Green-emitting Zn(II) halide complexes with N,N,N',N'-tetramethyl-P-indol-1-ylphosphonic diamide as ligand

Zn(II) derivatives are of current interest for photophysical applications because they can enhance the emission of fluorescent ligands upon coordination. In addition, LMCT transitions involving the low-lying s or p empty orbitals of the metal center have been reported [1,2]. Herein we report the synthesis and characterization of tetrahedral Zn(II) complexes having general formula [ZnX2L2] (X = Cl, Br, I; L = N,N,N',N'-tetramethyl-P-indol-1-ylphosphonic diamide). The species were isolated from the reaction between the proper anhydrous ZnX2 salt and L under mild conditions. The structure of all the three derivatives was ascertained by single-crystal X-ray diffraction. The three species revealed to be appreciably luminescent in the green region upon excitation with UV-light below 300 nm, with emission bands centered between 520 and 530 nm. The high Stokes shifts and the luminescence lifetimes in the μs range suggest that triplet excited states are involved in the emission. On the other hand, the absorption bands are essentially ascribable to the π*←π transition of the indolyl fragment, as confirmed by DFT calculations.