Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 137

8 Effects of Self-Assembled

Aggregation on Excited States

Tomáš Polívka

CONTENTS
8.1 Introduction .......................................................................................................................... 137
8.2 Excited States of Monomeric Carotenoids ........................................................................... 139
8.3 Excited States of Carotenoid Aggregates ............................................................................. 141
8.3.1 Excitonic Interaction: Origin of the Spectral Shifts ................................................. 141
8.3.1.1 Intermolecular Interaction ......................................................................... 141
8.3.1.2 Intensity of the Exciton Bands ................................................................... 142
8.3.1.3 Limitations ................................................................................................. 143
8.3.2 Absorption Spectra of Carotenoid Aggregates......................................................... 144
8.3.2.1 Effect of Carotenoid Structure................................................................... 147
8.3.2.2 Effect of Hydrogen Bonds.......................................................................... 148
8.3.2.3 Other Spectral Features in Absorption Spectra of
Carotenoid Aggregates............................................................................... 148
8.3.2.4 Organization and Stability of Aggregates ................................................. 149
8.3.3 Excited-State Dynamics ........................................................................................... 150
8.4 Summary and Outlook ......................................................................................................... 154
Acknowledgments .......................................................................................................................... 154
References ...................................................................................................................................... 155

8.1 INTRODUCTION
The central structural feature of all carotenoids, a linear conjugated chain, makes carotenoids highly
hydrophobic molecules. Since pioneering work carried out on carotenoids more than 40 years ago
(Buchwald and Jencks 1968), it has been known that this hydrophobicity promotes the formation of
carotenoid aggregates when dissolved in hydrated solvents and that aggregation is characterized by
dramatic changes in absorption spectra (Ruban et al. 1993, Gruszecki 1999, Simonyi et al. 2003).
A number of studies carried out since the observation of astaxanthin aggregation (Buchwald and
Jencks 1968) demonstrates that two types of carotenoid aggregates can be distinguished according
to their absorption spectra. The i rst type is termed an H-aggregate and is characterized by a large
blueshift of the absorption spectrum. The H-aggregate consists of molecules whose conjugated
chains are oriented parallel to each other and are closely packed (the card-pack arrangement). The
second aggregation type, the J-aggregate, is characterized by a redshift of the absorption spectrum,
and results from a head-to-tail organization of conjugated chains (Simonyi et al. 2003).
Numerous studies of carotenoid aggregates have focused on the molecular organization of the
aggregates (Simonyi et al. 2003), but little is known about aggregation-induced effects on carote-
noid excited states. Classical exciton theory can qualitatively explain the aggregation-induced shifts
of absorption bands (Section 8.3.1), but a detailed understanding of the parameters governing the

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