Photo-active units

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

Over the past 15 years, we have assisted in a huge development of the covalent chemical functionalization of the Ceo carbon sphere aimed at generating many new fullerene-based materials,6 7 in which the outstanding properties of the fullerenes would combine synergistically with those of other molecular materials, polymers, dendrons, liquid crystals, and more, in general, with photo-, electro-, or biologically active units (Scheme 9.1). 

The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. Przibilla, K.J. Lehn, J.-M., unpublished data). 

Currently, metal-polypyridine units are the molecular building blocks of choice whenever a compound with special electro- and/or photo-activity is to be designed. The research emphasis has somewhat shifted from fundamental studies of electron transfer reactivity and excited state properties of individual complexes to the design of new functional molecules and supermolecules with predetermined properties. 

Intriguing excited state electron transfer dynamics are displayed by Cu -complexed rotaxanes which combine several photo-redox active units [92, 331, 332], For example, a rotaxane [Cu(catphen)(phen-9,7-(C6o)2)] based on a Cu (phen)2... 

Polynuclear complexes, molecular dyads, triads, and other supermolecules composed of redox- and photo-active metal polypyridine units have a great promise as components of future molecular electronic or photonic devices as optical switches, relays, memories, etc. [38, 46],... 

Magnetic interactions of a paramagnetic metal center and a free radical ligand are of interest for the development of new types of molecular magnetic materials. Complexes of this type (such as (83)) change their magnetic properties upon irradiation by visible light and can serve as a basis for development of novel photo-activated memory units for electronic devices.346,347... 

In an effort to produce selective anion spectroelectrochemical reagents the Lewis acidic redox-active cobalticinium unit and the redox- and photo-active ruthenium(II) bipyridyl moiety have recently been incorporated into calixarene host structural frameworks to produce novel calix[4]arene anion receptors which are extremely rare. The calix[4]arene ditopic anion receptor (22) containing two cobalticinium moieties was prepared as shown in Scheme 4 and its crystal structure is illustrated in (Fig 7). Proton NMR titration experiments suggested (22) acts as a ditopic anion receptor forming (22) 2X"(X = Cl",Br",N03, HS04") stoichiometric solution complexes. Analogous titration... 

Sometimes light of the appropriate wavelength for initiating the above reaction with a particular photoinitiator is unavailable in sufficient intensity from the standard exposure units used. In these cases another photo-active compound is added with the photoinitiator this is usually referred to as a photosensitizer. The photosensitizer is chosen so that it absorbs the appropriate wavelength of light and also is matched to the photoinitiator in a way that it can transfer the absorbed energy to the latter, causing it to split into active radicals. Section 2.4.4(b) describes the mechanism in detail. 

Photo active iridium complexes are of potential in the organic EL device, which was also studied [83-86]. The introduction of the cationic luminescent iridium(III) complexes into negatively charged P(Glu) as a polymeric scaffold is allowed to perform the tuning of the emission properties in an aqueous media (Fig. 4.33) [87]. Increasing the ratio of the Glu unit of P(Glu) to the cationic cyclometalated... 

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

Recently, Kimura et al. have reported the encapsulation of electro chemically and photo chemically active groups such as a porphyrin unit (51) or a ruthe-nium(II) bis(terpyridyl) (52) unit in the interior of a 1,3,5-polyphenylene-based dendritic structure (Scheme 21). The dendritic porphyrins have been synthesiz-... 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

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