In the quest for low-molecular-weight metal sulfur complexes that bind nitrogenase-relevant small molecules and can serve as model complexes for nitrogenase, compounds with the [Ru(PiPr(3))('N(2)Me(2)S(2)')] fragment were found ('N(2)Me(2)S(2)'(2-)=1,2-ethanediamine-N,N'-dimethyl-N,N'-bis(2-benzenethiolate)(2-)). This fragment enabled the synthesis of a first series of chiral metal sulfur complexes, [Ru(L)(PiPr(3))('N(2)Me(2)S(2)')] with L=N(2), N(2)H(2), N(2)H(4), and NH(3), that meet the biological constraint of forming under mild conditions. The reaction of [Ru(NCCH(3))(PiPr(3))('N(2)Me(2)S(2)')] (1) with NH(3) gave the ammonia complex [Ru(NH(3))(PiPr(3))('N(2)Me(2)S(2)')] (4), which readily exchanged NH(3) for N(2) to yield the mononuclear dinitrogen complex [Ru(N(2))(PiPr(3))('N(2)Me(2)S(2)')] (2) in almost quantitative yield. Complex 2, obtained by this new efficient synthesis, was the starting material for the synthesis of dinuclear (R,R)- and (S,S)-[micro-N(2)[Ru(PiPr(3))('N(2)Me(2)S(2)')](2)] ((R,R)-/(S,S)-3). (Both 2 and 3 have been reported previously.) The as-yet inexplicable behavior of complex 3 to form also the R,S isomer in solution has been revealed by DFT calculations and (2)D NMR spectroscopy studies. The reaction of 1 or 2 with anhydrous hydrazine yielded the hydrazine complex [Ru(N(2)H(4))(PiPr(3))('N(2)Me(2)S(2)')] (6), which is a highly reactive intermediate. Disproportionation of 6 resulted in the formation of mononuclear diazene complexes, the ammonia complex 4, and finally the dinuclear diazene complex [micro-N(2)H(2)[Ru(PiPr(3))('N(2)Me(2)S(2)')](2)] (5). Dinuclear complex 5 could also be obtained directly in an independent synthesis from 1 and N(2)H(2), which was generated in situ by acidolysis of K(2)N(2)(CO(2))(2). Treatment of 6 with CH(2)Cl(2), however, formed a chloromethylated diazene species [[Ru(PiPr(3))('N(2)Me(2)S(2)')]-micro-N(2)H(2)[Ru(Cl)('N(2)Me(2)S(2)CH(2)Cl')]] (9) ('N(2)Me(2)S(2)CH(2)Cl'(2-) =1,2-ethanediamine-N,N'-dimethyl-N-(2-benzenethiolate)(1-)-N'-(2-benzenechloromethylthioether)(1-)]. The molecular structures of 4, 5, and 9 were determined by X-ray crystal structure analysis, and the labile N(2)H(4) complex 6 was characterized by NMR spectroscopy.
A stable nonlinear optical point light source is investigated, based on field enhancement at individual, pointed gold nanocones with sub-wavelength dimensions. Exciting these cones with near-infrared, focused radially polarized femtosecond beams allows for tip-emission at the second harmonic wavelength (second harmonic generation, SHG) in the visible range. In fact, gold nanocones with ultra-sharp tips possess interesting nonlinear optical (NLO) properties for SHG and two-photon photoluminescence (TPPL) emission, due to the enhanced electric field confinement at the tip apex combined with centrosymmetry breaking. Using two complementary optical setups for bottom or top illumination a sharp tip SHG emission is discriminated from the broad TPPL background continuum. Moreover, comparing the experiments with theoretical calculations manifests that these NLO signatures originate either from the tip apex or the base edge of the nanocones, clearly depending on the cone size, the surrounding medium, and illumination conditions. Finally, it is demonstrated that the tip-emitted signal vanishes when switching from radial to azimuthal polarization.
We report on the in-situ controlled tuning of the particle gap in single pairs of gold nanodisks by photochemical metal deposition. The optically induced growth of nanodisk dimers fabricated by electron beam lithography leads to a decrease of the interparticle gap width down to 0 nm. Due to the increasing particle size and stronger plasmonic coupling, a smooth redshift of the localized surface plasmon (LSP) resonances is observed in such particle pairs during the growth process. The interparticle gap width, and hence the LSP resonance, can be tuned to any desired spectral position. The experimental results we obtain with this nanoscale fabrication technique are well described by the so-called plasmon ruler equation. Consequently, both the changes in particle diameter as well as in gap width can be characterized in-situ via far-field read-out of the optical properties of the dimers.
For two-dimensional (2D) arrays of metallic nanorods arranged perpendicular to a substrate several methods have been proposed to determine the electromagnetic near-field distribution and the surface plasmon resonances, but an analytical approach to explain all optical features on the nanometer length scale has been missing to date. To fill this gap, we demonstrate here that the field distribution in such arrays can be understood on the basis of surface plasmon polaritons (SPPs) that propagate along the nanorods and form standing waves. Notably, SPPs couple laterally through their optical near fields, giving rise to collective surface plasmon (CSP) effects. Using the dispersion relation of such CSPs, we deduce the condition of standing-wave formation, which enables us to successfully predict several features, such as eigenmodes and resonances. As one such property and potential application, we show both theoretically and in an experiment that CSP propagation allows for polarization conversion and optical filtering in 2D nanorod arrays. Hence, these arrays are promising candidates for manipulating the light polarization on the nanometer length scale.
A detailed computational study of the wavelength-dependent efficiency of optical second-harmonic generation in plasmonic nanostructures is presented. The computations are based on a discontinuous Galerkin Maxwell solver that utilizes a hydrodynamic material model to calculate the free-electron dynamics in metals without any further approximations. Besides wave-mixing effects, the material model thus contains the full nonlocal characteristics of the electromagnetic response, as well as intensity-dependent phenomena such as the Kerr effect. To be specific, two prototypical nanostructures are studied in depth with the help of two independent computer codes. For an infinitely long metal cylinder, it is found that the spectral position of linear particle plasmon modes (dipolar modes, higher-order modes, and, for frequencies above the plasma frequency also bulk plasmon modes) and their associated relative strengths for scattering and absorption both at the fundamental and second-harmonic wavelengths largely control the conversion efficiency. Notably, Fabry− Perot resonances associated with longitudinal bulk plasmons may be detectable via background-free second-harmonic spectroscopy. For a more complex V-groove nanostructure, it becomes possible to engineer a doubly resonant scenario at the fundamental and the second-harmonic wavelength. This leads to an efficient enhancement of second-harmonic emission. Our work thus demonstrates that the careful design of nanostructures on the nonlocal linear level facilitates highly efficient nanoantennas for second-harmonic emission with applications in background-free imaging and frequency conversion systems.
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