Dimension and Valence Manipulation of Luminescent Manganese‐Based Perovskites via Alternating Cation Interactions

Manganese (Mn)‐based halide perovskites have attracted tremendous attention due to their low‐cost and environment‐friendly characteristics. Nevertheless, their applications are hindered by limited photoluminescence (PL) efficiency and insufficient stability. Dimensional engineering offers a viable pathway to modulate their photophysical properties and enhance their robustness. Herein, we design 2D@3D perovskites based on the dimensional reduction of CsMnCl 3 ·2H 2 O 3D perovskites via alternating cation interactions (ACIs) by employing chitosan as a polymeric spacer cation. ACI effectively stabilized the 2D@3D perovskite and passivated surface defects through enriched H‐bonding. As such, the PL intensity can be boosted by 50 times with a PL quantum yield (PLQY) of 18.1%. Intriguingly, 2D@3D perovskites experienced valence transition (VT: Mn 2+ → Mn 4+ ) at high temperatures, resulting in NH 4 CsMnCl 6 perovskite. Density functional theory calculations indicated that an interfacial orbital hybridization‐driven reaction mechanism triggered VT, which was initiated by the synergistic effect of octahedral distortion and ACI within 2D@3D perovskite. Notably, the proposed VT perovskites exhibited narrowband emission of Mn 4+ with remarkable air‐, photo‐, and thermally stability, achieving a PLQY up to 80.7%. This approach paves the way for exploring organic‐inorganic interactions in designing highly luminescent Mn‐based perovskites.