Electronic and conformational properties of the conjugated polymer MEH-PPV at a buried film/solid interface investigated by two-dimensional IR-visible sum frequency generation

We present the first measurement of the buried surface electronic states of the conjugated polymer poly[2methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) using two-dimensional (2D) IR-visible sum frequency generation (SFG). SFG electronic spectra were obtained by scanning the frequencies of both incident visible and IR beams and used to study the surface electronic transitions associated with the C-C stretching of benzene rings located at the backbone of MEH-PPV. Because of the surface confinement effects, the polymer conformation, and consequently the electronic states, at the film/solid interface are different from those of the bulk film. Theoretical analysis based on an oligomer model was employed to estimate the conjugation-length distributions of MEH-PPV at interfaces. Assuming a Gaussian conjugation-length distribution, it was found that the conjugation-length distribution at the MEH-PPV/solid interface was centered at 5.8 monomer units. Similar surface effects were also observed at the air/polymer interface, with a shorter average conjugation length of 5.1 monomer units.