  We report a family of two-dimensional hybrid perovskites (2DHPs) based on
phenethylammonium lead iodide ((PEA)$_2$PbI$_4$) that show complex structure in
their low-temperature excitonic absorption and photoluminescence (PL) spectra
as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl
group of the PEA cation with F, Cl, or Br to systematically increase the
cation's cross-sectional area and mass and study changes in the excitonic
structure. These single atom substitutions substantially change the observable
number of and spacing between discrete resonances in the excitonic absorption
and PL spectra and drastically increase the amount of hot exciton PL that
violates Kasha's rule by over an order of magnitude. To fit the progressively
larger cations, the inorganic framework distorts and is strained, reducing the
Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the
2DHP structure and steady-state and time-resolved spectra suggests the complex
structure of resonances arises from one or two manifolds of states, depending
on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a
manifold are regularly spaced with an energy separation that decreases as the
mass of the cation increases. The uniform separation between resonances and the
dynamics that show excitons can only relax to the next-lowest state are
consistent with a vibronic progression caused by a vibrational mode on the
cation. These results demonstrate that simple changes to the structure of the
cation can be used to tailor the properties and dynamics of the confined
excitons without directly modifying the inorganic framework.
