Phosphorescence at Low Temperature by External Heavy-Atom Effect in Zinc(II) Clusters

Luminescent Zn-II clusters [Zn4L4(mu(3)-OMe)(2)X-2] (X=SCN (1), Cl (2), Br (3)) and [Zn7L6(mu(3)-OMe)(2)(mu(3)-OH)(4)]Y-2 (Y=I- (4), ClO4- (5)), HL=methyl-3-methoxysalicylate, exhibiting blue fluorescence at room temperature (lambda(max)=416 approximate to 429 nm, phi(em)=0.09-0.36) have been synthesised and investigated in detail. In one case the external heavy-atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (lambda(max)=520 nm, tau=95.3 ms) at 77 K. Single-crystal X-ray structural analysis and time-dependent density-functional theory (TD-DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand-centred ((LC)-L-1) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH-I interactions.