  Whereas their photophysics exhibits an intricate interplay of carriers with
the lattice, most reports have so far relied on single compound studies. With
the exception of variations of the organic spacer cations, the effect of
constituent substitution on the photophysics and the nature of emitting
species, in particular, has remained largely under-explored. Here
PEA$_2$PbBr$_4$, PEA$_2$PbI$_4$, and PEA$_2$SnI$_4$ are studied through a
variety of optical spectroscopy techniques to reveal a complex set of excitonic
transitions at low temperature. We attribute the emergence of weak high energy
features to a vibronic progression breaking Kasha's rule and highlight that the
responsible phonons cannot be accessed through simple Raman spectroscopy.
Bright peaks at lower energy are due to two distinct excitons, of which the
upper is a convolution of a bright exciton and a localised state, whereas the
lower is attributed to shallow defects. Our study offers deeper insights into
the photophysics of two-dimensional perovskites through compositional
substitution and highlights critical limits to the communities' current
understanding of the photophysics of these compounds.
