Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin.
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.
Laboratory for Atomistic and Molecular Mechanics (LAMM), Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 1-235A&B, Cambridge, Massachusetts 02139, USA.
Chun-Teh
CT
Chen
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Chern
C
Chuang
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Jianshu
J
Cao
Faculté de Chirurgie Dentaire, Université de Strasbourg, 8 rue Sainte Elizabeth, 67000 Strasbourg, France and Unité INSERM 1121, 11 rue Humann, 67085 Strasbourg Cédex, France.
Vincent
V
Ball
Department for Advanced Materials and Structures, Centre de Recherche Public Henri Tudor, 5 rue Bommel, L-4940 Hautcharage, Luxembourg.
David
D
Ruch
1] Laboratory for Atomistic and Molecular Mechanics (LAMM), Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 1-235A&B, Cambridge, Massachusetts 02139, USA [2] Center for Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA [3] Center for Computational Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Markus J
MJ
Buehler
D065966:Absorption, Physicochemical; D004298:Dopamine / Q000737:chemistry; D004583:Electrons; D008543:Melanins / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D055620:Optical Phenomena; D010084:Oxidation-Reduction; D012996:Solutions; D013057:Spectrum Analysis
D008543:Melanins; D012996:Solutions; C041877:eumelanin; D004298:Dopamine