Effect of Ion Concentration on the Electro-Optic Response in Polymer-Stabilized Cholesteric Liquid Crystals

We have previously reported that the application of a DC field can adjust the position and/or bandwidth of the selective reflection notch in polymer-stabilized cholesteric liquid crystals (PSCLCs). The proposed mechanism of these electro-optic (EO) response is ion-facilitated electromechanical deformation of the polymer stabilizing network. Accordingly, the concentration of ions trapped within the polymer network should considerably influence the EO response of PSCLC. Our prior studies have indicated that photoinitiators can increase ion density in PSCLC by an order of magnitude. Here, we isolate the contribution of ionic impurities associated with liquid crystal monomers (LCMs) by utilizing initiator-less polymerization. PSCLCs prepared with LCM with low ion concentration show bandwidth broadening of the reflection band whereas PSCLCs prepared with LCM with high ion concentration exhibit a red shifting tuning of the reflection band. The extent of the tuning or bandwidth broadening of the CLC reflection band depends on the concentration of LCMs and the chirality of the LCM.</div

Identification of Lithocholic Acid as a Molecular Glass Host for Room-Temperature Phosphorescent Materials

Lithocholic acid was identified as a molecular glass host material for room temperature phosphorescent (RTP) chromophores. Differential scanning calorimetry (DSC) was performed on a series of structurally similar, biologically sourced molecules, including lithocholic acid, β-estradiol, cholesterol, and β-sitosterol, in an effort to identify new amorphous molecular glasses independent of plasticizing additives. DSC analysis revealed lithocholic acid and β-estradiol form stable molecular glasses post thermal processing unlike neat cholesterol and β-sitosterol. The ability of lithocholic acid and β-estradiol to stabilize high wt. % loadings of d10-pyrene and a mixture of d10-pyrene and an iridium chromophore, bis(2,4-difluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III) (FIr6), was also investigated. All β-estradiol formulations show β-estradiol cold crystallization. Optical microscopy and wide angle X-ray scattering measurements suggest spherulite β-estradiol crystals form during this process. Finally, time-resolved luminescence and phosphorescence quantum yield experiments show that the d10-pyrene RTP lifetime is longer and the d10-pyrene phosphorescence quantum yield is higher in lithocholic acid molecular glasses than in β-estradiol molecular glasses. The discrepancy in lifetime and quantum yield values is explained by quantitatively smaller rates of non-radiative decay from the triplet state of d10-pyrene in lithocholic acid

Effect of Amorphous Crosslinker on Phase Behavior and Electro-Optic Response of Polymer-Stabilized Blue Phase Liquid Crystals

Blue phase liquid crystals (BPLCs) composed of double-twisted cholesteric helices are promising materials for use in next-generation displays, optical components, and photonics applications. However, BPLCs are only observed in a narrow temperature range of 0.5&ndash;3 &deg;C and must be stabilized with a polymer network. Here, we report on controlling the phase behavior of BPLCs by varying the concentration of an amorphous crosslinker (pentaerythritol triacrylate (PETA)). LC mixtures without amorphous crosslinker display narrow phase transition temperatures from isotropic to the blue phase-II (BP-II), blue phase-I (BP-I), and cholesteric phases, but the addition of PETA stabilizes the BP-I phase. A PETA content above 3 wt% prevents the formation of the simple cubic BP-II phase and induces a direct transition from the isotropic to the BP-I phase. PETA widens the temperature window of BP-I from ~6.8 &deg;C for BPLC without PETA to ~15 &deg;C for BPLC with 4 wt% PETA. The BPLCs with 3 and 4 wt% PETA are stabilized using polymer networks via in situ photopolymerization. Polymer-stabilized BPLC with 3 wt% PETA showed switching between reflective to transparent states with response times of 400&ndash;500 &mu;s when an AC field was applied, whereas the application of a DC field induced a large color change from green to red

1,3-Bis(4-bromophenyl)propane

The title compound, C15H14Br2, obtained through the reduction of 4,4′-dibromochalcone, has monoclinic P21 symmetry at 100 K. No directional interactions could be identified in the crystal

Effect of Ion Concentration on the Electro-Optic Response in Polymer-Stabilized Cholesteric Liquid Crystals

We have previously reported that the application of a DC field can adjust the position and/or bandwidth of the selective reflection notch in polymer-stabilized cholesteric liquid crystals (PSCLCs). The proposed mechanism of these electro-optic (EO) response is ion-facilitated electromechanical deformation of the polymer stabilizing network. Accordingly, the concentration of ions trapped within the polymer network should considerably influence the EO response of PSCLC. Our prior studies have indicated that photoinitiators can increase ion density in PSCLC by an order of magnitude. Here, we isolate the contribution of ionic impurities associated with liquid crystal monomers (LCMs) by utilizing initiator-less polymerization. PSCLCs prepared with LCM with low ion concentration show bandwidth broadening of the reflection band whereas PSCLCs prepared with LCM with high ion concentration exhibit a red shifting tuning of the reflection band. The extent of the tuning or bandwidth broadening of the CLC reflection band depends on the concentration of LCMs and the chirality of the LCM

Synthesis, Electrochemical Characterization, and Linear Free Energy Relationship of 1,3-Diphenyl-6-alkyl/arylfulvenes

A series of 1,3-diphenyl-6-alkyl/arylfulvenes was prepared, and the electrochemical properties were investigated. The addition of phenyl groups about the fulvene raised the reduction potential and helped to stabilize the electrochemically generated radical anion. The addition of various functional groups onto the phenyl ring at the 6-position of 1,3,6-triphenylfulvene results in a linear free energy relationship between reduction potential and the Hammett substituent constant, σ. Further extending the conjugation at the 6-position of 1,3-diphenyl-6-arylfulvenes increases the reversibility of the redox reactions, but does not appear to further stabilize the generated radical anion. This in-depth investigation provides evidence that the compounds studied may have utility in light-harvesting applications

Synthesis of Elastomeric Liquid Crystalline Polymer Networks via Chain Transfer

Materials capable of complex shape changes have broad reaching applications spanning biomimetic devices, componentless actuators, artificial muscles, and haptic displays. Liquid crystal elastomers (LCE) are a class of shape programmable materials which display anisotropic mechanical deformations in response external stimuli. This work details a synthetic strategy to quickly and efficiently prepare LCEs through the usage of chain transfer agents (CTA). The polyacrylate materials described herein exhibit large, reversible shape changes with strains greater 475%, rivalling properties observed in polysiloxane-based networks. The approach reported here is distinguished in that the materials chemistry is readily amenable to surface alignment techniques. The facile nature of the materials chemistry and the compatibility of these materials with directed self-assembly methods could further enable paradigm shifting end uses as designer substrates for flexible electronics or as actuating surfaces

Improved Performance of Glucose Bioanodes Using Composites of (7,6) Single-Walled Carbon Nanotubes and a Ferrocene-LPEI Redox Polymer

The effect of incorporating different types of carbon nanotubes into composite films of a redox polymer (FcMe₂-C₃-LPEI) and glucose oxidase (GOX) was investigated. The composite films were constructed by first forming a high-surface area network film of either single-walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs) on a glassy carbon electrode (GCE) by solution casting of a suspension of Triton-X-100 dispersed SWNTs. Next a glucose responsive redox hydrogel was formed on top of the nanotube-modified electrode by cross-linking FcMe₂-C₃-LPEI with glucose oxidase via ethylene glycol diglycidyl ether (EGDGE). Electrochemical and enzymatic measurements showed that composite films made with (7,6) SWNTs produced a higher response (3.3 mA/cm²) to glucose than films made with (6,5) SWNTs (1.8 mA/cm²) or MWNTs (1.2 mA/cm²) or films made without SWNTs (0.7 mA/cm²). We also show that the response of the composite films could be systematically varied by fabricating SWNT films with different weight ratios of (7,6) and (6,5) SWNTs. Optimization of the (7,6) SWNTs loading and the redox polymer-enzyme film produced a glucose response of 11.2 mA/cm². Combining the optimized glucose films with a platinum oxygen breathing cathode into a biofuel cell produced a maximum power density output of 343 μW/cm²