Redox fluorescence switch

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

Oxidation of TTF and its derivatives induces the transformation from neutral species into cationic ones, namely, cation radicals (TTF +) and dications (TTF2+). Moreover, TTF, TTF +, and TTF2+ exhibit different absorption spectra. Taking these advantages of TTF new TTF-based redox fluorescence switches and chiral switches have been recently reported. 

Ferrocene has been widely investigated as an electron donor and its electron donating ability can be tuned by redox reactions. As anticipated, when a ferrocene unit is covalently connected to an electron acceptor moiety that shows intrinsic fluorescence, the fluorescence of the acceptor moiety would be largely quenched because of the photoinduced electron transfer between ferrocene and the fluorescent acceptor. For instance, triad 15 that contains perylene diimide flanked by two ferrocene moieties, shows rather weak fluorescence due to the photoinduced electron transfer between perylene diimide and ferrocene units. Either chemical or electrochemical oxidation of ferrocene unit lead to fluorescence enhancement. This is simply because the electron donating ability of ferrocene is reduced after oxidation and accordingly the photoinduced electron transfer is prohibited. In this way, the fluorescence intensity of 15 can be reversibly modulated by sequential electrochemical oxidation and reduction. Therefore, a new redox fluorescence switch can be established with triad 15.25... 

The efficient on/off switching of fluorescence from substituted zinc porphyrin-ferrocene dyads 16a and 16b is achieved through redox control of the excited-state electron transfer quenching.26 This redox fluorescence switch is based on the switching of the excited-state electron transfer from the ferrocene to the zinc porphyrin through the use of the ferrocene/ferrocenium (Fc/Fc +) redox couple. 

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... 

Nicotinamide is an important redox moiety in biological system. The nicotin-amide-perylene diimide dyad 23 can work as a redox-responsive fluorescence switch.33 Dyad 23, in which nicotinamide is on the oxidation state, exhibits strong fluorescence. However, it becomes nonfluorescent when nicotinamide is reduced due to the electron transfer from the reduced nicotinamide to the photoexcited perylene diimide. The fluorescence of dyad 23 can be reversibly switched off and on chemically by successive reduction with NaBH3CN and oxidation with tetrachlorobenzoquinone and switched electrochemically over 10 cycles without significant degradation. 

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... 

Scheme 15.8 The photo- and redox-controlled fluorescence switch based on bis-thiaxanthy-lidenes 49 and 50.

Keywords Molecular movements, Metal translocation, Scorpionate complexes, Fluorescence switches, M(III)/M(II) and M(II)/M(I) redox couples... 

Mangano C. (1998) Molecular Switches of Fluorescence Operating Through Metal Centred Redox Couples, Coord. Chem. Rev. 170, 31-46. 

Another redox switchable system is based on dyad 21 in which 2-chloro-1,4-naphthoquinone is covalently attached to 5-dimethyl-aminonaphthalene via a non-conjugated spacer. The intrinsic fluorescence of the dansyl excited state in dyad 21 is strongly quenched, due to the intramolecular electron transfer from the excited dansyl to the adjacent quinone acceptor. However, the fluorescence can be switched on by addition of a reducing agent. Apart from chemical switching, the fluorescence of dyad 21 can also be switched electrochemically. This can be realized using a photoelec -trochemical cell, and the solution starts to fluoresce upon application of a reductive potential.31... 

Fabbrizzi s S[36] is unusual in using a nonmetallic, redox-active guest with a metal center serving as the receptor. An anionic nitrobenzoate guest is held by coordination of the carboxylate to the free apical position in Zn2+. The electron-deficient nitrobenzoate engages in PET with the anthracene fluorophore to switch the fluorescence OFF . 

Bergonzi R, Fabbrizzi L, Licchelli M, Mangano C. Molecular switches of fluorescence operating through metal centred redox couples. Coord Chem Rev 1998 170 31-47. 

The ability of tetraazamacrocycles (106) to stabilize nickel ions in the +3 oxidation state has been employed to demonstrate redox switching of fluorescence. The wavelength of the emitted light is controlled through the choice of pendant group... 

The effect of the metal oxidation state on the emission intensity has been also investigated by performing controlled potential exhaustive electrolysis experiments. When an MeCN, poorly emissive solution of the Cu complex is reduced cathodi-cally to the corresponding colorless Cu species, a strong fluorescence enhancement is observed. As the redox process is fully reversible, fluorescence can be switched on/off at will, by setting the potential of the working electrode at the proper potentials for Cu -to-Cu (on) and Cu -to-Cu (off ) changes to take place. 

Figure 31 Redox switching of the dansyl fluorescence of the two-component system illustrated in Figure 30. The Ni" derivative, in aqueous ethanol, displays the dansyl emission (solid line) on addition of S20g , quick one-electron oxidation to the Ni" derivative takes place and fluorescence is quenched (dashed line) on subsequent addition of N02, Ni " is reduced to Ni" and fluorescence is revived (dotted line).

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... 

Fig. 7. A Schematic representation of a multicomponent system consisting of linked input and output units. B A pH-input fluorescence-output molecular switch. C A two-component redox-input fluorescence-output molecular switch. D A four-component redox-input fluorescence-output molecular switch. The fluorescence spectra in B, C and D are adapted from [32], [33] and [34], respectively, with permission...

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