Nondestructive readout

As for the relevant application, recently, a specific photochromic compound, 1,2-bis(2 -methyl-5 -phenyl-3 -thienyl)perfluorocyclopentene (BP-BTE), and the analogs of HBO, 2,5-bis(5,-0 rt-butyl-benzooxazol-2 -yl)hydroquinone (DHBO), were employed in the high-contrast, reversible, photochromic switching of fluorescence emission and its perfect nondestructive readout (Fig. 14). Due to the large... 

Fig. 14 The molecular structures of BP-BTE and DHBO above). Microsized erasable ESIPT-fluorescence photoimaging on a spin-coated BP-BTE/DHBO-loaded PMMA film and its nondestructive readout capability (a) initial open-form state (b) writing (c) erasing (d) rewriting and (e) continuous nondestructive reading under irradiation with relatively high-intensity 415 nm light (200 4W cm-2) for 30 min. The dark region represents the area irradiated with the 365 nm UV light below) (reprint from ref. [88], Copyright 2006 American Chemical Society)...

Lim SJ, Seo J, Park SY (2006) Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence a unique route to high-contrast memory switching and nondestructive readout. J Am Chem Soc 128 14542-14547... 

A charge injection device (CID) is very similar to a CCD as it consists also in a 2D grid of pixels on a semiconductor substrate. The main difference between the devices is that the readout process in a CID is a nondestructive readout... 

Naphthalene-2,3-dicarboxaldehyde Nicotinamide adenine dinucleotide N-Acetylneuraminic acid 4-Fluoro-7-nitrobenzoxadiazole Naphthalene-2,3-dicarboxaldehyde Nondestructive readout Near infrared Near infrared fluorescence Nuclear magnetic resonance 2-Nitrophenyl oxalate 1,1 -Oxalyldiimidazole Polycyclic aromatic hydrocarbon Principal component analysis Photosensitized chemiluminescence Pentachlorophenyl oxalate Polymerase chain reaction... 

Recently (08JPCA4765), new thermally irreversible photochromes 156-159 containing silicon or phosphorus atoms in the five-membered bridge have been synthesized. They have fluorescence in the closed forms, whereas their open forms do not exhibit fluorescence. Hence, they are highly promising for the nondestructive readout of optical information (07MI1). Their synthesis from 2- and 3-substituted butadienes was documented (09NJC1357). 

Substituents on the reactive position greatly influence the photoreactivity. 2-Thienyl derivatives have been reported to work very well (03JA3404). The control of the photoreactivity by external stimuli such as pH or electric potential is important for the practical application of diarylethenes, because a controlled photoreactivity, namely a gated reactivity, can provide a nondestructive readout capability. Pyridyl group is one of the external stimuli-responsive substituents that are quaternized by alkylating reagent or protic acid. Dithienylethene derivatives 250-253 have been reported. 

Y is then reconverted back to X (erase). Such a write-lock-read-unlock-erase cycle could constitute the basis for an optical memory system with multiple storage and nondestructive readout capacity. 

As discussed at the beginning of this chapter, photochromic systems represent potential molecular level memory devices. A number of problems, however, must be solved for practical applications. A challenge problem is to find systems with multiple storage and nondestructive readout capacity those in which the record can be... 

How can nondestructive readout be obtained It is still a serious question. There are several ways to try. Matsui et al.107 presented a fundamental idea for a nondestructive readout method based on finding the wavelength dependence of the bleaching quantum yield of the colored form of the fulgide. An optical disk sample with photochromic compound 56d has been made. The photochromic reactions are shown in Scheme 41. 

Y. Yokoyama and Y. Kurita, Photochromic fulgides applicable to optical information storage, discovery of new nondestructive readout method, J. Chem. Soc. Jpn., Chem. Indus. Chem., 1992, 998-1006. 

G. M. Tsivgolulis, and J.-M. Lehn, Photonic molecular devices reversibly photoswitchable fluorophores for nondestructive readout for optical memory, J Chem. Soc. Chem. Commun. 1995, 1119-1122. 

Practical optical memory media should have nondestructive readout capability. Photochromic reactions, in general, proceed in proportion to the number of photons absorbed by the compounds. Such linear-response characteristics cannot be used as the basis of memory media, because recorded memories are destroyed after many readout operations. There are two approaches to avoid such inconvenience one is to introduce gated photochromic reactivity to the molecules, and another is to read the recorded memories by using light whose energy cannot induce photochromic reactions. The former approach will be described in this section. The latter approach will be discussed in Section 7.4. 

Upon addition of an acid, such as trichloroacetic acid, the absorption maximum showed the hypsochromic shift and the cycloreversion quantum yield increased as much as 10 -fold. Protonation to the dimethylamino group decreased the electron-donating ability of the substituent and resulted in the change in reactivity. This acid-gated reactivity is potentially applicable to nondestructive readout in a polymer memory medium. 

When we intend to apply organic molecular materials, especially photochromic dyes, to optical memory media, the indispensable condition is stability, both thermal and photochemical. The photogenerated isomers are required never to return to the initial isomers in the dark, even at elevated temperatures, e.g., 80 °C. In addition, the coloration/decoloration can be cycled many times while the photochromic performance is maintained, and the memory media are provided with nondestructive readout capability. Although several molecules which fulfill the former condition have been developed, some problems still remain to gain access to molecules and systems which fully satisfy the latter condition. 

FIG. 16.5 Optical system for readout of 3D memory. For nondestructive readout, a near-IR, differential phase-contrast microscope is used. 

As described in Section 16.4.5, the detection of refractive-index change between two isomers in a photochromic material is the most promising technique for nondestructive readout.It is necessary to develop a readout system that is sensitive to refractive-index distribution. For this purpose, several methods, such as phase-contrast microscope, differential phase-contrast microscope, and reflection confocal microscope configurations have been proposed. 

FIG. 17.1 I Working principles of nondestructive readout method using photocurrent detection. 

Tsujioka, T, Hamada, Y., Shibata, K., Taniguchi, A., and Fuyuki, T. Nondestructive readout of photochromic optical memory using photocurrent detection. Appl. Phys. Lett. 78, 2282 (2001). 

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

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