However, Brønsted photobases predominantly function in organic solvent, rely on UV light and exhibit small shifts in pKa (~2 units), thereby limiting their utility in reversible CO2 capture. In this work, we show that a new class of reversible hydroxide emitters, 9-aryl-9-fluorenols, represent among the first highly reversible and tuneable Arrhenius photobases, capable of generating large pH swings reaching ca. pH 12 under illumination. We report the effect of ground state pKa on the efficiency of capture and release, validating a precise pH-matching principle that we had developed in the context of photoacids. Solutions of these materials can capture CO2 from streams as dilute as 400 ppm and then release that captured CO2 upon removal of the light source, resulting in output concentrations that are significantly higher than the input stream. Studies into the effects of excitedstate aromaticity, ground-state antiaromaticity, and the transient dynamics provide insight into the design principles underlying the effectiveness of these photobases for reversible CO2 capture compared to previously known hydroxide emitters. Acridinols, trityl alcohols, and fluorenols are examples of molecular switches that photochemically and reversibly release hydroxide. The best-known Arrhenius photobase, malachite green carbinol base (MGCB), reported by Irie in 1982, was found to engender large reversible pH swings by toggling between a neutral alcohol state and photoionized ion pair of a trityl carbocation and hydroxide, formed upon C-O heterolysis. This behaviour has been used to effect conformational changes in DNA/RNA and in light-driven drug-release applications, but otherwise, limited follow-up work has been published. We wondered whether the photoinduced pH swing of MGCB could be utilised to reversibly capture CO2 and accordingly, our attempts initially focused on engineering the structure of MGCB to satisfy the criteria of efficient CO2 capture under photoirradiation and CO2 release in darkness (Figure ).