Reversible Structural Phase Transitions in Zero‐dimensional Cu(I)‐Based Metal Halides for Dynamically Tunable Emissions

Exploring structural phase transitions and luminescence mechanisms in Zero‐dimensional (0D) metal halides poses significant challenges, that are crucial for unlocking the full potential of tunable optical properties and diversifying their functional capabilities. Herein, we have designed two inter‐transformable 0D Cu(I)‐based metal halides, namely (C19H18P)2CuI3 and (C19H18P)2Cu4I6, through distinct synthesis conditions utilizing identical reactants. Their optical properties and luminescence mechanisms were systematically elucidated by experiments combined with density functional theory calculations. The bright cyan‐fluorescent (C19H18P)2CuI3 with high photoluminescence quantum yield (PLQY) of 77% is attributed to the self‐trapped exciton emission. Differently, the broad yellow‐orange fluorescence of (C19H18P)2Cu4I6 displays a remarkable PLQY of 83%. Its luminescence mechanism is mainly attributed to the combined effects of metal/halide‐to‐ligand charge transfer and cluster‐centered charge transfer, which effects stem from the strong Cu‐Cu bonding interactions in the (Cu4I6)2‐ clusters. Moreover, (C19H18P)2CuI3 and (C19H18P)2Cu4I6 exhibit reversible structural phase transitions. The elucidation of the phase transitions mechanism has paved the way for an unforgeable anti‐counterfeiting system. This system dynamically shifts between cyan and yellow‐orange fluorescence, triggered by the phase transitions, bolstering security and authenticity. This work enriches the luminescence theory of 0D metal halides, offering novel strategies for optical property modulation and fostering optical applications.