Blue, green, and red polymerizable light‐emitting liquid crystals have been patterned photolithographically in a full‐color liquid‐crystal electroluminescent display (see Figure). A new hole‐transporting photoalignment copolymer is also reported and the spatial patterning of the polarization direction of emission is demonstrated.
Light-emitting liquid crystals for organic light-emitting diodes (OLEDs) require low-temperature liquid crystal phases for room-temperature processing and a range of molecular
energies for electron and hole injection, as well as tunable color and color purity for multicolor
OLEDs. We report a number of light-emitting polymerizable liquid crystals (reactive
mesogens) based on 2,7-disubstituted-9,9-dialkylfluorene, whose energy levels can be tuned
for optimized charge injection and light emission. As a consequence of these systematic
property/structure investigations small molecule reactive mesogens have been synthezised,
which exhibit low melting points, even below room temperature and nematic phases above
room temperature as single components. Many of the molecules retain a supercooled nematic
phase on cooling to room temperature. Simple binary eutectic mixtures of reactive mesogens
with identical aromatic cores form light-emitting nematic phases at room temperature with
a high clearing point to generate a high order parameter. The ionization potential of six-ring fluorene reactive mesogens can be tuned between 4.93 and 5.57 eV by chemical
modification of the aromatic cores. Similarly the emission spectrum can be tuned from blue
to green. A typical performance for an OLED using such liquid crystalline materials as a
cross-linked polymer network is described.
Polarized electroluminescence from a uniformly aligned nematic network, formed by photopolymerization of a LC monofluorene with diene end‐groups (see Figure), is reported. It is shown that macroscopic orientation of the chromophore is achieved with a photoalignment layer doped to allow hole transport. Thus standard photolithography can be used to make polarized, patterned, multilayer organic electroluminescent displays.
Light-emitting liquid crystals incorporating two photopolymerizable end groups have been synthesized for implementation in multilayer organic electroluminescent devices. Series of diene as well as diallylamine and methacrylate moieties are used as the photoreactive groups attached via spacers to both ends of a fluorene-based chromophore. Nematic glasses are formed upon cooling from the liquid crystalline phase. Ultraviolet radiation at room temperature is used to photopolymerize and cross-link the reactive units, resulting in the formation of insoluble nematic polymer networks. The quantum efficiency of photoluminescence from the fluorene-based chromophore is increased by cross-linking of the diene reactive end groups. Photopolymerization occurs more rapidly with methacrylate end groups, but the chromophore is somewhat degraded by the incident radiation. In materials incorporating the photopolymerizable 1,4-pentadien-3-yl group, the formation of the polymer network enhances the electroluminescence. An electron-transporting polymer containing an oxaziaole ring is deposited on top of the insoluble network. Electroluminescence is obtained with an unchanged spectrum.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.