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

Photochemical Pathways and Light-Enhanced Radical Scavenging Activity of 1,8-Dihydroxynaphthalene Allomelanin

Melanins play important roles in nature, particularly in coloration and photoprotection, where interaction with light is essential. Biomimetic melanins represent an advantageous alternative to natural melanin for technological applications, sharing the same unique biocompatibility, as well as optoelectronic properties. Allomelanin, derived from 1,8-dihydroxynaphthalene, has been reported to exhibit even better photoprotective and antioxidant properties than the most studied example of biomimetic melanin, polydopamine. However, the interaction of allomelanin with light remains largely unexplored. Here we report the excited state dynamics of allomelanin in a wide range of time windows from femtoseconds to microseconds to minutes, using different experimental techniques, i.e., ultrafast transient absorption, nanosecond transient absorption, X-band electron paramagnetic resonance and radical quenching assays. We find that the photophysics of allomelanin starkly differs from that of the widely studied polydopamine, with broadband excitonically coupled states funneling the absorbed energy to a lower energy species in less than 1 ps. Independent of the excitation wavelength, a long-lived (>450 μs) photoproduct is populated in ≈24 ps. Quantum chemistry calculations suggest that the photoproduct primarily exhibits the character of localized 1,8-naphthoquinone radical anions. This light-driven increase in the anionic semiquinone-like radical concentration enhances the antioxidant activity of allomelanin. These results suggest that the two mechanisms considered at the basis of photoprotection, light-extinction and antioxidant action, are indeed synergistic in allomelanin and not independent, paving the way for new applications of allomelanin in nanomedicine, photocatalysis, energy conversion and environmental remediation.

Sub-50 fs Formation of Charge Transfer States Rules the Fate of Photoexcitations in Eumelanin-Like Materials

Eumelanins play a crucial role as photoprotective agents for living organisms, yet the nature of the stationary and transient species involved in the light absorption and deactivation processes remains controversial. Moreover, the critical sub-100 fs time scale, which is key to the characterization of the primary excited species, has remained unexplored. Here, we study the eumelanin analogue polydopamine (PDA) and employ a combination of steady-state and transient optical spectroscopies to reveal the presence of spectrally broad coupled electronic transitions with, at least partial, charge-transfer (CT) character. We monitor the CT state dynamics using tunable sub-20 fs pulses. We find that high photon energy excitation results in accelerated (sub-20 fs) CT formation times while activating pathways, which lead to long-lived (≫1 ns), possibly reactive CT species. On the other hand, visible light excitation results in a slower (≈45 fs) formation of bound CT states, which, however, recombine on the ultrafast sub-2 ps time scale.

"Photon recycling" can enhance cutaneous response to lasers: A pilot human study

BACKGROUND

Depending on wavelength and pigmentation, human skin can reflect up to 70% of incident laser light.

AIMS

We tested the hypothesis that returning ("recycling") this diffusely reflected light to the site of laser exposure would increase cutaneous response.

MATERIALS AND METHODS

Thirteen adult volunteers with Fitzpatrick skin types I-IV participated in this IRB-approved study. Matched contralateral test sites on the volar forearms were exposed to a pulsed dye laser operated at 585 nm, 450 microseconds pulse duration in a uniform 5 mm circular exposure spot without skin cooling. On one arm, the laser handpiece was fitted with an aluminized hemispherical mirror with a reflectance of 67%. The minimum fluence causing skin purpura, and the purpura lesion diameter were measured.

RESULTS

The mean purpura threshold fluence with the reflector was 3.1 J/cm² (0.5 SD), and 3.7 J/cm² without the reflector (0.36 SD) (p < 0.001). The mean laser-induced purpura lesion diameter was approximately 5.3 mm with the reflector and 5.0 mm without the reflector.

CONCLUSION

Consistent with a theoretical model and in vitro measurements, this human study confirms that "recycling" reflected laser light can increase skin response. Potentially, the therapeutic response can also be improved with "photon recycling."

The Photophysics and Photochemistry of Melanin- Like Nanomaterials Depend on Morphology and Structure

Melanin-like nanomaterials have found application in a large variety of high economic and social impact fields as medicine, energy conversion and storage, photothermal catalysis and environmental remediation. These materials have been used mostly for their optical and electronic properties, but also for their high biocompatibility and simplicity and versatility of preparation. Beside this, their chemistry is complex and it yields structures with different molecular weight and composition ranging from oligomers, to polymers as well as nanoparticles (NP). The comprehension of the correlation of the different compositions and morphologies to the optical properties of melanin is still incomplete and challenging, even if it is fundamental also from a technological point of view. In this minireview we focus on scientific papers, mostly recent ones, that indeed examine the link between composition and structural feature and photophysical and photochemical properties proposing this approach as a general one for future research.

Paramagnetism and Relaxation Dynamics in Melanin Biomaterials

Spectroscopical characterization of melanins is a prior requirement for the efficient tailoring of their radical scavenging, ultraviolet-visible radiation absorption, metal chelation, and natural pigment properties. Electron paramagnetic resonance (EPR), exploiting the common persistent paramagnetism of melanins, represents the elective standard for the structural and dynamical characterization of their constituting radical species. Although melanins are mainly investigated using X-band (9.5 GHz) continuous wave (CW)-EPR, an integration with the application of Q-band (34 GHz) in CW and pulse EPR for the discrimination of melanin pigments of different compositions is presented here. The longitudinal relaxation times measured highlight faster relaxation rates for cysteinyldopa melanin, compared to those of the most common dopa melanin pigment, suggesting pulse EPR spin-lattice relaxation time measurements as a complementary tool for characterization of pigments of interest for biomimetic materials engineering.

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

Eumelanin, the brown-black pigment found in organisms from bacteria to humans, dissipates solar energy and prevents photochemical damage. While the structure of eumelanin is unclear, it is thought to consist of an extremely heterogeneous collection of chromophores that absorb from the UV to the infrared, additively producing its remarkably broad absorption spectrum. However, the chromophores responsible for absorption by eumelanin and their excited state decay pathways remain highly uncertain. Using femtosecond broadband transient absorption spectroscopy, we address the excited state behavior of chromophore subsets that make up a synthetic eumelanin, DOPA melanin, and probe the heterogeneity of its chromophores. Tuning the excitation light over more than an octave from the UV to the visible and probing with the broadest spectral window used to study any form of melanin to date enable the detection of spectral holes with a linewidth of 0.6 eV that track the excitation wavelength. Transient spectral hole burning is a manifestation of extreme chemical heterogeneity, yet exciting these diverse chromophores unexpectedly produces a common photoinduced absorption spectrum and similar kinetics. This common photoresponse is assigned to the ultrafast formation of immobile charge transfer excitons that decay locally and that are formed among graphene-like chromophores in less than 200 fs. Raman spectroscopy reveals that chromophore heterogeneity in DOPA melanin arises from different sized domains of sp²-hybridized carbon and nitrogen atoms. Furthermore, we identify for the first time striking parallels between the excited state dynamics of eumelanin and disordered carbon nanomaterials, suggesting that they share common structural attributes.

Seeing the colors in black: ultrafast transient hole burning spectroscopy reveals the absorption properties of discrete chromophores and their interactions in the skin pigment eumelanin.