Understanding the origin of the 535 nm emission band in oxidized poly(9,9-dioctylfluorene): The essential role of inter-chain/inter-segment interactions

We present a careful study of the effects of photo-oxidation on the emissive properties of poly (9,9-dioctylfluorene) (PFO) that addresses important issues raised by a recent flurry of publications concerning the degradation of blue light-emitting, fluorene-based homo and copolymers. The photoluminescence (PL) spectra of thin PFO films oxidized at room temperature comprise two major components, namely a vibronically structured blue band and a green, structureless component, referred to hereafter as the 'g-band'. These are common features in a wide range of poly(fluorene)s (PFs) and whilst the former is uniformly accepted to be the result of intra-chain, fluorene-based, singlet-exciton emission, the origin of the 'g-band' is subject to increasing debate. Our studies, described in detail below, support the proposed formation of oxidation-induced fluorenone defects that quench intra-chain, singlet-exciton emission and activate the g-band emission. However, whilst these fluorenone defects are concluded to be necessary for the g-band emission to be observed, they are considered not to be, alone, sufficient. We show that inter-chain/inter-segment interactions are required for the appearance of the g-band in the PL spectra of PFO and propose that the g-band is attributable to emission from fluorenone-based excimers rather than from localized fluorenone pi-pi* transitions as recently suggested.