The conformational control of a dibenzo-1,5diazacyclooctane (DACO) derivative with tertiary and secondary amino groups by stepwise protonation is investigated by single crystal X-ray diffraction analysis, computational simulation, and variable temperature NMR measurements. The DACO derivative undergoes two-step protonation, to afford bent-shape monoprotonated and diprotonated species, whose conformations are different from each other. The monoprotonated form is characterized by a Tröger'sbase-like proton-bridged pseudo bicyclic structure, while the diprotonated form features a deformed boat-shape eight membered ring structure. The degree of bending and conformational rigidity of the DACO skeleton become higher as the degree of the protonation is increased. Such a multistep conformational control of the DACO derivative would be useful for developing new stimuli-responsive supramolecular systems that exhibit a multistep change in properties.

Dibenzo‐1,5‐diazacyclooctane (DACO) derivative undergoes two‐step protonation and would be useful for developing multistep stimuli‐responsive systems. The first proton (the brown cat) affords a proton‐bridged Tröger's‐base‐like pseudo bicyclic structure, and the second proton (the blue and white cats) causes an electrostatic repulsion between positively charged nitrogen atoms, as if the blue and white cats pull the two nitrogen atoms apart, to afford a deformed boat‐shape eight membered ring structure. As a result, the degree of bending and conformational rigidity of the DACO become higher as the degree of the protonation is increased. More information can be found in the Communication by Fumitaka Ishiwari et al.