| Abstract
| - The lowest absorption band of fac-[Re(Cl)(CO)3(5-NO2-phen)] encompasses two close-lying MLCT transitions.The lower one is directed to LUMO, which is heavily localized on the NO2 group. The UV−vis absorptionspectrum is well accounted for by TD-DFT (G03/PBEPBE1/CPCM), provided that the solvent, MeCN, isincluded in the calculations. Near-UV excitation of fac-[Re(Cl)(CO)3(5-NO2-phen)] populates a triplet metalto ligand charge-transfer excited state, 3MLCT, that was characterized by picosecond time-resolved IRspectroscopy. Large positive shifts of the ν(CO) bands upon excitation (+70 cm-1 for the A‘(1) band) signifya very large charge separation between the Re(Cl)(CO)3 unit and the 5-NO2-phen ligand. Details of the excited-state character are revealed by TD-DFT calculated changes of electron density distribution. Experimentalexcited-state ν(CO) wavenumbers agree well with those calculated by DFT. The 3MLCT state decays with aca. 10 ps lifetime (in MeCN) into another transient species, that was identified by TRIR and TD-DFTcalculations as an intraligand 3nπ* excited state, whereby the electron density is excited from the NO2 oxygenlone pairs to the π* system of 5-NO2-phen. This state is short-lived, decaying to the ground state with a ∼30ps lifetime. The presence of an nπ* state seems to be the main factor responsible for the lack of emission andthe very short lifetimes of 3MLCT states seen in all d6-metal complexes of nitro-polypyridyl ligands.Localization of the excited electron density in the lowest 3MLCT states parallels localization of the extraelectron in the reduced state that is characterized by a very small negative shift of the ν(CO) IR bands (−6cm-1 for A‘(1)) but a large downward shift of the νs(NO2) IR band. The Re−Cl bond is unusually stabletoward reduction, whereas the Cl ligand is readily substituted upon oxidation.
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