Connectivity-Dependent Exciton–Phonon Coupling in Cesium Bismuth Halide Quantum Dots

Metal halide octahedra form the fundamental functional building blocks of metal halide perovskites, dictating their structures, optical properties, electronic structures, and dynamics. In this study, we show that the connectivity of bismuth halide octahedra in Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ quantum dots (QDs) changes with different halide elements. We use first-principles calculations to reveal the key role of the connectivity of bismuth halide octahedra on the wave function symmetry, Huang–Rhys factor, and exciton–phonon interaction strength. Following QD synthesis via a ligand-mediated transport method, the effect of connectivity is verified with transient absorption spectroscopy, where we contrast Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ QD exciton dynamics. In photoexcited Cs₃Bi₂I₉ QDs, phonons related to the vibrational motions of face-sharing [BiI₆]^3– bioctahedra couple strongly to the electronic state and drive rapid carrier relaxation. Equivalent signals are not observed for photoexcited Cs₃Bi₂Br₉ QDs, implying a lack of phonon involvement in band-edge absorption and subsequent exciton relaxation. Our findings suggest that structural engineering can effectively tune the exciton–phonon coupling and therefore influence exciton relaxation and recombination in perovskite nanomaterials.