Condensed Matter Seminar

Manfred J. Walter
University of Utah

Tuesday, September 23, 2008; 4:00 pm in JFB 334

Energy transfer in single polymer chains

When π-conjugated polymers are used in molecular electronic devices, energy transfer phenomena play a significant role in the operation characteristics. In this talk, I will present recent results on how single molecule fluorescence and Raman spectroscopy can contribute to the understanding of intrachain energy migration.[1] We performed photoluminescence excitation spectroscopy on single conjugated polymer chains over a wide wavelength range and observe an unexpectedly broad absorption characteristic. Also, the emission properties turn out to be dependent on the excitation energy, although absorbing and emitting units are spatially separated from each other. Further, polarization- and power-dependent studies show the distribution of absorbing chromophores - individual photophysical subunits on the chain - and the interruption of their interaction by the "exciton blockade" effect, respectively.[2] In the second part of the talk, single molecule fluorescence spectroscopy will be combined with another powerful nanoscopic characterization technique - single molecule surface-enhanced resonance Raman spectroscopy (SERRS). While resonance Raman spectra probe the absorbing chromophore on the polymer chain, fluorescence spectra display the vibrational characteristics (i.e. the phonon spectrum) of the emitting site. Comparing both spectra acquired simultaneously from a single chain allows us to determine whether or not energy transfer occurred.[3] Power-, polarization- and excitation energy-dependent single molecule fluorescence spectroscopy turns out to be a powerful means to study intramolecular energy transfer. In combination with the complementary method of SERRS, a new window to tracking excited state relaxation is opened.

[1] M. J. Walter et al., in "Single Particle Tracking and Single Molecule Energy Transfer", Eds. D. C. Lamb, J. Michaelis, and C. Bräuchle, Wiley, in press (2009).
[2] M. J. Walter et al., Nano Lett. (2008), DOI: 10.1021/nl801757p.
[3] M. J. Walter et al., Phys. Rev. Lett. 98, 137401 (2007).