Ultrafast Nonradiative Relaxation Dynamics of Eumelanin

Abstract

Degenerate pump−probe femtosecond spectroscopy is used to examine the primary photodynamics of eumelanin from Sepia officinalis following UV-A excitation. Exciting and probing at 320, 350, and 380 nm produces a transient absorption that rises within the instrument response and decays with a lifetime less than approximately 50 fs. Following the rapid decay of this transient signal, the recovery dynamics associated with the bleaching of the ground-state absorption are observed. The transient pump−probe data are compared to reported photoacoustic and time-resolved emission data. This comparison shows the dominant nonradiative process in eumelanin is repopulation of the ground electronic state and that the time constants revealed by the nonexponential decays of the emission and absorption data are associated with chemically distinct species.