A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices. We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry-Perot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action.

Microcavity effects and optically pumped lasing in single conjugated polymer nanowires

Strategies for the design and construction of non-linear, 2D and 3D conjugated macromolecules are presented in this critical review. The materials, termed here as star-shaped structures, feature a core unit which may or may not provide conjugated links between arms that radiate like spokes from a central axle. The arms of the macromolecules consist of linear oligomers or irregular conjugated chains lacking a formal repeat unit. The cores range from simple atoms to single or fused aromatic units and can provide a high level of symmetry to the overall structure. The physical properties of the star-shaped materials can be markedly different to their simple, linear conjugated analogues. These differences are highlighted and we report on anomalies in absorption/emission characteristics, electronic energy levels, thermal properties and morphology of thin films. We provide numerous examples for the application of star-shaped conjugated macromolecules in organic semiconductor devices; a comparison of their device performance with those comprising analogous linear systems provides clear evidence that the star-shaped compounds are an important class of material in organic electronics. Moreover, these structures are monodisperse, well-defined, discrete molecules with 100% synthetic reproducibility, and possess high purity and excellent solubility in common organic solvents. They feature many of the attributes of plastic materials (good film-forming properties, thermal stability, flexibility) and are therefore extremely attractive alternatives to conjugated polymers (210 references).

Star-shaped π-conjugated oligomers and their applications in organic electronics and photonics

Polyfluorene-based semiconductors combined with various periodic table elements for organic electronics

This review draws a rather comprehensive picture of how Suzuki polycondensation was discovered in 1989 and how it was subsequently developed into the most powerful polymerization method for polyarylenes during the last 20 years. It combines insights into synthetic issues with classes of polymers prepared and touches upon aspects of this method's technological importance. Because a significant part of the developmental work was carried out in industry, the present review makes reference to an unusually large number of patents.

Suzuki Polycondensation: Polyarylenes à la Carte

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.

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

Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions (Adv. Funct. Mater. 2004, 14, 765–781)

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=I D /I M , 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.

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

We report systematic measurements of the evolution of the emission characteristics of PFO whilst undergoing photo-oxidation. Pure PFO and highly diluted PFO/polystyrene blended films were prepared for the studies by spin-coating. Each film was oxidised by exposure to the 351 nm line of a cw Ar + laser. Both the kinetics of the various spectral components and the photoluminescence intensity for each film was monitored as a function of oxidation time and their respective behaviours were compared. Our results demonstrate that there is a strong tendency for singlet intrachain excitons initially created on pristine PFO segments to migrate to the fluorenone moieties produced by photo-oxidation. However, we conclusively show that emission from states localised at these defect sites cannot account for the appearance of the broad green emission band (g-band) that is well-known to occur in degraded polyfluorenes. Instead, it is shown that the g-band must emanate from interchain states that are formed after energy has been transferred to the fluorenone moieties (either via energy transfer from non-defective PFO segments or by direct excitation).

Aristidis Asimakis

Papers

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

Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions (Adv. Funct. Mater. 2004, 14, 765–781)

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

The interchain origin of the green emission band in oxidized poly(9,9-dioctylfluorene) (PFO)