**3.1 Measurement of ultrafast time-resolved optical absorption spectra**

From studies on peridinin and other carotenoids, it has been known that there are at least two important low-lying excited singlet states, denoted S1 and S2, which are related to the highly efficient energy transfer from peridinin to chlorophyll a (Fig. 10(A)). To elucidate the mechanism of this efficient energy transfer, it is important that we make clear the characteristics of these excited states and the energy transfer pathways such as those from S2 to QX and/or from S1 to QY. Recently, many researchers have tried to examine this particular mechanism. The conjugated double bonds of most carotenoids are symmetry and these double bonds can be regarded as polyenes described in terms of the idealized C2h point group in the spectroscopic fields (Hudson et al., 1973). The lowest excited singlet (S1) state is assigned to the 21Ag- state, and the second lowest singlet (S2) state is assigned to the 11Bu+ state. The excitation to S1 from the ground state is symmetry forbidden and is not directly accessible by one-photon processes in contrast to the allowed absorption to S2 state (Polivka et al., 2004). On the other hand, the conjugated double bond of peridinin and other carbonylcontaining carotenoids are asymmetric due to the presence of the conjugated carbonyl group, and these oxygenated carotenoids display a pronounced solvent dependence of its lowest excited singlet state lifetime (S1 lifetime). Namely, it has been proposed that the findings are consistent with the presence of an intramolecular charge transfer (ICT) state, which is uniquely formed in carotenoids containing the carbonyl group in conjugation with the -electron system of double bonds (Frank et al., 2000; Zigmantas et al., 2004). It has also been argued that changes in the position of the ICT state related to the S1 state rationalize the dependence on solvent polarity concerning S1 lifetime. In the case of peridinin, the relationship of these energy levels is well discussed based on the detailed experimental works. The ICT in the excited state manifold of peridinin is shown to be higher in energy than the S1 state in nonpolar solvents and shifts below S1 with increasing solvent polarity

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

of PCP complex (Akimoto et al., 1996).

**3.2 Measurement of Stark spectra** 

ICT state would be directly discernible.

solvent

*n*-hexane

lifetime (ps)

C33-DerivativeC35-Derivative Peridinin C39-Derivative

*max* (nm)

410 436 454 469

Fig. 11. (A) Result of S1 lifetime and (B) proposed energy level diagram

The precise relationship between S1 and ICT energy levels and also the nature of ICT are described in the previous chapter. On the other hand, there has also been a suggestion of the relationship between S2 and ICT states. The measurement of electroabsorption spectroscopy (Stark spectra) of peridinin has been reported (Premvardhan et al., 2005). Stark spectra can determine the change in electrostatic properties and estimate the change of the static dipole moment (||) between in the ground state and in the excited state. Thus, the value of this change represents by ||. Based on the observation, it is found that the absorption band from S0 to S2 showes large static dipolemoment change. In addition, it is suggested that in PCP complex there may be strong dipole-dipole coupling between peridinin and chlorophyll a. The large dipole moment would allow for strong dipolar interaction between peridinin and Chl a in PCP, and would contribute to high energy transfer. It has also been recently proposed that the presence of the ICT excited state promotes dipolar interactions with Chl a in the PCP complex and facilitates energy transfer via a dipole mechanism (Zigmantas et al., 2002). Although the magnitude of the static dipole moment is suggested to be very important, the relationship between the structural features of peridinin and the dipole moment has not been made clear. We then measured the Stark absorption spectra of perdinin along with its allene modified and polyene chain modified derivatives. Stark spectra is particularly suitable for peridinin and its derivatives, because the presence of the

The Stark spectra and the maximum absorption of the electronic spectra of peridinin (**1**) and the synthesized derivatives (**2**-**4, 6** and **7**) are summarized in Fig. 12. The Stark spectra of

*n*-hexane 4200±200 1000±100 186±4 41±1

(A) (B)

methanol 11±3 9±1 10±1 9±1

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 147

solvent, methanol, for all the peridinin analogues regardless of the extent of -electron conjugation. Potential energy level diagrams for four molecules in polar and nonpolar solvents are described as shown in Fig. 11 (B). Based on the results of S1 lifetime, althought S1 state gradually drops as longer polyene chain in hexane, the ICT state exists in the same position in methanol. We dramatically observed that the behavior of ICT states were obviously different from that of S1 states in the series of our synthesized peridinin derivatives including peridinin itself. These results strongly support the idea that the S1 and ICT states act as independent states. We can not, however, conclude clearly whether ICT state is separate or mixed energy level from S1 state. The unexpected phenomena, that the ICT state exists in the same position in methanol, is quite intereisting. We can presume that this nature of the ICT state is very important for energy transfers because the environment in methanol is considered to be the nearly same to that

C37-Peridinin

C33-Derivative C35-Derivative C39-Derivative

(Bautista et al., 1999) (Fig.10 (B)). In addition, it is suggested that the efficient energy transfers are related with this ICT state. Proposals for the nature of the ICT state include its being a separate electronic state from S1 (Vaswani et al., 2003; Papagiannakis et al., 2005), quantum mixed with S1 (Shima et al., 2003) or simply S1 itself. Although there are many discussions with experiments and calculations, the precise nature of the ICT state remains to be elucidated. Under these back-ground on the proposed attractive energy level, ICT energy level, a new approach, that the synthesis of a series of peridinin analogues followed by their spectroscopic measurements are investigated, has been started as a collaboration work between Connecticut University, Osaka City University and Kwansei Gakuin University of Hyogo. Thus, to explore the nature of the ICT state in carbonyl-containing carotenoids, both steady-state and ultrafast time resolved optical spectroscopy have been performed on peridinin and its synthetic derivatives.

Fig. 10. (A) Enegy transfer from peridinin to chl-a and (B) the nature of ICT state

The ultrafast time resolved optical absorption experiments of peridinin (**1**) and many other carbonyl-containing natural carotenoids such as fucoxanthin and spheroidenone were measured, and their S1 lifetimes were reported by the analysis of their ultrafast time resolved optical absorption (Frank et al., 2000). The lifetime of three natural carotenoid was reported to depend on the polarity of the measured solvents, and this effect is attributed to the presence of an intramolecular charge transfer (ICT) state in the manifold of the excited states of these molecules. We then measured the lifetime of the lowest excited single state of the four compounds, which are C33, C35 and C39 synthesized derivatives along with peridinin, and the results are listed in Fig. 11 (A). The data listed in Fig. 11 (A) show that the lifetime is shorter in the polar solvent, methanol, and is longer in a non-polar solvent, *n*-hexane. This means that the ICT states in the excited state manifold of peridinin and its three derivatives are higher energy than the S1 state in nonpolar solvents, and they shift to a lower energy than the S1 state in polar solvents. These experiments on peridinin and its derivatives revealed an increasing solvent effect with the decreasing -electron chain length. This result agrees with the experimental results carried out on conjugated apo-carotenoids (Ehlers et al., 2007). The lifetime of the lowest excited singlet state of C33 peridinin derivative **5** is the one most strongly dependent on the solvent polarity. In fact, this is the strongest solvent dependence on the lifetime of the carotenoid excited state so far yet reported. Moreover, the most striking observation in the data is that the lifetime of the ICT state converges to a value of 10 1 ps in the polar

(Bautista et al., 1999) (Fig.10 (B)). In addition, it is suggested that the efficient energy transfers are related with this ICT state. Proposals for the nature of the ICT state include its being a separate electronic state from S1 (Vaswani et al., 2003; Papagiannakis et al., 2005), quantum mixed with S1 (Shima et al., 2003) or simply S1 itself. Although there are many discussions with experiments and calculations, the precise nature of the ICT state remains to be elucidated. Under these back-ground on the proposed attractive energy level, ICT energy level, a new approach, that the synthesis of a series of peridinin analogues followed by their spectroscopic measurements are investigated, has been started as a collaboration work between Connecticut University, Osaka City University and Kwansei Gakuin University of Hyogo. Thus, to explore the nature of the ICT state in carbonyl-containing carotenoids, both steady-state and ultrafast time resolved optical spectroscopy have been performed on

Fig. 10. (A) Enegy transfer from peridinin to chl-a and (B) the nature of ICT state

The ultrafast time resolved optical absorption experiments of peridinin (**1**) and many other carbonyl-containing natural carotenoids such as fucoxanthin and spheroidenone were measured, and their S1 lifetimes were reported by the analysis of their ultrafast time resolved optical absorption (Frank et al., 2000). The lifetime of three natural carotenoid was reported to depend on the polarity of the measured solvents, and this effect is attributed to the presence of an intramolecular charge transfer (ICT) state in the manifold of the excited states of these molecules. We then measured the lifetime of the lowest excited single state of the four compounds, which are C33, C35 and C39 synthesized derivatives along with peridinin, and the results are listed in Fig. 11 (A). The data listed in Fig. 11 (A) show that the lifetime is shorter in the polar solvent, methanol, and is longer in a non-polar solvent, *n*-hexane. This means that the ICT states in the excited state manifold of peridinin and its three derivatives are higher energy than the S1 state in nonpolar solvents, and they shift to a lower energy than the S1 state in polar solvents. These experiments on peridinin and its derivatives revealed an increasing solvent effect with the decreasing -electron chain length. This result agrees with the experimental results carried out on conjugated apo-carotenoids (Ehlers et al., 2007). The lifetime of the lowest excited singlet state of C33 peridinin derivative **5** is the one most strongly dependent on the solvent polarity. In fact, this is the strongest solvent dependence on the lifetime of the carotenoid excited state so far yet reported. Moreover, the most striking observation in the data is that the lifetime of the ICT state converges to a value of 10 1 ps in the polar

peridinin and its synthetic derivatives.

solvent, methanol, for all the peridinin analogues regardless of the extent of -electron conjugation. Potential energy level diagrams for four molecules in polar and nonpolar solvents are described as shown in Fig. 11 (B). Based on the results of S1 lifetime, althought S1 state gradually drops as longer polyene chain in hexane, the ICT state exists in the same position in methanol. We dramatically observed that the behavior of ICT states were obviously different from that of S1 states in the series of our synthesized peridinin derivatives including peridinin itself. These results strongly support the idea that the S1 and ICT states act as independent states. We can not, however, conclude clearly whether ICT state is separate or mixed energy level from S1 state. The unexpected phenomena, that the ICT state exists in the same position in methanol, is quite intereisting. We can presume that this nature of the ICT state is very important for energy transfers because the environment in methanol is considered to be the nearly same to that of PCP complex (Akimoto et al., 1996).

Fig. 11. (A) Result of S1 lifetime and (B) proposed energy level diagram
