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

Abstract

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, [Zn₂(L²)Br₄]·2H₂O (1) and [Zn₂(L²)Cl₄]·2H₂O (2) (L² = tetra(1H-benzo[d]imidazol-2-yl)ethene), have been constructed via one-pot facile synthesis from bis(1H-benzo[d]imidazol-2-yl)methane (L¹) and zinc(II) salts. The ligand L² with a CC double bond was in situ formed by C–C coupling of two sp³-C atoms of L¹ 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.