Indolyl alkali-metal ion pairs in the excited state. Part 4.—Their complexes with macrocyclic ligands

Abstract

The complexes of indolyl (Idl^–) alkali-metal ion pairs with macrocyclic ligands in 2-methyl-tetrahydrofuran have been studied by temperature-dependent fluorescence spectroscopy. Their structures in the electronic ground state are discussed on the basis of the fluorescence ion-pair shifts at –180 °C. In the equilibrated lowest excited singlet state (S₁-eq) complexation induces a change in type of structure for Idl^–, Na⁺; in the uncomplexed ion pair the cation is coordinated via the π-electron cloud with Idl^–, but in the complexed one this occurs via the nitrogen lone-pair electrons. For Idl^–, Li⁺ and Idl^–, K⁺ such a change is not found, likewise the uncomplexed ion pairs; in S₁-eq the complexed Li⁺ ion pair has the σ-type structure and the K⁺ ion pair the π-type. Nevertheless, for Idl^–, K⁺ the solvent-excited solute relaxation is significantly affected by complexation: in the dicyclohexyl-18-crown-6 complex the relaxation time of intramolecular cation migration is increased by about three orders of magnitude.