Spin Crossover-Mediated Low-Energy Charge Transfer Excited States in a Heterogeneous Cobalt Photocatalyst

In molecular complexes, ligand-to-metal charge transfer (LMCT) excited states enable efficient photoinduced charge separation and strong redox reactivity, but their operation is typically limited to short ultraviolet excitation. Extending LMCT absorption into the visible region utilizing strongly donating ligands often leads to ligand dissociation and further decomposition, which highlights the intrinsic trade-off between spectral response and structural robustness in homogeneous systems. In this context, we reconfigure LMCT chemistry within heterogeneous single-atom catalysts, where rigid coordination environments that decouple electronic excitation from (photo)stability. Low-energy LMCT excited states are implemented into single-atom photocatalysts through incorporating site-specific Co₁-C₂N₁ moiety. Multimodal synchrotron x-ray spectroscopies reveal that the pseudo square-planar geometry fosters a low spin Co(II) state (s = 1/2) with pronounced Jahn-Teller distortion. Crucially, enhanced d_z ²-p_z orbital coupling gives access to visible-light responsive LMCT states, fundamentally different from the predominant metal-to-ligand charge transfer excitations in conventional Co₁-N₂ counterpart. Electron localization at Co─C pairs creates photoreduction centers in close proximity, facilitating selective benzyl alcohol oxidation via a singlet oxygen (¹O₂)-mediated pathway. This work establishes single-atom frontier-orbital engineering for exploring visible-light photochemistry in heterogeneous photocatalysts.