Low-Temperature Single-Molecule Spectroscopy of MEH-PPV Conjugated Polymer Molecules

Relatively narrow bandwidth fluorescence spectra were observed from isolated single molecules of the conjugated polymer, MEH-PPV (poly[2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene]) at low temperature. The spectroscopy reveals rich spectral information in energy relaxation pathways and structure of this important electroluminescent material. From room temperature to 20 K the fluorescence of MEH-PPV molecules was found to red-shift by ∼20 nm along with a decreased electron−phonon coupling. The fluorescence spectra give clear evidence for efficient electronic energy funneling to a small number (one in some cases) of low-energy sites, each with an increased effective conjugation length compared to room temperature.