Folding and anion-binding properties of fluorescent oligoindole foldamers

A series of oligoindole foldamers 1a-d that are highly fluorescent were prepared by using a biindole derivative as the repeating unit, and their folding and anion-binding properties were revealed by ¹H NMR and fluorescence spectroscopy. The oligoindoles exist in an extended conformation, but adopt a compact helical structure in the presence of an anion. The anion is entrapped inside the tubular cavity of the helical strand, comprising four aryl units per turn, by multiple hydrogen bonds with the indole NHs. These structural features were confirmed by ¹H NMR and fluorescence spectroscopy. When folded by anion binding, 1b-d show characteristic downfield shifts of the NH signals and upfield shifts of the aromatic CH signals by Δδ = 0.1-1.0 ppm. The average chemical shift for all the aromatic signals of 1a-d is more upfield shifted as the chain lengthens, as anticipated from the degree of overlapped aromatic surfaces in the helical strand. Moreover, 1a-d are strongly fluorescent in the absence of an anion. Upon binding an anion such as a chloride, the shorter oligoindoles la and b lead to negligible change in the emission spectra, where-as the longer ones 1c and d result in dramatic changes, that is, large hypochromic and bathochromic shifts (Δλ = 65 and 70 nm) of the emission band, confirming the helical folding. The association constants (K_a) between oligoindoles and tetrabutylammonium chloride strongly depend on the chain length; <1M^-1 for 1a, 630M ^-1 for 1b, 1.1×10⁵M^-1 for 1c, and 2.9×10⁵ M^-1 for 1d in 20% (v/v) MeOH/CH ₂Cl₂ at 24± 1°C. In addition, the association constants of 1c and 1d with other anions such as fluoride, bromide, iodide, azide, cyanide, acetate, and nitrate are determined to be in the order of 10³-10⁶ M^-1 under the same conditions.