Spectroscopic Study on Fluorescing Processes of (nd)10 Transition-Metal Complexes. 1. ZnX2(phen) (X = Cl, Br, I; phen = 1,10-Phenanthroline Monohydrate)

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). (iv) The fluorescence lifetimes 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.