Creation of Chiral Organic Photochromic Dyes Applicable to Ultra-high Density Optical Memory System

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When diarylethenes, one of the representative thermally irreversible photochromic families, cylclize photochemically, two stereogenic carbon atoms are produced. When an asymmetric carbon atom is introduced to the neighbor of the hexatriene moiety, the absolute stereochemistry of the stereogenic centers generated by cyclization will become highly biased as the result of the diastereoselective photochromism. We have already synthesized compound 1 possessing an asymmetric carbon atom at the end of the hexatriene moiety, which showed such a high diastereoselective photochromic ring-closing reaction of 88 - 94% de, by applying allylic 1,3-strain as the pilot of the stereocontrol of the ground-state conformations. However, when the carbon atom connecting the benzothienyl ring and hexafluorocyclopentene ring was changed from C-3 of benzothiophene to C-2, the diastereoselectivity of the ring closure dropped to only 47%, though the change in specific optical rotation of the compound (2) was as large as 1300 degrees. We designed and synthesized compound 3 which should exhibit higher diastereoselectivity due to the larger electronic repulsion which was present in 1 but absent in 2. As the result, the diastereoselectivity of photochemical ring closure of 3 raised to 90%. The change in specific optical rotation was 953 degrees. These results were published in J. Org. Chem.When the chiral pilot substituent was attached to both of the benzothienylethenes, the diastereoselectivity (4) was as high as 98% and the change in specific optical rotation was 1429 degrees. Unfortunately, bisnaphthothienylethene 5 and the compound 6 with benzothienyl and naphthothienyl rings did not give the cyclized compound sufficiently upon UV irradiation.