Ultrafast transient absorption of eumelanin suspensions: the role of inverse Raman scattering

Abstract

An ultrafast investigation is carried out on synthetic eumelanin suspended either in water or in DMSO-methanol. Upon photoexcitation by visible femtosecond pulses, the transient absorption (TA) dynamics of the suspensions are probed in a broad visible spectral range, showing clear nonlinearities. The latter arise from pump-probe interactions that induce the inverse Raman scattering (IRS) effect. We show how eumelanin TA dynamics are modified in proximity of the solvent Stokes and anti-Stokes scattering peaks, demonstrating that IRS affects the sign of TA but not the relaxation times. We compare the results obtained in both suspensions, unveiling the role of the surrounding environment. Eventually, the intrinsic response of synthetic eumelanin to ultrafast photoexcitation is evaluated.

Ultrafast spectral hole burning reveals the distinct chromophores in eumelanin and their common photoresponse

“…2, 3 and 7) reveal a surprisingly homogeneous response by DOPA melanin to photoexcitation, even when the absorbed photon energy is tuned by more than a factor of two from the UV to the visible. Whereas others have reported multiexponential decay kinetics that vary with pump and probe wavelengths, 21,24,25,42 our measurements reveal unexpectedly similar excited state deactivation, offering an explanation of how eumelanin is able to effectively dissipate light with wavelengths throughout its broad absorption spectrum. The spectral hole burning reported here provides evidence for the selective excitation of subensembles of chromophores.…”

“…7a). Whereas the authors of previous TA studies t their kinetic traces to 2 or 3 exponentials, 21,25,41 a log-log plot (Fig. 7b) reveals that the kinetics are actually biphasic in nature, and better ts are obtained with a similar number of parameters using a linear combination of a stretched exponential and a power law,…”

“…20,22,23 Our results also explain past reports of oppositely signed signals at the same probe wavelength. 21,24 Aloi et al 24 reported a negative TA signal at a probe wavelength of 504 nm when exciting synthetic eumelanin in DMSO-methanol solution at 550 nm, yet Ye and Simon 21 saw positive signals at this same probe wavelength when exciting at 303 nm. These observations are only apparently contradictory because spectral hole burning is only seen when the pump and probe wavelengths are sufficiently close as pointed out in pioneering work by Warren and coworkers.…”

“…3 and 4 claries the GSB behavior of eumelanin reported at just a few pump and probe wavelengths in earlier ultrafast TA experiments. 2,19,20,[22][23][24] In measurements on natural Sepia melanin in aqueous solution, Nofsinger, Ye, and Simon reported negative TA signals with same-wavelength excitation and probing at 320, 350, and 380 nm. 2 Warren and co-workers observed negative TA signals for several combinations of NIR pump and probe wavelengths.…”

“…8,19,20 Thus, top-down studies are crucial for studying how heterogeneity affects the excited state dynamics of eumelanin. To this end, transient absorption 2,[19][20][21][22][23][24][25] and photoluminescence 10,17,26,27 studies of natural and synthetic eumelanins have been carried out, but it has proven difficult to link dynamical observations with structural models.…”