Highly sensitive stimuli-responsive multifunctional luminescent materials are crucial for applications in optical sensing, high-level security, and anticounterfeiting. Here, we report two zero-dimensional (0D) hybrid copper(I) halides, (TEP) ₂Cu ₂Br ₄ and (TEP) ₂Cu ₄Br ₆, which are comprised of isolated [Cu ₂Br ₄] ^2- and [Cu ₄Br ₆] ^2- inorganic cluster units, respectively, separated by TEP ⁺ (TEP = tetraethylphosphonium) cations. (TEP) ₂Cu ₂Br ₄ and (TEP) ₂Cu ₄Br ₆ demonstrate ultrabright greenish-white and orange-red emissions, respectively, with near unity photoluminescence quantum yields. Optical spectroscopy measurements and density-functional theory (DFT) calculations reveal that photoemissions of these compounds originate from the formation of self-trapped excitons (STEs) due to the excited-state distortions in the copper(I) halide units. Single crystals of both compounds are radioluminescence (RL) active at room temperature under both X- and γ-rays exposure. The excellent energy resolution values and light yields up to 15,800 ph/MeV under 662 keV γ-rays of ¹³⁷Cs suggest their potential for scintillation applications. Remarkably, (TEP) ₂Cu ₂Br ₄ and (TEP) ₂Cu ₄Br ₆ are interconvertible through external chemical stimuli or reverse crystallization. In addition, both compounds demonstrate luminescence on-off switching upon thermal stimuli. The sensitivity of (TEP) ₂Cu ₂Br ₄ and (TEP) ₂Cu ₄Br ₆ to the chemical and thermal stimuli coupled with their ultrabright emission allows their consideration for practical applications such as solid-state lighting, sensing, information storage, and anticounterfeiting.