Dimensionality-Dependent Exciton-Phonon Coupling in 2D and Quasi-2D Perovskites Revealed by Transient Infrared Spectroscopy.

While the exceptional emission properties of two-dimensional Ruddlesden-Popper perovskites are fundamentally governed by exciton-phonon interactions, tracking these dynamic lattice couplings remains elusive due to the state-filling limitations of visible-spectrum spectroscopy. Herein, we utilize broadband mid-infrared transient absorption spectroscopy to directly probe the intraexcitonic transitions and dynamic lattice reorganizations across a dimensionality series (n = 1, 2, and 4) of phenethylammonium (PEA)-based perovskites. We observe a profound, dimensionality-driven tuning of the microscopic coupling mechanism. In the extreme confinement limit (n = 1), intense structural distortion washes out discrete transitions, yielding a broad continuum indicative of rapid self-trapping. Relaxing this confinement (n = 2) unveils beautifully resolved Franck-Condon progressions driven by ≈10 meV inorganic Pb-I optical phonons. By fitting this progression, we extract a Huang-Rhys factor of S ≈ 0.57, quantitatively confirming an intermediate coupling regime that prevents deep self-trapping. Further relaxation (n = 4) shifts the dominant interaction to higher-energy (≈33 meV) organic cation modes. This work demonstrates that structural dimensionality provides a direct lever to engineer the specific exciton-phonon bath, offering the fundamental physical blueprint required to optimize emission lineshapes for high-efficiency perovskite optoelectronics.