However, when MGHOH solutions were exposed to CO2 (1 atm) in the dark, a large and irreversible prompt absorbance at 620 nm was observed that is characteristic of the corresponding carbocation (Figure , green trace). The carbocation evolution indicates the transfer of hydroxide from MGHOH to CO2, generating a carbocation-bicarbonate ion pair. Thus, MGHOH is too basic and is thermodynamically unlikely to release CO2 in the absence of illumination. In line with this supposition, a spectrophotometric titration (SI Figure ) determined the pKaH´ of MGHOH to be 6.8. We sought to reduce the pKaH´ by destabilising the carbocation via para-functionalization of the phenyl group with electron-withdrawing substituents, Br and CF3. Whereas MGBrOH solutions (1 mM in 2:1 MeCN/water) exposed to CO2 (1 atm) in the dark also exhibited carbocation formation (Figure , orange trace), MGCF3OH, was less reactive with CO2 in the absence of light (Figure , red trace).