The enormous versatility of MOFs in terms of both structure and composition, allows for an almost ad-hoc design of their optical properties as well. However, it also imposes a challenge to determining and predicting those properties, since these materials possess nature between ordered extended solids, and molecular systems with highly-localized bonding interactions. Perhaps the most relevant parameter when thinking about combining MOFs and SERS (as it will be discussed in the next section) is the optical band gap (Eopt). Given the ambivalent character of MOFs (whether they can be treated as molecules or extended solids), it is worth to summarize the differences between the reported optical parameters that can be found in literature. Shortly, when addressing molecules, the Eopt refers to the lowest energy of electronic transition accessible via absorption of a single photon, and it is significantly below the fundamental gap (difference between ionization potential and electron affinity). This is contrary to the ionized states due to the electron-hole pair binding energy (since they remain electrostatically bound),. As the molecular size increases when turning small molecules into polymers, electronic bands are formed. Similar to classical semiconductors, this new band structure will present a valence and conduction band. However, the degree of delocalization of the wave functions will be less pronounced than for ordered structures, relying on the balance between electronic coupling and disorder in the structure. The band structure then allows defining a band gap between the conduction and valence bands. Being the equivalent to the molecular fundamental gap for materials, it must not be mistaken for the optical gap.