Temperature-Induced Crossover of Coherent Phonon Mechanisms in Chiral 2D Perovskites
The coupling between electronic excitations and lattice degrees of freedom fundamentally dictates the optoelectronic functionality of hybrid perovskites. While the potential energy surfaces (PESs) of the electronic excited states are typically considered static, albeit modulated by thermal disorder, the exact nature of their structural evolution with temperature remains elusive. Here, we demonstrate that the excited-state structural reconfiguration in two-dimensional metal-halide perovskites is explicitly temperature-evolving, governed by lattice compliance. We select a chiral perovskite framework with an exceptionally large, temperature-dependent bond angle variance to maximize the structural compliance. Through phase-resolved resonant impulsive stimulated Raman scattering, we measure the coherent phonon dynamics and resolve the real-time structural pathways of exciton-lattice dressing. We observe a temperature-induced crossover from field-driven Impulsive Stimulated Raman Scattering (ISRS) to population-driven Displacive Excitation of Coherent Phonons (DECP). While momentum-driven ISRS pathways dominate at low temperatures, increasing thermal energy softens the lattice and enhances coordinate-driven displacive pathways, allowing excitons to sample steeper, highly anharmonic regions of the excited-state PES. Our results show that temperature can actively modulate the excited-state structural coordinates of flexible 2D frameworks, offering a practical strategy to tune exciton-lattice interactions in chiral optoelectronics.