Characterization of Carotenoid Aggregates by Steady-State Optical Spectroscopy

The carotenoids have low-lying triplet excited states and can self-assemble in some solvents to form weakly or strongly coupled aggregates. These qualities make carotenoid aggregates useful for studies of singlet fission, where an outstanding goal is the correlation of interchromophoric coupling to...

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Bibliographic Details
Published in:The journal of physical chemistry. B 2012-09, Vol.116 (35), p.10617-10630
Main Authors: Wang, Chen, Berg, Christopher J, Hsu, Cheng-Chih, Merrill, Brittany A, Tauber, Michael J
Format: Article
Language:English
Subjects:
Aggregates
Carotenoids
Carotenoids - chemistry
Circular Dichroism
Excitation
Fluorescence
Heating
Joining
Monomers
Quantum Theory
Spectrometry, Fluorescence
Spectroscopy
Spectrum Analysis, Raman
Temperature
Xanthophylls - chemistry
Zeaxanthins
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Summary:The carotenoids have low-lying triplet excited states and can self-assemble in some solvents to form weakly or strongly coupled aggregates. These qualities make carotenoid aggregates useful for studies of singlet fission, where an outstanding goal is the correlation of interchromophoric coupling to the dynamics and yield of triplet excited states from a parent singlet excited state. Three aggregates of zeaxanthin, two weakly coupled and one strongly coupled, are characterized by steady-state spectroscopic methods including temperature-dependent absorption, fluorescence, and resonance Raman spectroscopy. The absorption spectra for each type of aggregate are distinct; however, an analysis of band positions reveals some important shared characteristics and suggests that the strongly coupled H-aggregate contains a subpopulation of weakly coupled constituents. Temperature-dependent absorption spectroscopy indicates that one of the weakly coupled aggregates can be converted to the other upon heating. The emission spectra of the three aggregates have similar profiles that are overall red-shifted by more than 1000 cm–1 relative to the monomer. The emission quantum yields of the aggregates are 5 to 30 times less than that of the monomer, with the lowest yield for the strongly coupled aggregate. The vibrational spectra of the chromophores support only slight perturbations from the structure of solvated monomers. Our findings support the conclusion that all three aggregates are best characterized as H-aggregates, in agreement with a prior theoretical study of lutein aggregates.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp3069514