State Crossing Affects Photolysis Efficiency of Iron(III) Aqua Hydroxo Complexes

The iron(III)-(hydro)oxides and hydrated iron(III) ion respond to important applications in many fields, but their ligand-to-metal charge-transfer (LMCT) reactions receive less attention due to short excited-state lifetime or degradation. Here, we apply the ab initio multireference method to investigate the charge transfer photolysis of iron(III) aqua hydroxo complexes. Our studies reveal that both Fe-OH₂ bond photolysis and photohydrolysis can occur simultaneously in iron(III) aqua complexes, with the second hydration shell promoting photohydrolysis by serving as a proton acceptor. During the photolysis of the Fe-OH₂ bond, the reactive sextet electronic states can additionally undergo intersystem crossing transitions to lower-energy, nonreactive quartet states, which further reduces the overall photolytic efficiency of iron(III) aqua complexes. In iron(III)-hydroxy complexes, direct photolysis of the Fe-OH bond is the predominant pathway. In dimeric iron(III) complexes, lower LMCT states would undergo internal conversion to reach the ground state during photolysis of the bridging Fe-OH/Fe-O bond, leading to a low excited-state reactivity. Our theoretical results provide an improved understanding of the iron-oxygen bond photolysis of iron(III) aqua hydroxo complexes and complement previous explanations to experimental observations. These results are also potentially helpful for understanding the surface photochemistry of iron oxides in aqueous solutions and LMCT-triggered water oxidation.