Tracking relaxation dynamics of polaritons and reservoir states in organic exciton-polaritons

Organic exciton-polaritons have garnered significant attention in optoelectronic devices and photochemistry, primarily due to their micrometer-scale diffusion lengths and tunable energy levels. Recent studies have underscored the critical role of reservoir states in the dynamical processes following polaritonic relaxation. However, questions remain regarding the peak assignments of reservoir states and their specific functions during relaxation. In this study, we employed pump-probe and two-dimensional electronic spectroscopy (2DES) coupled with model simulations to investigate photoexcited dynamics of organic exciton-polaritons. By comparing experimental data with simulations, we identified that reservoir states consist of dark states and excited-state molecules uncoupled to the cavity mode, each exhibiting distinct spectral characteristics. Analyses of the spectral evolution in 2DES reveal that reservoir states are generated through three pathways: direct photoexcitation, rapid relaxation from middle polaritons, and entropy-driven relaxation from lower polaritons. Moreover, we find that equilibrium is established between dark states and uncoupled excited-state molecules following polaritonic relaxation, regardless of the excitation wavelength. This equilibrium facilitates subsequent polaritonic regeneration. Modulating the equilibrium constant can be achieved by engineering the light-matter interaction strength, offering a strategy to control postrelaxation polaritonic dynamics. These insights advance our understanding of relaxation dynamics in organic exciton-polaritons that can benefit the design of next-generation optoelectronic devices.