Calculation of Electron Transfer Rate Constants from Emission Spectra in Re(I) Chromophore−Quencher Complexes

The photophysical properties of the chromophore−quencher complexes, fac-[(4,4‘-R2bpy)ReI(CO)3(LA)]n+ (4,4‘-R2bpy = 4,4‘-R2-2,2‘-bipyridine, R = Me or tBu, and LA = the quinone acceptor ligands, benz[g]isoquinoline-5,10-dione (BIQD), 2-oxy-1,4-naphthoquinone anion (ONQ-), 1/2 2,6-dihydroxyanthraquinone dianion (AFA2-), or the pyridinium acceptor, 1-methyl-6-oxyquinoline (OQD)) in 1,2-dichloroethane are described. Following ReI → 4,4‘-R2bpy metal-to-ligand charge transfer (MLCT) excitation, intramolecular electron transfer leads to the transients, fac-[(4,4‘-R2bpy)ReII(CO)3(LA•-)]n+. They have been characterized by emission spectral fitting and transient absorption measurements and, for LA = OQD, by transient infrared measurements. As shown by analysis of excited-state emission, there is weak-to-moderate electronic coupling between the electron donor and acceptor sites in the transients with HDA varying from 153 cm-1 for the BIQD complex to 3.9 cm-1 for the OQD complex. The transients are redox-separated (RS) states with the electronic configurations dπ5πLA*1 for BIQD or σ(Re−O)1πLA*1 for ONQ-, AFA2-, and OQD. They are weak emitters and return to the ground state largely by nonradiative decay which occurs by back electron transfer (kET). Reasonable agreement has been reached between kET and values calculated from kinetic parameters derived by emission spectral fitting and excited-state decay. The RS states for the AFA2- and OQD complexes are remarkably long-lived (τ = 4 μs for LA = AFA2- in DCE at 296 K and 16 μs for LA = OQD in DCE at 296 K) due to orbital and spin restrictions on back electron transfer.