Fluorescence−Tracking of ASite Cation Insertion: Resolving Nucleation and Growth in the PbBr2 to APbBr3 Perovskite Transformation

A-site cations (A+) play an important role in tailoring the structural, electronic, and excitonic landscape of metal‐halide perovskites, yet their dynamic insertion and impact on lead halide octahedron ([PbX6]4−) precursor assembly and subsequent crystal growth remain poorly understood. Here we introduce a fluorescence‐tracking methodology that monitors in situ the progressive embedding of A+, with alkyl chains from C1 (MA⁺) to C4 (BA⁺), into a PbBr2 lattice, capturing optical evolution from PbBr2 colloidal to perovskite nucleation formation. Steady‐state and time‐resolved photoluminescence reveal a universal two‐step transition: initial amplification of 610 nm shallow self‐trapped exciton (STE) emission upon surface passivation, followed by emergence of 565 nm deep‐STE emission as ABX₃ nuclei form, independent of cation length. Ultrafast transient absorption deciphers a switch from about 200 ps free‐exciton lifetimes to ps‐scale self‐trapping at high A⁺ loading. In the crystalline regime, increasing chain length drives [PbX6]4− octahedral connectivity from 0D clusters to corrugated 2D sheets and flat 2D layers, modulating the competition between free and self‐trapped exciton recombination and yielding tunable blue to white broadband emission. This two‐stage mechanistic framework decouples inorganic motif assembly from organic‐cation effects, underscoring A‐site engineering as a powerful handle for designing low‐dimensional perovskite emitters.