We report excellent correlations between the first negative threshold potentials (V THs) for electric conduction, electrochemical potentials, and computed lowest unoccupied molecular orbital energies ...in a series of phenylene−ethynylene oligomers bearing a sulfur-based anchoring unit and different electroactive substituents on the central benzene ring. The theoretical and electrochemical results strongly suggest that the peaks observed in the i − V curves have a true molecular origin and are associated with distinct unoccupied molecular levels of the compounds that are strongly localized on the central ring (except for compound I). This localization might account for the existence of a long-lived radical−anion state that permits lateral electron hopping and leads to charge trapping and storage.
A simple, direct method for photopatterning poly(p-phenylenevinylene) (PPV) from a xanthate precursor polymer is presented. The effect of UV exposure on the resultant PPV is examined by UV-Vis, XPS, ...FTIR, and DC conductivity measurements. By optimizing the photolithographic conditions, a spatial resolution of one micron is obtained, with minimal impact to the properties of the photopatterned PPV.
Plasmonic fluorescent nanocomposites are difficult to prepare due to strong quenching effects on fluorophores in the vicinity of noble metal nanoparticles such as gold (AuNPs). We successfully ...prepared plasmonic fluorescent nanocomposites of two cyanines (1 and 2) aggregating upon 2 – 40 nm AuNPs or streptavidin-conjugated 10 nm AuNPs. We used high throughput screening (HTS) for the first time to characterize the spectral properties, aggregation kinetics, aggregation density and photostability of the nanocomposites. Fluorescence from nanocomposites declined inversely with AuNPs size: 40 nm ≥ 20 nm > 10 nm > 5 nm > 2 nm. Sensitivity (limit of detection, LOD, 105 – 1011 AuNPs/mL), brightness of the nanocomposites and surface coverage of AuNPs by cyanine aggregates were all influenced by five factors: 1) AuNPs size; 2) cyanine type (1 or 2); 3) aggregate density; 4) distance between aggregates and AuNPs surface; and 5) streptavidin protein conjugation to AuNPs. We propose a model for plasmonic fluorescent nanocomposites based on these observations. Our plasmonic fluorescent nanocomposites have applications in chemical and biological assays.
Photolithographic patterning of a xanthate precursor to poly(3,4‐diphenyl‐2,5‐thienylene vinylene) is described. Unlike xanthate precursors to poly(p‐phenylene vinylene), the thienylene vinylene ...analogue patterns as a positive tone resist. Characterization of irradiated films reveals photooxidative cleavage of the vinylene linker decreases the molecular weight of the polymer (increasing the solubility of the UV‐exposed areas). As a result of the mechanism, the developed pattern sees no UV light exposure, which is a significant advantage compared with negative‐tone‐conjugated polymer resists. Single micron resolution of a low‐bandgap polymer is achieved in an efficient and scalable process.
A xanthate precursor to poly(3,4‐diphenyl‐2,5‐thienylene vinylene) is shown to photopattern as a positive tone resist. Photooxidative cleavage of the polymer backbone increases the solubility of the light‐exposed areas, allowing for pattern formation. Single micrometer resolution of an insoluble active material is achieved without compromise to the polymer's properties.
Conducting polymers are seeing ever-increased use in electronic and optoelectronic applications. While a variety of techniques are available to pattern conducting polymers, the demand for low cost, ...high throughput, and good spatial resolution continues to drive research efforts in this area. We have previously developed a method to pattern poly(p-phenylenevinylene) (PPV) using contact photolithography. Here, the synthesis, characterization, and photopatterning of alkoxy substituted PPVs (a much more commonly utilized derivative) is presented. Utilizing a photoacid generator, the polymer systems are demonstrated to pattern to one micron spatial resolution. The patterning process is demonstrated to have little effect on the polymers properties as the materials retain good optical characteristics and high conductivities upon doping.
Several potential molecular devices have been synthesized consisting of oligo(phenylene ethynylene) (OPE) backbones containing a terminal nitrile group alligator clip as a means of attachment to a ...metal surface. The synthesis of four new nitrile-containing OPEs is discussed, including an improved synthesis of an intermediate used in our prior production of OPEs containing acetate-protected thiol alligator clips.
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Several new classes of oligomers have been synthesized with functionalities designed to aid in the understanding of molecular device behavior, specifically when molecules are interfaced between ...proximal electronic probes. The compounds synthesized are series of azobenzenes, bipyridines and oligo(phenylene vinylene)s that bear acetyl-protected thiols for ultimate attachment to metallic surfaces. Some initial electrochemical and solid-state test results are also reported.
The synthesis and testing of a resetable molecular switch is described.
New syntheses of ethyl and nitro substituted oligo(phenylene ethynylene)s (OPEs) have been developed. To further explore whether the presence of nitro functionality in OPEs leads to switching and ...memory capabilities, new nitro substituted OPEs have been designed and synthesized. An isatogen-based system, a structure that is isomeric to the nitro OPE, has been synthesized. Additionally, pyridine-based and chromium-based compounds have been synthesized. We surmise that redox reactions of these candidates may impart switching capabilities and electrochemical studies are shown. U-shaped OPEs were synthesized to inhibit leakage of metals deposited during formation of top contacts on self-assembled monolayers (SAMs). The OPEs contain either thiol-based moieties or isonitrile groups to enable formation of SAMs on metal substrates.
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A novel electrograft method of forming covalent bonds directly to silicon hydride or platinum surfaces has been demonstrated with the use of iodonium salt precursors. Initially, a silicon substrate ...is treated with a dilute hydrofluoric acid wash to ensure a hydride-passivated surface, which is immersed in a solution of iodonium salt dissolved in a solution of tetrabutylammonium tetrafluoroborate in acetonitrile. Grafting is accomplished by applying a negative bias to the silicon substrate. Directed assembly has been demonstrated with Pt substrates.
Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on H-passivated Si(111) and studied by UHV STM and ...XPS. STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. STM I(V) data acquired at different tip−substrate separations reveals a factor of ∼10 enhancement in current for positive bias voltage when current flows through the 4-trifluoromethyl molecule when compared to the 4-methyl variant. The observed current enhancement in I(V) can be understood by comparing the projected density of states of the two molecule−Si systems calculated using a density functional theory local density approximation after geometry optimization was performed via the conjugate gradient method. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provide evidence that the molecules chemically react with the silicon surface.
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