Structure of contact ion pairs in the ground and first excited states. Aromatic closed-shell anions containing a five-membered ring

Abstract

The absorption and fluorescence spectra of the carbanion of indene have been recorded as a function of temperature, solvent and counter ion. In ethereal solvents indenyl-lithium forms contact and solvent-separated ion pairs, depending on the temperature, whereas only contact ion pairs occur when Na+ and K+ are the counter ions. The absorption spectra exhibit a blue shift on formation of contact ion pairs, but in the fluorescence spectra a red shift is observed.The results for the indenyl anion are compared with those for the carbanion of fluorene and the anion of carbazole. The differences in cation–anion attraction and fluorescence shift are discussed with the aid of energy level diagrams, and it is concluded that these differences are ultimately determined by the π-electron charge distribution of the anion in the ground and first excited states. Theoretically calculated charge densities confirm this conclusion; an exception is the carbazolyl anion, where the nitrogen lone pair electrons play an important role, leading to the formation of a σ-complex.The structures of the contact ion pairs of the two carbanions indenyl and fluorenyl in the ground and first excited states are inferred from the calculations. An apparent discrepancy with proton n.m.r. results is resolved by a theoretical examination of the effects of the cationic field on the proton chemical shifts.