Nondestructive read-out

Nondestructive read-out the detection method should not erase the stored information. 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

Control of the aggregation state in an organogel offers other attractive means for modulation of materials properties and nondestructive read-out. A photoactive gela-tor (40) was obtained by Shinkai et al.[80] by connecting 4-methoxyazobenzene through an ester linkage to cholesterol. The trans-isomer 40 formed a stable gel with... 

V. V Belov, V. A. Barachevsky, N. I. Bolondaeva, V.M. Kozenkov, N. T. Poteleshenko, A. A. Yastrebov, and P. P. Kisilitsa, Organic photochromic materials with nondestructive read-out information, in Abstracts of Papers on II All-Union Conference on Holography, 1975, Part I, pp. 50-51 (Russ.). 

Solutions could be found for many of these requirements. For instance, the use of photon-gated hole burning ensures nondestructive read out. Many materials have been discovered in which this concept could be successfully demonstrated however, attempts to unify all the requirements in one system have failed so far. 

To read out the recorded information with an adequate carrier-to-noise ratio (CNR), the medium must exhibit threshold recording behavior. The power level of the read beam can then be set at some value below threshold to guarantee nondestructive playback. For a variety of write-once media, the read power appears to be generally 3-5 times lower than the recording power. 

Berberich M, Natah M, Spenst P, Chiorboli C, Scandola F, Wiirthner F (2012) Nondestructive photoluminescence read-out by intramolecular electron transfer in a perylene bisimide-diarylethene dyad. Chem Eur J 18 13651-13664. doi 10.1002/chem.201201484... 

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