Acid-Base Strength and Acidochromism of Some Dimethylamino-Azinium Iodides. An Integrated Experimental and Theoretical Study

The effects of pH on the spectral properties of stilbazolium salts bearing dimethylamino substituents, namely, trans isomers of the iodides of the dipolar E-[2-(4-dimethylamino)styryl]-1-methylpyridinium, its branched quadrupolar analogue E,E-[2,6-di-(p-dimethylamino)styryl]-1-methylpyridinium, and three analogues, chosen to investigate the effects of the stronger quinolinium acceptor, the longer butadiene π bridge, or both, were investigated through a joint experimental and computational approach. A noticeable acidochromism of the absorption spectra (interesting for applications) was observed, with the basic and protonated species giving intensely colored and transparent solutions, respectively. The acid-base equilibrium constants for the protonation of the dimethylamino group in the ground state (pK_a) were experimentally derived. Theoretical calculations according to the thermodynamic Born-Haber cycle provided pK_a values in good agreement with the experimental values. The very low fluorescence yield did not allow a direct investigation of the changes in the acid-base properties in the excited state (pKa_a∗) by fluorimetric titrations. Their values were derived by quantum-mechanical calculations and estimated experimentally on the basis of the Förster cycle. (Figure Presented).