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Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

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  • Chun-Teh Chen

    (Laboratory for Atomistic and Molecular Mechanics (LAMM), Massachusetts Institute of Technology)

  • Chern Chuang

    (Massachusetts Institute of Technology)

  • Jianshu Cao

    (Massachusetts Institute of Technology)

  • Vincent Ball

    (Faculté de Chirurgie Dentaire, Université de Strasbourg, 8 rue Sainte Elizabeth, 67000 Strasbourg, France and Unité INSERM 1121)

  • David Ruch

    (Centre de Recherche Public Henri Tudor)

  • Markus J. Buehler

    (Laboratory for Atomistic and Molecular Mechanics (LAMM), Massachusetts Institute of Technology
    Center for Materials Science and Engineering, Massachusetts Institute of Technology
    Center for Computational Engineering, Massachusetts Institute of Technology)

Abstract

Eumelanin is a ubiquitous biological pigment, and the origin of its broadband absorption spectrum has long been a topic of scientific debate. Here, we report a first-principles computational investigation to explain its broadband absorption feature. These computations are complemented by experimental results showing a broadening of the absorption spectra of dopamine solutions upon their oxidation. We consider a variety of eumelanin molecular structures supported by experiments or theoretical studies, and calculate the absorption spectra with proper account of the excitonic couplings based on the Frenkel exciton model. The interplay of geometric order and disorder of eumelanin aggregate structures broadens the absorption spectrum and gives rise to a relative enhancement of absorption intensity at the higher-energy end, proportional to the cube of absorption energy. These findings show that the geometric disorder model is as able as the chemical disorder model, and complements this model, to describe the optical properties of eumelanin.

Suggested Citation

  • Chun-Teh Chen & Chern Chuang & Jianshu Cao & Vincent Ball & David Ruch & Markus J. Buehler, 2014. "Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin," Nature Communications, Nature, vol. 5(1), pages 1-10, September.
    • Handle: RePEc:nat:natcom:v:5:y:2014:i:1:d:10.1038_ncomms4859
    DOI: 10.1038/ncomms4859
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