Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains

We study the phase diagram and orientational ordering of guest liquid crystalline (LC) rods immersed in a quenched host made of a liquid crystalline polymer (LCP) matrix with mobile side chains. The LCP matrix lies below the glass transition of the polymer backbone. The side chains are mobile and can align to the guest rod molecules in a plane normal to the local LCP chain contour. A field theoretic formulation for this system is proposed and the effects of the LCP matrix on LC ordering are determined numerically. We obtain simple analytical equations for the nematic/isotropic phase diagram boundaries. Our calculation show a nematic-nematic (N/N) first order transition from a guest stabilized to a guest-host stabilized region and the possibility of a reentrant transition from a guest stabilized nematic region to a host only stabilized regime separated by an isotropic phase. A detailed study of thermodynamic variables and interactions on orientational ordering and phases is carried out and the relevance of our predictions to experiments and computer simulations is presented.     

Rigid orthogonal bis-TEMPO biradicals with improved solubility for dynamic nuclear polarization

The synthesis and characterization of oxidized bis-thioketal-trispiro dinitroxide biradicals that orient the nitroxides in a rigid, approximately orthogonal geometry are reported. The biradicals show better performance as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments as compared to biradicals lacking the constrained geometry. In addition, the biradicals display improved solubility in aqueous media due to the presence of polar sulfoxides. The results suggest that the orientation of the radicals is not dramatically affected by the oxidation state of the sulfur atoms in the biradical, and we conclude that a biradical polarizing agent containing a mixture of oxidation states can be used for improved solubility without a loss in performance.     

Detection of Per- and Polyfluoroalkyl Substances (PFAS) by Interrupted Energy Transfer

The ubiquitous presence of per- and polyfluoroalkyl substances (PFAS) in aqueous environments has aroused societal concern. Nonetheless, effective sensing technologies for continuous monitoring of PFAS within water distribution infrastructures currently do not exist. Herein, we describe a ratiometric sensing approach to selectively detect aqueous perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentrations of μg ⋅ L⁻¹. Our method relies on the excitonic transport in a highly fluorinated poly(p-phenylene ethynylene) to amplify a ratiometric emission signal modulated by an embedded fluorinated squaraine dye. The electronic coupling between the polymer and dye occurs through overlap of π-orbitals and is designed such that energy transfer is dominated by an electron-exchange (Dexter) mechanism. Exposure to aqueous solutions of PFAS perturbs the orbital interactions between the squaraine dye and the polymer backbone, thereby diminishing the efficiency of the energy transfer and producing a “polymer-ON/dye-OFF” response. These polymer/dye combinations were evaluated in spin-coated films and polymer nanoparticles and were able to selectively detect PFAS at concentrations of ca. 150 ppb and ca. 50 ppb, respectively. Both polymer films and nanoparticles are not affected by the type of water, and similar responses to PFAS were found in milliQ and well water.     

Defining space around conducting polymers: reversible protonic doping of a canopied polypyrrole

A canopy-shaped pyrrole derivative 2 was prepared, in which a sterically demanding pendant group is juxtaposed to the pyrrole fragment to minimize interstrand pi-pi stacking interactions in the resulting polymer. Anodic polymerization of 2 afforded highly conductive poly(2), the electronic structure of which was probed by various spectroelectrochemical techniques. A limited charge delocalization within poly(2) translates into a well-defined conductivity profile, properties important for resistivity-based sensing. Notably, the bulk conductivity was precisely modulated by a rapid and reversible deprotonation and reprotonation of the polymer backbone.     

High ionization potential conjugated polymers

We report the synthesis of a series of poly(p-phenylene ethynylene)s (PPEs) with high ionization potentials and associated high excited-state electron affinities. Their photophysical properties were investigated using steady-state and time-resolved fluorescence techniques. The ionization potentials of the polymer thin films were determined using ultraviolet photoelectron spectroscopy (UPS), and those with the highest ionization potentials displayed high sensitivity for the detection of electron-donating aromatic compounds. The effects of sterics, chemical structure, and electronic properties on the polymers' sensory responses were investigated by fluorescence quenching experiments in both solution and solid thin films. In addition, we report that in some cases the excited-state charge-transfer complexes (exciplexes) of the PPEs with analytes were observed. These latter effects provide promising opportunities for the formation of sensitive and selective chemical sensors.