JOURNAL OF MATERIALS SCIENCE LETTERS 9 (1990) 1296-1299 X-ray photoelectron spectroscopy studies of electrically conducting phenothiazine, phenoxazine and quinoxaline ladder polymers H. S. NALWA Department of Material Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Electrically conducting organic polymers have been a subject of intense research activities over the last decade because of their unique electronic properties [1]. Organic polymers that could be doped to metallic regime upon treatment either with electron acceptor or electron donor species possess a highly conjugated re-electron backbone. These conjugated polymers can be divided into two classes on the basis of charge- carrying species involved in conduction process [1-3]. Polyacetylene has a degenerate ground state and charge-carrying species have been modelled as solitons [4]. On the other hand, polypyrrole, polythiophene, poly(p-phenylene), and poly(p-phenylene sulphide) have a nondegenerate ground state and polarons and bipolarons have been considered as the charge- carriers [1, 5]. Heteroaromatic ladder polymers are another class of conjugated conducting polymers. Electron spin resonance (ESR) and optical studies of benzimidazobenzophenanthroline-type BBL polymer indicate the presence of charged defects referred to as polarons [6]. Pace and Kim [7] have suggested the solitonic contribution in polyphenothiazine ladder polymer. Polyphenothiazine (PTL), polyphenoxazine (POL), and poly-quinoxaline (PQL), as illustrated by the following chemical structures, are three represen- tatives of heteroaromatic ladder polymers. All these ladder polymers have conjugated backbone desirable to obtain high conductivity like polyacetylene and other conducting polymers. Kim [8, 9] investigated "NU " T NC Sy ".S/~ \ N / ~ S.-"~"~...--"-.N~ ~ \ PTL H H POL J I I H U PQL electrical conductivity of heteroaromatic ladder polymers including benzimidazobenzophenanthroline type BBL and BBB polymers. Phenothiazine, phen- oxazine and quinoxaline ladder polymers have nearly two dimensional structure and their electrical con- ductivities are influenced by the nature of hetero- atoms (X = S, O and NH). For example, electrical 1 296 conductivity of PTL polymer is about two orders of magnitude higher than that of POL polymer because of the large polarizability of sulphur atoms as compared with oxygen atoms [9]. It has been reported by Kim that reaction medium, i.e., solvent such as polyphosphoric acid has a profound effect upon the electrical conductivities of ladder polymers [9]. The possible contamination of polyphosphoric acid increases the electrical conductivity up to two orders of magnitude in ladder polymers, in case if polyphosphoric acid was not completely extracted, therefore, the importance of polymer-solvent charge- transfer interactions has been realized. X-ray photoelectron spectroscopy (XPS) has been found useful for studying electron acceptor-donor interactions in electrically conducting polymers such as polyacetylene [10], polypyrrole [11], polythiophene [12], polyaniline [13], and poly(p-phenylene vinylene) [14]. XPS-technique can detect redistribution of charges in the charge-transfer complexes. In the present study, XPS-technique has been employed to understand the role of polyphosphoric acid solvent. This communication reports the XPS studies of the charge-transfer interactions in PTL, POL and PQL ladder polymers. The nature of polymer-solvent interactions involved with the trend of electrical con- ductivity of PTL, POL and PQL polymers is discussed in accordance with XPS spectroscopy. Polyphenpothiazine was obtained by condensing 2,5-diamino-l,4-benzenedithiol with 2,5-dichloro-p- benzoquinone while polyphenoxazine was synthe- sized from 2,5-dichloro-p-phenylenediamine with 2,5- dihydroxy-p-benzoquinone. Polyquinoxaline was prepared from 1,2,4,5-tetraaminobenzene and 2,5- dihydroxy-p-benzoquinone. The polycondensation reactions were performed in polyphosphoric acid solvent under an inert atmosphere at 250 °C. The preparative methods of PTL and POL have been des- cribed in detail by Kim [9] and for PQL by Stille et al. [15]. All of the polymers were black powders and are insoluble and infusible materials. Polyquinoxaline did show sensitivity to moisture while polyphenothiazine and polyphenoxazine are comparatively stable in air atmosphere. Sensitivity to moisture occurs due to the presence of nitrogen in polymer backbone. X-ray photoelectron spectra of powdered samples were obtained by an X-ray photoelectron spectrometer (model 5100, ESCA System, Physical Electronics) using a MgK~ (1245.90 eV) radiation source. Electrical conductivity measurements of the pressed pellets were carried out by a conventional two probe method. The electrical conductivities of PTL, POL and PQL 0261-8028/90 $03.00 + .12 © 1990 Chapman and Hall Ltd.