Tuning the ion formation processes from triplet–triplet annihilation to triplet-mediated photoionization

Free ion formation in acetonitrile is examined through transient photoconductivity for a set of ketones excited at different wavelengths. According to the photophysical parameters of the ketones and the incident photon energy, two mechanisms can be operative: triplet–triplet annihilation (bimolecular process) and/or photoionization (monomolecular biphotonic process). By using a tunable laser, excited state mediated photoionization was studied. From the threshold energy (Ethr) required for this process to occur, ionization potentials in solution (IS) were deduced and compared to the corresponding values in gas phase (IG). A simple energetic model enables the determination of the oxidation potential (Eox) of the ketones that are compared to the corresponding values obtained through electrochemical measurements.