Direct observation of electron transfer in solids through X-ray crystallography

Nanoscale electron transfer (ET) in solids is fundamental to the design of multifunctional nanomaterials, yet its process is not fully understood. Herein, through X-ray crystallography, we directly observe solid-state ET via a crystal-to-crystal process. We first demonstrate the creation of a robust and flexible electron acceptor/acceptor (A/A) double-wall nanotube crystal ([(Zn2+)4(LA)4(LA=O)4]n) with a large window (0.90 nm × 0.92 nm) through the one-dimensional porous crystallization of heteroleptic Zn4 metallocycles ((Zn2+)4(LA)4(LA=O)4) with two different acceptor ligands (2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridine (LA) and 2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridin-9(10H)-one (LA=O)) in a slow-oxidation-associated crystallization procedure. We then achieve the bottom-up construction of the electron donor incorporated-A/A nanotube crystal ([(D)2⊂(Zn2+)4(LA)4(LA=O)4]n) through the subsequent absorption of electron donor guests (D = tetrathiafulvalene (TTF) and ferrocene (Fc)). Finally, we remove electrons from the electron donor guests inside the nanotube crystal through facile ET in the solid state to accumulate holes inside the nanotube crystal ([(D•+)2⊂(Zn2+)4(LA)4(LA=O)4]n), where the solid-state ET process (D - e- → D•+) is thus observed directly by X-ray crystallography.

Multifunctional π-expanded oligothiophene macrocycles

This tutorial review summarizes recent progress in the design, synthesis, and multifunctional properties of fully conjugated macrocyclic π-systems. We focus on the π-expanded oligothiophene macrocycles after a short survey of macrocyclic conjugated loops and belts such as [n]cycloparaphenylenes, cyclic[n]para-phenylacetylenes, [4]cyclo-2,8-crysenylenes, and cyclo[n]thiophenes. Fully conjugated π-expanded oligothiophene macrocycles possess shape-persistent but sometimes pliable π-frames, and the electronic and optoelectronic properties of the macrocycles largely depend on the π-systems inserted into the oligothiophene macrocycles. Among them, the π-expanded oligothiophene macrocycle composed of 2,5-thienylenes, ethynylenes, and vinylenes is one of the most widely applicable macrocycles for constructing multifunctional π-systems. These π-expanded oligothiophene macrocycles from small to very large ring sizes can be prepared via a short step procedure, and their various solid state structures can be determined by X-ray analysis. Since these macrocycles have inner and outer domains, specific information concerning structural, electronic, and optical properties is expected. Furthermore, π-expanded oligothiophene macrocycles with alkyl substituents exhibit various morphologies depending on nanophase separation of molecules, and a morphological change is observed for the molecular switch.

Self-rolled nanotubes with controlled hollow interiors by patterned grafts

By patterning surface grafts, we propose a simple and systematic method to form tubular structures for which two-dimensional grafted sheets are programmed to self-roll into hollow tubes with a desired size of the internal cavity. The repeating pattern of grafts utilizing defect sites causes anisotropy in the surface-grafted nanosheet, which spontaneously transforms into a curved secondary architecture and, thus, becomes a potential tool with which to form and control the curvature of nanotubes. In fact, the degree and the type of graft defect allow control of the internal cavity size and shape of the resulting nanotubes. By performing dissipative particle dynamics simulations on coarse-grained sheets, we found that the inner cavity size is inversely proportional to the graft-defect density, the difference in the graft densities between the two surface sides of the layer, regardless of whether the defects are patterned or random. While a random distribution of defects gives rise to a non-uniform local curvature and often leads to twisted tubes, regular patterns of graft defects ensure uniform local curvature throughout the sheet, which is important to generate monodisperse nanotubes. At a low graft-defect density, the sheet-to-tube transformation is governed by the layer anisotropy, which induces spontaneous scrolling along the long edge of the sheet, resulting in short tubes. Thus, the curve formation rate and the cavity diameter are independent of the pattern of the graft defects. At a high graft-defect density, however, the scroll direction owing to the graft pattern may conflict with that due to the layer anisotropy. To produce monodisperse nanotubes, two factors are important: (1) a graft-defect pattern parallel to the short edge of the layer, and (2) a graft-defect area wider than half of the graft coil length.

Self-assembled gelators for organic electronics

Nature excels at engineering materials by using the principles of chemical synthesis and molecular self-assembly with the help of noncovalent forces. Learning from these phenomena, scientists have been able to create a variety of self-assembled artificial materials of different size, shapes, and properties for wide ranging applications. An area of great interest in this regard is solvent-assisted gel formation with functional organic molecules, thus leading to one-dimensional fibers. Such fibers have improved electronic properties and are potential soft materials for organic electronic devices, particularly in bulk heterojunction solar cells. Described herein is how molecular self-assembly, which was originally proposed as a simple laboratory curiosity, has helped the evolution of a variety of soft functional materials useful for advanced electronic devices such as organic field-effect transistors and organic solar cells. Highlights on some of the recent developments are discussed.

Organogelators based on TTF supramolecular assemblies: synthesis, characterization, and conductive property

A closely related family of organogelators 1-2 appended one or two electroactive tetrathiafulvalene (TTF) residues, has been designed and readily synthesized by Sonogashira reactions. These compounds can gelate a variety of organic solvents in view of multiple intermolecular interactions, and compounds 2 with two TTF subunits exhibit higher gelation ability than their corresponding 1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigation of the xerogels from toluene gave a visual image showing that fibrillar aggregates are entangled in three-dimensional network structures. The columnar TTF cores stacking in the fiber, evidenced by the mixed-valence states absorption at around 2000 nm in ultraviolet-visible-near-infrared (UV-vis-NIR) spectra, provide an efficient pathway for the electron conduction. Upon oxidized by iodine, these xerogels exhibit semiconductive behaviors with moderate levels of conductivity. Additionally, the electrical conductivity of doped-xerogels 2 is 1 order of magnitude higher than that of 1 under identical conditions.

Facile preparation of concentration-gradient materials with radical spin of the mixed-valence tetrathiafulvalene in conventional polymer films

We describe here the facile methods for the stepwise regulation of the formation of tetrathiafulvalene (TTF) radical cation and the self-assembly to the mixed-valence state of TTF in various polymer films. The homogeneous polymer composites containing TTF and (1R)-3,3-dimethylbicyclo[2.2.1]hepta-2-one-1-triflate as a photoinduced acid generator in several kinds of conventional polymers were prepared, and the phototriggered conversion of TTF to radical cation via protonation in the films was executed. The amount of the TTF radical species can be controlled by the length of UV-irradiation time to the polymer composite films. In addition, the formation of the mixed-valence TTF can be observed in the composite films by annealing after UV irradiation when the glass transition temperature of the matrices was sufficiently higher than the reaction temperature during the photoirradiation. Finally, we presented the facile patterning method for preparing the composite with the concentration gradient of TTF radical and the mixed-valence TTF.

Photoinduced radical generation and self-assembly of tetrathiafulvalene into the mixed-valence state in the poly(vinyl chloride) film under UV irradiation

The photoinduced self-assembly and the formation of the mixed-valence state of tetrathiafulvalene (TTF) in the solid state are reported. The polymer composites containing TTF in poly(vinyl chloride) (PVC) were prepared, and oxidation of TTF by chlorine radical generated by UV irradiation in PVC was investigated. The formation of the mixed-valence state of TTF in the composite films by UV irradiation was observed, and the resulting TTF radical species, including the mixed-valence state after the photopatterning, exhibited extremely high stability in the composite films. Finally, we performed the fabrication of the gradient materials of the radical concentrations on the TTF/PVC composites with the photopatterning.

Halloysite clay nanotubes for controlled release of protective agents

Halloysite aluminosilicate nanotubes with a 15 nm lumen, 50 nm external diameter, and length of 800 +/- 300 nm have been developed as an entrapment system for loading, storage, and controlled release of anticorrosion agents and biocides. Fundamental research to enable the control of release rates from hours to months is being undertaken. By variation of internal fluidic properties, the formation of nanoshells over the nanotubes and by creation of smart caps at the tube ends it is possible to develop further means of controlling the rate of release. Anticorrosive halloysite coatings are in development and a self-healing approach has been developed for repair mechanisms through response activation to external impacts. In this Perspective, applications of halloysite as nanometer-scale containers are discussed, including the use of halloysite tubes as drug releasing agents, as biomimetic reaction vessels, and as additives in biocide and protective coatings. Halloysite nanotubes are available in thousands of tons, and remain sophisticated and novel natural nanomaterials which can be used for the loading of agents for metal and plastic anticorrosion and biocide protection.