Zinc/itaconate coordination polymers as first examples with long-lasting phosphorescence based on acyclic ligands

Abstract

Two novel coordination polymers (CPs) based on zinc(II) and acyclic dicarboxylate linkers, i.e. [Zn₂(μ₃-IA)₂(H₂O)₄]_n (1-W) and {[Zn₂(μ-IA)₂(DMF)]·H₂O}_n (2-DMF) (where IA²⁻ = itaconate) have been obtained as a result of a systematic study of some synthetic factors such as pH and solvents, which has allowed optimizing the conditions that render the best yields. 1-W is obtained as X-ray quality single crystals upon which diffraction analysis has been conducted, showing that it consists of a neutral 2D layered structure. Some structural details have been elucidated for 2-DMF based on a comparison of PXRD data with 1-W and in view of the similar composition of both crystalline phases. Photoluminescence (PL) measurements confirm that 2-DMF presents slightly different spectra from those of 1-W. Time-dependent density functional theory (TD-DFT) calculations of 1-W reveal that electronic transitions arise from both first excited singlet and triplet states involving ligand-centred emissions. Moreover, both compounds exhibit blue fluorescence and green phosphorescence that involves less frequent long-lasting phosphorescence (LLP) at 10 K. Exhaustive characterization of long-lasting phosphorescence by means of time-resolved spectroscopy reveals remarkably long lifetimes (τ) of ca. 352 ms (λ_em ≈ 550 nm) for 1-W and 547 ms (λ_em ≈ 500 nm) for 2-DMF.