Photoinduced electron transfer chelation

Figure 2.3. Examples of fluoroionophores based on cation control of photoinduced electron transfer, (a) Chelators (b) coronands (c) cryptands.

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

A great variety of covalently or non-covalently boimd porphyrin-terpyri-dine motifs are known to closely mimic the initial photoinduced electron transfer (PET) or excitation energy transfer (EET) events of natural photosynthetic reactions. A first example of covalent connection of a terpyridine group to the axial position of a phosphorus porphyrin as in 33 (Fig. 26b) was presented by Kumar and Maiya [81 ]. The terpy moiety itself is a strong chelator towards other transition metal ions (see also 24). Upon postirradiation, the axial terpy subimits act as a donor and P(V) porphyrin as an acceptor. In fluorescence titrations it was found that the PET and EET reactions are mod-... 

Thr), leucine (Leu), phenylglycine (Phg), and phenylalanine (Phe), in form of tetrabutylammonium salts were used in this study. Addition of amino acid anions to solution of 21 or 22 caused chelation enhanced fluorescence quenching (CHEQ) for both anion receptors. The CHEQ effect for host 21 can be ascribed to photoinduced charge transfer (PCX), whereas for 22 quenching can be attributed to photoinduced electron transfer processes (PET). The DNB protected amino acids displayed larger... 

A series of compounds, aimed to be used as fluorescent probes for biological ions, have been synthesized and the fluorescent properties of their free and ion-bound forms were studied. The fluorescent properties of these probes are due to the extended conjugation of chromophores such as substituted coumarins and benzothiazols, whereas their ion chelating properties are due to the presence of ionophores such as [1,10]phenanthrolin-5-amine, N,N-bis(2-pyridinylmethyl)amine, mono- and polyaza macrocyclics, and polycarboxylate moieties. Based on their structural features and their spectral profile, these probes are classified as Photoinduced Electron Transfer (PET) or Photoinduced Charge Transfer (PCT) indicators. Their ion selectivity is discussed in terms of the ionophore structure and the extent of conjugation in their framework. 

An approach, similar to that of the chelated heme model systems, has been adopted by many researchers to prepare porphyrins with appended quinone groups. Such systems have stimulated interest as possible models for the primary electron transfer event of photosynthesis, where photoinduced charge transfer occurs from excited singlet state chlorophyll donors to nearby quinone acceptors. 

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