Crystal-size-dependent Optical Properties of H-atoms on the Nodes of Ti-based Metal-organic Framework

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_max, have yet to be determined. The presented work determines these parameters using colloidally stable Ti-MIL-125 of three different crystallite sizes. These studies revealed that both parameters are highly dependent on the crystal size and are likely indicating a distortion of the Ti-oxo nodes at the crystal surface. Together, these highlight the importance of considering defects in understanding HAT reactions of otherwise structurally uniform and periodic MOFs.