Spacers polyene

A series of conjugated polyenes capped with chromophores and containing an androstane spacer were synthesized by Wittig or Wittig-type olefinations from epi-androsterone 5150. For example, vinyl carboxaldehyde 52, prepared from 51 in 60% yield as shown in equation 32, was treated with 9-anthrylmethylphosphonium bromide and n-butyllithium to give diene 53. Exocyclic diene 53 was subsequently oxidized to vinyl carboxaldehyde 54. The androsterone vinyl aldehyde intermediate could either be treated with a tetraphenylporphyrinpolyenyl phosphonium ylide, or, as shown below, the phosphonium salt of the androsterone (55) could be reacted with TPP polyeneal 56. The desired all-(E) isomer, 57, was obtained from the ( )/(Z)-isomeric mixture by chromatographic purification. 

Figure 14. Kb values, from intervalence measurements, on a log scale, as a function of the through-space metal-metal distance +, ruthenium bipyridylpolyene complexes (series 1) , ruthenium bis(terpyridyl) complexes (series 2) , ruthenium bis(cyclometallated) complexes, dipyridylbenzene family (series 3) , ruthenium bis(cyclometallated) complexes, phenylpyridine family (series 4) A, bis(ferrocenyl)polyenes (series 5). Additional measurements a, bis(cyclometallated) complexes, phenylpyridine family with diethynylanthracene as spacer b, bis(ferrocene) with three phenylene and four vinylene units as spacer c, bis[ruthenium(trisbipyridyl)] with five double bonds as spacer. Note that the Cp (dppe)Fe system bridged by octatetrayne (13 A, 0.32 eV) is outside the graph.

Larger cyclophanes have been used for metal coordination, too [92]. A recent example is the preparation of the disilver(I) complex of a conjugated polyene with [2.2.2]paracyclophane end groups (144) [93]. The NMR spectroscopic studies and X-ray structure analysis showed that the sites of complexation are exclusively the cyclophane groups and the olefinic spacers do not overly perturb the complexation properties of the cyclophanes. 

More than a decade ago, Frank et al. (1987) reported on the light-modulated EMR ofa number of synthetic porphyrin-polyene model systems that were linked by an amide group and in some cases by methylene spacers of various lengths. The triplet transfer efficiency increased as the link between the two pigments became shorter which is what one would expect, especially for Dexter type triplet exchange... 

An efficient electron transfer through dlacetylene as a spacer group with a rate constant of 5.0 X 10 s" was reported by the same authors [526] (30). The superexchange mechanism was evidently observed in the compounds 31 by Osuka et al. [527]. The decay parameter a in Eq. (26) was observed to be as small as 0.08 A" for polyene spacers and 0.1 A" for polyyne ones, suggesting substantial contribution of the superexchange mechanism through-bond. 

While this question is difficult to answer for homopolymers such as PPV or PTV, it can be addressed more directly for model compounds representing copolymer structures. Series of bisthienylpolyenes linked by saturated methylene spacers were studied by Spangler and Liu [45]. Absorption spectra of these model structures (99-103) before and after oxidative doping show clearly that both dithienyl polyene segments are oxidized to bi-polarons and that their signature spectra are essentially indistinguishable from the model monomer, as shown in... 

Fig. 2.16 Different approaches to the realization of molecular wires, a) Polyene compound designed by Jean-Marie Lehn and coworkers and proven to conduct electrons between the Ruthenium centers, b) Polyphenylene compound synthesized by Barigelletti et al. In contrast to appearance, the benzene rings in the spacer cannot be positioned in the same plane, as their hydrogen atoms would touch. Hence the possibilities for continuous overlaps of electronic orbitals are limited and it came as a surprise that luminescence energy can be electronically transported from the ruthenium to the osmium center.

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