Solvent‐Vapor‐Induced Reversible Single‐Crystal‐to‐Single‐Crystal Transformation of a Triphosphaazatriangulene‐Based Metal–Organic Framework

A triphosphaazatriangulene (H 3 L) was synthesized through an intramolecular triple phospha‐Friedel–Crafts reaction. The H 3 L triangulene contains three phosphinate groups and an extended π‐conjugated framework, which enables the stimuli‐responsive reversible transformation of [Cu(HL)(DMSO)⋅(MeOH)] n , a 3D‐MOF that exhibits reversible sorption characteristics, into (H 3 L⋅0.5 [Cu 2 (OH) 4 ⋅6 H 2 O] ⋅4 H 2 O), a 1D‐columnar assembled proton‐conducting material. The hydrophilic nature of the latter resulted in a proton conductivity of 5.5×10 −3  S cm −1 at 95 % relative humidity and 60 °C.