Structure of indolyl alkali-metal ion pairs in the ground state and the first excited state

Abstract

The absorption and fluorescence spectra of indolyl anion (Idl^–) have been recorded as a function of temperature, solvent and counter ion. The n.m.r. spectra of the counter ion ⁷Li⁺ have been measured in various solvents. Semiempirical m.o. calculations have been performed to give insight into the structure of the absorption spectrum, its dependence on contact ion pair formation, and the charge distributions in the ground state and several excited states.

From the experimental and theoretical results it is concluded that in ethereal solvents Idl^– forms contact ion pairs in the ground state. It is established that the counter ion Li⁺ is coordinated with the nitrogen lone pair of Idl^–(σ-complex); the same is likely to be true for Na⁺ and K⁺. Contrary results from the literature, derived from proton n.m.r. spectra, are discussed.

In the first excited (S₁) state Idl^– forms contact ion pairs exclusively, as in the ground state. From the temperature dependence of the fluorescence spectra of Idl^–,Na⁺ and Idl^–,K⁺ it is concluded that in the S₁ state two types of contact ion pair occur, i.e., σ- and π-complexes.