Ultrafast ligand-to-ligand electron and energy transfer in the complexes fac-[Re~Ⅰ(L)(CO)_3(bpy)]~(n+)

摘要

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 [Re~Ⅰ(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 [Re~Ⅰ(Medpe~+)(CO)_3(dmb)]~(2+). Optical excitation of [Re~Ⅰ(MQ~+)(CO)_3(dmb)]~(2+) populates a Re → dmb ~3MLCT 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 ~3MLCT(dmb) precursor state. Irradiation of [Re~Ⅰ(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 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 ~3IL 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 ~3IL 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~+ ~3MLCT state is populated, which decays to the ground state with ca. 100 ps lifetime by a Medpe~· → Re~Ⅱ electron transfer.