Exciton-Polaron in a Quasi-One-Dimensional Chain of\nHexyl-Diammonium-BiI<sub>5</sub> Octahedra

Lower-dimensional organic–inorganic hybrid perovskite materials\npromise to revolutionize the optoelectronics industry due to the tremendous\npossibilities of exotic control on excitonic properties driven via\nquantum confinement. Flexible organic cations acting as spacers and\nstabilizers enhance electron–phonon couplings, further amplifying\nthe potential for modular light–matter interactions in these\nmaterials. Herein we unravel the nature of excitons in a quasi-1D\nchain of corner-sharing bismuth iodide octahedra with an intrinsic\nquantum well structure stabilized by a hexyl-diammonium cation. Using\nbroadband femtosecond impulsive Raman spectroscopy and detailed electronic\nstructure calculations, we directly quantify the exciton lifetime\nalong with the electron–phonon coupling constants to fully\ndescribe the excitation as an exciton-polaron. We find ∼30\ntimes larger electron–phonon couplings beyond the standard\n2D-hybrid perovskite materials along with picosecond time-scale decoherences,\nthereby shedding light for the first time on the immense potential\nof these 1D perovskite analogues for developing novel materials for\nefficient light-conversion technologies.