Femtosecond studies of crown ethers: supramolecular solvation, local solvent structure and cation–π interaction

We report here femtosecond studies of microsolvation, host-guest recognition, and cation–π interaction of crown ethers in organic solvents. The side-armed indole ring of the supramolecule acts as an optical probe and a π-donor. Significantly-slow solvation dynamics (e.g. 206 ps in acetonitrile) were observed, revealing ordered local solvent structures. With encapsulation of the alkali-metal cation in the macroring, solvation dynamics become much faster by one order of magnitude, reflecting significant solvent reorganization during molecular recognition. The combined data suggest a folded supramolecular structure involving intercalated solvent molecules or a sandwiched metal cation, consistent with the enhanced local polarization and the strong electrostatic interactions.