Linear free energy relationships. Part VII. Synthesis and properties of a novel ‘double-barrelled’ solvatochromic compound

Abstract

The synthesis of bis-[2-(2-hydroxyphenyl)-1-(N-methyl-4-pyridinio)vinyl]methane di-iodide from 1, 3-bis-(4-pyridyl)propane was accomplished in 7% yield. This compound was of interest for the study of possible intramolecular interactions as a model for some analogous intermolecular complexes discovered and described earlier. The u.v.–visible absorption spectra of this diprotonated species, and its mono- and un-protonated (solvatochromic) conjugate bases have been examined, as well as their acid–base behaviour. A linear plot of absorption energy versus the solvent polarity parameter is consistent with the results of analogous species previously studied. The decreased acidity of the double-barrelled species is unexpected in view of its double positive charge and suggests an interaction (probably hydrogen-bonding) between the two halves. The lack of fluorescence of the double-barrelled solvatochromic species also suggests an intramolecular excited state complex (exciplex). Low solubility precluded detailed analysis by i.r. and n.m.r. spectroscopy.