ELSEVIER Synthetic Metals 69 (1995) 53-54 Film-thickness dependent optical properties of Durham truns-polyacetylene W. Graupner”, M. Mauria, H. Kreimaier”, G. Leising”, W. Fischer*, F. Stelzer* a Institut fiir Festkiirperphysik, Technische Universitgt Graz, Petersgasse 16, A-8010 Graz, Austria. * Institut fiir Chemische Technologie Organischer Stoffe, Technische Universitlt Graz, Stremayrgasse 16, A-8010 Graz, Austria. Abstract We present absorption measurements in the visible (Vis) and infrared (IR) spectral range of Durham trans-polyacetylene (PA) with the film thickness varying between 4 - 240 nm. Photoinduced absorption (PIA) measurements support our finding of improved order in the samples with decreasing sample thickness. We conclude this from an apparent narrowing of the distribution of the effective conjugation lengths (&jr) and a shift towards longer l,,,ff for films thinner than 60 nm. The results are interpreted by comparing the experimental facts to theoretical simulations based on the experimentally determined dielectric function of Durham trans-PA. 1. INTRODUCTION From measurements on polyenes [l]-[3] and early theo- retical work [4] it is well known that the energetic position of the it - x* transition increases with decreasing conjugation length. Samples of PA are characterized by a distribution of eflectiue conjugation lengths [5]. The width of the absorp- tion peak is therefore a measure for the breadth of this dis- tribution. In contrast to the degree of polymerization which determines the length of a PA chain, I,,,ff describes the length of a PA segment along which the delocalization of r-electrons can take place [6]. In this report we want to de- scribe the scaling of the thickness of PA samples of varying optical density (OD) with the optical properties. 2. EXPERIMENTAL The PA precursor polymer is prepared via ring-opening metathetical polymerization of 7,8-bis-(trifluoromethyl)- tricycle-[4,2,2,0]-deca-3,7,9-triene (BTFM-TCDT) [7] in chlorobenzene using a Mo(CHtBu)(NAr)(O-C(CF3)2CH3)2 [s] system as catalyst. The macromolecules consist of about 800 conjugated double bonds with a polydispersity index (PDI) of 1.1 (from size exclusion chromatography). Films are cast from a solution in methylethylketone on quartz, sapphire, KBr, Si and NaCl substrat.es to exclude any influence of the substrate on the observed phenomena. The precursor polymer is thermally converted to trans-PA by heating at 80°C for 40 minutes and at 14O’C for 20 mi- nutes. The samples are always kept in an Ar-atmosphere or dynamic vacuum. The measurement techniques are described in detail el- sewhere [9]. To obtain transmission, absorption and reflec- 0379-6779/95/$09.50 0 1995 Elsevier Science S.A. All rights resen SSDZ 0379-6779(94)02360-B tance of the system polyacetylene-substrate the PC- programme FilmstarTM was used. The surfaces and the interface sample/substrate are assumed to be ideally planar. Interference effects however are only taken into account wit- hin the PA layer. The input parameters are the thickness of sample and substrate as well as their dielectric functions. To check the influence of the substrate we investigated optical glass (BK7) and quartz. The dielectric function (ci,c~) of unoriented Durham trans-PA is taken from Ref. [lo]. The dielectric function (cl,cz) of SiO2 and BK7 are from [ll] and provided by the software respectively. 3. RESULTS AND DISCUSSION 25000 ( 1 s Y “, 20000 e s is 2 15000 .9 > 0 1 2 3 4 5 Optical Density Figure 1. Parameters up (O), AD (O), v,, (H) and fii (x) versus the optical density of PA samples. Lines should guide the eye. red