Unusually Large Band Gap Changes in Breathing Metal–Organic Framework Materials

Many
of the potential applications for metal–organic frameworks
(MOFs) focus on exploiting their porosity for molecular storage, release,
and separation, where the functional behavior is controlled by a subtle
balance of host–guest interactions. Typically, the host structure
is relatively unperturbed by the presence of guests; however, a subset
of MOFs exhibit dramatic phase-change behavior triggered by the adsorption
of guests or other stimuli, for which the MIL-53 material is an archetype.
In this work, we use density functional approaches to examine the
electronic structure changes associated with changes of phase and
density and find the associated change in band gaps can be larger
than 1 eV for known MIL-53-type materials and hypothecated structures.
Moreover, we show that internal pressure (via guest molecules) and
external pressure can exert a major influence on the band gap size
and gap states. The large response in electronic properties to breathing
transitions in MOFs could be exploitable in future applications in
resistive switching, phase-change memory, piezoresistor, gas-sensing
and thermochromic materials.