Direct Evidence of the Effect of Water Molecules Position in the Spectroscopy, Dynamics, and Lighting Performance of an Eco‐Friendly Mn‐Based Organic–Inorganic Metal Halide Material for High‐Performance LEDs and Solvent Vapor Sensing

Abstract Luminescent Mn(II)‐based organic–inorganic hybrid halides have drawn attention as potential materials for sensing and photonics applications. Here, the synthesis and characterization of methylammonium (MA) manganese bromide ((MA) n Br x Mn(H 2 O) 2 , ( n = 1, 4 and x = 3, 6)) with different stoichiometries of the organic cation and inorganic counterpart, are reported. While the Mn 2+ centers have an octahedral conformation, the two coordinating water molecules are found either in cis ( 1 ) or in trans ( 2 ) positions. The photophysical behavior of 1 reflects the luminescence of Mn 2+ in an octahedral environment. Although Mn 2+ in 2 also has octahedral coordination, at room temperature dual emission bands at ≈530 and ≈660 nm are observed, explained in terms of emission from Mn 2+ in tetragonally compressed octahedra and self‐trapped excitons (STEs), respectively. Above the room temperature, 2 shows quasi‐tetrahedral behavior with intense green emission, while at temperatures below 140 K, another STE band emerges at 570 nm. Time‐resolved experiments (77–360 K) provide a clear picture of the excited dynamics . 2 shows rising components due to STEs formation equilibrated at room temperature with their precursors. Finally, the potential of these materials for the fabrication of color‐tunable down‐converted light‐emitting diode (LED) and for detecting polar solvent vapors is shown.