ELSEVIER Synthetic Metals 102 (1999) 935-936 A Highly Luminescent Polymer For LEDs B.S. Chuaha, F. Caciallib, D.A. dos Santosc, N.Feedera, J.E. Daviesa, SC. Morattia, A.B. Holmesa, R.H. Friendb, J.L. BrCdasC aA4elville Laboratory for Polymer Synthesis & Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3R.4, UK bCavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK CCentre de Recherche en Electronique et Photonique A4ol&ulaires, UniversitP de Mans-Hainaut, Place du Part, 20, B-7000 Mans, Belgium Abstract A new 2,3-dibutoxy-1,4-phenylene vinylene is reported with high photoluminescence (-40%) and electroluminescence efficiencies. The unusual substitution pattern is effective in tuning the emission colour as the polymer exhibits unexpected blue- shifted emission to the green region. Crystal structures of model oligomers (PL efficiencies -80%) and theoretical calculations on optimised geometry offer insight into these interesting properties. Keywords: electroluminescence, photoluminescence, light-emitting devices, poly(phenylene vinylene), semiempirical calculations. 1. Introduction The aim of research in the field of organic semi- conducting materials for light-emitting device applications is to produce materials with high photoluminescence (PL) quantum yields, good processibility and improved charge transporting properties [l]. Poly[(2,5-dialkoxy)-1,4- phenylene vinylenels have been established as potential polymers for such applications. They emit in the red region with reasonable PL efficiencies (ca. 15-20%) [2, 31. A new PPV deriviative, the poly(2,3-dibutoxy)- 1,4-phenylene vinylene (DB-PPV, formerly referred as PDB PPV) 4 with the unconventional 2,3-dialkoxy substitution on the phenylene ring has been investigated. The polymer exhibited a significant blue-shift compared with its 2,5- dialkoxy-substituted counterpart while high PL and electroluminescence (EL) efficiencies were also recorded. 2. Results and Discussion The homopolymer 4 was obtained through the dehydrohalogenation polymerisation route using the bis(bromomethyl)benzene monomer 3 (Scheme 1). The model oligomer compound 7 was obtained using Wittig- Horner condensation of the dimethoxy-benzaldehyde 5 and the corresponding bisphosphonate 6 (Scheme 2). The PL and the absorption spectra of 4 show significant blue shifts to the yellow-green region (Figures 1 and 2) compared with the corresponding 2,5-dialkoxy-substituted analogues. The PL efficiencies in solid state were 40% (for polymer 4) and 80% for the model compound 7. In single layer devices with polymer 4, EL efficiencies of up to 0.07 Cd/A (Ca cathode) and ca. 0.03 Cd/A (Al) were observed at drive voltages of lo-15 V. In a bilayer light emitting device with PPV as the hole transporting layer, an efficiency of 0.68 Cd/A at ea. 7.5 V was recorded. At ca. 18 V, the device emitted light with a brightness in excess of 4,500 Cd/m*. The origin of the blue-shifted emission and high fluorescence efficiency of the polymer 4 is attributed to the steric bulk of the ortho-alkoxy substituents which presumably hinder effective orbital overlap of the lone pair electrons of the alkoxy-oxygen with the aromatic rings. The twisting of the aromatic rings to a significant extent as well as the cisoid-like disposition of the styryl substituents (seen in the X-ray analysis of the oligomer 7 in Figure 3) could also contribute to the disruption of the effective conjugation length of the polymer backbone. To support this hypothesis, semiempirical calculations were performed. Based on Austin-Model 1 (AMl) geometry optimisations [4], Intermediate Neglect of Differential Overlap calculations were then carried out and coupled to a single configuration interaction scheme (INDO-SCI) to simulate the absorption spectrum of the model compound. This concluded that both the twist of the polymer backbone as well as the steric effect of the alkoxy groups are responsible for differences in the absorption spectrum compared with the spectrum expected for a hypothetical planar 1,4-disubstituted model oligomer. High luminescence efficiencies may also result from the packing manner of the conjugated backbone and consequent relaxation processes involving the lowest excited state [4]. Differential scanning calorimetry measurements of polymer 4 did not show clear transition temperatures. Polarised light microscopy study however, revealed birefringent behaviour in the film. This phenomenon was further enhanced in the lyotropic state, (1:50 wt:wt) in 1,2- dichlorobenzene, showing patterns characteristic of a nematic liquid crystalline phase. 0379-6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00965-5