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1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Adv. Mater., DOI: /adma Dual-Responsive Nanoparticles and their Self-Assembly Sanjib Das, Priyadarshi Ranjan, Pradipta Sankar Maiti, Gurvinder Singh, Gregory Leitus, and Rafal Klajn*

2 SUPPORTING INFORMATION Dual-responsive Nanoparticles and their Self-assembly Sanjib Das, Priyadarshi Ranjan, Pradipta Sankar Maiti, Gurvinder Singh, Gregory Leitus, and Rafal Klajn* [*] Dr. R. Klajn, Dr. S. Das, P. Ranjan, P. S. Maiti, Dr. G. Singh Department of Organic Chemistry Weizmann Institute of Science Rehovot, (Israel) Dr. G. Leitus Department of Chemical Research Support Weizmann Institute of Science Rehovot, (Israel) This work was supported by the European Union Marie Curie Reintegration Grant, the G. M. J. Schmidt-Minerva Center for Supramolecular Architectures, the Helen and Martin Kimmel Center for Molecular Design, and the Minerva Foundation with funding from the Federal German Ministry for Education and Research. We thank Mr. Shachar Lerer (Prof. Gil Markovich group; Tel Aviv University) for the assistance with SQUID measurements. We thank Dr. Guohua Jia (Prof. Uri Banin group; Hebrew University of Jerusalem) for the assistance with XRD measurements. The EM studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging at the Weizmann Institute. R. K. is the incumbent of the Robert Edward and Roselyn Rich Manson Career Development Chair. Table of contents: 1. Synthesis of catechol-terminated azobenzene (AC) 2 2. Photoisomerization of AC in solution 7 3. Preparation of monodisperse Fe 3 O 4 nanoparticles 8 4. Structural characterization of Fe 3 O 4 nanoparticles 9 5. Functionalization of iron oxide nanoparticles with AC Spectrophotometric estimation of a surface area occupied by a single AC molecule on the surface of Fe 3 O Photoisomerization of AC on nanoparticles Reversible assembly-disassembly of AC-functionalized nanoparticles Magnetic properties of AC-functionalized magnetite NPs and their aggregates Additional SEM images of magnetic threads taken at various tilt angles Additional SEM images of extended Fe 3 O 4 NP assemblies undergoing coalescence in an end-to-end fashion 15 1

3 1. Synthesis of catechol-terminated azobenzene (AC). Synthesis of 1: To a stirred solution of 4-phenylazophenol (1.00 g; 5.04 mmol) in 30 ml of acetone was added, at room temperature, potassium carbonate (3.48 g; mmol), followed by 18-crown-6 (0.66 g; 2.52 mmol). After 15 min, methyl bromoacetate (2.31 g; 1.43 ml; mmol) was added and the resulting mixture was refluxed for 12 hr and then cooled down to room temperature. The reaction mixture was filtered and the solids were washed with dichloromethane. The combined filtrate was evaporated on a rotary evaporator and the resulting solid was washed with hexane several times to obtain compound 1 as a yellow solid. Yield: g (82.6 %). 1 H NMR (300 MHz, CDCl 3 ): δ (ppm) = 3.80 (s, 3H), 4.69 (s, 2H), 6.99 (d, 2H), (m, 3H), (m, 4H). 2

4 Synthesis of 2: Compound 1 (1.00 g; 3.69 mmol) was dissolved in a mixture of 0.5 M aqueous NaOH (40 ml) and THF (20 ml). The reaction mixture was refluxed for 2.5 hr, cooled down to room temperature, and acidified with 0.5 M aqueous HCl solution, resulting in the precipitation of 2. The precipitate was washed several times with water and dried under vacuum. Yield: 810 mg (85.4%). 1 H NMR (300 MHz, DMSO-d 6 ): δ (ppm) = 4.66 (s, 2H), 7.06 (d, 2H), (m, 3H), (m, 4H). Synthesis of 3: Compound 2 (410 mg; 1.59 mmol) was dissolved in dry DMF (10 ml) with stirring under a nitrogen atmosphere. The reaction mixture was cooled to 0 C in an ice bath and N,N -dicyclohexylcarbodiimide (380 mg; 1.84 mmol) was added while stirring. After 15 min N-hydroxy-succinimide (210 mg; 1.84 mmol) was added and stirring was continued for an additional 30 min, after which the ice bath was removed and the reaction mixture was stirred for 19 hr at room temperature. Addition of water (200 ml) resulted in the formation of a precipitate which was collected by filtration, washed several times with distilled water, and dried under vacuum. The crude product was purified by column chromatography on silica gel using a mixture of ethyl acetate and hexane (65:35) as the eluent. Compound 3 was obtained as an orange solid. Yield: 324 mg (57.0%). 1 H NMR (300 MHz, CDCl 3 ): δ (ppm) = 2.88 (s, 4H), 5.07 (s, 2H), 7.09 (d, 2H), (m, 3H), (m, 4H). 3

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6 Synthesis of AC: To a solution of dopamine hydrochloride (86 mg; 0.45 mmol) and N-methylmorpholine (92 mg; 100 μl; 0.91 mmol) in ethanol (20 ml) was added compound 3 (160 mg; 0.45 mmol) in CHCl 3 (10 ml). The resulting mixture was stirred 16 hr at room temperature. The crude reaction mixture was evaporated to dryness in a rotary evaporator. The crude product was purified by column chromatography on silica gel using ethyl acetate as the eluent. Compound AC was obtained as an orange solid. Yield: 120 mg (67.8 %). 1 H NMR (300 MHz, DMSO-d 6 ): δ (ppm) = (t, 2H), (t, 2H), 4.62 (s, 2H), (m, 1H), (m, 2H), 7.17 (d, 2H), (m, 3H), (m, 4H), (t, 1H), 8.71 (s, 1H), 8.82 (s, 1H). 5

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8 2. Photoisomerization of AC in solution. To determine the composition of the photostationary state of AC under UV light irradiation, the isomerization process was monitored by 1 H NMR. NMR is the technique of choice to follow the azobenzene isomerization since each of the two isomers has a distinct and non-overlapping set of NMR signals. Fig. S1 shows changes in the 1 H NMR spectra of trans-ac (in CD 3 OD) subjected to UV irradiation for increasing periods of time. Integrating signals assigned to trans- and cis-ac allows one to conclude that the photostationary state under these conditions consists of 93% of cis- and 7% of trans-ac. UV irradiation of this mixture for longer periods of time did not result in further enrichment in the cis isomer. Figure S1. Changes in partial 300 MHz 1 H NMR spectra of AC (31 mm solution in CD 3 OD) accompanying irradiation of the sample with UV light (wavelength, λ = 365 nm). The orange set of signals can be assigned to the trans configuration of azobenzene, while the purple set of signals is due to the cis isomer. The NMR data were then correlated with results collected by UV-Vis spectroscopy (Fig. S2). Given that the initial state (100% trans) has, at the given concentration, 339 nm (= λ max ) = 1.356, whereas the photostationary state (93% cis + 7% trans) has 339 nm = (see Fig. S2), the relative absorbance was calculated at 339 nm for the two isomers (A 339 nm / A 339 = 14.5; that is, a solution of pure cis-ac, should it exist, would have, at the same concentration, 339 nm = ). This reasoning allows calculation of the composition of the photostationary state as a function of 339 nm (see Fig. S3). 7

9 Figure S2. Changes in the UV-Vis spectra of AC (dissolved in methanol) accompanying irradiation with UV light. The most pronounced change is the decrease of the peak centered at λ = 339 nm and assigned to the π-π* transition in trans-azobenzene. The peak centered at λ = 285 nm originates from the catechol moiety. Figure S3. Reversible changes of absorbance at 339 nm and the % of the cis isomer of AC as a result of alternating exposure to UV and visible light in toluene. 3. Preparation of monodisperse Fe 3 O 4 nanoparticles. Monodisperse, single crystalline Fe 3 O 4 nanoparticles, ± 0.87 nm in diameter, were prepared by thermal decomposition of iron (III) oleate in the presence of oleic acid, as described previously. [16] First, iron (III) oleate was synthesized by reacting sodium oleate with iron (III) chloride. Briefly, sodium oleate (36.5 g, 120 mmol) (TCI, >97%; high purity of sodium oleate was critical for reproducible synthesis of high-quality, monodisperse Fe 3 O 4 NPs) and iron (III) chloride hexahydrate (10.8 g, 40 mmol) (Alfa Aesar, 98%) were dissolved in a mixture of solvents composed of 60 ml of distilled water, 80 ml of ethanol and 140 ml of hexane. The resulting solution was refluxed (bath temperature was set up to 70 C ) with vigorous stirring and under a gentle flow of nitrogen. The reaction was discontinued after four hours and the upper (hexane) layer containing the product was collected, washed three times with 30 ml of distilled water in a separatory funnel, and dried over magnesium sulfate. Removal of the solvent in vacuo gave the product in the form of a waxy solid in a near quantitative yield. A two-neck, 100 ml round bottom flask was charged with 1-8

10 octadecene (25 ml) (Aldrich, 90%), iron (III) oleate (3.200 g, 3.56 mmol), and oleic acid (1.78 g, 6.30 mmol, 2.00 ml). The solution was then heated with a constant heating rate of 2.8 C / min to T = 310 C, and was left at this temperature for 30 min. After cooling down to room temperature, the NPs were precipitated with a mixture of solvents composed of n-hexane, isopropanol and acetone (v / v / v = 1 : 2 : 2). The transparent supernatant was discarded and the solids were washed with a mixture of n-hexane and acetone (v / v = 1 : 2). Evacuation of the solvent in vacuo afforded monodisperse Fe3O4 nanoparticles (11.32 ± 0.87 nm in diameter) readily soluble in toluene. 4. Structural characterization of Fe3O4 nanoparticles. Fig. S4 shows transmission electron microscopy (TEM) pictures of as-prepared NPs deposited on carbon-coated copper grids. The micrographs were acquired using a Philips CM120 Super Twin microscope operating at 120 kv. In order to investigate the structure of the NPs, we performed powder X-ray diffraction (XRD) measurements using Cu Kα photons on a Philips PW1830/40 diffractometer operated at 30 ma and 40 kv. Samples were deposited as thin layers from toluene solutions on silica substrates. A typical powder XRD spectrum is showed in Fig. S5a in blue. Peak broadening is due to small particle sizes and their random orientation on the substrate. The recorded spectrum is in agreement with the standard JCPDS file for magnetite, and, importantly, does not show any indication of peaks characteristic for maghemite (γ-fe2o3) (such as 300 at 2Θ and 320 at 2Θ (B. Y. Yu, S.-Y. Kwak, J. Mater. Chem. 2010, 20, 8320). Selected area electron diffraction (SAED) patterns were collected from ensembles of as-prepared nanoparticles deposited on carbon-coated copper grids using a Philips CM120 Super Twin TEM operating at 120 kv. The patterns obtained at various locations (e.g., Fig. S5b) are in good agreement with those expected from Fe3O4 (e.g. S. Sun, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang, G. Li, J. Am. Chem. Soc. 2004, 126, 273; T.-I. Yang, R. N. C. Brown, L. C. Kempel, P. Kofinas, J. Magn. Magn. Mater. 2008, 320, 2714). Finally, the color of the solid sample (black) is a further indication that it is magnetite and not maghemite (which is brown; e.g., W. Cai, J. Wan, J. Colloid Inter. Sci. 2007, 305, 366). Figure S4. TEM images of as-prepared Fe3O4 nanoparticles at various magnifications. 9

11 Figure S5. a) Blue: Powder X-ray diffractogram of as-prepared Fe 3 O 4 NPs. Red: standard XRD data for Fe 3 O 4 (JCPDS file ). b) Electron diffractogram acquired from as-prepared Fe 3 O 4 NPs. Numbers in yellow correspond to hkl indices, in the order 1-7: 111, 220, 311, 400, 422, 511, Functionalization of iron oxide nanoparticles with AC. Iron oxide NPs dissolved in toluene (whic had been prepared according to Section 3 in the SI) were mixed with one volume of methanol, precipitated with centrifugation, and sonicated with pure methanol to remove any residual oleic acid. After washing with three portions of methanol, the black solids were dried in vacuo and redissolved in pure toluene. An excess of AC dissolved in a small volume of methanol (<5% of the volume of toluene) was added and the NPs were incubated with the ligand for several days with shaking. Functionalized NPs were purified by precipitating with two volumes of methanol (with centrifugation), the resulting solids were washed with methanol twice to remove any unbound AC, dried in vacuo, and redissolved in pure toluene to give a brown solution. The resulting solution is stable and shows no signs of aggregation for at least three months. The functionalization procedure did not affect the size and size distribution of the nanoparticles (compare Fig. S4 and S5). Figure S6. TEM images of AC-functionalized Fe 3 O 4 nanoparticles at various magnifications. 10

12 6. Spectrophotometric estimation of a surface area occupied by a single AC molecule on the surface of Fe 3 O 4. High absorbance of AC in the near-uv region (see Fig. S2) was the basis of our method used to establish the average surface area occupied by a single molecule of AC on the surface of Fe 3 O 4 NPs (Fig. S6). For the functionalization, a toluene solution of AC of a known concentration was used in excess. As-prepared Fe 3 O 4 NPs were purified from excess oleic acid so that the NPs subjected to functionalization contained only a monolayer / bilayer of oleic acid on their surfaces. The NPs were incubated with AC for several days to allow a complete ligand exchange. Addition of ten volumes of methanol followed by centrifugation led to a selective removal of functionalized NPs from the solution, whereas excess of free AC remained in the solution. The supernatant was then characterized spectrophotometrically for the absorbance at λ = 339 nm (the wavelength of maximum absorbance for trans-ac at which the absorbance of oleic acid, toluene and methanol is negligible). Then, having established the dependence of AC's absorbance on its concentration (Fig. S7), and knowing the average diameter of the NPs, their total mass and density, it was calculated that a single molecule of AC occupies 0.49 ± 0.03 nm 2 on the surface of Fe 3 O 4. Figure S7. Strategy used to estimate the surface area occupied by a single molecule of AC on the surface of Fe 3 O 4 NPs. Figure S8. Dependence of AC's absorbance at λ max (339 nm) on its concentration in methanolic solution. 11

13 7. Photoisomerization of AC on nanoparticles. Dilute solutions of AC and AC-functionalized NPs were irradiated with low-intensity UV light 2 ( I339nm 1.0 mw/cm ) and the isomerization progress was followed by UV-Vis spectrophotometry (Fig. S8). To record the spectra, a toluene solution of AC was used and AC-NPs were dissolved in a 4:1 toluene-methanol mixture, in which both trans- and cis-ac-nps are well-soluble. Oleic acidfunctionalized Fe 3 O 4 NPs (prior to functionalization with AC) dissolved in toluene (grey line in Fig. S8b) were used as a background. The compositions of the photostationary states were calculated as described in Section 2 of the SI. These results demonstate that immobilizing AC onto NPs does not significantly affect its photoswitching characteristics in both cases, it took ~120 sec to reach the photostationary states, whose compositions were similar (~93% and ~88% of the cis isomer in solution and on NPs, respectively). Figure S9. a) Changes in the UV-Vis spectra of trans-ac upon UV light irradiation. b, c) Changes in the UV-Vis spectra of surface-immobilized trans-ac upon UV light irradiation. d) Changes in λ max for trans-ac in solution and on NPs. 12

14 8. Reversible assembly-disassembly of AC-functionalized nanoparticles. To verify that the light-induced self-assembly process was fully reversible, SEM was used to image the particles following 5, 10, and 15 cycles of assembly-disassembly. Figure S10. SEM images of (from left to right) as-prepared AC-functionalized NPs, NPs irradiated with visible light after 5, 10, and 15 assembly-disassembly cycles. Scale bars = 200 nm. 9. Magnetic properties of AC-functionalized magnetite NPs and their aggregates. Magnetic properties of the NPs and their aggregates were investigated using a superconducting quantum interference device (SQUID) magnetometer MPMS XL-5. For single NPs, g of AC-functionalized Fe 3 O 4 NPs dissolved in 180 μl of toluene were applied onto a silicon wafer and the solvent was evaporated at once to avoid aggregation of NP during slow solvent evaporation (note that the small amount of the material used was well below the amount corresponding to a densely packed monolayer of NPs; we verified by SEM that the majority of NPs remained single after solvent evaporation while others formed small, monolayer-thick patches). For the aggregates, a solution of g of the same NPs in 180 μl of toluene was first irradiated with UV light (1.46 mw cm 2 ) for 5 minutes, then applied onto a silicon wafer and the solvent was evaporated at once. M-H curves were recorded at room temperature (300 K) at an external magnetic field, H, in the range from 50 to +50 koe. Zero field-cooled (ZFC) and field-cooled (FC) temperature dependences of the sample's magnetic moment were recorded at a weak external magnetic field, H = 500 Oe, after the samples had been cooled from temperature T = 300 K to T = 2 K at H = 0 (ZFC) and H = 500 Oe (FC). To subtract the magnetic contribution of pure silicon, measurements at the same temperature and field were carried out on bare silicon wafers of dimensions identical to those used as substrates for NP deposition. SEM was used to verify that the samples looked the same before and after the magnetic measurements. 13

15 10. Additional SEM images of magnetic threads taken at various tilt angles. Figure S11. A collection of scanning electron micrographs of 1D SPM NP assemblies taken at various magnifications and tilt angles. 14

16 11. Additional SEM images of extended Fe3O4 NP assemblies undergoing coalescence in an endto-end fashion. Figure S12. Visualizing end-to-end self-assembly of 1D assemblies. 15