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Size-Dependent Local Ordering in Melanin Aggregates and Its Implication on Optical Properties
journal contribution
posted on 2019-10-17, 20:44 authored by Johannes Träg, Patrick Duchstein, Matthias Hennemann, Timothy Clark, Dirk M. Guldi, Dirk ZahnWe present atomic
scale models of differently sized eumelanin nanoaggregates
from molecular dynamics simulations combined with a simulated annealing
procedure. The analysis reveals the formation of secondary structures
due to π-stacking on one hand, but on the other hand a broad
distribution of stack geometries in terms of stack size, horizontal
displacement angles, and relative torsion angles. The displacement
angle distribution, which is a measure of the occurrence of zigzag
and linear stacking motives, respectively, strongly depends on the
aggregate sizeand is hence controlled by the interplay of
surface and bulk energy terms. Semiempirical spectra calculations
of small stacks (up to five protomolecules) reveal a strong dependence
on the precise stack structure and allow for a direct structure–property
correlation. The observed spectral shifts result in an overall spectral
broadening and, hence, further support the geometric disorder model,
which complements the chemical disorder model in the interpretation
of eumelanin’s monotonically increasing broad-band absorption.
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Melanin Aggregatesshifts resultscale modelsstack geometriesdisplacement anglesSemiempirical spectra calculationsstack sizestack structureOptical Propertiesdisplacement angle distributionbulk energy termseumelanin nanoaggregatestorsion anglesdisorder modelbroad-band absorptionchemical disorder modeldynamics simulationsannealing procedure
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