21 July 2000 Ž . Chemical Physics Letters 325 2000 132–138 www.elsevier.nlrlocatercplett Excitation energy migration in highly emissive semiconducting polymers E.J.W. List a, ) , C. Creely a , G. Leising a , N.. Schulte b , A.D. Schluter b , ¨ U. Scherf c , K. Mullen c , W. Graupner d ¨ a Institut fur Festkorperphysik, Technische UniÕersitat Graz, 8010 Graz, Austria ¨ ¨ ¨ b Institut fur Chemie r Organische Chemie, Freie UniÕersitat Berlin, Berlin, Germany ¨ ¨ c Max-Planck-Institut fur Polymerforschung, Mainz, Mainz, Germany ¨ d Department of Physics, Virginia Tech, Blacksburg, VA 24060, USA Received 16 February 2000; in final form 18 April 2000 Abstract Ž . The process of excitation energy migration EEM in conjugated polymers, which is relevant both for light emitting Ž diodes and laser applications, is probed by doping the blue light emitting methyl-substituted ladder-type poly para-phenyl- . ene with small concentrations of a highly fluorescent p-conjugated macromolecule. The experimentally attained tempera- ture and concentration dependence of the steady state photoluminescence is modeled and discussed by means of a two-step Ž. Ž. EEM process: 1 a thermally actiÕated migration within the host and 2 transfer from the host to the guest. In particular we show that a Forster-type mechanism alone cannot account for the experimental facts in such a guest host system. q 2000 ¨ Published by Elsevier Science B.V. 1. Introduction Ž . Excitation energy migration EEM , the transfer of an excited entity from a donor to an acceptor, is a tool nature takes advantage of in conversion of solar wx energy by bacteria, algae and plants 1 . As demon- strated recently, by means of EEM one can signifi- cantly increase the overall performance of organic Ž . light emitting diodes OLED when combining a donor or host polymer or with an acceptor or guest w x polymer 2–4 . In very confined clusters of matter, such as quan- wx wx tum dots 5 or molecules 6 , the process of singlet ) Corresponding author. Fax: q43-316-873-8478; e-mail: F513ejwl@mbox.tu-graz.ac.at Ž . exciton SE transfer can be described within the dipole–dipole interaction mechanism as originally wx explained by Forster 7 as a single-step long-range ¨ resonance transfer. Triplet exciton transfer as shown wx in Ref. 8 can be described by exchange interaction, wx the Dexter process 9 or a Coulomb interaction type w x including higher multipole terms 10 . As we will show in this work in very extended molecular structures, such as polymers, the energy Ž. transfer mechanism includes at least two steps: 1 Ž. diffusion within the host polymer, 2 transfer from the host polymer to the guest as depicted in Fig. 1. We will demonstrate that in conjugated polymers EEM is the sum of two processes and has to be understood in the following way. Molecules with a weaker overlap of the p-electrons, exhibit a larger 0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V. Ž . PII: S0009-2614 00 00635-7