The Kick Inside: time-resolved mechanistic insights into the DUV-driven interconversion of pyrazole to imidazole

The interconversion of aromatic heterocyclic molecules <i>via</i> photochemical scaffold-hopping provides a clean and efficient formation route to otherwise synthetically challenging targets. The interconversion between pyrazole and imidazole is a widely used example in materials science and biochemical applications with arrow-pushing mechanisms used to define the reaction path. To study the photochemically driven isomerisation of pyrazole to imidazole, we combine femtosecond time-resolved photoelectron spectroscopy experiments with <i>ab initio</i> electronic structure calculations. Our results show that excitation to the ¹ππ* state in the gas-phase provides a directed 'kick' to the system, resulting in the breaking of the N-N bond and formation of a ring-opened biradical intermediate on the vibrationally hot electronic ground state on ultrafast (sub-90 fs) timescales. Once on the vibrationally hot electronic ground state, production of the imidazole photoproduct proceeds <i>via</i> the formation of a three membered ring that subsequently opens and shifts the relative position of the two nitrogen atoms.