Coordination units of Mn2+ modulation toward tunable emission in zero-dimensional bromides for white light-emitting diodes

Abstract

Organic–inorganic metal halides have become a multifunctional platform for manipulating photoluminescence due to highly efficient and tunable emissions, especially for lead-free Mn²⁺-based halides. Herein, the zero-dimensional (0D) bromides of (C₅H₁₄N₃)₂MnBr₄ and (CH₆N₃)₂MnBr₄ with different coordination environments were designed and synthesized by a solvent evaporation method. They exhibit green and red broadband emission peaks at 528 nm and 627 nm with high photoluminescence quantum yields of 86.83% and 61.91%, respectively, which are attributed to the d–d transition (⁴T₁(G) → ⁶A₁(S)) of [MnBr₄]²⁻ tetrahedral and [Mn₃Br₁₂]⁶⁻ octahedral units. The cases emphasize the effect of organic ligands on the intrinsic emissions of Mn²⁺ ions, thereby revealing the luminescence mechanism of Mn²⁺ ions in 0D isolated structures through the Tanabe–Sugano (TS) energy diagram. Thanks to their bright and stable emissions, the fabricated white light-emitting diode (LED) based on (C₅H₁₄N₃)₂MnBr₄ and (CH₆N₃)₂MnBr₄ provides an outstanding color rendering index (R_a) of 90.8 at a correlated color temperature (CCT) of 3709 K, along with the CIE chromaticity coordinates of (0.3985, 0.3979) and a luminous efficacy of 51.2 lm W⁻¹. This work aims at clarifying the relationship between the coordination units of Mn²⁺ and tunable emissions, and in particular, proposes a new strategy to explore phosphors excited by blue light for white LEDs.