Lead-Free Hybrid Metal Halides with a Green-Emissive [MnBr₄] Unit as a Selective Turn-On Fluorescent Sensor for Acetone

Organic-inorganic hybrid metal halides with zero-dimensional (0D) structure has emerged as a new class of light-emitting materials. Herein, a new lead-free compound (C₉NH₂₀)₂MnBr₄ has been discovered and a temperature-dependent phase transition has been identified for two phases (space group <i>P</i>2₁/<i>c</i> and <i>C</i>2/<i>c</i>) in which individual [MnBr₄]^2- anions connect with organic cations, (C₉NH₂₀⁺) (1-buty-1-methylpyrrolidinium⁺), forming periodic structure with 0D blocks. A green emission band, peaking at 528 nm with a high photoluminescence quantum efficiency (PLQE) of 81.08%, has been observed at room temperature, which is originated from the ⁴T₁(G) to ⁶A₁ transition of tetrahedrally coordinated Mn²⁺ ions, as also elaborated by density functional theory calculation. Accordingly, a fast, switchable, and highly selective fluorescent sensor platform for different organic solvents based on the luminescence of (C₉NH₂₀)₂MnBr₄ has been developed. We believe that the hybrid metal halides with high PLQE and the exploration of these as a fluorescence sensor will expand the applications scope of bulk 0D materials for future development.