A detailed single molecule spectroscopy study of the vibronic states and energy transfer pathways of the conjugated polymer MEH-PPV

The low energy onset of the optically accessible density of electronic states (OADOS) of the conjugated polymer MEH-PPV has been investigated by low temperature fluorescence single molecule spectroscopy (SMS). The first extensive set of high signal-to-noise SMS data have been acquired for MEH-PPV samples as a function of molecular weight confirming the previously reported bimodal energy gap distribution. These data have been acquired under sample and radiation conditions that inhibit photophysical dynamics that have distorted previous SMS studies. A molecule by molecule vibronic analysis has yielded an experimentally determined spectral density of the S0–S1 energy gaps for the individual chromophores for MEH-PPV. The SMS data has also led to new insights on the energy transfer pathways for conjugated polymers, and on the molecular nature of the “blue” and “red” sites of conjugated polymers. Finally, a combined analysis of the new SMS data with the previously obtained bulk absorption and emission data for MEH-PPV has offered clear evidence that the low energy onset of the OADOS for MEH-PPV, in analogy to the emission spectral density, confirms the existence of a bimodal energy gap distribution.