Proton NMR studies of the structure of trans-(CH)x

The proton NMR line width in undoped and AsF₅ doped polyacetylene shows an abrupt change as a function of temperature, indicative of motional narrowing. The characteristic temperature for the onset of narrowing decreases with increasing AsF₅ concentration. The results are discussed in terms of large amplitude librational or translational motions of the trans-(CH)_x chains. The magnitudes obtained for the second moment at temperatures below and above the narrowing transition are used to obtain information on the chain packing of trans-(CH)_x within the three-dimensional crystal structure.     

Theory of the rod-to-coil transition in polydiacetylene

A theory is proposed for the rod to coil transition in polydiacetylene 4BCMU and related polymers which is based on the hypothesis that the high-temperature (yellow) phase consists predominantly of the cis structure, while the low-temperature (red) phase is trans. Because the occurrence of a cis-trans interface is energetically costly, the correlation length for either isomer remains long and the transition is sharp, much like the helix-coil transition in the theory of Zimm and Bragg. The transition is driven by the higher entropy of the cis isomer, which is free to coil, unlike the trans or butatriene forms. The theory gives excellent agreement with optical absorption measurements and is consistent with all other experimental data for this system.     

Anisotropic optical properties of pure and doped polyacetylene

Optical absorption and reflection measurements have been carried out on pure and doped polyacetylene films. The absorption data are consistent with a model of (CH)_X as a direct gap quasi-l d semiconductor; doping with AsF₅ or iodine introduces absorption within the gap, but appears to leave the interband transition intact. The anisotropic reflectance from partially aligned films and the increased optical anisotropy upon doping provide evidence of quasi-l d behavior of the semiconducting pure polymer and the metallic doped polymer.     

Elastic properties of one dimensional compounds

We have investigated the elastic properties of several one-dimensional compounds using a vibrating reed technique. A model calculation shows that a vanishing Young's modulus will occur for elastic one-dimensionality. TTF-TCNQ exhibits a small modulus which decreases with increasing temperature extrapolating to zero at T ≈ 500–700K. The uniaxial stress dependence of the metal-insulator transition temperature is calculated to be $\text{¦}\text{d}\text{T}{}_{\mathrm{c}}\text{/}\text{d}\text{σ¦ ～ (10 ± 5)}\text{K/bar}$. KCP shows a much higher modulus and a phase transformation at ～ 35 K.     

Neutron scattering study of 49 K phase transition in TTF-TCNQ

Elastic neutron scattering measurements show the existence of three low temperature phase transitions in TTF-TCNQ and are interpreted within the framework of a recent theoretical investigation by Bak and Emery. Direct evidence for the new 49 K transition is presented in addition to the 38 and 54 K transitions reported previously.     

Functionalized methanofullerenes used as n-type materials in bulk-heterojunction polymer solar cells and in field-effect transistors

The synthesis of two well-solubilized [60]methanofullerene derivatives ( p- EHO-PCBM and p- EHO-PCBA) is presented for usage in organic solar cells and in field-effect transistors. The para position of the PCBM's phenyl ring was substituted with a branched alkoxy side chain, which contributes to higher solubility, facilitating synthesis, purification, and processing. We find a small change of the open-circuit voltage ( V oc) as a slight improvement in performance upon application in P3HT/[60]methanofullerene bulk-heterojunction-photovoltaic cells, when compared to PCBM, because of the electron donation of the alkoxy group. In the case of the devices with a TiO x layer, the best power conversion efficiencies (PCE, eta e) is observed in a layered structure of P3HT/ p- EHO-PCBA/TiO x (eta e = 2.6%), which slightly exceeds that of P3HT/PCBM/TiO x (eta e = 2.3%) under conditions reported here. This can be attributed, in part, to the carboxylic acid group in p- EHO-PCBA that leads to an effective interface interaction between the active layer and TiO x phase. In addition, n-channel organic field-effect transistor (OFET) devices were fabricated with thin films of p- EHO-PCBM and p- EHO-PCBA, respectively cast from solution on SiO 2/Si substrates. The values of field-effect mobility (mu) for p- EHO-PCBM and p- EHO-PCBA are 1 x 10 (-2) and 1.6 x 10 (-3) cm (2)/V.s, respectively. The results in this paper demonstrate the effects of a carboxylic acid group and an electron-donating substituent in [60]methanofullerenes as n-type materials with respect to organic solar cells and OFET applications.     

Polymer-polymer rectifying heterojunction based on poly(3,4-dicyanothiophene) and MEH-PPV

Thin, homogeneous films of a high electron affinity (n-dopable) polymer, poly(3,4-dicyanothiophene) (PDCTh), were prepared by in-situ thermal polymerization of 2,5-diiodo-3,4-dicyanothiophene on substrates. As a result of the presence of two cyano substituents on the thiophene backbone, PDCTh is an electronegative polymer, with the LUMO at ca. 3.6 eV and the HOMO at ca. 6.7 eV. We demonstrate the fabrication of polymer-polymer rectifying heterojunctions using PDCTh and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) polymer layers on ITO-glass. Rectification ratios in the current voltage characteristics of these devices exceed 10.³ This device exhibits a photovoltaic effect with a dc sensitivity at -3 V reverse bias of 4 × 10^-4 A/W or quantum yield of 0.1% electrons/photon. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3115–3120, 1998     

Comparison of the signaling and stability of electrochemical DNA sensors fabricated from 6- or 11-carbon self-assembled monolayers

We have characterized the solution-phase and dry storage stability of electrochemical E-DNA sensors fabricated using mixed self-assembled monolayers (SAMs) composed of 6- or 11-carbon (C6 and C11, respectively) alpha,omega-thiol alcohols and the analogous C6- or C11-thiol-terminated stem-loop DNA probe. We find that the solution-phase and dry storage stability of C6-based E-DNA sensors are limited and poorly reproducible. The use of stabilizing agents bovine serum albumin plus either glucose or trehalose significantly improves the dry storage shelf life of such sensors: when using these preservatives, we observe only 7-9% sensor degradation after 1 month of storage in air at room temperature. In comparison, the stability of C11-based E-DNA sensors is significantly greater than that of the C6-based sensors; we observe only minor (5-8%) loss of signal upon storing these sensors for a week under ambient solution conditions or for more than a month in air in the presence of preservatives. Moreover, whereas the electron-transfer rate through C11 SAMs is slower than that observed for C6 SAMs, it is rapid enough to support good sensor performance. It thus appears that C11 SAMs provide a reasonable compromise between electron-transfer efficiency and sensor stability and are well suited for use in electronic DNA-sensing applications.