Character and picosecond dynamics of the excited states of [Re(Br)(CO)(3)(alpha-diimine)] complexes as a function of the alpha-diimine ligand: 4,4'-di-methyl-2,2'-bipyridine, N,N-di-p-tolyl-1,4-diaza-butadiene and N,N-di-p-anisyl-1,4-diaza-butadiene

Excited state behavior of the [Re(Br)(CO)(3)(alpha-diimine)] complexes, alpha-diimine=4,4'-di-methyl-2,2'-bipyridine (bpy'),N,N-di-p-tolyl-1,4-diaza-butadiene (p-Tol-DAB) and N,N-di-p-anisyl-1,4-diaza-butadiene (p-An-DAB), was investigated by means of time resolved visible absorption spectroscopy studied over the time range 50 ps-205 ns following the laser pulse excitation at either 355 or 532 nm. Transient spectra obtained were compared with the absorption spectra of the [Re(Br) (CO)(3)(alpha-diimine)](-) anions produced by a (spectro)electrochemical reduction. The lowest excited state of [Re(Br)(CO)(3)(bpy')] was identified as a relatively long-lived (about 60 ns), strongly emissive Re --> bpy' (3)MLCT state. Both [Re(Br)(CO)(3)(DAB)] complexes have a very short-lived (150 ps for p-Tol-DAB and 250 ps for p-An-DAB) lowest excited state of (partly) Br- --> DAB ligand-to-ligand charge transfer character that is populated by the 532 nm excitation. High energy excitation at 355 nm populates both the MLCT and pi --> pi* intraligand excited states of [Re(Br)(CO)(3)(DAB)].