Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 154

generally considered as the absorption spectrum of a J-aggregate of a carotenoid actually consists
of contributions from both the J-aggregate and monomeric carotenoids.

8.4 SUMMARY AND OUTLOOK
The last decade has witnessed signii cant progress in the development of an understanding the excit-
ed-state properties of carotenoids. The majority of studies have focused on monomeric carotenoids
in organic solvents, but much has also been done to improve our knowledge of carotenoid aggre-
gates in hydrated solvents. The aggregation-induced shifts characteristic of the H- or J-aggregate
spectra are now qualitatively understood, and controlled the formation of both H- and J-aggregates
has been achieved for some carotenoids (Simonyi et al. 2003, Billsten et al. 2005, Avital et al. 2006).
The exact relationship between the carotenoid structure and its ability to form aggregates remains
incomplete. However, it is now clear that the presence of hydroxyl groups promotes the formation of
H-aggregates, most likely by the stabilization of the card-pack organization involving a hydrogen-
bonding network. In contrast, J-aggregates form when carotenoids lack end-ring functional groups
(e.g.,

-carotene), or when hydrogen-bond formation is prevented either by esterii cation (Simonyi et
al. 2003) or pH change (Billsten et al. 2005). Consequently, J-aggregates are usually less stable and
generated only in a narrow window of water/solvent ratios (Billsten et al. 2005, Avital et al. 2006).
Recent microscopy studies have also revealed the organization of aggregates on surfaces, establish-
ing that the average intermolecular distance in H-aggregates is in the 5-7 Å range (Zsila et al. 2001b,
Baro et al. 2003, Köpsel et al. 2005).
Despite the considerable progress that has been achieved in the last decade, signii cant challenges
remain. Very little is known about excited-state lifetimes and relaxation pathways in carotenoid
aggregates. While this topic has been extensively studied for monomeric carotenoids, zeaxanthin
is the only carotenoid whose excited-state lifetimes have been investigated in aggregated form
(Billsten et al. 2005). This study has demonstrated that signii cant changes occur in the S 1 lifetime
of zeaxanthin for both H- and J-aggregates. Additional experiments will be necessary to estab-
lish the aggregation-induced effects for other carotenoids. This is especially important for those
carotenoids known to form aggregates in both natural and artii cial systems. For example, some
apo-

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-carotenals that exhibit polarity-dependent behavior due to ICT state (Kopczynski et al. 2007)
have been used as TiO 2 sensitizers in thin i lms where they form H-aggregates (Gao et al. 2000,
Pan et al. 2004). The aggregation-induced effects on the ICT state, which may affect electron and/or
energy transfer properties, remain unknown. The behavior of other dark excited states, which may
be located within the S 1 - S 2 gap (Koyama et al. 2004, Polívka and Sundström 2004), is completely
unknown for carotenoid aggregates. Though the negligible dipole moment of these states prevents
large aggregation-induced shifts, the signii cant change in the S 1 lifetime upon aggregation (Billsten
et al. 2005) suggests that a comparable effect may also occur for other dark states. Another impor-
tant issue to tackle is the effect of environment on spectroscopic properties of carotenoid aggregates.
Most of studies have been carried out in hydrated solvents, but it is obvious that the spectroscopic
properties of aggregates of a particular carotenoid will differ when prepared in hydrated solvent, in
micelles (Avital et al. 2006), in lipid bilayers (Gruszecki 1999, Sujak et al. 2002), deposited on i lms
(Zsila et al. 2001b), or in solid phase (Hashimoto 1999). Because aggregated carotenoids in these
environments are functioning either in natural or artii cial systems and are promising candidates for
future applications in harnessing solar energy, both experimental and theoretical approaches will be
needed to reveal details of aggregation effects on carotenoid excited states.

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ACKNOWLEDGMENTS
The author thanks Tomáš Man c al for useful discussions, and Helena Billsten and Jingxi Pan for
important contributions to the work surveyed here. Financial support from the Czech Ministry of
Education (grants No. MSM6007665808 and AV0Z50510513) is gratefully acknowledged.