A model for the activated energy transfer within eumelanin aggregates

The emission properties of eumelanin from Sepia officinalis are examined fo llowing UV-A excitation. The emission decay is nonexponential, exhibiting d ecay components on the tens of picosecond to several nanosecond time scales . The corresponding depolarization dynamics are also nonexponential and rev eal that the emission becomes totally depolarized with an average time cons tant of similar to 80 ps at 20 degrees C. The depolarization of the emissio n is found to be activated; a simple Arrhenius fit to the depolarization ra te data gives an activation barrier of 21 +/- 3 kJ mol(-1). The nonexponent ial emission decay is concluded to be a reflection of the structural disord er of eumelanin. The rapid and nonexponential depolarization dynamics are a ttributed to energy transfer processes that occur within "spherical" subuni ts that comprise the eumelanin aggregates.