We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 21 helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.

We have studied the dynamics of optically generated excitations in spin-coated glassy films of poly͑9,9-dioctylfluorene͒ ͑PFO͒ and in ␤-phase PFO films using picosecond time resolved photoluminescence ͑PL͒ spectroscopy, performed both at room temperature ͑RT͒ and at 5 K. We also present measurements of the PL emission of PFO and ␤-phase PFO at RT and 5 K following continuous wave ͑cw͒ excitation. We show that the cw emission from ␤-phase PFO at 5 K is very highly resolved, permitting us to make an assignment of the different vibrational modes of the molecule that couple to the S 1 →S 0 transition. Via time-dependent spectroscopy measurements performed at 5 K, we are able to follow exciton diffusion and relaxation through an energetically broadened density of states to polymer chains having a longer conjugation length and lower energy gap. By comparing the relative emission intensity of the different vibronic transitions as a function of time, we are able to directly demonstrate that the lower energy emissive states are associated with longer conjugation length polymeric chains that have enhanced rigidity. At room temperature, we find that these relaxation processes occur faster than the resolution of our detector due to thermally assisted energy migration.

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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We report the effects that geometrical confinement has on both the glass transition temperature, T g, and the crystalline phase transition temperature, T c, for two widely studied conjugated polymers, namely poly(9,9-dioctylfluorene) [PFO] and poly(9,9-dioctylfluorene-co-benzothiadiazole) [F8BT]. The T g and T c values were determined for thin film samples via temperature-dependent ellipsometry measurements. The thickness-dependent T g (T c) behavior is characterized by three regimes, namely, (i) large d or bulk samples for which T g = T g bulk (T c = T c bulk), (ii) intermediate d samples for which T g > T g bulk (likewise for T c), and (iii) ultrathin samples for which T g drops again (likewise for T c). The intermediate regimes occur for 160 nm > d > 60 nm and 300 nm > d > 80 nm for PFO and F8BT, respectively. The higher-than-bulk T g and T c values offer the potential to design more robust and thermally stable polymer optoelectronic devices, including light-emitting diodes, lasers, and solar cells.

Here, the optical properties of a series of structurally well‐defined model compounds for oxidatively degraded poly(dialkylfluorenes) (PFs) are reported. Specifically, linear compounds comprising one, two, or four dihexylfluorene (F) moieties together with one fluorenone (O) moiety placed either at the end or in the center of each chain (i.e., FO, FFO, FOF, FFOFF) are studied. The results support the recent observation that the photophysics of the fluorenone‐centered “pentamer” (FFOFF) is most similar to that of oxidized PFs. They further demonstrate that molecule–molecule interaction is essential to activate the green emission band. Investigations by X‐ray diffraction (XRD) identify the solid‐state structure of a representative member of this class of compounds and reveal inter‐molecular interaction through dipole–dipole coupling between neighboring fluorenone moieties.

We report on the use of optical probes to determine the thermal transition temperatures of spin-coated films of the prototypical fluorene based conjugated polymer poly(9,9-dioctylfluorene) (PFO). In particular we focus here on the use of temperature dependent photoluminescence (PL) measurements that are well suited to the study of PFO and other intrinsically fluorescent polymers and blends. The integrated PL intensity reveals clear signatures of the glass transition temperature (T g ), the onset of crystallization (T c ) and subsequent melting into the nematic liquid crystalline mesophase (T m ). The PL intensity determined transitions are shown to be consistent with an independent optical probe, namely spectroscopic ellipsometry, and with differential scanning calorimetry measurements on bulk samples. The especially strong contrast in PL intensity at T c and T m is shown to be a consequence of changes in light out-coupling in the direction of PL detection, a conclusion that is confirmed by measurements of edge emitted waveguided PL and amplified spontaneous emission, and by analysis of ellipsometry data.As 'soft solids' with characteristically weak inter-molecular interactions, polymers have the ability to adopt a wide variety of different physical structures, the control of which provides a desirable means to tune functional properties. Thermal transitions between physical structures are of strong interest because thermal processing can be convenient but also because the * Based on a presentation to the American Physical Society March Meeting 2005.

This study examines the underlying nature of the green emission band observed as a result of oxidation in dialkyl-fluorene polymers. Specifically, we set out to further determine whether an inter-(excimeric) or intra-molecular fluorenone-based excited state is involved. The emission properties of poly(9,9-dihexylfluorene) dispersed at low concentration in a solid polystyrene matrix are carefully explored. In situ, time-resolved photoluminescence measurements are made during photo-oxidation of the blend and during subsequent exposure to an atmosphere saturated with the vapour of a good solvent. The polystyrene matrix suppresses the appearance of the green emission band during oxidation but the subsequent solvent vapour exposure then activates it. The same effect (activation of the green emission) can be achieved by thermally annealing the matrix above its glass transition temperature. Moreover, the activation of the green emission can be reversed by dissolving the film and re-casting. This behaviour is attributed to controlling the phase structure of the polyfluorene/polystyrene blend and is considered strong evidence for an origin of the green band emission in the formation of excimer-like states between co-facially arranged fluorenone moieties. The photoluminescence behaviour of 9-fluorenone and fluorene molecular mixtures in solution is also studied. This model system allows analysis of the green emission band independent of relative intra-and inter-molecular energy transfer effects since this system is affected only by inter-molecular energy transfer. These results provide further evidence for an excimeric origin of the green emission.