Thermal disorder prevents the suppression of ultra-fast photochemistry in the strong light-matter coupling regime
A. Dutta, V. Tiainen, I. Sokolovskii, L. Duarte, N. Markešević, D. Morozov, H.A. Qureshi, S. Pikker, G. Groenhof, J.J. Toppari, Nat. Comm. 15, 6600 (2024).
Strong coupling between molecules and confined light modes of optical cavities to form polaritons can alter photochemistry, but the origin of this effect remains largely unknown [1]. While theoretical models suggest a suppression of photochemistry due to the formation of new polaritonic potential energy surfaces [2], the key hypothesis for this is that the lifetime of the polariton is sufficiently long for the reactants to evolve over the modified potential energy surface. Despite recent suggestions that the lowest-energy polariton state can be very long-lived [3], polariton lifetimes are generally considered to be limited by the lifetime of the cavity photon [4,5], which for the metallic cavities used in experiments is much shorter than the reaction time.
In this work [6], we therefore investigate the influence of strong coupling on the ultra-fast photochemical reaction of 10-hydroxybenzo[h]quinoline (HBQ) which happens on a timescale of (about 15 fs) comparable to the lifetime of a cavity photon. By combined simulations and experiments we show that even the reaction is fast enough to proceed along the modified polaritonic energy surface, the thermal disorder among the molecules prevents the proper formation of this surface, and the suppression is due to radiative decay of the lossy cavity modes. Most of the theoretical models do not account for this energetic disorder, which is anyway unavoidable at ambient conditions [6,7]. We also show that the excitation spectrum under strong coupling is a product of the excitation spectrum of the bare molecules and the absorption spectrum of the molecule-cavity system, suggesting that polaritons can act as gateways for channeling an excitation into a molecule, which then reacts normally. Our results therefore imply that strong coupling provides a means to tune the action spectrum of a molecule, rather than to change the reaction. However, this channeling of the excitation energy could further be utilized as a basis of enhanced light-harvesting [8].
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