Low temperature single molecule spectroscopy of MEH-PPV conjugated polymer molecules

Electronic energy transfer and singlet-triplet exciton interactions in conjugated polymers are investigated by low temperature single molecule spectroscopy SMS. Low temperature SMS leads to narrower vibronic features allowing for a more detailed and definitive analysis. Low temperature spectroscopy also suppresses unwanted thermally assisted photochemical processes especially photo-oxidation. The SMS data obtained for MEH-PPV samples as a function of molecular weight reveal a bimodal distribution of SMS emission maxima, confirming previous room temperature SMS results. By analyzing the narrow bandwidth fluorescence spectra at 20 K, the results give new insights on the energy transfer pathways in single conjugated polymer, giving new evidence for efficient energy funneling to a small number of low energy sites. The results furthermore shed light on the molecular nature of “blue” and “red” sites. In addition to studies on MEH-PPV itself, energy transfer has been investigated in a diblock polymer, consisting of an energy donor and energy acceptor block. SMS results indicate that the addition of energy donor block may induce the significant morphology change of energy acceptor block, which modifies electronic energy transfer behavior. Microsecond dynamics of triplet exciton has been investigated by single molecule excitation intensity modulation spectroscopy leading to a quantitative analysis of the rate constants for triplet formation and singlet quenching by triplets. In addition, the effect of the sample preparation and excitation condition on the single molecule spectroscopy of conjugated polymers is examined at low temperature, revealing that high excitation intensity, environmental impurities and near-by interfaces may damage the MEH-PPV molecules photochemically.