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

Abstract

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.