Aiming to evaluate whether photodissociation would proceed with these derivatives, we subjected MGBrOH and MGCF3OH (1 mM in 2:1 MeCN/water) to UV light (λexc = 265 nm). Unexpectedly, instead of the photobasic behaviour characteristic of parent MGCB (MGHOH), both solutions displayed a rapid decrease in pH, which appeared to slightly reverse upon removal of the light source (Figure , red and orange traces). The photoacidity appears to be oxygen-dependent, suggesting the presence of a radical pathway that may generate hydrogen peroxide and/or organic hydroperoxides, though these products remain to be fully characterized. Nonetheless, such complications indicated that straightforward electronic tuning of MGCB derivatives was unlikely to realize reversible CO2 capture and release. As an alternative strategy to destabilize the trityl cation formed upon hydroxide dissociation (Figure ), we were interested in the introduction of antiaromaticity by connecting two of the aromatic rings into a fluorene substructure. Upon hydroxide dissociation, the fluorenyl cation core contains a 4πelectron cyclopentadienyl substructure, which could favour ion recombination and hence lower the pKaH´. This approach has several appealing secondary consequences. First, the enhanced conjugation of the fluorene core, which effectively enforces two of the benzene rings to be coplanar, bathochromically shifts the photobase absorption to enable the system to be driven by visible light, rather than the ultraviolet wavelengths used to operate Malachite green derivatives. Furthermore, DFT calculations in Figure and the Supplementary Information show that, while the carbocation is indeed disfavored in the ground state, on the excited-state surface, the evolution of Baird aromaticity provides a strong thermodynamic driving force for hydroxide elimination. Hence, the fluorenes may be expected to show better reversibility than their Malachite green analogues.