Infiltration as a frontier bandgap engineering strategy in MOFs: A critical review

Despite the outstanding intrinsic properties of Metal-Organic Frameworks (MOFs), several of these materials present a wide bandgap value, which limits their use in interesting applications such as conductive materials, photo-electrocatalysis, sensing, among others. Consequently, understanding the molecular and structural origin of bandgaps and its fine-tuning is crucial to create new semiconductor materials based on MOFs and coordination polymers, but this also could shed light in a general perspective to achieve such modulation. In this context, several authors have observed that the infiltration of different species into common MOFs of insulating nature have developed outstanding modulations of their bandgap and conductivity, while retaining the crystalline structure and the porosity of the original host. Thus, infiltrating "guests" into MOFs is an easy, but novel and also advantageous alternative to generate guest@MOF composites, and emerging a modulation of bandgap by conjugating/coupling these moieties. Consequently, herein, we collected, analyzed, and discussed recent research contributions (2013–2022) of guest@MOF systems. First, we established the fundamental principles that explained the modulation of bandgap in MOFs through the classical structures consisting of modification of the linkers and SBUs. Then, the infiltration strategies and evidence confirming the infiltration process are presented. Afterward, we systematized many examples in which the modulation of bandgap through the incorporation of guest species is observed; therefore in this section, we classified the guest species as i) non-conjugated molecules, ii) conjugated molecules, iii) inorganic species, and iv) fullerenes. The following two sections discuss the possible applications of guest@MOF systems and have established the most important mechanisms behind the induction of bandgap modulation in these composites. Finally, we presented the general conclusions of the work. The most important mechanisms found to develop bandgap modulation in guest@MOF systems, were i) creation of energetic intermediate states between VB and CB of pristine material; ii) redistribution of the electronic density in the host moiety of the composite; iii) changes in chemical structure of the host; iv) higher electronic delocalization; or v) creation of a new band structure associated only with the guest molecule. The most common and efficient mechanism is the development of new energetic states, where electron-withdrawing and electron-releasing guests induced the formation of a new CB or VB in the band structure of the host, respectively. Finally, we conclude that the strongest supramolecular interaction among the host–guest pair reaches the most effective modulation of bandgap. Therefore, herein, we endeavored to correlate such findings with the structural and general physicochemical observations of the studied materials, providing a more comprehensive and coherent understanding behind bandgap modulation. All the latter is turning this research field into a very promising pathway to obtain emergent properties of resulting materials, and at the same time, emergent applications.