Properties and applications of designable and photo/redox dual responsive surfactants with the new head group 2-arylazo-imidazolium

Article abstract of DOI:10.1039/c6ra04448d

2-Arylazo-imidazolium was investigated as a new hydrophilic head group for ionic liquid surfactants. With the azo moiety at the 2-C site of imidazolium, it was designed to expand the extensibility of the functionalization and controllability of assembly. Structural designability was displayed by the colloidal chemical properties like surface tension, conductivity and steady fluorescence quenching of the designed surfactants. Their corresponding responsiveness towards photo stimuli was compared. Further properties like the reversible electrochemical redox responsiveness and the instinctive fluorescence of the designed surfactants were discovered. A potential application was demonstrated by the photo-controllable fabrication of Au nanostars.

Full text of DOI:10.1039/c6ra04448d

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PAPER
Properties and applications of designable and
photo/redox dual responsive surfactants with the
new head group 2-arylazo-imidazolium†
a
Cite this: RSC Adv., 2016, 6, 51552
Changxu Lin, Long Yang,ab Mengchun Xu,a Qi An,c Zheng Xiangab
*
a
*
and Xiangyang Liu
2-Arylazo-imidazolium was investigated as a new hydrophilic head group for ionic liquid surfactants. With
the azo moiety at the 2-C site of imidazolium, it was designed to expand the extensibility of the
functionalization and controllability of assembly. Structural designability was displayed by the colloidal
chemical properties like surface tension, conductivity and steady fluorescence quenching of the
designed surfactants. Their corresponding responsiveness towards photo stimuli was compared. Further
properties like the reversible electrochemical redox responsiveness and the instinctive fluorescence of
the designed surfactants were discovered. A potential application was demonstrated by the photo-
controllable fabrication of Au nanostars.
Received 19th February 2016
Accepted 13th May 2016
DOI: 10.1039/c6ra04448d
www.rsc.org/advances
hydrogen bonded network12 for further assembly at the far end
of the N-site. At the X-site, there have been reports that counter
Introduction
anions incorporated with imidazolium surfactants make them
suitable as drug delivery vehicles13 and as part of a precursory
microemulsion for making an anion-responsive porous polymer
surface.14 The simultaneous delivery of function and a working
environment is an important feature of Im-IL surfactants, but
also make the imidazolium cation so crowded that it oen
encounters the lack of a functionalization point. Thus, it is
necessary to nd an easier structural tuning and a new deriva-
tive spot for the more integrated application of Im-IL
surfactants.
On the other hand, there have been efforts to adjust the
aggregation status of Im-IL surfactants, including commonly
variation of the alkyl chain length, topological options as
double tailed,14 gemini,11,13 bola,15 etc., and more conveniently,
anion exchange to adjust hydrophobicity16 and packing
parameters.11 Beyond the tuning of covalent scaffolds or
compositions, more instant adjustment was induced by
external stimuli, of which photo stimulation is the most typi-
cally selected due to its easy and diversied controllability,17–20
has also been broadly considered. Cationic surfactants have
been embedded with azobenzene at the ends21 or at the
boundaries21 of hydrophobic chains, spacers of gemini struc-
tures22,23 and the anion.22,24 With their help, the dispersity of
surfactant stabilized nanoparticles in different solvents21 and
the rheological behaviour of surfactant gels25 have been both
manipulated by light. The photoinduced modulation of prop-
erties more or less originate from the change of colloidal
chemical parameters in these systems. It is interesting to
explore closely how a conformation change at the imidazolium
head group could affect the colloidal chemistry of surfactants. Is
Since the origination of Room Temperature Ionic Liquids
(RTIL) in the 1990s, imidazolium-class ILs (Im-IL) have made
a major contribution to the concept of “Task-Specic Ionic
Liquids” (TSILs),1,2 which are widely employed in catalysis,3
nanomaterials, energy4 and the environment.5 Applications
have sprung mostly from the facile construction of desired
molecules by easy covalent combination at the N atom (N-site)
or even easier exchange of anions (X-site).6 With long alkyl
chains connected to them, imidazolium surfactants6–8 were
designed not only to introduce certain functions but also to set
up ourishing functioning environments such as micelles and
other assemblies credited with the exponential scaling up
extensibility based on their colloidal and interfacial chemistry
and easy structural diversication. The 1-N and 3-N (as N-Site)
on the imidazolium ring and the anion9 in the meantime
acted as cut-in points to endow Im-IL surfactants with func-
tionality. A zwitterionic imidazolium surfactant with a sulfonate
group (–SO3ꢀ) at the N-site was effective in increasing the dis-
persity of Pd nanoparticles.10 There are also examples of addi-
tional compositions like
a
polymerizable monomer11 or
aResearch Institute for Biomimetics and So Matter, Fujian Provincial Key Laboratory
for So Functional Materials Research, College of Physical Science and Technology,
Xiamen University, 361005 Xiamen, China. E-mail: lincx@xmu.edu.cn; liuxy@xmu.
edu.cn
bCollege of Material Science and Engineering, Huaqiao University, 361012 Xiamen,
China
cSchool of Materials Science and Technology, China University of Geosciences, Beijing
100083, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra04448d
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there any way to expand the functionalization site of Im-IL (from Iꢀ to Brꢀ) to determine that the as-made surfactants are of
surfactants and provide them with photo-controllability at the the proper solubility to perform water based surface tension
same time?
and conductivity tests or other experiments. Seven AzoIm+
To answer this question, we uncovered the possibility of the surfactants were synthesized accordingly to investigate steric
2-C position of imidazolium as a new site for connecting an and electronic effects.
arylazo moiety to ionic liquid surfactants. The new azo-
The steric effects of R0 at the N-site, which was the nal alkyl
functionalized head was intended to regulate species in the to nish the quarternisation of azo-imidazolium, could be
hydrophilic domain more directly, nely and strongly from revealed through the comparison of 1-Br (methyl) and 2-Br
additional p–p forces.26 Seven arylazo-imidazolium (AzoIm+) (ethyl). They had nearly identical original absorption peaks in
surfactants with minor structural variations in the cation and both aqueous and ethanol solutions. Yet a shorter R0 resulted in
anion were synthesized, and how such structural variations a better responsiveness to UV stimuli shown in the UV-Vis
affect their photoresponsive colloidal chemistry was investi- spectra (Fig. 1) with methoxyl as an example; the R at the
gated. That is, by examining how groups at the N-site or X-site para-position of azobenzene greatly inuenced the electronic
affect the different colloidal chemical properties, we would distribution. It resulted in a red shi of the maximum absorp-
construct a comprehensive understanding of the new head tion wavelength (lmax) and the weakest photo responsiveness of
group thus to obtain knowledge of the requirements for absorption coefficient (3) in both water and ethanol. The elec-
directing the construction of AzoIm+ molecules. Moreover, tronic structure of arylazo-imidazolium could be manipulated
a new 2-arylazo-imidazolium surfactant showed additional by introducing an electron donating/withdrawing group on the
photocontrollable instinctive uorescence, reversible respon- azobenzene. In the example using methoxyl the (p–p*)E peak of
siveness to electrochemical redox stimuli and the ability to 3-Br and 3-I red-shied to ꢁ397 nm in water and ꢁ405 nm in
modify the shape of Au nanoparticles. An understanding of this ethanol, and the overlapped (p–p*)Z peak was shied to ꢁ325
head group would add to and also cooperate with the extensive nm estimated from the peak shape (Fig. 1). This results from the
possibilities for RTIL class compounds.
electron donated to the conjugated system of arylazo-
imidazolium. These electrons also hinder Z–E conversion of
the azo bond. The principal effect of anions on solubility was
concluded. Both series are well soluble in ethanol, yet Brꢀ anion
surfactants were more soluble in water and have stronger
absorption than those with Iꢀ. The best responsiveness of
cation 3 was achieved by selecting Iꢀ as the anion and
measuring in ethanol.
Results and discussion
Synthesis of AzoIm+ surfactants and their photochemistry
Three basic steps led to the construction of AzoIm+ surfactants
(Scheme 1). (1) The diazotization of the 2-C site of imidazole,
which could introduce a phenyl or other aryl derivative to form
an arylazo-imidazole27 (methoxyphenyl as an example), (2)
alkylation and quarternisation to form an arylazo-imidazolium
and manipulation of the length of the hydrophobic chain (tet-
radecyl in our work) and of additional groups (methyl, ethyl or
sulfopropyl) on both the N sites, and (3) counter anion exchange
Fig.
cations: 1 (black), 2 (red), 3 (blue) and 4 (pink) and anions (a and b Brꢀ, c
Scheme 1 Structures and synthetic route of 2-arylazo-imidazolium and d Iꢀ), in the solvent of (a and c) H2O or (b and d) ethanol. A solid line
1 UV-Vis spectra of four surfactants with combinations of
surfactants.
indicates before UV exposure and a dashed line indicates after.
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The special zwitterionic 4 surfactant had a conspicuously low
solubility in water, showing a turbid baseline and scarcely any
photoresponsiveness even at the rather low test concentration
(0.04 mM) (Fig. 1a). So we could not perform a water based test
on the surface tension, pyrene probe uorescence and
conductivity of 4. On the contrary, in an ethanol solution, 4
showed good solubility and the strongest absorption at the
original state and the strongest photoresponsiveness (Fig. 1b).
The shorter alkyl at the N-site would not change its (p–p*)Z peak
wavelength, yet had a signicant effect on Z–E conversion in
solvents of different polarity. At the N-site, the decisive factor in
azo cis–trans conversion was the polarity of substituents rather
than their steric or electronic donor effects.
Surface properties and micellization
The measurement of surface tension (g) vs. surfactant concen-
tration (C) gives us an important tool for understanding the
relationship between molecular structure and aggregation
behaviour of surfactants. In our case we could gather much
information about the new imidazolium head groups and their
photoresponsiveness from the g–C plots in Fig. 2, (for a Brꢀ
anion) Fig. 3 (for an Iꢀ anion) and the calculated data in Table 1.
In general, the AzoIm+ surfactants had a remarkably lower
CMC and Amin than the imidazolium or N-aryl imidazolium29
surfactants (Table 1). Two important factors in their CMC and
Amin slump were: (1) a larger p-conjugated system to scatter the
positive charge of the imidazolium cation whose repulsion
prevented micelle formation or compact structure, and (2) the
higher hydrophobicity of the arylazo group making them
pseudo “double tailed” surfactants. In another aspect, b was
Fig. 3 Surface tension of [C14AzoIm]I aqueous solutions as a function
of their concentration. (a) Overall data for (-) 1-I, (C) 2-I, (:) 3-I; (b–
d) comparison of the corresponding compounds before or after UV
exposure of (b) 1-I (c) 2-I (d) 3-I. Solid symbol: before, empty symbol:
after UV exposure.
even higher than that of gemini imidazolium surfactants,30 not
to mention traditional imidazolium surfactants. Aer UV
exposure, the CMCg and gCMC of all involved surfactants
decreased, taken as a sign of the stronger micellar tendency and
surface activity. Compared to surfactants with an azo moiety in
the boundary of the hydrophobic chain,20 surface tension was
affected more mildly by a trans-to-cis conformation change. A
further decrease was mainly observed before rather than aer
reaching CMC concentration, indicating that the conformation
change of the hydrophilic head mainly occurred before the
surfactants assembled into micelles.
The better responsiveness shown in the UV spectra resulting
from a shorter R0 is supported by the critical micellar concen-
tration (CMCg and CMCk), surface tension (g) and conductivity
(k) data (Table 1). In addition, 1-Br had a lower CMC and lower
Amin which meant a better surface activity and a more com-
pacted arrangement of surfactants at the air–water interface
resulting from less steric stretching of methyl. The methoxyl of
3-I induced a lesser decrease in CMCg and CMCk than 3-Br aer
UV exposure. The methoxyl was projected to improve the elec-
tron dispersion of the p system which would result in a decrease
in CMCg, however it delivered the opposite outcome in the case
of 3-Br. An explanation may be a combination of the weak
charge transfer through the azo bond and the better hydro-
philicity given by the O atom. The example of 3-Br reveals the
rich possibilities that could be raised by modifying the structure
of azophenyl connected to the 2-C site. Once they had an Iꢀ
anion, AzoIm+ surfactants all showed a lower surface efficiency
(gCMC) and photo responsiveness. Their CMCs were lower than
those of Brꢀ. The reason might lay in the low solubility caused
Fig. 2 Surface tension of [C14AzoIm]Br aqueous solutions as a func-
tion of their concentrations. (a) Overall data for (-) 1-Br, (C) 2-Br, (:)
3-Br; (b–d) comparison before or after UV exposure of (b) 1-Br (c) 2-Br
(d) 3-Br. Solid symbol: before, empty symbol: after UV exposure.
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Table 1 Surface properties and micellization parameters of [C14AzoIm]I and [C14AzoIm]Br and reference imidazolium surfactants in aqueous
solution at 25 ꢂC
gCMC
(mN mꢀ1
Gmax
(mmol mꢀ2
ꢀ2
Surfactant
1-Br
2-Br
3-Br
1-I
Conditions
CMCg (mM)
)
)
Amin (A )
Original
UV
Original
UV
Original
UV
Original
UV
Original
UV
Original
UV
150.1
128.7
423.3
306.9
165.1
158
80.18
72.55
110.9
94.76
50.09
38.14
2800
610
43.1
30.7
40.2
39.6
41.5
41.2
42.9
43.2
40.1
41.3
52.0
48.6
39.2
38.3
33.6
5.99
5.98
3.59
3.44
3.8
3.67
4.19
5.20
3.90
3.82
3.83
5.07
1.96
2.21
1.23
27.7
27.7
46.3
48.3
43.7
45.2
39.6
32.0
42.6
43.4
43.3
32.7
84.7
75.1
135
2-I
3-I
[C14mim]Br28
[C14pim]Br29
[C12-2-C12im]Br2
30
550
by the Iꢀ anion, which might also cause the abnormal uctua- some remarks should be made. The results for [C14AzoIm]Br
tions of g just past the low concentration plateaus of 1-I and 2-I surfactants were in much better accordance than those of
in Fig. 3. Finally in this section, the morphology of each [C14AzoIm]I for the absolute value or photo induced change of
aggregate was studied using cryo-TEM and sorted in two groups CMC, among which 3-Br was almost same number of both the
(Fig. S4†). The majority except 2-Br had irregular grains of 10–20 original and aer UV absorption at 25 ꢂC. Concluding from the
nm in size, which were merged from smaller spherical micelles. overall sequence of specic conductivity, which is 2-Br > 1-Br ꢁ
The case of 2-Br showed giant micelles of ꢁ50 nm with 3-Br > 1-I ꢁ 2-I > 3-I, we suggest that the more compact cation 1+
a smoother surface, in accordance with the much higher CMC might act to compensate for mobility loss when linked with the
of 2-Br.
bigger anion, Iꢀ.
Based on the mixed electrolyte model of micelles in solution
developed,32 the degree of counterion binding (b), which is
essential information for the anion exchange chemistry of
imidazolium, can be obtained from the k–C plots. In general, an
unprecedentedly high b implying higher affinity between the
[C14AzoIm]+ cation and Brꢀ/Iꢀ anion was observed compared to
ordinary [C14mim]Br,31 CTAB (b ¼ 0.71 (ref. 33)) and previously
reported [C14pim]Br.29 Although it is a big head group,
[C14AzoIm]+ has additional electronic interactions with Brꢀ or
Iꢀ anions on 2-arylazo-imidazoliums to support such a high
affinity. Detailed analysis revealed different characteristics of
[C14AzoIm]Br and [C14AzoIm]I. [C14AzoIm]+ interacted closely
Thermodynamic analysis of micelle formation
The electrical conductivity (k) vs. concentration (C) correlation
of the IL surfactants and its behaviour under a temperature
sequence in aqueous solutions is an important tool to under-
stand the thermodynamics of micelle formation and cation–
anion interactions of the new AzoIm+ surfactants especially,
referring to the data for [C14AzoIm]Br in Fig. 4, and [C14AzoIm]]I
in Fig. 5. We performed this test for the [C14AzoIm]I surfactants
at 30 ꢂC for better solubility and therefore stability. Most
surfactants involved showed a similar CMC range and tendency
for photoresponsiveness as they did in tensiometry, though
Fig. 4 Specific conductivity as a function of concentration at different temperatures for (a) 1-Br, (b) 2-Br and (c) 3-Br. The dashed line represents
the data series after UV exposure.
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Fig. 5 Specific conductivity as a function of concentration at different temperatures for (a) 1-I, (b) 2-I, and (c) 3-I. Dashed lines for each data
series except the one after UV exposure.
with Iꢀ at room temperature, however the interactions are anion,36 and (4) the p–p interactions.37 Considering the
dramatically affected by increasing temperature.
extraordinarily high b discussed above, the sum of DH0m ion
Unlike this, the affinity of [C14AzoIm]Br exhibited a good interaction ((2) + (3)) should be positive. Thus the other two
temperature tolerance. Adaptation of [C14AzoIm]+ with bromide reasons dominate the enthalpy driven process together. Unlike
should be responsible for this special phenomenon, which [C14mim]Br,31 the TDSm0 of all [C14AzoIm]Br as well as 3-I
could be further explored by applying to other anions. Aer UV decrease from a high positive value with increasing tempera-
exposure, a marginal decrease of b that did not surpass ture. The positive TDS0m suggests that the micellization of
a difference of 5 ꢂC for each surfactant was observed. The [C14AzoIm]+ surfactants consists of the rearrangement of the 2-
photoinduced trans to cis conformation change slightly loos- azoaryl-imidazolium head group into a uniform micelle shell,
ened the cation–anion close pair. This might also be the reason which was affected by increasing temperature. In the case of 1-I
that CMCk aer UV exposure decreased far less than CMCg. The or 2-I the micelle was even more uniform aer the intercalated
close cation–anion micellar structure formed in these new Iꢀ was repelled by heat, along with the amplied exothermic
surfactants could provide anions with a front edge to be effect. Aer UV exposure, the Gibbs free energy change
involved in supramolecular and colloidal chemistry. An attempt decreased slightly, implying a weakening tendency of micelli-
to modify the C-site resulted in the optimization of the X-site zation. That might came from the difficulty brought to water
unexpectedly.
solvation process by more uniform cis head groups which needs
The thermodynamic parameters of micellization process: the a little extra energy.
standard Gibbs free energy change of DG0m, the standard
enthalpy change of DH0m and the standard entropy change of
DS0m could also be calculated with specic conductivity data
using the following equations:34
Fluorescence properties
Steady-state uorescence measurements were performed to
study micelle aggregation behaviour in H2O solution with pyr-
ene as the probe.38 The 2-arylazo-imidazolium was found to
highly hinder uorescent emission (Fig. S1†), keeping
[C14AzoIm]Br with a higher CMC from this test. There are ve
characteristic uorescence emission bands of 370–400 nm
under 335 nm excitation. The ratio of the rst band (I1, nm) to
the third band (I3, nm) positively correlates to the polarity of the
environment around the excited pyrene. Fig. 6a shows the
trends of I1/I3 ratio over the concentration of three surfactants at
DG0m ¼ (1 + b)RT ln XCMC
(1)
(2)
À
ꢀ Á
v DGm0
T
DHm0
¼
vð1=TÞ
TDS0m ¼ DH0m ꢀ DG0m
(3)
where XCMC is CMC in molar fraction and b is the degree of
counterion binding to micelles as above. Eqn (2) was applied by
tting DG0m/T ꢁ 1/T data to a quadratic function and then
derivation at a designated point. The corresponding data for
[C14AzoIm]Br and [C14AzoIm]I are listed in Tables 2 and 3.
The highly negative DHm0 is the major force in making
DG0m negative, which could lead to the micellization process of
AzoIm+ surfactants being enthalpy driven. This contribution
could be divided into: (1) the exothermic entrance into the
micelle of the tails of surfactants and release of solvating
water,35 (2) the exothermic self-repulsion of cations and anions,
(3) the endothermic attraction between the cation and the
ꢂ
30 C, which was changes stepwise with different behaviour.
The initial I1/I3 of 1-I and 2-I were signicantly lower than the
value in pure water (ꢁ1.8),39 but relatively stable below 0.02 mM.
Considering that the concentration was far below the CMC of
the corresponding surfactants, pyrene was more likely to be free
in solution than conned in micelles. Thus it was the better
sensitivity of the I1 vibration to 2-aryl-imidazolium quenching
that should be ascribed to this phenomenon rather than the low
polarity of the alkyl chain of the surfactants (Fig. S2†). At higher
concentrations than 0.04 mM, I1/I3 ratio began to decrease as
micelles gradually formed and the probe started to enter the low
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Table 2 Critical micelle concentration (CMCk) and specific conductivity at CMC (kCMC), degree of counterion binding (b), and thermodynamic
parameters of micelle formation for [C14AzoIm]Br in aqueous solutions at various temperatures and UV exposures
T
Surfactant
(ꢂC) & conditions
CMCk (mM)
b
DG0m (kJ molꢀ1
)
DH0m (kJ molꢀ1
)
ꢀTDS0m (kJ molꢀ1
)
1-Br
25
30
35
40
0.147
0.155
0.160
0.163
0.170
0.139
0.146
0.419
0.428
0.429
0.431
0.437
0.402
0.415
0.168
0.169
0.170
0.172
0.173
0.158
0.159
2.610
0.550
0.88
0.86
0.85
0.84
0.84
0.86
0.84
0.87
0.86
0.84
0.83
0.83
0.86
0.85
0.90
0.88
0.87
0.86
0.86
0.89
0.87
0.78
0.50
ꢀ60.78
ꢀ60.10
ꢀ59.40
ꢀ59.09
ꢀ58.84
ꢀ60.43
ꢀ59.68
ꢀ55.67
ꢀ55.17
ꢀ54.62
ꢀ54.25
ꢀ54.19
ꢀ55.47
ꢀ54.89
ꢀ60.83
ꢀ60.31
ꢀ59.78
ꢀ59.56
ꢀ59.53
ꢀ60.76
ꢀ60.27
ꢀ44.33a
ꢀ42.74
ꢀ112.21
ꢀ100.57
ꢀ89.316
ꢀ78.416
ꢀ67.860
51.426
40.473
29.913
19.330
9.015
45
25-UV
30-UV
25
30
35
40
45
25-UV
30-UV
25
30
35
40
45
25-UV
30-UV
25
2-Br
3-Br
ꢀ96.917
ꢀ87.448
ꢀ78.286
ꢀ69.417
ꢀ60.827
41.250
32.276
23.661
15.170
6.636
ꢀ103.59
ꢀ91.660
ꢀ80.116
ꢀ68.941
ꢀ58.117
42.757
31.351
20.335
9.377
ꢀ1.418
[C14mim]Br31
[C14pim]Br29
ꢀ15.13a
ꢀ55.06
ꢀ30.38
25
12.32
a
Measured from the data gure.
polarity zone. The I1/I3 ratio of 2-I which had ethyl on the imi- ramp up above 0.02 mM and nally attained equilibrium at 2.2.
dazolium decreased more quickly than that of 1-I with methyl at An increase in the ratio indicated an enhancement of polarity,
the same site. The longer hydrocarbon substituent might offer which even surpassed other highly polar solvents such as H2O
more affinity and easier entry of pyrene and thus a quicker or DMSO40 (Fig. 6a). It might be speculated that the pyrene was
decrease in the polarity of the microenvironment.
dragged closely to the polar and electron donating cation of 3-I,
The change in the I1/I3 ratio of 3-I over the concentration instead of alkyl chains inside micelles when only p–p interac-
range was distinct. Aer a stable I1/I3 ratio, the ratio began to tions are available from 1-I and 2-I (Fig. 6b). The high initial I1/I3
Table 3 Critical micelle concentration (CMCk) and specific conductivity at CMC (kCMC), degree of counterion binding (b), and thermodynamic
parameters of micelle formation for [C14AzoIm]I in aqueous solutions at various temperatures and UV exposures
T
Surfactant
(ꢂC) & conditions
CMCk (mM)
b
DG0m (kJ molꢀ1
)
DH0m (kJ molꢀ1
)
ꢀTDS0m (kJ molꢀ1
)
1-I
25
30
35
40
45
30-UV
25
30
35
40
45
30-UV
25
30
35
40
45
30-UV
0.101
0.111
0.116
0.120
0.124
0.106
0.102
0.111
0.117
0.120
0.125
0.106
0.065
0.070
0.072
0.078
0.089
0.064
0.93
0.86
0.78
0.70
0.62
0.85
0.94
0.87
0.79
0.75
0.65
0.84
0.90
0.82
0.73
0.63
0.61
0.77
ꢀ63.25
ꢀ61.65
ꢀ59.52
ꢀ57.87
ꢀ55.86
ꢀ61.55
ꢀ63.37
ꢀ61.76
ꢀ59.89
ꢀ59.33
ꢀ56.73
ꢀ60.96
ꢀ64.11
ꢀ62.12
ꢀ59.97
ꢀ57.26
ꢀ56.76
ꢀ61.02
ꢀ167.92
ꢀ171.12
ꢀ174.22
ꢀ177.22
ꢀ180.13
104.67
109.47
114.70
119.35
124.27
2-I
3-I
ꢀ146.99
ꢀ152.19
ꢀ157.22
ꢀ162.09
ꢀ166.81
83.617
90.431
97.332
102.76
110.08
ꢀ221.10
ꢀ200.09
ꢀ179.76
ꢀ160.09
ꢀ141.03
156.99
137.97
119.79
102.83
84.268
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Fig. 6 Fluorescence emission intensity of pyrene as a function of concentration for three 2-azobenzenyl-imidazolium surfactants at 30 ꢂC: (a) I1/
I3 ratio of (-) 1-I, (C) 2-I and (:) 3-I, (b) a schematic representation of donor–acceptor interactions between 3-I and pyrene, and (c) fluo-
rescence spectrum of 0.02 mM 3-I solution: solid line before UV exposure, dashed line after 30 min UV exposure.
might imply the existence of instinctive uorescence. The stabilization and strengthening of the hydrogenated azo bond
uorescence of pure 3-I solution was conrmed to have in the reduced state.
a maximum emission wavelength of 535 nm under excitation at
381 nm. Aer UV exposure the maximum emission shied to
528 nm (Fig. 6c) which is unique compared to previously re-
ported uorescent phenyl connected imidazolium surfactants.29
Even within the AzoIm+ surfactants, only the cation 3 showed
instinctive uorescence and that of 3-I was stronger than 3-Br
(Fig. S3†). This property has not yet been reported in imidazo-
lium compounds, and might be applied in ion detection.41
Manipulating the shape of Au nanoparticles
We take AuNPs as a demonstration of AzoIm+ surfactants’
application in the fabrication of various nanomaterials.
Electrochemical properties
Azo surfactants were used as redox-responsive agents 20 years
ago, yet are irreversible due to the decomposition of the azo
bond at medium pH.42 We found that 2-aryl-imidazolium (rep-
resented by 2-Br) acted reversibly to redox electrochemical
stimuli (Fig. 7). The reduction and oxidation peaks are centered
at ꢀ0.24 V and ꢀ0.17 V vs. Ag/AgCl, respectively, quite different
from conventional azobenzene surfactants.43 More to the point,
they were in a perfectly symmetrical reversible peak shape. We
suppose that the imidazolium had a major effect on the
Fig. 8 TEM and HRTEM images of Au NPs prepared when 2-Br was (a)
without UV exposure; (b and c) after UV exposure. (d) UV-Vis NIR
absorption spectra of the Au nanostructures obtained under the two
Fig. 7 Cyclic voltammetry of 0.1 mg mLꢀ1 2-Br in 0.1 M PBS and different conditions listed above. (e) Schematic illustration of the
possible mechanism of redox reactivity.
possible mechanism of Au nanostar formation.
51558 | RSC Adv., 2016, 6, 51552–51561
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Directed by 2-Br without UV exposure, random Au nanoparticles imidazole (0.1 mol) and anhydrous sodium carbonate and the
ꢂ
appeared with a round-cornered shape (Fig. 8a). Different from system was kept at a temperature under 5 C overnight. Then
this bland result, Au nanostars (Au NST) were fabricated with 2- the orange powder collected was washed well with water and
Br under UV exposure (Fig. 8b). The spike may result from the extracted successively with cold hydrochloric acid. The extract
micelle consisting of cis-2-Br which encouraged anisotropic was collected and basied with sodium carbonate to obtain the
growth of the (111) and (200) faces of Au nanostructures44 crude product. Aer further purication by washing with cold
(Fig. 8c). When compared to a benzene ring, the imidazolium deionized water, drying and recrystallization from alcohol, the
ring should have greater affinity for Au. However such compe- pure product was obtained. The yield was 83.7%. 1H NMR (400
tition didn’t really happen in the trans AzoIm+ which was MHz, CDCl3) d 7.95–7.92 (dd, 2H), 7.50 (m, J ¼ 5.1, 1.7 Hz, 3H),
supposed to interact with Au equatorially, meaning that both 7.32 (s, 2H). 13C NMR (100 MHz, CDCl3): d ¼ 153.71, 151.91,
benzene and imidazolium interact with Au. When AzoIm+ was 132.37, 129.28, 124.12, 123.36 ppm. 5b was synthesized using
in the cis state, the imidazolium should be more close. In the the same method, only replacing aniline with 4-aminophenol.
original state, the mixture of majority trans- and minority cis- The yield of MeOAzoIm was 79.6%. 1H NMR (400 MHz, CDCl3)
AzoIm+ surfactants made it difficult to maintain a regular d 7.96–7.91 (m, 2H), 7.27 (d, J ¼ 5.6 Hz, 3H), 7.03–6.98 (m, 2H),
growth edge. This situation was inversed aer UV exposure 3.90 (s, 3H). 13C NMR (100 MHz, CDCl3): d ¼ 163.26, 153.83,
(Fig. 8e). Meanwhile, the hydrophobic microenvironment 146.34, 125.54, 123.48, 114.50, 55.71 ppm.
induced by cis AzoIm+ kept the Au surface from direct contact
with and etching by Brꢀ (Fig. 8c). The Au NST show a broad PhAzoIm)
Synthesis of 2-(phenyldiazenyl)-1-tetradecyl imidazole (6a: C14-
and 2-((4-methoxyphenyl)diazenyl)-1-tetradecyl-
surface plasmon resonance band peak at ꢁ720 nm and might imidazole (6b: C14MeOAzoIm).. Briey for 6a, 5a (0.02 mol),
be benecial to SERS detection45 (Fig. 8d). These examples bromotetradecane (0.02 mol), tetrabutylammonium iodide
could be an illustration of the potential of the new 2-arylazo- (0.004 mol) and sodium hydroxide (0.0625 mol) were dissolved
imidazolium class of surfactants in controlling self-assembly in ethyl alcohol. The mixture was magnetically stirred and
and colloidal chemistry.
reuxed for 24 hours. Water was poured in and extracted with
ethyl acetate to remove the inorganics. The nal product was
obtained by silica gel column chromatography using CH2Cl2 as
Conclusions
1
an eluent giving a yield of 72.8%. H NMR (400 MHz, CDCl3)
In conclusion, we have synthesized a new series of 2-arylazo- d 7.98 (dd, J ¼ 7.8, 1.6 Hz, 2H), 7.53–7.47 (m, 3H), 7.28 (s, 1H),
imidazolium (AzoIm+) surfactants, characterized the colloidal 7.16 (s, 1H), 4.42 (t, J ¼ 7.1 Hz, 2H), 1.88 (p, J ¼ 7.0 Hz, 2H), 1.30–
chemistry and explored how those properties react to photo 1.20 (m, 22H), 0.87 (t, J ¼ 6.8 Hz, 3H). 6b was synthesized with
stimulation. We found that the 2-arylazo-imidazolium group the same method, only to replace 5a with 5b. The yield of
1
shows a unique stimuli/structural response when it serves as C14MeOAzoIm was 74.4%. H NMR (400 MHz, CDCl3) d 8.00–
the hydrophilic part in surfactants, making it a new option to 7.96 (m, 2H), 7.24–7.22 (m, 1H), 7.12–7.10 (m, 1H), 7.01–6.98
construct a colloidal working environment. It was veried that (m, 2H), 4.38 (t, J ¼ 8.8 Hz, 2H), 3.89 (s, 3H), 1.28 (dd, J ¼ 44.7,
applying 2-Br in Au NP fabrication could control nanoparticle 14.3 Hz, 24H), 0.87 (t, J ¼ 6.0 Hz, 3H).
shape into Au nanostars. The ne-tuning and diverse structure
Synthesis of 2-arylazo-1-tetradecyl imidazolium iodides: (1-I:
of 2-arylazo-imidazolium will make it an important extension of [C14PhAzoMeIm]I), (2-I: [C14PhAzoEtIm]I) and (3-I: [C14
-
ionic liquid surfactants and important in the research of smart MeOPhAzoMeIm]I). Briey for 1-I, 6a and iodomethane were
materials. The newly discovered redox-reversible properties dissolved in dry THF and reuxed under air tight conditions for
would also give more potential for the combination of photo 72 hours. The nal product was obtained by removing the
and electrochemical applications. Given the widespread usage solvent and then silica gel column chromatography with eluents
of imidazolium surfactants in nanotechnology or as green of CH2Cl2/CH3OH (20 : 1). Yield 75.3%. 1H NMR (400 MHz,
solvents, more expansive or deep research could be inspired by CDCl3) d 8.36 (d, J ¼ 1.9 Hz, 1H), 8.08 (d, J ¼ 2.0 Hz, 1H), 8.00 (s,
this work.
1H), 7.98 (d, J ¼ 1.2 Hz, 1H), 7.71 (dd, J ¼ 8.4, 6.3 Hz, 1H), 7.63 (t,
J ¼ 7.6 Hz, 2H), 4.66–4.61 (m, 2H), 4.33 (s, 3H), 1.34–1.20 (m,
24H), 0.87 (t, J ¼ 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): d ¼
152.56, 142.98, 135.88, 130.00, 126.36, 124.36, 124.13, 50.22,
38.38, 31.92, 30.45, 29.68, 29.64, 29.60, 29.53, 29.39, 29.36,
Experimental section
Synthesis of 2-arylazo-imidazolium surfactants
Materials. General grade solvents and reagents were 29.00, 26.36, 22.70, 14.14 ppm. 2-I was obtained similarly, only
purchased from commercial suppliers. Amberlyst A-26 (R+–OHꢀ replacing iodomethane with iodoethane. Yield 75.3%. 1H NMR
form) resin was obtained from Alfa Aesar. All chemicals were AR (400 MHz, CDCl3) d 8.01 (d, J ¼ 7.4 Hz, 2H), 7.53 (d, J ¼ 6.9 Hz,
grade and used without further purication.
3H), 7.31 (d, J ¼ 7.5 Hz, 1H), 7.20 (s, 1H), 4.45 (t, J ¼ 7.0 Hz, 2H),
Synthesis of 2-(phenyldiazenyl)imidazole (5a: PhAzoIm) and 2- 1.90 (d, J ¼ 6.2 Hz, 2H), 1.31 (d, J ¼ 41.1 Hz, 27H), 0.91 (t, J ¼ 6.1
((4-methoxyphenyl)diazenyl)imidazole (5b: MeOAzoIm).. 5a and Hz, 3H). 13C NMR (100 MHz, CDCl3): d ¼ 152.60, 142.51, 135.88,
5b were synthesized following a previously reported proce- 130.04, 124.87, 124.82, 124.28, 50.43, 46.14, 31.92, 30.48, 29.68,
dure.46 Briey for 5a, aniline (0.1 mol) was dissolved in dilute 29.64, 29.60, 29.53, 29.40, 29.36, 29.02, 26.36, 22.69, 15.93, 14.14
hydrochloric acid, and diazotized with sodium nitrite (0.103 ppm. 3-I was obtained similarly, only replacing 6a with 6b. Yield
1
mol). The solution was dropped slowly into a mixed solution of 77.8%. H NMR (400 MHz, CDCl3) d 8.27 (d, J ¼ 1.9 Hz, 1H),
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8.02–7.96 (m, 3H), 7.12–7.08 (m, 2H), 4.60–4.56 (m, 2H), 4.29 (s, Education of China (Grand No. 20133501120004) and China
3H), 3.98 (s, 3H), 1.96–1.88 (m, 2H), 1.34–1.20 (m, 22H), 0.88 (t, Postdoctoral Science Foundation (Grand No. 2015M582037).
J ¼ 6.9 Hz, 3H). 13C NMR (100 MHz, CDCl3): d ¼ 166.56, 147.32, Also supported by the “111” Project (B16029), NSFC (No.
143.55, 127.33, 125.50, 123.30, 115.46, 56.22, 49.84, 38.14, U1405226), Fujian Provincial Department of Science & Tech-
31.92, 30.44, 29.69, 29.65, 29.61, 29.54, 29.40, 29.36, 29.02, nology (2014H6022) and 1000 Talents Program from Xiamen
26.34, 22.69, 14.13 ppm.
University. We thank Prof. Hai Xu and Prof. Dong Wang at the
Synthesis of 2-arylazo-1-tetradecyl imidazolium bromides: (1-Br: Centre for Bioengineering and Biotechnology of China Univer-
[C14PhAzoMeIm]Br), (2-Br: [C14PhAzoEtIm]Br) and (3-Br: [C14
-
sity of Petroleum for the cryo-TEM characterization.
MeOPhAzoMeIm]Br). Compound 8–10 was synthesized following
an anion exchange process.47 1-Br: [C14PhAzoMeIm]Br: mass
ESI(ꢀ) 541.1 (M + Br), 543.1(M + Br + 2), 545.1(M + Br + 4). 2-Br:
[C14PhAzoEtIm]Br: mass ESI(ꢀ) 555.2 (M + Br), 557.2 (M + Br +
2), 559.2 (M + Br + 4). 3-Br: [C14MeOPhAzoMeIm]Br: MS ESI(ꢀ)
571.1 (M + Br), 573.1 (M + Br + 2), 575.1 (M + Br + 4). All
compounds have identical 1H NMR spectra as corresponding 1-
I, 2-I and 3-I.
Synthesis of 3-(2-(phenyldiazenyl)-1-tetradecyl-imidazolium-3-
yl)propane-1-sulfonate 4: C14PhAzoIm+C3SO3ꢀ. An equimolar
amount of 1,3-propanesultone (1.22 g, 0.01 mol) was slowly
added to 6a (3.68 g, 0.01 mol) dissolved in acetone at 0 ꢂC under
a nitrogen atmosphere. The mixture was then magnetically
stirred for 120 h at room temperature, ltered, washed with
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