Langmuir 2008, 24, 7635-7638
7635
Switch from Intra- to Intermolecular H-Bonds by Ultrasound: Induced Gelation and Distinct Nanoscale Morphologies Yaobing Wang, Chuanlang Zhan, Hongbing Fu,* Xiao Li, Xiaohai Sheng, Yongsheng Zhao, Debao Xiao, Ying Ma, Jin Shi Ma, and Jiannian Yao* Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, and Gradate School of Chinese Academy of Science, Beijing 100080, P. R. China ReceiVed May 15, 2008 During cooling of the (R)-N-Fmoc-Octylglycine (Fmoc-OG)/cyclohexane solution, gelation is observed exclusively when ultrasound is used as an external stimulus, while deposit is obtained without sonication. The xerogel consists of entangled fibrous network made by interconnected nanofibers, while the deposit comprises large numbers of unbranched nanowires. It is found that the Fmoc-OG molecules form bilayer structures in both the deposit and the gel. However, the ratio (R) between the Fmoc-OG molecules in a stable intramolecular H-bonding conformation and those in a metastable intermolecular H-bonding conformation can be tuned by the ultrasound, R (deposit) > R (gel). The increased population of the intermolecular H-bonding Fmoc-OG molecules induced by the ultrasonication facilitates to the interconnection of nanofibers for the formation of the fibrous network, and therefore gelation. The alteration in the morphologies and properties of the obtained nanomaterials induced by the ultrasound wave demonstrates a potential method for smart controlling of the functions of nanomaterials from the molecular level.
In recent years, nanomaterials of organic small molecules have attracted growing attention in the development of novel optoelectronic devices due to their low production cost and tunable optical and electronic properties.1–6 Properties of organic nanomaterials not only are decided by the properties of the precursory molecules, but also depend sensitively on the supramolecular aggregation in these materials. However, approaches available to control the supramolecular aggregation is still limited.7–9 Recently, the use of ultrasound waves has become a novel way to control the supramolecular aggregation by modifying the noncovalent intermolecular interactions. Sijbesma and co-workers reported that ultrasound can destroy the gel formed by palladium organometallic polymers by breaking the metalorganic coordination bonds.10–12 Conversely, Naoto and Takaya described the gelation of palladium metal complexes under ultrasound treatment, because the varied molecular conformations induced by ultrasound tend toward polymerization and gelation.13,14 This may provide a new methodology for smartly controlling the molecular self-assemble. * E-mail:
[email protected];
[email protected]. (1) Sarikaya, M.; Tamerler, C.; Jen, A. K.-Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577–585. (2) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (3) Channon, K. J.; Devlin, G. L.; Magennis, S. W.; Finlayson, C. E.; Tickler, A. K.; Silva, C.; MacPhee, C. E. J. Am. Chem. Soc. 2008, 130, 5487–5491. (4) Forrest, S. R. Chem. ReV 1997, 97, 1793–1896. (5) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S. H.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Nature 2006, 444, 913– 917. (6) ushey, M. L.; Nguyen, T. Q.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 8264–8269. (7) Tian, Z. Y.; Chen, Y.; Yang, W. S.; Yao, J. N.; Zhu, L. Y.; Shuai, Z. G. Angew. Chem., Int. Ed. 2004, 43, 4060–4064. (8) Wang, Y. B.; Fu, H. B.; Peng, A.; Zhao, Y. S.; Ma, J. S.; Ma, Y.; Yao, J. N. Chem. Commun. 2007, 1623–1625. (9) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J. L.; Oitker, R.; Yen, M.; Zhao, J. C.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390–7398. (10) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2006, 45, 2334–2337. (11) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2004, 43, 4460–4462. (12) Paulusse, J. M. J.; van Beek, D. J. M.; Sijbesma, R. P. J. Am. Chem. Soc. 2007, 129, 2392–2397.
Manipulation of the morphology of organic nanomaterial, by altering the supramolecular self-assembly by ultrasound, has met with limited success.15–22 Bardelang and co-workers reported that ultrasound reshapes the morphologies of dipeptide particles,15 due to the sonocrystallization effect. Yi and Huang have prepared organic materials with various shapes by ultrasound treatment from a cholesterol-based molecule, as the molecular conformation can be slightly altered in gel by either thermal process or sonication.17 Herein, we reported that, during cooling of the hot solution of Fmoc-OG/cyclohexane, the gel was formed exclusively by using the ultrasound as an external stimulus, while the deposit was obtained without sonication. We found that the ultrasound resulted in a switch from intramolecular H-bond to intermolecular H-bond. The varied molecule geometries and intermolecular interactions lead to the different nanoscale morphologies from unbranched nanowires to entangled fibrous network, which is responsible for the gelation under the ultrasonic treatment. Brief ultrasound irradiation (0.40 W/cm2, 40 KHz, R (gel)]; while in xerogel, the νCdO bands of carboxyl occur at 1718.6 cm-1, corresponding to the intermolecular lateral bonded and bifurcated carboxyl groups. The νCdO of the carbamate unit peaks at 1691.6 and 1695.4 cm-1 reveal that the carbonyl groups in the carbamate unit form from different H-bonding modes in both the deposit and the gel, respectively, likely intramolecular H-bonding with -OH of the carboxyl group in the deposit, while intermolecular H-bonding with the -NH unit in the gel by consideration of the possible H-bonding mode between carboxyl and carbamate groups.34,35 The νNH bands at 1540.8 cm-1 and 1541.9 cm-1 suggest that -NH groups form H-bonds both in the deposit and the gel, as supported by the temperature-dependent 1H NMR studies. The upfield shift of -NH from 5.0 to 4.8 ppm for the gel with increasing temperature and the -NH skipping from 5.1 to 4.8 ppm for the deposit indicate the H-bonded states of -NH groups at RT (Supporting Information Figure S2). On the basis of the above results, we propose a viable model to explain the formation of the different morphologies induced by ultrasound waves, as shown in Figure 4. When cooling the Fmoc-OG/cyclohexane solution without ultrasonic perturbation, the Fmoc-OG molecules incline to adopt the stable conformation by forming intramolecular H-bonds between -OH of carboxyl and -CdO of carbomate (Figure 4d). The intermolecular interaction between -CdO of carboxyl and -NH of carbamate and the π-π stacking of fluorene functions drive the Fmoc-OG molecules to self-assemble into nanowires.36,37 It is known that the ultrasound wave is able to provide heat and pressure on the
(29) Hirst, J. D.; Colella, K.; Gilbert, A. T. B. J. Phys. Chem. B 2003, 107, 11813–11819. (30) Oakley, M. T.; Hirst, J. D. J. Am. Chem. Soc. 2006, 128, 12414–12415. (31) Simonyi, M.; Bikadi, Z.; Zsila, R.; Deli, J. Chirality 2003, 15, 680–698. (32) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: exciton coupling in organic stereochemistry, 2nd ed.; Oxford University Press: New York, 1983.
(33) Moniruzzaman, M.; Sundararajan, P. R. Langmuir 2005, 21, 3802–3807. (34) Gao, P.; Zhan, C. L.; Liu, L. Z.; Zhou, Y. B.; Liu, M. H. Chem. Commun. 2004, 1174–1175. (35) Zhan, C. L.; Gao, P.; Liu, M. H. Chem. Commun. 2005, 462–464. (36) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263– 2266. (37) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133–3359.
Figure 3. Small-angle X-ray diffraction analysis of the deposit (upper panel) and the gel (lower panel) formed by Fmoc-OG. Insert: schematic packing molecules in the deposit (upper panel) and gel (lower panel) of Fmoc-OG.
7638 Langmuir, Vol. 24, No. 15, 2008
Letters
Figure 4. Proposed scheme of molecular packing: (a) molecular structure of Fmoc-OG; (b) nanowires in the deposit; (c) fibrous network in the gel; (d) bilayer structure formed by intramolecular H-bonds of Fmoc-OG; (e) bilayer structure formed by intermolecular H-bonds of Fmoc-OG.
nanosecond scale as well as the extreme cooling rates.38,39 The energy provided by the ultrasonication is enough to cleave the intramolecular H-bonding between the -OH of carboxyl and the -CdO of carbamate, offering possibilities for the formation of intermolecular H-bonding between carboxyl groups and carbamate groups, respectively (Figure 4e). The interpenetrating H-bonding interactions result in the extended fibrous network and therefore gelation. Differential scanning calorimetry (DSC) reveals that the different melting enthalpy could further confirm the ultrasound results in gelation and the distinct nanoscale morphologies. Supporting Information Figure S3 shows typical DSC traces obtained by heating the powder and the samples at 10.0 K min-1. The thermogram of the deposit shows the endothermic peaks at Tm ) 141.34/144.11 °C and melting enthalpy ∆Hm ) 37.09 KJ/mol, that of powder at Tm ) 141.54 °C and ∆Hm ) 45.09 kJ/mol, and that of xerogel at Tm ) 136.44 °C and ∆Hm ) 24.73 kJ/mol (Supporting Information Table S1). Thus, the xerogel tends to exhibit endothermic peaks at a lower temperature than that of the deposit. This is likely attributed to the fact that ultrasound did not induce a preferable molecular conformation in gel as compared with that in deposit.40,41 (38) Cravotto, G.; Cintas, P. Angew. Chem., Int. Ed. 2007, 46, 5476–5478. (39) Cai, Y.; Pan, X.; Xu, X.; Hu, Q.; Li, L.; Tang, R. Chem. Mater. 2007, 19, 3081–3083. (40) Zhang, Y.; Gu, H. W.; Yang, Z. M.; Xu, B. J. Am. Chem. Soc. 2003, 125, 13680–13681. (41) Binder, W. H.; Smrzka, O. W. Angew. Chem., Int. Ed. 2006, 45, 7324– 7328. (42) John, G.; Zhu, G. Y.; Li, J.; Dordick, J. S. Angew. Chem., Int. Ed. 2006, 45, 4772–4775.
In conclusion, during cooling of the Fmoc-OG/cyclohexane solution, gelation is observed exclusively when ultrasound is used as an external stimulus, while deposit is obtained without sonication. The xerogel consists of an entangled fibrous network made up of interconnected nanofibers, while the deposit comprises large numbers of unbranched nanowires. It is found that the Fmoc-OG molecules form bilayer structures in both the deposit and the gel. However, the ratio (R) between the Fmoc-OG molecules in a stable intramolecular H-bonding conformation and those in a metastable intermolecular H-bonding conformation can be tuned by the ultrasound, R (deposit) > R (gel). The increased population of the intermolecular H-bonding FmocOG molecules induced by the ultrasonication facilitates the interconnection of nanofibers for the formation of a fibrous network, and therefore gelation. The alteration in the morphologies and properties of the obtained nanomaterials induced by the ultrasound wave demonstrates a potential method for smart control of the functions of nanomaterials from the molecular level. Acknowledgment. This work was supported by the National Natural Science Foundation of China (nos 90301010, 50573084, 90606004), the Chinese Academy of Sciences (“100 Talents” program), and the National Research Fund for Fundamental Key Project 973 (2006CB806200, 2006CB932101, 2007CB936401). Supporting Information Available: Additional information: the NMR, fluorescence spectra, DSC and the modeling molecular structures profile were available. This material is available free of charge via the Internet at http://pubs.acs.org. LA801499Y
