Redox-Responsive Molecular Systems and Materials.

Redox reactions can alter the electronic, optical, and magnetic properties of molecules and their ensembles by adding or removing electrons. Here, the developments made over the past 10 years using molecular events are discussed, such as assembly/disassembly, transformation of ensembles, geometric changes, and molecular motions that are designed to be redox-responsive. Considerable progress has occurred in the application of these events to the realization of electronic memory, color displays, actuators, adhesives, and drug delivery. In these cases, systems behave in either a highly or a poorly correlated manner depending on the number of redox-active units involved, based on the method of integration. One of the great advantages of redox-responsive devices and materials is that they have the potential to be readily integrated into existing electronic technologies.

Self-Sorting in the Formation of Metal-Organic Nanotubes: A Crucial Role of 2D Cooperative Interactions.

A mixture of ferrocene-based tetratopic pyridyl ligands FcL1 and FcL2 undergoes self-sorting upon competitive coordination with AgBF4, affording homomeric nanotubes FcNT1 and FcNT2 as a mixture. No mutual interference for the nanotubular growth occurred between FcNT1 and FcNT2 even when one of these ligands was used in large excess with respect to the other. 2D X-ray diffraction analysis of unidirectionally oriented nanotube samples, prepared by using the capillary technique, revealed that although FcL1 as reported previously stacks helically in the resulting nanotube FcNT1 FcL2 prefers to stack with no discernible helical twist in FcNT2. Such a difference in their stacking geometries is most likely a major reason for why mixed-ligand metal-organic nanotubes are not constructed upon competitive coordination of FcL1 and FcL2 with AgBF4.

Metal-Organic Nanotube with Helical and Propeller-Chiral Motifs Composed of a C10-Symmetric Double-Decker Nanoring.

Coassembly of an achiral ferrocene-cored tetratopic pyridyl ligand (FcL) with AgBF4 in CH2Cl2/MeCN (7:3 v/v) containing chiral Bu4N(+) (+)- or (-)-menthylsulfate (MS*(-)) results in the formation of an "optically active" metal-organic nanotube (FcNT) composed of a C10-symmetric double-decker nanoring featuring 10 FcL units and 20 Ag(+) ions. The circular dichroism spectrum of FcNT along with its 2D X-ray diffraction (2D XRD) pattern indicates that the constituent metal-organic nanorings in FcNT stack one-handed helically on top of each other. A crystal structure of the dimeric double-decker model complex (Ag2(FcL')2) from a ditopic ferrocene ligand (FcL') and AgBF4 allowed for confirming the binding of MS*(-) onto the Ag(+) center of the complex. The results of detailed spectroscopic studies indicate that in its double-decker aromatic arrays, FcNT possibly possesses propeller-chiral twists in addition to the helically chiral structure, where the former is considerably more dynamic than the latter. Notably, both chiral structural motifs responded nonlinearly to an enantiomeric excess of MS*(-) (majority rule) though with no stereochemical influence on one another.

Manipulation of discrete nanostructures by selective modulation of noncovalent forces.

Covalent organic synthesis commonly uses the strategy of selective bond cleavage and formation. If a similar approach can be applied stepwisely to noncovalent synthesis, more exotic or challenging nanostructures might become achievable. Here, we report that ferrocene-based tetratopic pyridyl ligands, which can dynamically change their geometry by means of thermal rotation of their cyclopentadienyl rings in solution, assemble with AgBF4 into discrete metal-organic nanotubes with large and uniform diameters. The nanotubes can be cut into metal-organic nanorings through selective attenuation of the inter-nanoring interaction via ferrocene oxidation. The resultant nanorings can be transferred onto inorganic substrates electrostatically or allowed to reassemble to form the original nanotube by the reductive neutralization of their oxidized ferrocene units.

Propeller-shaped fused oligothiophenes: a remarkable effect of the topology of sulfur atoms on columnar stacking.

Propeller-shaped regioisomers of fused oligothiophenes F9T(endo), F9T(anti), and F9T(exo) were successfully synthesized. DFT calculations indicated that their core parts are distorted from planarity due to intramolecular steric repulsions involving large sulfur atoms. In contrast with soft crystalline F9T(anti) and F9T(exo), F9T(endo) self-assembles into a hexagonal columnar liquid crystal (Col(h) LC), displaying a clear X-ray diffraction (XRD) due to its stacked π-conjugated core. In each LC column, well-organized intermolecular S-S contacts are developed triple-helically along the columnar axis with a helical pitch of 4.04 nm. Among LC semiconductors reported to date, Col(h) LC F9T(endo) displays a top-class charge-carrier mobility (0.18 cm(2) V(-1) s(-1)) with a distinct ambipolar character featuring well-balanced hole and electron mobilities. A thin film, prepared by mixing F9T(endo) with soluble fullerene PCBM, shows a photovoltaic response, when the fullerene content is large enough to compensate a small absorptivity of F9T(endo) for visible light.

Coupling of a C2-chiral ferrocene with phenylalkynyl groups: novel ferrocenophanes carrying multiple chiral ferrocenyl units.

Upon treatment with CuCl/TMEDA (1:1) at a high [CuCl]/[1] ratio, enantiopure C(2)-chiral tetrasubstituted ferrocene 1 carrying phenylalkynyl groups is transformed into a novel family of chiral ferrocenophanes 2 and 3 with their ferrocenyl joints connected by a pi-conjugated rigid bridge. Compound 3, which is reminiscent of the Escher's endless staircase, is a single conformer with a one-handed helical chirality.