Andrew P. Monkman

Methods for Controlling Structure and Photophysical Properties in Polyfluorene Solutions and Gels

Knowledge of the phase behavior of polyfluorene solutions and gels has expanded tremendously in recent years. The relationship between the structure formation and photophysics is known at the quantitative level. The factors which we understand control these relationships include virtually all important materials parameters such as solvent quality, side chain branching, side chain length, molecular weight, thermal history and myriad functionalizations. This review describes advances in controlling structure and photophysical properties in polyfluorene solutions and gels. It discusses the demarcation lines between solutions, gels, and macrophase separation in conjugated polymers and reviews essential solid state properties needed for understanding of solutions. It gives an insight into polyfluorene and polyfluorene beta phase in solutions and gels and describes all the structural levels in solvent matrices, ranging from intramolecular structures to the diverse aggregate classes and network structures and agglomerates of these units. It goes on to describe the kinetics and thermodynamics of these structures. It details the manifold molecular parameters used in their control and continues with the molecular confinement and touches on permanently cross-linked networks. Particular focus is placed on the experimental results of archetypical polyfluorenes and solvent matrices and connection between structure and photonics. A connection is also made to the mean field type theories of hairy-rod like polymers. This altogether allows generalizations and provides a guideline for materials scientists, synthetic chemists and device engineers as well, for this important class of semiconductor, luminescent polymers.

Recent Advances in White Organic Light‐Emitting Materials and Devices (WOLEDs)

WOLEDs offer new design opportunities in practical solid-state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large-area devices and the development of low-cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.

Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

Polyaniline thin films for gas sensing

Thin films of polyaniline have been deposited by spinning, evaporation and by the Langmuir-Blodgett technique. The films are shown to possess slightly different in-plane electrical conductivities, reflecting differences in their chemical structure and layer morphology. The conductivity is found to depend on the gas ambient. All types of polyaniline films are sensitive to H2S and NOx at concentrations down to 4 ppm. However, only spun and evaporated films are responsive to SO2.

Measurements of Solid‐State Photoluminescence Quantum Yields of Films Using a Fluorimeter

In this article we show that a commercial spectrofluorimeter can be used successfully in combination with an integrating sphere to measure solid-state photoluminescence quantum yields of films. This approach significantly simplifies the experimental method as the need for specialized equipment on the excitation and detection side is relaxed. Two different light-emitting polymer systems are examined with this approach, and the results agree with those that have been reported previously using different methods.

Absolute measurements of photoluminescence quantum yields of solutions using an integrating sphere.

We demonstrate that absolute measurements of the photoluminescence quantum yield of solutions can be made using an integrating sphere and a conventional fluorimeter. With this method the need for measurements against a luminescence standard is overcome. The sphere is mounted inside a commercial fluorimeter, which gives flexibility in excitation and emission wavelength ranges. A number of compounds have been investigated and the results are compared to literature values and data obtained using a comparative method.

Low temperature synthesis of high molecular weight polyaniline

Abstract Oxidative chemical polymerization of aniline in hydrochloric acid solution at sub-zero temperatures leads to polyaniline with a molecular weight five to ten times higher than that prepared at room temperature. The optimum polymerization temperature to give a polymer with the highest yield and molecular weight has been determined, and solution state 13C n.m.r. spectroscopy shows that the improved polyaniline has fewer defect sites compared to the standard polymer. Despite these intrinsic improvements, the conductivity is of the same order of magnitude as standard polyaniline. A solvent-cast film of polyaniline camphorsulfonate had a conductivity of 295 S cm−1, although this rose to 820 S cm−1 along the stretch direction after the film had been uniaxially oriented. An oriented film cast from N-methyl-2-pyrrolidone solution and protonated with methanesulfonic acid showed a conductivity of 260 S cm−1 along the stretch direction and 31 S cm−1 perpendicular to this.

White polymeric light-emitting diode based on a fluorene polymer∕Ir complex blend system

Efficient white-polymeric light-emitting diodes (PLED) were fabricated as a single active layer containing blue-emitting poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) endcapped with bis(4-methylphenyl)phenylamine; (PF2∕6am4), and yellow-orange-emitting iridium [tri-fluorenyl] pyridine complex [Ir(Fl3Py)3]. The fluorene-like ligands in the blended device prevent phase segregation and also enhance energy transfer from the polymer host to the guest due to efficient overlap of wave function (Dexter process) and host singlet emission and guest absorption bands (Forster process) which reduces the loading level required to produce white emission. The two emitted colors complement each other and doping levels of 2%–3% produce white emission. Above a certain current density, depending on the doping level, the device Commission Internationale de L’Eclairage (CIE) coordinates become bias independent and a stabilized white emission can be obtained. A white-emitting PLED (coordinates 0.348, 0.367) of peak external quan...

Synthesis of high molecular weight polyaniline at low temperatures

Abstract The oxidative chemical polymerization of aniline in hydrochloric acid solution has been extensively studied at low temperatures. The resulting polyaniline has a molecular weight up to ten times higher than that prepared at room temperature. Polyaniline of high molecular weight may be obtained by carrying out the polymerization at low temperatures, typically between −25 and −30 °C. This requires the use of an inert solute (lithium chloride is the preferred choice) to keep the aqueous reaction medium mobile. Among the reaction parameters studied which affected the molecular weight of the polymer were: (a) reaction temperature; (b) solution pH at the start of the reaction; (c) molar ratio of oxidant:aniline; (d) oxidant addition time; (e) total reaction time.

Optical and electronic properties of stretch-oriented solution-cast polyaniline films

Abstract We have prepared and characterized polyaniline in the form of thin free-standing films and films formed by spin coating. The films are formed from solution with chemically prepared base polyaniline (emeraldine) which is dissolved in N -methyl−2-pyrrolidone. The films, as prepared and doped with HCl, have conductivities in the range 60–70 S/cm. Optical reflection spectra reveal metallic-like films. Furthermore, we have been able to align the polyaniline (PANi) chains using uniaxial stress. Elongation ratios of up to 650% have been achieved so far. When doped with HCl these films yield conductivities of up to 350 S/cm and anisotropy σ  /σ ⊥ of up to 24. Initial polarized reflection spectra reveal optical anisotropy.