**3.2 Measurement of Stark spectra**

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 ICT state would be directly discernible.

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

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

unique allene bond and the irregular C37 skeleton.

states behave independently.

the visible spectrum.

**5. Acknowledgment** 

**6. References** 

recieving the Scholar Ship of JSPS.

**4. Conclusion** 

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 149

The research on the efficient energy transfer mechanism of the primary photochemical events of photosynthesis have been exactly developed by the new approach based on the synthetic organic chemistry. Namely, the relationship between the characteristic structure of peridinin and the nature of its ICT state has been gradually made clear by synthesizing a series of peridinin analogues and comparing their characteristics to those of peridinin. Stark spectra of peridinin, allene modified, and polyene chain modified derivatives have showed that the dipole moment of the excited state (|| value) of peridinin is the largest among the series of six compounds, although the increasing -electron chain length generally shows a larger value of ||. These results apparently show that the allene group and C37 skeleton of peridinin effectively contributes to production of the large dipole moment in the molecule in excited state, which would result in the high energy transfer efficiencies to Chl a in the PCP complex. This is an answer of why peridinin possesses the

In addition, the ultrafast time resolved optical absorption spectra of polyene chain modified derivatives including peridinin show that the lifetime of the lowest excited singlet state of C33 peridinin derivative has the strongest solvent dependence so far yet reported. Furthermore, the data reveal the striking observation that the lifetime of the ICT state converges to a value of 10 1 ps in methanol for all peridinin analogues regardless of the extent of -electron conjugation. These data strongly support the notion that the S1 and ICT

On the other hand, comparing the stereochemical stability and spectral characteristics of the synthesized ylidenebutenolide modified analogues to those of peridinin has resulted in the conclusion that this particular functional group at least contributes to maintaining the stereochemistry of the conjugated double bonds in the all-*trans* configuration and giving rise to a max value desirable for the marine organism to absorb light in the blue-green region of

These inherent characteristics of peridinin are important clues for elucidating the energy transfer mechanism from the light-harvesting carotenoids to chlorophylls. The studies to measure the energy transfer efficiencies of peridinin derivatives are currently in progress to

We thank Dr. Thomas Netscher of DSM Nutritional Products, Ltd., for the donation of (-) actinol **18**. We also would like to thank Prof. H. A. Frank (University of Connecticut) with ultrafast experiments and Prof. H. Hashimoto (Osaka City University) with Stark spectra measurement. This work was supported by a Grant-in-Aid for Science Research on Priority Areas 16073222 from the Ministry of Education, Culture, Sports, Science and Technology, and Matching Fund Subsidy for a Private University, Japan. T. K. is also grateful for

Akimoto, S.; Takaichi, S.; Ogata, T.; Nishiyama, Y.; Yamazaki, I. & Mimuro, M. (1996).

Excitation energy transfer in carotenoid chlorophyll protein complexes probed by

further understand the exact role of these unique functional groups.

peridinin, allene modified and polyene chain modified derivatives were recorded in methyl methacrylate polymer at 77 K. The || values were corresponding to the CT absorption band. As the results of peridinin and allene modified derivatives, peridinin showed the largest || value among all of them. Namely, peridinin yielded a || value of 5.42 (x 10- 29 C・m), acetylene derivative **2** showed 2.47, olefin derivative **3** showed 4.22, and diolefin derivative **4** showed 4.25. A || value of peridinin was in agreement with a reported value (Premvardhan et al., 2005). The || value generally shows a larger number with the increasing -electron chain length theoretically. Although peridinin possesses fewer conjugating double bonds and shows a shorter max rather than that of diolefin derivative **4**, the || value of peridinin was the largest among the four compounds. The difference in the || value is evidently attributable to the difference in the functional groups. Thus, we have understood that the unique allene group contributes to production of the large dipole moment in the molecule. These results strongly suggest that the allene group of peridinin is essential for formation of the effective ICT state, which would allow the quantitative energy transfer to Chl a in the PCP complex. This is the first experimental evidence that shows the allene group in peridinin enhances the ICT character (Kusumoto et al., 2010).

In addition, as the results of peridinin and polyene chain modified derivatives, peridinin (**1**) also showed the largest || value among all of them. Namely, peridinin (**1**) yielded a || value of 5.42 (x 10-29 C・m), C35 peridinin derivative **6** showed 4.25, and C39 peridinin derivative **7** did 5.29. The || value generally shows a larger number with the increasing -electron chain length (Kajikawa et al., 2009b). Although peridinin possesses fewer conjugated double bonds and shows a rather shorter max than that of C39 peridinin derivative **7**, the || value of peridinin (**1**) was the largest among the three compounds. Thus, the C37 skeleton of peridinin (**1**) would also contribute to the large dipole moment of the molecule in the exited state to facilitate energy transfer. This would be at least a partial answer to the question of why peridinin (**1**) possesses the irregular C37 skeleton.

Fig. 12. Structure of Peridinin and its derivatives and the result of Stark spectra
