Dual-emitting Mn(II) and Zn(II) halide complexes with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide as ligand

The H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) acts as O-donor ligand towards manganese(II) and zinc(II) halides, with the formation of tetrahedral complexes having general formula [MX2(DOPO)2] (M = Mn, X = Cl, Br, I; M = Zn, X = Br). The structure of [ZnBr2(DOPO)2] was ascertained by single-crystal X-ray diffraction. All the compounds showed appreciable photoluminescence at the solid state, with an emission band in the red region of the spectrum attributed to radiative decay from triplet states of the coordinated DOPO ligands. Such an emission is superimposed to the 4T1(4G)→6A1(6S) transition around 530–540 nm in the case of the Mn(II) derivatives and to a fluorescent band centred at 380 nm for [ZnBr2(DOPO)2]. Excitation wavelength-dependent emission was observed for [MnCl2(DOPO)2] and [MnBr2(DOPO)2]. The absorption and emission features of the coordinated DOPO ligands were rationalized by means of TD-DFT calculations.