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 due to its potential to produce customized chemical systems with tailored photophysical properties. However, before such systems can be realized, it is essential to address the open questions concerning the nature and strength of electronic interactions between exciton-polaritons and localized excited states in chemical system as well as the proper way to measure such interactions. Here, we use transient optical spectroscopy to investigate dynamical interactions between exciton-polaritons, singlet excitons, and triplet excitons in a molecular singlet fission system that is strongly coupled to an optical microcavity. We identify some of the major limitations to modify molecular dynamics in the strong coupling regime. Simultaneous excitation of cavity polaritons and 'reservoir' states, defined as dark polaritons and dark excitons (e.g. triplets) from coupled molecules and excitons from uncoupled molecules, always occurs. In addition, slow conversion from reservoir states to cavity polaritons results in minimal changes to the overall population dynamics. Furthermore, we demonstrate how in addition to the usual population dynamics, transient optical measurements on microcavities reveal information pertaining to modification of the exciton-polariton transition energies due to changes in the population of molecular excited states and the exciton-photon coupling conditions. As a consequence of weak interactions between reservoir states and cavity polaritons, judicious design considerations are required to achieve modified chemical dynamics, necessitating the use of molecular systems with long excited-state lifetimes or strong coupling approaches that require a small number of molecules.