Abstract
Strong constant spectral shifts are recorded as stated in the preceding chapters when benzene rings are fused together to form the acene or phene series. These shifts are constant and from them an order number K or K p for each member in the annellation series may be derived. In striking contrast to this, very different spectral shifts are produced by the successive connection of two diphenyl systems to benzene (I) forming triphenylene (II) and tetrabenzanthracene (III), which is formally a symmetric annellation. The first shift is much bigger than the second one. The nature of this electronic asymmetry becomes obvious if the second shift is deducted from the β-band (in √Å) of triphenylene (II). This means the elimination of the influence of the angular rings. The result is a β-band at 2237 Å, which is almost identical with the β-band found in the absorption spectrum of naphthalene which is at 2210 Å in alcoholic solution. It is obvious that one of the two annellations produces an induced benzenoid ring, marked “Bz”, and extends the central aromatic system of the benzene ring to a naphthalene system, whilst the second annellation does not extend it to an anthracene system but forms an empty ring, marked “E”. It must be concluded therefore that each inherent benzenoid ring (as indicated by the circles) can transfer two π-electrons only to the induced benzenoid ring (Bz), a conclusion which was reached in the preceding chapters for different reasons. It appears that the formation of an induced π-electron sextet (Bz) has preference over a symmetric distribution of π-electrons. If this interpretation is correct then the same principle should apply to the central acene system in a higher tetrabenzacene, without regard to the number of rings. There is only one sextet in an acene independent of its length and accordingly only two transferable π-electrons able to migrate from one ring to the other.
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Clar, E. (1964). Asymmetric Annellation Effects. In: Polycyclic Hydrocarbons. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-01665-7_8
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DOI: https://doi.org/10.1007/978-3-662-01665-7_8
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