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 se miconducting polymer MEH-PPV embedded into the channels of oriented, hexago nal nanoporous silica glass allow control over energy transfer and exciton migration in the polymer. The composite samples are characterized by two po lymer environments: randomly oriented and film-like segments with short con jugation-length outside the channels, and well aligned, long conjugation se gments that are isolated by encapsulation within the porous glass. Ultrafas t emission anisotropy measurements show that excitons migrate unidirectiona lly from the polymer segments outside the pores to the oriented chains with in the pores, leading to a spontaneous increase in emission polarization wi th time. Because the chains in the pores are isolated, the observed increas e in polarization can take place only by exciton migration along the polyme r backbone. The anisotropy measurements show that energy migration along th e backbone occurs more slowly than Forster energy transfer between polymer chains; transfer along the chain likely takes place by a thermally-activate d 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 optoe lectronic devices. (C) 2001 Elsevier Science B.V. All rights reserved.