Ultrafast spectroscopic study of pheomelanin: Implications on the mechanism of superoxide anion formation

UV-A excitation of pheomelanin results in the activation of molecular oxygen through the production of the superoxide radical anion (O-2(.-)). Formation of this species has been attributed to rapid reaction between O-2 and solvated electrons produced by one-photon ionization of pheomelanin. Herein, ultrafast spectroscopy is used for the first time to directly examine the primary photodynamics of pheomelanin following UV-A excitation (320-400 nm). Immediately following photolysis (t similar to 100 fs) a transient absorption centered at 720 nm is observed. This feature decays nonexponentially; the decay dynamics are described by a sum of three exponential lifetimes, 0.84, 6.3, and 120 ps. While this transient species has the same absorption maximum as the solvated electron, the absorption line shapes differ. The data establish O-2(.-) production does not arise from oxygen scavenging of electrons produced by direct photoionization of pheomelanin. The transient species is assigned to the first excited singlet state of pheomelanin, and the nonexponentiality of the decay is attributed to different aggregated structures present in solution.