Phenalenone (PN) derivatives are involved in plant defense strategies, producing molecular singlet oxygen in a photosensitization process. Many experimental and theoretical studies determined that PN can performe this process with a quantum yield close to 1. However, it has been observed that the efficiency of some of its derivatives is much lower. This is the case of 9-phenylphenalenone (9-PhPN). To elucidate the factors that determine the different photochemistry of PN and its derivate 9-PhPN, we developed a complete active space self-consistent field/multi-configurational second-order perturbation theory study where several deactivation paths through the lowest excited states were explored. We found that the characteristics of the low-lying excited states are similar for both PN and 9-PhPN in the areas near the geometry of excitation. Consequently, the first processes that take place immediately after absorption are possible in both systems, including the population of the triplet state responsible for oxygen sensitization. However, 9-PhPN can also undergo cyclization by a bond formation between the carbonyl oxygen and a carbon atom of the phenyl substituent. This process competes favorably with population of triplet states and is responsible for the decrease of the quantum yield of oxygen sensitization in 9-PhPN relative to PN.