The ultraviolet spectra of the 5-methoxy-3,3-dimethyl and -1,3,3-trimethyl-3H-indolium cations

Abstract

5-Methoxy-3,3-dimethyl-3H-indole, its methiodide, and its trimer have been synthesised and their ultraviolet (u.v.) spectra studied in strong acid solution in order to determine the structural features responsible for the recently reported bathochromically shifted long-wavelength maximum in the u.v. spectrum of a 3H-indolium cation. This shift depends on the presence of a 5-alkoxyl and two 3-alkyl substituents but not a 1-alkyl substituent on the 3H-indolium cation nucleus. Proton magnetic resonance (p.m.r.) and u.v. spectral results also indicate that 5-methoxy-3,3-dimethyl-3H-indole trimer completely de-trimerises into its parent monomer on standing in neutral solution at room temperature.