Molecular gauge blocks for building on the nanoscale

Abstract

Molecular gauge blocks, based on 1-7, 9-11 paraxylene rings, have been synthesized as part of a homologous series of oligoparaxylenes (OPXs) with a view to providing a molecular tool box for the construction of nano architectures-such as spheres, cages, capsules, metal-organic frameworks (MOFs), metal-organic polyhedrons (MOPs) and covalent-organic frameworks (COFs), to name but a few-of well-defined sizes and shapes. Twisting between the planes of contiguous paraxylene rings is generated by the steric hindrance associated with the methyl groups and leads to the existence of soluble molecular gauge blocks without the need, at least in the case of the lower homologues, to introduce long aliphatic side chains onto the phenylene rings in the molecules. Although soluble molecular gauge blocks with up to seven consecutive benzenoid rings have been prepared employing repeating paraxylene units, in the case of the higher homologues it becomes necessary to introduce hexyl groups instead of methyl groups onto selected phenylene rings to maintain solubility. A hexyl-doped compound with seven substituted phenylene rings was found to be an organogelator, exhibiting thermally reversible gelation and a critical gelation concentration of 10 mM in dimethyl sulfoxide. Furthermore, control over the morphology of a series of hexyl-doped OPXs to give microfibers, microaggregates, or nanofibers, was observed as a function of their lengths according to images obtained by scanning electron microscopy. The modular syntheses of the paraphenylene derivatives rely heavily on Suzuki-Miyaura cross-coupling reactions. The lack of π-π conjugation in these derivatives that is responsible for their enhanced solubilities was corroborated by UV/Vis and fluorescent spectroscopy. In one particular series of model OPXs, dynamic 1H NMR spectroscopy was used to probe the stereochemical consequences of having from one up to five axes of chirality present in the same molecule. The Losanitsch sequence for the compounds with 1-3 chiral axes was established, and a contemporary mathematical way was found to describe the sequence. The development of the ways and means to make molecular gauge building blocks will have positive repercussions on the control of nanostructures in general. Their incorporation into extended structures with the MOF-74 topology provides an excellent demonstration of the potential usefulness of these molecular gauge blocks. Walking along a fine line: The synthesis and characterization of a series of high-aspect-ratio oligoparaxylenes as molecular gauge blocks are reported. The lengths of the molecules range from 10 Å for the shortest member of the series with two phenylene rings up to 50 Å for the longest member with eleven phenylene units (see figure). Minimalistic design criteria led to a walk along a fine line between keeping the molecules slender, yet also rendering them soluble. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.