Photoresponsive Self-Assembly of Surface Active Ionic Liquid

Article abstract of DOI:10.1021/acs.langmuir.6b01937

A novel photoresponsive surface active ionic liquid (SAIL) 1-(4-methyl azobenzene)-3-tetradecylimidazolium bromide ([C₁₄mimAzo]Br) with azobenzene located in the headgroup was designed. Reversible vesicle formation and rupture can be finely controlled by photostimuli without any additives in the aqueous solution of the single-tailed ionic liquid. The photoisomerization of the azobenzene derivative was investigated by ¹H NMR and UV-vis spectroscopy. Density functional theory (DFT) calculations further demonstrate that trans-[C₁₄mimAzo]Br has less negative interaction energy, which is beneficial to aggregate formation in water. The incorporation of trans-azobenzene group increases the hydrophobicity of the headgroup and reduces the electrostatic repulsion by delocalization of charge, which are beneficial to the formation of vesicles. However, the bend of cis-azobenzene makes the cis-isomers have no ability to accumulate tightly, which induces the rupture of vesicles. Our work paves a convenient way to achieve controlled topologies and self-assembly of single SAIL.

Full text of DOI:10.1021/acs.langmuir.6b01937

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Article
Photo-Responsive Self-Assembly of Surface Active Ionic Liquid
Aoli Wu, Fei Lu, Panpan Sun, Xinpei Gao, Lijuan Shi, and Li-Qiang Zheng
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01937 • Publication Date (Web): 22 Jul 2016
Downloaded from http://pubs.acs.org on July 22, 2016
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PhotoꢀResponsive SelfꢀAssembly of Surface
Active Ionic Liquid
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Aoli Wu,† Fei Lu,† Panpan Sun,† Xinpei Gao,† Lijuan Shi,‡ Liqiang Zheng*†
†Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry
of Education, Jinan 250100, China
‡Key Laboratory of Coal Science and Technology of Ministry of Education and
Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China
ABSTRACT: A novel photoꢀresponsive surface active ionic liquid (SAIL)
1ꢀ(4ꢀmethyl azobenzene)ꢀ3ꢀtetradecylimidazolium bromide ([C14mimAzo]Br) with
azobenzene located in the head group was designed. Reversible vesicle formation and
rupture can be finely controlled by photo stimuli without any additives in the aqueous
solution of the singleꢀtailed ionic liquid. The photoisomerization of the azobenzene
derivative was investigated by 1H NMR and UVꢀvis spectroscopy. Density functional
theory (DFT) calculations further demonstrate that transꢀ[C14mimAzo]Br has less
negative interaction energy, which is beneficial to aggregate formation in water. The
incorporation of transꢀazobenzene group increases the hydrophobicity of head group
and reduces the electrostatic repulsion by delocalization of charge, which are
beneficial to the formation of vesicles. However, the bend of cisꢀazobenzene makes
the cisꢀisomers have no ability to accumulate tightly, which induces the rupture of
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vesicles. Our work paves a convenient way to achieve controlled topologies and
selfꢀassembly of single SAIL.
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1. INTRODUCTION
Amphiphiles with specific functional groups is crucial to achieve rich hierarchically
complex architectures by controllable selfꢀassembly. Systems with controllable
responsivity are potential candidates in catalysis, drug delivery, biosensors, and
diagnostics.1ꢀ4 Amphiphilicity of the wellꢀorganized structures can be tuned by
external stimulus such as temperature,5 CO2,6 redox reagent,7 salinity,8 enzyme9 as
well as combined stimuli.10,11 In particular, light is easy to obtain and can be precisely
manipulated in a clean environment without pollution to system, which makes it more
competitive as an ideal external stimulus to modulate molecular selfꢀassembly at
nanoscale or macroscopic scale.12ꢀ14 Photoꢀinduced isomerization of the molecules
might trigger the reꢀarrangement of the building blocks to bring about morphological
transitions, which have been considered as promising photoꢀresponsive materials.
The most common lightꢀresponsive systems contain phenylazosulfonate, stilbene,
cinnamate or azobenzene.15,16 Compounds containing azobenzene usually show
higher medium sensitivity. Moerover, they are capable of undergoing reversible
transꢀcis transition under UV and visible light irradiation, and widely applied in drug
delivery,17 nanomaterials,18 catalysis,19 biochemical engineering20 and commodity
field.21 Up to now, controlled selfꢀassembly systems, such as solutions,22,23 liquid
crystals,24 gels,25 and solids26,27 formed by amphiphiles with azobenzene groups, have
been successfully constructed. Huang et al. have investigated a lightꢀactive
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selfꢀassembled
system,
consisting
of
azobenzene
derivative
sodium
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(4ꢀphenylazoꢀphenoxy)ꢀacetate (AzoNa) and conventional surfactant CTAB.28 The
mixture of AzoNa and CTAB can exhibit wormlike micelles, vesicles, lamellar
structure and small micelles in aqueous solution depending on illumination time.
Photoꢀresponsive amphiphiles consisted of oligooxyethylene, alkyl chain and
azobenzene which located in the hydrophobic tail have been designed by Hughes and
coworkers.29 The lyotropic liquid crystal phase can be significantly changed by
photoisomerization. Because of the hydrophobicity, azobenzene is often used as an
additive or as hydrophobic chain. The complex systems will always introduce
inorganic salts, which is not conducive to understand the weak interactions
responsible for aggregate formation. Putting the azobenzene in the hydrophilic head
group may be an alternative pathway to tune their aggregation behavior, so as to
provide a versatile platform to fabricate novel controllable selfꢀassembly materials.
The selfꢀassembled aggregates based on ionic liquids (ILs) have attracted increasing
attentions during the last ten years due to their extraordinary properties, such as
negligible vapor pressure, low melting point, large electrochemical window and
thermal stability.30,31 Ionic liquids with long alkyl chains are regarded as amphiphiles
named as surface active ionic liquids (SAILs), which combine the properties of ILs
and surfactants.32ꢀ35 SAILs can selfꢀassemble into a rich variety of hierarchically
complex architectures with controllable functions depending on the structure of
SAILs and the composition of solution.36ꢀ39 Based on the designability of SAILs, it
will be expected to achieve the functionalization of selfꢀassembled aggregates by
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tailoring the structures of SAILs.
Herein, we designed
a
photoꢀresponsive
SAIL
1ꢀ(4ꢀmethyl
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azobenzene)ꢀ3ꢀtetradecylimidazolium bromide ([C14mimAzo]Br) with azobenzene
located in the head group (Scheme 1). The incorporation of azobenzene group has a
significant influence on the surface activity and aggregate structures. Before UV
irradiation, two kinds of aggregates, micelles and vesicles, can be observed in
[C14mimAzo]Br aqueous solution with increasing concentration. After UV irradiation,
transꢀcis photoisomerization of azobenzene in the headgroup induces the rupture of
vesicles. Significantly, the rapid and reversible responses of the single SAIL open a
door to new fields for special applications.
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Scheme 1. Chemical structure and transꢀcis photoisomerization of [C14mimAzo]Br
2. EXPERIMENTAL SECTION
2.1. Materials. Nitrosobenzene (98%), pꢀtoluidine (99%), 1ꢀbromotetradecane (98%),
imidazole (99%), Nꢀbromosuccinimide (99%) and benzoyl peroxide (98%) were
purchased from J&K Scientific Ltd. Glacial acetic acid, ethanol, petroleum ether,
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ethyl acetate, tetrahydrofuran, dichloromethane, tetrachloromethane, methanol and
sodium hydroxide were purchased from Shanghai Chemical Co. All the above
materials were used without further purification. Ultraꢀpure water was used
throughout the whole experiment.
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2.2. Synthesis of [C14mimAzo]Br.
Synthesis of 1ꢀtetradecylimidazole (1).40
Imidazole (2.16 g), NaOH (1.38 g) and ultraꢀpure water (1.38 mL) was added to a
neck round bottom flask. 1ꢀBromotetradecane (8 g) was dissolved in THF and added
dropwise into the above mixture. The system was refluxed for 3 days. After that, THF
was removed by rotary evaporation. The crude product was extracted three times with
CH2Cl2/H2O. The organic layer was concentrated dried under vacuum to obtain white
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solid. H NMR (CDCl3, δ): 0.86ꢀ0.90 (t, 3H), 1.25ꢀ1.30 (m, 22H), 1.74ꢀ1.81 (m, 2H),
3.89ꢀ3.94 (m, 2H), 6.90 (s, 1H), 7.05 (s, 1H), 7.45 (s, 1H).
Synthesis of 4ꢀmethylazobenzene (2).41
Nitrosobenzene (3 g, 27.8 mmol) and pꢀtoluidine (3 g, 27.8 mmol) were dissolved in
50 mL glacial acetic acid. The mixture was stirred in a round bottom flask under an
atmosphere of nitrogen for 24 h at room temperature. After removing the solvent by a
rotary evaporator, the resulting solid was recrystallized from EtOH/H2O for three
times, followed by purification by column chromatography (petroleum ether and ethyl
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acetate). H NMR (CDCl3, δ): 2.44 (s, 3H), 7.26ꢀ7.33 (m, 2H), 7.45ꢀ7.54 (m, 3H),
7.82ꢀ7.85 (m, 2H), 7.89ꢀ7.92 (m, 2H).
Synthesis of 4ꢀ(bromomethyl)azobenzene (3).41
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4ꢀMethylazobenzene (0.81 g), Nꢀbromosuccinimide (NBS 0.73 g) and benzoyl
peroxide (BPO 0.014 g) were dissolved in 24 mL CCl4. The mixture was refluxed for
24 h under a nitrogen atmosphere. After completion of the reaction, the solution was
filtrated. The filtrate was removed by a rotary evaporator. The crude product was
purified by column chromatography on silica gel (petroleum ether and ethyl acetate).
1H NMR (CDCl3, δ): 4.56 (s, 2H), 7.48ꢀ7.56 (m, 5H), 7.88ꢀ7.94 (m, 4H).
Synthesis of [C14mimAzo]Br (4).
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4ꢀ(Bromomethyl)azobenzene (0.87 g) and 1ꢀtetradecylimidazole (0.84 g) were
dissolved in CH2Cl2. The mixture was stirred under a nitrogen atmosphere for 24 h at
room temperature. The CH2Cl2 was removed by rotary evaporated, and the product
was purified by silica gel column chromatography (dichloromethane and methanol).
1H NMR (CDCl3, δ): 0.86ꢀ0.89 (t, 3H), 1.24ꢀ1.32 (m, 22H), 1.91ꢀ1.94 (m, 2H),
4.27ꢀ4.31 (t, 2H), 5.78 (s, 1H), 7.21ꢀ7.29 (m, 2H), 7.50ꢀ7.52 (m, 3H), 7.67ꢀ7.69 (m,
2H), 7.90ꢀ7.93 (m, 4H).
Scheme 2. Preparation of [C14mimAzo]Br.
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2.3. Photoisomeric Experiments. In order to achieve transꢀcis transition, the samples
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in quartz tube were irradiated with a LUYORꢀ3109 UV light until it reached the
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maximum degree of conversion. This experiment was performed at 35 C. After
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photoisomerization, all the samples were wrapped with aluminum foil to prevent the
transformation from cis to tans.
2.4. Characterizations.
UVꢀvis Spectroscopy. The UVꢀvis spectroscopy measurements of 0.1 mM aqueous
solutions before and after different UV irradiation time were carried out on a
UVꢀ4100 spectrophotometer. The ultraꢀpure water was utilized as a blank in the
experiments.
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1H NMR Measurements. H NMR spectra were recorded on a Bruker AMꢀ400
spectrometer equipped with a pulse field gradient module (Z axis). The samples were
placed in a 5 mm BBO probe and dissolved in CDCl3.
Surface Tension. Surface tension measurements were determined by Model
JYMꢀ200B tensiometer (accuracy ± 0.1 mN/m) using the ring method. Temperature
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was controlled at 35 ± 0.1 C using a thermostatic bath. The platinum ring was
washed with ultraꢀpure water and cleaned thoroughly by heating through flame till red
hot. All measurements were repeated until the values were reproducible.
Electrical Conductivity. Electrical conductivity measurements were carried out on a
lowꢀfrequency conductivity analyzer (model DDSꢀ307. Shanghai Precision &
Scientific Instrument Co., Ltd., accuracy ± 1%). The thermostatic bath was also used
to control the temperature at 35 ± 0.1 oC.
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Dynamic Light Scattering. The size distributions of the aggregates at different
conditions were employed using a dynamic light scattering (DLS; DynaPro NanoStar,
Wyatt Instrument Co.) with an argonꢀion laser operating at 658 nm. All samples were
properly filtered through 0.45 μm filters prior to measurements.
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CryoꢀTEM. A pipette was used to load a small drop of sample on a copper grid, and a
thin film was produced by blotting off the redundant liquid with two pieces of filter
papers. This thin film was then quickly plunged into liquid ethane, which was cooled
by liquid nitrogen. Observation of the sample was carried out at ꢀ183 oC.
3. RESULTS AND DISCUSSION
3.1. Photoisomerization of [C14mimAzo]Br.
The synthesized SAIL may undergo transꢀcis transition depending on the wavelength
of incident light. The photoisomerization in Scheme 1 suggests that the area of the
head group will be significantly altered by the isomerization of the N=N double bond,
leading to the change of critical packing parameter.16 At the same time,
transꢀazobenzene has no dipole moment while the dipole moment of the
cisꢀcompound is 3.0 D.42 That is, the hydrophilicꢀlipophile balance of [C14mimAzo]Br
can be adjusted by UV and visible light.
The transꢀcis transition of [C14mimAzo]Br can be confirmed by UVꢀvis spectroscopy.
Figure 1a shows the absorbance spectra of 0.1 mM [C14mimAzo]Br aqueous solution
irradiated by UV light for different times. Before UV irradiation, the strong absorption
band at 320 nm is ascribed to the πꢀπ* transition of transꢀ[C14mimAzo]Br. UV
stimulus give rise to a remarkable decrease in intensity and an obvious blue shift of
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the absorption peak at 320 nm. Concomitantly, a slight increase of a band located at
about 425 nm is observed, which can be ascribed to the nꢀπ* bands of the cisꢀisomer.
After irradiation for 5 minutes, the transꢀcis photoisomerization has been completely
finished and longer irradiation time causes no change in the UVꢀvis spectra. The
transꢀ[C14mimAzo]Br can be obtained again when the irradiated sample is then
treated with visible light. The trans/cis photoisomerization of the synthesized SAIL
can be repeated many times without decomposition of the components (Figure 1b). It
is anticipated that [C14mimAzo]Br can be used in photoꢀresponsive molecular
selfꢀassembly.
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Figure 1. (a) UVꢀvis absorption spectra of an aqueous [C14mimAzo]Br solution (0.1
mM) at different times of UVꢀlight irradiation; (b) the absorbance at 320 nm of
[C14mimAzo]Br solution by alternate irradiation at UV and visible light, respectively.
1H NMR technique is also used to confirm the transꢀcis photoisomerization. As
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shown in the H NMR spectra (Figure 2), discernible proton signals undergo obvious
changes upon UV irradiation. Because of the significant shielding effect caused by the
πꢀπ conjugation of the azobenzene group and magnetically anisotropic effect caused
by the change of polarity, UV irradiation results in an upfield shift of attached protons
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on cisꢀ[C14mimAzo]Br compared with the transꢀisomer.43,44 As calculated from the 1H
NMR spectrum of aromatic protons, almost all of the compounds are transꢀisomer
before UV irradiation (Figure 2a). After UV lamp treatment over 30 minutes, the
percentage of transꢀisomer is reduced to 39% while the cisꢀform is increased to 61%
(Figure 2b). Complete conversion cannot be achieved even irradiated by UV light for
longer time.
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Figure 2. 1H NMR spectra of [C14mimAzo]Br in CDCl3 (a) before and (b) after
UVꢀlight irradiation
3.2. Photo-responsible variation in surface properties and micellization
parameters
The aggregation behaviors of [C14mimAzo]Br before and after UV irradiation are
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investigated by surface tension and conductivity measurements at 35 C. Figure 3
shows the surface tension (γ) curves as a function of the [C14mimAzo]Br
concentration. The appearance of break point indicates certain kinds of selfꢀassembled
aggregates begin to be formed in the solution at the concentration of the transition
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point. It is intriguing to find that there are two plateaus in the plots of [C14mimAzo]Br
before UV irradiation. This phenomenon is also described in other literatures.45ꢀ48 The
two critical aggregate concentrations denoted as cac1 and cac2 suggest that two kinds
of selfꢀassembled aggregates are formed with increasing concentrations. Interestingly,
the dualꢀplatform in the γꢀC curve disappears after UV irradiation. This phenomenon
indicates the presence of only one kind of aggregates.
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Figure 3. Plots of the surface tension of [C14mimAzo]Br aqueous solution as a
function of [C14mimAzo]Br concentration at 35 oC
The cac values of the two isomeric [C14mimAzo]Br are further investigated by
conductivity measurement. Electrical conductivity curves of [C14mimAzo]Br aqueous
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solution with different concentrations at 35 C are depicted in Figure 4. For the
transꢀ[C14mimAzo]Br, the conductivity profiles fit into three straight lines with
different slops originating from different aggregates. After irradiated by UV light, the
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conductivity profiles of the same solution exhibit only two straight lines. The result
obtained by this technology is consistent with the surface tension method.
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Figure 4. Plot of conductivity against the concentration of [C14mimAzo]Br aqueous
solutions at 35 oC (a) before and (b) after UV irradiation
The values of A and Γ can reflect the arrangement of SAIL molecule at the
air/liquid interface. By utilizing the Gibbs adsorption isotherm to the surface tension
versus concentration plot, the amount of adsorbed molecules (Γ) at the air/solution
interface and the interfacial area (A) occupied by a SAIL molecule are calculated as
follows:49,50
ꢂ
ꢇꢈ
ꢌ
ꢍꢎ
mmol/m
Γ ꢀ ꢁ ꢃꢄꢅ ꢆꢇꢉꢃꢊ
ꢋ
(1)
(2)
ꢅ
ꢂ
A ꢀ ꢏ ∙ꢑ ꢒꢓ 10ꢍꢔÅꢍꢕ
ꢐ
where R represents the gas constant (8.314 J⋅molꢀ1⋅Kꢀ1), T (K) is the absolute
temperature of the test, γ is the surface tension (mN⋅mꢀ1), C refers to the surfactant
concentration (mol⋅Lꢀ1), ∂γ/∂lnC is the slope of the γ versus lnC curve when the
concentration is near cac, and NA is Avogadro’s number (6.022×1023 molꢀ1). Before
UV irradiation, almost all of the molecules are transꢀisomer. The prefactor n is 1.
While after UV light treatment, the coexist of the transꢀ and cisꢀisomers indicates that
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there are two kinds of molecules in solution. So, the value of n is taken as 2.51,52
The cac values obtained by surface tension and conductivity methods and
micellization parameters are listed in Table 1, together with the data for
1ꢀtetradecylꢀ3ꢀmethylimidazolium bromide (C14mimBr) obtained from reference.53 As
shown in Table 1, the cac1 for transꢀ[C14mimAzo]Br is only 0.028 mM, which is
much lower than C14mimBr, indicating that the incorporation of hydrophobic
azobenzene group favors the micellization. The main reason is that the electrostatic
repulsion among the imidazolium cations can be significantly reduced due to the
delocalization of charge and the πꢀπ stacking interaction between azobenzene. The
hydrophilicity of the cisꢀisomer28,54 and the curved structure are not conducive to the
molecule aggregation. Consequently, the cac value of cisꢀ[C14mimAzo]Br is higher
than the transꢀform.
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It is well demonstrated that the size of hydrophilic head group plays an important
role in the values of A and Γ.53 Both of the two forms of isomers have significantly
larger Γ value and smaller A value than C14mimBr indicating a tighter arrangement of
synthesized SAIL. This result is due to the decreased electrostatic repulsion among
imidazolium cations and enhanced πꢀπ stacking interaction caused by two phenyl
rings. Furthermore, the photoisomerization of azobenzene significantly alter the
micellization parameters and aggregation behavior of [C14mimAzo]Br in aqueous
solution. From Table 1, it can be found that, 0.15 mM aqueous solution of
cisꢀ[C14mimAzo]Br has a significantly smaller Γ value and a correspondingly larger A
than transꢀ[C14mimAzo]Br. It is likely that, the bend of azobenzene increases the area
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of head group. The change of hydrophobicity and polarity can also influence the
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Table 1. Surface properties and micellization parameters of [C14mimAzo]Br and
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C14mimBr in aqueous solution
cac (mM)
γcmc
Γmax
Amin
(Å2)
SAIL
P
surface
tension
(mN
⋅
mꢀ1) (mmol
⋅
mꢀ2)
conductivity
transꢀ
[C14mimAzo]Br
cac1 0.028
cac2 0.070
0.032
0.085
0.100
2.6
44.11
36.85
37.50
39.2
0.0036
0.0062
0.0029
0.00196
43.22
26.98
79.45
84.7
0.46
0.78
0.26
/
cisꢀ[C14mimAzo]Br
0.090
2.8
C14mimBr
3.3. Photo-responsible variation in the self-assembly of [C14mimAzo]Br
DLS is utilized to observe the variation of aggregate size of the aqueous
[C14mimAzo]Br solution upon UV irradiation. The size distributions under different
irradiation conditions are shown in Figure 5. Aggregate size of 30 nm was observed
when the concentration of transꢀ[C14mimAzo]Br is 0.05 mM. When the concentration
of transꢀ[C14mimAzo]Br increases to 0.15 mM, the size of the aggregates increases
significantly to 250 nm. This great change in aggregate sizes can be proposed to a
transformation from micelles to vesicles. After UV lamp treatment, the aggregate size
of 0.15 mM solution decreases to 25 nm which indicates a manifest structural
transition. Of great interest, aggregate size at about 150 nm can be obtained again
followed by visible light exposure. This phenomenon indicates that transformation
between different aggregates is reversible.
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Figure 5. Size distributions of [C14mimAzo]Br aqueous solutions.
CryoꢀTEM was applied to confirm the above proposal. As shown in Figure 6a,
spherical micelles are formed in 0.05 mM aqueous solution. When the concentration
is increased to 0.15 mM, unilamellar and mutilamellar vesicles with diameters from
about 50 to 500 nm can be observed in Figure 6b. As marked in the chart, the vesicles
with a mean thickness of 3.2 nm are obtained. After UV irradiation to this system,
vesicles are ruptured and then transformed into spherical micelles (Figure 6c). More
importantly, vesicles can be reꢀformed after visible light irradiation (Figure 6d). This
is consistent with the DLS results above. It can be concluded that the [C14mimAzo]Br
system experiences a phase transition from vesicles to micelles after UV and visible
light irradiation. This phenomenon indicates that both of concentration and UV
treatment play important roles in controlling the size and morphology of aggregates.
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Figure 6. CryoꢀTEM images of 0.05 mM [C14mimAzo]Br solution before UV
irradiation (a), 0.15 mM before (b) and after (c) UV irradiation and 0.15 mM after UV
irradiation followed by visible light exposure (d).
3.4. Mechanism of photo-responsive aggregates transformation
Density functional theory (DFT) calculation can be used to understand the interaction
of SAIL with water and further comprehend the aggregates transformation mechanism.
Gaussian 09 package with the hybrid B3LYP functional and the 6ꢀ31G (d,p) basis set
were performed. Figure 7 shows the optimized structures of the two binary complex
systems (transꢀ and cisꢀ[C14mimAzo]Br with water). Their interaction energies (Eint)
are listed in Table 2. The calculated interaction energy of the
transꢀ[C14mimAzo]Br/H2O complex is smaller than that of cisꢀ[C14mimAzo]Br/H2O,
indicating that the transꢀ[C14mimAzo]Br is more readily to form aggregates in
water.55,56 This result is in agreement with the surface tension result. The length of
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single trans/cisꢀ[C14mimAzo]Br molecule are also evaluated as 21.2 Å and 21.1 Å
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respectively.
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Figure 7. B3LYP/6ꢀ31G(d,p)ꢀoptimized structures: (a) transꢀ[C14mimAzo]Br, (b)
cisꢀ[C14mimAzo]Br. Color code for atoms: dark gray, carbon; light gray, hydrogen;
blue, nitrogen; red, oxygen.
Table 2. Interaction energy (ꢀEint) of SAIL with H2O
complex system
transꢀ[C14mimAzo]Br cisꢀ[C14mimAzo]Br
167.832 173.292
interaction energy (−Eint/kJ⋅mol)
To propose the mechanism of phase transition, critical packing parameter (P) values
are calculated according to the surface tensionꢀconcentration curve, and the results are
also displayed in Table 1. The calculation of P is described as P=V/AL, where V
represents the effective hydrophobic chain volume, A represents the effective head
group area, and L is the alkyl chain length of SAIL. The values of L and V can be
estimated as follows:57
L ꢀ 1.5 ꢖ 1.265N
(3)
(4)
VꢗꢒÅꢔꢕ ꢀ 27.4 ꢖ 26.9N
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where N is the number of carbon atoms in the hydrophobic chain. The calculation of P
for the prediction of the aggregates is still valid to the novel SAIL systems. According
to the molecular packing parameter theory, the selfꢀassembly structures of the SAILs
are predicted as micelles, vesicles and micelles respectively according to obtained
values (Table 1). The results are in agreement with above test.
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The reasons for the vesicle formation and rupture of the synthesized SAIL at different
conditions can be explained as follows. Before UV irradiation, all the molecules are
transꢀisomer, and the formation of micelles can be observed. The incorporation of
azobenzene group, on one hand, could increase the steric hindrance between
molecules due to the large size of the head group. On the other hand, the charge
delocalization caused by azobenzene could reduce the electron density and induce the
molecules to pack more compactly and then reduce the adverse effects of steric
hindrance to aggregation. More importantly, the hydrophobicity of the
transꢀazobenzene makes the configuration of the head group more appropriate to fit
the critical packing parameter required for micelle formation.58 Our group has a
similar result about a longꢀchain Nꢀaryl imidazolium bromide incorporated by a
2,4,6ꢀtrimethylphenyl group.59 Due to the stronger hydrophobic interaction between
the alkyl chains and lower polarity of the azobenzene group, the expressions of
molecules give the chance of vesicle formation.46 The thickness of vesicle is smaller
than a biꢀmolecular length of [C14mimAzo]Br (4,2 nm), which indicates an overlap
among the hydrophobic chains during the formation of vesicles according to the
wellꢀestablished theory of vesicle formation.14 The azobenzene is hid in the
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membrane of the vesicle to avoid exposing in water while the imidazole contact with
water. When a part of the transꢀforms are transformed into more hydrophilic
cisꢀforms under UV light, the curvature of the head group increases the A value
threefold. Therefore, the obtained P value indicates that this is not conducive to the
formation of vesicles. The steric hindrance caused by the bend of azobenzene makes
the cisꢀisomers have no ability to accumulate tightly by weak πꢀπ staking interaction.
Therefore, it is anticipated that the variation of structure will cause a significant
reorganization of the SAIL aggregates. Based on the analysis above, the possible
accumulation mode of molecules and the transition between different aggregates are
described in Scheme 3.
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Scheme 3. Schematic illustration of the proposed packing structures of aggregates
formed by [C14mimAzo]Br and the aggregates transition under different conditions.
4. CONCLUSIONS
In conclusion, photoꢀcontrollable selfꢀassembly based on SAIL with azobenzene
located in the head group was achieved. The cac obtained for [C14mimAzo]Br is much
lower than that for C14mimBr, suggesting that the delocalization of charge caused by
azobenzene group reduces the electron density of headꢀgroup and facilitates the
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formation of vesicles. The transꢀcis transition of the azobenzene can be manipulated
by external light signal without any pollution to the system. The change in geometry
and dipole moment of headꢀgroup in [C14mimAzo]Br influence the critical packing
parameters and hydrophilicꢀhydrophobic balance and leads to reversible transition
between vesicles and micelles in the absence of additives. We believe that this work
can help to achieve the controllability of selfꢀassembled structures by external
stimulus, which is benefit for the application of responsive materials.
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AUTHOR INFORMATION
Corresponding Author
Eꢀmail: lqzheng@sdu.edu.cn. Phone: +86ꢀ531ꢀ88366062. Fax: +86ꢀ531ꢀ88564750.
Notes
Any additional relevant notes should be placed here
ACKNOWLEDGEMENTS
The authors are grateful to the National Basic Research Program (2013CB834505),
the National Natural Science Foundation of China (no. 21573132), and the National
Natural Science Foundation of China (no. 21403151) for financial support.
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