Neutrinoless double beta decay (bb0n) processes constitute a very promising method (perhaps the only practical way) to demonstrate that neutrinos are they own antiparticles. Demonstrating this hypothesis would constitute a major discovery in physical sciences and cosmology. One useful bb0n nuclear reaction involves the Xe-136 isotope and leads to the emission of two electrons, whose drift and energy can be monitored in an ETD (Energy-Tracking Detector), together with a barium cation. Therefore, the second essential component of the TPC (Time Projection Chamber) to monitor this reaction is the BTD (Barium Tagging Detector). The BTD must incorporate a fluorescent sensor whose photophysical properties are ade-quate to distinguish the free and Ba2+-bound states, thus giving rise to a bicolour fluorescent indicator (FBI) potentially useful for barium tagging in NEXT-BOLD experiments. The different emission spectra for the free and coordinated states stem from the de-coupling between the aromatic components of the fluorophore upon barium coordination. In this lecture, details about the design and chemical synthesis of FBIs, their photophysical properties and structure-activity relationships will be presented, with special emphasis on DFT and TD-DFT calculations that provide a rationale for the observed behaviour.