Halide-Dependent Change of the Lowest-Excited-State Character from MLCT to XLCT for the Complexes Re(X)(CO)3(α-diimine) (X = Cl, Br, I; α-diimine = bpy, iPr-PyCa, iPr-DAB) Studied by Resonance Raman, Time-Resolved Absorption, and Emission Spectroscopy

Complexes of the type Re(X)(CO)3(α-diimine) (X = Cl, Br, I; α-diimine = bpy, iPr-PyCa, iPr-DAB) exhibit a significant influence of X on the energies and intensities of their lowest-energy electronic transitions. Resonance Raman experiments revealed a change in character of the lowest energy transitions of these complexes from Re → α-diimine (MLCT) to X → α-diimine (XLCT) upon going from Cl to Br. This halide influence can be explained by different extents of mixing of the dπ(Re) and pπ(X) orbitals. All complexes under study are emissive at 80 K in a glass; the bpy complexes are also emissive at room temperature in fluid solution. The emission from the XLCT excited state is characterized by a longer lifetime, due to smaller knr and kr values, than the MLCT emission. Nanosecond time-resolved absorption spectroscopy of Re(X)(CO)3(bpy) revealed that the halide determines the excited state character of the complexes also in fluid solution. The transient absorption maximum shifts to lower energy going from Cl to Br to I, and the excited state lifetime increases from 50 to 57 to 79 ns, respectively. Variation of the α-diimine has less influence on the properties of the XLCT state than on those of the MLCT state.