ce(ClO4)3ꢂnH2O
PtCl2ꢂ2PhCN
K2PtCl4
Zn(ClO4)2ꢂ7H2O
IrCl3ꢂnH2O
PdCl2
G(O)a (yellow)
G(O) (green)
G(T)b (yellow)
Pc
G(O) (beige)
P
Id
G(T)
I
Se
I
I
P
G (O)f
G (O)
G(O) (dark red)
G(O) (orange)
G(O) (yellow)
1j
1k
1l
PGg
NiCl2ꢂ6H2O
I
I
a
b
c
G(O) = opaque gel. G(T) = transparent gel. P = precipitate.
a
5
mg of each compound in 0.5 ml of water was subjected
to ultrasound irradiation (0.18 W cmꢀ2, 40 kHz, 5 min) and the
The same behaviour is observed when the gel is treated with
Zn(ClO4)2ꢂ7H2O (Table 2, entry 7). The addition of other
cations, such as Ir(III), Pd(II) and Ni(II) (Table 2, entries 8–10)
gave rise to the formation of less stable gels, as they collapsed
within a few hours.
b
resulting state was checked after 1 h. CGC = critical gel concentra-
c
tion. G(T) = transparent gel. I = insoluble. S = soluble. G(O) =
d
e
f
g
opaque gel. PG = partial gel.
To investigate the morphology of the gels, dried samples
were analyzed by scanning electron microscopy (SEM). In
each case, well developed retiform structures were observed.
As shown in the two representative examples in Fig. 2,
the fibers are bundled and have a general length of several
micrometres, with diameters approximately less than one
micrometre.
We have investigated how the gelation process was influenced
by the presence of different counterions and substituents on the
phenyl ring of the terpyridine unit. The results are summarized in
Table 1. No notable changes to the gelation were noted when the
counterion10,15c,17 was changed from Brꢀ to Clꢀ ( 1c), while the
gel formation did not occur in the presence of a doubly charged
counterion such as the sulfate ( 1d) obtaining a precipitate. Also,
under basic conditions the sodium salt 1e was not able to gel
water since it resulted in an isotropic solution, both before and
after ultrasound irradiation.
Further images of the gels are reported in the ESI.w It can be
observed that both hydrogels 1a (Fig. 2a) and 1c display a
The conversion of the –COOH group into the corresponding
methyl ester (compound 1f) makes the substrate insoluble in
water and no gelation was observed (Table 1). The same
behaviour is displayed by the Br or OH substituted deriva-
tives, compounds 1k and 1l, respectively.
When a CN or OMe group is present in the 4-position of the
phenyl ring, compounds 1h and 1i, respectively, an opaque gel
was formed, even if the CGC values are higher (1.9 ꢁ 10ꢀ2 M
and 2.0 ꢁ 10ꢀ2 M, respectively) than for 1a (0.6 ꢁ 10ꢀ2 M).
Finally for compound 1j, when Ar = mesityl, a transparent
partial gel was observed (Table 1).
Therefore, the results summarized in Table 1 suggest that, in
addition to the bisprotonated terpy-core, the presence of a
highly polar substituent is essential for promoting the formation
of gels.
Owing to the importance of including/incorporating metals
in supramolecular gels,10,15,18 and considering the ability of the
terpyridine unit to coordinate metal ions, the effect of different
cations on the gel was tested. Various metal salts (1 equivalent)
were directly added to the formed hydrogel of 1a (pH = 2.3).
Then sonication of the resulting mixtures was performed in
order to obtain a solution and the formation of a new gel was
checked after 1 h.
As reported in Table 2 in entries 1–3 and 5, after the addition
of EuCl3ꢂ6H2O, CuCl2ꢂ6H2O, CeCl3ꢂ7H2O, and PtCl2(PhCN)2,
very stable gels were obtained. On the contrary, insoluble
precipitates were recovered after the treatment of the 1a-based
gel with metal salts containing different counterions,10,17 such
as Ce(ClO4)3ꢂnH2O and K2PtCl4 (Table 2, entries 4 and 6).
Fig. 2 SEM image of a dried film of: (a) 1a-hydrogel and (b) CeCl3ꢂ
7H2O– 1a-hydrogel.
c
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flattened fiber-like structure, with only a small size difference
between the fiber morphologies of the gel forming counter-
anions Brꢀ and Clꢀ. The presence of metal cations such as
Eu(III) and Ce(III) (Fig. 2b) seems to influence the gel structure
towards the formation of branched fibers, whereas upon
addition of Pt(II) and Cu(II) salts no significant changes are
observed in the morphology of the corresponding gels.
The stable 1a-gels containing the metal salts were charac-
terized by UV/Vis absorption and emission spectroscopy,
performed on xerogel solid substrates. For this purpose the
gel samples were drop casted onto a microscope slide and let
dry before measuring.
Even though the photophysical characterization is not com-
plete and it will require further investigations, the preliminary
measurements indicate that in most cases the metal ions
interact with the ligand 1a. The UV/Vis absorption spectra
are very similar for all the gels and the presence of a metal
seemed to affect only slightly the absorption of the 1a-xerogel,
as the recorded spectra of the metal-containing films resemble
the absorption profile of the metal-free film (Fig. 3). The latter
one displays a band in the UV region (up to 380 nm) due to the
p* ’ p transition centered on the terpyridine moiety. This
band broadens for the metal-containing films. As shown in
Fig. 3 (see also ESIw for the corresponding diluted solutions),
the incorporation of copper(II) broadens the band in the UV
portion of the spectrum, while the other metal-containing films
display new features at long wavelengths. These spectral
changes in the light absorption are likely due to any inter-
action occurring between the metals and 1a within the supra-
molecular gel network.
Fig. 4 Normalized emission spectra of films of 1a-hydrogels containing
different metal salts (lexc = 350 nm).
radiative processes. The detected red luminescence upon exci-
tation of 1a within the soft material along with a lifetime value
of 159.64 ms indicates that a sensitization of the lanthanide ion
takes place (antenna effect).21
On the other hand, the gel emission spectrum is slightly
modified by the presence of CeCl3ꢂ7H2O. In particular, even
though the emission band of the ligand at approximately
440 nm increased in intensity in relation to the band at 530 nm,
the luminescence lifetimes do not change significantly when
compared to those of 1a (see also Table 3).
An even less pronounced effect in the emission spectrum is
observed in the case of the gel obtained by doping with
PtCl2(PhCN)2, whose emission band recalls the one of 1a,
showing the same vibronic progression (within the range
100–1200 cmꢀ1). However, the elongation of the lifetime to
the microsecond scale which is observed in the presence of the
metal suggests the probable triplet nature of the emitting
excited state (Table 3).
The emission spectrum of the gel containing 1a shows two
bands centered at 430 and 530 nm. The high energy emission
is due to the non aggregate terpyridine derivative and is
attributed to p* ’ p transitions.19 The excited state lifetime
is also in good agreement with such assignment. The lower
energy band is due to the aggregate form of 1a in the gel.
The emission features of the gels depend on the nature of the
metal (Fig. 4).20 For example, in the case of the 1a-gel
containing 1 equivalent of EuCl3ꢂ6H2O, exciting at 350 nm,
the emission of 1a, in the aggregate form, at 530 nm is com-
pletely quenched in favour of the characteristic Eu(III)-based
Conversely, the gel containing the CuCl2ꢂ6H2O salt shows a
structureless broad emission band, ranging from 400 to
700 nm, and a decay very similar to that of 1a.
In conclusion, we reported the formation of stable hydrogels
with small organic molecules, such as terpyridine derivatives.
The gelation occurs only under acidic conditions and in the
presence of a highly polarized substituent. The ability of
1a-hydrogel to coordinate metal cations was tested and various
examples are reported in which the gelation still occurs.
Table 3 Emission lifetimes of the studied hydrogels, at 440 and
530 nm unless otherwise mentioned, recorded on films
Lifetimes
lem = 440 nm
lem = 530 nm
Hydrogel
1a
0.92 ns (60%)
6.63 ns (40%)
0.88 ns (43%)
2.27 ns (57%)
1.50 ms (29%)
7.44 ms (71%)
1.34 ns (42%)
5.35 ns (58%)
159.64 msa
0.87 ns (55%)
5.76 ns (45%)
0.83 ns (40%)
2.96 ns (60%)
1.77 ms (16%)
8.18 ms (84%)
0.98 ns (31%)
4.80 ns (69%)
—
1a + CeCl3
1a + PtCl2
1a + CuCl2
1a + EuCl3
Fig. 3 Normalized UV/vis absorption spectra of films of 1a-hydrogels
a
Detected at lem = 612 nm.
containing different metal salts.
c
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Studies are in progress in our laboratories in order to
optimize the gelation process and, in particular, to further
understand whether the photophysical behavior of various
metal complexes can be influenced by the formation of corres-
ponding terpyridine-based gels.
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Experimental
General procedure for the synthesis of terpyridine derivatives
1a–l
Compounds 1a22 and 1b23 were synthesized according to
previously reported procedures. Compound 1f was obtained
by treating 1b with TMSCH2N2. The corresponding neutral
forms of compounds 1h–l were synthesized starting from the
suitable aromatic aldehydes, according to a previously reported
procedure.24 Compounds 1c–e and 1g–l were obtained by
treatment of the neutral forms with an excess of the desired
concentrated acid (HBr, HCl or H2SO4) or NaOH. Detailed
experimental descriptions and NMR analysis of the com-
pounds are reported in ESI.w
General procedure for the 1a-gel preparation
In a 5 ml vial, 1.3 ml of water was added to 4 mg (7.8 ꢁ
10ꢀ3 mmol) of 1a. The suspension was put in the ultrasound bath
(Branson 2200, 0.18 W cmꢀ2, 40 kHz) for 5 min. A transparent
solution was obtained. After 30 min standing at room tempera-
ture the formation of the gel occurred.
General procedure for the preparation of 1a-gels containing a
metal salt
After the preparation of a 1a-hydrogel sample, the desired
metal salt (1 equiv.) was added. The mixture was submitted to
ultrasound irradiation (Branson 2200, 0.18 W cmꢀ2, 40 kHz)
for 5 min. The resulting solution was left at room temperature
for at least 1 h and then the formation of the gel was checked.
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18 F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680.
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c
2096 New J. Chem., 2010, 34, 2093–2096 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010