Determination of acid–base dissociation constants of azahelicenes by capillary zone electrophoresis

CZE was employed to determine acid–base dissociation constants (p K a ) of ionogenic groups of azahelicenes in methanol (MeOH). Azahelicenes are unique 3‐D aromatic systems, which consist of ortho ‐fused benzene/pyridine units and exhibit helical chirality. The p K a values of pyridinium groups of the studied azahelicenes were determined from the dependence of their effective electrophoretic mobility on pH by a nonlinear regression analysis. The effective mobilities of azahelicenes were determined by CZE at pH range between 2.1 and 10.5. Thermodynamic p K a values of monobasic 1‐aza[6]helicene and 2‐aza[6]helicene in MeOH were determined to be 4.94 ± 0.05 and 5.68 ± 0.05, respectively, and p K a values of dibasic 1,14‐diaza[5]helicene were found to be equal to 7.56 ± 0.38 and 8.85 ± 0.26. From these values, the aqueous p K a of these compounds was estimated.