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Fluorescence, phosphorescence, and the triplet spin sublevel properties (zero-field splitting, total decay rate constants, and relative radiative decay rate constants for individual sublevels) are investigated for a series of complexes of the type MX 2 (phen), where M is Zn(II) or Cd(II), and X is Cl, Br, or I. The fluorescence and the phosphorescence are almost identical to those of free phen indicating that the emission is from the phen localized ππ * state. The rate constants for spin forbidden transitions increase as the atomic number of halogen increases. All the kinetic data as well as the zero-field splitting parameters are satisfactorily interpreted by a configurational mixing between the phen localized ππ * state and the halogen p to phen π * ligand-ligand charge-transfer state. Zero-field splitting and radiative properties of the triplet spin sublevels of the “ligand localized” state of the complexes of closed shell metal ions such as Zn(II) and Cd(II) are all interpreted satisfactorily in terms of the configurational mixing between phen localized ππ * state and the halogen p to phen π * ligand-ligand charge-transfer state. Metal d orbitals do not participate in determining excited state properties. Apparent difference experimentally observed is traced back to the difference of the electronegativity of Zn(II) and Cd(II).

The photophysical properties (the absorption and the emission spectra and the fluorescence decay profiles in the picosecond region) of the lowest singlet excited state for ZnX2(phen) (X = Cl, Br, I; phen = 1,10-phenanthroline monohydrate) were investigated. The experimental findings are as follows: (i) The band structures of the absorption and fluorescence spectra of ZnX2(phen) are essentially the same as those of phen. (ii) The S0 → S1 absorption and the fluorescence spectra of ZnX2(phen) are red-shifted from those of phen. (iii) The phosphorescence/fluorescence intensity ratios vary in the order ZnCl2(phen) phen > ZnBr2(phen) > ZnI2(phen). These results are systematically and satisfactorily interpreted by a model invoking Heitler−London type mixing of LLCT (halogen p to phen π* charge transfer) with phen locally excited electronic configuration.