Abstract

Recently a whole class of conjugated organic polymers has been discovered, the members of which undergo a transition to a conductive state on doping with electron acceptors or donors. One of the most prominent members of this class is poly-(paraphenylene) (PPP), which exhibits a conductivity increase as high as 18 orders of magnitude on appropriate doping [1]. The basic chemical unit of PPP is shown in Fig. 1. Doped PPP and similar polymers in many respects show transport and optical properties reminiscent of those of conventional semiconductors like silicon. Undoped PPP-type polymers composed of coupled aromatic rings are very heat resistant and have already been used as materials which must stand against high temperatures. We have performed measurements of the excitation and emission spectra of the photoluminescence of PPP in the wavelength region between 200 nm and 800 nm. Samples have been prepared by two different methods, the one designed by Kovacic et al. [2], which is known to yield material with a higher degree of polymerization, and by the Yamamoto [3] method. Both synthetical routes not only result in high-molecular weight insoluble PPP, but also in products which are soluble in toluene and exhibit a much lower molecular weight. The emission spectra for the Kovacic-product show a series of three bands in the blue region of the spectrum and a broad maximum around 625 nm, whereas the Yamamoto-samples show similar bands in the blue region and an additional band around 505 nm. A comparison of these spectra with photoluminescence spectra of the commercially available oligomer p-terphenyl is shown in Fig. 2 together with the excitation spectra for p-terphenyl (dashed curve a) and for PPP (dashed curve b), which coincide for both types of samples. These measurements on two differently produced kinds of PPP allow a critical discussion of the role of impurities for the strong photostimulated luminescence of PPP.

© 1984 Optical Society of America