Luminescence properties of heterodinuclear Pt–Eu complexes from unusual nonadentate ligands

Abstract

The synthesis of ligand L₁, based on the tetramethyl ester of [4′-ethynyl-(6,6″-bis(aminomethyl))-2,2′:6′,2″-terpyridine]tetrakisacetate, and its acidic form L₂, are described. Using a Cu-assisted coupling reaction, L₁ was connected to the Pt atom of a [(tpy)Pt] precursor (tpy = 2,2′:6′,2″-terpyridine) to afford the [(tpy)PtL₁](BF₄) metallosynthon, from which a hydrolysis reaction gave Na₃[(tpy)PtL₂]. The photophysical properties of the metallosynthons were studied by means of absorption and steady state emission spectroscopic techniques in various solvents, which revealed a dramatic impact of the solvent polarity. DFT and TDDFT calculations (B3LYP) were used to investigate the absorption and emission properties of the [(tpy)PtL₁]⁺ system both in vacuo and in different solvents. Spectrophotometric titrations of [(tpy)PtL₁]⁺ with EuCl₃·6H₂O in acetonitrile revealed the formation of complicated mixtures of complexes with different [(tpyPtL₁)_xEu_y] stoichiometries, all of which display the typical Eu^III centred luminescence upon excitation into the Pt centred ¹MLCT absorption band. The energy transfer from the Pt subunit of [(tpy)PtL₂]³⁻ to Eu^III is inefficient in polar solvents, but it is restored in a TFA–CH₂Cl₂ mixture, together with the observation of a new emission band at 684 nm, likely arising from a charge transfer process involving reduction of Eu^III.