This journal is c The Royal Society of Chemistry 2013 Chem. Commun.

Cite this: DOI: 10.1039/c3cc47288d

Light- and solvent-driven morphologicaltransformations of self-assembled hydrogen-bondednanostructures†

Kuo-Pi Tseng,a Yu-Tang Tsai,b Chung-Chih Wu,b Jing-Jong Shyue,c

Dario M. Bassani*d and Ken-Tsung Wong*ae

The morphology of aggregates formed by an E-azobenzene deriva-

tive possessing terminal phenylenebiuret hydrogen-bonding groups

can be manipulated by the solvent composition and UV irradiation.

Supramolecular self-assembly can be used to organize functionalmolecular units into ordered nanostructures, thus offering abottom-up approach to develop novel materials based on therational design of building blocks endowed with targetedfunctionality.1 A feature of self-assembled materials is theirability to autonomously adjust to their environment, either forself-repair or as a non-linear response to an outside event.2 Forexample, some polymeric amphiphile systems undergo morpho-logical transitions between various canonical architectures, suchas spherical to elongated micelles upon exposure to hydrophobicor hydrophilic environments.3 In contrast, similar behavior ofsmall-molecule assemblies is rare,4 as they tend to precipitate orcrystallize as the environment is changed to one of less favorablesolvation. Considerable work using azobenzene-based moleculeshas led to systems displaying light-driven morphological modula-tions, such as micelles to vesicles,5 rods to spheres,6 or tubes tofibers.7 In these systems, E,Z isomerization of the azobenzene coreinduces a structural variation at the molecular level that resultsin a variation of the packing mode of the molecules. In thiscommunication, we report an azobenzene-based compound

(1, Scheme 1) incorporating self-complementary hydrogen-bonding(H-B) units which responds to both light and solvent polarity toreversibly convert between spherical and fibrillar morphologies.

Compound 1 was designed to incorporate functional blocks tocontrol the formation of nanostructures and is based on a photo-responsive azobenzene core with terminal phenylbiuret H-B units.The latter are known to form a six-membered ring through intra-molecular H-B,4 providing two molecular recognition sites that leadto the formation of two-dimensional H-B sheets. The interplayof H-B and van der Waals interactions leads to the formation ofordered nanoscopic structures which can be manipulated throughthe use of external stimuli that alter the balance between theintermolecular forces. The synthesis of 1 is shown in Scheme 1 andis based on the Suzuki coupling between diazobenzene 5 andboronic ester 68a (see ESI† for details). The presence of alkoxychains provides solubility and promotes hydrophobic interactions.

The electronic absorption spectrum of 1 shows three bandscentered at 285, 340, and 445 nm. The strong 445 nm absorptionband of 1 is assigned to the p–p* transition of the azobenzene thatis bathochromically shifted with respect to 5 (Fig. S1, ESI†) andoverlaps with the n–p* transition band now located at 500 nm.9

Irradiation of 1 (1.0 � 10�5 M in THF) at 377 nm leads to thedecrease of the 445 nm band to reach a photostationary state (PSS)after 5 minutes (Fig. 1). As is frequently observed for azobenzenederivatives, the E isomer can be quantitatively recovered upon

Scheme 1 Synthetic pathway of azobenzene-cored biuret-capped molecule 1.

a Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan.

E-mail: kenwong@ntu.edu.twb Department of Electrical Engineering, Graduate Institute of Electro-optical

Engineering and Graduate Institute of Electronics Engineering,

National Taiwan University, Taipei, 10617, Taiwanc Research Center for Applied Sciences, Academia Sinica, 128 Academia Road,

Nankang, Taipei 115, Taiwand Institut des Sciences Moleculaires, CNRS UMR 5255, Univ. Bordeaux 1,

33405 Talence, France. E-mail: d.bassani@ism.u-bordeaux1.fre Institute of Atomic and Molecular Sciences, Academia Sinica,

Taipei, 10617, Taiwan

† Electronic supplementary information (ESI) available: Experimental details,synthesis and characterization, 1H, 13C spectra of new compounds, UV-visabsorption spectra, 1H-NMR spectra, IR, DLS, and GIXS analysis of molecule 1

before and after irradiation and in different media, TEM of 1 in THF–Hex andTHF–H2O systems. See DOI: 10.1039/c3cc47288d

Received 24th September 2013,Accepted 15th October 2013

DOI: 10.1039/c3cc47288d

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exposure to long-wavelength UV light (10 min at 450 nm) orthermally (12 h at RT). The high photostability of 1 was verifiedby alternately irradiating at 377 and 450 nm (Fig. S2, ESI†).The composition of the PSS reached upon irradiation at 377 nmwas determined using 1H-NMR spectroscopy (Fig. S3, ESI†) tobe 64/36 E/Z.

The SEM and TEM images of the self-assembled nanostructuresformed by E-1 upon drop-casting from THF solution (10�4 M) andof an E–Z (64/36) mixture obtained by irradiation at 377 nm areshown in Fig. 2. The SEM images (Fig. 2a) indicate that theintermolecular interactions in E-1 lead to the formation of networksof fibers that are approximately 100–300 nm in width and severalmicrometers in length (Fig. 2b). In contrast, the deposition of asolution of 1 containing a PSS composition of E and Z isomers(Fig. 2c) clearly indicates that the morphology of the sample waschanged from fibers into randomly dispersed spheres with anaverage diameter of ca. 500 nm. TEM analysis (Fig. 2d) confirmedthat the spherical assemblies are hollow. Interestingly, for shortexposure time (15 min), both fibrillar and spherical aggregates arepresent simultaneously (Fig. 2e), which is indicative of a progressiveinterconversion between the two morphologies. Dynamic lightscattering (DLS) analysis of the irradiated 1 solution (THF,10�4 M) indicates the presence of objects possessing a meandiameter of ca. 430 nm (Fig. S4, ESI†). This size is consistentwith the results observed by SEM of drop-cast samples in whichsimilar-sized spheres are observed, supporting the conclusion thatthe spherical structures are spontaneously formed in THF solutionupon irradiation.8b,c Although the photostationary state is com-posed of a majority of the E isomer, it is worthwhile to note thatthe minority component (36% Z isomer) is enough to drive thecomplete transition to spherical aggregates. More importantly,

the photo-driven morphological changes are reversible: treatmentof a solution of spherical aggregates obtained upon 377 nmirradiation using a long-wavelength light (450 nm, 45 min)produced a solution that upon deposition revealed the exclu-sive formation of fibrous assemblies (Fig. 2f–h).

To better understand the self-assembly process involved in thespontaneous organization of 1, grazing-incidence X-ray diffraction(GIXRD) and IR spectroscopy were used to analyze the molecularorganization within the aggregates and the role of H-B. The IRspectrum of 1 (Fig. S5, ESI†) shows that the N–H stretching/amide-Ibands of fibers and spheres are at lower energies (3438 and 1633 vs.3442 and 1635 cm�1, for the stretching and amide I vibrations ofthe fibrous and spherical aggregates, respectively) as compared tothose of the isolated monomer state (3465 and 1693 cm�1)obtained under highly diluted conditions (o10�6 M). Theseresults indicate that intermolecular H-B between the biuret groupsis present in the condensed phases. Elucidation of the supra-molecular ordering in the fibrous and spherical aggregates wasobtained by the GIXRD experiments. The XRD diffraction patternof fibers assembled from E-1 (Fig. S6, ESI†) shows a diffractionsignal in the small angle region that corresponds to a d-spacing of42.8 Å. The remaining three peaks correspond to d-spacings of30.0, 15.1, and 10.2 Å with the ratios of 1 : 1/2 : 1/3, and indicatethat the fibrous morphology is organized into a lamellarstructure.10 The broad band present in the wide-angle regionis typically associated with an interdigitated packing of thealkoxyl chains.11 After UV-light irradiation, the diffractionpattern obtained for the spherical aggregates was significantlymodified, with the appearance of new diffraction peaks corre-lating with a d-spacing of 39.5 Å and d-spacings of 29.3 and 14.6 Åwith a ratio of 1 : 1/2, indicating a distorted lamellar stacking.

Combining the results obtained from IR and XRD analyses andthe known intermolecular interaction behavior of biuret groups,4

we propose that, as illustrated in Fig. 3, the intermolecular H-B ofthe biuret groups in the more extended E-1 molecules induces theirassembly into a planar, sheet-like structure. Assuming a zigzag con-figuration along the molecular long axis, the XRD d-spacing of 42.8 Å(shorter than twice the molecular length) corresponds well to theperiodic spacing of the repeating biuret groups. The van der Waalsinteractions between alkoxyl chains and p–p stacking of azo-benzene cores allow the sheets to further stack into a lamellararchitecture, thus leading to fibrous networks. This situation

Fig. 1 Time evolution of the absorption spectra of molecule 1 solution (10�5 M, THF)under (a) 377 nm irradiation (E - Z), (b) 445 nm irradiation (Z - E).

Fig. 2 SEM (a, c) and TEM (b, d) images of 1 solution (10�4 M in THF) before andafter UV-light (377 nm, high power Xe lamp) treatment (30 min) and SEM images of 1(e) UV irradiation at 377 nm over 15 min, (f) before UV irradiation, (g) after 30 min UVirradiation at 377 nm and (h) further 45 min visible light irradiation at 450 nm.

Fig. 3 Schematic representation of the photoinduced morphological transition:E,Z isomerization leads to the formation of a photostationary state (64/36 E/Zcomposition) that has a more compact and twisted molecular structure due to stericinteractions between the alkoxy chains. The reduction in repeat distance observed byGIXRD analysis (8%) is in agreement with the expected 10% reduction in the averagemolecular length based on the composition of the photostationary state.

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differs for the Z isomer, which should adopt a more compact andtwisted conformation due to intramolecular steric repulsionswithin the substituted azobenzene moiety. Although E,Z isomeri-zation does not impede H-B interactions, GIXRD results show thatthe periodic distance between the repeating units is smaller thanin aggregates composed of the pure E isomer (39.5 vs. 42.8 Å).Assuming that the molecules retain a zigzag configuration, thereduction in intermolecular spacing upon photoisomerization canbe understood by taking the weighted average of the reduction inlength. Thus, by considering an E–Z mixture composed of 64%E (l = 25.9 Å) and 36% Z (l = 18.1 Å), a weighted average molecularlength of 23.1 Å is calculated, which represents a 10% decrease inmolecular length with respect to the E isomer. The latter value is ingood agreement with the experimentally observed 8% reductionin the average repeat distance observed by GIXRD analysis uponphotoirradiation of the solution. Steric hindrance between thedialkoxy-phenyl groups in the Z-configuration will lead to twistingof the sheet-like structure, which would introduce curvature andthus lead to the formation of spherical aggregates.

As noted in the introduction, morphology transitions can alsobe governed by the polarity of the medium.4,12 The TEM image ofa sample cast from a solution of E-1 in a non-polar environmenthexane–THF (10�4 M, v/v = 5/1) showed folding and entanglingof the fiber bundles (Fig. S7a, ESI†). Switching to a more polarH2O–THF (v/v = 2/1) mixture led to irregular blade-like structures(Fig. 4a and e), whereas isolated flat ribbons that are 70–150 nm inwidth and several mm in length (Fig. 4b and f) are obtained at higherwater content (H2O–THF, v/v = 5/1). Interestingly, as the aqueouscontent of the solvent further increases (H2O–THF, v/v = 10/1), theformation of inter-connected spheres and sphere-fused nano-sheetsis observed (Fig. 4c and g). Finally, as the ratio of H2O–THFreaches v/v = 20/1, spherical aggregates with an average diameterof ca. 50 nm are formed (Fig. 4d and h). These spherical structuresare not hollow, unlike the vesicle-like aggregates formed from thedeposition of PSS mixtures of E- and Z-1, and are similar tomicelles in morphology (Fig. S7b, ESI†).

The formation of the nanospheres in high-polarity solutions wasalso evidenced by DLS, which evidences a decreased mean diameterof the aggregates upon increasing the aqueous content (Fig. S8,ESI†). GIXRD analysis indicates that the original lamellar structurepresent in neat THF is transformed into a rectangular columnararrangement at higher water content (THF–H2O, v/v = 1/2), fromwhich it gradually shifts to a less-ordered structure as the watercontent further increases (Fig. S10, ESI†). The solvent effects can be

rationalized on the basis of hydrophobic interactions: in THF–hexane, hydrophobic interactions are weak and induce E-1 toorganize into amorphous networks. In contrast, as the mediumhydrophilicity increases, p–p stacking and hydrophobic inter-actions of the alkoxyl chains are strengthened and lead to theformation of tighter and more ordered sheets. As the hydro-philicity is further increased, the sheets curve to reduce theirsurface energy and finally reorganize into spheres.12b

In conclusion, a photoresponsive azobenzene-based compoundpossessing H-B groups was prepared to demonstrate the feasibilityof combining light- and solvent-triggered morphological trans-formations. Thanks to the cooperative effects of non-covalentinteractions, even partial E,Z isomerization is sufficient to triggerthe reversible transition of nanofibers into hollow spheres uponUV-light irradiation. We can compare this behaviour to that of abis-biuret derivative possessing a 1,4-distyrylbenzene core that doesnot undergo photoinduced isomerization using visible light.8a

In the latter case, only the formation of fibrous aggregates wasobserved. Our results demonstrate that biuret units with photo-chromic azobenzene cores can be used as photosensitive buildingblocks for the design of adaptive materials.

The authors gratefully acknowledge the financial supportfrom the National Science Council of Taiwan (NSC-99-2923-M-002-002-MY3) and the Agence Nationale de la Recherche (ANR-09-BLAN-0387) and the technical assistance from TechnologyCommons, College of Life Science and Precision InstrumentationCenter, NTU with TEM analysis.

Notes and references1 (a) S. S. Babu, S. Prasanthkumar and A. Ajayaghosh, Angew. Chem.,

Int. Ed., 2012, 51, 1766; (b) D. Gorl, X. Zhang and F. Wurthner,Angew. Chem., Int. Ed., 2012, 51, 6328.

2 J.-M. Lehn, C. R. Chim., 2011, 14, 348.3 R. B. Grubbs and Z. Sun, Chem. Soc. Rev., 2013, 42, 7436.4 M.-C. Kuo, H.-F. Chen, J.-J. Shyue, D. M. Bassani and K.-T. Wong,

Chem. Commun., 2012, 48, 8051.5 (a) D. Faure, J. Gravier, T. Labrot, B. Desbat, R. Oda and D. M. Bassani,

Chem. Commun., 2005, 1167; (b) Y. Wang, N. Ma, Z. Wang andX. Zhang, Angew. Chem., Int. Ed., 2007, 46, 2823.

6 (a) Y. Wu, S. Wu, X. Tian, X. Wang, W. Wu, G. Zou and Q. Zhang,Soft Matter, 2011, 7, 716; (b) X. Ran, H. Wang, P. Zhang, B. Bai, C. Zhao,Z. Yu and M. Li, Soft Matter, 2011, 7, 8561; (c) C. Wang, Q. Chen, H. Xu,Z. Wang and X. Zhang, Adv. Mater., 2010, 22, 2553; (d) L. Ma, J. Jia,T. Yang, G. Yin, Y. Liu, X. Sun and X. Tao, RSC Adv., 2012, 2, 2902.

7 (a) N. Kameta, A. Tanaka, H. Akiyama, H. Minamikawa, M. Masudaand T. Shimizu, Chem.–Eur. J., 2011, 17, 5251; (b) Y. Lin, Y. Qiao,P. Tang, Z. Li and J. Huang, Soft Matter, 2011, 7, 2762.

8 (a) F.-C. Fang, C.-C. Chu, C.-H. Huang, G. Raffy, A. Del Guerzo, K.-T. Wongand D. M. Bassani, Chem. Commun., 2008, 6369; (b) K.-P. Tseng, F.-C.Fang, J.-J. Shyue, K.-T. Wong, G. Raffy, A. Del Guerzo and D. M. Bassani,Angew. Chem., Int. Ed., 2011, 50, 7032; (c) S. K. P. Velu, M. Yan, K.-P. Tseng,K.-T. Wong, D. M. Bassani and P. Terech, Macromolecules, 2013, 46, 1591.

9 (a) J. Zeitouny, C. Aurisicchio, D. Bonifazi, R. De Zorzi, S. Geremia,M. Bonini, C.-A. Palma, P. Samorı, A. Listorti, A. Belbakra and N. Armaroli,J. Mater. Chem., 2009, 19, 4715; (b) Q. Tang, X. Meng, H. Jiang, T. Zhou,C. Gong, X. Fua and S. Shi, J. Mater. Chem., 2010, 20, 9133.

10 (a) B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc.,2002, 124, 14410; (b) S. Qu, X. Chen, X. Shao, F. Li, H. Zhang,H. Wang, P. Zhang, Z. Yu, K. Wu, Y. Wang and M. Li, J. Mater.Chem., 2008, 18, 3954.

11 (a) A. J. Lampkins, O. Abdul-Rahim, H. Li and R. K. Castellano,Org. Lett., 2005, 7, 4471; (b) H.-F. Hsu, M.-C. Lin, W.-C. Lin, Y.-H. Laiand S.-Y. Lin, Chem. Mater., 2005, 15, 2115.

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Fig. 4 SEM (top) and TEM (bottom) images of E-1 in THF–water co-solventsystem in volume ratio from (a, e) 1 : 2, (b, f) 1 : 5, (c, g) 1 : 10 and (d, h) 1 : 20.

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