Divide-to-Conquer: A Kinetic Model for Singlet Oxygen Photosensitization.

Photosensitized singlet oxygen generation occurring in a PS-O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS-O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations.