Ultrafast Vibrational Spectroscopy of the Eumelanin pigment
Authors:   , Thorn Karen 
Published: March 2021

<p>Solar ultraviolet (UV) radiation is a highly toxic carcinogen prevalent in our environment. Eumelanin pigment is a photo-stable biopolymer naturally produced in the skin's pigmentary system, providing the skin a unique photo-protection mechanism against exposure to UV radiation. The large macro-molecule rapidly dissipates 99 % of incident UV photons as thermal energy on ultra-fast femtosecond (fs) - picosecond (ps) time scales, before damage can occur to the underlying cells. The fundamental nature of eumelanin's structure and its vibrational energy dissipation mechanism is not yet fully understood, with complexities in the molecule's highly disordered chemical structure, and the ultrafast time-scale on which the energy dissipation occurs, rendering its characterisation elusive.
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<p>Indeed, the absorption spectrum of eumelanin gives little away, rising monotonically in wavelength towards the UV - quite unusual in organic polymers. It is proposed that due to the highly disordered structure of the molecule, multiple chromophores of overlapping energies may be selectively excited with differing irradiation wavelengths. This theory is further supported by eumelanin’s transient absorption signatures and its wavelength dependant photo-luminescence spectra, however the fundamental non-radiative relaxation pathways are not yet understood.
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<p><br></p><p>To bridge this knowledge gap, we present here the application of femtosecond stimulated Raman spectroscopy (FSRS) to eumelanin pigment. FSRS reveals ultra-fast vibrational dynamics on fs – ps time scales, allowing excited state vibrational pathways to be mapped, providing essential structural information of this intriguing molecule.
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<p><br></p><p>Following the introduction of eumelanin’s known photo-physical and structural properties as presented in chapter 1, an introduction to FSRS is presented in chapter 2. The build method and optical construction of the FSRS experiment are presented in chapter 4 including a novel bandwidth compression method used to generate a narrow-band Raman pump using frequency-domain nonlinear optics, presented in chapter 3. Here, using a 1.5 kHz, 800 nm Ti:Sapphire pulsed laser at a power of 3 W, conversion efficiencies of up to 30 % are achieved, generating intense second harmonic Raman pump pulses centred at 400 nm with <20 cm-1 bandwidths. Additionally, the well-known spatial filtering technique is used to generate tuneable, narrow-band pulses centered at the fundamental 800 nm laser pulse.
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<p><br></p><p>The application of FSRS to the study of eumelanin’s indole subunits, 5-6-dihydroxyindole (DHI) and its carboxylated form, 5-6-dihydroxyindole-2-carboxylic acid (DHICA) (and their oligomers) are presented in chapter 5. These studies provide direct evidence for excited state proton transfer (ESPT) in DHICA, and a reference for the interpretation of the complex eumelanin macro-molecule, as well as its dynamic vibrational signatures, which are discussed in chapter 6. Here, vibrational dynamics are resolved on

From these FSRS studies, ESPT – proposed as a relaxation pathway in eumelanin’s subunit DHICA – is demonstrated. Direct vibrational relaxation measurement mapped using density functional theory (DFT) provides strong evidence of excited state de-activation of DHICA via this mechanism. Further, eumelanin’s vibrational mode deactivation pathways are presented, providing evidence of specific mode excitations of the macro-molecule upon photo-excitation in both the UV (267 nm) and visible (400 nm) regions.
</p><p><br></p><p>Resolving eumelanin’s excited state vibrational modes and kinetics using FSRS provides dynamic structural information of eumelanin's thermal energy transfer system, including that of its indole subunits. Determining FSRS signatures as a function of excitation wavelength in both the visible and UV regions, reveals insights into this highly efficient UV absorbing material.
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