Radialenes significance

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

I. 355(4) A, 1.358(3) A. The exocyclic CC double bonds in 8 appear to be significantly longer than in 7. This is, however, not surprising, as the exocyclic double bonds in 8 are cross-conjugated with the ethynyl substituents. The experimental results available for [3]radialenes are in good agreement with the calculated results for the parent compound (Table 20). 

Dewar and coworkers39 have pointed out that an adequate description of polyenes has to be based on a set of reference molecules and reference bonds that comprises not only a C(sp3)—C(sp3) bond, but also C(sp2)—C(sp3), C(sp2)—C(sp2) reference bonds in order to separate normal cases of bond conjugation from exceptional cases which show bond delocalization. When this extended set of reference bonds is used, polyenes as well as radialenes and many cyclopolyenes are described as normal conjugated 7t-systems without any significant extra stabilization from bond delocalization39. 

By connecting double and single bonds, formally five classes of hydrocarbons can be constructed which differ considerably from one another not only chemically and physically but also in terms of their practical significance [1] the linear polyenes 1, the annulenes 2, which consist exclusively of endocyclic double bonds , the radialenes 3, polyolefins which are characterized by semicyclic double bonds, the fulvenes 4, hybrids containing endo-and semicyclic double bonds, and finally, the dendralenes 5 [2] which are acyclic cross-conjugated polyenes... 

The calculated bond distances in the C-C bonds in [60]fiillerene reveal significant differences between the [5,6]- and [6,6]-bonds with values of 1.45 and 1.38 A, respectively. Because of the mixed character of 1,3,5-cyclohexatrienes and [5] radialenes, Qo behaves as a highly strained electron-deficient olefin whose chemical reactivity is mainly driven by strain relief. Therefore, addition reactions have been widely used [13]. Interestingly, although similar reactivity patterns have also been observed for higher fullerenes, the chemical reactivity tends to decrease significantly with their size [14-16]. 

The first radialene to be reported was all-( )-7,8,9,10,ll,12-hexamethyl[6] radialene [1]. The first syntheses of the parent systems [3]radialene 2 [2], [4]radi-alene 3 [3], and [6]radialene 5 [4-6] were reported in 1965,1962 and 1977/1978, respectively, yet unsubstituted [5]radialene 4 (see below, however note added in proof) as well as monocyclic [ ]radialenes with n>6 are still unknown. On the other hand, significant research activities have been devoted recently to the so-called expanded radialenes, which have general structural formulae such as 6 and 7 ( > 1), with ethyne, butadiyne, or benzene spacers being inserted between the corners of the parent [ ]radialenes they are covered in Chapter 9 of this book. 

Nickel(0)-mediated [2-1-2] cyclodimerization reactions of [5]cumulenes 79 have been investigated in detail by lyoda and coworkers [50a, 51, 75]. Here again, the regioselectivity depends on the steric demand of the substituents and on the reactivity of the double bonds as well. Thus, [4]radialene structures 80-82 have been obtained, and in the presence of Ni(CO)2(PPh3)2 the [5]radialenone 83 was isolated as a significant by-product in some cases (Scheme 4.16). For example, tetraarylhexapentaenes form extended octaaryl[4]radialenes 80 (see 57, Scheme 4.13) treatment of tetra-ferf-butylhexapentaene with Ni(CO)2(PPhj)2... 

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