Interchain and intrachain exciton transport in conjugated polymers: ultrafast studies of energy migration in aligned MEH-PPV/mesoporous silica composites

In this paper, we show how composite samples consisting of chains of the semiconducting polymer MEH-PPV embedded into the channels of oriented, hexagonal nanoporous silica glass allow control over energy transfer and exciton migration in the polymer. The composite samples are characterized by two polymer environments: randomly oriented and film-like segments with short conjugation-length outside the channels, and well aligned, long conjugation segments that are isolated by encapsulation within the porous glass. Ultrafast emission anisotropy measurements show that excitons migrate unidirectionally from the polymer segments outside the pores to the oriented chains within the pores, leading to a spontaneous increase in emission polarization with time. Because the chains in the pores are isolated, the observed increase in polarization can take place only by exciton migration along the polymer backbone. The anisotropy measurements show that energy migration along the backbone occurs more slowly than Forster energy transfer between polymer chains; transfer along the chain likely takes place by a thermally-activated hopping mechanism. Similar time scales for intra- and interchain energy transfer are also observed for MEH-PPV chains in solution. All the results provide new insights for optimizing the use of conjugated polymers in optoelectronic devices.