Ultrafast ligand-to-ligand electron and energy transfer in the complexes fac-[ReI(L)(CO)3(bpy)]n+

Depending on the ligands L and N,N (=polypyridine, alpha-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(+)), isonterizable 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 center dot- -> MQ(+) interligand electron transfer (ILET) from a (MLCT)-M-3 state of [Re-I(MQ(+))(CO)(3)(dmb)](2+) (dmb = 4,4'-dimethyl-2,2'-bipyridine), (ii) intramolecular energy transfer from a (MLCT)-M-3 state of the Re(CO)(3)(bpy) unit of [Re(t-stpy)(CO)(3)(bpy)](+) to the intraligand (IL)-I-3 pi pi* 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 (MLCT)-M-3 excited state [Re-II(MQ(center dot))(CO)(3)(dmb)](2+). Ultrafast dmb(center dot-) -> MQ(+) ILET follows, producing a Re -> MQ(+) MLCT excited state [Re-II(MQ(center dot))(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 (MLCT)-M-3 excited state, which converts into a (IL)-I-3 state of the t-stpy ligand with a 3.5 ps time constant. This (MLCT)-M-3 -> (IL)-I-3 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 (IL)-I-3 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 (IL)-I-3 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 (IL)-I-3 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(+) (MLCT)-M-3 state is populated, which decays to the ground state with ca. 100 ps lifetime by a Medpe(center dot) -> Re-II electron transfer. (c) 2006 Elsevier B.V. All rights reserved.