Mechanochromic Luminescence of Four Zn(II)/Cd(II) Complexes Based on Same Schiff-base Ligand with Different Coordination Modes

Research based on derivatives of salicylaldehyde Schiff bases and their complexes has established that most of them crystallize predominantly in the enol form rather than the keto form. However, the self-assembly of 2-(4-aminophenyl)­benzimidazole, 4-(diethylamino)­salicylaldehyde, and metal salts could afford four Zn­(II)/Cd­(II) complexes in which the ligands exist in enol and keto forms with different coordination modes. Ligands of complexes 1 and 4 are in the neutral keto form (HL^k), and ligands of complexes 2 and 3 are in the enol form (L^e) as an anion. The crystal structure and fluorescence of complex 1 are temperature-dependent. At low temperature, the aromatic ring could rotate to increase the coplanarity of the ligand. All of them exhibit mechanochromic luminescence (MCL) properties. The emission peaks of crystals 1–4 as prepared are located at 637, 650, 542/612 and 643/615 nm and changed to 603, 602, 555, and 595 nm after grinding, respectively. The MCL mechanism of complexes 1, 3, and 4 could be attributed to the transformation from the crystalline to the amorphous state. In addition, the MCL mechanism of 2 could be mainly attributed to the self-absorption effect. A series of xerogels PAA-2 with variable and adjustable fluorescence were synthesized to further explain the effect of the molecular packing of 2.