Aromaticity acenes

Figure 2.8. Wavelengths of Lb, L and bands of condensed aromatic hydrocarbons plotted versus the number of benzene rings a) linearly annelated aromatics (acenes) b) angularly annelated aromatics (phenes) (by permission from Badger. 1954).

In 1993, Scherf and Chmil described the first synthesis of a ladder-type poly(pflra-phenylene-czs-vinylene) (116) [138]. On the one hand, ladder polymer 116 represents, a planar poly(phenylene) containing additional vinylene bridges on the other hand, it is a poly(phenylenevinylene) with aryl-aryl linkages in the polymeric main chain. The target macromolecules, as fully aromatic ladder polymers, are composed of all-carbon six-membered rings in the double-stranded main chain (an example of angularly annelated poly(acene)s). 

The present section is organized as follows. Firstly, the reactivity and aromaticity of the different rings that compose an acene system as a reactant is analyzed, and secondly, the aromaticity of the TS structures of pericyclic and pseudopericyclic reactions is discussed. 

We earlier recalled that one can build a huge number of molecules by simply assembling CeH rings, a scheme that was summarized in Table 1.4, and that a solid such as graphite can be obtained in this gedanken synthesis. Let us briefly discuss the case of linear acenes, which are linear polycyclic aromatic hydrocarbons (PAHs) composed of laterally fused CeHe rings. 

Fused aromatic systems are named by prefixing the largest parent trivial names with combining forms such as benz(o)- and naphth(o)-. Hydrocarbons that contain five or more fused benzene nngs in a linear arrangement aie named fiom a numerical prefix followed by -acene. ... 

Clar 1957-1960 Spectra of acenes, aromatic sextet rules... 

Anthracene is the smallest linearly-fused aromatic species considered to exhibit true acene character (Fig. 14.2) and has been the smallest acene typically studied for electronic materials applications [12]. Single-crystal anthracene does exhibit relatively high intrinsic hole mobility, with values as high as 3 cm2 XT1 s 1 measured by the time-of-flight technique and as high as 0.02 cm2 V-1 s 1 measured from a FET constructed on a single crystal [13], Films of functionalized anthracene derivatives have also yielded FETs with reasonable hole mobilities (see... 

Oxidation of anthracene or higher acenes also occurs in the absence of a dye, since the same substrates act as sensitizers in singlet oxygenation [9c,42a]. These compounds react mainly at the mexo-positions of the aromatic nucleus [49a], but electron-donating groups have a strong directing effect when placed at the 1- and 4-positions (Sch. 33) [49b]. 

The thermal stability of the endoperoxides of aromatic compounds enhances going from the, only spectroscopically detectable, endoperoxides of benzene derivatives to those from acenes, which are stable enough to be stored for several days. The main rearrangement is reversion to singlet oxygen and the starting materials and depends on the aromatic system and on the type of substituents in the meso positions when they carry the... 

Similar endoperoxides108 of anthracene189 to hexacene170 and of many meso-substituted aromatic hydrocarbons are known.9,10,188 The nature of the meso substituent has a strong effect on peroxide formation. Endoperoxides are not formed by compounds with cyano, carboxy, or carboalkoxy substituents in the meso position.171 Endoperoxides, e.g., 165, have also been obtained from acenes containing a condensed heterocycle.172-174... 

The names of polycyclic aromatic hydrocarbons containing live or more fused benzene rings in a straight linear arrangement are formed by a numerical prefix followed by -acene. 

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