Takeshi Iwasa

Isolation, structure, and stability of a dodecanethiolate-protected Pd1Au24 cluster

A dodecanethiolate-protected Pd(1)Au(24)(SC(12)H(25))(18) cluster, which is a mono-Pd-doped cluster of the well understood magic gold cluster Au(25)(SR)(18), was isolated in high purity using solvent fractionation and high-performance liquid chromatography (HPLC) after the preparation of dodecanethiolate-protected palladium-gold bimetal clusters. The cluster thus isolated was identified as the neutral [Pd(1)Au(24)(SC(12)H(25))(18)](0) from the retention time in reverse phase columns and by elemental analyses. The LDI mass spectrum of [Pd(1)Au(24)(SC(12)H(25))(18)](0) indicates that [Pd(1)Au(24)(SC(12)H(25))(18)](0) adopts a similar framework structure to Au(25)(SR)(18), in which an icosahedral Au(13) core is protected by six [-S-Au-S-Au-S-] oligomers. The optical absorption spectrum of [Pd(1)Au(24)(SC(12)H(25))(18)](0) exhibits peaks at approximately 690 and approximately 620 nm, which is consistent with calculated results on [Pd(1)@Au(24)(SC(1)H(3))(18)](0) in which the central gold atom of Au(25)(SC(1)H(3))(18) is replaced with Pd. These results strongly indicate that the isolated [Pd(1)Au(24)(SC(12)H(25))(18)](0) has a core-shell [Pd(1)@Au(24)(SC(12)H(25))(18)](0) structure in which the central Pd atom is surrounded by a frame of Au(24)(SC(12)H(25))(18). Experiments on the stability of the cluster showed that Pd(1)@Au(24)(SC(12)H(25))(18) is more stable against degradation in solution and laser dissociation than Au(25)(SC(12)H(25))(18). These results indicate that the doping of a central atom is a powerful method to increase the stability beyond the Au(25)(SR)(18) cluster.

Luminescent Mechanochromic 9-Anthryl Gold(I) Isocyanide Complex with an Emission Maximum at 900 nm after Mechanical Stimulation

Upon mechanical stimulation, 9-anthryl gold(I) isocyanide complex 3 exhibited a bathochromic shift of its emission color from the visible to the infrared (IR) region, which is unprecedented in its magnitude. Prior to exposure to the mechanical stimulus, the polymorphs 3α and 3β exhibit emission wavelength maxima (λem,max) at 448 and 710 nm, respectively. Upon grinding, the λem,max of 3αground and 3βground are bathochromically shifted to 900 nm, i.e., Δλem,max (3α) = 452 nm or 1.39 eV. Polymorphs 3α and 3β thus represent the first examples of mechanochromic luminescent materials with λem,max in the IR region.

Experimental and theoretical studies on the electronic properties of vanadium-benzene sandwich cluster anions, VnBzn+1− (n = 1-5)

Vanadium-benzene cluster anions, V(n)Bz(n+1)(-) (Bz = C(6)H(6)) were generated by laser ablation and supersonic jet methods, and studied using photoelectron spectroscopy. The density functional theory was employed to compute their geometric and electronic structures. It is concluded that the V(n)Bz(n+1)(-) anions exhibit multiple-decker sandwich structures similar to their corresponding neutrals, and the adiabatic electron affinity increases with the cluster size. Our computation shows that the excess electron of the anion occupies the d orbitals of the vanadium atoms and that it is delocalized one-dimensionally. Furthermore, a very large HOMO-LUMO gap difference between majority and minority spin orbitals is observed for both the neutrals and the anions, and the V(n)Bz(n+1)(0∕-) clusters are found to be completely spin-polarized. These facts confirm the possibility of using V(n)Bz(n+1) clusters as spin filters.

Optical readout of hydrogen storage in films of Au and Pd

For hydrogen sensor and storage applications, films of Au and Pd were (i) co-sputtered at different rates or (ii) deposited in a sequentially alternating fashion to create a layered structure on a cover glass. Peculiarities of hydrogen uptake and release were optically monitored using 1.3 μm wavelength light. Increase of optical transmission was observed for hydrogenated Pd-rich films of 10-30 nm thickness. Up to a three times slower hydrogen release took place as compared with the hydrogen uptake. Compositional ratio of Au:Pd and thermal treatment of films provided control over the optical extinction changes and hydrogen uptake/release time constants. Higher uptake and release rates were observed in the annealed Au:Pd films as compared to those deposited at room temperature and were faster for the Auricher films. Three main parameters relevant for sensors: sensitivity, selectivity, stability (reproducibility) are discussed together with the hydrogenation mechanism in Au:Pd alloys.

Generalized theoretical method for the interaction between arbitrary nonuniform electric field and molecular vibrations: Toward near-field infrared spectroscopy and microscopy

A theoretical method to compute infrared absorption spectra when a molecule is interacting with an arbitrary nonuniform electric field such as near-fields is developed and numerically applied to simple model systems. The method is based on the multipolar Hamiltonian where the light-matter interaction is described by a spatial integral of the inner product of the molecular polarization and applied electric field. The computation scheme is developed under the harmonic approximation for the molecular vibrations and the framework of modern electronic structure calculations such as the density functional theory. Infrared reflection absorption and near-field infrared absorption are considered as model systems. The obtained IR spectra successfully reflect the spatial structure of the applied electric field and corresponding vibrational modes, demonstrating applicability of the present method to analyze modern nanovibrational spectroscopy using near-fields. The present method can use arbitral electric fields and thus can integrate two fields such as computational chemistry and electromagnetics.

Experimental and theoretical studies of the structural and electronic properties of vanadium–benzene sandwich clusters and their anions: VnBzn0/− (n = 1–5) and VnBzn−10/− (n = 2–5)

One end open V(n)Bz(n)(-) (n = 1-5; Bz = benzene) and both ends open V(n)Bz(n-1)(-) (n = 2-5) vanadium-benzene cluster anions were studied using anion photoelectron spectroscopy and density functional calculations. The smaller (n ≤ 3) V(n)Bz(n) and V(n)Bz(n-1) clusters and corresponding anions were found to have structural isomers, whereas full-sandwiched V(n)Bz(n+1) clusters preferred to form multiple-decker sandwich structures. Several isomeric V2Bz2 structures were identified theoretically and the anion photoelectron spectra of V2Bz2(0/-) were explained well by the coexistence of two isomeric structures: (1) a V2-core structure sandwiched between benzene molecules and (2) an alternating sandwich structure with the spin state strongly dependent on the structure. The adiabatic electron affinity of both V(n)Bz(n) and V(n)Bz(n-1) was found to increase with the cluster size at larger sizes (n = 4 or 5) and approaches to that of V(n)Bz(n+1). The evolution of the structural and electronic properties of V(n)Bz(m) and V(n)Bz(m)(-) (m = n and n - 1) with size is discussed in comparison with V(n)Bz(n+1) and V(n)Bz(n+1)(-).

Near-field-induced optical force on a metal particle and C{sub 60}: Real-time and real-space electron dynamics simulation

Optical forces induced by a near field are calculated for a 1-mm-sized metal particle mimicked by a jellium model and for C{sub 60} in the framework of real-time and real-space time-dependent density-functional theory combined with a nonuniform light-matter interaction formalism, fully taking account of multipole interaction. A highly localized near field nonuniformly polarizes these molecules. The locally induced polarization charges in the molecules are partly canceled by the screening charges. The polarization and screening charges generally contribute to the attractive and repulsive forces, respectively, and a sensible balance between these charges results in several peaks in the optical force as a function of the frequency of the near field. The resonance excitation does not necessarily maximally induce the net force, and the force exerted on the molecules strongly depends on the details of their electronic structures. The optical force is larger in the metal particle than in C{sub 60}. We also found that the optical force depends linearly on the intensity of the near field.