Computational exploration of heterolytic halogen-carbon bond scission photoreactions in ruthenium polypyridyl...

Energy wasting charge recombination is an efficiency limiting process in efforts to achieve solar energy storage. Here, density functional theory is used to explore the thermodynamics of photochemical energy storage reactions in several ruthenium polypyridyl complexes where heterolytic halogen-carbon bond scission occurs after light-induced formation of the triplet metal to ligand charge transfer ((3)MLCT) state, as seen in the following reaction: [Ru(II)(A)(n)(L-X)](2+) + hν → [Ru(III)(A)(n)(L-X)(•-)](2+)* → [Ru(III)(A)(n)(L·)](3+) + X(-) (L = polypyridine ligand; X = Cl, Br, and I; A = ancillary ligand). A thermochemical cycle is employed to determine structural and electronic factors influencing ΔE(rxn). Significant energetic penalties in the oxidation of the metal center are mitigated through methylation of ancillary ligands or introduction of amine ancillary ligands. Methylation of the halogenated ligand maintains energy stored in the (3)MLCT state. Reduction in ΔE(rxn) is obtained by exploiting strain in the coordination geometry or in sterically encumbered ligands that is released upon bond breaking. Formation of a contact ion pair is significantly more favorable than complete separation of charged products, and shows negative ΔE with respect to the (3)MLCT state in certain cases. Future tunability in stored energy may be achieved through careful manipulation of ligand structure and charge on ancillary ligands.