Benzenoids phenes

Figure 2.8 shows how the experimentally observed wavelengths for two series of cata-condensed benzenoid hydrocarbons, the linearly annelated acenes and the angularly annelated phenes, change with increasing number of benzene rings. The shifts in the Lb and Bb positions are parallel to one another in both series, whereas the bathochromic shift of the Lj, band of acenes upon annelation is so pronounced that even in anthracene it masks the Lb band. 

Here again, these thiophene derivatives are much less stable than their benzenoid counterparts, unless the ring is provided with other substitution. " The unsubstituted amino-thiophenes (thiophenamines) can be obtained by reduction of the nitro-thiophenes, "" but in such a way as to isolate them as salts - usually hexachlorostannates - or via Beckmann rearrangements " or Hofmann degradation, " as acyl-derivatives, which are stable. 3,4-Dinitration of 2,5-dibromothiophene, then reduction, produces 3,4-diaminothio-phene. " " Many substituted amines have been prepared by nucleophilic displacement of halogen in nitro-halo-thiophenes. In so far as it can be studied, in simple cases, and certainly in substituted amino-thiophenes the amino form is the only detectable tautomer. " ... 

Two other common subcategories of PAH are the acenes and the phenes. The acenes consist of benzoid rings fused in a linear arrangement, e.g., naphthalene, anthracene, naphthacene, etc. The phenes consist of benzenoid rings fused in an angular arrangement, e.g., phenanthrene, benzo[a]anthracene, etc. 

The helicenes are a subclass of phenes consisting of benzenoid rings fused in a helical arrangement. These are informally named by a numerical prefix followed by helicene. 

For the acene and phene series, the UV-vis and fluorescence spectra shift dramatically when the number of the phenyl rings increases. In contrast, the shifts for all-benzenoid PAHs with either armchair or cove -type edges are small and they show a high chemical stability. Very recently, graphitic molecules with partial zig-zag periphery such as 68a, 68b and 73 (Scheme 3.19) were synthesized. It was found that the introduction of two or six extra re-centers onto the all-benzenoid graphitic molecules dramatically influences their electronic properties, chemical reactivity and two- and three-dimensional self-assembly [64]. 

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