How β-Phase Content Moderates Chain Conjugation and Energy Transfer in Polyfluorene Films

Poly(9,9-dioctylfluorene) (PFO) is a blue-light-emitting polymer exhibiting two distinct phases, namely, the disordered "glassy" phase and a more ordered β-phase. We investigate how a systematic increase in the fraction of β-phase present in PFO films controls chain conformation, photoluminescence quantum efficiency (PLQE), and the resonant energy transfer from the glassy to the β-phase. All films are prepared by the same technique, using paraffin oil as an additive to the spin-coating solution, allowing systematic tuning of the β-phase fraction. The PFO films exhibit high PLQE with values increasing to 0.72 for increasing fractions of β-phase present, with the β-phase chain conformation becoming more planar and including more repeat units. Differences in Förster radii calculated from the overlap of steady-state absorptance and emission spectra and from time-resolved ultrafast photoluminescence transients indicate that exciton diffusion within the glassy phase plays an important role in the energy transfer process.