Phototoxic phytoalexins. Processes that compete with the photosensitized production of singlet oxygen by 9-phenylphenalenones

Experiments were performed to elucidate the excited-state behavior of 9-phenylphenalenones, which are phototoxic plant secondary metabolites involved in mechanisms of light-mediated plant defense. Using a combination of time-resolved and steady-state UV/visible spectroscopies, time-resolved IR absorption spectroscopy, time-resolved singlet oxygen phosphorescence measurements and cyclic voltammetry, we provide evidence of an intramolecular charge-transfer process in the excited singlet and the triplet states of 9-phenylphenalenones that modulates the photosensitized production of singlet oxygen.