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 reported. 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 emission band shapes and quantum yields. For all size fractions, absorption and emission overlap significantly. The emission yield for small eumelanin aggregates in 5.7 times greater than that for large eumelanin aggregates. Time-resolved population decays reveal that small eumelanin aggregates are responsible for long-lived emission dynamics (lifetimes greater than 1 ns), while large eumelanin aggregates are the source of short emission decay (lifetimes less than 1 ns). Polarized emission decays for the large and small aggregates 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 Förster theory.