Spectroscopic study on fluorescing processes of (nd)(10) transition-metal complexes .1. ZnX(2)(phen) (X=Cl, Br, I; phen=1,1O-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 ZnX(2)(phen) (X = Cl, Br, I; phen phenanthroline monohydrate) were investigated. The experimental findings are as follows: (i) The band structures of the absorption and fluorescence spectra of ZnX(2)(phen) are essentially the same as those of phen. (ii) The S-0 --> S-1 absorption and the fluorescence spectra of ZnX(2)(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 pi* charge transfer) with phen locally excited electronic configuration.