A unique case of combined enhanced mechanical performance and tunable structural color for chiral composites from cellulose nanocrystals (CNCs) arises by adding nanofibrillar wood-derived polymers with similar chemical compositions. When amorphous polysaccharides (pullulan, dextran, and xylan) are added, they intercalate seamlessly into the original helicoidal organization via interstitial volumes within nanocrystals and between nematic monolayers. The polysaccharides fill the available free volume in between packed nanocrystals until 40 wt% content is reached, with no phase separation occurring. Due to the inter-nanocrystal intercalation, these natural polysaccharide-cellulose composites show a nearly twofold increase in toughness and modulus. Beyond improved mechanics, the preserved iridescence shows a dramatic red shift from blue to near-infrared region, expanding the initial pitch length without disturbing the long-range chiral ordering. In the mechanistic model suggested here, the individual backbones first form intercalated morphologies in the interstitial volumes between tightly packed nanocrystals. After this, the polysaccharides form a monolayer, and eventually double layer, between nanocrystal monolayers, thus incrementally "unwinding" initial chiral organization. The resulting CNC-polysaccharide films maintain their vivid iridescence with broad color appearance and are among the first entirely biobased composites to maintain iridescence with improved mechanics.
We fabricated a yellow organic light-emitting diode (OLED) based on the star-shaped donor compound tri(9-hexylcarbazol-3-yl)amine, which provides formation of the interface exciplexes with the iridium(III) bis[4,6-difluorophenyl]-pyridinato-N,C2']picolinate (FIrpic). The exciplex emission is characterized by a broad band and provides a condition to realize the highly effective white OLED. It consists of a combination of the blue phosphorescent emission from the FIrpic complex and a broad efficient delayed fluorescence induced by thermal activation with additional direct phosphorescence from the triplet exciplex formed at the interface. The fabricated exciplex-type device exhibits a high brightness of 38 000 cd/m(2) and a high external quantum efficiency.
We
showed large area uniformly aligned chiral photonic bioderived films
from a liquid crystal phase formed by a cellulose nanocrystal (CNC)
suspension placed in a thin capillary. As a result of the spatial
confinement of the drying process, the interface between coexisting
isotropic and chiral phases aligns perpendicular to the long axis
of the capillary. This orientation facilitates a fast homogeneous
growth of chiral pseudolayers parallel to the interface. Overall,
the formation of organized solids takes hours vs weeks in contrast
to the slow and heterogeneous process of drying from the traditional
dish-cast approach. The saturation of water vapor in one end of the
capillary causes anisotropic drying and promotes unidirectional propagation
of the anisotropic phase in large regions that results in chiral CNC
solid films with a uniformly oriented layered morphology. Corresponding
ordering processes were monitored in situ at a nanoscale, mesoscale,
and microscopic scale with complementary scattering and microscopic
techniques. The resulting films show high orientation order at a multilength
scale over large regions and preserved chiral handedness causing a
narrower optical reflectance band and uniform birefringence over macroscopic
regions in contrast to traditional dish-cast CNC films and those assembled
in a magnetic field and on porous substrates. These thin films with
a controllable and well-identified uniform morphology, structural
colors, and handedness open up interesting possibilities for broad
applications in bioderived photonic nanomaterials.
A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10 and 2 × 10 cm V s, respectively, at electric fields higher than 5.3 × 10 V cm. The resulting WOLED exhibited a maximum luminance of 6579 cd m with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell.
The starburst carbazole derivative
and phosphorescent bis-cyclometalated
iridium(III) complex (IC2) were used for the preparation
of multilayered “warm-white” organic light-emitting
diodes (OLEDs), the emission spectra of which are modulated by the
thickness of the phosphorescent layer. It was shown that the electroluminescence
spectra of the fabricated devices are more extended into the visible
region compared with the photoluminescence spectra of both component
materials. The observed extension of the electroluminescence spectra
can be assigned to the phosphorescent emission of the low-energy exciplex
formed at the interface of the emissive layers. The quantum-chemical
calculations performed by the DFT and (TD) DFT methods support the
formation of the low-energy triplet exciplex at the interface of the IC2 layer and the neighboring layer of the starshaped carbazole-based
compound, (4,4′,4″-tris[3-methylphenyl(phenyl)amino]
triphenylamine, tri(9-hexylcarbazol-3-yl)amine (THCA). Indeed, the triplet excited state of such bimolecular complex corresponds
to intermolecular charge transfer between IC2 and THCA. The experimentally observed electrophosphorescence of
these exciplexes is induced by strong spin–orbit coupling in
the THCA:IC2 complexes due to the Ir(III) heavy atom
effect. With dependence on the iridium(III)-complex film thickness
(5–9 nm), the CIE coordinates changed from (0.41, 0.41) to
(0.52, 0.47), corresponding to the warm white and orange color. The
brightness of the fabricated OLEDs at the 15 V bias was in the range
from 500 to 6000 cd/m2.
Diphenilamino-substituted carbazoles were used as guest compounds for the preparation of highly efficient blue organic light-emitting diodes based on the phenomenon of delayed fluorescence. It was shown that the spectra of the delayed fluorescence of host−guest systems are identical to those of the prompt fluorescence and in general coincide with the photoluminescence spectra of the guest films. The congruence of the prompt and delayed fluorescence spectra is explained by the effective intermolecular triplet−singlet (T → S) energy transfer from the excited T states of the host to the S states of the guest molecules. High external electroluminescence efficiency of the fabricated electroluminescent devices, reaching 17%, is comparable to that achieved in phosphorescence-based organic light-emitting diodes.
A new triaryl molecule based on a benzene–benzothiadiazole–benzene core has been applied in a WOLED device. This very simple molecule emits from a combination of emissive states (exciton/electromer/exciplex/electroplex) to give white light with CIE coordinates of (0.38, 0.45) and a colour temperature of 4500 K
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