Tethered electron transfer

In 1990, Wright and DeVoe [50] described a sulfonium salt with a tethered electron transfer sensitizer. 

Hyun et al. [345] prepared PbS Q-dots in a suspension and tethered them to Ti02 nanoparticles with a bifunctional thiol-carboxyl linker molecule. Strong size dependence due to quantum confinement was inferred from cyclic voltammetry measurements, for the electron affinity and ionization potential of the attached Q-dots. On the basis of the measured energy levels, the authors claimed that pho-toexcited electrons should transfer efficiently from PbS into T1O2 only for dot diameters below 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of the PbS/Ti02 assembly were consistent with electron transfer from small Q-dots. The measured charge transfer time was surprisingly slow ( 100 ns). Implications of this fact for future photovoltaics were discussed, while initial results from as-fabricated sensitized solar cells were presented. 

Ketoaldehydes and 1,4-diketones are reduced to cA-cyclopentanediols 258 1,5-Diketo compounds can be cyclized to cyclopentanediols, again with a preference for cis-diols.259 These reactions are believed to occur through successive one-electron transfer, radical cyclization, and a second electron transfer with Sm2+ ether serving as a tether and Lewis acid, as well as being the reductant. 

Fig. 4 Coupling of the redox participants to the DNA w-stack is requisite to DNA-mediated charge transport. Rapid (>109 s 1) photoinduced electron transfer occurs between the metallointercalators, [Ru(phen)2dppz]2+ and [Rh(phi) 2phen]3+, when they are tethered to opposite ends of a DNA duplex over 40 A apart. Conversely, electron transfer does not occur between non-intercalated Ru(II) and Rh(III) complexes tethered to DNA...

Fig. 5 Charge transfer occurs through the DNA /r-stack and is strongly dependent on minor base stack perturbations. Photoinduced electron transfer is observed from tethered intercalated ethidium to a rhodium intercalator bound to DNA up to 30 A away. The efficiency of electron transfer is drastically reduced in the presence of a stacking disruption, here a single base-pair mismatch... 

Other radical cyclization approaches to the synthesis of piperidines include a CAN-mediated stereoselective cyclization of epoxypropyl cinnamyl amines <06TL705> and a cyclization of (-trimethylsilylmethylamine radical cation, generated via a photoinduced electron transfer reaction to a tethered -functionality <06JOC8481>. 

The CgQ surface coverage was determined to be 2.0 10 mol cm . The monolayer can be further modified with monomeric amine reagents, which demonstrates the potential of the self-assembly process for growing three-dimensional fullerene structures. Different surfaces such as quartz. Si-oxide [105] or ITO [102] were coated with multilayers of fullerene up to stacks of 9 layers. An imidirectional electron transfer is possible across the fullerene mulhlayers [102]. Not only can multiple layers of fullerenes be connected to a certain surface but amino-functionalized can also serve as a linker between two different surfaces. 3-Aminopropyl-tethered glass plates could be linked via a Cgg layer to 3-aminopropyl covered zeolite crystals [106]. 

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

The intramolecular photoinduced electron transfer reaction of N-(o-chlorobenzyl)aniline 440 in the presence of sodium hydroxide in aqueous acetonitrile afforded, 9,10-dihydrophenanthridine and its dimer, which is reasonably explained by dechlorination from the radical anion of chlorobenzene chro-mophore followed by the cyclization (Scheme 130) [481], Similar photocyclization 9-(io-anilinoalkyl)-10-bromophenantherens 441 takes place to give spiro compounds, cyclized products, and reduction products dependent on the methylene chain length. The efficient intramolecular photocyclization occurs when the methylene tether is n = 3 [476] (Scheme 131). 

A PET reporter site tethered to the rim of the bucket allows the chemosensing strategy of Scheme 12 to be developed. In this construct, the bucket simply acts as a scaffold for the PET reporter site. The receptor is not required for analyte recognition and large changes in conformation are not required for signal transduction. Rather, electron transfer from a lone pair to the frontier orbitals of the excited reporter quenches luminescence, which is recovered by the interaction of the analyte with the lone pair (see Fig. 6). 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

Quinone functionalities appear as components in organic switches, and the coupled redox chemistry of quinones with transition metals may provide the basis for an organotransition metal switch [164]. A system that may exhibit light-induced switching was studied in the example of the quionone-tethered form of Ru(bipy)3+ [242], but the charge-separated state that results from the Ru(II) —Q electron transfer is short-lived [164,242],... 

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

The Butler-Volmer formulation of electrode kinetics [16,17] is the oldest and least complicated model constructed to describe heterogeneous electron transfer. However, this is a macroscopic model which does not explicitly consider the individual steps described above. Consider the following reaction in which an oxidized species, Ox, e.g. a ferricenium center bound to an alkanethiol tether, [Fe(Cp)2]+, is converted to the reduced form, Red, e.g. [Fe(Cp)2], by adding a single electron ... 

Willner et al. [52] have created some elegant interfacial supramolecular assemblies to address this issue by removing the non-covalently bound flavin adenine dinucleotide (FAD) redox center from glucose oxidase and immobilizing the enzyme on a tether consisting of cystamine chemisorbed on a gold surface, a pyrroloquinoline quinone (PQQ) link and FAD. The mediator potential and electron transfer distances of this assembly were carefully chosen so that transfer of electrons from the FAD to the PQQ and to the electrode is very fast. A maximum rate of 900 150 s-1 for the enzymatic reaction within this monolayer assembly was obtained, which is indistinguishable from the value of about 1000 s-1 obtained for the enzyme in solution. While monolayers can offer molecular-level control of the interfacial structure, the... 

Cyclopentanols 214 and 217 are synthesized by intramolecular ketyl radical addition to tethered olefin by a photochemically induced electron-transfer activation of 8,co-unsaturated ketones 213 and 216, respectively, using either hexamethyl-phosphoric triamide (HMPA) or triethylamine in CH3CN [98] as solvent (see Scheme 8.59). 

Aryl radical cations generated by electron-transfer processes from methoxy substituted arenes to DCN, tethered by oxygen, nitrogen as well as carbon nucleophile leads to intramolecular cyclizations (Scheme 8.67). The synthetic potentials of... 

Pandey, G., Devi Reddy, G., and Kumaraswamy, G. (1994) Photoinduced electron transfer (PET) promoted cydisations of l-[N-alkyl-N-(trimethyl-silyljmethyljamines tethered to proximate olefin mechanistic and synthetic perspectives. Tetrahedron, 50, 8185-8194. 

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