Computational Exploration of Heterolytic Halogen−Carbon Bond Scission Photoreactions in Ruthenium Polypyridyl Complexes

Abstract

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.

Experimental and Computational Exploration of Ground and Excited State Properties of Highly Strained Ruthenium Terpyridine Complexes

“…It represents the result of electron transfer of a photoexcited electron residing in a ligand π* orbital to a bromine σ* orbital concomitant with carbon–bromine homolytic bond cleavage resulting in a bromide anion Coulombically bound to a triply charged ruthenium-complex cation (with a carbon-based radical). Our previous report calculated that formation of the CIP from the 3 MLCT state in 2 was exoergic by ∼0.17 eV, suggesting that the CIP may be energetically accessible in 3 as well . In support of this assignment, it is noted that a π* ← π* transition of a ligand-based radical anion, which we are probing at 375 nm, would be particularly sensitive to CIP formation (resulting, as we have observed, in a fast decay of the magnitude of the π* ← π* absorption signal as the photochemistry proceeds and the radical anion on the ligand is lost).…”

“…This bond length is calculated to be only 0.3% larger than the crystallographic value. The calculated Ru–N2 bond lengths are overestimated relative to the crystal structure by only 0.9%, which represents a significant improvement relative to the typical 2–3% overestimation of Ru–N distances that result from B3LYP calculations of similar complexes. ,, …”

“…One way to introduce strain in metal–ligand complexes is via the attachment of bulky functional groups such as bromine substituents in the ligands themselves and in particular at the 6 and 6″ positions of a 2,2′:6′,2″-terpyridine (tpy). Previous density functional theory (DFT) results by our group as well as those discussed herein indicate that, when such ligands are incorporated in bis-terpyridyl complexes of Ru(II), the bromine substituents are in van der Waals contact with the opposing terpyridine ligand, causing interligand strain. The release of strain upon carbon–halogen bond scission causes the calculated bond dissociation energy (BDE) to be reduced by 0.45 eV as compared to a related unstrained system …”

“…Energy-wasting charge recombination is a limiting factor in the use of chromophoric materials in photochemical or photoredox transformations such as those needed in solar energy conversion schemes. − Many photoactive systems ranging from natural photosynthetic constructs to artificial material assemblies within organic photovoltaics or within dye-sensitized solar cells expend significant redox potential as electron transfer (ET) driving force in order to rapidly separate charge carriers by large distances to avoid recombination. − However, this necessarily reduces the efficiency of such systems. , Our research has previously focused on utilizing ligand structure and nuclear motions to control the photophysics of materials with the goal of delaying the charge recombination process after ET without relying exclusively on expending driving force. − Recently we have explored a different approach and computationally considered ruthenium polypyridyl systems designed to store energy via photoinduced dissociative ET (DET) inclusive of scission of a carbon–halogen bond . While photoinduced DET in small organic systems has been readily observed, − we determined that in typical Ru(II) polypyridyl systems containing aryl-halogen bonds within the ligand framework, the presence of the charged metal center significantly modifies the reaction thermodynamics such that the desired DET product is higher in energy than the metal-to-ligand charge-transfer ( 3 MLCT) excited state from which the reaction would originate.…”

Probing the photophysical capability of mono and bis(cyclometallated) Fe(ii) polypyridine complexes using inexpensive ground state DFT

“…In conjunction with an empirical dispersion correction, 18 the PBE0 functional 19 has been shown to give excellent geometrical parameters for Ru(II) [20][21][22][23][24][25][26] and Fe(II) 27,28 polypyridine complexes. Overall the replacement of a pyridine ring by a cyclometallating ring induces minor geometrical changes.…”