A Comparative Study of the Photophysics in Polyfluorenes and Polyfluorenes with Polyphenylene Dendron Sidechains
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A Comparative Study of the Photophysics in Polyfluorenes and Polyfluorenes with Polyphenylene Dendron Sidechains E. J. W. List1,2, A. Pogantsch1, F.P. Wenzl1, C.-H. Kim2, J. Shinar2, M. A. Loi3*, G. Bongiovanni3, A. Mura3, S. Setayesh4, A. C. Grimsdale4, H.G. Nothofer4, K. Müllen4, U. Scherf5, G. Leising6 1 Institute for Solid State Physics, Technical University Graz, Petergasse 16, A-8010 Graz, Austria. 2 Ames Laboratory-USDOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 3 Dipartimento di Fisica and INFM, Università di Cagliari, I-09042 Monserrato (CA), Italy * Present address: Linz Institute for organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria. 4 Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany 5 Institut für Physikalische und Theoretische Chemie, Universität Potsdam, D-14476 Golm, Germany 6 AT&S Research&Technology, Fabriksgasse 13, A-8700 Leoben, Austria; Institute of Nanostructured Materials and Photonics, Joanneum Research Inc., Franz-Pichler-Strasse 30, A-8160 Weiz, Austria ABSTRACT The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar. INTRODUCTION Poly(p-phenylene) (PPP) and its derivatives, such as polytetrahydropyrene (PTHP) [1], polyfluorene (PF) [2], poly-2,8-indenofluorene (PIFO) [3], and ladder-type PPP (LPPP), [4] have emerged as an attractive class of high-HOMO-LUMO gap blue-emitting materials for applications in organic optoelectronic devices [5,6,7]. In order to realize an injection polymer laser it is highly desirable to attain intrinsically stable materials with high carrier mobilities and a large HOMO-LUMO gap. The latter is of special importance since in materials with a large energy gap one finds a weaker overlap between the emission band and the excited stateabsorption bands. Quenching of the emission by these bands has been identified as the dominant loss mechanism in optically [8] and electrically [9] pumped device structures. Yet, despite the ongoing improvements in terms of stability and colour purity of polymer light emitting devices (PLED)s fabricated from PF type materials, most of these PLEDs su
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