Exciton energy transfer in conjugated polymers
Exciton energy transfer in conjugated polymers is well described in the framework of Förster-type resonance energy transfer. It can be observed in real-time by measuring the dynamic Stokes shift or the depolarization kinetics on a picosecond timescale.
We have developed a Monte Carlo model that describes exciton energy transfer along conformationally disordered polymer chains. For an accurate calculation of the electronic coupling the model takes the shape of the exciton wavefunctions into account (line-dipole approximation). It also disentangles the homogeneous and inhomogeneous contributions of the absorption and emission spectra for the calculation of the spectral overlap.[1]
|
Sketch of exciton energy transfer in density of states and in the spatial domain.
|
Lasing in polymer gain media
High optical gain is observed in various conjugated polymers. When these materials are inserted in a feedback structure lasing can be achieved. In 1996, this was observed for the first time in our group in a microcavity containing PPV (Tessler et. al, Nature,1996, 382, (6593), 695-697). Since then, remarkable efforts have been made in order to reduce the lasing threshold through the optimization of the materials and the device structures. However an electrically-pumped device has not yet been shown and this future target provides a strong motivation for our research.
In this project we explore the tunability and the efficiency of devices based on 1- and 2-D distributed feedback (DFB) structures.
|
A 2D periodic structure providing distributed feedback.
|
Conformational disorder of conjugated polymers
Ultrafast depolarization experiments are not only a probe for exciton energy transfer dynamics but also for the conformational disorder of the polymer chain. We were able to quantify the conformational disorder of a polythiophene. We found that torsional rotation between the thiophene rings is the most important source of disorder.[2]
|
Example of a dissolved polyhiophene chain. The molecular model to obtain the chain conformations is described in Ref. 2.
|
Torsional dynamics of conjugated polymers
The dynamic Stokes shift of conjugated polymers is usually assigned to incoherent exciton energy transfer. Recently we were able to show that conformational relaxation also contributes to this Stokes shift. Fig. 3 shows that by site-selectively exciting the red edge of the absorption spectrum where energy transfer is suppressed (2.13 eV) a dynamic Stokes shift is observed on a picosecond timescale. We assign this to torsional relaxation[3].
|
The dynamic Stokes shift of films and solutions of a polythiophene measured by femtosecond transient absorption spectroscopy. The excitation energies are indicated in the figure and the fits are explained in Ref. 1.
|
