© J D White SMS : Illuminating the photo-physics of Luminescent Conjugated Polymers– Results 1...

SMS : Illuminating the photo-physics of Luminescent Conjugated Polymers– Results 1© J D White

Results & Discussion• Results: observing the PL time trace’s:

– Intensity – Spectrum– Spectral evolution

• Discussion– Modeling the energy transfer– Ensemble Absorption/PL spectrum– Spectral Shift Coefficient histogram


PL time trace


Insights from the PL time trace• Changes in intensity

abrupt.• 2 types: recoverable &

non-recoverable• In general, emission at

3-5 discrete intensities• Lifetime dependent on O2 exclusion (<1s to >30 min)• data consistent with view of polymer as composed of a

number of independent excitons– at max emission, all active

– decrease in emission when exciton disabled...


Spectrum of PL time trace• Each polymer has a

unique spectral signature.

• Ensemble average is the same as that in solution.

• Single molecule spectrum much narrower than ensemble average.

F each step might exhibit different spectral characteristics


Time Evolution of PL spectra (1)

• Provides information on the source of photons and nature of excitons emitting them– if exciton migration inefficient along backbone,

absorption and emission occur at same conjugated segment

– if efficient, then energy will be funneled from absorbing to one lower energy emission site

• Energy transfer or its lack, will be seen in the behavior of the spectral shift coefficient during an abrupt intensity change.



• Efficient energy transfer– on-off blinking– no spectral shift at lower emission intensity– blue-shift spectrum at lower emission intensity

• Inefficient energy transfer– each chromophore absorbs/emits independently– random spectral shifts with lower emission intensities

depending on which chromophore is quenched.


Time evolution of PL: blue shift


Time evolution of PL: red shift


Time evolution of PL: no shift



• > 46 individual polymers considered– 11 : blue shift with lower intensity– 15 : red shift with lower intensity– 6 : combination of red and blue shifts– 5 : no spectral shift

• Implication is that energy transfer is inefficient


Histogram of Spectral Shift Coefficients for 30 polymers

• if energy transfer efficient, expect lower energy excitons to bleach first

• result should be progressively blue shifted spectrum

• But only a slight blue shift is observed.

• --> inefficient energy transfer


Modeling the Data

• Molecular exciton model

• Appearance of different conjugated segments governed by a distribution function

• In this model rapid energy transfer is assumed.


Ensemble average spectrum (rapid energy transfer model)

• Excellent agreement between theory and experiment for single long chain DOO-PPV polymers

• Poor agreement for short chain polymers.

• Suggest interchain energy transfer efficient in long chain polymers


Modifying the molecular exciton model

Replace assumption of rapid energy transfer with the assumption of no energy transfer.

Comparison with experiment, plot over the histogram

Good fit!


ReferencesC. F. Wang, J. D. White, T. L. Lim, J. H. Hsu, S. C. Yang, W. S. Fann, K. Y.

Peng, S. A. Chen, "Illumination of Exciton Migration in Rod-Like Luminescent Conjugated Polymers by Single Molecule Spectroscopy", Phys. Rev. B, 67, 035202 (2003)

© J D White SMS : Illuminating the photo-physics of Luminescent Conjugated Polymers– Results 1...

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