Electronic energy transfer in single conjugated polymer molecules revealed by phase-modulated pulse-pair controlled single molecule spectroscopy

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.