In Situ Solid‐State Reactions Monitored by X‐ray Absorption Spectroscopy: Temperature‐Induced Proton Transfer Leads to Chemical Shifts

The dramatic colour and phase alteration with the solid‐state, temperature‐dependent reaction between squaric acid and 4,4′‐bipyridine has been probed in situ with X‐ray absorption spectroscopy. The electronic and chemical sensitivity to the local atomic environment through chemical shifts in the near‐edge X‐ray absorption fine structure (NEXAFS) revealed proton transfer from the acid to the bipyridine base through the change in nitrogen protonation state in the high‐temperature form. Direct detection of proton transfer coupled with structural analysis elucidates the nature of the solid‐state process, with intermolecular proton transfer occurring along an acid‐base chain followed by a domino effect to the subsequent acid‐base chains, leading to the rapid migration along the length of the crystal. NEXAFS thereby conveys the ability to monitor the nature of solid‐state chemical reactions in situ, without the need for a priori information or long‐range order.