Pressure-induced tunable emission colors and irreversible bandgap narrowing in organic–inorganic manganese bromide hybrids

Abstract

Manganese (Mn)-based organic–inorganic metal halides (OIMHs) are extensively employed in various optical applications due to their non-toxicity, superior optical properties, and tunable emission advantages. Developing effective strategies to enhance the optical properties of these materials and clarifying their structure–optical property relationships continue to be of significant interest. Here, we report that the zero-dimensional (0D) (C₂₄H₂₀P)₂MnBr₄ exhibits remarkable pressure emission tuning that shifts from green to red, a phenomenon driven by the enhancement of the crystal field splitting energy and the reduced energy difference of the lowest d–d transition (⁴T₁ → ⁶A₁) motivated by the shortening of Mn–Br bonds. Additionally, the band gap value of (C₂₄H₂₀P)₂MnBr₄ becomes 3.13 eV, showing an unexpected decrease by 1.09 eV from an initial value of 4.22 eV, when the pressure is released from 31.0 GPa. The incomplete recovery of the distortion in [MnBr₄]²⁻ tetrahedra, impeded by the distortion of soft organic macromolecules, is believed to be responsible for band gap narrowing. Combined with computations and subsequent experiments, it is inferred that pressure induced reduction in the Mn–Br bond distance contributes to the distortion of tetrahedral [MnBr₄]²⁻ and larger-sized organic cations. Our work not only elucidates the structure–optical property relationships in Mn-based OIMHs, but also opens new avenues for modulating their optical properties and devising innovative strategies for designing novel Mn-based OIMHs.