Linear Free Energy Relationships. IV. Synthesis and Spectral Characterization of Some New Solvatochromics. N-(p-Substituted)benzyl-2′-hydroxy-4-stilbazolium Betaines
The spectral data for the present series of compounds indicate that the long wavelength π → π* transition in each case involves transfer of negative charge from the phenol–quinone moiety to the pyridyl ring. Two separate features bear on this point: the transition energy dependence on (i) substituent and (ii) solvent. The plots of transition energy υs. Hammett substituent constant for the salts and the betaines have negative slopes as expected for such a transition. The plots of transition energy for the betaines υs. the solvent polarity parameter have large positive slopes, which are evidence for a large decrease in dipole moment upon excitation, consistent with such a transition. Through study of the pH dependence of the spectrum, the pKa of the parent compound was found to be 8.37 at 22°. From this pKa the apparent σortho for the N-benzylstilbazolium substituent was estimated to be 0.75.