ELSEVIER ELSEVIER Synthetic ?&.tals 84 (1997) 507-5 10 Origin and Stabilization of Photoexcitations in Conjugated Polymers W. Graupner”, S. Edera, K. Petritscha, G. Leisinga, U. Scherf6 a Institut fiir Festkcrperphysik, Technische Uniuersitdt Grat, Petersgasse 16, A-8010 Graz, Austria. b Max-Planck-Institut fiir Polymerforschung, Ackermannweg 10, D-55128 hfainz, Germany. Abstract We report studies on the nature of photoexcitations in onedimensional organic semiconductors using photoluminescence, absorption and photoinduced absorption techniques. The polymers of interest -ladder-type poly(para-phenylenes) (LPPPs) - show high intrachain order and a small distribution of effective conjugation lengths, which makes them a perfect model system for conjugated polymers. The influence of the effective conjugation length on the energetic position and the stabilization of photoexcited charged and neutral states is investigated. Interchain interactions are discussed on the base of optical spectroscopy under hydrostatic pressure. Keywords: Conjugated Polymers, Photoinduced absorption spectroscopy, Photoluminescence, UV- Vis-NIR absorption. 1 Introduction In treating the photoexcitations of conjugated polymers one always had to distinguish between intrinsic and extrinsic ef- fects. The latter are caused by chemical or conformational defects along the polymer backbone, which lead to a distri- bution of effective conjugation lengths within the samples [I]. Therefore to obtain the intrinsic properties of these polymers one either has to investigate oligomers of defined conjugation length and extrapolate to longer chains or rely on polymers of known high intrachain order. Such high in- trachain order can be induced in Poly(para-phenylene viny- lene) (PPV) by improving the synthetic route [a], or by em- bedding the PPV in polyethylene and subsequent stretch orienting [3]. For the class of poly(pura-phenylenes) (PPPs) high in- trachain order can be obtained by forcing the PPP- backbone into a planar ladder-type structure (LPPP) [4], therefore avoiding the torsional degree of freedom and sup- pressing side reactions such as branching and ortho- or meta-substitution of the phenyl rings. While oligophenyls are known for decades as laser dyes [5], the new synthe- tic route also made the polyphenyls increasingly interesting. The small Stokes shift [6] coupled with a high fluorescence quantum yield [7] and the excellent solubility due to the large sidegroups made these materials promising candida- tes for application in light emitting devices, which were in fact built [8],[9]. In the subsequent paragraphs we want to describe the photophysics of the LPPPs in terms of char- ged and neutral excited states. We will also address the question of interchain interactions by optical experiments under pressure. 2 Experimental The synthesis of the LPPPs (see Fig. 1) is described in [4]. Throughout this work we have used LPPPs of different de- grees of polymerization with a hydrogen at the Y-position in Fig. 1 and also m-LPPP, which is distinguished by a me- thyl substituent at the Y-position and very high intrachain order. R YO R’ P%l \I / / A R yO Figure 1: hydrogen CsH13. L I J R Chemical structure of the LPPPs, Y is either or a methyl group (m-LPPP), R= CrsH2l, R’= The photoinduced absorption (PIA) experiments were performed with polymer films on quartz substrates, moun- ted in an optically acccessible liquid nitrogen cryostat. The probe beam originates from a tungsten halogen lamp pas- sing through the sample and a grating monochromator (CVI CM110) subsequently, before impinging on a Si-photodiode. The pump beam, delivered by selected lines of an Ar-ion la- ser, is modulated by a mechanical chopper, which also pro- vides the reference frequency for an EG&G 5210 lockin am- plifier. While the latter measures the pump beam induced 0379-6779/97/$17.00 Q 1997 Elsevier Science S.k All rights reserved PII SO379-6779(96)04030-1