Photoinduced electron transfer in a bilirubin-oxygen complex: the triplet-reactant computational challenge

Although bilirubin photochemistry is central to neonatal jaundice phototherapy, the mechanism of bilirubin photooxidation remains unclear. Here, we use a comprehensive computational approach to investigate whether this mechanism might be initiated by photoinduced electron transfer (PET) to molecular oxygen (O₂), generating superoxide (O₂˙^-). For this purpose, we employed a simplified bilirubin model compound-tetramethyldipyrrinone (TMD). We use a combination of multireference, coupled cluster methods, and density functional theory techniques to assess the feasibility of the TMD-O₂ complex formation and PET from TMD to O₂. Our results support the feasibility of PET in the TMD-O₂ complex and suggest that PET could potentially initiate bilirubin photooxidation. Beyond the bilirubin case, this work underscores the need for efficient and accurate protocols to compute binding free energies of weak O₂ encounter complexes with organic chromophores in solution, and it highlights the broader challenge of modeling triplet photochemistry with dense manifolds of near-degenerate states, crossings, and strongly state-specific solvent response.