pH-Controlled isomerization kinetics of ortho-disubstituted benzamidines: E/Z isomerism and axial chirality.

pH-Responsive molecular switches and motors are a class of organic molecules whose three-dimensional structure can be changed by acid-base stimuli. To date, pH-responsive molecular switches have been developed using various functional groups, but further advances require expanding the range of pH-responsive systems and discovering new molecular architectures. Here, we investigate the pH-responsive behavior of ortho-disubstituted benzamidine, which generates atropisomers and E/Z isomers. The amidine moiety allows modulation of the C-N and C-N/C-C rotational barriers by protonation, providing a novel approach to control the kinetics of isomerization via pH adjustment. The results showed that protonation of the amidine moiety significantly suppresses both C-N bond rotation and C-N/C-C concerted rotation, demonstrating the potential of ortho-disubstituted benzamidine derivatives as a novel pH-responsive molecular switch.