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46628-G6
Structural Dynamics in Conducting Polymer Systems Probed with In-Situ Electrochemical Two-dimensional Infrared Spectroscopy

Aaron M. Massari, University of Minnesota, Twin Cities

Minneapolis, MN 55455-0431

I. Instrumentation Development: The first stages of this project involved the construction of the ultrafast 2D-IR spectrometer.  The light generation for the instrument was a commercial regeneratively amplified Ti:Sapphire laser system (Spectra Physics) producing 40 fs pulses at 800 nm.  The near IR pulses were then down converted in a commercial Spectra Physics optical parametric amplifier that was modified by the PRF-supported graduate student, Audrey Eigner, to include a re-timing and difference frequency generation stage where short pulses of mid-IR light are generated.  The pulses produced are tunable from roughly 1600 to 3300 cm^-1, have a full width at half max (FWHM) of roughly 80 fs, and spectral bandwidth of 200 cm^-1.  The mid-IR pulse train is split by a series of beam splitters into 3 equal intensity beams of approximately 1 microjoule per pulse.  Two of the beams are sent through reflective delay stages that allow a sequence of the three pulses to be computer controlled to within +/- 2 fs.  This produces the 2D-IR signals, which we have thus far measured as a homodyne detected, spectrally-resolved vibrational echoes.  As an example, Figure 1 shows a series of vibrational echo decays measured with this instrument for a model compound in organic solvent.  The decay times of each of these scans has a Fourier relation to the homogeneous (dynamic) lineshape, and the progressive shifting of each decay curve is the result of structural dynamics on the timescale of tens of ps.

II. Conducting Polymer Dynamics:  In the early work of this project, we endeavored to measure 2D-IR signals from nitrile containing poly(aniline) (PANI) thin films with little to no success.  The underlying limitation to that approach seemed to be the absorbing strength of the CN oscillator. Also an issue with that approach was that the functionalized PANI films had little to no conductivity, which would make many of the goals of this project unobtainable.  We modified our approach by embedding IR-active chromophores as dynamics reporters into several different polymer films.  Figure 2 shows the 2D-IR vibrational echo decay data collected from the reporting species embedded in a semiconducting PANI thin film.  The decay times are clearly longer than those of the solution-phase probe (Figure 1), which is intuitive since the chemical environment should fluctuate faster in solution.  We obtained the same datasets for CN-PPV, an n-type semiconducting polymer film, and note that the dynamics reported by the embedded molecule are noticeably different.  Of particular interest, both polymers show structure motions on the timescales of tens of fs that are not resolved by our instrument, but the CN-PPV also shows dynamics on the tens of ps timescale.  We have fit the data for both polymers to quantify these differences, and have determined that the CN-PPV has a unique structural dynamic component with a time constant of 44 ps that is absent in the PANI dynamics.  Building on these early successes, we have approached this project as a means to catalogue the timescales of motion that are present and unique in a range of conducting and semiconducting polymers.  This effort is ongoing during the second year of funding.

In the process of measuring structural dynamics in various polymers, we attempted to embed our reporter into regioregular poly(3-hexylthiophene) (P3HT), a semicrystalline semiconducting polymer with applications in photovoltaics and molecular electronics.  We were surprised to find that the polymer and the reporter molecule can be controllably mixed and phase segregated.  What is particularly intriguing about our observations of this system is that it appears that we can measure the structural dynamics for different domains and sub-environments within the same polymer.  By spectrally resolving the 2D-IR echo signals, we are able to selectively measure the decay curves that correspond to the blended and segregated samples.  We currently have a manuscript in progress for this system that we hope to submit before the end of the year.  Preliminary results with iodine doping indicate that we should be able to perform in-situ conductivity and dynamics measurements with this sample, which is another ongoing effort for the project in the second year of funding.

Figure 1.  2D-IR vibrational echo decays collected from a reporter molecule (ruthenium(II)tetraphenylporphyrin) in organic solvent.

Figure 2.  2D-IR vibrational echo decays from a reporter molecule embedded in poly(aniline).

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