Ligand Structure, Conformational Dynamics, and Excited-State Electron Delocalization for Control of Photoinduced Electron Transfer Rates in Synthetic Donor-Bridge-Acceptor Systems

Synthesis, ground-, and excited-state properties are reported for two new electron donor-bridge-acceptor (D-B-A) molecules and two new photophysical model complexes. The D-B-A molecules are [Ru(bpy)2(bpy-ϕ-MV)](PF6)4 (3) and [Ru(tmb)2(bpy-ϕ-MV)](PF6)4 (4), where bpy is 2,2′-bipyridine, tmb is 4,4′,5,5′-tetramethyl-2,2′-bipyridine, MV is methyl viologen, and ϕ is a phenylene spacer. Their model complexes are [Ru(bpy)2(p-tol-bpy)](PF6)2 (1) and [Ru(tmb)2(p-tol-bpy)](PF6)2 (2), where p-tolyl-bpy is 4-(p-tolyl)-2,2′-bipyridine. Photophysical characterization of 1 and 2 indicates that 2.17 eV and 2.12 eV are stored in their respective 3MLCT (metal-to-ligand charge transfer) excited state. These values along with electrochemical measurements show that photoinduced electron transfer (D*-B-A → D+-B-A−) is favorable in 3 and 4 with ΔG°ET = −0.52 eV and −0.62 eV, respectively. The driving force for the reverse process (D+-B-A− → D-B-A) is also reported: ΔG°BET = −1.7 eV for 3 and −1.5 eV for 4. Transient absorption (TA) spectra for 3 and 4 in 298 K acetonitrile provide evidence that reduced methyl viologen is observable at 50 ps following excitation. Detailed TA kinetics confirm this, and the data are fit to a model to determine both forward (kET) and back (kBET) electron transfer rate constants: kET = 2.6 × 1010 s−1 for 3 and 2.8 × 1010 s−1 for 4; kBET = 0.62 × 1010 s−1 for 3 and 1.37 × 1010 s−1 for 4. The similar rate constants kET for 3 and 4 despite a 100 meV driving force (ΔG°ET) increase suggests that forward electron transfer in these molecules in room temperature acetonitrile is nearly barrierless as predicted by the Marcus theory. The reduction in electron transfer reorganization energy necessary for this barrierless reactivity is attributed to excited-state electron delocalization in the 3MLCT excited states of 3 and 4, an effect that is made possible by excited-state conformational changes in the aryl-substituted ligands of these complexes.