A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

Abstract

Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states.

Fig. 1. Molecular orbital diagram of an octahedral transition metal complex with π donating ligands.

Fig. 2. Examples of accessible excited states from d⁰, d⁵, and d⁷ complexes, which most commonly exhibit LMCT. In addition, it is important to note the bonding character of the metal-based t_2g orbitals depends on the nature of the coordinating ligands. Strong π donors facilitate antibonding t_2g orbitals, σ donors promote non-bonding t_2g orbitals, and π acceptors facilitate bonding t_2g orbitals.

Fig. 3. VLIH mechanism of ferrioxalate to form ferrous oxalate. Further decomposition of the C₂O₄˙⁻ with a second equivalence of ferric oxalate yields an additional equivalence of ferrous oxalate and two equivalents of CO₂.

Fig. 4. Irradiation of Ni^IIIX₃(dppe), shown here, and other Ni^IIIX₃PP complexes results in VLIH of the Ni–X bond, where the resulting halogen radical is stabilized by arene substituents on the diphosphine ligand.

Fig. 5. Structures of three classes of compounds known to facilitate ES-ET reactions (where M = Re or Tc). A summary of their role as photooxidants and/or photoreductants are included, alongside their respective excited state lifetimes.

Fig. 6. Excited state protonation of [Mo₂X₈]⁴⁻ yields [Mo₂(μ-H)(μ-X)₂(X)₆]³⁻.

Fig. 7. Irradiation of Cp₂MR₂ complexes (R = Ar) results in photo-induced reductive elimination to form biphenyl derivatives.

Fig. 8. Irradiation of arylalkynyl titanocene complexes results in photo-induced reductive elimination to form an enyne product.

Fig. 9. Summary of reactions for (A) [(η⁵-C₅Me₄H)₂Zr(Tol)₂] and (B) (^MePMP^Me)₂ZrBn₂.

Ann Marie May

Jillian L. Dempsey