Do thermal treatments influence the ultrafast opto-thermal processes of eumelanin?

After light absorption, melanin converts very rapidly the energy gained into heat. The time scale of this process ranges from tens of femtoseconds to a few nanoseconds. Femtosecond transient absorption allows for exploration of such photo-induced carrier dynamics to observe the de-excitation pathways of the biological complex. Here, we report on the ultrafast relaxation of suspensions of Sepia melanin in DMSO at room temperature using a femtosecond broadband pump and probe technique by photoexciting in the UV and probing in the entire visible range. In particular, we focus on the possible role that different heat treatments, performed in the temperature range 30-80 °C might have on the relaxation of charge carriers photogenerated by UV radiation in such suspensions. Experimental data indicate that in all the investigated suspensions, photoexcited carriers always follow a tri-exponential route to relaxation. Moreover, we find that the relaxation time constants are essentially the same in all cases, within the experimental error. We take this as evidence that all the investigated suspensions essentially exhibit the same relaxation dynamics, regardless of the temperature at which the heat treatment has been performed and of the heat-induced denaturation of the proteinaceous compounds bound to the photoactive pigment. Our experiments represent a significant step towards the understanding of the stability of melanin with respect to temperature changes.