Green-Emissive Zn2+ Complex Supported by a Macrocyclic Schiff-Base/Calix[4]arene-Ligand: Crystallographic and Spectroscopic Characterization

The macrocyclic calix[4]arene ligand H2L comprises two non-fluorescent 2,6-bis-(iminomethyl)phenolate chromophores, which show a chelation-enhanced fluorescence enhancement upon Zn2+ ion complexation. Macrocyclic [ZnL] complexes aggregate in the absence of external coligands via intermolecular Zn-N bonds to give dimeric [ZnL](2) structures comprising two five-coordinated Zn2+ ions. The absorption and emission wavelengths are bathochromically shifted upon going from the liquid (lambda(max,abs) (CH2Cl2)=404 nm, lambda(max,em) (CH2Cl2)=484 nm) to the solid state (lambda(max,abs)=424 nm (4 wt%, BaSO4 pellet), lambda(max,em)=524 nm (neat solid)). Insights into the electronic nature of the UV-vis transitions were obtained with time-dependent density functional theory (TD-DFT) calculations for a truncated model complex.