Photoinitiated multi-step charge separation and ultrafast charge transfer induced dissociation in a pyridyl-linked photosensitizer-cobaloxime assembly

Using visible and near-infrared transient absorption spectroscopy to track distinct excited state, cation, and anion signals, we report a detailed kinetic analysis of photoinitiated multi-step charge separation and ultrafast charge transfer induced dissociation in a self-assembled donor-bridge-acceptor- cobaloxime triad. The donor-bridge-acceptor ligand consists of a perylene chromophore linked via a xylene bridge to a pyridyl-substituted 1,8-naphthalimide electron acceptor. Coordination of the ligand to the catalyst [Co(dmgBF₂)₂(L)₂], where dmgBF₂ = (difluoroboryl)dimethylglyoximato and L = water or a solvent molecule, yields a donor-bridge-acceptor-catalyst triad assembly. Photoexcitation with 416 nm laser pulses generates the perylene S₁ excited state. Subsequent electron transfer from perylene to the acceptor occurs in τ = 9.0 ± 0.1 ps followed by electron transfer to the catalyst in τ = 6 ± 1 ps. Of the charge-separated state population formed, 79 ± 1% undergoes charge recombination to either the singlet ground state (τ = 0.8 ± 0.1 ns) or the perylene triplet state (τ = 4.3 ± 0.1 ns). Co(i)-pyridyl bond dissociation with τ = 2.4 ± 0.2 ns competes with intramolecular charge recombination resulting in a 21 ± 1% yield of dissociated oxidized photosensitizer and reduced catalyst. Subsequent diffusional charge recombination occurs with k = (1.8 ± 0.2) × 10¹⁰ M ^-1 s^-1. This detailed analysis of the electron transfer and dissociation dynamics of an integrated photosensitizer-catalyst system will inform the rational design of novel molecular assemblies that efficiently absorb photons, transfer electrons, and catalyze fuel-forming reactions.