Effect of Ultrasonication on Optical Properties and Electronic States of Conjugated Polymer MEH-PPV

Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene](MEH-PPV) solutions with different concentrations were prepared in chloroform for different ultrasonication times. The ultraviolet absorption and photoluminescence(PL) spectra of the MEH-PPV solutions were measured, and the electronic states of the polymer chains under different experimental conditions were studied. The results showed that the effects of ultrasonication on the dilute and concentrated solutions were different. After ultrasonication, the intensity of the absorption peak at 280 nm significantly decreased, relative to the absorption peak at 500 nm for both dilute and concentrated solutions, indicating that the proportion of the two excited states in the polymer chains had changed. For dilute MEH-PPV solutions, the blue-shifted absorption(at about 500 nm) and PL spectra show that ultrasonication also led to polymer chain degradation and thus shortened the effective conjugation length. For concentrated solutions, however, the peak positions of the absorption spectra remained unchanged. In addition, the effects of the solution temperatures on the optical spectra for the MEH-PPV solutions were also discussed.