Proton-coupled electron transfer (PCET) reactions are fundamental to energy storage and conversion processes. By coupling electrons with protons, the net charge neutrality is retained, preventing electrode decomposition due to charge imbalance. PCET reactions with equimolar amounts of protons and electrons can be considered as a net H-atom transfer (HAT) reaction. Many redox-active metal-organic frameworks (MOFs) have demonstrated that the inorganic nodes and/or the organic linkers can be tailored to undergo HAT reactions. In particular, the Ti-oxo nodes of the MOF focused on this work, Ti-MIL-125, can accept up to two H-atoms. H-atom binding on the nodes of Ti-MIL-125 has long been known to correlate with the color change in the crystals from white to blue-black, but its exact optical properties, such as molar extinction coefficient (ϵ) and wavelength with maximum ϵ, λ