Electrochemical probing of hydrogelation induced by the self-assembly of a donor-acceptor complex comprising pyranine and viologen

Article abstract of DOI:10.1039/c6cc09465a

A long-tailed methyl viologen (DMV) forms a co-assembly (1?:?1) with pyranine to result in pronounced hydrogelation. The systematic evolution of the hydrogel promoted by the donor-acceptor interactions could be probed electrochemically in a non-invasive manner.

Full text of DOI:10.1039/c6cc09465a

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Electrochemical probing of hydrogelation induced
by the self-assembly of a donor–acceptor
complex comprising pyranine and viologen†
Cite this:DOI: 10.1039/c6cc09465a
Received 28th November 2016,
Accepted 2nd January 2017
ab
a
ab
Sougata Datta,‡ Nilanjan Dey and Santanu Bhattacharya*
DOI: 10.1039/c6cc09465a
rsc.li/chemcomm
A long-tailed methyl viologen (DMV) forms a co-assembly (1 : 1) with cationic radical species, further reduction of which leads to the
pyranine to result in pronounced hydrogelation. The systematic evolu- formation of a yellow colored, neutral quinoid moiety. Zhang et al.
tion of the hydrogel promoted by the donor–acceptor interactions reported a pH-responsive hierarchical self-assembly derived from the
could be probed electrochemically in a non-invasive manner.
CT-complex made of 1 : 1 viologen derivatives and water-soluble
donors (8-hydroxy-1,3,6-pyrene-trisulfonic acid trisodium salt or
8
The formation of various supramolecular architectures mediated 6,8-dihydroxypyrene-1,3-disulfonic acid disodium salt). However, no
by self-assembly continues to inspire the scientific community to gelation was observed. Also, negatively charged Au-nanoparticles and
1
unlock the secrets hidden in many natural phenomena. In this an anionic dye were trapped in the inner cavity of a redox-active
context, the design and synthesis of diverse classes of low cationic polymeric nanotube made of viologen via electrostatic inter-
molecular-weight gelators (LMWGs) along with their application actions. The entrapped fluorescent dye was subsequently released
2
for various purposes are the subject of growing attention. There from the cavity of the nanotubes via electrochemical or chemical
9
has been increasing interest in the exploitation of two-component reduction. George and co-workers reported a co-assembly of a non-
LMWGs which offer a novel opportunity to tune their properties covalent donor–acceptor amphiphilic pair made of coronene and a
by modulation of their functionalities, thereby controlling the viologen derivative which transformed into cylindrical micelles to
1
0
interactions in the complex by their aggregation process.
generate a hydrogel. However, there are hardly a handful of reports
11
Charge transfer (CT)-based self-assemblies comprise a unique on viologen-derived gels. Recently, we reported a redox-responsive
class of gelators which form gels in a particular solvent in the two-component hydrogelator consisting of an L-alanine based amphi-
presence of appropriate electron-rich or electron-deficient partners phile and a redox-active viologen moiety. Interestingly, the incor-
11a
3
often accompanied by visual changes in color. CT-based organo- poration of SWNTs in the electrochemically irreversible hydrogel
gelation involving pyrene derivatives in the presence of various transformed it into a quasi-reversible electrochemical system.
4
acceptors have been reported. An example of persistent
Herein, we present a new two-component donor–acceptor pair
CT-interactions in a sugar based dual-component system is known, hydrogelator system comprising n-dodecyl-tailed methyl viologen
2
+
where both the donor and the acceptor moieties act as gelators (CH
3
-V -12 = acceptor) and 8-hydroxy-1,3,6-pyrene-trisulfonic acid
5
3ꢀ
independently. Recently, Ghosh and co-workers described a trisodium salt (pyranine = Py = donor). Cetyl pyridinium bromide
CT-mediated vesicle-to-fiber (2D-to-1D) morphological transfor- (P -16) was chosen as a control salt which could neither form a
+
mation of a bis-hydrazide-functionalized naphthalene diimide- CT-complex nor a hydrogel in the presence of pyranine (Fig. 1a).
6
2+
based bolaamphiphile in aqueous media.
Since the viologen core of CH -V -12 senses changing environ-
3
0
3ꢀ
4
,4 -Bipyridinium derivatives (viologens) are well known ments in the presence of different amounts of Py , the incumbent
redox-active systems which show two successive reversible, self-assembly phenomena involved in the hydrogelation process
7
reductive processes, both chemically and electrochemically.
have been successfully explained in detail by an electrochemical
The first reductive step induces the formation of a blue colored analysis. There are limitations for various physical methods to
probe supramolecular gelation. For example, NMR resonances of
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore, India.
E-mail: sb@orgchem.iisc.ernet.in; Fax: +91-80-23600529; Tel: +91-80-22932664
Director’s Research Unit (DRU), Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700 032, India
gel materials are considerably broader than in their sol state.
Hence, it is difficult to assign the peaks of the gelator in most of
the cases owing to the spectral broadening. Similarly, crystallization
b
12
of a gelator from the gelling solvent is rare. Gels and crystals
are generally formed from different experimental conditions (ca.
solvent). Hence, it is difficult to confirm the difference in molecular
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc09465a
Present address: Department of Chemistry, University of Utah, 315 South 1400
East, RM 2020, Salt Lake City, Utah 84112, USA.
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3ꢀ
Fig. 1 (a) Molecular structures of the donor (Py ) and the potential
2
+
2+
+
acceptors (CH
3 3 3
-V -12, CH -V -CH and P -16) used in the present
2+
3ꢀ
study; photographs showing aq. sol of (b) CH
c) P -16–(Py ) and (d–f) S
CT-hydrogel of CH
3
-V -CH
2ꢀ
4
O triggered gel-to-sol transition of the
3
–(Py ) and
+
3ꢀ
(
2
2+
3ꢀ
3
-V -12–(Py ).
1a
packing between single crystals and gels. A non-invasive approach
such as electrochemical analysis provides fundamental insights
when the components involved in the gelation are electroactive.
Notably, this approach has not been applied to investigate the
hydrogelation mechanism involving electroactive partners that
participate in a CT process during self-assembly. To the best of
Fig. 2 (a) SEM (inset: magnified image) and (b) AFM images of the xerogel
2+
3ꢀ
derived from CH
spectra of (c) CH
3
-V -12–(Py ) at a concentration of 9 mM; Raman
2+
3ꢀ
2+
3ꢀ
+
3ꢀ
3
-V -12, Py and CH
3
-V -12–(Py ); (d) P -16, Py
and P -16–(Py ) each at the concentration of 9 mM.
+
3ꢀ
our knowledge, this is the first report where an electrochemical ground state inter-molecular interaction between the donor and
analysis of the CT-phenomenon provides critical insights into the the acceptor molecules in such self-assemblies (Fig. S9a, ESI†).
hydrogelation process.
Moreover, a new weaker absorption band appeared near B485 nm
3
ꢀ
The gelation ability of pyranine (Py ) in water (Fig. 1b–d) was upon directly probing the existence of CT-phenomenon between
+
2+
11d
examined in the presence of each of P -16, CH
3
-V -CH
3
and CH
3
-
the donor and the acceptor.
Furthermore, an addition of a
2+
2+
V -12 using the ‘‘stable-to-inversion protocol of a test tube’’ method. colorless aq. solution of CH
3
-V -12 to the greenish aq. solution
-V -12 formed a dark-red colored hydrogel of Py resulted in the formation of a brownish solution which
minimum gelator concentration [mgc] = 9 mM) in the presence of again indicates the manifestation of CT-interactions between
2
+
3ꢀ
It was observed that CH
3
(
3ꢀ
Py at 1 : 1 molar stoichiometry. The red coloration is an indication the donor and the acceptor (Fig. S9b–d, ESI†). The prevalence of
3
ꢀ
2+
of the effective CT between Py and CH
3
-V -12. Addition of excess CT-interaction was further confirmed by the dramatic quenching
3ꢀ
3ꢀ
Py over 1 : 1 ratio however, led to the abolition of gelation. In of the fluorescence emission intensity of Py in the presence of
+
2+
13
contrast, mixing of P -16 resulted in only a viscous solution devoid of CH
3
-V -12 (Fig. S10b–d, ESI†). Thus, the emission intensity at
3
ꢀ
any change in color of Py and the mixture never formed a gel even B511 nm diminished monotonically with the increasing amount
upon prolonged aging. This result indicates the absence of of CH
2+
3
-V -12 (Fig. S10a, ESI†).
+
3ꢀ
CT-interaction between P -16 and Py . Interestingly, addition of
Raman spectroscopy is an invaluable tool for investigating
2+
3ꢀ
14
15
CH -V -CH to a solution of Py resulted in the formation of a red viologens, polyaromatics, carbon allotropes (i.e., carbon
3
3
1
6
17
colored solution under the same circumstances, suggesting the nanotube, graphene ) etc. The changes in the electronic
existence of CT-interaction between CH
2
+
3ꢀ
3 3
-V -CH and Py . However, structure of graphene due to the CT-interaction with tetrathia-
the resulting CT-complex could not immobilize water presumably fulvalene (TTF), donor or tetracyanoethylene (TCNE), acceptor
1
7
because of the lack of a long hydrocarbon tail in the acceptor have been examined by Raman spectroscopy. A softening of
2
+
2
(CH
3 3
-V -CH ) which may be important for hydrogel formation. the G-band (a characteristic of the planar sp bonded carbon
The microstructures formed by the self-assembly in the hydrogel configuration) occurred progressively with the increasing concen-
were examined by scanning electron microscopy (SEM) and atomic tration of TTF, an electron donor, whereas, a stiffening of the
force microscopy (AFM). Well-organized and dendritic structures, band was found with the increasing concentration of TCNE, an
maintaining high degrees of ordering, were observed in the SEM electron acceptor. Lee and co-workers observed electron transfer
2
+
3ꢀ
0
ꢀ1
image (Fig. 2a) of the freeze-dried gel of CH
3
-V -12–(Py ). from reduced viologen (V ) to graphene by detecting a 2 cm red-
ꢀ
1
18
However, a closer examination of the corresponding sample of shift (1591 to 1589 cm ) of the G band of the graphene.
:1 P -16–(Py ) showed the presence of regular fibrous networks Graphene may be considered as an indefinitely large aromatic
+
3ꢀ
1
only (Fig. S7, ESI†). Distinct networks of fibers with an average molecule. Hence, polyaromatic hydrocarbons may be considered
diameter of 0.1–0.5 mm along with bundles were clearly visible as a small fragment of graphene. Fig. 2c shows a strong band
2
+
3ꢀ
ꢀ1
in the xerogel of 1 : 1 CH
3
-V -12–(Py ) under AFM (Fig. 2b). at 1648 cm associated with the C–C inner ring vibration of
2
+
ꢀ1
However, long tape-like structures of 2–5 mm width were observed CH -V -12 which red-shifted to 1620 cm in the CT-complex
3
+
3ꢀ
2+
3ꢀ 14
3ꢀ
in the case of 1:1 P -16–(Py ) (Fig. S8, ESI†).
CH -V -12–(Py ). Furthermore, relative to Py , the Raman
3
3ꢀ
ꢀ1
The UV-Vis absorption bands of Py at B364 and 400 nm band at 1625 cm which is characteristic of the CQC bond
shifted to higher wavelengths with concomitant decreases in the vibration of Py showed a B20 cm blue-shift in the xerogel
intensity and broadening of the absorption band with the increasing of CH -V -12–(Py ), corroborating the formation of a donor–
3
3
ꢀ
ꢀ1
2
+
3ꢀ
2+
10
15,18,19
CH
3
-V -12 concentration. This suggests the existence of a strong acceptor type CT-complex.
Hence, the present situation is
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complementary to the case reported by Lee et al. because electron
Another interesting phenomenon has been observed in the
3ꢀ
2+
3 p
transfer occurs here from Py to CH -V -12. However, the Raman cyclic voltammetry experiment (Fig. 3). The peak current (i )
shift is significantly larger in this case, because of the smaller size of corresponding to each of the half-wave reduction potentials
3
ꢀ
18
3ꢀ
Py as compared to that of graphene. However, no noticeable monotonically decreased with increasing Py stoichiometry in
+
3ꢀ
shift was observed in the case of P -16–(Py ) because of the absence the donor–acceptor complexes [i (n = 0) 4 i (n = 0.5) 4 i (n = 1)].
of the CT-interactions between P -16 and Py (Fig. 2d).
p
p
p
+
3ꢀ
p
The peak current (i ) is proportional to the square root of the
Kaifer and co-workers probed various asymmetric dendrimers diffusion coefficient (D) as described by the Randles–Sevcik
1
1a,20
containing redox-active-viologen or the ferrocene moiety using cyclic equation,
20
voltammetry. These authors showed that the half-wave reduction
potentials of the viologen core shifted to more negative values with the
1
/2
i
p
= 0.4463 nFAC (nFvD/RT)
(1)
20
increasing size of the Newkome-type of dendrimers. This indicates a
thermodynamic hindrance towards the reduction process for the
higher dendrimer molecular mass (destabilization of the reduced
species). Recently, the utility of this concept in the supramolecular
hydrogelation mediated by a two-component self-assembly, compris-
ing an L-alanine based amphiphile and a redox-active viologen based
In this equation, ‘n’ is the number of electrons appearing in the
half-reaction for the redox couple; ‘v’ is the rate at which the potential
ꢀ
1
ꢀ1
is swept (V s ); ‘F’ is the Faraday’s constant (96 485 C mol );
2
‘
(
A’ is the electrode area (cm ); ‘R’ is the universal gas constant
ꢀ
1
ꢀ1
8.314 J mol
K ); ‘T’ is the absolute temperature (K) and ‘D’
is the analyte’s diffusion coefficient (cm s ).
2
ꢀ1
11a
partner was demonstrated.
It was shown that the half-wave
If the temperature is 25 1C (298.15 K), the Randles–Sevcik
equation can be rewritten in a more concise form as,
reduction potentials (E1/2) associated with the viologen ring shifted
to more negative values with the increasing amount of the amine
component suggesting that a higher extent of salt formation hinders
the reduction of the viologen moiety.
5
3/2 1/2 1/2
i
p
= (2.687 ꢁ 10 )n
v
D
AC
(2)
Herein, we have shown the systematic build up of the hydrogelation
by successive increases in the extent of the donor–acceptor interactions.
Towards this end, we have recorded cyclic voltammograms of various
CT complexes at different donor–acceptor stoichiometries. CH -V -12
3
alone showed two distinguishable, sharp peaks associated with its two
We calculated the diffusion coefficients of various CT com-
plexes (n = 0, 0.5 and 1) associated with the first half-wave
reduction potential using the Randles–Sevcik equation listed in
2+
2
ꢀ3
ꢀ3
Table S2 (ESI†) (n = 1, A = 0.09 cm , C = 0.009 ꢁ 10 mol cm
,
ꢀ3
ꢀ1
v = 100 ꢁ 10 V s ). The diffusion coefficient of the medium
redox-states in the cathodic as well as anodic scans (Fig. 3). The E1/2
2+
3ꢀ
ꢀ6
2
ꢀ1
containing CH -V -12 in the absence of Py is 10 ꢁ 10 cm s
,
3
values for both the reduction steps are clearly affected by an augmenta-
ꢀ
6
2
ꢀ1
which decreases to 1.32 ꢁ 10 cm s in the presence of 0.5 equiv.
tion of the supramolecular aggregation with the increasing extent of CT
activity. Both half-wave reduction potentials shifted to more negative
3
ꢀ
ꢀ7
2
ꢀ1
of Py and finally reaches 1.36 ꢁ 10 cm s in the case of
2
+
3ꢀ
1
3ꢀ
3
CH -V -12–(Py ) .
values with increasing Py stoichiometry accompanied by a concomi-
tant drop in the peak current intensity (Table S1, ESI†). Increase in the
Furthermore, the diffusion coefficient (D) is inversely
proportional to the viscosity (Z) of the medium as described
3ꢀ
stoichiometry of Py promotes the CT process to a greater extent and
this leads to an enhancement in the negative charge density around the
viologen moiety, which hinders the reduction (loss of positive charge) of
the viologen nucleus.
1
1a,20
by the Stokes–Einstein equation,
D = kT/6pZr
(3)
In this equation, ‘D’ is the diffusion coefficient of the medium;
k’ is the Boltzmann’s constant, ‘T’ is the absolute temperature
K); ‘Z’ is the viscosity of the medium, and ‘r’ is the radius of the
‘
(
spherical particle.
On the basis of the eqn (2) and (3), the trend in the fall of peak
current could be rationalized to be a consequence of the increase
in the viscosity of the medium with the increasing extent of
CT interaction, thus resulting in supramolecular hydrogelation.
Though it is important to note that the diffusion coefficient
measured is an average of the corresponding free molecules
and those forming part of the aggregates which could be in fast
exchange in the time scale of the measurements.
In order to probe this finding further, a rheological experi-
ment was performed to acquire information pertaining to the
structural correlation with the viscoelastic properties (Fig. S11,
3
ꢀ
2+
3ꢀ
)
n
2
+
3ꢀ
Fig. 3 Cyclic voltammograms of Py , CH
3
-V -12–(Py
+ 1 equiv. Na on glassy carbon
electrode in water in the presence of 0.1 M LiCl as a supporting electrolyte
(n = 0, 0.5
ESI†). At a fixed applied frequency of 1 Hz, CH -V -12–(Py )0.75
3
2+
3ꢀ
and 1) and CH
3
-V -12–(Py )
1
2 2 4
S O
began to flow at a shear stress of 14 Pa critical stress, whereas
2+
3ꢀ
2+
3ꢀ
ꢀ1
3 3
CH -V -12–(Py )0.85 and CH -V -12–(Py ) at identical concen-
1
at a scan rate of 100 mV s at the concentration of 9 mM in each case.
Inset shows the magnification of the peaks associated with the first redox
tration started to flow at a shear stress of approximately 40 and
100 Pa, respectively, thus ascribing significantly higher viscoelastic
3
ꢀ
2+
3ꢀ
potentials of Py and CH
3
-V -12–(Py )
n
(n = 0.5 and 1).
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2
+
3ꢀ
1
solid-like behavior of CH
3
-V -12–(Py
)
compared to that of
This work was supported by a J. C. Bose fellowship of DST
and by DRU at IACS to S. B. S. D. thanks IACS for a research
2
+
3ꢀ
the CH
3
-V -12–(Py
)
0.85
.
In order to further probe, a control experiment was performed. associateship.
2
+
Though CH -V -CH is capable of forming a CT-complex with
3
3
3ꢀ
Py at 1 : 1 molar ratio, the resultant CT-complex, however, did
not form a hydrogel. In the cyclic voltammetry experiment, the half-
wave reduction potentials associated with the viologen core of
Notes and references
1
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(
(
b) D. K. Kumar and J. W. Steed, Chem. Soc. Rev., 2014, 43, 2080;
c) M. D. Segarra-Maset, V. J. Nebot, J. F. Miravet and B. Escuder,
2
+
3ꢀ
3 3
CH -V -CH shifted to more negative values with increasing Py
stoichiometry because of the CT-complexation (Fig. S12, ESI†).
However, significant drops in the peak current were not observed
because the viscosity of the media remained unaffected with
Chem. Soc. Rev., 2013, 42, 7086; (d) J. W. Steed, Chem. Soc. Rev., 2010,
9, 3686; (e) J. H. Jung, G. John, K. Yoshida and T. Shimizu, J. Am.
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(b) S. Datta and S. Bhattacharya, Chem. Commun., 2015, 51, 13929;
3
2
3ꢀ
increasing Py stoichiometry in this case. A slight drop in the
(
2
c) D. Mandal, S. K. Mandal, M. Ghosh and P. K. Das, Chem. – Eur. J.,
015, 21, 12042; (d) R. Zou, Q. Wang, J. Wu, J. Wu, C. Schmuck and
peak current intensity is presumably because of the larger size of
2
+
the resultant CT-complex compared to that of CH
3 3
-V -CH . It
H. Tian, Chem. Soc. Rev., 2015, 44, 5200; (e) S. K. Samanta and
S. Bhattacharya, Chem. Commun., 2013, 49, 1425; ( f ) S. Datta,
S. K. Samanta and S. Bhattacharya, Chem. – Eur. J., 2013, 19, 11364;
may be important to note that the peak current corresponding to
2
+
each half-wave potential of CH
3
-V -CH
-V -12 because of its tendency toward aggregation
Fig. S12, ESI†).
The CT-hydrogel derived from CH -V -12 and Py showed a
3
is higher compared to
(
2
g) S. K. Samanta and S. Bhattacharya, J. Mater. Chem., 2012,
2, 25277; (h) S. R. Jadhav, P. K. Vemula, R. Kumar, S. R. Raghavan
2
+
that of CH
3
(
and G. John, Angew. Chem., Int. Ed., 2010, 49, 7695; (i) A. R. Hirst,
B. Escuder, J. F. Miravet and D. K. Smith, Angew. Chem., Int. Ed., 2008,
2+
3ꢀ
3
47, 8002; ( j) S.-I. Kawano, N. Fujita and S. Shinkai, J. Am. Chem. Soc.,
2ꢀ
selective supramolecular response towards a dithionite (S O ) anion.
2
4
2004, 126, 8592; (k) M. George, S. L. Snyder, P. Terech, C. J. Glinka and
R. G. Weiss, J. Am. Chem. Soc., 2003, 125, 10275.
M. Kumar, K. V. Rao and S. J. George, Phys. Chem. Chem. Phys., 2014,
Addition of 1 equiv. of Na S O to the red colored hydrogel resulted in
2
2 4
3
4
a dark blue colored sol immediately, which subsequently transformed
1
6, 1300.
to a yellow colored sol (Fig. 1d–f). Na
2
S
2
O
4
reduced the viologen core of
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2+
3ꢀ
CH
3
-V -12; this disrupted its CT-interaction with Py and finally
induced gel-to-sol transition. This was further supported by the
5
3ꢀ
appearance of a strong fluorescence emission associated with Py
under long UV-light (365 nm) upon addition of 1 equiv. of Na S O to
6 M. R. Molla and S. Ghosh, Chem. – Eur. J., 2012, 18, 9860.
2
2 4
7
S. M. Kim, J. H. Jang, K. K. Kim, H. K. Park, J. J. Bae, W. J. Yu,
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the CT-hydrogel (Fig. S13a–c, ESI†). However, no fluorescence emis-
ꢀ
ꢀ
ꢀ
ꢀ
sion was observed in the presence of other anions e.g., AcO , F , Br ,
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ 2ꢀ
Cl , I , CN , H
2
PO
4
, HPO
4
, N
3
, NO
2
, NO
3
, SO
4
, S , SCN
etc. (Fig. S14a–c, ESI†). In the cyclic voltammetry experiment, both
9
1
2
+
3ꢀ
half-wave reduction potentials of CH
3
-V -12–(Py ) further shifted to
selectively
more negative values upon addition of 1 equiv. of Na S O
2 2 4
1
1 (a) S. Datta and S. Bhattacharya, Chem. – Eur. J., 2016, 22, 7524;
due to the reduction of the viologen core (Fig. 3). In addition, a
remarkable increment in the peak current was observed owing to the
(
(
b) K.-P. Wang, Y. Chen and Y. Liu, Chem. Commun., 2015, 51, 1647;
c) Q. Zhang, D.-H. Qu, X. Ma and H. Tian, Chem. Commun., 2013,
2
ꢀ
S O
induced gel-to-sol transition. However, no significant change
49, 9800; (d) Y. Liu, Y. Yu, J. Gao, Z. Wang and X. Zhang, Angew.
Chem., Int. Ed., 2010, 49, 6576.
2 (a) S. Bhattacharjee and S. Bhattacharya, Chem. Commun., 2015, 51, 7019;
2
4
in the half-wave reduction potentials as well as peak current was
observed in the presence of other anions (Fig. S15, ESI†).
1
(b) S. Bhattacharjee and S. Bhattacharya, Langmuir, 2016, 32, 4270.
In conclusion, we have demonstrated the first instance of 13 (a) S. Gamsey, A. Miller, M. M. Olmstead, C. M. Beavers, L. C. Hirayama,
S. Pradhan, R. A. Wessling and B. Singaram, J. Am. Chem. Soc., 2007,
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1
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been explained by UV-Vis, fluorescence and Raman spectroscopy.
Considerably organized dendritic textures of the aggregates in the
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5 G. R. Loppnow, L. Shoute, K. J. Schmidt, A. Savage, R. H. Hall and
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Y. F. Lian, T. Nakahodo, T. Tsuchiya, T. Akasaka, J. Lu, X. W. Zhang,
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0 (a) W. Ong and A. Kaifer, J. Am. Chem. Soc., 2002, 124, 9358;
perceptible shift in the Raman spectra of the xerogel. Such an
approach may find widespread utilty in probing other complex
2
fluids and soft materials.
(b) C. Cardona and A. Kaifer, J. Am. Chem. Soc., 1998, 120, 4023.
Chem. Commun.
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