No abstract

Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.

To study the supramolecular polymerisation mechanisms of a self-assembling system, concentration- and temperature-dependent measurements can both be used to probe the transition from the molecular dissolved state to the aggregated state. In this report, both methods are evaluated to determine their effectiveness in identifying and quantifying the self-assembly mechanism for isodesmic and cooperative self-assembling systems. It was found that for a rapid and unambiguous determination of the self-assembly mechanism and its thermodynamic parameters, temperature-dependent measurements are more appropriate. These studies allow the acquisition of a large data set leading to an accurate determination of the self-assembly mechanism and quantification of the different thermodynamic parameters that describe the supramolecular polymerisation. For a comprehensive characterisation, additional concentration-dependent measurements can be performed.

The conditions required for supramolecular electronics, e.g. nano-sized optoelectronic devices, will be illustrated on the basis of the programmed self-assembly of pi-conjugated systems into individual nanosized wires. Using the supramolecular design rules nanowires can be created from almost any polymeric and oligomeric pi-conjugated system. In the case of oligomers it is even possible to construct individual wires having a uniform diameter of one molecule thickness. The construction of wires on a substrate is possible by self-assembly in solution or during the deposition. The transfer of the supramolecular stacks from solution to a solid support is a very delicate process. A comprehensive knowledge of all intermolecular interactions gives rise to controlled transfer of pi-conjugated assemblies to specific surfaces. There are a large number of very appealing targets that should be reached before supramolecular electronics can serve as an attractive alternative in between single molecule electronics and bulk devices. Nevertheless, the combination of exciting scientific results and intriguing technological challenges creates an interesting future for supramolecular electronics.

Comparative studies on hydrogen-bonded versus covalently linked donor-acceptor-donor dye arrays obtained from oligo(p-phenylene vinylene)s (OPVs) as donor and bay-substituted perylene bisimides (PERYs) as acceptor dyes are presented. Both systems form well-ordered J-type aggregates in methylcyclohexane, but only hydrogen-bonded arrays afford hierarchically assembled chiral OPV-PERY dye superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study). In the case of hydrogen-bonded arrays, the stability of the aggregates in solution increases with increasing conjugation length of the OPV unit. The well-defined co-aggregated dyes presented here exhibit photoinduced electron transfer on subpicosecond time scale, and thus, these supramolecular entities might serve as valuable nanoscopic functional units.