An efficient synthesis of novel functionalized 2-aryl-4-(indol-3-yl)-4H-chromenes has been developed, in the presence of a catalytic amount of iodine, from easily available starting materials, 2-hydroxychalcone derivatives. Indole, substituted indoles and 7-azaindole are suitable for this transformation. The possible domino Michael addition-intramolecular cyclization reaction mechanism is proposed.

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Molecular clip 1 remains monomeric in water and engages in host•guest recognition processes with suitable guests. We report the Ka values for 32 1•guest complexes measured by 1H NMR, UV/Vis, and fluorescence titrations. The cavity of 1 is shaped by aromatic surfaces of negative electrostatic potential and therefore displays high affinity and selectivity for planar and cationic aromatic guests which distinguishes it from CB[n] receptors that prefer aliphatic over aromatic guests. Electrostatic effects play a dominant role in the recognition process whereby ion-dipole interactions may occur between ammonium ions and the C=O groups of 1, between the SO3− groups of 1 and pendant cationic groups on the guest, and within the cavity of 1 by cation-π interactions. Host 1 displays high affinity toward dicationic guests with large planar aromatic surfaces (e.g. naphthalene diimide NDI+ and perylene diimide PDI+) and cationic dyes derived from acridine (e.g. methylene blue and azure A). The critical importance of cation-π interactions was ascertained by a comparison of analogous neutral and cationic guests (e.g. methylene violet vs. methylene blue; quinoline vs. N-methylquinolinium; acridine vs. N-methylacridinium; neutral red vs. neutral red H+) whose affinities differ by up to 380-fold. We demonstrate that the high affinity of 1 toward methylene blue (Ka = 3.92 × 107 M−1; Kd = 25 nM) allows for the selective sequestration and destaining of U87 cells stained with methylene blue.

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