Abstract:
The science of catalysis underpins some of the most crucial aspects of our world: namely the biochemical reactions that constitute life, and the synthesis of almost all the materials we use on a day-to-day basis. This work demonstrates a new approach to the problem of catalysis by exerting control over chemical reactions via strong interactions between light and matter inside an optical cavity. A hybrid state of light and matter (polariton) was created by performing the reaction between two mirrors (a cavity) tailored to be in resonance with a vibrational transition within the reacting molecule. The cavity confined reaction acceleration was demonstrated on a model unimolecular migratory insertion reaction for simplicity: the insertion of a thiocarbonyl ligand (CS) within iridacyclopentadiene to form the corresponding iridabenzene. The reaction was monitored using a variety of spectroscopic tools (IR absorption, IR cavity resonance shifts, UV-Vis spectroscopy, and NMR spectroscopy). The reaction rate inside a cavity resonant with a vibrational transition dominated by a CS stretch was accelerated several times relative to an uncoupled reaction, and there was a clear decrease in the Arrhenius activation barrier. The changes in the thermodynamics of the system were rationalized using arguments based on the effect of light on the molecular wavefunction of the system. The formation of a polariton stabilizes and strengthens the CS bond, with corresponding changes in the π-backbonding interactions and thus reactivity towards migratory insertion into an Ir-C bond. When the molecule is not in resonance with the cavity, the entropy change with bond insertion is negative. In resonance with the cavity, however, that same reaction experiences a positive entropy change. This change implies that the cavity induces order in the ground state reactant. If this approach can be generalized to other reaction classes, it has the potential to provide important new directions for a field in which it is normal to spend years optimising a single catalytic reaction.