Site-selective fluorescence studies of poly(p-phenylene vinylene) and its derivatives.

We report fluorescence measurements on poly( p-phenylene vinylene!, PPV, and four derivatives of this polymer, all of which show strong luminescence and can be used as emissive materials in electroluminescent diodes. We measure the variation of the emission spectrum with excitation energy at low temperature, and find a threshold energy above which emission is independent of excitation energy and below which the emission energy tracks with the excitation energy. This information makes it possible to separate out the effects of spectral diffusion by exciton migration from other forms of excited-state relaxation. We find that PPV and two derivatives with asymmetric, branches side chains show little or no excited-state relaxation. In contrast, the other two derivatives ~one with bromine and dodecyloxy attachments at the two and five positions on the phenylene, the other with hexyloxy attachments at these sites, and cyano groups at the vinylic carbons! show further relaxation by about 0.25 eV. We consider that emission in these two polymers is from an interchain excimer excited state. Supporting evidence for the cyano-PPV is seen in the differences between the dilute solution and solid-state fluorescence spectra. @S0163-1829~96!00523-1#