Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 149

of the monomer. Moreover, even when the H-band dominates the absorption spectrum, a weak band
below the low-energy edge of the monomeric absorption spectrum is often present, Figure 8.5. Even
though this feature may be interpreted as the J-band, indicating that a fraction of the molecules is
in the head-to-tail arrangement, l uorescence anisotropy measurements of conjugated oligomers
proved that this low-energy band has a polarization nearly identical to the H-band and therefore
cannot be interpreted as a J-band (Spano 2006). Very little is known about the emission proper-
ties of carotenoid aggregates. A rare measurement of the emission spectrum of an H-aggregate of
lycopene organized in a Langmuir-Blodgett i lm (Ray and Mishra 1997) demonstrated a large
redshift of the emission spectrum, but no information about polarization was provided. It is also
worth mentioning that the spectral shift of the main H-band in respect to the 0-0 origin of the
S 0 - S 2 transition of a monomeric carotenoid can exceed 6,000 cm −1 , see Table 8.1, indicating that
the lower, forbidden exciton state of the H-aggregate may be found below 15,000 cm −1 . Since the
carotenoid lowest excited state, S 1 , bears a negligible dipole moment and is thus barely affected
by the dipole-dipole interaction, Equation 8.3, the lower exciton state of the H-aggregate may be
energetically very close to the S 1 state. Therefore, the large redshift of the lycopene H-aggregate
emission observed by Ray and Mishra (1997) may be interpreted as emission either from the lower
exciton state of the H-aggregate or from the S 1 state. To clarify the origin of the emission band and
to determine the nature of the bands in the carotenoid H-aggregate absorption spectrum, further
emission data on carotenoid aggregates are clearly needed.
The absorption spectra of J-aggregates always contain bands coinciding with vibrational bands
of monomeric carotenoids. Although these bands were earlier interpreted as due to the vibrational
bands of the J-aggregate (Zsila et al. 2001b), studies of the excited state dynamics of the zeaxan-
thin J-aggregate showed that excitation of the “true” J-band at 540 nm produces distinctly different
excited-state dynamics than excitation into the vibrational bands (Section 8.3.3). Since the 480 nm
excitation generates excited-state dynamics similar to that of the monomeric carotenoid, the obvious
assignment of the vibrational bands in the J-aggregate spectrum is that they are due to monomeric
carotenoid molecules coexisting with the J-aggregate (Billsten et al. 2005).

8.3.2.4 Organization and Stability of Aggregates
A useful tool for determining the large-scale organization of carotenoid aggregates is CD spec-
troscopy. While monomeric carotenoids are usually nonchiral molecules, chirality is induced upon
aggregation. CD spectra of carotenoid aggregates exhibit large Cotton effects that could be used
to evaluate the large-scale arrangement of the aggregate, because the sign of the Cotton effect
indicates the torsion angle between the neighboring molecules within the aggregate (Simonyi
et al. 2003). In a set of studies, Zsila et al. demonstrated a helical arrangement of H-aggregates of
certain carotenoids (Zsila et al. 2001a-e). These authors have shown that, depending on carotenoid
structure, either right- or left-handed helical structures may be formed. For capsanthol, a spontane-
ous transition between a right-handed and a left-handed helical structure has even been observed
in the time course of 2 h (Zsila et al. 2001e). When H-aggregates form on surfaces, STM reveals a
neat card-pack organization. For example, astaxanthin deposited on a graphite surface exhibited an
arrangement of card-packed molecules with intermolecular distances of

6 Å (Köpsel et al. 2005).
The large-scale organization of J-aggregates is less understood, but the combination of CD studies
and atomic force microscopy of J-aggregates in i lms indicates that J-aggregates may be organized
into layers of nematic crystals (Zsila et al. 2001b). J-aggregates were also observed when

∼

β -carotene

was deposited on the Cu(111) surface. STM images reveal a grid of -carotene molecules clearly

organized in the head-to-tail arrangement (Baro et al. 2003), indicating that

β

-carotene forms
predominantly J-aggregates not only in hydrated solvents, but also when deposited on surfaces.
In some cases, a transition between J- and H-aggregates has also been observed. Mori et al.
(1996) showed that astaxanthin H-aggregates transform into J-aggregates in a few hours. These
authors studied the transformation in the 2°C-32°C temperature range and concluded that assem-
blies corresponding to H- and J-aggregates are separated by an energy barrier that allows the H to J

β