Formation of Dimer Radical Anions of Aromatic Acetylenes during Pulse Radiolysis and γ-Radiolysis

Abstract

Dimerization of the radical anions of aromatic acetylenes (A•-) such as diphenylacetylene and its derivatives with substituents on the benzene ring (1•-), 1,4-diphenyl-1,3-butadiyne (2•-), and intramolecular dimer model compounds having two diphenylacetylene chromophores linked by several methylene chains (3•-) has been studied with pulse radiolysis of A in solutions at room temperature and γ-radiolysis in rigid matrices of A at 77 K. The transient absorptions of A•- decayed with the formation of new bands assignable to the dimer radical anions of A•- and A. Because the decay and formation depend on the concentration of A, the bimolecular rate constants of kb = 7.3 × 106 to 6.6 × 107 M-1 s-1 were estimated for the intermolecular dimerization at room temperature. The spectral changes were also observed upon warming of 77 K rigid matrices of A•-. It is suggested that A•- dimerizes with A through the formation of one C−C bond between two sp carbons, giving σ-type dimer radical anions (σ-A2•-) with a diene-type structure. Absorption spectra similar to those of 1•- were initially observed in 3•- generated by the radiolyses but changed to those assignable to the intramolecular dimer radical anions similar to σ-12•-. The yield of the dimer radical anions of 3•- with a tetramethylene chain was the largest among the dimer radical anions with several methylene chains.