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

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.

Pump‐Probe Spectroscopy in Organic Semiconductors: Monitoring Fundamental Processes of Relevance in Optoelectronics

“…[ 93 , 94 ] There are cases however where dissociation centres may be constituted by chemical sites such as oxidized polymer segments. [ 95 ] Some of these traps are likely to be present in every single study reported in literature since 100% material purity and oxygen free atmosphere are both very difficult to achieve. Their concentration however will differ from one study to another and so their infl uence on CPG.…”

Electronic Structure of Self-Assembled Amorphous Polyfluorenes

“…1 Of all conjugated polymers, polyfluorenes, including poly-(9,9=-dioctylfluorene) (PFO), have attracted a special interest because of their efficient blue photoluminescence and one of the highest hole mobility values 2,3 (10 Ϫ3 to 10 Ϫ2 cm 2 /Vs) in combination with excellent chemical and thermal stability. 4,5 These features render PFOs with great application potential as blue-light-emitting diodes (LED), 6,7 optical switches, 8 thin film transistors, 9 solar cells, 6 and lasers. 10 A distinct structural feature, which is responsible for many unique physicochemical properties of fluorenes, is the presence of the bridging nonconjugated (sp 3 ) carbon atom between rigid planar biphenyl units ( Figure 1b).…”

“…4 The tetrahedral character of the sp 3 -bonds of the bridging carbon allows for selforganization of PFO polymers into several distinct supramolecular structures at room temperature. 4 In films, PFO samples typically exhibit two morphological formations: amorphous glassy phase (␣ phase), in which PFO molecules have large torsional angles between the adjacent biphenyls, and the so-called ␤ phase with a high degree of organization and planar PFO molecules. 5,11,12 A strong variation of film morphology leads to a vast difference in photophysics of PFO materials.…”

Deep-red electrophosphorescence from a platinum(II)–porphyrin complex copolymerised with polyfluorene for efficient energy transfer and triplet harvesting

“…S2) can be fit with a bi-exponential function yielding time constants of ∼ 400 ps and 6.6 ns that are consistent with PFO exciton and fluorenone excimer respectively. [25] The radiative lifetime taken near the peak emission wavelength of the PFO (440 nm) also confirms the presence of efficient energy transfer from the PFO to the porphyrins. In fact, the lifetime of the pure PFO of about ∼ 400 ps is reduced upon loading with the MPP(Pt) and drops below the instrument detection limit ( ∼ 150 ps).…”