A fundamental, Quantum Mechanical description of photoactivation of a generic
photosensitizer and the ensuing transfer of energy to endogenous
oxygen as part of the Type II pathway to photodamage during
photodynamic therapy (
PDT) is presented. The PS and molecular
oxygen are approximated as two-electron molecules. Conservation of energy and of angular momenta of the two molecule system are abided via selection rules throughout the four-stage process, including initial states, absorption of a photon by the PS, conversion of the PS to an excited spin triplet via intersystem crossing (ISC), and the transition of molecular
oxygen to an excited spin singlet state via a Triplet-Triplet Exchange of electrons with the PS. The provided description of
photosensitization will provide students and researchers with a fundamental introduction to
PDT, while offering the broader population of Quantum Mechanics and Physical Chemistry students an advanced example of quantum systems in an applied, medical context.