Protic-switching

Figure 7. Protic-switching of cubic NLO response.s in ruthenium cr-acetylide/vinylidene complexes. The imaginary component of the second hyperpolarizability y i.s related to cr, [112]...

While many metal centers can be reversibly cycled between two (or more) oxidation states, few organic moieties can match such reversibility especially in protic media. Nevertheless, the first supramolecular example of an electroswitch-able luminescent device involved the benzoquinone-hydroquinone couple. The luminescence of 55 " is switched off due to PET in the benzoquinone state of the redox couple. Electrochemical or chemical reduction of the benzoquinone under protic conditions to hydroquinone recovers the luminescence of the tris(2,2 -bipyridyl) Ru(II) unit. It is noted that the luminescence of tris(2,2 -bipyridyl) Ru(Il) itself is electroswitchable. Indeed tris(2,2 -bipyridyl) Ru(II) came to fame as a solar energy material from more humble beginnings as a luminescent redox indicator. However 55 achieves the same switching at a lower magnitude of reduction potential. Here lies the advantage of the supramolecular design. Like tris(2,2 -bipyridyl) Ru(II), many lumophores show electroswitchable luminescence. An... 

The interaction with the cation noticeably affects the reactivity of the anion in media of low dielectric constant. Thus, on switching from the large tetra u-butylammonium to the small lithium cation, the reactivity sequence of the corresponding halides in the nucleophilic substitution of n-butyl 4-bromobenzenesulfonate in the weakly dissociating acetone is completely reversed (Eq. 3). Whereas the order obtained with bulky quaternary onium salts (Cl >Br >I ) corresponds to that of free, nonassociated halides in dipolar non-HBD solvents (e.g.. acetone), the sequence of the lithium halides (Cl
protic solvents, is mainly due to the increasing deactivation of the anion in the ion pair on increasing the cation charge density ... 

Activation of the contractile machinary by means of the interaction of Ca " with Troponins (TN) is amenable to a new interpretation in the framework of the proto-osmotic approach. It is suggested that the above interaction serves to establish the hydrogen-bonded systems of F-actin. Each TN overlaps two G-actins and is thus in a position to interfere with their mutual linkages and break the protic circuit of F-actin in the absence of Ca ". The interaction of Ca " " with TN-C and of the latter with TN-I and TN-T may restore the normal linkages between the affected G-actins and switch on the circuit of H along the F-actin. All other modes of activation are also suggested to be by means of the establishment of proton conduction paths in the myofilaments, from SI to the Mline and the Z disc. 

Switching the cubic nonlinearity of ruthenium alkynyl complexes by a protmi-ation/deprotonation sequence (via a vinylidene complex) was demraistrated by fs Z-scan studies at 800 nm several years ago [41]. Recently, protic and electrochemical switching were demonstrated in the ruthenium alkynyl cruciform complex 11 for which distinct linear optical and NLO behavior were noted for the vinylidene complex and the Ru(II) and Ru(III) alkynyl complexes [42]. Because the oxidation/ reduction and protonation/deprotonation procedures are independent, this system corresponds to switching by orthogonal stimuli. 

Implementing this concept, we have successfully found reaction conditions for two independent routes to access either 2,3-syn-107 or 2,3-syn-108 selectively, and our results are collected in Table 10.4. First of all, contrary to our ejq)ectation, addition of 2,3-antz-106 to an aprotic THF solution of tert-BuOK at 0 °C (conditions A) led to formation of 2,3-syn-108 rather than 2,3-syn-107 in excellent chemical yields as well as selectivity starting from the epoxides with (Z)-geometry (entries 4-6). However, clean results were not obtained from (E)-epoxides (entries 1-3). On the other hand, switching to the protic solvent MeOH (conditions B) yielded the anticipated products 2,3-syn-107 in high yields as well as excellent diastereoselectivity (entries 7-12). 

Protic ILs can be switched to the neutral amine or imidazole in the presence of a base... 

Bryce-Smith, D. and Gilbert, A, The organic photochemistry of benzene. II, Tetrahedron, 40, 2459,1976 Ohashi, M., Yoshino, A., Yamazaki, K., and Yonezawa, T, Use of the zinc chloride in the 1,2-photoaddition of acrylonitrile to benzene. Tetrahedron Lett., 3395, 1973 Tada, M., Saiki, H., Miura, K., and Shinozaki, H., The photoreaction of benzyl 1-cycloalkenyl ketone in acidic or protic media. Bull. Chem. Soc. Jpn., 49, 1666, 1976 Mori, X, Wada, X, and Inoue, Y, Perfect switching photoreactivity by acid photochemical decarboxylation versus transesterification of methyl cyclohexanecarboxylate, Org. Lett., 2, 3401,2000. 

Protic tautomerism [1], which involves reversible intramolecular transfer of protons, usually between electronegative atoms, is an important feature not only of biologically relevant heterocycles [2] but also of a variety of potentially useful synthetic systems [3]. For instance, several current research themes involve the study of tautomeric processes as memory elements or as molecular switches with a view to developing future molecule-scale devices [4, 5]. 

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