Organic Cation Modulation Triggered Second Harmonic Response in Manganese Halides with Bright Fluorescence

Zero-dimensional (0D) hybrid manganese halides have been recently synthesized and exhibited rich functional properties including fluorescence, ferroelectrics, and ferromagnetism. However, few studies on second-harmonic generation (SHG) behaviors of manganese halide crystals have been reported, presumably owing to the d–d transitions. Here, we report three manganese bromides, [TEA]2MnBr4 (TEA+ = tetraethylammonium; 1), [BTEA]2MnBr4 (BTEA+ = benzyltriethylammonium; 2), and [BTMA]2MnBr4 (BTMA+= benzyltrimethylammonium; 3), with linear and nonlinear optical properties via benzyl or ethyl/methyl substitution strategies. They feature 0D structures containing isolated [MnBr4]2– anions and quaternary ammonium cations with different sizes inserted for charge balance. They all show green phosphorescence, and 2 possesses good luminescence efficiency with a quantum yield of 97.8%, which is larger than those of 1 (79%) and 3 (72%). Specifically, acentric 1 and 3 present effective SHG responses about 0.48 and 0.59 times that of KDP, respectively. The result throws light on the new properties of the hybrid manganese halides and provides a new way to develop novel nonlinear optical-active organic–inorganic hybrid metal halides.