Fluorescence switches

The cycloaddition product of 7i-extended TTF and Cjq functions as a fluorescence switch (Scheme 4.3) [16]. The fluorescence can be turned off by attaching the fluorophore to Cjq, which leads to effective quenching of fluorescence. Release of the addend by heating the solution to 80 °C turns the fluorescence on since intermolecular quenching is not very effective. 

C.H. Tung, Q. Zeng, K. Shah, D.E. Kim, D. Schellingerhout, R. Weissleder, In vivo imaging of beta-galactosidase activity using far red fluorescent switch. Cancer Res. 64 (2004) 1579-1583. 

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.

Fig. 4 Effect of pH on the fluorescence switching of compound 7 in a methanol-water mixture (volume fraction [( > = 0.5]).

Green plant photosynthesis, which feeds the world, runs on photoinduced electron transfer (PET). 121 This principle was developed in chemical contexts by Albert Weller over three decades ago, 131 and became adapted for use in fluorescent switching contexts in the late 1970s and early 1980s. 14-211 A general design principle emerged soon afterwards. 221... 

In 2008, Kim H-J et al. reported a photochromic fluorescence switching of porphyrin-bridged dithienylethenes, in which two dithienylethene derivatives were axially coordinated to the two sides of porphyrinato tin complex [35]. As depicted in Scheme 7, this photochromic fluorophore (Sn(TTP)(DTE)2) was synthesized by the inter-molecular etherification of trans-dihydroxo (5,10,15,20-tetratolylporphyrinato) tin and two phenolic derivatives of 1,2-dithienylethene. 

In contrast with the dibenzo-18-crown-6 integrated fluorophore-receptor system 4,33,34 the first use of PET systems with benzocrown ether receptors arrived a decade later in the form of 22.59 Receptor 22 shows fluorescence switching on by Na+ with excellent selectivity against protons. Earlier PET systems employed azacrown... 

Recently, proton sponge was used as a fluorescent indicator for protons257. By incorporating the residue of 1 into the structure of the known fluorophore, 4-aminonaphthalimide, the researchers designed the fluorescent switch 274, whose synthesis was conducted in 5 steps with overall yield ca 10%. 

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