Radiative relaxation of Sepia eumelanin is affected by aggregation

The steady-state and time-resolved emission properties of aqueous solutions containing different aggregation state distributions of eumelanin are repo rted. Excitation spectra of the size-selected samples reveal, for the first time, differences in absorption bands due to varying levels of aggregation . These size-dependent absorption properties result in size-dependent emiss ion band shapes and quantum yields. For all size fractions, absorption and emission overlap significantly. The emission yield for small eumelanin aggr egates is 5.7 times greater than that for large eumelanin aggregates. Time- resolved population decays reveal that small eumelanin aggregates are respo nsible for long-lived emission dynamics (lifetimes greater than 1 ns), whil e large eumelanin aggregates are the source of short emission decay (lifeti mes less than 1 ns). Polarized emission decays for the large and small aggr egates reveal that energy transfer occurs both within the same and between the separate fundamental building blocks of eumelanin, The observed energy transfer dynamics can be accounted for using Forster theory.