The first series of monodisperse glass-forming, nematic oligofluorenes was synthesized
following a divergent−convergent approach. Both chain length and pendant structure were
found to affect solid morphology and phase transition temperatures. With optimized
structural parameters, a T
g close to 150 °C and a T
c beyond 375 °C were achieved. This
material class is characterized by its ability to form monodomain glassy nematic films via
spin-coating, thermal annealing at 10 to 20 °C above T
g for 15−30 min, and cooling to room
temperature without encountering crystallization. The absorption and emission dichroic
ratios of 80−90-nm-thick films increase from 7 to 17 with an increasing molecular aspect
ratio, while the photoluminescence quantum yield varies erratically from 43 to 60%. Superior
stability of emissive color and of emission dichroic ratio and that against thermally activated
crystallization were also demonstrated through prolonged heating of a film at 10 °C above
T
g under argon.
Light‐emitting conjugated oligomers comprising anthracene, naphthalene, and fluorene units have been synthesized to investigate three configurations of blue organic light‐emitting diodes (OLEDs) that are designed to identify the origins of device instability. The transient OLED technique is employed to measure hole mobilities, which are found to be 3.1 × 10–4, 8.9 × 10–5, and 3.6 × 10–5 cm2 V–1 s–1 for three different blue‐light‐emitting model compounds with varying fluorene content. A higher hole mobility through the emissive layer results in a wider recombination zone, which, in turn, is responsible for a longer device lifetime and a lower drive voltage at the expense of luminance yield.
Polymers containing 6-and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the parts of the reacted and the unreacted coumarin moieties. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and the unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction.
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