The naphthalene dication 3 is appreciably more unstable than the ethene and benzene analogues 1 and 2, respectively. This was revealed by oxidation experiments with 2,3,6,7-tetrakis(dimethylamino)naphthalene. Apparently, the bi-cyclic framework of 3 is so strongly planarized that a planar arrangement of the mesomeric cyanine systems is hindered by steric interaction between the neigh-boring NMe2 groups.
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Elbl‐Weiser et al. (1990) studied this question.
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