The parent fluorenyl photobase (PBHOH) was synthesized via Grignard reaction of 3,6bis(dimethylamino)fluorenone (Figure ). We measured the pKaH´ to be 5.9 (SI Table ), confirming that the effective basicity is reduced relative to MCGB (pKaH´ = 6.8). Additional tuning of the basicity was achieved by preparing derivatives substituted on the 9-aryl ring: PBCF3OH, PBClOH, PBMeOH, and PBOMeOH. By titration, their pKaH´ values spanned 2.7 units (SI Table ), with the -CF3 and -Cl para-substituents destabilizing the carbocation and the -Me and -OMe para-substituents slightly stabilizing the carbocation, but all being less basic than MGHOH.