Tuning up of chromism, luminescence in cadmium-viologen complexes through polymorphism strategy: Inkless erasable printing application

In our work, polymorphism strategy has been successfully applied to tune up chromism and luminescence properties of viologen-based materials. Two polymorphs of viologen-based complexes of α-CdBr2(PHSQ)2(H2O)2 (1) and β-CdBr2(PHSQ)2(H2O)2 (2) (PHSQ = N-(4-sulfophenyl)-4,4′-bipyridinium) were synthesized by changing the solvent. They can both respond to UV light and electricity in the manner of chromism visible to the naked eye and the coloration states have good reversibility, through which an inkless erasable printing model has been established. But the coloration contrast of 1 is higher compared to 2. Meanwhile, they both exhibit photoluminescence properties and the intensity of 1 is twice that of 2, which is accompanied by photoquenching upon continuous UV light irradiation. The only divergence of disordered/ordered O atoms in the two crystalline compounds leads to significantly different chromic and luminescent properties. Further explorations simultaneously demonstrate that the different chromic performance between 1 and 2 should attribute to the alteration of stimulus-induced (light/ electricity) electron transfer channels caused by the ordered/disordered O atoms in the complexes, which is achieved through C-H···O and O-H···O interactions to change crystal arrangement and structural rigidity, thus affect luminescent properties.