Indolyl alkali-metal ion pairs in the excited state. Part 2. The influence of temperature on the position of the counterion

Abstract

The structures of the fluorescent ion pairs of indolyl (Idl^–) alkali-metal salts in 2-methyltetrahydrofuran (MTHF) as a function of temperature have been elucidated on the basis of the absorption and fluorescence ion-pair shifts. For the determination of these shifts the complex of IdlK with the cryptand [2.2.2] has been used as a reference system. From the values of the ion pair shifts and the Stokes shifts it has been concluded that at –180 °C the emitting Li⁺, Na⁺, and K⁺ ion pairs have a σ-type structure. At this temperature the solvent-excited solute relaxation of Idl^– in the first excited singlet (S₁) state is slow on the fluorescence time scale so that the counterion cannot change its position before emission occurs. The temperature dependence of the fluorescence band position strongly suggests that for Idl^–, Li⁺ this relaxation only involves a reorientation of the neighbouring solvent molecules, but that for Idl^–, Na⁺, and K⁺ intramolecular migration of the counterion takes place simultaneously. The ‘stable’ ion pair structure in the S₁ state is a σ-complex for Idl^–, Li⁺, but a π-complex for Idl^–, Na⁺, and Idl^–, K⁺. The possible positions for the counterion in the π-complexes have been discussed and for this purpose also the ion-pair shifts of the 7-methylindolyl salts have been measured.