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 $\mathrm{poly}\text{[(2-}\mathrm{methoxy}\text{-5-}\mathrm{hexyloxy}\mathrm{-p-}\mathrm{phenylene}\mathrm{)\hspace{0.17em}}\mathrm{vinylene}]$ (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.