Portal de Periódicos da CAPES

Excited States of Nitro-Polypyridine Metal Complexes and Their Ultrafast Decay. Time-Resolved IR Absorption, Spectroelectrochemistry, and TD-DFT Calculations of f ac- [Re(Cl)(CO) 3 (5-Nitro-1,10-phenanthroline)]

2005; American Chemical Society; Volume: 109; Issue: 28 Linguagem: Inglês

10.1021/jp051677h

1520-5215

Anders Gabrielsson, Pavel Matousek, Michael Towrie, František Hartl, Stanislav Záliš, Antonı́n Vlček,

Lanthanide and Transition Metal Complexes

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 absorption spectrum is well accounted for by TD-DFT (G03/PBEPBE1/CPCM), provided that the solvent, MeCN, is included in the calculations. Near-UV excitation of fac-[Re(Cl)(CO)3(5-NO2-phen)] populates a triplet metal to ligand charge-transfer excited state, 3MLCT, that was characterized by picosecond time-resolved IR spectroscopy. Large positive shifts of the ν(CO) bands upon excitation (+70 cm-1 for the A'(1) band) signify a 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. Experimental excited-state ν(CO) wavenumbers agree well with those calculated by DFT. The 3MLCT state decays with a ca. 10 ps lifetime (in MeCN) into another transient species, that was identified by TRIR and TD-DFT calculations as an intraligand 3nπ* excited state, whereby the electron density is excited from the NO2 oxygen lone pairs to the π* system of 5-NO2-phen. This state is short-lived, decaying to the ground state with a ∼30 ps lifetime. The presence of an nπ* state seems to be the main factor responsible for the lack of emission and the 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 extra electron in the reduced state that is characterized by a very small negative shift of the ν(CO) IR bands (−6 cm-1 for A'(1)) but a large downward shift of the νs(NO2) IR band. The Re−Cl bond is unusually stable toward reduction, whereas the Cl ligand is readily substituted upon oxidation.