End-to-end assembly of gold nanorods via oligopeptide linking and surfactant control

We report two novel approaches for fabricating self-assembled chains of end-to-end linked Au nanorods separated by a nanogap. In one approach, bi-functional cysteine end-capped oligopeptides of different lengths are used as the linking agent. The widths of the produced nanogaps scale with the length and tertiary structure of the peptide linker. Functionalized oligopeptides containing an acetylene group are also employed as a linker, and the functional group is uniquely identified using surface-enhanced Raman spectroscopy. The development of an oligopeptide-linking platform is motivated by the ease of synthesis and high modularity of peptides; these features enable the possibility to integrate diverse functionality into molecular nanogap junctions - synthesized in water. The stepwise nanochain formation is followed via the evolution of the longitudinal plasmon absorption band in combination with transmission electron microscopy. The reaction rate and extent is tuned by controlling the concentration of the stabilizing CTAB surfactant in the solution. At very low surfactant concentrations, spontaneous end-to-end linking of the Au nanorods is observed even in the absence of linking peptide. The assembled AuNRs may act as next-generation electrodes in a platform for molecular electronics and synthetic biology.     

Combustion chemistry of enols: possible ethenol precursors in flames

Before the recent discovery that enols are intermediates in many flames, they appeared in no combustion models. Furthermore, little is known about enols' flame chemistry. Enol formation in low-pressure flames takes place in the preheat zone, and its precursors are most likely fuel species or the early products of fuel decomposition. The OH + ethene reaction has been shown to dominate ethenol production in ethene flames although this reaction has appeared insufficient to describe ethenol formation in all hydrocarbon oxidation systems. In this work, the mole fraction profiles of ethenol in several representative low-pressure flames are correlated with those of possible precursor species as a means for judging likely formation pathways in flames. These correlations and modeling suggest that the reaction of OH with ethene is in fact the dominant source of ethenol in many hydrocarbon flames, and that addition-elimination reactions of OH with other alkenes are also likely to be responsible for enol formation in flames. On this basis, enols are predicted to be minor intermediates in most flames and should be most prevalent in olefinic flames where reactions of the fuel with OH can produce enols directly.     

Dissecting and reducing the heterogeneity of excited-state energy transport in DNA-based photonic wires

Molecular photonic wires are one-dimensional representatives of a family of nanoscale molecular devices that transport excited-state energy over considerable distances in analogy to optical waveguides in the far-field. In particular, the design and synthesis of such complex supramolecular devices is challenging concerning the desired homogeneity of energy transport. On the other hand, novel optical techniques are available that permit direct investigation of heterogeneity by studying one device at a time. In this article, we describe our efforts to synthesize and study DNA-based molecular photonic wires that carry several chromophores arranged in an energetic downhill cascade and exploit fluorescence resonance energy transfer to convey excited-state energy. The focus of this work is to understand and control the heterogeneity of such complex systems, applying single-molecule fluorescence spectroscopy (SMFS) to dissect the different sources of heterogeneity, i.e., chemical heterogeneity and inhomogeneous broadening induced by the nanoenvironment. We demonstrate that the homogeneity of excited-state energy transport in DNA-based photonic wires is dramatically improved by immobilizing photonic wires in aqueous solution without perturbation by the surface. In addition, our study shows that the in situ construction of wire molecules, i.e., the stepwise hybridization of differently labeled oligonucleotides on glass cover slides, further decreases the observed heterogeneity in overall energy-transfer efficiency. The developed strategy enables efficient energy transfer between up to five chromophores in the majority of molecules investigated along a distance of approximately 14 nm. Finally, we used multiparameter SMFS to analyze the energy flow in photonic wires in more detail and to assign residual heterogeneity under optimized conditions in solution to different leakages and competing energy-transfer processes.     

Dihydroazulene-Boron Subphthalocyanine Conjugates with Oligo(phenyleneethynylene) Bridging Unit: Photoswitchable Fluorophores

In this work, we have linked the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo-/thermoswitch and the boron subphthalocyanine (BsubPc) fluorophore via an axial oligo(phenyleneethynylene) bridging unit into new DHA-BsubPc conjugates. The objectives were to elucidate the influence of BsubPc on the DHA/VHF switching reactions and the influence of DHA/VHF on the BsubPc fluorescence in these conjugates for which the entire axial substituent connected to boron comprises one large, conjugated scaffold. We present the synthesis and properties of DHA-BsubPc conjugates with varying peripheral substituents on the BsubPc core, being either unsubstituted (H₁₂BsubPc) or partially fluorinated (F₆BsubPc). Fluorination of the BsubPc core provided a remarkable increase in the reversibility of the DHA-VHF interconversions promoted by light and heat, respectively, and accompanied by on/off switching of the BsubPc fluorescence. Synthetically, the units were connected using Sonogashira coupling reactions of suitable acetylenic building blocks.