Our pKa tuning and CO2 capture/release investigations have shown a delicate thermodynamic balance must be maintained to achieve light-driven capture and dark release (Figure ). The photobase should be neutral in the dark, and hydroxide dissociation unfavourable to inhibit CO2 capture without light. After photoexcitation the potential energy of the system is driven out-of-equilibrium resulting in favourable ionization to free hydroxide and a carbocation. The hydroxide ion can then sequester CO2 as bicarbonate. Upon removal of the external energy stimulus, cation-hydroxide recombination is energetically favourable, and CO2 is therefore liberated. PBMeOH (pKaH´= 6.1) was found to perform best among tested photobases, consistent with the hypothesis that the pKaH ' of the photobase should approximately match the pKa of carbonic acid (6.4).