Novel nonafluorenes with a varying extent of pendant chirality were synthesized for an investigation of the origins of chiroptical activities in neat films. Thermal annealing of 4-microm-thick sandwiched films and of 90-nm-thick spin-cast films, all on surface-treated substrates, produced monodomain glassy films characterized as a right-handed cholesteric stack with a helical pitch length ranging from 180 to 534 nm and from 252 to 1151 nm, respectively. The observed strong circular dichroism (CD) and g(e) as functions of helical pitch length in single-substrate monodomain glassy cholesteric films were quantitatively interpreted with a circularly polarized fluorescence theory accounting for light absorption, emission, and propagation in a cholesteric stack. Although intertwined molecular helices were likely to be present, cholesteric stacking of rodlike molecules seemed to be the predominant contributor to the strong chiroptical activities. All the cholesteric stacks comprising a polydomain glassy film on an untreated substrate were found to contribute to CD and g(e) largely to the same extent as in a monodomain film. A circularly polarized blue organic light-emitting diode containing a nonafluorene film resulted in a g(e) of 0.35 with a luminance yield of 0.94 cd/A at 20 mA/cm(2), the best performance to date.
Colloidal quantum dots which can emit red, green, and blue colors are incorporated with a micro-LED array to demonstrate a feasible choice for future display technology. The pitch of the micro-LED array is 40 μm, which is sufficient for high-resolution screen applications. The method that was used to spray the quantum dots in such tight space is called Aerosol Jet technology which uses atomizer and gas flow control to obtain uniform and controlled narrow spots. The ultra-violet LEDs are used in the array to excite the red, green and blue quantum dots on the top surface. To increase the utilization of the UV photons, a layer of distributed Bragg reflector was laid down on the device to reflect most of the leaked UV photons back to the quantum dot layers. With this mechanism, the enhanced luminous flux is 194% (blue), 173% (green) and 183% (red) more than that of the samples without the reflector. The luminous efficacy of radiation (LER) was measured under various currents and a value of 165 lm/Watt was recorded.
A new series of morphologically stable, glassy chiral-nematic liquid crystals were
synthesized following three distinct deterministic approaches. A glass transition temperature
from 67 to 82 °C and a clearing temperature from 175 to 225 °C were accomplished. A helical
pitch length from 188 to 210 nm emerged with enantiomeric 1-phenylethylamine as the
precursors, giving rise to selective reflection in the UV region. With (+)-estrone as the chiral
moiety, a selective reflection in the near-infrared was observed. The concepts of high-performance circular polarizers and optical notch filters and reflectors were illustrated with
a mixture of enantiomeric glassy chiral nematics. Selective reflection wavelength can be
readily tuned by varying chemical composition in terms of the ratio of enantiomers or that
of chiral-nematic to nematic components.
Glass‐forming liquid crystals combine the ability of liquid crystals to self‐organize and that of polymers to undergo glass transition (instead of crystallization) upon cooling. Here is reported the synthesis and implementation of novel right‐ and left‐handed chiral‐nematic glass‐forming liquid crystals in optical notch filters and reflectors, which have various applications in liquid‐crystal displays. The observed attenuation of incident unpolarized light in 3.7 optical density units is the highest ever accomplished with organic and polymeric materials.
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