Fabrication of nanofibres with azopyridine compounds in various acids and solvents

Abstract

Supramolecular self-organization behaviours of one azopyridine compound were systematically studied in a series of inorganic acids and various organic solvents. Different morphologies of the resultant low-molecular-weight compounds were obtained in diverse environments. The acid dissociation constant has a critical effect on the self-assembly since the nanofibres were successfully obtained only when the acid dissociation constant is between 2.12 and −3.00. Besides, the organic solvent also influenced the process of fibre formation. In acetone, the self-organized fibrous material resulted in the occurrence of gelation. The driving force for nanofibre formation may be due to the existence of several non-covalent interactions, such as hydrogen bonds, ionic bonds and the aggregation of chromophores. Then, a possible schematic illustration for the fabrication of supramolecularly self-assembled fibres was proposed based on TEM and XRD measurements. That is, visible micron-fibres were made from nanofibres through stacking interaction forces of azopyridine chromophores.

---

1. Introduction

In general, myelinated axons play an important role in the living body, which has been inspiring us to artificially design them by fabricating nanofibres.^1,2 These have been successfully obtained upon supramolecular self-assembly by choosing special low-molecular-weight (LMW) compounds or macromolecular materials in suitable environments.^3–6 Among various approaches, the amphiphilic self-assembly of LMW soft matter in acidic environments, which usually exist in biological systems (e.g., gastric juice), has been attracting much attention.¹ In this regard, azopyridine derivatives have been ideal candidates among the promising LMW compounds for preparing nanofibres in recent years, because of their unique combination of amphiphilic properties, light-active performance related to azobenzene chromophores and a capability for self-assembly through pyridyl groups via supramolecular functions.^7–12

In the fabrication process using LMW compounds, the control and modulation of the self-assembled nanofibres are the preliminary concerns for endowing them with special functionalities. Usually, the self-assembled processes are often influenced by the chemical structure of the component LMW molecule and environmental factors.¹³ By way of chemical modification, the regulation of nanofibres has been easily achieved recently.^2,8,14,15 For example, we reported two types of aggregation based on amphiphilic LMW azopyridine derivatives by changing the alkyl substituent on benzene groups.^7,8,14 Meanwhile, Aoki et al. showed two ways of controlling the macroscopic morphology of fibrous organizations of amphoteric azopyridine carboxylic acids by tuning the strength of the π–π stacking among the component molecules.¹³ However, environmental factors such as acidity, solvent or temperature have seldom been considered in the past few years.¹⁶ Hence, the development of such a method still remains a major challenge, and the related research should undoubtedly broaden the scope of designing soft matter like nanofibres.

In this paper, we report the fabrication of self-organized fibres using a relatively simple molecule of azopyridine (A12AzPy) with a series of inorganic acids in diverse organic solvents, and control the fibrous morphologies upon supramolecular self-assembly by changing the acid dissociation constant (pK_a values) or the solution. We try to find out the regulation method for the resultant nanofibres. The self-organization process and the driving forces for fibre formation, such as hydrogen bonds, ionic bonds and the aggregation of chromophores, will also be studied in detail.

2. Experimental

2.1 Fabrication of the fibres

As shown in Scheme 1, the materials for fabrication of the self-assembled fibres were prepared from one azopyridine compound (A12AzPy) in organic solutions with the addition of an excess of inorganic acids. Here, tetrahydrofuran (THF), acetone, dimethyl formamide (DMF) and dimethylsulfoxide (DMSO) were used. The fibrous materials were spontaneously formed upon evaporation of one drop of the resultant solution on the surface of a glass substrate under room light.

Scheme 1 Illustration of the fabrication process of the supramolecular fibres with one azopyridine compound A12AzPy in a series of inorganic acids and organic solvents.

2.2 Characterization

¹H NMR spectra of the compounds were recorded on a Bruker Avance III 400. Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer DSC 8000 with a heating and a cooling rate of 10 °C min⁻¹ under a dry nitrogen purge. The morphology of the fibres was observed by using scanning electron microscopy (SEM, Zeiss EVO18, Germany), polarizing optical microscopy (POM, Zeiss Axio Scope A1 Microscope) and transmission electron microscopy (TEM, Tecnai G² F20). The fluorescence quantum yield was measured using a luminescence spectrometer (Perkin-Elmer LS55). The powder X-ray diffraction (XRD) analysis was carried out on a Philips X’pert pro with a heating rate of 10 °C min⁻¹. Fourier infrared spectroscopy (FTIR) was performed using a Nicolet 510P IR spectrometer. UV and visible absorption spectroscopy was performed with a Perkin-Elmer lambda 750 spectrometer.

3. Results and discussion

3.1 Fibres formed in inorganic acids

Fig. 1 shows optical micrographs and SEM pictures of the self-organized materials formed from A12AzPy in various inorganic acids upon evaporation of THF. The pK_a values of all the inorganic acids used are given in Table S1.† All of the molecular assemblies exhibited characteristic structures. Among them, the self-assembled structures with nitric acid (Fig. 1a and b), sulfuric acid (Fig. 1e and f) and phosphoric acid (Fig. 1i and j) showed fibrous morphologies, and their pK_a values are −1.3, −3.0 and 2.1, respectively. On the other hand, no fibres were obtained using perchloric acid (Fig. 1c and d) and boric acid (Fig. 1g and h), and their pK_a values are −10 and 9.2, respectively. These results indicate that the self-assembled fibres can be obtained when the pK_a values of the inorganic acids used are neutral or slightly negative. When their pK_a values are exorbitant or low, the pyridine groups in the azopyridine derivatives cannot be protonated, and the ionic interactions cannot be obtained because of the weak activity between the pyridine group and the acid in these cases. As a result, azopyridinium ions were formed only when the pK_a values were appropriate, acquiring amphiphilic properties from the hydrophobic azopyridine compound.^7,8 From analysis of the SEM images, the width of the supramolecular fibres was in the range of several millimetres to 100 micrometres. Moreover, all the fibres exhibited obvious birefringence upon POM observation, demonstrating ordered structures that self-assembled upon fibre formation.⁸

Fig. 1 Characterization of the fabricated fibres formed with 12AzPy in inorganic acids from THF solutions (25 mg mL⁻¹). Nitric acid (a and b), perchloric acid (c and d), sulfuric acid (e and f), boric acid (g and h) and phosphoric acid (i and j). Pictures a, c, e, g and i are SEM images, and b, d, f, h and j are optical images.

3.2 Fibres formed in halogen acids

Fig. 2 shows optical micrographs and SEM images of the self-assembled materials formed in halogen acids from THF solutions. A12AzPy self-organized into fibrous material only in hydrochloric acid (Fig. 2c and d), with a pK_a value of −2.2, demonstrating a similar result to those obtained from the above-mentioned inorganic acids. Consequently, these morphological correlations suggest that the pK_a value of the acid plays an important role in the process of fibre formation via self-assembly, since the strength of ligand binding (i.e. the binding free energy) is usually dependent on the protonation state of the ionisable residues and functional groups in the active site.¹⁵ Moreover, upon ligand binding, the pK_a values of the ionisable groups may be altered, resulting in the uptake or release of protons.^17–20

Fig. 2 Characterization of the fabricated fibres formed in hydrofluric acid (a and b), hydrochloric acid (c and d), hydrobromic acid (e and f) and hydroiodic acid (g and h). Pictures a, c, e and g are SEM images, and b, d, f and h are optical images (THF, 25 mg mL⁻¹).

From the above-mentioned results, self-assembled fibres were successfully fabricated using an amphiphilic LMW azopyridine compound (A12AzPy) in THF solutions with the existence of hydrochloric acid, phosphoric acid, nitric acid or sulfuric acid. These inorganic acids show pK_a values ranging between 2.12 and −3.00. Meanwhile, the pH values of A12AzPy with various inorganic acids in THF were also measured to obtain detailed information on the fibre formation conditions, as shown in Table S3.† The change in the pH value of the A12AzPy solutions before and after addition of the acids indicated that the formation of azopyridinium ions should be due to ligand binding.⁸

3.3 Fibres formed in different organic solvents

The morphologies of the fabricated fibres obtained using several polar solvents were investigated with SEM, as shown in Fig. 3, and the corresponding optical and POM pictures are given in Fig. S2.† Interestingly, network structures consisting of bundles of fibres with diameters of 1–2 μm were obtained from acetone (Fig. 3a), which are smaller than the fibres from THF (Fig. 1e and f). These entangled fibrous materials might be responsible for the immobilization of the solvent molecules to form organogels. Meanwhile, the diameters of the fibres formed from DMF and DMSO were in the range of 20–40 μm, as shown in Fig. 3b and c. Additionally, fibrous morphologies were formed in apolar toluene or weakly-polar chloroform, whereas polar dichloromethane and apolar petroleum ether did not produce fibrous morphologies, as presented in Fig. S3.† These results suggest that the organic solvent greatly affected the fabricated morphology of the self-organized fibres and that the fibrous morphologies have no direct correlation with the solvent polarity. Furthermore, the gelating process of the fibres might depend on the properties of the solvent.

Fig. 3 SEM images of the fabricated fibres formed with sulfuric acid from different organic solvents at room temperature. (a) Acetone, (b) DMF, and (c) DMSO.

3.4 TEM analysis

Fig. 4 shows the TEM image of the fabricated fibres formed from A12AzPy in sulfuric acid upon evaporation of THF, which is far different from that of the pure A12AzPy (see Fig. S4†). The lattice spacing of the fibre crystals was obtained as 0.344 nm, which should be ascribed to the molecular spacing of the nanofibres.⁷ Comprehensive analysis of the results from the TEM, SEM and optical micrographs allows us to propose a reasonable packing arrangement upon fibre formation in different inorganic acids. Taking advantage of these characterization results from the molecular and nano to micrometer scales, it can be inferred that the visible fibres should be formed with packing of the nanofibres.

Fig. 4 TEM image of the fabricated fibres formed from A12AzPy in sulfuric acid upon evaporation of THF.

3.5 ¹H NMR analysis of the LMW fibres

The ¹H NMR spectra of the samples in DMSO-d6 were examined to further study the self-assembly process, as shown in Fig. 5. Compared to the pristine molecule A12AzPy, the protons of the pyridine rings of the LMW fibres experienced a pronounced downfield shift upon the addition of sulfuric acid, indicating the occurrence of a strong electron-withdrawing effect on the pyridine ring. Especially, there are three protons that exhibited an obvious change in their chemical shift. The proton (a) at 8.8 ppm shifted to 9.0 ppm, and the proton (b) at 7.7 ppm moved to 8.2 ppm. The third proton (c) at 7.9 ppm also shifted a little downfield (to about 8.0 ppm). More importantly, the relative positions of protons (b) and (c) showed a reversal. This remarkable evidence undoubtedly indicates a strong interaction between A12AzPy and sulfuric acid,^21,22 leading to the formation of azopyridinium ions and self-assembly, which has been thought to play an important role in the fabrication of LMW fibres.^7,8

Fig. 5 The ¹H NMR spectra of the samples in DMSO-d6 at room temperature. (a) A12AzPy with the addition of sulfuric acid, (b) pure A12AzPy.

3.6 FT-IR analysis

FT-IR spectra of the pristine compound and the self-assembled fibres were obtained, and the results are shown in Fig. 6 and S5.† Generally, the stability of the hydrogen bonds in the fibres can be directly confirmed from temperature-dependent FT-IR spectra.²³ With heating of the self-assembled fibre sample, the appearance of three bands centered at 2489 cm⁻¹, 1967 cm⁻¹ and 2048 cm⁻¹ became clear, as shown in Fig. 6c, which should originate from the formation of hydrogen bonds as seen from the ν_OH of sulfuric acid and Fermi resonance bands of the pyridyl groups (–OH⋯N).^24,25 While after further heating to 150 °C, the hydrogen bonds became very weak. These results indicate that the hydrogen bonds still existed even for the azopyridinium ions when the supramolecular fibres were formed from A12AzPy in sulfuric acid.

Fig. 6 The temperature dependent FT-IR spectra of the samples. (a) A12AzPy, (b) H₂SO₄ and (c) self-assembled fibres.

3.7 UV-vis analysis

To investigate the intermolecular interactions of A12AzPy with inorganic acids, UV-vis absorption spectra of the samples in different states were measured. As shown in Fig. 7a, the maximum absorption peak (λ_max) of A12AzPy was observed at 355 nm in THF solution, whereas λ_max of the LMW fibres appeared at 394 nm in THF solution (Fig. 7b). This red-shift of 39 nm demonstrates the formation of ionic bonding between A12AzPy and H₂SO₄ in THF.⁸ Similar results (red-shifts) were also observed in acetone, DMF and DMSO solutions, as shown in Fig. S6.† Compared with the spectra of the LMW fibres in THF solution, a reduced absorption and a blue shift were obtained for the film state (Fig. 7b and c). This blue shift should be caused by the H-aggregation of the chromophores (Fig. 7f), which correlates with molecular packing models and may ultimately be used to confirm the formation of a supramolecular architecture.^26–34 This type of H-aggregation refers to the angle between the coplanar transition dipoles and their interconnecting axis being greater than 54.7°, which indicates a parallel orientation of the neighbouring transition dipoles as shown in Fig. 7f.^35–39 In the present system, besides the normal hydrogen bonding and ionic bonding between the acid and the pyridyl group of A12AzPy, H-aggregation also exists among the azopyridine groups, which provides an additional attractive interaction in the film state.

Fig. 7 UV-vis absorption spectra (above figure) and packing modes (below scheme) obtained with the help of inorganic acids. Pure A12AzPy in THF solution (a and d), self-assembled fibres in THF solution (b and e) and in film state (c and f).

Taking into account that some azobenzene nanofibres show characteristics of enhancement of aggregation-induced emission,^34,40 the fluorescence quantum yield of the LMW fibres was also examined. As shown in Fig. S7,† the fibres exhibited a very weak emission fluorescence at around 523 nm and the measured fluorescence quantum yield (Φ_f) was 0.065. This demonstrates that the self-organization of A12AzPy in inorganic acid should be different from other compounds.^34,40

3.8 XRD analysis

Powder XRD measurements were carried out to investigate the periodical molecular arrangement of the self-organized materials.^41–43 The obtained d-spacings correspond to either the thickness of a molecular sheet or the diameter of a micellar rod.^24,25 Fig. 8 shows the temperature-dependent XRD patterns of A12AzPy and the self-assembled fibres at different temperatures. As shown in Fig. 8a, many sharp reflection peaks appeared for the LMW fibres compared to the pristine A12AzPy, demonstrating the existence of novel organization structures in the fabricated fibres upon self-assembly.

Fig. 8 XRD patterns of the LMW fibres and A12AzPy upon heating at different temperatures. (a) 25 °C, (b) 60 °C, (c) 80 °C, (d) 95 °C, and (e) 105 °C.

The XRD results are also consistent with the foregoing results derived from DSC and FT-IR measurements. As shown in Fig. 8b, the largest d-spacings (d = 2.76, 1.37, 0.91 nm) for the LMW fibres disappeared at 60 °C, but several smaller d-spacings still existed. These can be attributed to the deconstruction of bigger micron-fibres into smaller ones, leading to broken hierarchical structures. On heating to 80 °C, just beyond the melting point of A12AzPy (74 °C), the organization structures of A12AzPy disappeared, as shown in Fig. 8c. On the other hand, the ordered structures of the LMW fibres can still be observed at this temperature, indicating that the LMW fibres possess more stable ordered structures than A12AzPy. These results also demonstrate that stronger interactions could be induced in the fibres fabricated from A12AzPy with the help of inorganic acids. On further heating, up to 95 °C, the larger d-spacings (d = 0.57, 0.47 nm) of the LMW fibres disappeared, and the two smallest d-spacings (d = 0.35, 0.30 nm) still exist in Fig. 8d. These changes might be ascribed to the deconstruction of smaller fibres into nanofibres. Finally, these smallest ordered structures in the LMW fibres disappeared as they were heated up to 105 °C (Fig. 8e). Obviously, the most stable microstructures are highly in accordance with the observed results of the nanofibres from the TEM picture shown in Fig. 4.^44,45

Based on these results, it can be concluded that several interactions such as hydrogen bonds, ionic bonds and H-aggregation should contribute to the formation of self-organized fibres. In addition, two possible steps could be involved in the self-assembled processes, which can be described as shown in Fig. 9. One is self-assembly at the molecular and nano level, in which molecular stacking happens under the function of hydrogen bonding, ionic bonding and aggregation of chromophores. These interactions are formed simultaneously upon addition of the inorganic acids into A12AzPy solutions. The molecular spacing of the nanofibres is about 0.344 nm, corresponding to the TEM and XRD results. The other process is the micron-level self-assembly, in which the nanofibres further stack together into visible micron-fibres upon evaporation of the organic solvents. Such bigger fibres can be easily identified from POM and SEM observation. Obviously, the second process of self-assembly is more unstable than the first process, and can be broken by the deconstruction of micron-level fibres into nanofibres upon heating.

Fig. 9 Pictorial presentation of the possible mechanism of the self-organization fibre formation.

4. Conclusions

In conclusion, a series of supramolecularly self-assembled fibres were fabricated with one azopyridine compound in diverse inorganic acids and various organic solvents. Both the pK_a values of the inorganic acids and the solvents greatly influence the formation of fibrous morphologies. Fibres can be successfully obtained only when the pK_a values of the acid are between 2.12 and −3.00. Moreover, smaller fibres were obtained in volatile solvents such as acetone, resulting in the formation of an organogel upon supramolecular self-assembly. A possible mechanism of fibre formation was proposed through two processes. Nanofibres are first formed on the molecular and nano scale by molecular stacking under the function of hydrogen bonds, ionic bonds and H-aggregation. Then the micron-level fibres are obtained through further stacking of the nanofibres. The control and modulation of self-assembled nanofibres using acidity and solvents will be of great significance for broadening the scope of the designing of soft matter, like nanofibers and organogels.

Acknowledgements

Yang is thankful for the support from the Major Project of International Cooperation of the Ministry of Science and Technology (Grant no. 2013DFB50340), the Key Project of the National Natural Science Foundation of China (Grant no. 51333001), and the Doctoral Fund of Chinese Ministry of Education (Grant no. 20120001130005). Yu acknowledges the National Natural Science Foundation of China (Grant no. 51322301) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry for the support. Quan is thankful for the support from the National Natural Science Foundation of China (Grant no. 51403017).

Notes and references

1. N. Kimizuka, Adv. Polym. Sci., 2008, 219, 1–26.
2. A. Klug, Angew. Chem., Int. Ed. Engl., 1983, 22, 565–582.
3. C. Zhan, P. Gao and M. Liu, Chem. Commun., 2005, 4, 462–464.
4. V. K. Praveen and J. George, Acc. Chem. Res., 2008, 41, 769–782.
5. H. Kong and J. Jang, Langmuir, 2008, 24, 2051–2056.
6. K. Fukuhara, T. Yamada and A. Suzuki, Ind. Eng. Chem. Res., 2012, 51, 10117–10123.
7. W. Zhou, T. Kobayashi, H. Zhu and H. F. Yu, Chem. Commun., 2011, 47, 12768–12770.
8. W. Zhou and H. F. Yu, ACS Appl. Mater. Interfaces, 2012, 4, 2154.
9. Y. J. Chen, H. F. Yu, L. Y. Zhang, H. Yang and Y. F. Lu, Chem. Commun., 2014, 50, 9647–9649.
10. Y. J. Chen, H. F. Yu, M. H. Quan, L. Y. Zhang, H. Yang and Y. F. Lu, RSC Adv., 2015, 5, 4675.
11. H. Liu, T. Kobayashi and H. F. Yu, Macromol. Rapid Commun., 2011, 32, 378–383.
12. H. F. Yu, H. Liu and T. Kobayashi, ACS Appl. Mater. Interfaces, 2011, 3, 1333–1340.
13. M. Moriyama, N. Mizoshita and T. Kato, Bull. Chem. Soc. Jpn., 2006, 79, 962–964.
14. W. Zhou and H. F. Yu, RSC Adv., 2013, 3, 22155.
15. K. Aoki, M. Nakagawa and K. Ichimura, J. Am. Chem. Soc., 2000, 122, 10997–11004.
16. J. Li, B. Yuan, X. Liu, R. Wang and X. Wang, Soft Matter, 2013, 9, 435–442.
17. D. C. Bas, D. M. Rogers and J. H. Jensen, Proteins: Struct., Funct., Bioinf., 2008, 73, 765–783.
18. Y. Gao, Z. H. Xu, S. W. Chen, W. W. Gu, L. L. Chen and Y. P. Li, Int. J. Pharm., 2008, 359, 241–246.
19. T. K. Harris and G. J. Turner, IUBMB Life, 2002, 53, 85–98.
20. M. Oliveberg, V. L. Arcus and A. R. Fersht, Biochemistry, 1995, 34, 9424–9433.
21. Y. P. Zhang, C. S. Liang, H. X. Shang, Y. Ma and S. M. Jiang, J. Mater. Chem. C, 2013, 1, 4472–4480.
22. L. J. McAllister, C. Präsang, J. P.-W. Wong, R. J. Thatcher, A. C. Whitwood, B. Donnio, P. O’Brien, P. B. Karadakov and D. W. Bruce, Chem. Commun., 2013, 49, 3946.
23. W. L. He, G. H. Pan, Z. Yang, D. Y. Zhao, G. G. Niu, W. Huang, X. T. Yuan, J. B. Guo, H. Cao and H. Yang, Adv. Mater., 2009, 21, 2050–2053.
24. M. Nakagawa, D. Ishii, K. Aoki, T. Seki and T. Iyoda, Adv. Mater., 2005, 17, 200–205.
25. M. Nakagawa and K. Ichimura, Chem. Lett., 1999, 11, 1205–1206.
26. B. Qi and Y. Zhao, Langmuir, 2007, 23, 5746–5751.
27. A. Zouni, R. J. Clarke and J. F. Holzwarth, J. Phys. Chem., 1994, 98, 1732–1738.
28. M. I. Viseu, A. S. Tatikolov, R. F. Correia and S. M. B. Costa, J. Photochem. Photobiol., A, 2014, 280, 54–62.
29. V. V. Shelkovnikov, F. A. Zhuravlev, N. A. Orlova, A. I. Plekhanov and V. P. Safonov, J. Mater. Chem., 1995, 5, 1331–1334.
30. M. E. Daraio and E. S. Román, Helv. Chim. Acta, 2001, 84, 2601–2614.
31. S. Das and P. V. Kamat, J. Phys. Chem. B, 1999, 103, 209–215.
32. M. Morikawa and N. Kimizuka, Chem. Commun., 2012, 48, 11106–11108.
33. H. Menzel, B. Weichart, A. Schmidt, S. Paul, W. Knoll, J. Stumpe and T. Fischer, Langmuir, 1994, 10, 1926–1933.
34. H. Ren, D. Chen, Y. Shi, H. F. Yu and Z. F. Fu, Polym. Chem., 2015, 6, 270–277.
35. T. Kunitake, Angew. Chem., Int. Ed. Engl., 1992, 31, 709–726.
36. M. Han and K. Ichimura, Macromolecules, 2001, 34, 90–98.
37. X. Tong, L. Cui and Y. Zhao, Macromolecules, 2004, 37, 3101–3112.
38. M. Kasha, H. R. Rawls and M. Ashraf El-Bayoumi, Pure Appl. Chem., 1965, 11, 371–392.
39. S. Wu, L. F. Niu, J. Shen, Q. J. Zhang and C. Bubeck, Macromolecules, 2009, 42, 362–367.
40. M. Han, S. J. Cho, Y. Norikane, M. Shimizu, A. Kimura, T. Tamagawa and T. Seki, Chem. Commun., 2014, 50, 15815–15818.
41. K. Aoki, M. Nakagawa, T. Seki and K. Ichimura, Bull. Chem. Soc. Jpn., 2002, 75, 2533–2539.
42. L. Leiserowitz and M. Tuval, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1978, 34, 1230–1247.
43. T. Shimizu and M. Masuda, J. Am. Chem. Soc., 1997, 119, 2812–2818.
44. H. Zhang, R. Hao, J. K. Jackson, M. Chiao and H. F. Yu, Chem. Commun., 2014, 50, 14843–14846.
45. H. F. Yu, J. Mater. Chem. C, 2014, 2, 3047–3054.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02089a

---