Conformational order of spacer cations dictates coherent phonon oscillations in two-dimensional organic-inorganic perovskites

Phonon coherence can reflect electron‒phonon coupling information and has been proven to modulate elec-tronic states and charge transport. The manipulation of phonon coherence through spacer cation engineering in organic‒inorganic hybrid perovskites (OIH-Ps) has been extensively demonstrated; however, the underlying structural ori-gin remains elusive at the molecular level. Herein, we present molecular structure and temperature-dependent coherent phonon studies via a combination of sum frequency generation vibrational spectroscopy (SFG-VS) and transient absorption spectroscopy (TAS). The conformational order of spacer cations dictates the coherent phonon oscillations in 2D OIHPs. Our study further analyzes the static order and dynamic disorder in 2D perovskites. This work provides molecular-level insights in-to the role of spacer cations in tuning structural order and may provide valuable guidance for advancing emergent optoelecltronics development.