Rigid spacer

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

If the ligand is not directly linked to the porphyrin periphery but is held at a distance from the chromophore by a rigid spacer, more open... 

Similar work was carried out with ligands featuring different flexible or rigid spacers (X = -C(0)0-, -C=C-, -CH2-CH2-) and terminal alkoxy groups where n = 9-12. These compounds and the carbene complexes derived therefrom by addition of alcohols also show mesogenic properties (Scheme 49).211 212... 

Okayama, Japan. The synthetic procedures for S-D dyads and their reference compounds shown in Fig. 6 are illustrated in Fig. 7, where a naphthalene and a ferrocene moiety are used as an S and a D moiety, respectively. For comparison of photoinduced electron transfer rates between a single alkyl chain and a triple alkyl chain as the spacers of the S-D dyads with the same length of four-carbons, S-D dyads with a rigid spacer with a bicyclo[2.2.2]octane were synthesized [39]. The synthetic procedure for the S-D dyads with the rigid spacer is also shown in Fig. 8 [39], Other amphiphilic compounds and chemicals were commercially available. 

Figure 8. Synthetic routes for S-D dyads with a rigid spacer consisting of a bicyclo[2.2.2]octane.

Figure 9. Top Two tetraurea calixarene monomers connected by a rigid spacer at their bottom rim display diverging hydrogen-bonding sites ideally suited for polymerization. The polymer (53) bears capsules of ca. 1.6 nm x 2.2 nm dimensions like beads on a string. Bottom Photomicrographs of typical Schlieren textures of 53 in chloroform (top row, left) and p-difluorobenzene (top row, middle) as viewed...

Molecular wires conduct an electrical signal (which could be just one electron) between two connected components over a long distance. This function can be brought about by linking a donor and acceptor by means of a rigid spacer. 

Fig. 12 Left. ICT-PET probe 34 with a small and rigid spacer (thick black), decoupling the PET active donor = receptor unit from the 7c-conjugated ICT fluorophore. Middle and right probes 35 and 36 combining a conventional ICT and a virtually decoupled ICT process with two identical receptor units (35, for Hg2+) and two different receptor units (36, tetraoxa monoaza crown for Na+ and dithia monooxa monoaza crown for Ag+). For color code, see Fig. 10...

The Forster resonance energy transfer can be used as a spectroscopic ruler in the range of 10-100 A. The distance between the donor and acceptor molecules should be constant during the donor lifetime, and greater than about 10 A in order to avoid the effect of short-range interactions. The validity of such a spectroscopic ruler has been confirmed by studies on model systems in which the donor and acceptor are separated by well-defined rigid spacers. Several precautions must be taken to ensure correct use of the spectroscopic ruler, which is based on the use of Eqs (9.1) to (9.3) ... 

The donor and acceptor molecules can be covalently linked by a flexible or rigid spacer, conjugated to macromolecules, or simply mixed together. To illustrate some features of these approaches, it may be useful to display some simulated results. 

To search for the inverted region, experiments were designed so that the donor-acceptor distances were kept fixed by attaching them to a covalent network of rigid spacers, frozen media, electrostatic com-plexation, and protein frameworks. In all of these cases, the... 

The tether directed cyclopropanations of Cjq discussed so far take advantage of specifically designed open-chain malonates with quite rigid spacers. This is a particularly suitable method for the synthesis of bisadducts. However, for an entry to tris- and higher adducts the design of a suitable steric arrangement becomes more and more difficult... 

The validity of the Forster theory was tested and confirmed in a number of model studies with compounds that contained a donor and an acceptor separated by well-defined rigid spacers. This work has been reviewed 5] In a classical study, a naphthyl group (donor) was attached to the C-terminal and a dansyl group (acceptor) to the N-terminal of poly-L-proline oligomers (1-12 proline residues) 61 These proline oligomers assume a trans helical conformation in ethanol and thus represent spacers of well-defined length (12-46 A). A continuous decrease in the transfer efficiency from 100% at a donor-acceptor separation of... 

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