To meet the increasing demand of anti-counterfeiting technology, the development of materials with novel stimuli-response property is essential.
Luminescent materials with thermo-responsible properties hold great promise in high level anti-counterfeiting. However, most of the reported thermo-responsible materials can only respond once to a given temperature, that is, on/off switching of photoluminescent (PL) or emission colour switching, which are easily counterfeited.
In a study published in Angew. Chem. Int. Ed., a research group led by Prof. HUNAG Xiaoying from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported a thermally induced ‘on-off-on’ luminescent switching mode in inorganic-organic metal halides (IOMHs), which was firstly achieved in thermo-responsible luminescent materials. The existence of triple luminescent states and switching modes in the switching process may largely enhance the forgery difficulty.
Through the combination of changeable coordination number of antimony, crystallization-induced luminescence of IOMHs, and low volatility of ionic liquids (ILs), researchers achieved the reversible extraction and insertion of the IL of [Bzmim]Cl (Bzmim = 1-benzyl-3-methylimidazolium) in compounds [Bzmim]n-3SbCln (n = 6 for 1 and 5 for 2). Compound 1 exhibits green emission (Quantum Efficiency = 87.5%), while 2 exhibits blue and red emissions, respectively, under the irradiation of 310 and 396 nm light.
Meanwhile, both 1 and IL@2 can be crystallized from the molten of [Bzmim]3SbCl6 by using different cooling methods. The extraction of ILs from 1 and the insertion of ILs into 2 could be triggered by moisture and heat, respectively. The mechanism was analyzed by comparing the crystal structures of 1 and 2.
The melting points (Tm) of 1 and 2 are 410 K and 348 K, respectively. Heating IL@2 would cause triple-mode luminescent switching, which depends on the temperature (T).
At T > Tm1, the luminescence of IL@2 would quench due to the crystallization-induced luminescence of IOMHs, that is, ‘on-off’ switching mode. At T < Tm2, the luminescence would directly switch from red to green due to the insertion of ILs into 2, that is, ‘colour switching’ mode. At Tm2 < T < Tm1, the emission of 2 would be quenched. However, the ILs would then react with 2 to form 1 and a green emission would arise from the non-emission molten, and an ‘on-off-on’ luminescent switching mode is finally achieved.
Besides, researchers revealed that the competition between kinetic and thermodynamic crystallization processes provides opportunity to fabricate rewritable luminescent paper. The low melting point of the two compounds makes it possible to write on the paper directly by laser, which is mask and ink free.
This study provides a new strategy to design novel stimuli-responsible materials, and expands the research for the applications of IOMHs in anti-counterfeiting and rewritable luminescent paper.
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