Rational Design of Multiple-Stimulus-Responsive Materials via

Article pubs.acs.org/JPCC

Rational Design of Multiple-Stimulus-Responsive Materials via Supramolecular Self-Assembly Yanjun Gong,† Tiliu Jiao,‡ Qiongzheng Hu,§ Ni Cheng,† Wenwen Xu,† Yanhui Bi,† and Li Yu*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China ‡ China Research Institute of Daily Chemical Industry, Taiyuan 030001, People’s Republic of China § Department of Chemistry, University of Houston, Houston 77204, United States S Supporting Information *

ABSTRACT: In this report, multiple-stimulus-responsive materials were synthesized via supramolecular self-assembly. One-dimensional nanorods were constructed by the selfaggregation of 4-(phenyl-azo)benzoic acid (PBA) molecules in aqueous solution at pH 3.2. As the pH of the solution was increased to 6.5, these nanorods transformed into twodimensional polygons. Upon UV irradiation, the as-prepared nanorods disappeared completely, and nanospheres were subsequently obtained. On the basis of the weak interactions between PBA and additive molecules, for example, N-alkyl-N′carboxymethyl imidazolium bromide, β-cyclodextrin, and cetyltrimethylammonium bromide, materials with various morphologies were also fabricated by a surfactant-assistant self-assembly strategy. Noteworthy is that Salvia officinalisshaped material is among them. To the best of our knowledge, this type of microstructured material has been rarely reported. In addition, slender fibers, sphere-like particles, and aggregates of spheres were also observed. These results suggest that the rational fabrication of materials with desired shapes and sizes can be achieved by changing external environments during the self-aggregation of PBA molecules. Both cyclic voltammogram experiments and density functional theory calculations exhibit the optoelectronic behavior of these materials, which is expected to have potential applications in the fabrication of photoelectronic nanodevices.

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INTRODUCTION Supramolecular self-assembly based on small organic molecules is of great interest for materials science.1−5 Among them, selfassembly of organic functional small molecules6,7 has drawn particular attention because of its excellent electrical and optical properties,8,9 which also depends on the sizes, structures, and shapes of the materials.10 The supramolecular materials are always constructed on the basis of noncovalent interactions, such as hydrogen bond, π−π stacking, hydrophobic interaction, and electrostatic interaction. Therefore, their properties can be readily manipulated by a change in external environment conditions.11 Stimuli-responsive nanomaterials are fascinating and attractive because tremendous changes in microstructure can be triggered by small changes in the environment. Thus, development of functional nanomaterials supported by the response to the external environment is fundamentally important to fabricate devices for opto-electrical, switching, and controlled drug release.12,13 © XXXX American Chemical Society

The azobenzene moieties can cause reversible trans−cis photoisomerization reactions, leading to considerable changes in molecular size, shape, and dipole moments. They have been widely used in numerous applications, such as heterogeneous separation, molecular switching, and drug delivery vesicles. Wu’s group presented a photodriven polyoxometalate complex with photoresponsive azobenzene units and its heterogeneous separation. Kim and Jung constructed stimulus-responsive supramolecular structures in the form of fibers, gels, and spheres, derived from an azobenzene-containing benzenetricarboxamide. Raghaven et al. reported a class of reversible photorheological fluids prepared by combining the azobenzene derivative 4-azobenzene carboxylic acid with the cationic surfactant erucyl bis(2-hydroxyethyl)methylammonium chlorReceived: April 2, 2015 Revised: June 13, 2015

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DOI: 10.1021/acs.jpcc.5b03220 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Chemical Structures of PBA (a), NnMI (n = 10, 12, 14) (b), CTAB (c), and β-CD (d)

Scheme 2. Proposed Model of the Formation of the Supramolecular Materials

properties (Scheme 2) but also display stable optoelectronics

ide. Although photoswitchable azobenze units have previously been reported, small azobenzene molecules’ response to multiple stimuli in the context of supramolecular self-assembly has rarely been investigated.14−17 Herein, we applied 4-(phenylazo)benzoic acid (PBA) (Scheme 1a) to fabricate nanorods (Scheme 2) in aqueous solution at pH 3.2. The obtained nanorod materials not only exhibit pH- and photoresponsive

behaviors. In addition, we employed a surfactant-assistant selfassembly (SAS) strategy to construct materials with various morphologies (Scheme 2). In the SAS process, with the aid of the PBA molecules, supramolecular architectures can be further built by the rational design of synthesis methodology, such as B


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The Journal of Physical Chemistry C

Figure 1. SEM (a, b) images of PBA self-assemblies at different pHs: 3.2 (a) and 6.5 (b). TEM image of the materials upon UV light irradiation for 1 h (c) and 1H NMR spectra (d) of PBA self-assembled structures before and after UV irradiation.

midazole (99%), and bromoacetic acid (99%) were bought from Aladdin Chemistry Co., Ltd. of China. Benzophenone (chemically pure) was obtained from Sinopharm Chemical Reagent Co., Ltd. All the above reagents were used without further purification. Triply distilled water was used in all the solutions. Synthesis. COOH-functionalized SAILs were prepared according to our previous report.18 N-alkylimidazole was also prepared as described previously.19 Briefly, bromoacetic acid (86 mmol) was added to 50 mL of a methanol solution of Nalkylimidazole (95 mmol) under continuous stirring. The mixture was refluxed at 70 °C for 6 h under the protection of nitrogen. After the methanol was removed, the residue was recrystallized five times from methanol/ether to obtain NnMI (20 g, 63% yields). The chemical structures were ascertained by 1 H NMR spectroscopy with a Bruker Avance 300 spectrometer. The 1H NMR peak of D2O (δ = 4.70 ppm) was used as the reference in determining the proton chemical shifts of NnMI. The elemental analysis measurements were performed on a Vario EL III elementar analyzer (Elementar). The mass analysis data were performed on Agilent 6510Q-TOF. Take N12MI as an example. 1HNMR (D2O, ppm): δ = 8.80 (s, 1 H, CH), 7.46 (d, 1 H, CH), 7.46 (d, 1 H, CH), 4.88 (s, 2 H, CH2), 4.18 (t, 2 H, CH2), 1.83 (m, 2 H, CH2), 1.27 (m, 14 H, CH2), 0.80 (t, 3 H, CH3). Calcd for [N12MI]: C, 69.40; H, 10.20; O, 10.88; N, 9.52. Found: C, 69.25; H, 10.78; O, 10.72; N, 9.25. MS: m/z (%) calcd: 379 [C14H31N2O2 + H]+; found: 379.33. Preparation of Supramolecular Structures. The pH of SPBA aqueous solutions (10 mL, 0.5 mM) was initially adjusted to 3.2 by adding HCl. After stirring for 4 h, the obtained products were collected by filtration, then washed three times with water to remove salts and possible precursors. The final products were dried under vacuum at 55 °C for 24 h.

changing the hydrophobic interaction, introducing host−guest interaction, and using the template. First, we observed transitions of the materials from onedimensional nanorods to two-dimensional polygons by changing the pH of the PBA solution from 3.2 to 6.5. Next, irradiating the nanorods with UV light (365 nm) for 1 h resulted in the disappearance of the self-assembled materials and emergence of nanosphere structures. A series of COOHfunctionalized imidazolium-based surface active ionic liquids (SAILs), N-alkyl-N′-carboxymethyl imidazolium bromide (NnMI, n = 10, 12, 14) (Scheme 1b), were synthesized and co-assembled with PBA to form aggregates with variant shapes and sizes based on the discrepancy of the hydrophobic interactions. The β-cyclodextrin (β-CD) molecule with seven glucose units provides a hydrophobic interior cavity, and its exterior side is exposed to a hydrophilic surface (Scheme 1c). It has a strong host−guest interaction to combine transazobenzene while cannot binding with cis-azobenzene because of the mismatch of the host and guest molecules. Then, NnMI (n = 10, 12, 14) was added to the sodium 4-(phenylazo)benzoate (SPBA)/β-CD systems to further modulate the supramolecular structures. It is noteworthy that the Salvia officinalis-shaped microstructure was rarely observed. In addition, slender fibers and sphere-like particles could also be observed. In the end, we employed a self-induced template growth method to produce coral-like structures of PBA in the presence of cetyltrimethylammonium bromide (CTAB) molecules (Scheme 1d).

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EXPERIMENTAL SECTION Materials. Sodium 4-(phenylazo)benzoate (99%), β-cyclodextrin (99%), and CTAB (99%) were purchased from J&K Chemical Technology, China. 1-Bromodecane (98%), 1bromododecane (98%), 1-bromotetradecane (98%), N-alkyliC


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Figure 2. TEM (a,b) and SEM (c) images of SPBA/β-CD complex in N10MI solution. TEM (d) image of the SPBA/β-CD complex in N10MI solution upon UV (365 nm) light irradiation.

2. RESULTS AND DISCUSSION 2.1. pH- and Light-Responsive Characteristics of PBA Self-Assemblies. For the PBA self-assembly, the morphologies of the supramolecular materials formed in the pH 3.2 aqueous solution were examined by scanning electron microscopy (SEM). As shown in Figure 1a, nanorods with a length up to two micrometers were formed. The surface of the material is very clean and smooth. The self-assembled nanorods could transit into micrometer polygons as the pH of solution was increased to 6.5 (Figure 1b). The polygonal materials disappeared when the pH was continually increased. Of particularly interest is that we captured the intermediate structures from nanorods to polygons (Figure S1a,b). The formation of one-dimensional nanorods can be attributed to the cooperation of H-bonding and π−π stacking between the PBA molecules at pH 3.2. As the pH of the aqueous solution increased, the PBA molecules partially deprotonated into SPBA. Therefore, the electrostatic repulsion was generated. The change of π−π stacking was observed and confirmed by UV−vis absorption spectra (Figure S2). The strong absorption of PBA molecules occurs around 300 nm (π−π*) at pH 3.2. When the pH was changed to 6.5, the absorption was redshifted by 20 nm as a result of the weakened π−π stacking. Both the electrostatic repulsion and weakened π−π stacking can alter the stacking structure of materials.20 This is proven by the shift of the SAXS diffraction peak (Figure S3). To obtain better insight into the stacking of PBA molecules, we also carried out computational studies on small aggregates formed by these molecules. Noncovalent weak interactions, such as hydrogen bonding, electrostatic interaction, and π−π stacking, are satisfactorily captured by ωB97X-D/6-31G(d,p). Hence, it is desirable for predicting the change of the stacking structure at different pH. As shown in Figure S4, the stacking structure of the PBA molecules changes considerably at different pHs. This

For the preparation of the pH-responsive materials, NaOH was added to adjust the pH of the PBA solutions from 3.2 to 5.7 and 6.5, respectively, followed by stirring for 4 h. For the preparation of the light-responsive materials, the SPBA solution (pH = 3.2) was irradiated with UV light (365 nm) for 1 h. Characterizations of Supramolecular Structures. The nanostructures were characterized by transmission electron microscopy (TEM) (JEM-100CX II (JEOL). The sizes and morphologies of supramolecular materials were characterized by scanning electron microscopy (SEM, JEOL JSM-7600F) operated at 5.0 kV. Ten microliters of the sample solution was applied to a carbon-coated copper grid for 3 min after removal of excess solution with filter paper. The 1H NMR spectra were recorded using a Bruker AV-300 NMR spectrometer with a pulse field gradient module (Z-axis) and a 5 mm sample tube. The instrument was operated at a frequency of 300.13 MHz at 25 °C with tetramethylsilane as an internal reference. Deuterated dimethyl sulfoxide (DMSO) was selected as the solvent. AFM images were collected in air under ambient conditions using the tapping mode with a NanoscopeIII/ Bioscope scanning probe microscope from digital instruments. UV−vis spectra were measured in a quartz cell (light path of 1 cm) by using a HP 8453E instrument. Cyclic voltammetric (CV) measurements were performed in a standard threeelectrode cell, with Pt/C as the working electrode and Pt as the auxiliary electrode and a Ag/AgCl electrode (saturated KCl) as the reference electrode. A 1 mM PBA solution was prepared in CHCl3 and methenol (MeOH). Tetrabutylammonium perchlorate (0.1 M) was used as supporting electrolyte, and ferrocene was used as the internal reference electrode with the scanning rate set to 50 mV/s. Initial voltage, −1.9 V; segment, 2; sensitivity, 1e-4 A/V; scan range, −1.9 to 0 V. D


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Figure 3. AFM image (a) of PBA/β-CD complex in N10MI solution upon UV light irradiation for 1 h. Cross-sectional height profile (b) and 3Drendered representation (c).

crucial role in the formation of the S. officinalis-shaped materials. To further study the formation mechanism of the interesting S. officinalis-shaped materials, the sample was irradiated by UV light (365 nm), and the morphological changes were observed by TEM (Figure 2d) and AFM (Figure 3a). Irradiation for 1 h resulted in disappearance of the “thorns”, but the nanobelts with clean and smooth surfaces still existed (Figure 3b,c). This implies that the formation of thorns is related to the photorespective PBA molecules. In addition, it is generally known that the formation process of inclusion complexes requires an equilibrium of the host and guest molecules, with some of the guests being included or spread in the solution.23−25 Therefore, we speculate that the formation mechanism of the S. officinalis-shaped materials may involve the following three processes. First, SPBA/β-CD complexes are converted into PBA/β-CD complexes that can further form novel supramolecular polymeric structures in aqueous solution by accepting the hydrogen protons provided by the −COOH groups of N10MI molecules. These supramolecular “polymers” constituting the thorn structures are formed by bridging PBA/β-CD complex “monomers” via noncovalent interaction, such as hydrogen bonding, hydrophobic interaction, and π−π interaction26 (Figure 4). Meanwhile, the free SPBA molecules spreading in the solution obtain hydrogen protons to transform into PBA. Subsequently, the PBA is formed into nanobelts as a result of the template effect of the N10MI molecules. Eventually, the “thorns” tightly stick to the surfaces of the nanobelts to reduce the surface energy. After UV irradiation for 1 h, PBA molecules underwent a trans−cis isomerization process. Because of the mismatch in size for cis-PBA and β-CD molecules, the supramolecular polymatic structures were broken, leading to the disappearance of the “thorns” on the nanotubes. This mechanism can be proved by 1H NMR. Figure S7 shows that the PBA and β-CD molecules are observed, which suggests that the materials are composed of PBA and β-CD molecules; however, the intensity of β-CD proton signal is weaker than that of PBA, which also proves that the S. officinalis-shaped materials may be composed of PBA/β-CD complexes and free PBA molecules. This suggests that the compositions of the belts and “thorns” are different. To understand the effect of the hydrophobic chain length of SAILs on the formation of supramolecular materials, N12MI and N14MI, respectively, were used to replace N10MI. When 0.5

also supports the experimental results of UV−vis and SAXS measurements. To study photoinduced morphological transition, the sample was irradiated with UV light (365 nm) for 1 h, which leads to the nanorode-to-nanosphere shape transition (Figure 1c). This transformation process at pH 3.2 can be ascribed to the photoisomerization of the PBA molecule from the trans form to its cis form upon irradiation with UV light, which is proved by 1 H NMR spectra (Figure 1d). 2.2. Effects of SAILs and β-CD. Surface-active ionic liquids (SAILs), ionic liquids containing long alkyl chains that exhibit an amphiphilic character, have emerged as a novel kind of surfactants. COOH-functionalized imidazolium-based SAILs, NnMI (n = 10, 12, 14), were employed to investigate their regulating effect on the morphologies of the materials. N10MI (0.5 mM; 10 mL) was added to the 0.5 mM (10 mL) SPBA aqueous solution. As shown in an SEM image (Figure S5a), nanorod materials were generated, similar to the morphology of materials formed by the self-assembly of PBA molecules in the pH 3.2 solution at room temperature (Figure 1a). This suggests the main role of NnMI plays in the formation process of nanorod structures is to provide a hydrogen proton to achieve the transformation from SPBA to PBA molecules in aqueous solution. In addition, we also examined whether SPBA can be associated with β-CD and how strong the association is. Therefore, 0.5 mM β-CD (10 mL) was added into 0.5 mM SPBA (10 mL) aqueous solution under ultrasonic conditions for 1 h. UV−vis absorption of SPBA molecules occurs around 325 nm (Figure S6). Upon the addition of β-CD, the maximum absorption is red-shifted by 7 nm, and the absorption intensity is obviously enhanced, which suggests that the SPBA/β-CD complex could be formed in this solution.21,22 Then N10MI was added to the SPBA/β-CD complex solution under ultrasonic conditions. As a result, a TEM image shows the unexpected supramolecular materials with S. officinalis shape (Figure 2a,b) composed of nanobelts (with a width of ∼400 nm) covered with “thorns”. The nanobelt structures of the S. officinalisshaped materials were further observed from the SEM measurements (Figure 2c). More interestingly, nanobelts could further roll up to form nanotubes (Figure 2c). It is noted that without ultrasound, the S. officinalis-shaped nanoarchitectures could not be obtained, leading only to some irregular structures (Figure S5b). Thus, ultrasound plays a E


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and N12MI molecules did not exist in the materials, which suggests they mainly act as templates (Figure S8). Sphere-like particles with a diameter of 500 nm were produced in a 0.5 mM SPBA solution upon addition of 0.5 mM N14MI (Figure 5c). Because the cmc of N14MI is 0.237 mM,19 it can form spherical micelles at 0.5 mM. In the presence of N14MI, the pH of the solution is 2.7, and the SPBA molecules can turn into PBA molecules, which are solubilized into the micelles of N14MI and further self-assembly to form sphereshaped nanostructures as a result of the H-bond and π−π stacking interactions between the PBA molecules. When 0.5 mM N14MI (10 mL) was added into the PBA/β-CD complex solution, irregular particles were formed (Figure 5d). These results demonstrate that an appropriate hydrophobic chain length of the SAILs is essential to the fabrication of the supramolecular materials with the S. officinalis shape. 2.3. Effect of CTAB. Generally, the template of selfassemblies of surfactants plays a vital role in the formation of materials. A traditional surfactant, CTAB, was chosen to investigate how it influences the morphology of the as-prepared materials. We added PBA/ethanol solution (1 mL, 5.0 mM) to 5 mL of CTAB aqueous solution with different concentrations and stirred for 10 min. When the concentration of CTAB was 0.225 and 0.9 mM, a milky white slurry was immediately produced (Figure 6a I, II). However, upon addition of 4.5 mM CTAB (above its cmc, 0.9 mM),27,28 a transparent yellowish solution was obtained (Figure 6a III). In 0.225 and 0.9 mM CTAB systems, PBA molecular aggregates precipitated completely, and the solution became colorless and transparent after 24 h (Figure 6b I′, II′). The solution remained yellow and transparent (Figure 6b III′) in the presence of 4.5 mM CTAB after 30 days. This can be attributed to the CTAB micelles, in which PBA molecules can be solubilized, inducing the

Figure 4. Proposed model of supramolecular polymeric structure formed by PBA/β-CD upon UV light irradiation.

mM N12MI (10 mL) was added to a 0.5 mM SPBA (10 mL) solution or PBA/β-CD complexes solution, the resulting precipitates were irregular. Even the concentration of the N12 MI increased to 5 mM, above its critical micelle concentration (cmc) of 1.69 mM,18 and upon addition to the 0.5 mM SPBA solution, irregular precipitates were still obtained. When 5 mM N12MI solution was added into the PBA/β-CD complex solution, fibers with a length ranging from tens of micrometers to hundreds of micrometers were fabricated (Figure 5a, b). A 1H NMR spectrum shows β-CD

Figure 5. SEM (a,b) images of PBA/β-CD complexes (on the copper grids) in N12MI solution. TEM images of PBA (c) and SPBA/β-CD complexes (d) in N14MI solution. F


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Figure 6. Photographs of PBA molecules in the different concentrations of CTAB (I, I′: 0.225 mM; II, II′: 0.9 mM; III, III′: 4.5 mM) for 25 min (a) and 30 days (b). SEM images (c, d) of PBA materials in the presence of 4.5 mM CTAB solution.

Figure 7. Proposed model of the CTAB micelles inducing growth of PBA self-assembled materials.

Figure 8. Cyclic voltammogram of PBA self-assembled materials in different solvent: (a) CHCl3 and (b) MeOH. Fc = ferrocene with a scanning rate of 50 mV/s at 25 °C.

2.4. Cyclic Voltammetry Measurements. Generally, because of intermolecular π−π stacking and other interactions, organic semiconductors can form conducting pathways that have potential applications for photoelectrical nanodevices.29−31 A cyclic voltammogram (CV) of nanorods selfassembled by PBA molecules in CHCl3 is depicted in Figure 8a

fabrication of the aggregates of spheres. SEM images show that the aggregates of spheres were observed (Figure 6c,d). A 1H NMR spectrum (Figure S9) proves that CTAB was not observed in the materials. This implies that the formed micelles act as templates directing the growth of PBA self-assembled nanomaterials (Figure 7). G



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and shows an irreversible reduction peak with Ereduction = −1.33 eV. On the basis of the onset potentials for the reduction, we performed a series of calculations. The zero vacuum level of ferrocene was taken as 4.8 eV. The equations employed for calculations of LUMO and HOMO energies are ELUMO = −e(Ereduction − Efer + 4.8), ELUMO = EHOMO − hc/λoffset, where hc = 1240 eV, λoffset = 380 nm, and Efer = 0.39 eV (versus Ag/ AgCl). The LUMO energy of PBA self-assemblies calculated from CV is −3.08 eV. The LUMO energy is higher than that of TiO2, −4.2 eV; perylene diimides, −3.5/-3.7 eV;32 and naphthalene diimide, −3.55/-3.68 eV.33 The HOMO energy of PBA nanomaterial was estimated to be −6.34 eV. In addition, we performed CV in MeOH, where PBA did not form aggregates. Compared with the aggregated state of PBA in CHCl3, an irreversible reduction peak shifted to −1.47 (Figure 8b). The calculated LUMO and HOMO energies are −2.94 and −6.20 eV. The difference can be ascribed to the disappearance of π−π stacking in MeOH. DFT calculations at the level of B3LYP/6-31G were performed for PBA self-assembled materials. The calculated HOMO and LUMO energies are −5.86 and −2.50 eV (Figure S10), respectively, which basically agree with the results obtained by the CV measurements with minor difference. This is because neither solvent effects nor intermolecular interaction are involved in the theoretical calculations. It is worth noting that the band gap obtained by the CV measurements and DFT calculations are in good agreement. It is possible to tune the HOMO and LUMO energies deliberately by choosing appropriate electron donors and acceptors as well as linkers that connect the electron donors and acceptors covalently.29 On the basis of the above information, the LUMO energy of the materials is low enough to enable efficient electron injection from electrodes. the HOMO−LUMO energy level implies that the PBA supramolecular material shows electron injection capabilities.11,34

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2014GSF117001), and the Natural Science Foundation of Shandong Province of China (No. ZR2011BM017).

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In this paper, we employed a supramolecular self-assembly strategy to build stimulus-responsive materials that contain both pH- and photoresponsive properties. Furthermore, we use a SAS strategy to guide and construct supramolecular materials with various morphologic characteristics. Both CV measurements and DFT calculations indicate the optoelectrical behavior of the materials self-assembled by the PBA molecules. This work provides examples for the possibility of constructing nanomaterials that are expected to have potential applications in optoelectronics and fast switching, etc.

S Supporting Information *

Further characterization (TEM and SEM), spectroscopic data (UV/vis, SAXS, and 1H NMR) and theoretical calculation of synthesized compounds are reported. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03220.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. H


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