An ideal photobase must be a sufficiently weak ground-state base to avoid CO2 capture in the dark but photodissociate a large concentration of hydroxide for maximum CO2 capture. As shown in Figure , only PBOMeOH (pKaH´ = 6.7) showed significant light-independent carbocation formation after CO2 exposure for 2 min, indicating that this candidate was too basic for reversible CO2 capture, as would be expected from the similarity of its basicity to MGHOH. Next, we evaluated the light-activated hydroxide release from each of the photobases by tracking pH of an MeCN/water solution under photoirradiation. Figure shows the solution pH of ca. 7 prior to irradiation jumping to pH 11. were unable to retain a steady pH under photoirradiation and some loss of hydroxide occurred before the light source was removed. We concluded that electron-withdrawing groups clearly inhibited hydroxide ion photodissociation, and, although the effect is not as pronounced as for the MGCB derivatives, PBClOH and PBCF3OH were unsuitable candidates for reversible CO2 capture. Overall, the most promising compound for reversible CO2 capture was PBMeOH, as it showed no CO2 capture in the dark and the largest pH increase under photoirradiation. The identification of this candidate is consistent with the optimal pKaH´ for photobasic CO2 capture to be ca. 6. An additional criterion for photobasic CO2 capture is that the photoswitch must be stable to many cycles between the neutral and the basic ionized state. As reference, Figure shows the absorption spectra of PBMeOH and its conjugate acid PBMe + . Irradiating PBMeOH solution with 390 nm light resulted in a large increase in carbocation absorption which gradually decayed over 90 min (Figure ). We repeated this entire experiment 10 times over a 15 h period (Figure ) and found that the maximum absorbance decreased by only ca. 1% per cycle and the minimum absorbance remained unchanged, signifying an acceptable level of photostability.