Abstract
Detailed understanding of the electronic energy transfer dynamics in conjugated polymer molecules and their conformation dependence is central for improving the photophysical properties as well as the performance of devices based on conjugated polymers. In this work, we demonstrate simultaneous measurement of the absorption and emission sites in single conjugated polymer poly[2,7-(9,9-dioctyluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) molecules based on polarization-resolved confocal fluorescence microscopy with excitation of phase-modulated ultrashort pulse pairs. The evolution of absorbing chromophores can be derived by modulating the relative phase between ultrashort pulse pairs and extracting modulation information in phase-dependent fluorescence. Meanwhile, the emitting chromophore can be measured by polarization-resolved emission. Simultaneous absorption and emission measurements give new insights into the evolution of energy transfer pathways in individual conjugated polymer molecules. The results suggest that the conformation of single conjugated polymer chains can be influenced by solvents. Single PFO-DBT conjugated molecules spin-cast from toluene solution have relatively fixed absorption and emission dipole moments. In contrast, single conjugated polymer molecules prepared with chloroform show multichromophore behavior that is responsible for distribution of absorption and emission in a single chain. The proposed scheme paves the way for further understanding of conformation dependent photophysical properties and the possible role of quantum effects in the energy transfer pathway in both natural and artificial light harvesting systems in the nanoscale.
Highlights
Electronic energy transfer (EET),12–14 which exists between chromophores on conjugated polymer chains, contributes to the photophysics of conjugated polymers and plays a significant role in their applications mentioned above
Both the absorption and emission dipole moments on single conjugated polymer molecules are found to show dynamic evolution, which results in the evolution of energy transfer pathways in conjugated polymer chains after excitation
Our results show evidence for the influence of solvents on the conformation of single conjugated polymer molecules by revealing the distribution of absorption and emission dipole moments of single chains prepared with different solvents
Summary
Conjugated polymers have attracted considerable attention because of their attractive semiconducting and optical properties as well as their potential application in optical sensors, lightemitting diodes, and solar cells. Electronic energy transfer (EET), which exists between chromophores on conjugated polymer chains, contributes to the photophysics of conjugated polymers and plays a significant role in their applications mentioned above. Single molecule spectroscopy, in combination with polarization-resolved excitation, emission, or both, has been used to reveal the conformation of single conjugated polymer molecules as well as the correlation between EET and their conformations.. Single molecule spectroscopy, in combination with polarization-resolved excitation, emission, or both, has been used to reveal the conformation of single conjugated polymer molecules as well as the correlation between EET and their conformations.24–27 These methods are enabled either by performing under low temperature to achieve resolvable polarization dependent spectra or by accumulating the whole polarizationresolved fluorescence trajectory of a single molecule to get time-averaged anisotropy. The emission dipole can be monitored by polarization-resolved fluorescence detection based on confocal fluorescence microscopy
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