Linear Self - Assembled Systems and the Effect of Capping Defects

Molecularself-assemblyplaysan important role inmany systems, ranging from zeolites1 to biological systems2-4 andmodified surfaces.5 Ofparticular interest are systems that assemble into single-molecule thick fibers that may be millimetres in extent. Such linear self-assembled supramolecular structureshave technological importance in the areas of molecular fibers,3,6-11 wires,12,13 and channels.13-15 Systems that exhibit linear self-assembly are thought to associate through ionic interactions, hydrogen bonding, or aromatic π-stacking, which are facilitated by the molecule’s shape, polarizability, or solubility. Moleculeshavingelectronrichcomponents that are flat and stackable, such as polyaromatics or metalloporphyrins, self-assemble into linear structures that mightserveasmolecularwires. Moleculeshavingahollow interior, such as cyclodextrins and crown ethers, or the flexibility to assemble into open helices can serve as molecular channels. Inmost of the literature systems, the formationof linear aggregates is reversible and the system exhibits an exceptionally lowcriticalaggregationconcentration (CAC). Below the CAC, molecules are largely unassociated; however, above the CAC, supramolecule self-assembly leads toabroaddistribution inaggregate lengths. Itwould be a technological advantage to use the reversibility of the system to control the size or length of the aggregates and then to convert the reversible supramolecules into a permanent structure through polymerization. For example, a self-assembled channel needs to be tailored to the thickness of themembrane that it is to span, whereas a molecular wire should be long and defect free. This note uses equilibrium concepts to demonstrate that the additionofdefectmolecules canshift themeanaggregation length, narrow the length distribution, and significantly raise the CAC.