Molecular Mode Selective Diels–Alder Cycloreversion under Vibrational Strong Coupling

We demonstrate mode-selective control of chemical reactivity under vibrational strong coupling (VSC) through Diels–Alder cycloreversion of a cyclopentadienone (CPD) dimer. Selective coupling of specific carbonyl stretching vibrations of the dimer to an optical mode of a Fabry–Pérot microcavity leads to a measurable shift in the monomer–dimer equilibrium and significant modulation of reaction kinetics. VSC of the allylic carbonyl stretch at 1690 cm –1 results in a pronounced change in reactivity, whereas coupling to a spectrally distinct localized carbonyl mode at 1772 cm –1 produces minimal effect, highlighting the importance of vibrational mode selectivity. Frontier orbital analysis along the reaction coordinate shows that the allylic carbonyl mode is more vibronically coupled to the reaction center than the other localized carbonyl (1772 cm –1 ) in the dimer, which is likely to be modulated by VSC. These findings demonstrate that such orbital interactions, and consequently chemical reactivity, can be tuned through targeted vibrational coupling, offering a general strategy for controlling reaction pathways via light–matter interactions.