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

Abstract

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.