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Abstract Highly sensitive stimuli‐responsive luminescent materials are crucial for applications in optical sensing, security, and anticounterfeiting. Here, we report two zero‐dimensional (0D) copper(I) halides, (TEP)2Cu2Br4, (TEP)2Cu4Br6, and 1D (TEP)3Ag6Br9, which are comprised of isolated [Cu2Br4]2−, [Cu4Br6]2−, and [Ag6Br9]3− polyanions, respectively, separated by TEP+ (tetraethylphosphonium [TEP]) cations. (TEP)2Cu2Br4 and (TEP)2Cu4Br6 demonstrate greenish‐white and orange‐red emissions, respectively, with near unity photoluminescence quantum yields, while (TEP)3Ag6Br9 is a poor light emitter. Optical spectroscopy measurements and density‐functional theory calculations reveal that photoemissions of these compounds originate from self‐trapped excitons due to the excited‐state distortions in the copper(I) halide units. Crystals of Cu(I) halides are radioluminescence active at room temperature under both X‐ and γ‐rays exposure. The light yields up to 15,800 ph/MeV under 662 keV γ‐rays of 137Cs suggesting their potential for scintillation applications. Remarkably, (TEP)2Cu2Br4 and (TEP)2Cu4Br6 are interconvertible through chemical stimuli or reverse crystallization. In addition, both compounds demonstrate luminescence on‐off switching upon thermal stimuli. The sensitivity of (TEP)2Cu2Br4 and (TEP)2Cu4Br6 to the chemical and thermal stimuli coupled with their ultrabright emission allows their consideration for applications such as solid‐state lighting, sensing, information storage, and anticounterfeiting. |
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