Crystal chemistry and luminescence properties of tetrabutylphosphonium tetrakis(8-quinolinato)lanthanidate [P4444][Ln(Q)4]·2X (X = H2O and (CH3)2CO)

Abstract

Since the discovery of AlQ₃ (Q = 8-quinolinolato) quinolinato complexes, they have been extensively scrutinized as emitter materials for organic lighting. Herein, we report on the first representatives of a series of tetrabutylphosphonium tetrakis(8-quinolinolato)lanthanidate complexes [P₄₄₄₄][Ln(Q)₄]·2X (Ln = Dy–Lu and Y; X = H₂O for Ln = Dy–Tm, Lu and Y and (CH₃)₂CO for Ln = Yb), which are synthesised by a simple metathesis reaction of the respective potassium tetrakis(8-quinolinolato)lanthanidate salts with tetrabutylphosphonium bromide in acetone at room temperature. Single-crystal X-ray diffraction reveals that Ln(III) is coordinated by four bidentate 8-quinolinato ligands in the form of a distorted square antiprism. The distinct [Ln(Q)₄]⁻ anions interact with the [P₄₄₄₄]⁺ cations through secondary bonding interactions, such as CH–π and van der Waals interactions, in addition to electrostatic coulombic interactions. Although these compounds contain crystal water/solvent molecules (and their synthesis does not require an inert atmosphere), they do not enter the metal coordination sphere but form pairwise intramolecular hydrogen bonds with the two 8-quinolinato ligands of the complex lanthanide anions. Combined differential scanning calorimetry–thermogravimetric analysis indicates that crystal water is lost at around 100 °C and [P₄₄₄₄][Ln(Q)₄] is formed, which is stable up to 300 °C, where further degradation occurs. All compounds feature strong emission in the green region, originating from the π* → π transitions within the 8-quinolinato ligand, with lifetimes in the nanosecond range. The luminescence colour changes from blue-green to yellow-green depending on Ln³⁺, which opens up additional directions in the colour tuning of emitters for organic lighting applications.