Vesicular Nanostructure Formation by Self-Assembly of Anisotropic

Nov 23, 2015 - The cumulative effect of these interactions is responsible for the stability of vesicular structures even on the removal of solvent. Th...
0 downloads 0 Views 3MB Size
Download PDF
Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

Vesicular Nanostructure Formation by Self-assembly of Anisotropic Penta-phenol Substituted Fullerene in Water Vaishakhi Mohanta, Debayan Dey, Suryanarayanarao Ramakumar, and Satish Patil Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03340 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Vesicular Nanostructure Formation by Self-assembly of Anisotropic Penta-phenol Substituted Fullerene in Water Vaishakhi Mohanta,a Debayan Dey,b Suryanarayanarao Ramakumar,b and Satish Patila* a

b

Solid State and Structural Chemistry Unit, Department of Physics, Indian Institute of Science, Bangalore 560012, India. Amphiphilic fullerene, vesicle, encapsulation, MD simulation

ABSTRACT: A study on self-assembly of anisotropically substituted penta-aryl fullerenes in water has been reported. The penta-phenol substituted amphiphilic fullerene derivative (C60Ph5(OH)5) exhibited self-assembled vesicular nanostructures in water under the experimental conditions. The size of the vesicles was observed to depend on the kinetics of selfassembly and could be varied from ~300 nm to ~70 nm. Our mechanistic study indicated that, the self-assembly of C60Ph5(OH)5 is driven by extensive intermolecular as well as water-mediated hydrogen bonding along with fullerenefullerene hydrophobic interaction in water. The cumulative effect of these interactions is responsible for the stability of vesicular structures even on removal of solvent. The substitution of phenol with anisole resulted in different packing and interaction of the fullerene derivative as indicated in the molecular dynamics studies, thus resulting in different selfassembled nanostructures. The hollow vesicles were further encapsulated with hydrophobic conjugated polymer and water-soluble dye as guest molecules. Such confinement of π-conjugated polymers in fullerene has significance in bulk heterojunction devices for efficient exciton diffusion.

The exploration of non-covalent interactions towards construction of supramolecular structures with distinct morphology and understanding the interactions guiding the self-assembly are some of the major interests among material scientists.1 Over the years, a particular interest has developed towards design of fullerene derivatives with unique structures by the process of self-assembly.2-3 In particular, the realization of biological relevance of fullerenes such as antioxidant, photodynamic activity and phototoxicity has driven the exploration of amphiphilic fullerene for application in biology.4 The hydrophobic fullerene can be made amphiphilic by covalent attachment of polar functional groups like, amine, carboxylic or hydroxyl groups. The self-assembly of these amphiphilic derivatives into vesicles needs fulfilment of the packing parameter requirement and hence, depends substantially on the geometry of the molecules.5 Fullerenes functionalized with carboxylic group (carboxy fullerenes)6 and hydroxyl groups (polyhydroxy fullerenes (PHFs) or fullerenols)7-8 are reported to form spherical aggregates in water while vesicle (10-70 nm) formation has been reported for C60-N,N-dimethylpyrrolidinium9 in DMSO/water mixture. Dendro-fullerene with carboxylic acid group has been reported to self-assemble in water as rods and spherical micelle;10 another molecule of the type is reported to form bilayer vesicle, but with a co-surfactant.11 Martín et al investigated the self-assembly behaviour of dendrofullerenes having varying number of carboxylic groups and observed vesicle formation for two of the reported molecules in THF and THF/water mixture.12 A new class of fullerene amphiphiles gained interest when Nakamura and co-workers observed spherical bilayered vesicle formation in penta-phenyl

substituted fullerene anion C60Ph5¯K+.13 The amphiphilic character is introduced in the fullerene derivative by generation of cyclopentadiene anion which exhibits high stability against protonation in water. Interestingly, fullerene derivatives with hydrophobic alkyl chains at the para position of phenyl rings; amphiphiles with nonpolar/polar/nonpolar (n-p-n’) motif; also associate in water to form bilayer vesicles like conventional nonpolar-polar (n-p) derivatives of fullerene.14 Cheng et al.

reported typical hydrophilic head and hydrophobic tail amphiphile based on fullerene derivative.15 The fullerene acted as polar head due to multiple functionalization with carboxy groups which are negatively charged at basic pH and polystyrene is the hydrophobic chain. Vesicles are formed in DMF/ water or dioxane/water mixture. Further, Janus particles based on amphiphilic [60]fullerene derivatives were synthesized by using the regioselective Bingel–Hirsh reaction and the click reaction.16 These particles contain carboxylic acid functional groups, a hydrophilic fullerene, and a hydrophobic C60 and one of the derivatives showed vesicle formation in pure THF. The previously reported fullerene derivatives for vesicle formation either requires multistep tedious synthesis, or uses high boiling polar solvents like DMF, DMSO or dioxane which are difficult to remove post assembly formation. The vesicles described by Nakamura and coworkers requires bulky counter ion like K+ for stabilization of vesicle in water.

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Molecular anisotropy is important for the generation of self-assembled complex hierarchical superstructures. In this regard, selective functionalization on one side of fullerene with hydroxyl group has been reported.17-18 The Janus fullerenols (C60(OH)8) were observed to form spherical aggregates of ~93 nm in water. In our study, the fullerene is anisotropically substituted with five phenols and the resultant amphiphilic fullerene (C60Ph5(OH)5)19 which is more flexible than the fullerenols due to rotation around the phenyl rings, exhibited self-assembled vesicular nanostructures in water. Remarkably, stable vesicles were observed even after removal of water. The NMR studies indicated the existence of hydrogen bonding (Hbonding) between peripheral hydroxyl groups. Such Hbonding and pristine fullerene-fullerene interactions could play pivotal role for the growth of vesicular structures. We have further shown that the size of vesicles can be tuned by controlling the process parameters like rate of diffusion of non-s0lvent, water. The substitution of hydroxyl groups with methoxy breaks this H-bonding interaction and the resultant molecule C60Ph5(OCH3)5 did not form vesicles in water, instead self-assembled into needle and plate-like morphologies. Further, the difference in the interaction of these two compounds in water has been analysed theoretically by molecular dynamics simulation. We also demonstrated that, the fullerene vesicles could efficiently encapsulate large guest molecules such as hydrophobic π-conjugated polymer and watersoluble dye. Nakamura and coworkers showed loading of small hydrophobic molecules in fullerene interior of vesicles by using post-loading approach in contrast to preloading approach.20. Post-loading approach would be unsuitable for loading of long chain polymers as one would expect only surface absorption of polymers on the vesicles. In our pre-loading approach the polymers and fullerene derivatives co-assemble during the formation of vesicles as a result one would expect more ordering in the polymer chain due to interaction with fullerene during the assembly formation. The supramolecular ordering of π-conjugated polymer in confined environment of fullerene is critical for efficient bulk heterojunction devices, as ordering results into efficient exciton diffusion and charge transport.21 The conceptual novelty of this design provides plethora of opportunities in the field of optoelectronic devices.

EXPERIMENTAL SECTION Materials Fullerene C60 (99.9%, sublimed) is procured from MER corporation, US. Copper bromide dimethyl sulphide (CuBr.SMe2), 1,2-dichloro benzene (ODCB) and p-toluic sulphonic acid (TsOH·H2O) are bought from Sigma Aldrich. Dihydropyran and 4-bromophenol are bought from Spectrochem. All materials were used without further purification. Synthetic Procedures

Page 2 of 12

1,4,11,15,30-Pentakis(4-hydroxyphenyl)-2H1,2,4,11,15,30-hexahydro-[60]fullerene (C60Ph5(OH)5) (2). The hydroxyl group of 4-bromophenol was first protected by converting it to tetrahydropyran. A mixture of 4-bromophenol and a catalytic amount of p-toluic sulphonic acid was taken in excess of dihydropyran and it was kept for stirring at room temperature for 3 h. The reaction was quenched by addition of 5% NaOH and extracted in dichlormethane (DCM). Column chromatography was done with 2% ethyl acetate/hexane to get a colorless liquid, which on cooling gives white solid. The resultant compound THPOC6H4Br was converted to its Grignard THPOC6H4MgBr using dry THF as solvent. THPOC6H4MgBr (3.64 mmol) was then cannulated to a suspension of CuBr.SMe2 (0.86 g, 4.2 mmol) in THF (10mL) at 0 oC and stirred for 10 min. C60 (100 mg, 0.14 mmol) was dissolved in 1,2-dichlorobenzene (7mL) by sonication and then added to the above reaction mixture. It was stirred for 2 h at 35 oC before quenching the reaction by addition of NH4Cl (aq). The mixture was diluted with toluene and purified by washing with water in a separating funnel and then filtered through a pad of celite and silica gel. The solution was concentrated with rotary evaporator and then precipitated with hexane. The obtained orange solid was dried in vacuo. This orange solid was dissolved in CH2Cl2/MeOH (30 mL, 50% v/v), and a pinch of TsOH·H2O was added to the solution. After the mixture was stirred for 1 day, it was neutralized with NaHCO3. Insoluble products were removed by filtration with a pad of Celite. Solvent was evaporated, and the residue was dissolved with a small amount of methanol. Upon addition of hexane, orange powder precipitated. The precipitates were collected and washed with hexane, and then dried in vacuo to obtain title compound. 1

H NMR (400 MHz, DMSO-d6) δ 9.56 (s, 2H, OH), 9.46 (s, 1H, OH), 9.46 (s, 2H, OH), 7.71 (d, J = 8.8 Hz, 4H, aromatic CH), 7.51 (d, J = 8.8 Hz, 4H, aromatic CH), 7.09 (d, J = 8.8 Hz, 2H, aromatic CH), 6.78 (d, J = 8.8 Hz, 4H, aromatic CH), 6.63 (d, J = 8.8 Hz, 4H, aromatic CH), 6.53 (d, J = 8.8 Hz, 2H, aromatic CH), 5.55 (s, 1H, C60H); MALDITOF : m/z = 1186.145. 1,4,11,15,30-Pentakis(4-methoxyphenyl)-2H1,2,4,11,15,30-hexahydro-[60]fullerene (C60Ph5(OCH3)5) (3). The 4-bromo anisole was converted to its Grignard CH3OC6H4MgBr using dry THF as solvent. CH3OC6H4MgBr (3.64 mmol) was then cannulated to a suspension of CuBr.SMe2 (0.86 g, 4.2 mmol) in THF (10mL) at 0 oC and stirred for 10 min. C60 (100 mg, 0.14 mmol) in 1,2-dichlorobenzene (7mL) was added to it and the reaction mixture was stirred for 2 h at 35 oC. The reaction was quenched by addition of NH4Cl (aq). The mixture was diluted with toluene and DCM and then purified by washing with water in a separating funnel. It was then filtered through a pad of celite and silica gel. The solution was concentrated with rotary evaporator and then precipitated with hexane. The obtained orange solid was dried in vacuo.

ACS Paragon Plus Environment

Page 3 of 12

Langmuir

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H NMR (400 MHz, CDCl3) δ 7.68 (d, J = 8.8 Hz, 4H, aromatic CH), 7.51 (d, J = 8.8 Hz, 4H, aromatic CH), 7.32 (d, J = 8.8 Hz, 2H, aromatic CH), 6.87 (d, J = 8.8 Hz, 4H, aromatic CH), 6.74 (d, J = 8.8 Hz, 4H, aromatic CH), 6.70 (d, J = 8.8 Hz, 2H, aromatic CH), 5.21 (s, 1H, C60H), 3.84 (s, 6H, CH3), 3.79 (s, 6H, CH3), 3.76 (s, 3H, CH3); MALDITOF: m/z = 1257.250 Preparation of Nano-structured Self-Assemblies The nano-assemblies of fullerene derivatives were formed in water by desolvation approach. A 500 µL of THF solution of the fullerene derivatives (0.1 mg/mL) was taken and then self-assembly was induced by addition of water (4 mL). THF was then removed by purging with N2. The rate of precipitation was varied by controlling the rate of addition of water. In one case, water was added rapidly under sonication. In the second case, water was added slowly in dropwise manner over a period of 5 min to the sample under stirring. Encapsulation of guest molecules The vesicles are loaded with water-soluble dye, Rhodamine B (RhB) and hydrophobic polymer, MEH-PPV. For loading of MEH-PPV, a 10 µL MEH-PPV solution in THF (0.5 mg/mL) was added to a 100 µL solution of C60Ph5(OH)5 in THF (0.5 mg/mL) and final volume of resultant mixture was adjusted to 500 µL using THF. The mixture was kept for stirring for 5 min. 2 mL of water was added dropwise over a period of 5 min. For loading of water soluble dye, 2 mL of aqueous solution of RhB (0.1 mg/mL) is added to 500 µL of C60Ph5(OH)5 in THF (0.1 mg/mL) dropwise. The RhB loaded vesicles were then dialyzed for 2 days to remove free RhB. Computational studies Molecular packing analysis and dynamics: The molecular geometry, highest occupied molecular orbital (HOMO) energy, LUMO energy, HOMO–LUMO gap and dipole moment (μ) for each penta-phenol substituted fullerene structure was calculated with the Jaguar density functional theory (DFT) package using the B3LYP hybrid density functional. The Pople triple-ζ polarized basis set, 6-311G** was used in these calculations. As a prerequisite for simulation of various packing schemes these molecules can adapt, different conformers of that molecule are determined using computational means. We searched for different low energy conformers of C60Ph5(OH)5 and C60Ph5(OCH3)5 using Boltzmann jump methods employed on conformers module in Accelrys Materials Studio 6.0 A universal force field is applied for this purpose implemented in Materials Studio 6.0. To understand the molecular packing of C60Ph5(OH)5 and C60Ph5(OCH3)5, we took few low energy conformers and simulated the packing using Polymorph module of Accelrys Materials Studio 6.0.

Classical molecular dynamics simulation of the system consisting of 24 penta-phenol substituted fullerene in different packed arrangements with explicit water was performed with AMBER Package. The partial charges are assigned using UCLA Chimera software using Gasteiger calculations. We employed the Generalized Amber Force Field (GAFF) for simulation and SHAKE algorithm was applied to fix all of the C-H bonds and the O-H bond, as well as the C-C bonds of the fullerene molecule. LJ and real space electrostatic interactions were cut off at 12 Å. The systems were then solvated in a truncated octahedral periodic box and TIP3P water model was used for solvation. The model systems were maintained at constant temperature of 300 K using Langevin thermostat and pressure of 1.0 bar with using the Berendsen algorithm with periodic boundary conditions (PBC). An energy minimization of 10000 cycles was performed first by steepest descent for 5000 cycles and then using conjugate gradient algorithms. 100 ps equilibration were separately conducted under NPT before final production runs. The subsequent production runs were performed under NPT ensemble with an integration time step of 2 fs for a total of 30 ns. Characterization Scanning Electron Microscopy (SEM). The morphologies of the nano-structures were observed under SEM. The SEM images were recorded on ULTRA 55, Field Emission Scanning Electron Microscope (Karl Zeiss). Samples were prepared by drop-casting on piranha treated Silicon wafer and then dried overnight under vacuum to ensure complete removal of moisture. Samples were gold sputtered prior to imaging. Transmission Electron Microscopy (TEM). For transmission electron microscopy (TEM) studies, the samples were dropcast on carbon coated copper TEM grids with 300 mesh and dried overnight in vacuum. TEM characterization was performed on a JEOL 2100F instrument operating at 200 kV. Dynamic Light Scattering (DLS) studies. PALS Zeta Potential Analyzer Ver 3.54 (Brookhaven Instrument Corps.) was used for DLS measurement. Confocal Laser Scanning Microscopy (CLSM). CLSM images were recorded on Zeiss LSM 510 META Confocal Microscope. Laser of 543 nm is used for excitation. The samples were prepared just before the imaging in order to avoid drying of sample. Nuclear Magnetic Resonance (NMR) Spectrometer. 1H-NMR spectra were recorded using Bruker Advance NMR spectrometer with 400 MHz frequency with DMSOd6 and CDCl3 as solvents. TMS was used as an internal standard when CDCl3 was used as solvent. Mass Spectrometry. Mass spectra were obtained using Ultraflex MALDI TOF/TOF (Bruker Daltonics).

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 12

TCSPC Measurements. The fluorescence lifetimes of MEH-PPV solution in THF and MEH-PPV entrapped in fullerene vesicles were obtained from time-correlated single photon counting (TCSPC) measurements. The samples were excited by 410 nm laser pulse, generated by frequency doubling of output frequency (700-860 nm) of a Ti:sapphire oscillator. Fluorescence decay profiles were obtained at the laser repetition rate of 4 MHz, using a micro-channel plate photomultiplier (model R2809; Hamamatsu Corp.) coupled to a time-correlated single photon counting setup. The instrument response function (IRF) at 410 nm was obtained using a dilute colloidal suspension of dried nondairy coffee whitener and found to be 115 ps. The emission of the samples was collected at 600 nm. The emission was monitored at the magic angle (54.7°) to eliminate the contribution from the anisotropy decay.

RESULT AND DISCUSSION The five-fold addition of phenols on fullerene was achieved using previously reported method17 wherein CuBr·Me2S was used as catalyst (Scheme1). Unlike the monoaddition of Grignard or organolithium reaction, the use of organo-copper reagent leads to selective 5-fold addition of phenols on one side of fullerene with high yield. The molecule lacks the typical polar head and hydrophobic tail structures of surfactants or lipids; both the hydrophobic (fullerene and the phenyl groups) and the hydrophilic part (the hydroxyl groups) have limited conformational flexibility. However, due to the phenyl rings the molecule can adopt more conformations than when the hydroxyl groups are directly attached to the fullerene as in the case of fullerenols. The rigidity along with anisotropy is also an important factor for determining the selfassembled structures.22 In addition to molecular structure and geometry, the shape and size of self-assembled structures are determined by the external parameters, i.e.; the process of solvation which includes the polarity of solvent and its addition rate.5,23 In this study, the nanostructures of C60Ph5(OH)5 were prepared by controlled addition of water to a solution of the compound in THF. THF was the choice of solvent as it is miscible with water and due to its low boiling point it is easy to remove after the formation of aggregates. The C60Ph5(OH)5 molecule self-assembled in water as spherical vesicles which were robust and retained their spherical morphology on drying, thus allowing us to characterize them from SEM and TEM. The kinetics of self-assembly was varied by controlling the rate of addition of non-solvent (water). The slow addition of water to C60Ph5(OH)5 solution resulted in larger vesicles of mean diameter of 288.3 nm as estimated from DLS, with monomodal size distribution (Figure 1 (inset)).

Scheme 1. Schematic representation of synthetic route to compounds C60Ph5(OH)5 (2) and C60Ph5(OCH3)5 (3). Smaller vesicles of ~69.6 nm were formed when water was added rapidly under sonication (Figure 2). The hollowness of the structures was evident from the SEM images where we observed dents in the spherical morphology due to removal of solvent from the interior on drying. The TEM images of the vesicles (Figure 3 and ESI, Figure S1) further ascertain the hollow morphology of the structures.

Figure 1. SEM image of vesicles of C60Ph5(OH)5 prepared by slow addition of water, (inset) size distribution of the vesicles from DLS measurement.

ACS Paragon Plus Environment

Page 5 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. SEM image of vesicles of C60Ph5(OH)5 prepared by rapid addition of water, (inset) size distribution of the vesicles from DLS measurement.

Figure 3. TEM image of vesicles of C60Ph5(OH)5 prepared by slow addition of water.

Figure 4. (a) Concentration dependent NMR of C60Ph5(OH)5. (b) and (c) shows the shifts in peak of hydroxyl and water, respectively, with increasing concentration, due to H-bond formation.

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The self-assembly process is controlled by the interactions among the fullerene molecules and their interaction with water. The spontaneous aggregation of fullerene can be attributed to strong Van der Waals attraction among the fullerene moieties resulting from a high interfacial atomic density of fullerene molecule (the C60–C60 dispersion interaction has energy of –30 kJmol-1),24 in concert with hydrophobic interactions between the molecules in presence of water. The H-bonding interaction among hydroxyl groups may also contribute to the stabilization of the assembly. We have performed concentration dependent NMR studies in DMSO-d6 to establish the ability of C60Ph5(OH)5 molecule to undergo H-bond formation. On increasing the concentration of C60Ph5(OH)5 in DMSO, there was evident change in the chemical shift of OH protons towards downfield (Figure 4 and ESI, Figure S2), indicating inter-molecular H-bond formation with oxygen of hydroxyl groups of fullerene or water in the proximity.25 A similar change in chemical shift was observed for protons of water as well. In order to comprehend the role of hydroxyl group in formation of vesicle structures, we have substituted the phenyl rings with methoxy group instead of hydroxyl and studied the self-assembly behaviour of the derivative C60Ph5(OCH3)5 (compound (3), structure provided in Scheme 1) in water. The substitution with methoxy eliminates the possibility of inter-molecular hydrogen bonding, however, there is a possibility that oxygen of methoxy can participate in hydrogen bonding formation with water molecules. The ratio of hydrophobicity-hydrophilicity is changed due to addition of hydrophobic CH3. The assembly formation in water was carried out in same fashion as that of C60Ph5(OH)5 ( both slow and rapid addition of water). The dispersion formed was quite stable, not susceptible to rapid agglomeration. However, The vesicle formation was not observed for C60Ph5(OCH3)5, instead the molecule assembled in needle-like or plate-like structures as observed from SEM (Figure 5 and ESI, Figure S3). The difference in the packing behavior of C60Ph5(OH)5 and C60Ph5(OCH3)5 in water which results in different assembly formation in the two cases was also observed from MD simulations. The treatment of the fullerene derivatives with potassium tert-butoxide leads to removal of cyclopentadine proton generating a cyclopentadiene anion.26 In case of C60Ph5(OH)5, the hydroxyl groups also get deprotonated generating phenoxide anions leading to disruption of the assembly in water due to electrostatic repulsion as observed from TEM (ESI, Figure S4). The treatment of C60Ph5(OCH3)5 with the base led to generation of anion C60Ph5(OCH3)5¯K+ which showed aggregates of size 4–5 nm (ESI, Figure S5) in water. The fullerene anion C60Ph5(CH3)5¯K+ was reported to form multilayer vesicle of ~37.6 nm,27 however, introduction of oxygen in the structure changed the self-assembly property.

Page 6 of 12

Figure 5. SEM image of self-assembled nanostructures of C60Ph5(OCH3)5 prepared by slow addition of water, (inset) size distribution of the particles from DLS measurement.

Computational Studies Different conformations from ab initio models, were found from analyzing a set of favorable dihedral angles in C60Ph5(OH)5 and C60Ph5(OCH3)5 (ESI, Figure S6). Due to rotation around phenyl ring, dihedral angle φ and φ±180 are same. The stable structure has greater intra-molecular stabilization in form of aromatic or hydrophobic interactions. We have chosen the lowest energy conformation of both C60Ph5(OH)5 and C60Ph5(OCH3)5 for further simulation. Fullerene derivatives have a strong stacking tendency which is observed in many crystal structures. Previous studies on pentaarylfullerene have illustrated the role of the different derivatives of pentaaryl group on its crystal packing.28 The addend group or “feathers” comes up from the quasi fivefold axis of these molecules and creates a socket. The nature, volume and polarity of the socket along with the solvent determine its packing. In previous studies, it was observed that C60Ph5(CH3)5, C60Ph5(C2H5)5 , C60Ph5(C3H7)5 and C60Ph5(C4H9)5 forms straight or zigzag shuttlecock stacking interaction, in organic solvents.28 Due to the hydrophobicity in the socket region, a compact packing is favored when the fullerene “head” latches to the socket “tail”. A few other packing arrangements are also possible in polar addend groups which include feather in cavity, head-to-tail layer and dimeric stacking (Figure 6). Geometric parameters are defined to categorize the different nature of packing interactions. Within each stack, fullerene–fullerene centroid distance SD [Å] and intra-stack angle Θ [°] defines the fundamental relationship between two neighboring molecules. The distance between centroid of fullerenes in neighboring stack is given as ID[Å]. The details of conformational analysis, packing and simulation are discussed in ESI.

ACS Paragon Plus Environment

Page 7 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 6. Schematic representation of different packing schemes in pentaarylfullerenes. a) straight shuttlecock packing (single stack), b) zigzag shuttlecock packing (single stack), c) dimer packing Type II, d) dimer packing type I, e) shuttlecock packing Type I (two stacks), f) shuttlecock packing Type II (four stacks), g) feather in cavity, h) geometric features in stacked packing where SD is inter molecule distance between the centroids of fullerene within the stack, ID is the distance across the stack and Θ is angle of stacking. Formation of supramolecular assembly of polymers through the process of non-covalent interactions like hydrogen bonding, hydrophobic effects etc. have created many design opportunities. Recently a polypeptide derivative of poly(γ-benzyl-L-glutamate) (PBLG) functionalized to synthesize C60–PBLG, has shown controlled self-assembly in water.29 To understand the organization of the building block molecules in water, we performed a molecular dynamics study of a pre-packed structure of C60Ph5(OH)5 and C60Ph5(OCH3)5. The choice of the packing scheme used in our simulation is guided through experimental observations. Both C60Ph5(OH)5 and

C60Ph5(OCH3)5 can occur as polymorphs of both dimer like and shuttlecock like packed schemes in crystal packing simulation studies, although NMR shift experiment on C60Ph5(OH)5 hints the formation of Hbonding between the hydroxyl group or H-bonding mediated through water. We observed that in type II dimer model of C60Ph5(OH)5 (packed in Pna21 space group) (Figure 7a), the hydroxyl groups are in proximity to each other which can facilitate H-bonding. In the shuttlecock packed model of C60Ph5(OH)5 (packed in Pbca space group) (Figure 7b), the formation of H-bonding is very restricted due to geometric constrains although not impossible. Similarly we observed that both dimer like (Pna21 space group, Figure 7c) as well as shuttlecock packing in C60Ph5(OCH3)5 (P1 space group, Figure 7d) doesn’t involve H-bond mediated interaction and is completely dependent on hydrophobic/ aromatic interactions. Thus, we performed a 30 ns molecular dynamics simulation of both C60Ph5(OH)5 and C60Ph5(OCH3)5 in two different packing schemes, to understand the water mediated rearrangements and its molecular organization (Figure 7e). As observed from experimental results, C60Ph5(OH)5 in water forms a vesicular structure, whereas C60Ph5(OCH3)5 in water forms thin crystalline plates. We observed that the dimer packed organization of C60Ph5(OH)5 forms H-bonding more strongly than the shuttlecock like organization, as the number of inter molecule H-bonding during the simulation was found to be higher in dimer stacked model than the other (Figure 7f). Interestingly, we observed that both dimer stacked and shuttlecock organization of C60Ph5(OCH3)5 tends to form a mixed packing organization with elements of both shuttlecock and type I dimer stacking features. Effectively, it forms a shifted columnar stack of C60Ph5(OCH3)5 molecules which prevent any water mediated interactions. This might explain the formation of crystalline plate, as observed in the experiments. Encapsulation of Guest Molecules in the Vesicles Vesicles have hollow aqueous interior and a hydrophobic shell; as a result they can serve as reservoirs for hydrophilic as well hydrophobic molecules. In this study, we have entrapped hydrophobic π-conjugated polymer MEHPPV in the vesicles. The loading of MEH-PPV is analyzed by UV-vis absorbance fluorescence studies (Figure 8).

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a)

b)

c)

d)

e)

f)

Page 8 of 12

H-bonding

Figure 7: a) C60Ph5(OH)5 molecules in Type II dimer stacking packed in Pna21 space group. b) C60Ph5(OH)5 in shuttlecock stacking packed in Pbca space group. c) C60Ph5(OCH3)5 molecules packed as type I dimer in Pna21 space group d) C60Ph5(OCH3)5 molecules in shuttlecock stacking in P1 space group. e) Octahedral solvated box with 24 C60Ph5(OH)5 molecules after 30ns. f) Section of the simulation box showing intermolecular H-bonding between the hydroxyl groups of C60Ph5(OH)5

ACS Paragon Plus Environment

Page 9 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir image showed that the morphology of vesicles was retained on incorporation of polymer (ESI, Figure S7). The possible interaction that holds MEH-PPV in the vesicle shell is the π-π interaction between the aromatic groups.30 This approach shows a great promise to organize πconjugated polymers and fullerene and could be a useful combination for bulk heterojunction devices. The hydrophilic dye, Rhodamine B (RhB) can be trapped both in the shell due to π-π interaction with fullerene as well as in the aqueous interior. The absorbance spectra of RhB trapped in the vesicles showed a bathochromic shift with broadening of the peak (ESI, Figure S8a) and the fluorescence of RhB is drastically quenched due to effective electron transfer (ESI, Figure S8b). The SEM image confirms the retention of vesicular morphology (ESI, Figure S9). The encapsulation was further confirmed from confocal microscopy, where we observed the red fluorescence due to encapsulated RhB (Figure 9).

(a)

Figure 8. UV-vis (a) and fluorescence (b) spectral analysis for loading of MEH-PPV in vesicles of C60Ph5(OH)5; (inset) photographic images before (a) and after (b) UV irradiation: (1) MEH-PPV (THF), (2) MEH-PPV– C60Ph5(OH)5 (THF), (3) MEH-PPV (water) and (4) MEH-PPV– C60Ph5(OH)5 (vesicle). (c) Timeresolved emission decay profiles of MEH-PPV. Broadening in absorbance spectra is observed for MEHPPV in aggregated state in water and when incorporated in vesicles accompanied with red-shift in the absorbance onset (Figure 8a). Solution of MEH-PPV in THF has fluorescence maxima at 553 nm. The addition of C60Ph5(OH)5 leads to insignificant fluorescence quenching with no shift in the spectrum, ruling out effective electron transfer between MEH-PPV and the fullerene derivative in the dilute solution. The incorporation of MEH-PPV in vesicles leads to a significant quenching of the fluorescence along with bathochromic shift in the spectra. A control experiment in absence of C60Ph5(OH)5 shows similar shift in the spectra of MEH-PPV due to aggregation of the polymer. However, the quenching of fluorescence is not that significant as in vesicles, proving electron transfer from MEHPPV to fullerene is significant in vesicles because of their close proximity. The lifetime of MEH-PPV was obtained from time-resolved emission decay profiles (Figure 8c). The MEH-PPV in THF solution had a lifetime of ~460 ps while the MEH-PPV inside the fullerene vesicles was quenched to ~145 ps. This quenching indicates electron transfer between fullerene and the polymer. The SEM

(b)

Figure 9. (a) SEM image of C60Ph5(OH)5 vesicles loaded with RhB, (inset) size distribution of the vesicles from DLS measurement. (b) Confocal laser scanning microscopic (CLSM) image of the vesicles. The red fluorescence is due to presence of RhB.

CONCLUSION In summary, we have shown vesicular nanostructure formation by anisotropic fullerene amphiphile C60Ph5(OH)5 in water. The molecule C60Ph5(OH)5 is an example of one of the simplest substitutions of polar groups to generate amphiphilicity in fullerenes and show vesicle formation without need for any counter ion. The work presented in this manuscript demonstrates, the self-assembly of fuller-

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ene vesicles can be controlled by modifying the kinetics with varying rate of addition of water. The vesicles are quite robust and retain their morphology in dried state. The vesicle formation was stabilized by fullerenefullerene interaction along with intermolecular Hbonding among the fullerene derivatives and with water. The presence of five hydroxyl groups on fullerene is anticipated to strengthen the H-bonding interaction and the rotation around the phenyl rings provide the required conformational flexibility for self-assembly. The substitution of hydroxyl group with methoxy breaks this Hbonding interaction resulting in loss of vesicular structures in water. Our molecular packing simulations concur with the experimental results and give a glimpse to the possible structural arrangements and interactions stabilizing vesicle formation. The applicability of the vesicles was shown by encapsulation of guest molecules – hydrophobic polymer, MEH-PPV and water-soluble dye, RhB. Such organized p and n-type π-conjugated system may come over the grand challenge of organizing the molecules at all length scales for applications like solar cells.

ASSOCIATED CONTENT Supporting Information. SEM and TEM images, Details of computational studies “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: +91-80- 22932651; Fax: +91-80-23601310

ACKNOWLEDGEMENT Authors acknowledge Prof. Jyotishman Dasgupta from TIFR, Mumbai for TCSPC data and Ms. Deepti Bopat of Confocal Facility, IISc for assistance in confocal studies. We thank Proteomics facility and NMR Research Centre, IISc for MALDI and NMR facility, respectively.

REFERENCES (1) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418-2421. (2) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Ordering Fullerene Materials at Nanometer Dimensions. Acc. Chem. Res. 2005, 38, 38-43. (3) Babu, S. S.; Möhwald, H.; Nakanishi, T. Recent Progress in Morphology Control of Supramolecular Fullerene Assemblies and Its Applications. Chem. Soc. Rev. 2010, 39, 4021-4035. (4) Nakamura, E.; Isobe, H. Functionalized Fullerenes in Water. The First 10 Years of Their Chemistry, Biology, and Nanoscience. Acc. Chem. Res. 2003, 36, 807-815. (5) Mai, Y.; Eisenberg, A. Self-Assembly of Block Copolymers. Chem. Soc. Rev. 2012, 41, 5969-5985. (6) Yin, J.-J.; Lao, F.; Fu, P. P.; Wamer, W. G.; Zhao, Y.; Wang, P. C.; Qiu, Y.; Sun, B.; Xing, G.; Dong, J. The Scavenging of Reactive Oxygen Species and the Potential for Cell Protection by Functionalized Fullerene Materials. Biomaterials 2009, 30, 611621. (7) Jiao, F.; Liu, Y.; Qu, Y.; Li, W.; Zhou, G.; Ge, C.; Li, Y.; Sun, B.; Chen, C. Studies on Anti-Tumor and Antimetastatic Activities

Page 10 of 12

of Fullerenol in a Mouse Breast Cancer Model. Carbon 2010, 48, 2231-2243. (8) Kokubo, K.; Matsubayashi, K.; Tategaki, H.; Takada, H.; Oshima, T. Facile Synthesis of Highly Water-Soluble Fullerenes More Than Half-Covered by Hydroxyl Groups. ACS Nano 2008, 2, 327-333. (9) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Self‐Assembling Supramolecular Nanostructures from a C60 Derivative: Nanorods and Vesicles. Angew. Chem. Int. Ed. 1999, 38, 2403-2405. (10) Burghardt, S.; Hirsch, A.; Schade, B.; Ludwig, K.; Böttcher, C. Switchable Supramolecular Organization of Structurally Defined Micelles Based on an Amphiphilic Fullerene. Angew. Chem. Int. Ed. 2005, 44, 2976-2979. (11) Hao, J.; Li, H.; Liu, W.; Hirsch, A. Well-Defined SelfAssembling Supramolecular Structures in Water Containing a Small Amount of C60. Chem. Commun. 2004, 602-603. (12) Muñoz, A.; Illescas, B. M.; Sánchez-Navarro, M.; Rojo, J.; Martín, N. Nanorods Versus Nanovesicles from Amphiphilic Dendrofullerenes. J. Am. Chem. Soc. 2011, 133, 16758-16761. (13) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Spherical Bilayer Vesicles of Fullerene-Based Surfactants in Water: A Laser Light Scattering Study. Science 2001, 291, 19441947. (14) Homma, T.; Harano, K.; Isobe, H.; Nakamura, E. Preparation and Properties of Vesicles Made of Nonpolar/Polar/Nonpolar Fullerene Amphiphiles. J. Am. Chem. Soc. 2011, 133, 6364-6370. (15) Yu, X; Zhang, W-B; Yue, K; Li, X; Liu, H; Xin, Y; Wang, CL; Wesdemiotis, C; Cheng, S. Z. D. Giant Molecular Shape Amphiphiles Based on Polystyrene–Hydrophilic [60]Fullerene Conjugates: Click Synthesis, Solution Self-Assembly, and Phase Behavior. J. Am. Chem. Soc 2012, 134, 7780-7787. (16) Lin, Z.; Lu, P.; Hsu, C-H.; Yue, K; Xue-Hui Dong, X-H.; Liu, H.; Guo, K.; Wesdemiotis, C.; Zhang, W-B.; Yu, X.; Cheng, S. Z. D. Self-Assembly of Fullerene-Based Janus Particles in Solution: Effects of Molecular Architecture and Solvent. Chem. Eur. J. , 2014, 20, 11630-11635. (17) Zhang, G.; Liu, Y.; Liang, D.; Gan, L.; Li, Y. Facile Synthesis of Isomerically Pure Fullerenols and Formation of Spherical Aggregates from C60 (Oh) 8. Angew. Chem. 2010, 122, 5421-5423. (18) Liu, Y.; Zhang, G.; Niu, L.; Gan, L.; Liang, D. Assembly of Janus Fullerenol: A Novel Approach to Prepare Rich Carbon Structures. J. Mater. Chem. 2011, 21, 14864-14868. (19) Sawamura, M.; Kawai, K.; Matsuo, Y.; Kanie, K.; Kato, T.; Nakamura, E. Stacking of Conical Molecules with a Fullerene Apex into Polar Columns in Crystals and Liquid Crystals. Nature 2002, 419, 702-705. (20) Harano, K; Gorgoll, R. M.; Nakamura, E. Binding of aromatic molecules in the fullerene-rich interior of a fullerenebilayer vesicle in water. Chem. Commun. 2013, 49, 7629-7631. (21) Najafov, H.; Lee, B.; Zhou, Q.; Feldman, L. C.; Podzorov, V. Observation of long-range exciton diffusion in highly ordered organic semiconductors. Nat. Mater. 2010, 9, 938-943. (22) Percec, V.; Ahn, C. H.; Ungar, G.; Yeardley, D. J. P.; Moller, M.; Sheiko, S. S. Controlling Polymer Shape through the Self-Assembly of Dendritic Side-Groups. Nature 1998, 391, 161164. (23) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of Self-Assembly of Hydrocarbon Amphiphiles into Micelles and Bilayers. J. Chem. Soc. Faraday Trans. 2 1976, 72, 1525-1568. (24) McHedlov-Petrossyan, N. O. Fullerenes in Liquid Media: An Unsettling Intrusion into the Solution Chemistry. Chem. Rev. 2013, 113, 5149-5193.

ACS Paragon Plus Environment

Page 11 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(25) Huggins, C. M.; Pimentel, G. C.; Shoolery, J. N. Proton Magnetic Resonance Studies of the Hydrogen Bonding of Phenol, Substituted Phenols and Acetic Acid. J. Phys. Chem. 1956, 60, 1311-1315. (26) Matsuo, Y.; Nakamura, E. Selective Multiaddition of Organocopper Reagents to Fullerenes. Chem. Rev. 2008, 108, 3016-3028. (27) Burger, C.; Hao, J.; Ying, Q.; Isobe, H.; Sawamura, M.; Nakamura, E.; Chu, B. Multilayer Vesicles and Vesicle Clusters Formed by the Fullerene-Based Surfactant C 60(Ch 3) 5k. J. Colloid Interface Sci. 2004, 275, 632-641. (28) Kennedy, R. D.; Halim, M.; Khan, S. I.; Schwartz, B. J.; Tolbert, S. H.; Rubin, Y. Crystal‐Packing Trends for a Series of 6, 9, 12, 15, 18‐Pentaaryl‐1‐Hydro [60] Fullerenes. Chemistry-A European Journal 2012, 18, 7418-7433. (29) Mazzier, D.; Mba, M.; Zerbetto, M.; Moretto, A. Bulky toroidal and vesicular self-assembled nanostructures from fullerene end-capped rod-like polymers. Chem. Commun., 2014, 50(35), 4571-4574. (30) Harano, K.; Gorgoll, R. M.; Nakamura, E. Binding of Aromatic Molecules in the Fullerene-Rich Interior of a Fullerene Bilayer Vesicle in Water. Chem. Commun. 2013, 49, 7629-7631.

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Content:

ACS Paragon Plus Environment

Page 12 of 12