In vivo brain uptake 3-Fluorophenyl Tz derivatives possess rate constants >70,000 M -1 s -1 . They can be labeled with fluorine-18 and modified with a second handle, i.e., their physicochemical properties can also be manipulated. Consequently, these scaffolds are ideal to develop a BBB penetrant Tz. Since previous studies have shown that addition of a single methyl group can dramatically affect the pharmacokinetics and distribution of CNS tracers, a homologation approach was employed stepwise increasing the length/bulk of the chosen side chain