Controlling electron transfer through the manipulation of structure and ligand-based torsional motions: a computational exploration of ruthenium donor-acceptor systems using density functional theory.

Computational studies using density functional theory (DFT) are reported for a series of donor-acceptor (DA) transition metal complexes and related excited-state and electron transfer (ET) photoproduct models. Three hybrid Hartree-Fock/DFT (HF/DFT) functionals, B3LYP, B3PW91, and PBE1PBE, are employed to characterize structural features implicated in the dynamical control of productive forward and energy wasting back ET events. Energies and optimized geometries are reported for the lowest energy singlet state in [Ru(dmb)(2)(bpy-phi-MV)](4+) (DA1), [Ru(dmb)(2)(bpy-o-tolyl-MV)](4+) (DA2), [Ru(dmb)(2)(bpy-2,6-Me(2)-phi-MV)](4+) (DA3), and [Ru(tmb)(2)(bpy-2,6-Me(2)-phi-MV)](4+) (DA3'), where dmb is 4,4'-dimethyl-2,2'-bipyridine, tmb is 4,4',5,5'-tetramethyl-2,2'-bipyridine, MV is methyl viologen, and phi is a phenylene spacer. These indicate that the dihedral angle theta(1) between the aryl substituent and the bipyridine fragment to which it is bound, systematically increases with the addition of steric bulk. Energies, optimized geometries, and unpaired electron spin densities are also reported for the lowest energy triplet state of [Ru(dmb)(2)(4-p-tolyl-2,2'-bipyridine)](2+) (D1*), [Ru(dmb)(2)(4-(2,6-dimethylphenyl)-2,2'-bipyridine)](2+) (D2*), [Ru(dmb)(2)(4-mesityl-2,2'-bipyridine)](2+) (D3*), and [Ru(tmb)(2)(4-mesityl-2,2'-bipyridine)](2+) (D3'*). Each of these serves as a model of a reactant excited state in the forward electron-transfer photochemistry allowing us to qualify and quantify the role of excited-state intraligand electron delocalization in driving substantial geometry changes (especially with respect to theta(1)) relative to its respective DA counterpart. Next, energies, optimized geometries, and spin densities are reported for the lowest energy triplet of each DA species: (3)DA1, (3)DA2, (3)DA3, and (3)DA3'. These are used to model the ET photoproduct and they indicate that theta(1) increases following ET, thus, verifying switch-like properties. Finally, we report data for geometry optimized DA1 and (3)DA1 in a continuum model of room temperature acetonitrile. This study shows a complete recovery of theta(1) to its ground state value which has implications in efforts to trap electrons in charge-separated states.