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Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9-dioctylfluorene): The Essential Role of Inter-Chain/Inter-Segment Interactions

Authors


  • We thank Mike Inbasekaran, Mark Bernius, Rob Fletcher, and Jim O'Brien of The Dow Chemical Company for the poly(9,9-dioctylfluorene) polymer that we have studied, and N. Avlonitis for programming assistance. We also thank the UK Engineering and Physical Sciences Research Council (Quota studentship to MS, Advanced Fellowship to DGL, Research grants GR/R97085 and GR/R26641), The Sardinian Regional Government of Italy (Scholarship to MA), and the Commission of the European Community (Powerplay project GRD-2000-25820)) for financial support.

Abstract

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 π–π* transitions as recently suggested.

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