Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 153

coupling. This argument, however, does not provide an explanation for the long decay component
(>300 ps) observed in H-aggregates of both zeaxanthin and ACOA, Figure 8.8, because a change in
the vibrational coupling cannot account for the dramatic change of the S 1 lifetime. Instead, Billsten
et al. (2005) showed that the spectrum of this long-lived component for zeaxanthin resembles fea-
tures attributable to a triplet state. These authors proposed an enhancement of intersystem crossing
induced by an H-type aggregation.
Studying the excited-state dynamics following excitation at different wavelengths has helped to
assign spectral bands in the absorption spectrum of the J-aggregate, Figure 8.9. The excitation of the
530 nm band of the zeaxanthin J-aggregate results in an ESA spectrum peaking at 605 nm. This spec-
trum shows no resemblance to the S 1 - S N ESA spectrum of monomeric carotenoids, either in position
or shape. In addition, the distinct bleaching band below 550 nm coni rms that this spectrum originates
from molecules forming the characteristic red band of the J-aggregates. On the contrary, the ESA
bands observed following 400 and 485 nm excitations are dominated by a band at 560 nm, Figure 8.9,
which is very close to that of monomeric zeaxanthin, Figure 8.7. Although H-zeaxanthin has an ESA
band at the same position, kinetics shown in the inset of Figure 8.9 exclude assigning this band as
originating from H-zeaxanthin; the 9 ps decay component that is present only at 560 nm after 400 and
485 nm excitation matches well the known S 1 lifetime of monomeric zeaxanthin in solution.
Thus, based on the excitation wavelength dependence, it is obvious that while excitation at 525 nm
selectively excites J-aggregates, excitation at higher energies results in excited-state dynamics cor-
responding to carotenoid monomers in solution. This indicates that monomers contribute signii -
cantly to the absorption spectrum of the J-aggregates. The presence of a shoulder at 605 nm in the
transient absorption spectrum measured after excitation at 400 and 485 nm shows that zeaxanthin
J-aggregates must be excited even at these wavelengths, suggesting that the absorption spectrum
of the J-aggregates extends to 400 nm (Billsten et al. 2005). These results suggest that what is

Energy (cm -1 )

18,000 17,000 16,000 15,000

1

0

1

0

-1

0 20 40 60 80

Time (ps)

550 600 650 700

Wavelength (nm)

FIGURE 8.9 Transient absorption spectra of the zeaxanthin J-aggregates recorded 3 ps following excitation
at 525 (full squares), 485 (open circles), and 400 nm (full triangles). All spectra are normalized to the maxi-
mum. (Inset) Kinetics of the zeaxanthin J-aggregates measured at 560 (full squares) and 605 nm (open circles)
following excitation at 400 nm. Solid lines represent multiexponential i ts of the data.