The Role of Long-Lived Excitons in the Dynamics of Strongly Coupled Molecular Polaritons

The concept of modifying molecular dynamics in strongly coupled exciton-polariton systems is an emerging topic in photonics because of its potential to produce customized chemical systems with tailored photophysical properties. However, there is no consensus on the types of molecular systems whose dynamics can be modified using strong coupling or the conditions under which these modifications can be realized. These open questions stem from persistent uncertainties concerning the lifetime and conversion dynamics of exciton-polaritons and localized excited states, as well as the proper way to measure such interactions in the time domain. Here, we provide a framework for measuring dynamical interactions between exciton-polaritons and a diverse manifold of singlet, triplet, and multiexciton states, using a model molecular spin conversion (singlet fission) system that is strongly coupled to an optical microcavity. In addition to the usual population dynamics, transient optical measurements on microcavities reveal information pertaining to transient modifications of the exciton-polariton transition energies and exciton-photon coupling conditions. This approach allows to identify major practical limitations for modifying molecular dynamics in the strong coupling regime, including (1) absorption into the large number of "reservoir" states, defined as dark excitons (e.g., triplets) and the degenerate manifold of dark states from the collective strongly coupled molecules and all excitons from uncoupled molecules, compared to polariton states and (2) the slow conversion from reservoir states to cavity polaritons. As a consequence of weak interactions between reservoir states and cavity polaritons, judicious design considerations are required to achieve modified photophysical dynamics, necessitating the use of molecular systems with long excited-state lifetimes or architectures that enhance exciton-photon coupling strength such that a small number of molecules is required.