Advanced Materials

Fishing of graphitic nanotubes with a macroscopic glass hook: A ∼30 mm long fiber (see figure and cover) is readily processed from a suspension of self-assembled nanotubes with one-handed helical chirality, formed from the (R)- or (S)-enantiomer of chiral amphiphile 2. The majority of the nanotubes in the fiber are unidirectionally oriented along the fiber axis. Upon doping with I₂, the fiber displays an anisotropic electrical conduction along the fiber axis more than an order of magnitude greater than that across the fiber axis.

Highly luminescent Cu^I complexes with one phenanthroline and one (bis[2-(diphenylphosphino)phenyl]ether) ligand show an emission quantum yield of up to 28 % upon deactivation of the metal-to-ligand charge-transfer excited states; their X-ray crystal structure shows a distorted tetrahedral geometry (see figure and inside cover). One of these compounds is used as an active material in a light-emitting electrochemical cell; it exhibits an efficiency similar to that of Ru^II-type complexes.

This Special Issue on supramolecular approaches to organic electronics and nanotechnology, borne out of a symposium of the European Materials Research Society meeting in 2004, brings together a number of excellent contributions from many leading scientists in supramolecular-systems research. Stimulating highlights of the design, fabrication, characterization, and exploitation of supramolecular objects are provided.

Transition metal-containing catenanes and rotaxanes are ideally suited to building “molecular machines”. This review describes some examples of prototypes elaborated in Strasbourg, focusing on light-driven systems using ruthenium(II) complexes. An electrochemically driven system and a linear rotaxane dimer whose behavior is reminiscent of that of muscles (see figure) are also presented.

Porphyrins, phthalocyanines, and perylenes often serve as building blocks for molecular materials with unique electronic, magnetic, and photophysical properties. Some recent efforts to construct highly organized arrays of these chromophores by means of self-assembly and by attaching them to polymer scaffolds (see figure) are described.

Enhanced molecular absorption of dyes in the visible range, mediated by surface plasmon excitations on metallic hole arrays, is reported. The combination of these structures with a photochromic cover layer can be used to create an all-optical switching system, essential for the development of active plasmonic devices. The figure shows transmission images of the coated hole array before (left) and after (right) UV irradiation.

An L-proline-derived amphiphilic squaraine dye displays unique self- assembly behavior in solution, forming new aggregates with large and symmetrical exciton splitting, while forming layered structures by an unprecedented “self-templating” effect on graphite surfaces. This unusual aggregation behavior is most likely the result of the dye's chiral molecular structure (see figure). These observations should facilitate the design of related self-assembled materials for (supra)molecular electronics.

Supramolecular fibers possessing an intrinsic large dipole moment can be manipulated with a DC electric field, triggering their alignment in preferred directions on a patterned surface made of Au electrodes supported on an insulating SiO_x/Si substrate. The figure shows the electric-field stream lines over the electrodes used to align the nanofibers, calculated using finite element analysis.

Supramolecular electronic coupling in chiral supramolecular nanostructures built with functionalized oligothiophenes (see figure) is investigated using optical spectroscopy. These are model systems to investigate such electronic interactions due to thermotropic reversibility of the assembly process. The electronic coupling of neutral excitations depends strongly on their spatial extent.

A monolayer-pattern replication concept is demonstrated based on direct electrochemical transfer of information through nanometric water bridges between two contacting organosilane monolayers self-assembled on silicon wafer surfaces (see figure). These findings bear particular relevance to the advancement of a supramolecular chemical approach to nanofabrication.

Langmuir films of a tricationic rotaxane and its dumbbell-shaped component are formed by cospreading with the amphiphilic anion dihexadecylphosphate at an air/water interface (see figure). The rotaxane behaves as an acid–base-controllable molecular shuttle in solution. Langmuir–Blodgett multilayers of either the rotaxane or the dumbbell form thin films that exhibit reversible switching behavior upon exposure to acids and bases.

Fishing of graphitic nanotubes with a macroscopic glass hook: A ∼30 mm long fiber (see figure and cover) is readily processed from a suspension of self-assembled nanotubes with one-handed helical chirality, formed from the (R)- or (S)-enantiomer of chiral amphiphile 2. The majority of the nanotubes in the fiber are unidirectionally oriented along the fiber axis. Upon doping with I₂, the fiber displays an anisotropic electrical conduction along the fiber axis more than an order of magnitude greater than that across the fiber axis.

Fast resonant energy transfer (RET) takes place from oligo(phenylenevinylene) (OPV_n) segments to C₆₀ in OPV_n–C₆₀ dyads (see figure) as a result of the presence of multiple energy-transfer pathways and of significant electronic couplings (when going beyond the point-dipole model).

Inkjet printing of a hydrogen-peroxide-based “ink” is used to oxidize conducting polymer thin films to produce patterned organic LEDs. Picoliter droplets of hydrogen peroxide oxidize the PEDOT film, rendering well- defined areas on the electrode non- electroluminescent. The figure shows a computer-generated image and the resulting patterned OLED.

Highly luminescent Cu^I complexes with one phenanthroline and one (bis[2-(diphenylphosphino)phenyl]ether) ligand show an emission quantum yield of up to 28 % upon deactivation of the metal-to-ligand charge-transfer excited states; their X-ray crystal structure shows a distorted tetrahedral geometry (see figure and inside cover). One of these compounds is used as an active material in a light-emitting electrochemical cell; it exhibits an efficiency similar to that of Ru^II-type complexes.

Nanoscale phase-segregated functional architectures can be formed both in monolayers at the solid/liquid interface and in the bulk by self-assembling electron-acceptor–electron-donor dyads and triads based on hexa-peri-hexabenzocoronene (HBC) and perylene monoimide (PMI). The figure shows charge transfer through the vertically segregated HBC and PMI channels in the bulk phase.

Molecular junctions based on self- assembled monolayers (SAMs), illustrated in the figure, are attractive alternatives to solid-state materials for molecular electronics because their resistivity can be tuned by altering their electronic structure. Like macroscale solid-state transistors, these molecular junctions can also show negative differential resistance when their redox potential is controlled electrochemically.