Abstract
Spectroscopic fluorescence polarization (P) measurements have been used to investigate exciton dynamics in conjugated polymers. We apply photoluminescence anisotropy to ensembles of non-interacting organic semiconductor molecules to explore exciton migration. An experimental observation shows linearly decreasing P values as the emission wavelength increases in partially oxidized poly[2-methoxy-5-(2-ethyloxy)-1,4-phenylenevinylene] (MEH-PPV) ensemble molecules. We discuss the origin of the experimental data with a computational simulation and P values for single chromophore perylene diimide dye molecules. We propose that the physical mechanism responsible for this behavior is the presence of exciton confined and blocking states at blue-shifted emission sites, which arise from excitons in partially oxidized parts of MEH-PPV.
Highlights
Exciton dynamics of individual polymers and conjugated systems have been characterized to a great degree through employing various optical techniques at the single molecule level,1–9 and some of the work utilized optical anisotropy measurements.1–5,9–16 Generally, this has been done using techniques that investigate optically anisotropic signals of individual molecules, and the results have provided great insight into the conformation of multichromophoric molecules and aggregates.9,11–13 Variations of this technique have been used with great success, but there have been only a few comments on the nature of charge carrier dynamics within the molecules.9,11,12 This is because the individual molecules that are typically studied require great time spans in order to produce photoluminescence (PL) signals that can be resolved by energy or emission wavelength
Emission polarization for ensembles of perylene diimide (PDI)-C3 and PTCDIPh is measured to show the contrast in P values between single chromophore molecules and multi-chromophore polymers (Fig. 4)
The high P values greater than 0.4 across each major emission wavelength indicate that the emission from these non-interacting small molecules is highly anisotropic as expected for ensembles of non-interacting PDI molecules, which is consistent with the previous work
Summary
Exciton dynamics of individual polymers and conjugated systems have been characterized to a great degree through employing various optical techniques at the single molecule level, and some of the work utilized optical anisotropy measurements. Generally, this has been done using techniques that investigate optically anisotropic signals of individual molecules, and the results have provided great insight into the conformation of multichromophoric molecules and aggregates. Variations of this technique have been used with great success, but there have been only a few comments on the nature of charge carrier dynamics within the molecules. This is because the individual molecules that are typically studied require great time spans in order to produce photoluminescence (PL) signals that can be resolved by energy or emission wavelength. Exciton dynamics of individual polymers and conjugated systems have been characterized to a great degree through employing various optical techniques at the single molecule level, and some of the work utilized optical anisotropy measurements.1–5,9–16 This has been done using techniques that investigate optically anisotropic signals of individual molecules, and the results have provided great insight into the conformation of multichromophoric molecules and aggregates.. Multiphoton excitation, energy transfer, and rotational diffusion can lead to different limiting values for emission anisotropy and polarization.. Multiphoton excitation, energy transfer, and rotational diffusion can lead to different limiting values for emission anisotropy and polarization.31 Another popular metric for optical anisotropy measurements exists that is known as modulation depth (M) that is used in many experimental reports.. The results suggest that average trends for charge carrier dynamics can be understood by analyzing emission polarization across individual wavelengths in a PL spectrum
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