Redox switch, luminescent

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Goulle, V., Harriman, A., Lehn, J. M., An electro-photoswitch - redox switching of the luminescence of a bipyridine metal-complex. J. Chem. Soc., Chem. Commun. 1993, 1034-1036. 

Systems 13 and 14 represent prototypes of molecular switches of luminescence which are operated through a redox input the system consists of a luminescent unit (the light bulb of the everyday life) and of a metal-containing redox unit (the true switch) [8], The on/off situation is achieved when one of the two stable oxidation states of the metal quenches the nearby excited fluorophore and the other does not. In principle, redox switches of luminescence based on an electron transfer mechanism can be obtained by properly assembling a photoactive fragment and a metal-centered redox couple, possibly hosted by a cyclic framework. A further example based on the Cu VCu redox couple will be discussed in Section 5.4. 

True multicomponent switches are usually based on the model in Fig. 7 A [31]. One component performs the input function (e.g. by photoswitching or electroswitching), while the second acts as the output . The output component has a function (e.g. fluorescence, absorbance) that is inhibited by the input component in one of its states. Figure 7B shows a switch (15) consisting of a fluorescent difluoroboradiaza-s-indacene connected to a tertiary amine through a benzene moiety [32]. In this (unprotonated) state the excited state of the dye is quenched by intramolecular electron transfer from the amine, and no fluorescence is observed (curve a). If the amine is protonated, however, electron transfer is blocked, and fluorescence is clearly observed (curve b). Similarly, the dyad 16 (Fig. 7C) offers the redox switching of luminescence [33]. While the quinone component effectively quenches the... 

Krishna, V., Y.-H. Ho, and K. Rajeshwar. 1991. Luminescence probe and voltammetry study of ion transport during redox switching of poly(pyrrole) thin films. J Am Chem Soc 113 3325. 

Enclosure also changes the redox properties of a compound, its color, and other physical properties (1,2). On this basis nonlinear optical materials, luminescence markers, controlled light switches, and other high-tech devices might be designed and prepared (15,17,137). 

As Figure 3 illustrates, redox active guests introduce PET processes almost by definition and luminescent on-off switching is the norm. However, the inhibitions outlined in Section 5 have not prevented the designers of switchable luminescent devices from exploring systems which bind redox active guests. The combined forces of inorganic coordination chemistry and supramolecular science have proved to be too attractive in many of these instances. It is to be hoped that some of this effort will filter across to the examination of more on-off systems like 17 and 18. 

Cu(II) is one of the best examples of a redox active guest, but apparently not when it is imprisoned in a cryptand such as 53. In this case, the Cu(II) is silent over a wide potential range during cyclic voltammetry. System 53 is designed as a lumophore-spacer-receptor system such as 28-30 and 33-34 in Section 1 with multiple lumophores. It also shows similar luminescence off-on switching with and even with Cu(II). The possibility of Cu(II) induced production of from moisture appears to have been ruled out. The absence of EET is a mystery which can only be dispelled by further studies on this interesting system. 

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 anthraquinone exhibits a similar redox behavior as benzoquinone. Thus, redox luminescence switch can also be constructed with fluorophore linked to anthraquinone. For example, the luminescence of molecule 22, a ruthenium complex with an appended anthraquinone moiety, can be reversibly tuned through the interconversion between the anthraquinone and the corresponding hydroquinone.32... 

Similarly, the luminescence of complexes 38,39,40,41, and 42 can be modulated by changing the redox states of the respective metal ions. Complexes 38 and 39 show emission in the Ni(II) state, whereas the emission is quenched in the Ni(III) state generated after oxidation.46 The fluorescence due to the naphthalene unit in complex 40 is observed in the Ni(II) state after reduction to the corresponding Ni(I) state the naphthalene fluorescence is distinctly reduced.47 Water-soluble complexes 41 and 42 also works as redox-responsive fluorescence switches in a similar way 48... 

Electrochemical switching of the emission of a metal complex occurs in a system in which a luminescent (tris-bipyridine)ruthenium(ll) centre is linked to a quinone unit. Interconversion of the redox couple 124-125 allows the reversible switching... 

Another example of a photoredox molecular switch is based on a ferrocene-ruthenium trisbipyridyl conjugate, in which the luminescent form 4 switches to the non-luminescent form 5 upon electrochemical oxidation (Figure 2/bottom)171. Biological systems exploit the interplay of redox and molecular recognition to regulate a wide variety of processes and transformations. In an attempt to mimic such redox systems, Deans et al. have reported a three-component, two-pole molecular switch, in which noncovalent molecular recognition can be controlled electrochemically. x Willner et al. have reported on their research activities in developing novel means to achieve reversible photostimulation of the activities of biomaterials (see Chapter 6).[91 Recently, we have shown that it is possible to switch the luminescence in benzodi-furan quinone 6 electrochemically. 101 The reduction in THF of the quinone moiety... 

Fig. 2 Redox luminescence switching in trisbipyridyl metal complexes.

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