Calculation of R0, a ruler for LRET experiments, in the pair of Tb3+ ion bound to LBT and Ts1-S14R-Bodipy. (a) Analytical LCMS data for Ts1-S14R-Bodipy; Calc. 7278.1 Da (average isotopes), Obsd. 7278.4 ± 0.3 Da. (b) Overlap between the emission spectrum of Tb3+ bound to LBT (black solid line) and the absorption spectra of Ts1-S14R-Bodipy (blue solid line). The emission spectrum of Ts1-S14R-Bodipy is also shown (green solid line). (c) Energy transfer efficiency as a function of distance.