FORMATION OF DIMER RADICAL-ANIONS OF AROMATIC ACETYLENES DURING PULSE-RADIOLYSIS AND GAMMA-RADIOLYSIS

Dimerization of the radical anions of aromatic acetylenes (A(.-)) such as diphenylacetylene and its derivatives with substituents on the ben zene ring (1(.-)), 1,4-diphenyl-1,3-butadiyne (2(.-)), and intramolecu lar dimer model compounds having two diphenylacetylene chromophores li nked by several methylene chains (3(.-)) has been studied with pulse r adiolysis of A in solutions at room temperature and gamma-radiolysis i n rigid matrices of A at 77 K. The transient absorptions of A(.-) deca yed with the formation of new bands assignable to the dimer radical an ions of A(.-) and A. Because the decay and formation depend on the con centration of A, the bimolecular rate constants of k(b) = 7.3 x 10(6) to 6.6 x 10(7) M(-1) s(-1) were estimated for the intermolecular dimer ization at room temperature. The spectral changes were also observed u pon 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 s p carbons, giving sigma-type dimer radical anions (sigma-A(2)(.-)) wit h a diene-type structure. Absorption spectra similar to those of 1(.-) were initially observed in 3(.-) generated by the radiolyses but chan ged to those assignable to the intramolecular dimer radical anions sim ilar to sigma-1(2)(.-). The yield of the dimer radical anions of 3(.-) with a tetramethylene chain was the largest among the dimer radical a nions with several methylene chains.