Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 150

transformation above 21°C. Although the J-aggregates in the study by Mori et al. were stable in the
whole temperature range, the reverse, J to H transition, was observed for zeaxanthin aggregates in
TX-100 micelles (Avital et al. 2006). Immediately after the insertion of zeaxanthin into micelles
from tetrahydrofuran, J-aggregates formed, but in the course of 2 h, they spontaneously turned into
the card-packed H-aggregates. Stabilization of H-aggregates in TX-100 micelles was also observed
for the synthetic carotenoid 7 ′ -apo-7 ′ ,7 ′ -dicyano- β -carotene, but the precursor was the monomer

rather than the J-aggregate (He and Kispert 1999).

8.3.3 E XCITED -S TATE D YNAMICS
Although the aggregation-induced shifts of absorption spectra have been largely investigated, very
little is known about excited-state dynamics of carotenoid aggregates. Time-resolved data with sufi -
cient time resolution are so far available only for two carotenoids, zeaxanthin (Billsten et al. 2005) and
8 -carotenoic acid (ACOA) (T. Polívka, unpublished). Transient absorption spectra of carote-

noid aggregates, compared with the corresponding monomeric carotenoids, are shown in Figure 8.7.
The transient absorption spectra of aggregates are reminiscent of those recorded for monomers. They
are dominated by an ESA band, which rel ects the spectral proi le of the S 1 - S N transition (see Section
8.2). For both zeaxanthin and ACOA, the peak position of the main ESA band of the H-aggregate
is close to that of the monomeric carotenoid, but the spectral band is markedly broader. The nega-
tive band centered at 525 nm for the H-aggregate of zeaxanthin is superimposed on the high-energy
wing of the ESA spectrum, giving the impression of a separate ESA band at

′ -apo- β

500 nm. This negative
feature originates from a ground state bleach of the weak 525 nm band of H-zeaxanthin.

∼

Zeaxanthin

1

0

J-aggregate

H-aggregate
Monomer

-1

(a) 500 550 600 650 700

ACOA

1

H-aggregate

Monomer

0

500 550

Wavelength (nm)

600 650 700

(b)

FIGURE 8.7 Transient absorption spectra recorded 3 ps following excitation for zeaxanthin (a) and ACOA
(b). The spectra were measured with excitation at 400 nm (H-aggregates), 485 nm (monomers), and 525 nm
(J-aggregates).