Electronic Transitions of Gold Nanoclusters

Abstract

Size-dependent luminescence is one of the most promising properties in semiconductor nanocrystals.1 -3 Such sizedependent luminescence is also found in various metal nanocrystals like the gold nanoclusters.4 -6 For example, blue luminescence was observed from Au 8 , which was stabilized and encapsulated by dendrimers. The color of the luminescence was extended to near-IR, as the size of the gold nanoclusters increased, 7,8 which was attributed to the confinement of the cluster sizes comparable to the Fermi wavelength of electrons (approximately 0.7 nm). 9 We recently studied the luminescence of the gold nanoclusters, especially the blue luminescence of Au 8 , 10 which provided a clue to the origin of luminescence in the gold nanoclusters. 11 -13 In this work, we examined the electronic transitions of Au 8 to elucidate the photoluminescence spectrum of the gold nanoclusters in more detail. The calculated geometries and energies of the isomers in Au 8 indicated that one isomer was more stable than the others, implying that a single isomer was the main contributor to the photoluminescence spectrum of Au 8 . The calculated spectrum of the most stable isomer agreed with the experimental spectrum, suggesting that several excited states of the single isomer contributed to the photoluminescence spectrum, whose shape was broad and asymmetric.The photoluminescence of the gold nanoclusters encapsulated by dendrimers was previously reported. 10 The emission maximum at 460 nm (2.7 eV) matched the emission energy of Au 8 (2.75 eV) in the spherical jellium approximation (Figure 1 (a)).8 On the other hand, the photoluminescence band was broad and asymmetric, which was ascribed to the spectral overlapping of emission bands.10 Indeed, the photoluminescence spectrum could be deconvoluted by two Gaussian functions. The low-energy band at 535 nm (2.32 eV) happened to agree with the emission energy of Au 13 (2.34 eV) in the spherical jellium model. 8 In this regard, another nanocluster was previously assigned as Au 13 , 10 because Au 13 was also the stable size of the gold nanoclusters. 8 For better understanding of the broad and asymmetric shape, the electronic transitions of Au 8 were calculated using the GAUSSIAN program.14 At first, the stable geometries and energies of Au 8 were obtained, by analytical gradients with full optimization using ab initio method such as the Møller-Plesset perturbation theory (MP2) and the density functional theory (DFT) such as Becke three-parameter Lee-Yang-Parr (B3LYP) and Perdew-Wang 1991 (B3PW91) gradient corrected exchange and correlation functional at two basis sets (CEP-4G and LANL2DZ). These levels of calculation have been frequently employed to obtain the stable geometries and energies of small gold nanoclusters. 15 -17 Since the emission energy of the dendrimer-encapsulated Au 8 matched the spherical jellium model, three-dimensional geometries were mainly examined, although the most stable geometry was known to be two-dimensional in the gas phase.15 -17 Among the optimized three-dimensio...

Conjugation of Cisplatin Analogues and Cyclooxygenase Inhibitors to Overcome Cisplatin Resistance

“…[14a, 15] However, the beneficial effect of increased lipophilicity reaches an upper limit when the corresponding drop in water solubility limits the bioavailability of the complex. [40] Thus, the high lipophilicity of the indomethacin conjugates might impede passing of cell membranes and accumulation in cells.…”

Bifunctional SO4/ZrO2catalysts for 5-hydroxymethylfufural (5-HMF) production from glucose

“…The spectra were recorded in the range of 0-1350 cm −1 using 514 nm source, 5x lens, 2 second exposure time, 100 accumulation and 100 % laser power. Diffuse Reflectance Infra-red Fourier 75 Transform (DRIFT) spectra were obtained using a Nicolet Avatar 370 MCT with Smart Collector accessory, mid/near infrared source and mercury cadmium telluride (MCT-A) photon detector at -196 °C (liquid N 2 ). Samples were diluted with KBr powder (10 wt% in KBr) for analysis then loaded into an environmental 80 cell and subjected to additional drying under vacuum at 110 °C for 10 min prior to measurements to remove moisture physisorbed during air exposure.…”

“…67 However, it should be noted that for homogeneous processes, the conversion of glucose is reported to be exhibit good first order behaviour and is independent of 65 concentration over the range 0.01-0.16 M. 68 Likewise, the kinetics of HMF formation from fructose are not reported to show a strong concentration dependence when operating at a fructose concentration ~0.7 M. 69 We thus expect that any changes in reactivity at higher initial glucose concentration would result in 70 an increased probability of side reactions when more concentrated HMF containing mixtures are produced. In light of this working at particularly high glucose concentrations may not be advisable and suggest operation in a biphasic system under more dilute glucose concentrations conditions could improve 75 HMF selectivity under continuous processing conditions. We are presently investigating this and the synthesis of high area, nanoporous SZ analogues to afford even greater control over ZrO 2 crystallisation, and opportunities to tune catalyst hydrophobicity.…”

“…Water/MIBK), which allows reactive extraction of 5-HMF from the aqueous phase thereby limiting side reactions, is a promising approach to continuous 5-HMF production, 8,27 when catalysed by liquid 8,28,29 or solid acids. [30][31][32] A tandem homogeneous 75 Lewis/Brönsted acid catalysed process, utilising AlCl 3 and HCl in a biphasic system of water/2-sec-butylphenol to respectively isomerise glucose to fructose and then dehydrate fructose to 5-HMF, recently achieved a 62 % 5-HMF yield. 33 Other homogeneous metal halides, 34, 35 including Cr(III) Zn(II) and Sn(IV) and more water tolerant lanthanide chloride, 36 can also effect glucose conversion, but confer lower HMF selectivity of ~45-48%.…”

Síntesis química, estudios de caracterización y reactividad de un material catalítico a base de ZrO2-H3PW12O40

“…Este efecto estabilizador se ha observado también en óxidos de circonio modificados con iones sulfato, fosfato y borato. [28][29][30] Los materiales sintetizados presentaron isotermas del tipo IV según la clasificación de Brunauer-Deming-Teller, 31,32 generalmente este tipo de isoterma se observa para sólidos mesoporosos (Figura 1). Las muestras Z500 y ZH500 desarrollaron ciclos de histéresis del tipo H1.…”