Synthesis and Conformational Characterization of Tethered, Self-Complexing 1,5-Dialkoxynaphthalene/1,4,5,8-Naphthalenetetracarboxylic Diimide Systems

Chemists are beginning to explore the abiotic folding of synthetic chains, and the term “foldamers” has been used to characterize oligomers with a strong inclination to adopt specific, compact conformations. The characterization of folded structure in solution is one of the difficult challenges facing the foldamer field. Aedamers were the first foldamers to make use of aromatic−aromatic interactions in water to direct folding and were designed to have several spectroscopic handles with which to probe folding conformations in solution. Herein is reported the synthesis and spectroscopic characterization of eleven aedamer dimers, with linkers chosen to provide a spectrum of lengths and flexibilities. The dimers, composed of one electron rich (1,5-dialkoxynaphthalene) and one electron deficient (1,4,5,8-naphthalenetetracarboxylic diimide) aromatic group tethered by a linker, are the smallest aedamer folding unit. The powerful spectroscopic handles associated with the stacked aedamer groups were exploited in a comprehensive spectroscopic analysis of conformation that included UV−vis absorption spectroscopy, fluorescence measurements (including time-resolved studies), as well as detailed NMR studies. The spectra were interpreted in the context of molecular modeling/spectral prediction and structural models were developed for the different dimers in aqueous solution. In most instances, the observed data was best described by an ensemble of predicted structures as opposed to one or few conformers. Thus, in the case of these aedamer dimers, “folding” does not appear to imply a two-state model with a rigid, unique conformation. Rather, the reported analysis indicates the data can best be described by a more dynamic model in which a given molecule spends its time in different folded conformations that are related by having a characteristic face-to-face stacking arrangement of the aromatic units.