Depending on the ligands L and N,N (=polypyridine, ␣-diimine), the complexes [Re(L)(CO) 3 (N,N)] n+ undergo different photophysical and photochemical processes. Herein, we compare the behavior of complexes with an electron-accepting ligand L = N-methyl-4,4 -bipyridinium (MQ + ), isomerizable ligand L = trans-4-styrylpyridine (t-stpy), and L = trans-N-methyl-4,4 -dipyridiniumethylene (Medpe + ), which can undergo either reduction, or isomerization, or both. In particular, we discuss the dynamics and mechanisms of (i) NN •-→ MQ + interligand electron transfer (ILET) from a 3 MLCT state of [Re I (MQ + )(CO) 3 (dmb)] 2+ (dmb = 4,4 -dimethyl-2,2 -bipyridine), (ii) intramolecular energy transfer from a 3 MLCT state of the Re(CO) 3 (bpy) unit of [Re(t-stpy)(CO) 3 (bpy)] + to the intraligand 3 IL * state of the axial ligand t-stpy and its subsequent isomerization, and (iii) competition between charge and energy transfer/isomerization in [Re I (Medpe + )(CO) 3 (dmb)] 2+ .
Optical excitation of [Re I (MQ + )(CO) 3 (dmb)] 2+ populates a Re → dmb 3 MLCT excited state [Re II (MQ + )(CO) 3 (dmb •-)] 2+ . Ultrafast dmb •-→ MQ + ILET follows, producing a Re → MQ + MLCT excited state [Re II (MQ • )(CO) 3 (dmb)] 2+ . The ILET rate (8-18 ps, depending on the solvent) is much faster than predicted by conventional electron-transfer theories, being accelerated by a combination of large (ca. 130 cm -1 ) electronic coupling through the Re II atom and vibrational excitation of the 3 MLCT(dmb) precursor state. Irradiation of [Re I (t-stpy)(CO) 3 (bpy)] + populates a Re → bpy 3 MLCT excited state, which converts into a 3 IL state of the t-stpy ligand with a 3.5 ps time constant. This 3 MLCT → 3 IL conversion amounts to an intramolecular energy transfer from the electronically excited chromophore Re(CO) 3 (bpy) + to the t-stpy ligand. The t-stpy ligand in the 3 IL state undergoes a 12 ps twist around the C C bond to the perpendicular geometry, followed by further 18 ns twist to the ground state and the cis isomer. The same isomerization mechanism operates for [Re I (Cl)(CO) 3 (t-stpy) 2 ] + whose 3 IL state is populated directly. Excitation of [Re I (Medpe + )(CO) 3 (bpy)] 2+ leads to both Re → bpy and Re → Medpe + MLCT states. A ps energy transfer to a 3 IL state follows, triggering rotation around the C C bond. In contrast, the electron-acceptor character of the Medpe + ligand prevails in [Re I (Cl)(CO) 3 (Medpe + ) 2 ] 2+ , where only a Re → Medpe + 3 MLCT state is populated, which decays to the ground state with ca. 100 ps lifetime by a Medpe • → Re II electron transfer.