Computational Mechanisms of Photosensitization, Oxygen Trapping, and Singlet Oxygen Release of N-Substituted Bisphenalenyl Compounds

Gaining mechanistic insight into singlet oxygen sensitization, binding, and release is critical to the rational design of triggered delivery. Here, the intermediates and transition states for photosensitization, binding, and release of molecular oxygen with three N-substituted bisphenalenyl molecules (1-3) bearing substituents with varying electronic effects (CF₃, H, and OCH₃) were calculated with density functional theory. Previously, it was seen that more electron donation speeds up the binding of oxygen, and both electron-donating and electron-withdrawing groups slow down the oxygen release. All of the transition states found were concerted additions and releases of the molecular oxygen. Computationally, there is a strong driving force for the bisphenalenyl molecules to form precomplexes with ground-state molecular oxygen. This allows for fast photosensitization and direct binding to form endoperoxides. Thus, the measured binding rate seems to be governed by complex formation rather than by the energy transfer or binding activation energy. Singlet oxygen is subsequently released from the substituted endoperoxides (1-EPO and 3-EPO) through a concerted mechanism. The unsubstituted 2-EPO proceeds through nonconcerted mechanisms for both binding and release, which single-reference calculations were unable to elucidate. These findings provide valuable insights into the design of efficient oxygen-sensitive systems for the storage and controlled delivery of singlet oxygen.