Localization and migration of excess electron in aromatic substituted polyacetylenes in solution

Optical absorption spectra of radical anions of poly(1-phenyl-1-propyne)(PPPr) and poly(phenylacetylene)(PPA) were studied in 2-methyltetrahydrofuran (MTHF). The radical anions of the polymers were obtained by γ-radiolysis at 77 K (and 4 K) and pulse radiolysis at ambient temperature and 140 K. There were two different trapping sites for electrons in PPPr dissolved in MTHF. The electron produced by ionizing radiation was initially trapped by a site to form the radical anion which gave rise to an absorption spectrum with λ_max at 390 and 560 nm and a shoulder at ca 800 nm. In liquids or warmed matrices, the spectrum relaxed to a new one which was similar to that of the radical anion of trans-β-methylstyrene, a model compound of the polymer. The result suggested that an electron trapped by a shallow site was ultimately localised on a deeper site of the size of a monomer unit. In contrast, the absorption spectrum of PPA^- showed a strong IR band which was attributed to a dimer-type radical anion. It was concluded therefore that the excess electron was delocalized over two monomer units in PPA. The difference was interpreted in terms of the twisted main chain structure of PPPr arising from the repulsive interactions between bulky phenyl groups and methyl groups of the polymer.