Abstract
Abstract The electronic spectra of TCNE complexes of acyclic and cyclic diacetylenes and cyclic tetraacetylenes were measured. They show a maximum in the narrow region 423–431 nm regardless of ring strain in the donor molecules and proximity of two diacetylene groups. Crystalline two-to-one TCNE complexes of [m.n]paracyclophadiynes were prepared, the formation of one-to-one TCNE complexes in solution being established by means of the Benesi-Hildebrand plots using various molar ratios of the two components. The electronic spectra of some 1: 1 [m.n]paracyclophadiyne–TCNE complexes were measured. The spectra suggest that the complexes are equilibrium mixtures consisting of three isomeric complexes, i.e., a diacetylene-site complex and two pseudo-conformers of benzene-site complex. The longer wavelength band of [m.n]paracyclophadiyne complexes shows a marked red-shift as compared with the other bands. This is interpreted in terms of transannular π-π interaction between the diacetylene group and the benzene ring fixed closely with each other by shortening of methylene bridges. The contribution of the three isomeric complexes on the red-shift is discussed.
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