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Dynamic Triplet-Excited Region in Retinal As Revealed by Deuteration Effects on the Quantum Yields of Isomerization via the T1 State (Okumura, Koyama, unpublished results)

The concept of a triplet-excited region that causes changes in bond order in the conjugated chain and triggers the CTI is now experimentally established in retinal. However, the detailed process of atomic rearrangements in the T1 excited state, resulting in the isomerization from the ground-state cis to trans configuration around a particular cis double bond, still remains to be determined. The rotational motion around a double bond must accompany sequential changes in the electronic structure, that is, sp2 fi sp3 fi sp2, during which changes in the directions of a pair of the olefinic C-H groups are expected to take place. Ohmine and Morokuma [11,12] theoretically calculated the process of isomerization in butadiene, and predicted that the rotation around one of the double bonds should accompany both the flapping of the C-H bonds attached to it and the rotation around the neighboring double bond. Stimulated by this prediction, we have examined the effects of the 7,8-, 11,12-, and 14,15-deuterations on the quantum yields of triplet-sensitized isomerization of retinal starting from the 7-cis, 9-cis, 11-cis, and 13-cis isomers, although Waddell et al. [13] showed that the 14,15-deu-teration did not affect the quantum yields of both the 13-cis fi all-trans and alltrans fi 13-cis isomerizations.

Figure 3.9 shows the effects of double deuteration of the C7=C8 or C11=C12 double bond and that of the C14-C15 single bond on the triplet-state CTI starting from the set of four cis isomers. The results can be summarized as follows: (1) The 7,8-deuteration (7,8-D2) reduces the quantum yield of isomerization from the 7-cis to the all-trans isomer that includes rotation around the particular double bond to which deuterium substitution was made, and also, the quantum yield of isomerization from the 9-cis to the all-trans isomer around the neighboring double bond on the right-hand side of the retinal molecule (see Scheme 3.1). (2) The 11,12-deu-teration (11,12-D2) reduces the quantum yields of isomerization from the 7-cis, 9-cis, and 11-cis isomers to the all-trans isomer that include rotation around the particular cis-double bond to which deuterium substitution was made, and also, that around the neighboring double bonds on the left-hand side of the molecule. (3) The 14,15-deuteration (14,15-D2) slightly reduces the quantum yield of isomerization from the 11-cis isomer. (4) Practically no deuteration effects on the quantum yields of isomerization are seen at all starting from the 13-cis isomer [13]. Table 3.1 lists the quantum yields of isomerization per triplet species generated for the undeuterated and variously deuterated retinal isomers.

Table 3.1 Quantum yields of isomerization per triplet species generated in undeuterated and deuterated retinals (Okumura, Koyama, unpublished results).

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