Exciton migration inβ-phase poly(9,9-dioctylfluorene)

We have studied the dynamics of optically generated excitations in spin-coated glassy films of poly(9,9-dioctylfluorene) (PFO) and in $\beta$-phase PFO films using picosecond time resolved photoluminescence (PL) spectroscopy, performed both at room temperature (RT) and at 5 K. We also present measurements of the PL emission of PFO and $\beta$-phase PFO at RT and 5 K following continuous wave (cw) excitation. We show that the cw emission from $\beta$-phase PFO at 5 K is very highly resolved, permitting us to make an assignment of the different vibrational modes of the molecule that couple to the $S_1\to S_0$ transition. Via time-dependent spectroscopy measurements performed at 5 K, we are able to follow exciton diffusion and relaxation through an energetically broadened density of states to polymer chains having a longer conjugation length and lower energy gap. By comparing the relative emission intensity of the different vibronic transitions as a function of time, we are able to directly demonstrate that the lower energy emissive states are associated with longer conjugation length polymeric chains that have enhanced rigidity. At room temperature, we find that these relaxation processes occur faster than the resolution of our detector due to thermally assisted energy migration.