Spin photochromic

Attempts to photoswitch the intramolecular magnetic interaction based on photo-chromism of diarylethenes have been overviewed. The switching between the disjoint and non-disjoint structures caused a change in the interaction between the separated spins. Photochromic diarylethene is one of the most favorable photoswitching units for magnetic interactions. This system has the possibility to be applied to the molecular-scale information processing system [73-77]. 

Few photochromes exhibit paramagnetism as evidenced by magnetic susceptibility and electron spin resonance measurements. In spite of this selectivity, these are probably the most powerful techniques to aid in the elucidation of mechanisms for certain photochromic systems. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Photochromism is a reversible phototransformation of a chemical species between two forms having different absorption spectra [17-20], Photochromic compounds reversibly change not only the absorption spectra but also their geometric and electronic structures. The geometric and electronic structural changes induce some changes in physical properties, such as fluorescence, refractive index, polarizability and electric conductivity. When the photochromic compounds are used as spin couplers , the magnetic interaction can be controlled by photoirradiation. 

To realize the above-mentioned systems, we carefully chose suitable switching units and radical moieties. As an initial attempt, we employed l,2-bis(2-methyl-l-benzothiophen-3-yl)perfluorocyclopentene (9a) as a photochromic spin coupler (Scheme 9.2). Compound 9a is one of the most fatigue-resistant diarylethenes [21]. Nitronyl nitroxide was chosen for the spin source, because this radical is jr-conjugative. Thus, we designed molecule 10a, which is an embodiment of the simplified model 8a [37, 62]. 

T. Hayashi, K. Maeda, and M. Takeuchi, Kinetic study of the photochromism of 2,2, 4,4, 5,5 -hexaphenyl-l,l -biimidazolyl with electron spin resonance. Bull. Chem. Soc. Jpn. 1964, 37, 1717. 

The traditional fluorescence and electron-spin resonance methods for recording molecular collisions do not allow the study of translational diffusion and rare encounters of molecules in a viscous media because of the short characteristic times of these methods. To measure the rate constants of rare encounters between macromolecules and to investigate the translation diffusion of labelled proteins and probes in a medium of high viscosity (like biomembranes), a new triplet-photochrome labeling technique has been developed (Mekler and Likhtenshtein, 1986 Mekler and Umarova, 1988 Likhtenshtein, 1993 Papper and Likhtenshtein, 2001). 

An additional step in the cascade reaction scheme is the quenching of the sensitizer triplet state with relatively low-concentration radicals (Fig. 1.5) (Papper et al 1999, 2000 Papper and Likhtenshtein, 2001). The entire investigated reaction that is shown in Fig. 1.5 is the sequence of the four kinetic processes and serves as a basis for the spin-triplet-photochrome labeling technique. This technique combines the three types of biophysical probes stilbene photochrome probe, triplet probe and stable nitroxide-radical spin probe, which depresses the sensitiser exited triplet state. 

Parkhomyuk-Ben Arye, P., Strashnikova, N. V., Iikhtenshtein G. I. (2001) Stilbene photochrome-fluorescence-spin molecules covalent immobilization on silica plate and applications as redox and viscosity probes, J. Biochem. Biophys. Methods (in press)... 

V. Malatesta, R. Millini, and L. Montanari, Key intermediate product of oxidative degradation of photochromic spiro-oxazines. X-ray crystal structure and electron spin resonance analysis of its 7,7,8,8-tetracyanoquinodimethane ion-radical salt, J. Am. Chem. Soc., 117, 6258-6264 (1995). 

Two methods could be used to prepare the disk samples. One is spin coating. Compound 96 and poly (methyl methacrylate) (PMMA) were dissolved in cyclohexanone and the solution spun coated onto the disk substrate to prepare a photosensitive PMMA thin film on the disk doped with compound 96. The second method is direct evaporation of pure compound 96 to the disk under high vacuum conditions. The structure of the photochromic optical disk sample is shown in Figure 4.3. 

FIG. 5.8 Absorption spectra of the photochromic IsoSm copolymers (A) KW40 (chloroform tion, 12 mg/l) (B) spin-coated films of (/ ) KM55, (2) SK28, (3) KW40, and (4) SK5. 

Laser flash-photolysis (LFP) studies in the nanosecond region carried out on 47 in hexane solution also suggested the presence of a triplet biradical, which, however, would not lie on the main reaction coordinate of the photochromic process, being instead involved in the dimerization of 47.49 As already mentioned, it is not possible to directly detect triplet biradicals in solutions by means of EPR spectroscopy, yet indirect indications of their presence in fluid benzene solutions of some spiro compounds50 could be obtained using the so-called double spin trapping technique.51... 

More recently, the new photochromic spin probe 74 was synthesized.67 As the paramagnetic unit is provided by a 4-(2,2,6,6,-tetramethylpiperidinyl-l-oxyl) (TEMPO) moiety, it is not unexpected that the spectral parameters of this radical, i.e., aN= 15.5 G and g = 2.0055, did not differ from those of TEMPO itself. 

B. Luccioni-Houze, P. Nakache, M. Campredon, R. Guglielmetti, and G. Giusti, Synthesis of new photochromic compounds containing a spin-trap or a spin-label, Res. Chem. Interned. 22, 449-458 (1996). 

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