Joint Theoretical and Experimental Characterization of the Structural and Electronic Properties of Poly(dioctylfluorene-alt-N-butylphenyl diphenylamine) J. C. Sancho-Garcı ´a, † C. L. Foden, ‡ I. Grizzi, ‡ G. Greczynski, § M. P. de Jong, | W. R. Salaneck, | J. L. Bre ´ das,* ,†,⊥ and J. Cornil †,⊥ Laboratory for Chemistry of NoVel Materials, Center for Research in Molecular Electronics and Photonics, UniVersity of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium, Cambridge Display Technology, Greenwich House, Madingley Rise, Madingley Road, Cambridge CB3 0TX, United Kingdom, Thin Films Electronics, Westmansgatan 27, S-582 16 Linko ¨ping, Sweden, Department of Physics, IFM, Linko ¨ping UniVersity, S-581 83 Linko ¨ping, Sweden, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: January 27, 2004; In Final Form: March 5, 2004 Fluorene-based copolymers are currently attracting considerable interest for use in a wide range of optoelectronic devices. Here, we present the results of a joint quantum-chemical and experimental characterization of the structural, electronic, and optical properties of an alternating fluorene-triphenylamine copolymer. We compare the results from this study with those from similar studies of polyfluorene. Although calculations are performed for the gas phase and experiments are performed on the solid state, the results from the two methodologies are in good agreement: the relevant electronic levels, HOMO and LUMO, of polyfluorene are found to be destabilized by incorporation of triphenylamine units in the conjugated backbone, whereas the optical properties of polyfluorene chains are largely unperturbed by the presence of triphenylamine. 1. Introduction The understanding and control of the organic electrolumi- nescence phenomenon is undoubtedly one of the major achieve- ments of recent years. 1,2 Current materials research in the field of organic light emitting diodes (LEDs) for displays is focused on the development of new high luminescence efficiency and stable (long lifetime) materials. Poly(paraphenylenevinylene), PPV, and its derivatives are frequently the materials of choice for the active layer in organic LEDs. 3,4 More recently, poly- fluorene (F) and fluorene-based copolymers have emerged as very promising systems. The growing number of experimental 5-12 and theoretical studies 13-16 dedicated to this new class of materials all indicate their suitability in LEDs for commercial applications: highly efficient emission in the whole UV-visible range, low-operating voltage, high stability, and long lifetime. Triarylamine-based derivatives have also reached a prominent position as hole-transporting materials in OLEDs and incorpora- tion of tertiary aromatic amines into a fluorene backbone has produced copolymers with hole mobilities reaching values as high as 10 - 3 cm 2 /Vs, 17 comparable with the mobility obtained for the widely used small-molecule hole-transporting material TPD. Therefore, fluorene-amine copolymers easily compete with small molecules in terms of mobility but offer the additional advantage of solution processability; they can be deposited by spin coating or ink-jet printing. Finally, a recent study in which fluorene-amine hole-accepting copolymers were blended with electron-accepting polyfluorene derivatives showed that exciplex (i.e., a stable charge-separated excitonic state that can emit light) formation in the blend plays a key role in determining the excellent device stability and lifetime, 18 increasing interest in the properties of these materials. Quantum-chemical calculations have proven useful in gaining insight into the optoelectronic properties of conjugated materi- als. 19,20 The results of quantum chemical calculations are used here in conjunction with experimental measurements to char- acterize the structural, electronic and optical properties of an alternating copolymer containing triphenylamine and fluorene moieties, TFB. The molecular structure of TFB is sketched in Figure 1. The good agreement obtained in the results from the combined study clearly demonstrates how it is possible to design materials appropriate to the applications, by fine-tuning the key parameters prior to chemical synthesis. The paper is organized as follows: sections 2 and 3 describe the experimental and theoretical methods, the main findings are reported in section 4, and the summary and conclusions are in the last section. 2. Experimental Section The cyclic voltammograms (CV) reported here were recorded with a computer-controlled EG&G potentiostat/galvanostat at a constant scan rate of 1000 mV/s. A three-electrode configu- ration undivided cell was used. The working electrode was glassy carbon (3 mm diameter), with a Pt wire auxiliary * To whom correspondence should be addressed. † University of Mons-Hainaut. ‡ Cambridge Display Technology. § Thin Films Electronics. | Linko ¨ping University. ⊥ Georgia Institute of Technology. Figure 1. Chemical structure of polyfluorene (left) and poly(di- octylfluorene-co-N-(4-butylphenyl)diphenylamine) (right). 5594 J. Phys. Chem. B 2004, 108, 5594-5599 10.1021/jp049631w CCC: $27.50 © 2004 American Chemical Society Published on Web 04/10/2004