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IRTG / Soft Matter Science
Freiburger Materialforschungszentrum
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79104 Freiburg, Germany

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You are here: Home Events Prof. Anna Köhler "Order-disorder transitions in conjugated polymers"

Prof. Anna Köhler "Order-disorder transitions in conjugated polymers"

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EPII and BIMF, Department of Physics, University of Bayreuth, 95440 Bayreuth, Germany

What
  • Seminar
When Jun 12, 2013
from 02:15 PM to 03:00 PM
Where FRIAS, Alter Hörsaal Anatomie, Albertstr. 19, Freiburg
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Some conjugated polymers such as MEH-PPV, Polyfluorene and P3HT are known to exist in two morphologically distinct species, a disordered coiled conformation and an ordered, planar conformation. For MEH-PPV, we have recently shown that the transition from the disordered to the ordered conformation is a critical phenomenon that can be quantitatively described as a phase transition with a critical temperature Tc.[1] The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π-system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition in MEH-PPV, PFO and P3HT by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. Whereas the chains have short effective conjugation lengths consistent with a disordered conformation at room temperature, upon cooling below the critical temperature Tc, the red phase with increased effective conjugation lengths forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character, that can be described in the context of a HJ-aggregate model.[2]

 

[1] A. Köhler, S. T. Hoffmann, H. Bässler
An Order−Disorder Transition in the Conjugated Polymer MEH-PPV
J. Am. Chem. Soc. 134 (2012) 11594−11601

[2] H. Yamagata, N. J. Hestand, A. Köhler, S.T. Hoffmann, C. S. Scharsich, H.Bässler, F. C. Spano
HJ-Aggregates of MEH-PPV, in preparation
 

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