“…The phase space of 2D halide perovskites has been populated by four primary structure subtypes, based on the charge of the organic spacer cation and the relative stacking of the inorganic layers: Ruddlesden–Popper (RP) structure (A′) 2 (A) n −1 M n X 3 n +1 , ,, Dion–Jacobson (DJ) structure (A′)(A) n −1 M n X 3 n +1 , , alternating cations in the interlayer space (ACI) type (A′)(A) n −1 M n X 3 n +1 and alkyl diammonium cations (NH 3 C m H m NH 3 )(CH 3 NH 3 ) n −1 M n X 3 n +1 . Among these, the (100) RP layered halide perovskites are the most prevalent, as the majority of 2D perovskites published belong to this substructure family. ,,,− , Less recurrent are layered perovskites of n ≥ 2, incorporating functional groups (e.g., unsaturated bonds, heteroatoms) within the organic layers that diversify significantly the explored phase space. ,− In this context, there are also n = 1 perovskites incorporating optical active organic layers that participate in the configuration of the optical properties of the 2D perovskite. − Finally, 2D hybrid halide perovskites provide a viable architecture to perform small-molecule reactivity in the solid state by utilizing the chemical reactivity of functional organic layers to orchestrate covalent and noncovalent interactions for technological use, as in (photo)polymerization, chemisorption, electrochemical ion cycling, etc., − as well as small-molecule intercalation that has been achieved with the intercalation of neutral or polarizable molecules, affording a final material with distinct structural and/or electronic properties. ,− The inclusion of designer organic molecules with functional groups within the organic layers has been demonstrated to influence the optoelectronic and photophysical properties of 2D perovskite materials. , Previously, the Sargent group showed that the organic cation influences the quantum-well distribution, as 2D perovskite films and devices incorporating allylammonium (AA) resulted in superior ...…”