Ultra-fast photochemistry in the strong light-matter coupling regime

Abstract

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. While theoretical models suggest a suppression of photochemistry due to the formation of new polaritonic potential energy surfaces, many of these models do not account for the energetic disorder among the molecules, which is unavoidable at ambient conditions. Here, we combine experiments and simulations to show that for an ultra-fast photochemical reaction such thermal disorder prevents the modification of the potential energy surface and that suppression is due to radiative decay of the lossy cavity modes. We demonstrate that by increasing the coupling strength we can reduce such losses and enhance reactivity of the strongly coupled system, in contrast to the theoretical paradigm, which would predict stronger suppression. 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.

Controlling Molecular Photoisomerization in Photonic Cavities through Polariton Funneling

“…87 Instead, this overall rate enhancement effect can be understood as arising from a cavity-induced renormalization of the photonic density of states, as was recently inferred for different reactions. 56,88 This effect does not require strong light-matter coupling. Nevertheless, our results indicate that enhanced absorption at the LP branch followed by conversion of LPs into dark states does enable more efficient use of photons resonant with the LP branch compared to that outside the cavity.…”

“…89,90 The trend we observe in γ (Figure 3d) can be rationalized within this context and suggests that photochemistry in our systems is principally driven by (relatively) localized dark states: as the detuning between the excitation and bare exciton resonance increases, we expect a smaller LP → dark state transfer rate. 41,62,88 The other decay pathway for LPs is to the ground state, typically on sub-100 fs time scales, dictated primarily by cavity losses. Given this kinetic competition between polariton localization into dark states and polariton decay to the ground state, large detuning (small LP → dark state rate) results in a lower dark state yield and thus lower photochemical yields (Figure 3d).…”

“…Therefore, cavity coupling, particularly under conditions where the LP is significantly detuned from dark states through ultrastrong coupling or large detuning, can yield photoisomerization suppression even without fundamentally modifying molecular potential surfaces. 41,88 We emphasize, however, that the absolute photoisomerization rate enhancement (per irradiation photon rather than absorbed photon, RE in Figure 3a) can be large in microcavities since it allows low-energy light below the molecular resonance to be efficiently absorbed by the system and transferred into reactive dark states.…”

“…These results suggest that strong light-matter coupling in chemical mixtures allows steering chemical dynamics in a potentially more profound way than by acting as an optical filter, which was recently suggested to be the case for chemical systems with single-component reactants. 56,88,102 This approach will be particularly valuable to adopt in mixtures of reactants that have drastically different photochemical pathways and product states. Electronic strong-coupling in this context would provide a compelling platform to achieve precise reaction specificity, product selectivity, including suppression of undesired products, and reconfigurable photoswitching.…”