  Molecular polaritons are hybrid light-matter states that emerge when a
molecular transition strongly interacts with photons in a resonator. At optical
frequencies, this interaction unlocks a way to explore and control new chemical
phenomena at the nanoscale. Achieving such a control at ultrafast timescales,
however, is an outstanding challenge, as it requires a deep understanding of
the dynamics of the collectively coupled molecular excitation and the
nanoconfined electromagnetic fields. Here, we investigate the dynamics of
collective polariton states, realized by coupling molecular photoswitches to
optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an
ultrafast collapse of polaritons to a single-molecule transition triggered by
femtosecond-pulse excitation at room-temperature. Through a synergistic
combination of experiments and quantum mechanical modelling, we show that the
response of the system is governed by intramolecular dynamics, occurring one
order of magnitude faster with respect to the unperturbed excited molecule
relaxation to the ground state.
