Berlin 2012 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 13: Focus Session: Structural Ordering and Electronic Transport II (jointly with CPP)

HL 13.2: Talk

Monday, March 26, 2012, 15:30–15:45, ER 270

Rod-coil transitions in the polymers MEH-PPV and PFO — •Anna Köhler — Universität Bayreuth, Bayreuth, Germany

In the current work we explore the effect of aggregation of three conjugated polymers, MEH-PPV , PFO and MeLPPP, dissolved in MeTHF. In MEH-PPV, the absorption spectra are a superposition of a broad ("blue") band and a vibrationally well resolved spectrum at lower energies (the "red"-phase)that appears at 190 K and grows at the expense of the blue phase. Obviously there is a temperature-induced transformation from the blue to the red phase. While this is observed in solutions of 5 x 10-6 mol/L , the red-phase is absent in a 10-7 mol/L solution. This proves that the formation of the red-phase requires aggregation. A similarly temperature dependent superposition of blue and red features can also observed for PFO, but upon dilution only the red feature survives. Finally for MeLPPP, the absorption and fluorescence spectra are mono-modal at all temperatures.The results are indicative of a rod-coil transition in the case of MEH-PPV and PFO with a temperature dependence that is characteristic of a order-disorder transition, albeit with different driving forces. In the case of MEH-PPV the planarization of the chain requires chain aggregation. Quantum chemical calculations show the potential for torsional motion of the repeat units is so shallow that the motion the solvent molecules above the glass temperature is sufficient to prevent chain elongation unless chain pairing occurs. In PFO, however, the tendency towards chain planarization is obviously strong enough to facilitate the elongation of single chains and inter-chain interaction obstructs rather than helps chain elongation.