<p>Although organic-inorganic manganese(II) halides have achieved significant advancements, research on the relationship among their structure, photophysical properties, and scintillation performance remains understudied, especially in neutral manganese(II) chlorides. In this work, we design and synthesize three neutral manganese(II) chlorides ((1-<i>t</i>Bu)₂MnCl₂, (2-<i>t</i>Bu)₂MnCl₂, and (3-<i>t</i>Bu)₂MnCl₂) by incorporating phosphine oxide ligands with tunable steric hindrance. Interestingly, the photoluminescence quantum yields (PLQYs) of these green emissive complexes are highly dependent on the combinations of the benzene ring and <i>tert</i>-butyl (<i>t</i>Bu) group, resulting in the PLQYs in the range of 1.7% to 55.6%. The PLQY of (2-<i>t</i>Bu)₂MnCl₂ represents the highest value up to 55.6% among neutral monodentate manganese(II) chlorides. The rigidity of the benzene ring and steric hindrance of the <i>t</i>Bu groups effectively suppress the non-radiative transition processes under UV excitation. Notably, as the X-ray scintillator, (1-<i>t</i>Bu)₂MnCl₂ and (2-<i>t</i>Bu)₂MnCl₂ exhibit high light yields of 13021.7 photons/MeV and 14562.3 photons/MeV, respectively, with a spatial resolution exceeding 10 lp/mm for X-ray imaging. This work demonstrates the rational design of the of <i>t</i>Bu-modified manganese(II) chlorides as the efficient luminescent materials for scintillators, thereby paving the way for the development of high-performance neutral monodentate manganese(II) complexes.</p> Graphical Abstract <p></p>