Uncovering the mechanism of the ultrafast UV-energy dissipation in the eumelanin pigment

Abstract

The black-brown pigment eumelanin acts as a vital barrier to the harmful UV radiation. With an ingenious ability to effectively dissipate 99.9% of the incoming UV energy to the heat within few picoseconds, eumelanin serves as a natural photo-protectant. To unravel the nature of the energy dissipation we study the eumelanin pigment and its major building block using ultrafast broadband transient absorption spectroscopy. The excited state decay was found to be fluence independent for all excitation energies, suggesting that electronic excitations are also spatially localized. The short excited state lifetime – on the order of a few picoseconds – leads to a suggestion that the energy is dissipated through excited-state intramolecular proton transfer, which we examined via comparison with pH-dependent TA spectroscopy of the DHICA and related building blocks.