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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.
ZUSCHRIFTEN gen rnit nahegelegenen Methylgruppen der SiiPr,-Gruppen. Die funffach koordinierten P-Atome weisen eine verzerrt tetragonal-pyramidale Koordination auf; die Li-P-Abstande betragen 2.449-2.562(5) A, d. h. sie entsprechen denen in Lithiumdisilyl-phosphanidenr3-41.Die Struktur von 8 resultiert aus der Offnung von zwei Na-P-Kanten im hypothetischen Si,P,Na,-Rhombendodekaeder durch zwei Molekule Toluol als Donor. Toluol ist bekanntlich ein relativ schwacher Donor gegenuber Alkalimetall-Zentren. Warum bevorzugt 8 nun nicht die rhombendodekaedrische, sondern eine offene Polyederstruktur? Dafur ist im wesentlichen der GroRenunterschied zwischen Lithium und Natrium maDgeblich: Das hypothetische Si,P,Na,-Rhombendodekaedergerust hat wegen der sehr unterschiedlichen Si1-P-und Na-P-Abstande (ca. 2.2 A versus 2.8 A) und der restriktiven, tripodalen Geometrie des Sip,-Inkrements eine drastische Ringspannung, was eine Polyederoffnung pradestiniert. Dagegen sind die entsprechenden Abstandsverhaltnisse in 7 (2.229-2.275(2) 8, vs.2.449-2.562(5) A) und in 9r111 (Ge-As: 2.442-2.448(2) A vs. Li-As: 2.52(2)-2.62(1) A) offenbar derart passend, daD diese Verbindungen auch in etherischen Losungsmitteln in Donorsolvens-freier Form entstehen. Die drei inaquivalenten Na-Sorten in 8 sind wegen der besonders sperrigen Ligandensphare koordinativ ungeslttigt: Die Na-Zentren sind rnit Ausnahme von Na2 an drei P-Atome gebunden und haben kurze Kontakte mit H-Atomen von Methin-und Methylgruppen (Na-H-C-Wechselwirkungen, Na-H-Abstande: 2.44-2.54 A) von nahegelegenen SiiPr,-Gruppen; dagegen erlangt das Na2-Zentrum im wesentlichen durch Bindungen zu jeweils zwei P-und Toluol-C-Atomen seine elektronische Stabilisierung. Die Na-C-Abstande betragen 2.882(5) und 2.968(5) A. In anderen Komplexen rnit x-Koordination von Na-Zentren an Arenliganden" wurden ahnliche Na-C-Abstande gefunden.Wir prufen derzeit, ob die molekularen Alkalimetalloligophosphanide wie 4, 7 und 8 zu Wirtgittern ausgebaut werden konnen. Experimentelles 2: 780 mg (0.65 mmol) 4 (s.u.) werden in ca. 50 mL siedeiidem Toluol rnit frisch hergestelltem, wasserfreiem LiCl im UberschuB umgesetzt. Nach dem Abfiltrieren (GIV-Fritte) von LiCl in der Wirme wird die klare Losung auf ca. 10 mL eingeengt und bei 0°C kristallisiert. wobei sich 2 in mikrokristalliner Form isolieren IHBt. Ausbeute: 430mg (0.17 mmol, 55%). 4: Eine Losung von 2.33 g (6.58 mmol) I in 50 mL Toluol wird bei -30°C mit einem Aquivalent nBuLi lithiiert; die klare gelbe Losung wird auf Raumtemperatur erwirmt und init einer Losung von 1.93 g (6.58 mmol) Ph,SiCI in Toluol vetsetzt. Durch Filtration (GIV-Fritte) wird von LiCl abgetrennt und die klare Losung anschlieoend mit zwei Aquivalenten nBuLi versetzt. Die hellgelbe Losung wird ddnach 2 h aufca. 60 "CerwHrmt, um die Umsetzungzu vervollstandigen. Anschlie-Bend wird das Solvens im Vakuum vollstindig entfernt und der feste Ruckstand mit drei Portionen i 20 mL Hexan gewaschen. Umkristallisation liefert fwbloses, analy-
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