Green-emitting tetrahedral Mn(II) complexes with [O=P]-donor ligands

Tetrahedral Mn(II) complexes are characterized by green luminescence related to the 4T1(4G)→6A1(6S) transition.[1] These derivatives can be of potential interest for lighting applications, for instance as replacement of expensive rare earths-based phosphors in fluorescent lamps.[2] On the basis of recent outcomes on dihalide derivatives with phosphine oxides, phosphoramides, arylphosphonic diamides, amidophosphates and phosphonates were used as ligands L for the preparation of tetrahedral Mn(II) complexes having general formula [MnX2L2] (X = Cl, Br, I).[3] The complexes were isolated from the reaction between the proper anhydrous MnX2 salt and L under mild conditions. The structure of the derivatives was ascertained by single-crystal X-ray diffraction. The species revealed to be appreciably luminescent in the green region upon excitation with UV-light, with emission maxima centered between 510 and 535 nm. The luminescence lifetimes are in the hundreds of μs range, strongly influenced by the choice of the coordinated halide. In general, the lifetime decreases passing from the chloro- to the bromo- and the iodo- derivative as a consequence of the increased spin-orbit coupling (SOC) effect. In the case of N,N,N’,N’-tetramethyl-P-naphtalen-2-ylphosphonic diamide the first tetrahedral complexes exhibiting wavelength-dependent emission were isolated, characterized by two bands, one centred in the green and the other one in the red region of the spectrum. The latter is probably related to a ligand-centred tripled excited state (3LC), as further confirmed by computational calculations.[4] In most of the cases, the emission from Mn(II) is related, besides the direct excitation of the metal center, to the energy transfer from the excited states of the coordinated ligands. The absorption bands are essentially ascribable to the π*←π transition of the aromatic moiety, as further confirmed by DFT calculations. The presence of a rigid structure and a light harvesting substituent such as the indolyl or the carbazolyl fragment in the skeleton of the [O=P]-donor ligands revealed to play a key aspect for the sensitization of Mn(II) luminescence.