Infrared-driven topochemical reaction of a diacetylene.

Driving chemical reactions with infrared (IR) vibrational excitation in condensed phases remains challenging due to rapid energy dissipation through solvent collisions and intramolecular vibrational redistribution (IVR). Using 2,4-heptadecadiynoic acid (DA-17) microcrystals as a model system, we show that topochemical lattice preorganization enables the infrared-driven reaction that proceeds approximately 14-fold faster than thermal activation at the same temperature. Spectral deconvolution, variable-temperature X-ray diffraction, and quantum chemical calculations reveal three distinct monomer populations: two topochemically confined layer motifs (aligned and misaligned) that are reactive under IR or UV excitation and an unconfined amorphous fraction that remains inert. Selective excitation with a tunable mid-IR laser identifies the symmetric alkyne stretch as a key reaction-coordinate-coupled mode under confinement-this mode selectivity is lost in the pre-molten, disordered state. The infrared-irradiated films exhibit self-acceleration characteristic of topochemical propagation, whereas thermally heated films at an equivalent "kinetic temperature" show no such behavior, suggesting that the enhancement arises, at least in part, from a mechanism distinct from simple heating. These findings establish a physical-organic model system in which preorganization directs vibrational energy into the reaction coordinate before it is lost to random conformational exchange, offering a design principle for mode-selective solid-state chemistry.