Centimeter‐Sized Tetraphenylphosphonium Manganese Bromide Single Crystal: A Platform for Revealing Latent Blue Emission and Enabling High‐Resolution Scintillation

ABSTRACT Large, high‐quality single crystals are essential for uncovering intrinsic photophysics in organic–inorganic hybrid metal halides (OIHMs), yet their growth remains challenging. We report a solvent‐engineered strategy that provides controlled solubility and a wide metastable zone, enabling the growth of centimetre‐sized C 24 H 20 P‐MnBr crystals, the largest reported Mn‐based OIHM crystal to date. These bulk crystals reveal, for the first time, an intrinsic blue emission (λ em = 430 nm, τ ≈ 228 ps) originating from the organic cation, and polarized spectroscopy confirms directional energy transfer to Mn 2 + , revising the excited‐state dynamics of this system. The crystals exhibit excellent scintillation performance with a light yield of 66,468 photons/MeV and an energy resolution of 44.1% at 50 keV. Our solvent‐based approach is general and applicable to other hybrid halides, demonstrating that large single crystals are crucial for uncovering complex photophysics and advancing radiation‐detection devices.