Electronic Structure and Optical Response of Electroluminescent Conjugated Polymers
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2 J.L. BREDAS', J. CORNIL" , D. BELJONNE', D.A. DOS SANTOS', Z. SHUAIV, R. SILBEY 'Service de Chimie des Mat6riaux Nouveaux, Centre de Recherche en Electronique et Photonique Mol6culaires, Universit6 de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium; 2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA 02139, USA.
ABSTRACT In this contribution, we investigate by means of correlated quantum-chemical calculations the influence of intermolecular interactions on the absorption and emission properties of conjugated chains. Various strategies are suggested to avoid a substantial decrease in fluorescence quantum yield in condensed media. Finally, the reliability of our theoretical approach is validated by showing the remarkable agreement obtained between the experimental data and the calculated optical properties of clusters formed by sexithienyl molecules. INTRODUCTION Since the first report of electroluminescence from polyparaphenylene vinylene, PPV [1], interest in conjugated polymers has been renewed and has opened the way to the fabrication of electro-optic devices exploiting their semiconducting properties and high luminescence yield. Efficient organic light-emitting diodes (LED's) [2] are now available in the whole UV-visible range [3-5] and the commercialization stage is approaching fast. Conjugated polymers are also used as active element in light-electrochemical cells [6], photodiodes [7,8], and solid-state lasers [9-11]. In spite of outstanding achievements in the field of device fabrication, a full understanding of the intrinsic electronic and optical properties of the conjugated chains is still far from being reached [12]. This is partly due to the fact that the experimental results strongly depend on the purity and/or morphology of the samples. In this context, the theoretical techniques of quantum-chemistry can prove to be useful in order to better apprehend the electronic and optical properties of the chains, for instance to characterize the nature of the excited states playing an essential role in the electro-optic devices. According to a large number of experimental studies, the most stable photogenerated species in the lowest excited states of conjugated chains are electron-hole pairs bound by Coulomb attraction and associated to a local deformation of the backbone, i.e., polaron-excitons [13]. A good insight into the properties of these species can be provided by quantum-chemical calculations performed on isolated chains, as illustrated for instance by some of our recent studies focusing on the lowest excited state in PPV oligomers (i.e., the state acting as source of light emission in LED's and lasers) [14-16] The choice of finite-size systems to modelize the properties of actual polymer samples can be justified by the often reduced degree of delocalization along the chains, resulting from the presence of physical or chemical impurities [17]. Many recent experimental studies have, however, clearly demonstrated that interchain effects can play an important role [18-23].
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