Use of picosecond fluorescence dynamics as an indicator of exciton motion in conjugated polymers: Dependence on chemical structure and temperature

The utility of picosecond fluorescence dynamics as an experimental indicator of exciton motion in conjugated Zn polymers is assessed by examining the fluorescence dynamics of phenylene vinylene polymers and oligomers as a function of temperature and chemical derivatization. Both polycrystalline poly(p-phenylene vinylene) (PPV) and an oligomeric analogue in polystyrene exhibit similar picosecond fluorescence Stokes shifts at all temperatures, indicating that this shift is due mostly to intramolecular relaxation. In PPV, the decay time of the integrated fluorescence decreases from 800 ps at 16 K (close to the oligomer lifetime of 930 ps) to 200 ps at 290 K and is accompanied by an increase in the anisotropy decay. A simple model for the dependence of the fluorescence decay on the exciton diffusion yields an activation energy of 170 cm(-1) for exciton motion in PPV. In contrast to the unsubstituted PPV, poly[ (2-methoxy-5-hexyloxy-p-phenylene) vinylene] (MH-PPV) forms an amorphous solid whose fluorescence Stokes shift is twice that observed for isolated MH-PPV molecules dissolved in a poly(vinyl chloride) (PVC) host polymer. The increased Stokes shift and spectral chancres in MH-PPV provide evidence for excimer formation which is absent in PPV. The 250 ps decay of the integrated fluorescence in MH-PPV exhibits almost no temperature dependence and is considerably faster than the decay rate of the isolated polymer in PVC. The differences between PPV and MH-PPV are rationalized based on differences in the solid-state intermolecular interactions, This work shows how the excited-state emission dynamics in these materials may be due to local phenomena like intramolecular vibrational relaxation (PPV) or excimer formation (MH-PPV) as opposed to exciton diffusion.