Thermodynamic constants for excimer formation and dissociation in oxidised poly(9,9-dioctylfluorene) (PFO)

We have studied the temperature-dependent photoluminescence (PL) characteristics of oxidised PFO thin films at temperatures above 298K. We find the relative strength of the green emission band (g-band) to increase greatly at temperatures corresponding to the onset of crystallisation. Based on the proposal that the g-band arises from a fluorenone-based excimer, this finding would seem to indicate that a close approach of neighbouring fluorenone-containing segments may be energetically favourable. Finally, by successfully identifying diffusion-limited and dynamic equilibrium regimes within the temperature dependence of the ratio R=ID/IM, we extract an activation energy for excimer formation of ∼ 0.05 eV and an excimer binding energy of ∼ 0.51 eV. This is a relatively high binding energy for an excimer and lends credibility to the notion of an energetically favourable fluorenone:fluorenone coupling configuration, perhaps as a result of the considerable ground state dipole moments associated with the ketone group.