An examination of the optical absorption spectrum of poly( p-phenylene sulphide) films when exposed to arsenic pentafluoride: comparison with poly( p-phenylene)

Abstract

The oxidative doping of poly( p-phenylene sulphide) (PPS) films with arsenic pentafluoride has been followed by transmission absorption spectroscopy in the range 0.5 to 6.0 eV. A uniform dopant distribution resulted from the doping of very thin melt-formed films, allowing resolution of the bands assigned to both the polaron and bipolaron defects. In contrast to the predictions of simple theory, the polaron levels are not symmetrically displaced about the band-gap centre, and this may be attributed to the dopant-ion ‘impurity’ potentials. Published spectra for doped poly( p-phenylene) can be similarly interpreted; in particular, the polaron levels are asymmetrically displaced about mid-gap and both materials show the transition between the two polaron levels in the gap to be unexpectedly weak. Prolonged exposure of the PPS films to AsF 5 resulted in reaction towards a poly(benzothiophene) structure. The rate and extent of this reaction was much greater for crystalline, solution-formed films than for less crystalline melt-formed films. The fully-transformed polymer had a band edge at 2.5 eV and π − π ∗ gap of 2.9 eV. Redoping of this material gave below-gap features consistent with the formation of bipolaron and asymmetrically displaced polaron states.