Photoluminescence properties of tetrahedral zinc(ii) complexes supported by calix[4]arene-based salicylaldiminato ligands

Abstract

The synthesis and photophysical properties of four new hybrid salicylaldiminato-calix[4]arene ligands and their corresponding zinc(II) complexes are described. The Schiff bases were obtained from condensation reactions between cone-25,27-di(aminoethoxy)-26,28-dihydroxy-calix[4]arene and salicylaldehyde (H₂L¹) or o-vanillin (H₂L²) and 1,3-alt-25,27-di(aminoethoxy)-26,28-di(n-propyloxy)-calix[4]arene and 3,5-di-tert-butyl-salicylaldehyde (H₂L³) or o-vanillin (H₂L⁴). Complexation reactions were investigated by ESI-MS, IR, NMR, UV-vis absorption and steady-state and time resolved fluorescence spectroscopy, and X-ray crystallography. All ligands support 1 : 1 complexes (ZnL¹–ZnL⁴), with equilibrium constants derived from absorption spectrophotometry in the range log K₁₁ = 5.5–8.2 (MeCN or MeOH/CH₂Cl₂, I = 0.01 M). The zinc complexes show blue fluorescence, both in solution as well as in the solid state, with λ_em, Φ_f, and τ ranging from 472–504 nm, 0.11–0.60, and 2–9 ns, respectively. The nature of the substituents on the salicylaldiminato fragments was found to be the main parameter that influences the photophysical properties of the zinc complexes. Insights into the electronic nature of the UV-vis transitions were obtained with time dependent density functional theory (TD-DFT) calculations.