Water Clusters Gather Luminescent Zinc(II) Complexes around Hydrogen-Bonded Framework Structures and Associated Fluorescence Modulation

Luminescent complexes [Zn(phen)(dca)₂] (1) and [Zn(bpy)(aba)₂] (2) (phen = 1,10-phenanthroline, dca = 4-dimethylamino cinnamate, bpy = 2,2‘-bipyridyl, aba = 4-dimethylamino benzoate) are formed readily at room temperature when the reactants are allowed to react in a 1:1:2 molar ratio. Complex 1 self-assembles into a hydrogen-bonded framework structure in aqueous ethanol to form {[Zn(phen)(dca)₂]·3H₂O·EtOH}_n (3), in which four complex units gather around tetrameric water clusters. Similarly, complex 2 forms a different hydrogen-bonded framework structure with the empirical formula {[Zn(bpy)(aba)₂]·4H₂O}_n (4), in which octameric water clusters assemble [Zn(bpy)(aba)₂] complex units. Compounds 3 and 4 exhibit significant red shifts of the emission bands with respect to 1 and 2, respectively. On heating, compound 3 loses water to form 1 and then later upon recrystallization from aqueous ethanol forms 3 as monitored by X-ray powder patterns. Similarly, the transformation between 2 and 4 is also reversible.