Studies of inter- and intra-molecular donor-acceptor interactions—IV: Intramolecular charge transfer phenomena in substituted N-aralkyl-pyridinium ions

Abstract

The absorption spectra of compounds with the general structure D-(CH2)n-A, in which D is a substituted Ph group and A a 4-cyanopyridinium group show long-wavelength absorption not attributable to transitions within the separate chromophores. This absorption at long-wavelength results from an intramolecular Charge Transfer (CT) interaction between D and A. The CT nature of the absorption is proved by the linear relationship between the ionization potential of D and the energy of the CT transition. Furthermore the intramolecular CT is comparable with the intermolecular CT interaction between analogous D and A systems, both in transition energy and solvent dependence.The appearance of an intramolecular CT band in the spectra of compounds with n = 1 can qualitatively be predicted from the character of the highest and one but highest filled molecular orbital in the D part of the molecule. Simple MO calculations indicate that the 4-cyanopyridinium ion has stronger acceptor properties than the 2- and 3-cyano isomers, which agrees with our previous findings,3 for intermolecular CT complexes.