ELSEVIER Synthetic Metals 84 (1997) 293-294 New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs A. Luxa, A.B. Holme~~?~, R. Cervinia, J.E. Daviesa, S.C. Morattib , J. Gri.inerc, F. Caciallic, R.H. FriendC aDepartment of Chemistry, University Chemical Laboratory University of Ccnnbridge, Lemfield Rwd, Cambridge CB2 IEW, U.K. bMelville Laboratory for Polymer Synthesis, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3R.4, U.K. ‘Department of Physics, Cavendish Laborafory Madingley Rod Cambridge CB2 OHE, U.K. Abstract New CF+ubstituted PPV derivatives have been synthesised by Wittig-Horner polycondensation. To obtain a better understanding of the relationship between absorption and luminescence properties and structure a single crystal X-ray analysis of a model oligomer has been performed. The effect of electron withdrawing trifluoromethyl groups at vinylidene linkages in PPVs on absorption, luminescence, hole blocking and electron injecting properties has been investigated. Keywords: Wittig-Horner reaction, electron withdrawing groups, hole blocking layer, light emitting diodes 1. Introduction The discovery of semiconducting polymers which emit light as a result of double charge injection has stimulated extensive research [l, 21. Our current research interest is focused on the design of materials which operate in devices with improved efficiencies. Calculations have shown that the introduction of an electron withdrawing substituent onto the vinyl group of poly(p-phenylenevinylene) (PPV) would lower HOMO and LUMO energies of the compounds, thus permitting the construction of a device utilizing a higher work function metal [3]. The use of a high electron affinity PPV derivative containing electron withdrawing cyan0 groups as an emissive layer with PPV as hole transport layer and aluminium as cathode produced a highly efficient (4%) device [4]. We now report the synthesis of new PPV derivatives bearing electron withdrawing CFg-groups at the vinylidene unit and show that this modification of structure provides a new class of light emitting, hole blocking materials possessing a highly im- proved photooxidation stability. 2. Results and discussion A facile synthetic pathway leading to PPV derivatives incor- porating monosubstituted vinylidene linkages is given by Wittig-Horner polycondensation of aromatic diketones and diphosphonates and was reported first by HGrhold et ai. [5, 61 for the synthesis of phenylsubstituted PPVs. Following this procedure, new CF3-substituted polymers 3 were obtained by condensation of 2,5-dialkoxy-p-xylylene-bis(diethylphos- phonate) 2 with appropriate bis(perfluoromethyl)ketones 1 in refluxing toluene under argon using ‘BuOK as base, accor- ding to Scheme 1. The assigned structures of new polymers 3 were established by NMR, IR and UV-VIS spectroscopy, GPC and microanalysis data. Both polymers were isolated as yellow, solution processible powders with good film-forming properties. Although polymer 3a exhibits a fully conjugated n-electron system along the polymer backbone there is no significant red shift in the absorption maximum (Table 1) in comparison to polymer 3b in which modified distyrylbenzene chromo- 03796779/97/317.00 Q 1997 Elsevier Science S.k All rights reserved PII s0379-6779(96)04012-x 3a X= -Q- Scheme 1 phores are linked by flexible, nonconjugated spacers. This can be explained using the results of a single crystal X-ray analysis (Fig. 1) of model oligomer 4a, also synthesised by Wittig-Horner reaction according to Scheme 2. These show that the CFg-substituents not only force the lateral phenyl groups into a cis relationship but also cause them to be con- siderably twisted out of plane by an angle of 84’. ‘0uOK * - toluen., reflux 4a R=H 4b R = OCeH,, 4 Scheme 2 Fig. 1. Single crystal X-ray structure of oligomer 4a