ELSEVIER Synthetic Metals 84 (1997) 401402 NEWLY SYNTHESIZED CONJUGATED COPOLYMERS FOR LIGHT EMITTING DIODES I. Benjamin”Tb, E. Z. Faraggi a,b,c, G. Cohenc, H. Chayet ‘. D. Davidovc, R. Neumannb and Y. Avnya aDepartment of Organic Chemistry , b Casali Institute of Applied Chemistry, c Racach Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, Israel 9 1904 ABSTRACT Copolymers derived from PPV in whjch naphthalene and 2,3,5,6-tetrafluorobenzene were incorporated into the PPV backbone were synthesized. The dark DC conductivity, electroluminescence (EL) and photoluminescence (PL) of the copolymers are reported. Keywords: Poly(phenylenevinylene) and derivatives. photoluminescence, electroluminescence INTRODUCTION The growing interest in recent years in conjugated polymers arises from recognition of their eiectro-optical properties’ and their use as organic light emitting diodes’, LEDs. A particular advantage of polymer-based LED devices is the ability to tune the color of light emitted by chemical modifications. Electronic tuning of the energy gap may be acheived by incorporatin, 0 segmers with different reduction potentials into the polymer backbone. The effect of such segmers on the HOMO-LUMO band gap can be estimated theoretically using Hiickel molecular orbital calculationsA. The present work deals with various copolymers of PPV. Copoly( 1,4-phenylenevinylene-1,4-naphthylenevinylene), co( 1,4-NV-PV), and copoly( 1,4-phenylenevinylene-2,6- naphthylenevinylene), co(2,6-NV-PV)“, copoly[ 1,4-(2,3,5,6- tetrafluorophenylenevinylene)-phenylenevinylene],G co(TFPV- PV) and ter-copoly( 1,4-phenylenevinylene, 1,4- naphthylenevinylene, 2,3,5,6-tetrafluorophenylenevinylene) co (1,4-NV-TFPV-PV), fig. I. Monomer to copolyeiectrolyte conversion, monomer to final copolymer yield, dark DC conductivity, PL and EL of these copolymers are reported. 3jgj+J$ w CO(l ,&V-PV)n co(2,6NV-PV) co(TFPV-PV) co(NV-TFPV-PV) Figure 1. Representation of copolymers RESULTS AND DISCUSSION Copolymers Synthesis The copolymers were synthesized using the electrolyte precursor technique according to the procedure developed by Wessling7 and Lenz8. The copolyelectrolytes were prepared using different comonomer concentrations. The presence of the naphthalene moiety was verified by 1H NMR spectroscopy of the copolyelctrolytes. The typical peak of naphthalene was found at 7.8 ppm. The presence 2,3,5,6-tetrafluorobenzene was verified by fluorine analysis. Conversion and yield of I,4-naphthaiene increase as a function of the amount of I ,4-naphthalene monomer. Conversion and yield of 2,6 naphthalene behaves oppositely, as the molar concentration of the 2,6-naphthalene monomer increases, thay decrease up to 6%. The 2,3,5,6-tetrafluorobenzene monomer behaves as the 2,6-naphthalene G. In the terpoiymer, maintaining the PPV monomer concentration at 4Q% and varying the I ,4- naphthalene and the TFPV monomer concentration between IO- 50% led to a constant monomer conversion of about 43%. The yield of co( 1,4-NV-PV) and the conversion to the precursor copolyelectrolyte are practically the same. On the other hand, for c0(2,6-NV-PV), co(TFPV-PV)O and the terpolymer. the conversion is always higher than the yield. The difference between conversion and final yield is due to the presence of low molecular weight copolyelectrolytes which are removed during the dialysis. This is not the case in the copolymerization of co( 1,4-lNV-PV). In fact, a difference in homopolymerization of I ,4-PNV and 2,6-PNV was already reported by Lenzg,to. 1,4- PNV was obtained in good yield9 whereas 2,6-PNV was obtained in low yield and low molecular weightto. The different reactivities in the 1,4- IX~SLIS2,6-naphthalene polymerization can be explained mechanistically. In the past, two possible mechanisms for polyelectrolyte formation have been suggested, either an ionic” or a free radical one’?. In both mechanisms a quinoid-like intermediate is required. A comparison of the quinoid-like structures of 1,4-naphthalene case I,CTSLLS the 2,6- naphthalene case shows that for the former aromiticity in one ring is retained whereas in the latter the aromaticity is totally lost. Thus, the formation of such an intermediate in the case of the I .4-naphthalene is easier. Electron withdrawtng fluorine also decreases the stability of the quinoid intermediate and leads to the same effect. The lower reactivity of these monomers is responsible both for the low molecular weight and low polymer yield. Dark DC Conductivity Figure 2 shows the dcpcndencc of the activation energy’s, Ea, on the naphchalene concentration for both copolymers, One may observe a decrease in the activation energy with increasing amounts of naphthalene in the copolymer. One can also see that the activation energy reaches a minimal value at the higher naphthalene concentrations and that the value is higher for co(l,4-NV-PV) than for co(2,6-NV-PV). The decrease of E,-with increasing naphthalene incorporation is taken as evidence for the increase in 7~ electron delocalization in both copolymers. This is expected because of the higher reduction potential of naphthalene \X~SLIS benzene. 0379-6779/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PII SO379-6779(96)039574