Dual ligand-based fluorescence and phosphorescence emission at room temperature from platinum thioxanthonyl complexes

New square-planar platinum(II) complexes of the type trans-[Pt(PEt3)2(Tx)(X)] (X = Br, Cl, I or CN) bearing a σ-bonded thioxanthon-2-yl (Tx) ligand have been prepared and characterised by X-ray crystallography, cyclic voltammetry, and by NMR and electronic absorption and luminescence spectroscopy. The ligand X hardly influences the electronic transitions, which indicates that the relevant molecular orbitals are largely confined to the Pt-Tx chromophore. In agreement with TD-DFT calculations the energetically lowest electronic transition is assigned as the Tx-centred π→π* HOMO → LUMO excitation. All four complexes display dual emission from the σ-bonded Tx ligand at ca. 450 nm and at ca. 510 nm, which are assigned as fluorescence originating from the (1)π*-state and as phosphorescence originating from the (3)π*-state, respectively. The phosphorescence quantum yield increases with increasing σ-donor strength of the ligand X and reaches a uniquely high value of 18.8% for the chlorido complex Pt-Cl. Switching-on of Tx phosphorescence emission by the Pt(PEt3)2(X) fragment goes along with a reduction of the lifetime of the Tx triplet state from several ms in purely organic derivatives to ca. 2 μs in the complexes.