Manganese-based organic-inorganic metal halides (Mn-based OIMHs) scintillators have aroused a research upsurge due to their outstanding photoluminescence performance, low toxicity, and tunable structures. Herein, we demonstrate a molecular engineering strategy that synergistically enhances luminescence efficiency and radiation sensitivity by systematic elongation of branched alkyl chains in phosphonium-based cations. The (4-DEATBP)MnBr