  Quantum dynamics of the photoisomerization of a single
3,3'-diethyl-2,2'-thiacynine iodide molecule embedded in an optical microcavity
was theoretically studied. The molecule was coupled to a single cavity mode via
the quantum Rabi Hamiltonian, and the corresponding time-dependent
Schr\"odinger equation starting with a purely molecular excitation was solved
using the Multiconfigurational Time-Dependent Hartree Method (MCTDH). We show
that, for single-molecule strong coupling with the photon mode, nonadiabatic
molecular dynamics produces mixing of polariton manifolds with differing number
of excitations, without the need of counterrotating light-matter coupling
terms. As a consequence, an electronic excitation of the molecule at {\it cis}
configuration leads to the generation of photon pairs in the {\it trans}
configuration upon isomerization. Conditions for this phenomenon to be
operating in the collective strong light-matter coupling regime are discussed
and found to be unfeasible for the present system, based on simulations of two
molecules inside the microcavity. Yet, our finding suggests a new mechanism
that, without ultrastrong coupling, achieves photon down-conversion by
exploiting the emergent molecular dynamics arising in polaritonic
architectures.
