Ultrafast excited-state dynamics preceding a ligand trans-cis isomerization of fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and fac-[Re(t-4-styrylpyridine)(CO)3(2,2′bipyridine)]+

UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)(3)(stpy)(2)] and fac-[Re(stpy)(CO)(3)(bpy)(+) (stpy = t-4-styrylpyridine, bpy = 2,2'-bipyridine) show that their lowest absorption bands are dominated by stpy-localized intraligand (IL) pi pi* transitions. For the latter complex a Re -> bpy transition contributes to the low-energy part of the absorption band. Optical population of the (IL)-I-1 excited state of fac-[Re(Cl)(CO)(3)(stpy)(2)] is followed by an intersystem crossing (<= 0.9 ps) to an (IL)-I-3 state with the original planar trans geometry of the stpy ligand. This state undergoes a similar to 90 degrees rotation around the stpy C=C bond with a 11 ps time constant. An electronically excited species with an approximately perpendicular orientation of the phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)(3)(bpy)](+) show the same behavior. Moreover, it was found that the planar reactive (IL)-I-3 excited state is rapidly and efficiently populated after optical excitation into the Re -> bpy (MLCT)-M-1 excited state. A (MLCT)-M-1 -> (MLCT)-M-3 intersystem crossing takes place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the Re-1(CO)(3)-(bpy) chromophore to a stpy-localized 3 IL state with a 3.5 ps time constant. The fast rate ensures complete conversion. Coordination of the stpy ligand to the Re-1 center thus switches the ligand trans-cis isomerization mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)(3)(bpy)](+), opens an indirect pathway for population of the reactive (IL)-I-3 excited state via MLCT states.