Fullerene Derivatives That Bear Aliphatic Chains as Unusual Surfactants: Hierarchical Self-Organization, Diverse Morphologies, and Functions

A compound eye-like morphology formed through hexagonal array of hemispherical microparticles where an alkyl-fullerene derivative self-assembled at atmosphere-sealed air/water interface

Self-assembly processes are widely used in nature to form hierarchically organized structures, prompting us to investigate such processes at the macroscopic scale. We report an unprecedented approach toward the self-assembly of alkyl-fullerene (C60) derivatives into a hexagonal array of hemispherical microparticles akin to the morphology of a compound eye. The method includes casting solvated alkyl-C60compound on an air/water interface followed by controlled evaporation of the solvent under atmosphere-sealed conditions. This leads to the formation of a thin film floating on water with a diameter of up to 1.3 centimeters and exhibiting a hexagonally-packed hemispherical structure with a diameter of approximately 38µm. Various measurements of the formed film reveal that amorphousness is necessary for suppressing uncontrollable crystallization, which affects the microparticle size and film formation mechanism. We tested the feasibility of this approach for the self-assembly of a relatively common C60derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), resulting in the formation of a film with a similar pattern of hexagonally-packed larger microparticles approximately 152µm in size of diameter.

Characterization of an Amphiphilic Janus-Type Surface in the Cellulose Nanofibril Prepared by Aqueous Counter Collision

Cellulose nanofibrils, which attract extensive attention as a bio-based, sustainable, high-performance nanofibril, are believed to be predominantly hydrophilic. This study aimed to prove the presence of an amphiphilic "Janus-type fiber surface" in water with hydrophobic and hydrophilic faces in a cellulose nanofibril (ACC-CNF) that was prepared by the aqueous counter collision method. We clarified the surface characteristics of the ACC-CNF by confocal laser scanning microscopy with a carbohydrate-binding module and congo red probes for the hydrophobic planes on the cellulose fiber surfaces and calcofluor white as hydrophilic plane probes. The results indicated the presence of both characteristic planes on a single ACC-CNF surface, which verifies an amphiphilic Janus-type structure. Both hydrophobic probes adsorbed onto ACC-CNFs for the quantitative evaluation of the degree of ACC-CNF surface hydrophobicity by Langmuir's adsorption theory based on the optimal maximum adsorption amounts for various starting raw material types.

Molecular dynamics simulations on fullerene surfactants with different charges at the air-water interface

The interfacial activity of fullerene surfactants at the air-water interface is studied via molecular dynamics and metadynamics simulations. Fullerene surfactants with different charges show different surface activity. Meanwhile, studies show that fullerene surfactants with zero or one positive charge show interesting interface behaviour, i.e. the hydrophobic fullerene of the fullerene surfactant with zero charge orients to bulk water while the fullerene surfactant with one positive charge can be a hydrophilic and hydrophobic rotator at the air-water interface.

Synthesis, crystal structure, self-assembly of C60 derivatives bearing rigid pyridine substituents

Microstructures of fullerene derivatives formed via self-assembly strategy facilitate the versatile applications of these zero-dimensional molecules. However, the accurate elucidation of formation mechanism of fullerene microstructures is a challenge issue. A novel fullerene derivative 2 with rigid pyridine substituent was synthesized and characterized by X-ray crystallography. Using the strategy of liquid-liquid interfacial precipitation, self-assembly of 2 affords a micrometer-sized flowerlike and a discoid morphology. Based on the crystal packing of 2, the proper formation mechanism of different morphologies was proposed. Meanwhile, the photoelectrochemical properties of different morphologies of 2 was also unveiled.

Fullerene Building Blocks with Tailor-Made Solubility and New Insights into Their Hierarchical Self-Assembly

Herein, the synthesis of fullerene derivatives with adjustable polarities and lyotropic aggregation properties is reported. The polarity range spans from superhydrophobic to hydrophilic, while simultaneously providing a further reactive position with a view to graft them onto other materials. The synthetic strategy relies on a selective protection with an isoxazoline moiety. The remaining octahedral positions were further functionalized with the desired groups to tune their solubility, yielding mixed [5:1] hexakisadducts. The subsequent deprotection by clean photolytic reaction led to fullerene pentakisadducts with an incomplete octahedral addition pattern, which are useful forerunners for the synthesis of building blocks. Their hydrophobic/hydrophilic behavior has been characterized both in solution and surface through octanol/water partition coefficients (log P) and contact angle measurements. Furthermore, these derivatives can form supramolecular constructions which have been studied by dynamic light scattering (DLS) and cryo‐TEM.

Supramolecular Differentiation for Construction of Anisotropic Fullerene Nanostructures by Time-Programmed Control of Interfacial Growth

Supramolecular assembly can be used to construct a wide variety of ordered structures by exploiting the cumulative effects of multiple noncovalent interactions. However, the construction of anisotropic nanostructures remains subject to some limitations. Here, we demonstrate the preparation of anisotropic fullerene-based nanostructures by supramolecular differentiation, which is the programmed control of multiple assembly strategies. We have carefully combined interfacial assembly and local phase separation phenomena. Two fullerene derivatives, PhH and C12H, were together formed into self-assembled anisotropic nanostructures by using this approach. This technique is applicable for the construction of anisotropic nanostructures without requiring complex molecular design or complicated methodology.

Supramolecular One-Dimensional n/p-Nanofibers

Currently, there is a broad interest in the control over creating ordered electroactive nanostructures, in which electron donors and acceptors are organized at similar length scales. In this article, a simple and efficient procedure is reported en-route towards the construction of 1D arrays of crystalline pristine C₆₀ and phenyl-C₆₁-butyric acid methyl ester (PCBM) coated onto supramolecular fibers based on exTTF-pentapeptides. The resulting n/p-nanohybrids have been fully characterized by a variety of spectroscopic (FTIR, UV-Vis, circular dichroism, Raman and transient absorption), microscopic (AFM, TEM, and SEM), and powder diffraction (X-ray) techniques. Our experimental findings document the tendency of electroactive exTTF-fibers to induce the crystallization of C₆₀ and PCBM, on one hand, and to afford 1D n/p-nanohybrids, on the other hand. Furthermore, photogenerated radical ion pairs, formed upon visible light irradiation of the n/p-nanohybrids, feature lifetimes on the range of 0.9–1.2 ns.

Fulleropeptide esters as potential self-assembled antioxidants

The potential use of amphiphilic fullerene derivatives as a bionanomaterial was investigated by cyclic voltammetry (CV), scanning electron microscopy (SEM), and the ferrous ion oxidation-xylenol orange (FOX) method. Despite the disrupted delocalization of the π-electronic system over the C60 sphere, its antioxidant capacity remained high for all twelve derivatives. The compounds expressed up to two-fold and 5-12-fold better peroxide quenching capacity as compared to pristine C60 and standard antioxidant vitamin C, respectively. During precipitation and slow evaporation of the solvent, all compounds underwent spontaneous self-assembly giving ordered structures. The size and morphology of the resulting particles depend primarily on the sample concentration, and somewhat on the side chain structure.

Fulleropyrrolidines derived from dioxa- and trioxaalkyl-tethered diglycines

The synthesis, characterization, morphological, electrochemical and antioxidant properties of fulleropyrrolidines bridged by polyoxaalkyl chains are described.

Electronic and electrochemical properties as well as flowerlike supramolecular assemblies of fulleropyrrolidines bearing ester substituents with different alkyl chain lengths

Flowerlike supramolecular architectures, obtained from a series of energetically PCBM-like and high C₆₀ content (76–79%) fulleropyrrolidines FP1–FP4, were lamellar structures with alkyl chain length dependent thickness of a bilayer structure.

Nanoplasmonic modification of the local morphology, shape, and wetting properties of nanoflake microparticles

Inducing a phase transition of a self-organized object may trigger its structural transformation. Here, we demonstrate local control of the morphology and shape of self-organized microparticles with a nanoflake outer surface by nanoplasmonic heating. To increase the photothermal efficiency of the microparticles, gold nanoparticles (AuNPs) or single-walled carbon nanotubes (SWCNTs) were incorporated. AuNPs and SWCNTs, which have excellent photothermal activity, acts as photoresponsive heat converters. Because they have distinct absorption characteristics, visible or near-infrared lasers can be used to induce local heating. The photothermal effect was used to spatially confine the melting to the space within the particle and the aggregate; as a result, microparticles with various shapes and morphologies have been fabricated. Such morphological changes lead to a superhydrophobic-hydrophobic wetting transition, which was confirmed by the films constituting the microparticles. The work presented is seen useful for anisotropic particle synthesis, local wetting control, lithography, and morphological control of functional materials.

Optoelectronic functional materials based on alkylated-π molecules: self-assembled architectures and nonassembled liquids

The engineering of single molecules into higher-order hierarchical assemblies is a current research focus in molecular materials chemistry. Molecules containing π-conjugated units are an important class of building blocks because their self-assembly is not only of fundamental interest, but also the key to fabricating functional systems for organic electronic and photovoltaic applications. Functionalizing the π-cores with "alkyl chains" is a common strategy in the molecular design that can give the system desirable properties, such as good solubility in organic solvents for solution processing. Moreover, the alkylated-π system can regulate the self-assembly behavior by fine-tuning the intermolecular forces. The optimally assembled structures can then exhibit advanced functions. However, while some general rules have been revealed, a comprehensive understanding of the function played by the attached alkyl chains is still lacking, and current methodology is system-specific in many cases. Better clarification of this issue requires contributions from carefully designed libraries of alkylated-π molecular systems in both self-assembly and nonassembly materialization strategies. Here, based on recent efforts toward this goal, we show the power of the alkyl chains in controlling the self-assembly of soft molecular materials and their resulting optoelectronic properties. The design of alkylated-C60 is selected from our recent research achievements, as the most attractive example of such alkylated-π systems. Some other closely related systems composed of alkyl chains and π-units are also reviewed to indicate the universality of the methodology. Finally, as a contrast to the self-assembled molecular materials, nonassembled, solvent-free, novel functional liquid materials are discussed. In doing so, a new journey toward the ultimate organic "soft" materials is introduced, based on alkylated-π molecular design.

Photoconductivity and enhanced memory effects in hybrid C60-graphene transistors

We describe the observation of photoconductivity and enhanced memory effects in graphene devices functionalized with clusters of alkylated C(60) molecules. The alkylated C(60) clusters were adsorbed on chemical vapor deposition-grown graphene devices from an aprotic medium. The resulting alkylated C(60)-graphene hybrid devices showed reproducible photoconductive behavior originating from the electron-accepting nature of the C(60) molecules. Significantly enhanced gate hysteresis was observed upon illumination with visible light, thereby enabling the use of C(60)-graphene hybrid devices in three-terminal photo-memory applications.

Location of [60]fullerene incorporation in lipid membranes

We confirmed that most C(60) fullerene units are located in the hydrophobic core of the lipid bilayer membrane in water-soluble lipid membrane incorporated C(60) (LMIC(60)) complexes using differential scanning calorimetry (DSC) and (13)C NMR spectra in the presence of radical labels.

Tunable one-, two-, and three-dimensional self-assemblies from an acceptor-donor fullerene-N,N-dimethylaminoazobenzene dyad: interfacial geometry and temporal evolution

Controllable fabrication of spontaneously ordered and varied geometry fullerene C(60) based molecular architecture was achieved upon hierarchical self-assembly of the fullerene-N,N-dimethylaminoazobenzene acceptor-donor hybrid (DPNME). Simple preparation techniques, such as Langmuir-Blodgett (LB), solution-cast, and immersion at the liquid-air and solid-air interfaces, were used without templates as a function of DPNME concentration, media pH, time, and supporting substrate characteristics. The resulting structures depending upon the preparation methods were investigated with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and molecular modeling, which revealed a delicate role of intermolecular donor-acceptor, π-π, and van der Waals interactions between the electron deficient fullerene core and the N,N-dimethylaminoazobenzene electron donor under neutral conditions. Upon protonation, the electrostatics associated with the charged DPNME moiety and the dominant intermolecular fullerene-fullerene interactions guided the self-assembly process. Increased time scales led the molecular subunits to grow by maximizing the most favored orientations and yielded one-dimensional (1D) and two-dimensional (2D) structures in neutral and acidic conditions, respectively, which upon solvent evaporation formed the final multipods or stacked squares upon oriented attachment. For the protonated DPNME, 2D lamellar sheets formed from the bilayers gained cohesive energy, forming ultimately rectangular sheets. Interestingly, the Si(100) supported multilayer DPNME Langmuir films as a function of surface pressure and pH yielded a uniform and directional structure pattern in comparison with the geometry obtained from drop casting methods. This controllable structure architecture of the fullerene-azobenzene hybrid opens up a new alley in fullerene C(60) based self-assembly.

Multifunctional, polymorphic, ionic fullerene supramolecular materials: self-assembly and thermotropic properties

An N-methylfulleropyrrolidine (2) bearing three eicosyloxy chains on the laterally substituted phenyl group can be further functionalized to give the ionic fullerene derivative, i.e., N,N-dimethylfulleropyrrolidinium iodide (1). The spectroscopic, electrochemical, self-assembly, and liquid crystalline properties of 1 have been investigated and compared to its neutral precursor 2. Changes in electronic structure upon ionization are observed in the UV spectra. Additionally, a positive potential shift of electrochemical reductions for 1 compared to those of 2 is noted in both homogeneous solution and film state. Driven by the π-π, van der Waals, and electrostatic interactions, the ionic compound 1 is able to form a variety of functional and polymorphic self-assembled structures both from solution and on substrates, including hierarchically organized flakelike microparticles with high water repellency, doughnut-shaped objects with rough surfaces, and long one-dimensional C(60) nanowires (>1 μm). The thermotropic behavior of 1 has also been investigated, and a smectic liquid crystalline phase was observed at elevated temperatures. Further investigations of the thermotropic behavior of 1 revealed that a deionization back-reaction from 1 to the neutral precursor 2 gradually occurred. The mechanism of this deionization reaction is presented and discussed. These investigations provide insight into the effects of added ionicity to alkylated fullerene derivatives, in particular on their self-assembly features and functionality.

Thermosolutal self-organization of supramolecular polymers into nanocraters

The ability of two complementary molecular modules bearing H-bonding uracilic and 2,6-(diacetylamino)pyridyl moieties to self-assemble and self-organize into submicrometer morphologies has been investigated by means of spectroscopic, thermogravimetric, and microscopic methods. Using uracilic (3)N-BOC-protected modules, it has been possible to thermally trigger the self-assembly/self-organization process of the two molecular modules, inducing the formation of objects on a mica surface that exhibit crater-like morphology and a very homogeneous size distribution. Confirmation of the presence of the hydrogen-bonding-driven self-assembly/self-organization process in solution was obtained by variable-temperature (VT) steady-state UV-vis absorption and emission measurements. The variation of the geometric and spatial features of the morphologies was monitored at different T by means of atomic force microscopy (AFM) and was interpreted by a nonequilibrium diffusion model for two chemical species in solution. The formation of nanostructures turned out to be affected by the solid substrate (molecular interactions at a solid-liquid interface), by the matter-momentum transport in solution (solute diffusivity D(0) and solvent kinematic viscosity ν), and the thermally dependent cleavage reaction of the BOC functions (T-dependent differential weight loss, θ = θ(Τ)) in a T interval extrapolated to ∼60 K. A scaling function, f = f (νD(0), ν/D(0), θ), relying on the onset condition of a concentration-driven thermosolutal instability has been established to simulate the T-dependent behavior of the structural dimension (i.e., height and radius) of the self-organized nanostructures as ⟨h⟩ ≈ f (T) and ⟨r⟩ ≈ 1/f (T).

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials

The self-assembly and self-organization behavior of chromophoric acetylenic scaffolds bearing 2,6-bis(acetylamino)pyridine (1, 2) or uracyl-type (3-9) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2, thanks to a combination of solvophilic/solvophobic forces and π-π stacking interactions, undergo self-organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide-field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2, a more uniform size distribution is found (80-200 nm) compared to 1 (20-1000 nm). Temperature scans in the range 283-353 K show that the self-organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen-bonding units 3-9. Depending on the specific geometrical structure of 3-9, different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3, which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis-uracyl derivative 4 or 5, a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3, which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self-assembly was studied in parallel with modules 8 and 9, in which the uracyl recognition sites are protected with tert-butyloxycarbonyl (BOC) groups. This strategy allows further control of the self-assembly/self-organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC-protected ditopic modules 8 self-assemble and self-organize with 1 into ordered linear nanostructures, whereas BOC-protected tritopic system 9 gives rise to extended domains of circular nano-objects in combination with 1.