Effects of conformational change and segmental aggregation on photoemission of illuminophores in conjugated polymer MEH-PPV: Blue shift versus red shift

摘要

In this contribution, we report experimental results to demonstrate the effects of chain collapse and segmental aggregation of conjugated polymer, poly[2-methoxy, 5-(2'-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), on its photophysics. We have found that the photoemission spectrum of isolated chains in toluene constitutes of high- and low-energy peaks at approx 385 nm and approx 558 nm. This is attributed to simultaneous emissions of short and long illuminophores within the same conjugated chain. When the system is perturbed by addition of a poor solvent, the chains minimize polymer-solvent interactions by undergoing segmental aggregation and/or chain collapse causing drastic change of the optical properties. We demonstrate that the aggregation is responsible for red-shift peaks in absorption and emission spectra detected at approx 550 nm and approx 580 nm, respectively. The chain collapse, on the other hand, causes a blue shift of the spectra. Furthermore, the measurements of emission spectra by using excitation wavelength of 350 nm detect novel high-energy peaks with lambda_(max) at approx 414nm. The results obtained from temperature-dependent experiments suggest that the detection of high-energy peaks arises from the collapse of individual chains.