Tetracoordinated Manganese(II) Halide with Multimode Thermochromic Luminescence for Anticounterfeiting and Encryption

Manganese(II) halides have received a booming development owing to their flexible structures and remarkable optical properties, whereas those with multimode luminescence color changes under specific stimuli are rarely reported. In this study, we synthesized a zero-dimensional (C5H14NO)2MnBr4 single crystal, which experiences an order-disorder phase transition from P21/c to P2/m at 268 K. Intriguingly, the emission color of (C5H14NO)2MnBr4 exhibits a "symmetric" switching of green-yellow-orange-red-orange-yellow-green within the temperature range of 80-420 K. For the low-temperature phase (LTP) below 268 K, the green and red lights at 510 and 640 nm are derived from the d-d electron transition of Mn(II) and the emission of self-trapped excitons (SETs), respectively. For the high-temperature phase (HTP) above 268 K, the dual emission peaks at 530 and 640 nm can be attributed to the simultaneous existence of trapping and detrapping processes of the STEs. Taking advantage of the unique thermochromic behaviors of (C5H14NO)2MnBr4, the temperature-responsive anticounterfeiting labels and encryption codes with multilevel design and high security have been constructed. This work provides new insights for the preparation of metal halides with multiple luminescence performances, facilitating the development of anticounterfeiting and information protection technologies.