Indolocarbazole-based anion receptors and molecular switches

A number of indolocarbazole-based anion receptors were prepared and their anion-binding behaviors were characterized in solution and in the solid state. First, chain-length-dependent binding affinities of chloride ion were revealed using a series of indolocarbazoles that consisted of one to four indolocarbazole units. The binding affinities were steadily enhanced from monomer to dimer, then to trimer by Gibb’s free energy (–∆∆G) = 2.4 ± 0.1 kcal/mol, and then nearly saturated. Second, a water-soluble trimer folded to generate an internal helical cavity with six convergent NHs, wherein small halides bound in water in the order of Cl– (65 M–1) > F– (46 M–1), and Br– (19 M–1). Third, X-ray crystal structures clearly proved helical folding of a trimer in the presence of sulfate ion, in which left- and right-handed helices stacked alternatively. It was also shown that the selectivity of anion binding could be varied by the modification of the spacer groups connecting indolocarbazole units. Finally, we prepared chiral indolocabazole dimers that adopted helical structures by intramolecular hydrogen bonds and displayed complete inversion of the helical sense upon anion binding. The dimers gave characteristic optical readouts in a reversible manner according to chemical stimuli, thus functioning as chirooptical molecular switches.