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, α-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 3MLCT state of [ReI(MQ+)(CO)3(dmb)]2+ (dmb = 4,4′-dimethyl-2,2′-bipyridine), (ii) intramolecular energy transfer from a 3MLCT state of the Re(CO)3(bpy) unit of [Re(t-stpy)(CO)3(bpy)]+ to the intraligand 3IL ππ* state of the axial ligand t-stpy and its subsequent isomerization, and (iii) competition between charge and energy transfer/isomerization in [ReI(Medpe+)(CO)3(dmb)]2+.Optical excitation of [ReI(MQ+)(CO)3(dmb)]2+ populates a Re → dmb 3MLCT excited state [ReII(MQ+)(CO)3(dmb−)]2+. Ultrafast dmb− → MQ+ILET follows, producing a Re → MQ+ MLCT excited state [ReII(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 ReII atom and vibrational excitation of the 3MLCT(dmb) precursor state. Irradiation of [ReI(t-stpy)(CO)3(bpy)]+ populates a Re → bpy 3MLCT excited state, which converts into a 3IL state of the t-stpy ligand with a 3.5 ps time constant. This 3MLCT → 3IL 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 3IL state undergoes a 12 ps twist around the CC 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 [ReI(Cl)(CO)3(t-stpy)2]+ whose 3IL state is populated directly. Excitation of [ReI(Medpe+)(CO)3(bpy)]2+ leads to both Re → bpy and Re → Medpe+ MLCT states. A ps energy transfer to a 3IL state follows, triggering rotation around the CC bond. In contrast, the electron-acceptor character of the Medpe+ ligand prevails in [ReI(Cl)(CO)3(Medpe+)2]2+, where only a Re → Medpe+3MLCT state is populated, which decays to the ground state with ca. 100 ps lifetime by a Medpe → ReII electron transfer.