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

Abstract

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.

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Introduction

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-Specific 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 surfactants^6–8 were designed not only to introduce certain functions but also to set up flourishing 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 diversification. 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 functionality. A zwitterionic imidazolium surfactant with a sulfonate group (–SO₃⁻) at the N-site was effective in increasing the dispersity of Pd nanoparticles.¹⁰ There are also examples of additional compositions like a polymerizable monomer¹¹ or hydrogen bonded network¹² for further assembly at the far end of the N-site. At the X-site, there have been reports that counter 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 often encounters the lack of a functionalization point. Thus, it is necessary to find an easier structural tuning and a new derivative 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 typically selected due to its easy and diversified 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 structures^22,23 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 properties 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 there any way to expand the functionalization site of Im-IL surfactants and provide them with photo-controllability at the same time?

To answer this question, we uncovered the possibility of the 2-C position of imidazolium as a new site for connecting an arylazo moiety to ionic liquid surfactants. The new azo-functionalized head was intended to regulate species in the hydrophilic domain more directly, finely and strongly from additional π–π forces.²⁶ Seven arylazo-imidazolium (AzoIm⁺) surfactants with minor structural variations in the cation and anion were synthesized, and how such structural variations affect their photoresponsive colloidal chemistry was investigated. That is, by examining how groups at the N-site or X-site affect the different colloidal chemical properties, we would construct a comprehensive understanding of the new head group thus to obtain knowledge of the requirements for directing the construction of AzoIm⁺ molecules. Moreover, a new 2-arylazo-imidazolium surfactant showed additional photocontrollable instinctive fluorescence, reversible responsiveness to electrochemical redox stimuli and the ability to modify the shape of Au nanoparticles. An understanding of this head group would add to and also cooperate with the extensive possibilities for RTIL class compounds.

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 (tetradecyl in our work) and of additional groups (methyl, ethyl or sulfopropyl) on both the N sites, and (3) counter anion exchange (from I⁻ to Br⁻) to determine that the as-made surfactants are of the proper solubility to perform water based surface tension and conductivity tests or other experiments. Seven AzoIm⁺ surfactants were synthesized accordingly to investigate steric and electronic effects.

Scheme 1 Structures and synthetic route of 2-arylazo-imidazolium surfactants.

The steric effects of R′ at the N-site, which was the final alkyl to finish the quarternisation of azo-imidazolium, could be revealed through the comparison of 1-Br (methyl) and 2-Br (ethyl). They had nearly identical original absorption peaks in both aqueous and ethanol solutions. Yet a shorter R′ resulted in a better responsiveness to UV stimuli shown in the UV-Vis spectra (Fig. 1) with methoxyl as an example; the R at the para-position of azobenzene greatly influenced the electronic distribution. It resulted in a red shift of the maximum absorption wavelength (λ_max) and the weakest photo responsiveness of absorption coefficient (ε) in both water and ethanol. The electronic structure of arylazo-imidazolium could be manipulated by introducing an electron donating/withdrawing group on the azobenzene. In the example using methoxyl the (π–π*)_E peak of 3-Br and 3-I red-shifted to ∼397 nm in water and ∼405 nm in ethanol, and the overlapped (π–π*)_Z peak was shifted to ∼325 nm estimated from the peak shape (Fig. 1). This results from the 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 Br⁻ anion surfactants were more soluble in water and have stronger absorption than those with I⁻. The best responsiveness of cation 3 was achieved by selecting I⁻ as the anion and measuring in ethanol.

Fig. 1 UV-Vis spectra of four surfactants with combinations of cations: 1 (black), 2 (red), 3 (blue) and 4 (pink) and anions (a and b Br⁻, c and d I⁻), in the solvent of (a and c) H₂O or (b and d) ethanol. A solid line indicates before UV exposure and a dashed line indicates after.

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 fluorescence 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 (π–π*)_Z peak wavelength, yet had a significant 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 (γ) vs. surfactant concentration (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 γ–C plots in Fig. 2, (for a Br⁻ anion) Fig. 3 (for an I⁻ anion) and the calculated data in Table 1.

Fig. 2 Surface tension of [C₁₄AzoIm]Br aqueous solutions as a function of their concentrations. (a) Overall data for (■) 1-Br, (●) 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.

Fig. 3 Surface tension of [C₁₄AzoIm]I aqueous solutions as a function of their concentration. (a) Overall data for (■) 1-I, (●) 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.

Table 1 Surface properties and micellization parameters of [C₁₄AzoIm]I and [C₁₄AzoIm]Br and reference imidazolium surfactants in aqueous solution at 25 °C

Surfactant	Conditions	CMCγ (μM)	γCMC (mN m^−1)	Γmax (μmol m^−2)	Amin (Å^2)
1-Br	Original	150.1	43.1	5.99	27.7
	UV	128.7	30.7	5.98	27.7
2-Br	Original	423.3	40.2	3.59	46.3
	UV	306.9	39.6	3.44	48.3
3-Br	Original	165.1	41.5	3.8	43.7
	UV	158	41.2	3.67	45.2
1-I	Original	80.18	42.9	4.19	39.6
	UV	72.55	43.2	5.20	32.0
2-I	Original	110.9	40.1	3.90	42.6
	UV	94.76	41.3	3.82	43.4
3-I	Original	50.09	52.0	3.83	43.3
	UV	38.14	48.6	5.07	32.7
[C14mim]Br^28		2800	39.2	1.96	84.7
[C14pim]Br^29		610	38.3	2.21	75.1
[C12-2-C12im]Br2 ^30		550	33.6	1.23	135

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In general, the AzoIm⁺ surfactants had a remarkably lower CMC and A_min than the imidazolium or N-aryl imidazolium²⁹ surfactants (Table 1). Two important factors in their CMC and A_min slump were: (1) a larger π-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, β was even higher than that of gemini imidazolium surfactants,³⁰ not to mention traditional imidazolium surfactants. After UV exposure, the CMC_γ and γ_CMC 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 after 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 R′ is supported by the critical micellar concentration (CMC_γ and CMC_κ), surface tension (γ) and conductivity (κ) data (Table 1). In addition, 1-Br had a lower CMC and lower A_min which meant a better surface activity and a more compacted 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 CMC_γ and CMC_κ than 3-Br after UV exposure. The methoxyl was projected to improve the electron dispersion of the π system which would result in a decrease in CMC_γ, 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 hydrophilicity 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 I⁻ anion, AzoIm⁺ surfactants all showed a lower surface efficiency (γ_CMC) and photo responsiveness. Their CMCs were lower than those of Br⁻. The reason might lay in the low solubility caused by the I⁻ anion, which might also cause the abnormal fluctuations of γ just past the low concentration plateaus of 1-I and 2-I in Fig. 3. Finally in this section, the morphology of each aggregate was studied using cryo-TEM and sorted in two groups (Fig. S4†). The majority except 2-Br had irregular grains of 10–20 nm in size, which were merged from smaller spherical micelles. The case of 2-Br showed giant micelles of ∼50 nm with a smoother surface, in accordance with the much higher CMC of 2-Br.

Thermodynamic analysis of micelle formation

The electrical conductivity (κ) vs. concentration (C) correlation of the IL surfactants and its behaviour under a temperature sequence in aqueous solutions is an important tool to understand 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 °C for better solubility and therefore stability. Most surfactants involved showed a similar CMC range and tendency for photoresponsiveness as they did in tensiometry, though some remarks should be made. The results for [C₁₄AzoIm]Br surfactants were in much better accordance than those of [C₁₄AzoIm]I for the absolute value or photo induced change of CMC, among which 3-Br was almost same number of both the original and after UV absorption at 25 °C. Concluding from the overall sequence of specific conductivity, which is 2-Br > 1-Br ∼ 3-Br > 1-I ∼ 2-I > 3-I, we suggest that the more compact cation 1⁺ might act to compensate for mobility loss when linked with the bigger anion, I⁻.

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.

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.

Based on the mixed electrolyte model of micelles in solution developed,³² the degree of counterion binding (β), which is essential information for the anion exchange chemistry of imidazolium, can be obtained from the κ–C plots. In general, an unprecedentedly high β implying higher affinity between the [C₁₄AzoIm]⁺ cation and Br⁻/I⁻ anion was observed compared to ordinary [C₁₄mim]Br,³¹ CTAB (β = 0.71 (ref. 33)) and previously reported [C₁₄pim]Br.²⁹ Although it is a big head group, [C₁₄AzoIm]⁺ has additional electronic interactions with Br⁻ or I⁻ 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 with I⁻ at room temperature, however the interactions are dramatically affected by increasing temperature.

Unlike this, the affinity of [C₁₄AzoIm]Br exhibited a good temperature tolerance. Adaptation of [C₁₄AzoIm]⁺ with bromide should be responsible for this special phenomenon, which could be further explored by applying to other anions. After UV exposure, a marginal decrease of β that did not surpass a difference of 5 °C for each surfactant was observed. The photoinduced trans to cis conformation change slightly loosened the cation–anion close pair. This might also be the reason that CMC_κ after UV exposure decreased far less than CMC_γ. The close cation–anion micellar structure formed in these new surfactants could provide anions with a front edge to be involved in supramolecular and colloidal chemistry. An attempt to modify the C-site resulted in the optimization of the X-site unexpectedly.

The thermodynamic parameters of micellization process: the standard Gibbs free energy change of ΔG⁰_m, the standard enthalpy change of ΔH⁰_m and the standard entropy change of ΔS⁰_m could also be calculated with specific conductivity data using the following equations:³⁴

ΔG⁰_m = (1 + β)RT ln X_CMC (1)

TΔS⁰_m = ΔH⁰_m − ΔG⁰_m (3)

where X_CMC is CMC in molar fraction and β is the degree of counterion binding to micelles as above. Eqn (2) was applied by fitting ΔG⁰_m/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.

Table 2 Critical micelle concentration (CMC_κ) and specific conductivity at CMC (κ_CMC), degree of counterion binding (β), and thermodynamic parameters of micelle formation for [C₁₄AzoIm]Br in aqueous solutions at various temperatures and UV exposures

Surfactant	T (°C) & conditions	CMCκ (mM)	β	ΔG^0m (kJ mol^−1)	ΔH^0m (kJ mol^−1)	−TΔS^0m (kJ mol^−1)
a Measured from the data figure.
1-Br	25	0.147	0.88	−60.78	−112.21	51.426
	30	0.155	0.86	−60.10	−100.57	40.473
	35	0.160	0.85	−59.40	−89.316	29.913
	40	0.163	0.84	−59.09	−78.416	19.330
	45	0.170	0.84	−58.84	−67.860	9.015
	25-UV	0.139	0.86	−60.43
	30-UV	0.146	0.84	−59.68
2-Br	25	0.419	0.87	−55.67	−96.917	41.250
	30	0.428	0.86	−55.17	−87.448	32.276
	35	0.429	0.84	−54.62	−78.286	23.661
	40	0.431	0.83	−54.25	−69.417	15.170
	45	0.437	0.83	−54.19	−60.827	6.636
	25-UV	0.402	0.86	−55.47
	30-UV	0.415	0.85	−54.89
3-Br	25	0.168	0.90	−60.83	−103.59	42.757
	30	0.169	0.88	−60.31	−91.660	31.351
	35	0.170	0.87	−59.78	−80.116	20.335
	40	0.172	0.86	−59.56	−68.941	9.377
	45	0.173	0.86	−59.53	−58.117	−1.418
	25-UV	0.158	0.89	−60.76
	30-UV	0.159	0.87	−60.27
[C14mim]Br^31	25	2.610	0.78	−44.33^a	−15.13^a	−30.38
[C14pim]Br^29	25	0.550	0.50	−42.74	−55.06	12.32

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Table 3 Critical micelle concentration (CMC_κ) and specific conductivity at CMC (κ_CMC), degree of counterion binding (β), and thermodynamic parameters of micelle formation for [C₁₄AzoIm]I in aqueous solutions at various temperatures and UV exposures

Surfactant	T (°C) & conditions	CMCκ (mM)	β	ΔG^0m (kJ mol^−1)	ΔH^0m (kJ mol^−1)	−TΔS^0m (kJ mol^−1)
1-I	25	0.101	0.93	−63.25	−167.92	104.67
	30	0.111	0.86	−61.65	−171.12	109.47
	35	0.116	0.78	−59.52	−174.22	114.70
	40	0.120	0.70	−57.87	−177.22	119.35
	45	0.124	0.62	−55.86	−180.13	124.27
	30-UV	0.106	0.85	−61.55
2-I	25	0.102	0.94	−63.37	−146.99	83.617
	30	0.111	0.87	−61.76	−152.19	90.431
	35	0.117	0.79	−59.89	−157.22	97.332
	40	0.120	0.75	−59.33	−162.09	102.76
	45	0.125	0.65	−56.73	−166.81	110.08
	30-UV	0.106	0.84	−60.96
3-I	25	0.065	0.90	−64.11	−221.10	156.99
	30	0.070	0.82	−62.12	−200.09	137.97
	35	0.072	0.73	−59.97	−179.76	119.79
	40	0.078	0.63	−57.26	−160.09	102.83
	45	0.089	0.61	−56.76	−141.03	84.268
	30-UV	0.064	0.77	−61.02

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The highly negative ΔH⁰_m is the major force in making ΔG⁰_m 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 anion,³⁶ and (4) the π–π interactions.³⁷ Considering the extraordinarily high β discussed above, the sum of ΔH⁰_m ion interaction ((2) + (3)) should be positive. Thus the other two reasons dominate the enthalpy driven process together. Unlike [C₁₄mim]Br,³¹ the TΔS⁰_m of all [C₁₄AzoIm]Br as well as 3-I decrease from a high positive value with increasing temperature. The positive TΔS⁰_m suggests that the micellization of [C₁₄AzoIm]⁺ surfactants consists of the rearrangement of the 2-azoaryl-imidazolium head group into a uniform micelle shell, which was affected by increasing temperature. In the case of 1-I or 2-I the micelle was even more uniform after the intercalated I⁻ was repelled by heat, along with the amplified exothermic effect. After UV exposure, the Gibbs free energy change decreased slightly, implying a weakening tendency of micellization. That might came from the difficulty brought to water solvation process by more uniform cis head groups which needs a little extra energy.

Fluorescence properties

Steady-state fluorescence measurements were performed to study micelle aggregation behaviour in H₂O solution with pyrene as the probe.³⁸ The 2-arylazo-imidazolium was found to highly hinder fluorescent emission (Fig. S1†), keeping [C₁₄AzoIm]Br with a higher CMC from this test. There are five characteristic fluorescence emission bands of 370–400 nm under 335 nm excitation. The ratio of the first 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 30 °C, which was changes stepwise with different behaviour.

Fig. 6 Fluorescence emission intensity of pyrene as a function of concentration for three 2-azobenzenyl-imidazolium surfactants at 30 °C: (a) I₁/I₃ ratio of (■) 1-I, (●) 2-I and (▲) 3-I, (b) a schematic representation of donor–acceptor interactions between 3-I and pyrene, and (c) fluorescence spectrum of 0.02 mM 3-I solution: solid line before UV exposure, dashed line after 30 min UV exposure.

The initial I₁/I₃ of 1-I and 2-I were significantly 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 confined 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 polarity zone. The I₁/I₃ ratio of 2-I which had ethyl on the imidazolium decreased more quickly than that of 1-I with methyl at the same site. The longer hydrocarbon substituent might offer more affinity and easier entry of pyrene and thus a quicker decrease in the polarity of the microenvironment.

The change in the I₁/I₃ ratio of 3-I over the concentration range was distinct. After a stable I₁/I₃ ratio, the ratio began to ramp up above 0.02 mM and finally attained equilibrium at 2.2. An increase in the ratio indicated an enhancement of polarity, which even surpassed other highly polar solvents such as H₂O or DMSO⁴⁰ (Fig. 6a). It might be speculated that the pyrene was dragged closely to the polar and electron donating cation of 3-I, instead of alkyl chains inside micelles when only π–π interactions are available from 1-I and 2-I (Fig. 6b). The high initial I₁/I₃ might imply the existence of instinctive fluorescence. The fluorescence of pure 3-I solution was confirmed to have a maximum emission wavelength of 535 nm under excitation at 381 nm. After UV exposure the maximum emission shifted to 528 nm (Fig. 6c) which is unique compared to previously reported fluorescent phenyl connected imidazolium surfactants.²⁹ Even within the AzoIm⁺ surfactants, only the cation 3 showed instinctive fluorescence and that of 3-I was stronger than 3-Br (Fig. S3†). This property has not yet been reported in imidazolium compounds, and might be applied in ion detection.⁴¹

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 (represented 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 stabilization and strengthening of the hydrogenated azo bond in the reduced state.

Fig. 7 Cyclic voltammetry of 0.1 mg mL⁻¹ 2-Br in 0.1 M PBS and possible mechanism of redox reactivity.

Manipulating the shape of Au nanoparticles

We take AuNPs as a demonstration of AzoIm⁺ surfactants’ application in the fabrication of various nanomaterials. Directed by 2-Br without UV exposure, random Au nanoparticles appeared with a round-cornered shape (Fig. 8a). Different from this bland result, Au nanostars (Au NST) were fabricated with 2-Br under UV exposure (Fig. 8b). The spike may result from the micelle consisting of cis-2-Br which encouraged anisotropic growth of the (111) and (200) faces of Au nanostructures⁴⁴ (Fig. 8c). When compared to a benzene ring, the imidazolium ring should have greater affinity for Au. However such competition didn’t really happen in the trans AzoIm⁺ which was supposed to interact with Au equatorially, meaning that both benzene and imidazolium interact with Au. When AzoIm⁺ was in the cis state, the imidazolium should be more close. In the original state, the mixture of majority trans- and minority cis-AzoIm⁺ surfactants made it difficult to maintain a regular growth edge. This situation was inversed after UV exposure (Fig. 8e). Meanwhile, the hydrophobic microenvironment induced by cis AzoIm⁺ kept the Au surface from direct contact with and etching by Br⁻ (Fig. 8c). The Au NST show a broad surface plasmon resonance band peak at ∼720 nm and might be beneficial to SERS detection⁴⁵ (Fig. 8d). These examples could be an illustration of the potential of the new 2-arylazo-imidazolium class of surfactants in controlling self-assembly and colloidal chemistry.

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 different conditions listed above. (e) Schematic illustration of the possible mechanism of Au nanostar formation.

Conclusions

In conclusion, we have synthesized a new series of 2-arylazo-imidazolium (AzoIm⁺) surfactants, characterized the colloidal chemistry and explored how those properties react to photo stimulation. We found that the 2-arylazo-imidazolium group shows a unique stimuli/structural response when it serves as the hydrophilic part in surfactants, making it a new option to construct a colloidal working environment. It was verified that applying 2-Br in Au NP fabrication could control nanoparticle shape into Au nanostars. The fine-tuning and diverse structure of 2-arylazo-imidazolium will make it an important extension of ionic liquid surfactants and important in the research of smart materials. The newly discovered redox-reversible properties would also give more potential for the combination of photo and electrochemical applications. Given the widespread usage of imidazolium surfactants in nanotechnology or as green solvents, more expansive or deep research could be inspired by this work.

Experimental section

Synthesis of 2-arylazo-imidazolium surfactants

Materials. General grade solvents and reagents were purchased from commercial suppliers. Amberlyst A-26 (R⁺–OH⁻ form) resin was obtained from Alfa Aesar. All chemicals were AR grade and used without further purification.
Synthesis of 2-(phenyldiazenyl)imidazole (5a: PhAzoIm) and 2-((4-methoxyphenyl)diazenyl)imidazole (5b: MeOAzoIm).. 5a and 5b were synthesized following a previously reported procedure.⁴⁶ Briefly for 5a, aniline (0.1 mol) was dissolved in dilute hydrochloric acid, and diazotized with sodium nitrite (0.103 mol). The solution was dropped slowly into a mixed solution of imidazole (0.1 mol) and anhydrous sodium carbonate and the system was kept at a temperature under 5 °C overnight. Then the orange powder collected was washed well with water and extracted successively with cold hydrochloric acid. The extract was collected and basified with sodium carbonate to obtain the crude product. After further purification by washing with cold deionized water, drying and recrystallization from alcohol, the pure product was obtained. The yield was 83.7%. ¹H NMR (400 MHz, CDCl₃) δ 7.95–7.92 (dd, 2H), 7.50 (m, J = 5.1, 1.7 Hz, 3H), 7.32 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 153.71, 151.91, 132.37, 129.28, 124.12, 123.36 ppm. 5b was synthesized using the same method, only replacing aniline with 4-aminophenol. The yield of MeOAzoIm was 79.6%. ¹H NMR (400 MHz, CDCl₃) δ 7.96–7.91 (m, 2H), 7.27 (d, J = 5.6 Hz, 3H), 7.03–6.98 (m, 2H), 3.90 (s, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 163.26, 153.83, 146.34, 125.54, 123.48, 114.50, 55.71 ppm.
Synthesis of 2-(phenyldiazenyl)-1-tetradecyl imidazole (6a: C₁₄PhAzoIm) and 2-((4-methoxyphenyl)diazenyl)-1-tetradecyl-imidazole (6b: C₁₄MeOAzoIm).. Briefly for 6a, 5a (0.02 mol), bromotetradecane (0.02 mol), tetrabutylammonium iodide (0.004 mol) and sodium hydroxide (0.0625 mol) were dissolved in ethyl alcohol. The mixture was magnetically stirred and refluxed for 24 hours. Water was poured in and extracted with ethyl acetate to remove the inorganics. The final product was obtained by silica gel column chromatography using CH₂Cl₂ as an eluent giving a yield of 72.8%. ¹H NMR (400 MHz, CDCl₃) δ 7.98 (dd, J = 7.8, 1.6 Hz, 2H), 7.53–7.47 (m, 3H), 7.28 (s, 1H), 7.16 (s, 1H), 4.42 (t, J = 7.1 Hz, 2H), 1.88 (p, J = 7.0 Hz, 2H), 1.30–1.20 (m, 22H), 0.87 (t, J = 6.8 Hz, 3H). 6b was synthesized with the same method, only to replace 5a with 5b. The yield of C14MeOAzoIm was 74.4%. ¹H NMR (400 MHz, CDCl₃) δ 8.00–7.96 (m, 2H), 7.24–7.22 (m, 1H), 7.12–7.10 (m, 1H), 7.01–6.98 (m, 2H), 4.38 (t, J = 8.8 Hz, 2H), 3.89 (s, 3H), 1.28 (dd, J = 44.7, 14.3 Hz, 24H), 0.87 (t, J = 6.0 Hz, 3H).
Synthesis of 2-arylazo-1-tetradecyl imidazolium iodides: (1-I: [C₁₄PhAzoMeIm]I), (2-I: [C₁₄PhAzoEtIm]I) and (3-I: [C₁₄MeOPhAzoMeIm]I). Briefly for 1-I, 6a and iodomethane were dissolved in dry THF and refluxed under air tight conditions for 72 hours. The final product was obtained by removing the solvent and then silica gel column chromatography with eluents of CH₂Cl₂/CH₃OH (20 : 1). Yield 75.3%. ¹H NMR (400 MHz, CDCl₃) δ 8.36 (d, J = 1.9 Hz, 1H), 8.08 (d, J = 2.0 Hz, 1H), 8.00 (s, 1H), 7.98 (d, J = 1.2 Hz, 1H), 7.71 (dd, J = 8.4, 6.3 Hz, 1H), 7.63 (t, J = 7.6 Hz, 2H), 4.66–4.61 (m, 2H), 4.33 (s, 3H), 1.34–1.20 (m, 24H), 0.87 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 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, 29.00, 26.36, 22.70, 14.14 ppm. 2-I was obtained similarly, only replacing iodomethane with iodoethane. Yield 75.3%. ¹H NMR (400 MHz, CDCl₃) δ 8.01 (d, J = 7.4 Hz, 2H), 7.53 (d, J = 6.9 Hz, 3H), 7.31 (d, J = 7.5 Hz, 1H), 7.20 (s, 1H), 4.45 (t, J = 7.0 Hz, 2H), 1.90 (d, J = 6.2 Hz, 2H), 1.31 (d, J = 41.1 Hz, 27H), 0.91 (t, J = 6.1 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ = 152.60, 142.51, 135.88, 130.04, 124.87, 124.82, 124.28, 50.43, 46.14, 31.92, 30.48, 29.68, 29.64, 29.60, 29.53, 29.40, 29.36, 29.02, 26.36, 22.69, 15.93, 14.14 ppm. 3-I was obtained similarly, only replacing 6a with 6b. Yield 77.8%. ¹H NMR (400 MHz, CDCl₃) δ 8.27 (d, J = 1.9 Hz, 1H), 8.02–7.96 (m, 3H), 7.12–7.08 (m, 2H), 4.60–4.56 (m, 2H), 4.29 (s, 3H), 3.98 (s, 3H), 1.96–1.88 (m, 2H), 1.34–1.20 (m, 22H), 0.88 (t, J = 6.9 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): d = 166.56, 147.32, 143.55, 127.33, 125.50, 123.30, 115.46, 56.22, 49.84, 38.14, 31.92, 30.44, 29.69, 29.65, 29.61, 29.54, 29.40, 29.36, 29.02, 26.34, 22.69, 14.13 ppm.
Synthesis of 2-arylazo-1-tetradecyl imidazolium bromides: (1-Br: [C₁₄PhAzoMeIm]Br), (2-Br: [C₁₄PhAzoEtIm]Br) and (3-Br: [C₁₄MeOPhAzoMeIm]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⁺C3SO3⁻. 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 °C under a nitrogen atmosphere. The mixture was then magnetically stirred for 120 h at room temperature, filtered, washed with acetone twice and dried under vacuum to obtain C₁₄PhAzoIm⁺C3SO3⁻. Yield 61%. ¹H NMR (400 MHz, CDCl₃) δ 8.43–8.35 (m, 1H), 8.04 (s, 1H), 8.00–7.97 (m, 2H), 7.65 (dt, J = 2.6, 1.9 Hz, 1H), 7.62–7.56 (m, 2H), 4.85 (t, J = 6.4 Hz, 2H), 4.56 (t, J = 7.4 Hz, 2H), 2.92 (t, J = 6.4 Hz, 2H), 2.46–2.38 (m, 2H), 1.93–1.84 (m, 2H), 1.30–1.19 (m, 27H), 0.88–0.86 (m, 2H).

Apparatus and procedures. TEM was performed on a JEOL JEM 2100. SEM was performed on a Hitachi SU-70. UV exposure was performed with a high pressure mercury light (175 W) and a filter of 365 nm. Exposure time was set to 30 min unless otherwise specified. UV-Vis spectra were gathered from with PerkinElmer Spectrophotometer lambda 750 at 25 °C. The surface tension was recorded using a DCAT11EC surface tension meter (Dataphysics Germany) at 25 °C. Conductivity was recorded using a DDS-307A Digital conductivity instrument (INESA Instrument Shanghai). Fluorescence spectra were obtained using a Fluoromax-4 Fluorospectro photometer (Horiba) at 25 °C and excitation at 337 nm for the pyrene probe. Electrochemical measurements were performed in a conventional three-electrode system with an Ag/AgCl electrode as the reference electrode and a Pt wire as the counter electrode. 0.1 M PBS (pH = 7) buffer was used as the electrolyte. Cyclic voltammetry (CV) tests were performed on a CHI 660C workstation (Shanghai Chenhua). Cryo-TEM was performed on a JEOL JEM 1400Plus.

Preparation of Au nanostars. Gold seed suspension and the growth solutions were prepared following a step-wise growth method,⁴⁸ only replacing CTAB with 2-Br in an equimolar amount. Au nanoparticle/nanostar preparation was performed according to the reference or with exposure to UV for 1 h beforehand.

Acknowledgements

This research was supported by the China NSFC (Grand No. 21403077), Research Fund for the Doctoral Program of Higher Education of China (Grand No. 20133501120004) and China Postdoctoral Science Foundation (Grand No. 2015M582037). Also supported by the “111” Project (B16029), NSFC (No. U1405226), Fujian Provincial Department of Science & Technology (2014H6022) and 1000 Talents Program from Xiamen University. We thank Prof. Hai Xu and Prof. Dong Wang at the Centre for Bioengineering and Biotechnology of China University of Petroleum for the cryo-TEM characterization.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04448d

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