Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 140

S N

3 A -

S 2 (1 B + )

_

1 B u

S *

S 1 (2 A - )

ICT

S 0 (1 A - )

(a)

Energy (cm -1 )

28,000 25,000 22,000 19,000 16,000 13,000 10,000 7,000

S 0 - S 2 S 1 - S N S 2 - S N S 1 - S 2

1

0

400 500 600 700 800 1,000 1,200 1,600

Wavelength (nm)

(b)

FIGURE 8.2 (a) Simplii ed energy-level scheme of a carotenoid molecule. The solid arrow represents the
absorbing S 0 - S 2 transition, the dotted arrows are transitions corresponding to transient signals occurring after
excitation. The S N state in this scheme represents only a symbolic i nal state for S 1 - S N and S 2 - S N transitions.
In reality, the i nal states of these transitions must be of different symmetry and therefore the S N state in the
scheme consists actually of two different states. (b) Spectral bands corresponding to various transitions for
monomeric carotenoids.

on these states may provide valuable information about the properties of these, so far, poorly
described states.
The aggregation-induced effects on carotenoid excited states discussed in this chapter are lim-
ited to the S 1 and S 2 states. Although knowledge about the S 2 energy is readily obtained from the
absorption spectrum, energy of the S 1 state had not been directly measured until the end of the last
century when a few different approaches, resonance Raman spectroscopy (Sashima et al. 1999),
detection of weak S 1 l uorescence (Fujii et al. 1998), two-photon absorption (Krueger et al. 1999),
and time-resolved S 1 - S 2 absor ption (Polívka et al. 1999), emerged. These methods provided valuable