A Model for the Activated Energy Transfer within Eumelanin Aggregates

The emission properties of eumelanin from Sepia officinalis are examined following UV-A excitation. The emission decay is nonexponential, exhibiting decay components on the tens of picosecond to several nanosecond time scales. The corresponding depolarization dynamics are also nonexponential and reveal that the emission becomes totally depolarized with an average time constant of ∼80 ps at 20 °C. The depolarization of the emission is found to be activated; a simple Arrhenius fit to the depolarization rate data gives an activation barrier of 21 ± 3 kJ mol -1 . The nonexponential emission decay is concluded to be a reflection of the structural disorder of eumelanin. The rapid and nonexponential depolarization dynamics are attributed to energy transfer processes that occur within “spherical” subunits that comprise the eumelanin aggregates.