Giant Negative Area Compressibility Tunable in a Soft Porous Framework Material

A soft
porous material [Zn­(L)2(OH)2]n·Guest (where L is 4-(1H-naphtho­[2,3-d]­imidazol-1-yl)­benzoate, and Guest
is water or methanol) exhibits the strongest ever observed negative
area compressibility (NAC), an extremely rare property, as at hydrostatic
pressure most materials shrink in all directions and few expand in
one direction. This is the first NAC reported in metal–organic
frameworks (MOFs), and its magnitude, clearly visible and by far the
highest of all known materials, can be reversibly tuned by exchanging
guests adsorbed from hydrostatic fluids. This counterintuitive strong
NAC of [Zn­(L)2(OH)2]n·Guest arises from the interplay of flexible [−Zn–O­(H)−]n helices with layers of [−Zn–L−]4 quadrangular puckered rings comprising large channel voids.
The compression of helices and flattening of puckered rings combine
to give a giant piezo-mechanical response, applicable in ultrasensitive
sensors and actuators. The extrinsic NAC response to different hydrostatic
fluids is due to varied host–guest interactions affecting the
mechanical strain within the range permitted by exceptionally high
flexibility of the framework.