Strong light-matter coupling: Effects of Stokes shift and thermal disorder

Strong coupling (SC) between photoactive molecules and confined light modes — such as surface plasmons (SPs) or cavity photons (CPs) — leads to the formation of hybrid light–matter states known as polaritons, with energies split above and below the original molecular transition. We investigate the dynamics of polaritons formed between organic molecules and either SPs or CPs, focusing on systems with varying Stokes shifts. Our experiments reveal distinct relaxation pathways depending on the magnitude of the Stokes shift. Molecules with a typical Stokes shift exhibit conventional molecular relaxation followed by radiative pumping into polaritonic states. In contrast, systems with negligible Stokes shift relax to the lower polariton via vibrationally assisted scattering, while molecules with very large Stokes shifts can relax to energies well below the lower polariton.

The formation of polaritons has been shown to influence photochemical processes, yet the underlying mechanisms remain incompletely understood [1]. While theoretical models often attribute these effects to the emergence of modified polaritonic potential energy surfaces (PES) [2], many neglect the role of energetic disorder among molecules, which is unavoidable under ambient conditions. Furthermore, for polariton-modified chemistry to occur, the polariton lifetime must be sufficiently long to allow nuclear motion on these altered PES. Although it has been proposed that the lowest-energy polariton state can exhibit extended lifetimes [3], polariton lifetimes are generally limited by the cavity photon lifetime [4], typically on the order of tens of femtoseconds.

To minimize the impact of finite lifetimes and isolate the effect of polariton formation on photoreactivity, we study excited-state intramolecular proton transfer (ESIPT) in 10-hydroxybenzo[h]quinoline (HBQ), a process that occurs on a timescale faster than the decay of the cavity used in our experiments [5]. By combining molecular dynamics simulations with fluorescence and excitation spectroscopy, we show that — contrary to theoretical predictions — this reaction is not suppressed under strong coupling. We attribute this to thermal disorder, which inhibits the formation of a well-defined, delocalized lower polariton state.

Moreover, we observe that the excitation spectrum of the strongly coupled system can be described as the product of the absorption and excitation spectra of the uncoupled molecules. This suggests that polaritons act as efficient gateways, channeling excitation energy into individual molecules that subsequently undergo standard photochemical reactions. Finally, our simulations demonstrate that introducing an inert molecule with strong absorption at the same energy as HBQ within the same cavity leads to rapid localization of excitation energy—initially delocalized across all molecules—onto a single reactive HBQ molecule [6]. This effect may offer a pathway toward enhanced light-harvesting applications.