Optical Spectroscopic Studies of a Soluble Fluorene-Based Conjugated Polymer: A Hydrostatic Pressure and Temperature Stu
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Optical Spectroscopic Studies of a Soluble Fluorene-Based Conjugated Polymer: A Hydrostatic Pressure and Temperature Study S. Guha1, J.D. Rice1, C. M. Martin2, W. Graupner3, M. Chandrasekhar2, H.R. Chandrasekhar2 and U. Scherf4 1 Dept. of Physics, Astronomy and Materials Science, Southwest Missouri State University, Springfield MO 65804 (USA). 2 Dept. of Physics and Astronomy, University of Missouri, Columbia MO 65211 (USA). 3 Dept of Physics and Astronomy, Virginia Tech, Blacksburg, VA 24061; Present address: Austriamicrosystems, AG; Schloss Premstaetten, A 8141 Austria. 4 Institut für Physikalische und Theoretische Chemie, Universitaet Potsdam, Germany. ABSTRACT Spectroscopic properties of conjugated molecules/polymers have varying degrees of sensitivity to backbone conformation. Optical studies are presented as a function of temperature and hydrostatic pressure, using photoluminescence and Raman scattering from two polymers with distinct differences in their backbone conformation, namely, polyfluorene (PF) and ladder type poly(para-phenylene)(m-LPPP). In contrast to the photoluminescence (PL) vibronics in mLPPP, the 0-0 PL vibronic peak in PF shows a red-shift with increasing temperatures. Pressure studies reveal that the PL spectrum of PF red-shifts and broadens with increasing pressures. The phonon lines in PF show an antiresonance effect at higher pressures indicating a strong electronphonon interaction. INTRODUCTION Conjugated organic molecules such as short-chain oligomers and longer-chain polymers are very promising active materials for low-cost, large-area optoelectronic and photonic devices. The ability of these materials to transport charge (holes or electrons) due to the π-orbital overlap of neighboring molecules defines their semiconducting and conducting properties. Devices such as organic light-emitting diodes (OLEDs), transistors, and photodiodes are currently attracting much attention [1]. Blue electroluminescent materials are of particular interest for organic displays since blue light can be converted into red and green quite easily by color-changing media (fluorescent dyes). Both polyfluorene (PF) and the methyl substituted ladder-type poly(para-phenylene) (m-LPPP) are of technological importance due to their strong blue luminescence. PF based conjugated polymers show the highest photoluminescence (PL) quantum efficiency (55%) compared to other conjugated polymers/molecules in solid state [2] and also have a high hole mobility at room temperature [3]. Figure 1 shows the chemical structure of m-LPPP and PF (poly 9,9-bis-2-ethylhexyfluorene2,7-diyl). The backbone of m-LPPP is planar and shows no torsional degree of freedom between the neighboring phenyl rings due to the methyl bridges. The planar backbone in m-LPPP results in a high intrachain order and low defect concentration. This is attributed to its synthesis, which selectively forms only certain bonds and hence reduces the number of defects. PF, on the other hand, is planar within a single monomer unit but shows a torsional degree of freedom
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