“…In recent years, macrocyclic pyclen (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene), featuring a pyridine moiety in its backbone, has garnered significant attention as a promising platform for various applications, including MRI CAs, luminescent complexes, and radiometal chelators. − Albelda et al developed a pyclen-based complex exhibiting pH-responsive behavior, as the number of coordinated water molecules varies at different pH values from 1 to 2, which results in a change of relaxivity . Tircso et al reported a pH-sensitive MRI probe, PC2A-EA, which contains an ethylamine moiety that can respond within the biologically relevant pH range .…”