Examples of optical gaps are included in Figure and they refer to the lowest energy of molecular state transition facilitated by a photon absorption. Again, MOFs will then lay somewhere in between these two worlds, with their optical properties explained and discussed either in terms of the HOMO-LUMO energy difference, or in terms of band structure. Chemical versatility of MOFs relies also on the use of linkers within the same family but featuring different dangling groups. This is nicely explored in literature for the case of UiO-66 for example.