Recent experimental progress in the collective strong coupling regime of organic molecules with optical cavity or plasmonic modes has shown light-induced modifications of material properties. Experimental and theoretical endeavors go in the direction of charge and energy transport, Förster resonance energy transfer (FRET) enhancement, modified chemical reactivity etc. Oftentimes experiments rely on theoretical models developed for standard cavity quantum electrodynamics with two-level quantum emitters. Molecular systems however have an increased complexity as molecular vibrations and level disorder play a crucial role. We provide a theoretical formalism to tackle the light-electronic-vibrations dynamics modeled via the Holstein-Tavis-Cummings Hamiltonian . We analytically describe aspects such as: polariton asymmetry, molecular branching ratio modification in the Purcell regime and cavity-mediated donor-acceptor FRET processes.
 M. Reitz, C. Sommer and C. Genes, Langevin approach to quantum optics with molecules, Phys. Rev. Letts 122, 203602 (2019)