Under these conditions, TPyP-α produced no measurable amount of hydrogen over a five-hour period. By contrast, TBAP-α was even more active with a HER of 3108 µmol g -1 h -1 . (SI, Figure ). This large change in HER can be rationalized by considering the driving forces of the two half reactions occurring in the system. DFT calculations performed on isolated molecules immersed in water at pH 2.6 (the expected pH of 0.1 M ascorbic acid solution, Figure ) suggest that all three materials should have a large driving force for proton reduction and reasonable driving force for the overall oxidation of ascorbic acid. The driving force for the initial one-hole oxidation of ascorbic acid is very small for TBAP and TPyP and actually slightly negative for TPhP. Thus, we might expect the oxidation of the scavenger to be rate limiting in these systems, accounting for the increased activity of TBAP-α when changing the pH level from pH 2.6 to pH 7, because the driving force for ascorbic acid oxidation increases. This change also reduces the driving force for proton reduction, however, even at pH 7, the driving force for proton reduction remains large (> 1.5 V) and, crucially, larger than that for ascorbic acid oxidation (0.9 V for the 2-hole and 0.5 V for the intermediate one-hole oxidation). This does not explain the decrease in rate of TPyP-α when changing the pH level from pH 2.6 to pH 7, but we note that the partial dissolution of the material and the influence of protonation on the substrate electronics under acidic conditions could play a role.