The role of long-lived dark states in the photoluminescence dynamics of phenylene vinylene conjugated polymers

The role of intermediate nonluminescent states in the relaxation of singlet excitons in the conjugated polymer poly~phenylene vinylene! ~PPV! and its soluble derivative poly@(2-methoxy-5-hexyloxy-p-phenylene) vinylene# ~MH-PPV! is investigated. Time-resolved luminescence and absorption measurements provide evidence for a long-lived, weakly emissive species in PPV at 17 K, in addition to the luminescent singlet state. Ground state recovery times at this temperature provide evidence that up to 40% of the initially excited chromophores end up in a state that does not relax back to the ground state on the 5 ns time scale of the experiment. As the temperature is raised from 17 K to 290 K, the redshifted emission disappears, the fluorescence decay becomes more rapid, and the magnitude of the long-time bleach decreases. These results can be understood in terms of a three-level model where the initially excited singlet state decays nonradiatively via two separate channels: thermally-activated direct relaxation to the ground state, and nanosecond relaxation into a third, long-lived dark state. As the temperature increases, the thermally-activated process increases at the expense of both the fluorescence and the intermediate dark state population. Using this model, a temperature-independent dark state formation time of 1.8 ns was found for PPV, and 1.1 ns for MH-PPV. Our data and modeling provide no evidence for a subpicosecond relaxation channel in the decay of the luminescent excitons in these phenylene vinylene polymers. © 2002 American Institute of Physics. @DOI: 10.1063/1.1481760#