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Optical spectroscopy of carotenoids in self-assembled aggregates and in avian plumage


Carotenoids are strongly absorbing pigments that can self- assemble to form aggregates. These aggregates provide an opportunity to study exciton-coupling, which is the delocalization of an excitation over multiple molecules. Two zeaxanthin aggregates, previously denoted in our group as "J1" and "J2", exhibit red-shifted absorption relative to the monomer, and a third, "H", displays a strongly blue -shifted absorption. Raman scattering cross-sections as a function of excitation wavelength are reported for these three zeaxanthin aggregates and the monomer. The aggregates were modeled as dimers to probe the strength of exciton-coupling, intermolecular geometry, and vibrational displacements. The simulations of J1 and J2 suggest that these two aggregates consist of weakly-coupled, card- stacked carotenoids. J2 was found to have a smaller distance and larger rotation angle between the two chromophores, relative to J1. The H-aggregate was fit to a strongly coupled card-stacked orientation. The possibility of exciton-coupling in carotenoid-containing feathers was investigated with in situ resonance Raman spectroscopy, coupled with measurements of diffuse reflectance and transmittance. Rhodoxanthin is the major carotenoid pigment in five different feathers, but there is a large variation in color. Red-shifts in the absorption profiles of these feathers correlated with a decrease in the frequency of the ethylenic mode and an increase in intensity of hydrogen-out-of-plane modes. Isomeric effects were found to be insignificant in terms of the color of the feathers. These studies of zeaxanthin and rhodoxanthin allow for a better understanding of exciton-coupling and illustrate how environment and structure affect the photophysics of carotenoids

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