Temperature- and excitation wavelength-dependent emission in a manganese(II) complex

A mononuclear manganese(II) complex with a chelating 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine ligand (L), [MnL2Cl2]center dot H2O, shows intriguing excitation wavelength-dependent emission. Depending on the excitation wavelength, the complex demonstrates three emission bands with the maxima at 380 nm, 440 nm and 495 nm. The 380 nm and 440 nm emissions originate from the pi ->pi* and n ->pi* ligand-centered transitions. The long-wave 495 nm emission with microsecond lifetimes is related to the d-d transitions and/or metal-to-ligand and halogen-to-ligand charge transfer. The emission behavior of this complex is strongly temperature-dependent: upon cooling from 300K down to 77K, the intensity of emission considerably increases. The enhancement of the luminescence upon cooling is accompanied by the appearance of the vibrational structure. This complex is the first example of manganese(II) complexes demonstrating excitation wavelength-dependent emission.