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$Recently^{1}$ the primary step in the one electron photooxidation of $N, N, N^{1}, N^{1}$-tetramethyl-p-phenylenediamine (TMPD) dissolved in rigid 3-methylpentane at $77^{\circ}K$ was investigated spectro-scopically. The photoproduct is the conjugated free radical cation, Wurster's Blue (WB). It was found that the quantum yields based on a one photon ionization mechanism displayed large variations with wavelength over the first absorption band of TMPD in contrast with the essentially wavelength independent fluorescence and phosphorescence yields. The thermally equilibrated singlet and triplet states were therefore ruled out as intermediates. Since then, photoconductivity $studies^{2}$ of the same system (and several others) have shown that the initial rate of producing mobile electrons depends quadratically on the incident light intensity, indicating a two photon mechanism. In studies now completed we have found that the initial rate of production of WB also displays a quadratic dependence on incident light intensity. As in the photoconductivity work it is found possible to separate the first and second photon steps in the photooxidation by using very near u. v. radiation (just outside of the TMPD absorption band) in conjunction with primary excitation of TMPD. With an intense near u. v. beam the initial rate of WB production becomes linearly dependent on the intensity of the primary exciting light. The quantum yields of the first and second photon steps have been determined and give very different results from a similar decomposition made for the initial rates in the photoconductivity work. Polarized photoselection studies indicate that the second photon step is isotropic when averaged over the band width of the near u.v. beam. It is also found that oxygen saturation inhibits the production of WB. (It does not inhibit the initial rate of producing mobile electrons.)$^{2}$ At the same time oxygen partially quenches the phosphorescence and, independently observed, diminishes the optical density of the triplet-triplet transition in the red region to the same extent. Other experiments underway should help to identify more positively the triplet state as the major intermediate in the photoproduction of WB. If the triplet state is the intermediate it follows that the ionization potential of TMPD in this system is about 6.2 ev (it is 6.6 ev in the gaseous state.) and the ionization potential of the lowest triplet state is about 3.2 ev.


This work has been supported in part by Public Health Service research grant A-3415 from the National Institute of Arthritis and Metabolic Diseases, and in part by the Advanced Research Projects Agency, contract SD-68.
Author Institution: Department of Chemistry, Cornell University