Extended 7r-systems

Planar coordination compounds with aromatic ligands, especially those having extended 7r-systems, show 7r-7r interactions in the solid state. The ligands shown in Figure 8 form complexes with silver(I) which have a supramolecular structure through 7r-7r interactions.586-596... 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

A similar situation holds for the extended 7r-systems of type 17-19. Comparison of the chemical proton shifts observed in compounds of type 17-19 with those of their corresponding (benzimidazolylvinyl)pyridinium salts 36-38 (ASH, Scheme 6) reveals a remarkably constant difference, irrespective of the substitution pattern between the rr-excessive moiety... 

Different dipole moments measurements were determined for several examples of the ensemble of unconventional extended 7r-systems 17-20, and the perturbing dominance of self-association has been a serious drawback (92PC1, 92TH1). It was, however, possible to record the dipole... 

Radialene 52 has been envisaged as a precursor to hexaethynyl[3]radialene, a potential building block for carbon networks. However, desilylation under very mild conditions led to an unstable product of so far unknown identity. In this context, it should be mentioned that according to thermochemical calculations, the still unknown hexaethynyl[3]radialene has an increased conjugation energy with respect to vinylacetylene, probably because of partial relief of strain in the radialene core . A remarkable aspect of 52 is its color the crystals are deep-red and a hexane solution has a purple color. In this respect, 52 differs from the yellow radialenes 50 and 51 and resembles [3]radialenes such as 27 and 38, which have much more extended 7r-systems. 

ESR spectra were reported for various phenoxyl-type radicals with extended 7r-systems, e.g. biphenyP , tetraphenyE , naphthalene , anthracene and phenyl systems conjugated with aliphatic double bonds or triple bonds. In all these cases, the ESR parameters indicate spin density distribution over the extended 7r-system with presumably lower spin density on the phenoxyl oxygen. For example, the hfs constants (in mT) for a- and /3-naphthoxyl are ... 

With more complex carbonyl compounds, especially those containing extended 7r-systems the alternative Sn2 type of mechanism [Eq. (22) and (23)] has to be considered in addition ... 

Paracyclophane 1 is the smallest stable member of the [m,n]paracyclophane series (Scheme 1). The close proximity of the two benzene rings leads to a strong interaction of the 7r-systems. In the dibenzoannelated analogue 2 two pairs of orthogonal 7r-systems can be found. Both molecules have been used to synthesize extended 7r-systems, in which 2 allows a mutually orthogonal arrangement of their subunits. 

Because of their extended 7r-system, porphyrins and related compounds absorb visible light. Therefore, their potential application as chromophores for harvesting solar energy, as a means to mimic the natural role of chlorophyll, has been under intense study. In addition, porphyrins can act as mediators of electron transfer, both as oxidants and as reductants. Therefore, the role of porphyrins as catalysts for various reductions and oxidations also has been studied by many authors. 

Some ligands can bond in more than one way to transition elements, particularly those with more than one double bond or with extended 7r-systems. 

Only few attempts have been made so far to use conducting polymers or other organic materials as contacts. However, TCNQ has been used recently to optimize the contact resistance between a-4T and Au contacts [333, 334]. The influence of TCNQ is assumed to make a layer in which a-4T is doped by TCNQ. Due to the fact that TCNQ is a larger molecule with extended 7r-system it does not diffuse in high electrical fields. (For the influence of mobile dopants on FET characteristics, compare [335]). 

Iridium Compiexes with Extended 7r-system Ligands... 

The observation of a photoinduced electron transfer in highly extended 7r-systems focused the synthetic effort on the preparation of intramolecular donor-acceptor systems bearing heterocyclic rings. This research opens up the way to interesting photoelec-tronic chemical systems with potential applications in the field of new organic materials. 

A complementary point, following this analysis, is the hierarchy of the electronic correlations and the electron-phonon interactions. In the presence of extended 7r-systems, U and V Hubbard parameters decrease and they become about the same order of magnitude as the interactions with the phonon lattice and molecular vibrations. In such a situation a polaronic process occurs even the case of bipolarons associated with a two electron transfer has been predicted with the possible occurrence of a Bose condensation [333a]. 

Figure 13.2 shows some additional phthalocyaninato derivatives eontaining extended 7r-systems like 2,3-naphthalocyaninato- (2,3-NcM), 1,2-naphthalocyani-... 

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