COMMUNICATIONSphotostationary states with ratios of M-4 and P-3 of 70:30 and 10:90, respectively (Table 1, Fig. 3). The results demonstrate that the photochemical isomerization of both P-3 and M-4 is stereospecific and also shows high wavelength-dependent diastereoselectivity. At the same time the photochemical and thermal stability of 3 and 4 towards racemization has greatly improved.Unlike other known potential optical memory systems[' -41 the key feature of the bistable molecules described here is that switching is accompanied by inversion of the chirality. These chiral compounds are suited as optical write-read-erase information storage systems.['. 61 Photochemical writing in the visible and reading by chiroptical means either in the visible (ORD) or in the UV (CD) may be possible. Furthermore an Erasable Direct Read After Write (EDRAW) process might be executed by using for instance 365nm light for reading and erasing ( P -3 e M-4 isomerizationj . The fact that the switching process occurs between photostationary states in which either of the pseudoenantiomers P-3 or M-4 are present in large excess makes this switch relatively insensitive towards racemization or degradation processes, since these processes will lower the C D signals of these states but will not change their signs. Studies on incorporating the highly selective chiral switchable molecules in polymeric films or depositing them on surfaces to develop macroscopic information storage devices are currently in progress.Experimental Procedure 8 and 9: A stirred solution of hydrazone 5 (350 mg, 1.53 mmol) in anhydrous CH,CI, (40 mL) was cooled to -30°C. whereupon MgSO, (approximately 1 g). Ag,O (530 mg. 230 mmol, 1.5 equiv), and a saturated solution of KOH in methanol (1.2 mL) were successively added. After 30 min the deep red solution was filtered into another ice-cooled flask, and the remaining residue was washed with cold CH,CI, (15 mL). To this Clear solution was added thioketone 7 in small portions. Evolution of nitrogen was observed, and the deep red color slowly disappeared. The thioketone was added until the evolution of nitrogen had ceased; a total of 200 mg (0.63 mmol) was necessary. After 3 h of stirring and a standard workup, the red residue was purified by flash column chromatography (SO,. CH,Cl,jpentane l j l . R, = 0.30). The orange crystalline product consisted of isomers 8 and 9 (60:40). (37700) 252,2 (33900) 273.0 (25500) 360.0 (5300); CD (P isomer. n-hexane. A,,, (-4.6). Irradiations of 2 x lo-' M solutions of enantiomerically pure 3 and 4 were performed in cylindrical quartz cells. Low-pressure mercury lamps (28 x 1 cm; 8 W) were used for irradiations at 300 and 350 nm. and the irradiation time was 10 s. For irradiations at 313, 365, 405, and 435 nm a 200 W high-pressure mercury lamp equipped with interference filters was employed. Typical irradiation times for this setup were 20 to 30 min.
