Picosecond time-resolved spectroscopy of the excited state in a soluble derivative of poly(phenylene vinylene): Origin of the bimolecular decay

Picosecond time-resolved spectroscopy studies are performed on solutions and thin films of ${\mathrm{poly}[2-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}-5-(2}^{\prime }-\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}-\mathrm{h}\mathrm{e}\mathrm{x}\mathrm{y}\mathrm{l}\mathrm{o}\mathrm{x}\mathrm{y})-1,4-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{}\mathrm{vinylene}]$ (MEH-PPV) in order to examine the excited state. Interactions between excitations on different chains result in bimolecular decay dynamics. Since a contribution from bimolecular decay is observed at excitation densities above ${10}^{18}{\mathrm{cm}}^{\mathrm{-}3},$ excitations on different chains interact in the solid state within the picosecond time regime over distances of at least 10 nm. We analyze the time dependence of the interaction dynamics predicted by different models and conclude that Förster transfer makes a significant contribution to the bimolecular interaction. However, in order to explain both the magnitude of the bimolecular decay coefficient and the interaction range (at least 10 nm in the picosecond time regime), quantum delocalization well beyond a single chain must be assumed. We conclude that the wave function of the photoexcitations extends over about 5 nm.