Facile preparation, mechanochromic luminescence and excitation wavelength-dependent emission of tetra(1-benzo[]imidazol-2-yl)ethene Zn(II) complexes and their applications.

Nowadays, benzimidazole and its derivatives are widely assembled into multifunctional materials with various properties such as mechanochromism, photochromism, thermochromism and electrochromism. Herein, two novel zinc(II) coordination compounds, [Zn(L)Br]·2HO (1) and [Zn(L)Cl]·2HO (2) (L = tetra(1-benzo[]imidazol-2-yl)ethene), have been constructed one-pot facile synthesis from bis(1-benzo[]imidazol-2-yl)methane (L) and zinc(II) salts. The ligand L with a CC double bond was formed by C-C coupling of two sp-C atoms of L in solvothermal synthesis, which provides a new strategy to generate the conjugation system conveniently. Impressively, complex 1 shows distinct fluorescence and phosphorescence dual emission due to the heavy atom effect of bromide ions. Complexes 1 and 2 exhibit high-contrast mechanochromic luminescence properties due to the transformation from a crystalline state to an amorphous state. In addition, they also exhibited excitation wavelength-dependent luminescence in crystalline states and DMF systems. As the excitation wavelength increased, their luminescence color in DMF solution changed significantly from blue to yellow. Theoretical calculations show that the complexes have multiple emission centers due to the locally excited nature and intramolecular charge transfer. Due to their excellent excitation wavelength-dependent luminescence properties, PMMA films based on complexes 1 and 2 were prepared and could be applied in anti-counterfeiting and UV protection.