  Organic-inorganic layered perovskites are two-dimensional quantum wells with
layers of lead-halide octahedra stacked between organic ligand barriers. The
combination of their dielectric confinement and ionic sublattice results in
excitonic excitations with substantial binding energies that are strongly
coupled to the surrounding soft, polar lattice. However, the ligand environment
in layered perovskites can significantly alter their optical properties due to
the complex dynamic disorder of soft perovskite lattice. Here, we observe the
dynamic disorder through phonon dephasing lifetimes initiated by ultrafast
photoexcitation employing high-resolution resonant impulsive stimulated Raman
spectroscopy of a variety of ligand substitutions. We demonstrate that
vibrational relaxation in layered perovskite formed from flexible alkyl-amines
as organic barriers is fast and relatively independent of the lattice
temperature. Relaxation in aromatic amine based layered perovskite is slower,
though still fast relative to pure inorganic lead bromide lattices, with a rate
that is temperature dependent. Using molecular dynamics simulations, we explain
the fast rates of relaxation by quantifying the large anharmonic coupling of
the optical modes with the ligand layers and rationalize the temperature
independence due to their amorphous packing. This work provides a molecular and
time-domain depiction of the relaxation of nascent optical excitations and
opens opportunities to understand how they couple to the complex layered
perovskite lattice, elucidating design principles for optoelectronic devices.
