Green Photoluminescence From Platinum(ii) Complexes Bearing Silylacetylide Ligands

The synthesis, structural characterization, photoluminescence properties, and density functional theory analysis of three Pt(II) diimine complexes, Pt(dbbpy)(CdropCR)(2) [dbbpy = 4,4\u27-di(tert-butyl-2,2\u27-bipyridine; R = -SiMe3, -CdropC-SiMe3, or -t-Bu], are presented. The Pt(dbbpy)(CdropC-tBu)(2) complex serves as a carbon-based ligand structure for which the photophysical properties of the two silicon-bearing complexes are compared in dichloromethane. Pt(dbbpy)(CdropC-SiMe3)(2) and Pt(dbbpy)(CdropC-CdropC-SiMe3)(2) display visible absorptions with strong green emission (lambda(emmax) = 526 and 524 nm, respectively) while Pt(dbbpy)(CdropCt-Bu)(2) displays efficient, long-lived yellow emission (lambda(emmax) = 557 nm). Direct side by side comparisons of Pt(dbbpy)(CdropC-SiMe3)(2) and Pt(dbbpy)(CdropC-t-Bu)(2) suggest that the difference in excited state energy results from the relative sigma-donor strength of the acetylide ligands

Platinum(ii) Diimine Diacetylides: Metallacyclization Enhances Photophysical Properties

The synthesis, structural characterization, and photoluminescence properties of a new platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan- 2,2\u27-diacetylide (tda), Pt(dbbpy)( tda) [dbbpy) = 4,4\u27-di-tert-butyl-2,2\u27-bipyridine], are described. In CH2Cl2, Pt( dbbpy)( tda) exhibits a strong visible charge-transfer absorption and broad emission centered at 562 nm. The photoluminescence quantum yield and excited-state lifetime are 0.52 and 2.56 mu s, respectively, at room temperature. These parameters indicate that the planarization and rigidity introduced by the cyclic diacetylide leads to a lower-energy-absorbing species displaying enhanced photophysics relative to the analogous Pt( dbbpy)( C = CPh)(2). Time-dependent density functional theory calculations, which include solvation by CH2Cl2 via the polarizable continuum model, are used to reveal the nature of the excited states in these molecules that are responsible for the charge-transfer transitions. The 77 K emission spectra of the two compounds in EtOH/MeOH glasses are compared, uncovering tda-based ligand-localized phosphorescence in the title compound

Luminescent Charge-transfer Platinum(ii) Metallacycle

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan-2,2\u27-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4\u27-di-tert-butyl-2,2\u27-bipyridine] (1), are compared with two reference compounds, Pt(dbbpy)(C CPh)(2) (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane] (3), respectively, The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1, whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet manifold. In CH2Cl2, 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77 K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for 3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as (3)tda in nature, implying that the charge-transfer states are raised in energy, relative to the (3)tda levels in 1 in the low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined for 1-3 in CH2Cl2 and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed at room temperature. As 3 does not possess any charge-transfer character, excitation into the pi pi* transitions of the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence are responsible for the absorption transients. ESR spectroscopy of the anions 1(-) and 2(-) and reductive spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the pi* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH2Cl2 continuum revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions in these molecules. A Delta SCF-method-derived shift applied to the calculated transition energies in the solvent continuum yielded good agreement between theory and experiment for each molecule in this study

Platinum(II) Charge Transfer Chromophores: Electrochemistry, Photophysics, and Vapochromic Sensing Applications

Square planar platinum(II) complexes have demonstrated promise in a range of applications due to their interesting ground and excited state properties. It is therefore vital to understand how to control the physical and chemical properties of such metal-organic chromophores in order to be able to tune their photophysical properties according to application-specific requirements. Hence, this dissertation research focuses on the photophysical, electrochemical, and spectroelectrochemical properties of a variety of organometallic platinum(II) complexes which possess accessible ligand-centered and charge transfer excited states. In addition to these interesting solution phase properties, metal-metal interactions in the solid state can be profoundly perturbed by vapor adsorption, rendering significant changes to both sample color and photoluminescence in the solid state. This process of vapochromism was systematically investigated in this dissertation. All molecules investigated herein are composed of a Pt(II) metal center, a single substituted diimine or triimine ligand, and the remaining coordination site generally bears systematically altered alkyl- and arylacetylides. The most successful vapochromic materials are terpyridyl-based cationic complexes where the ancillary ligand is chloride. The first part of this dissertation describes the electrochemical and photophysical properties of novel Pt(II) complexes with systematically varied polyimine and acetylide ligands. The photophysical properties have been investigated by a variety of instrumentation techniques, including absorption spectrophotometry, steady-state and time-resolved photoluminescence spectroscopy, and nanosecond laser flash photolysis/transient absorption spectroscopy. The electrochemical and spectroelectrochemical properties were studied by cyclic, pulse and wave voltammetry techniques, controlled potential electrolysis, and chronoamperometry. Chapter 2 describes the details of the instrumentation and methods employed for the compilation of this study. In the following chapter, two Pt(II) polyimine complexes that possess peryleneylacetylide ligand(s) are reported. Near-IR phosphorescence at room temperature in fluid solution is observed from these chromophores where the singlet metal-to-ligand charge transfer (MLCT) transitions are utilized to sensitize the perylenylacetylide-localized triplet excited states (3π-π*). These long-lived triplet states possess characteristic excited state absorption properties and have been shown to photochemically sensitize singlet oxygen, which was identified by its unique emission spectrum centered near 1270 nm in the near-IR. The final chapter focuses on the synthesis, characterization and the vapochromic sensing applications of mononuclear Pt(II) salts in addition to other Pt(II) complexes synthesized in our laboratory. A novel approach taken here involved the utilization of alkoxy-substituted terpyridine ligands known to facilitate π-stacking in the solid state. It was anticipated that ligand-based π-π interactions would help promote d8-d8 metal-metal interactions resulting in a new generation of vapochromic sensors based on synthetically facile materials. The development of chemical sensors that detect volatile organic compounds (VOCs) by unique and reversible color and luminescence changes were demonstrated by developing cross-responsive microarrays that are composed of various Pt(II) polyimine chromophores. These “artificial noses” were successful in rapidly detecting a variety of chemical analytes and in several instances the processes are completely reversible suggesting re-useable sensor arrays based on these Pt(II) materials

Probing the surface of platinum nanoparticles with 13CO by solid-state NMR and IR spectroscopies

2040-3364The synthesis and full characterization of platinum nanoparticles (Pt NPs) prepared by decomposition of the Pt(dba)2 complex in the presence of CO and H2 and stabilized either sterically by a polymer, polyvinylpyrrolidone or chemically by a ligand, diphenylphosphinobutane, are reported. In these studies, 13CO was used as a probe molecule to investigate the surface of the particles, using IR and solid-state NMR spectroscopies with magic angle spinning (MAS-NMR). Three nanosystems with different sizes are described: Pt/PVP/13CO (monomodal: 1.2 nm), Pt/dppb/13CO (bimodal: 1.2 nm and 2.0 nm) and Pt/dppb/H2 (monomodal: 2.0 nm) NPs. Spectroscopic data suggest a modification of the electronic state of the nanoparticles between 1.2 nm and 2.0 nm which can be related to the presence of Knight shift

alpha-olefin selectivity of Fe-Cu-K catalysts in Fischer-Tropsch synthesis: Effects of catalyst composition and process conditions

Fischer-Tropsch (FT) synthesis has been studied over a series of precipitated Fe catalysts promoted to investigate the effect of K and Cu promoters on the carbon number distribution of FT product. The main intention is to maximize the alpha-olefin selectivity by controlling the secondary reactions of alpha-olefin as a function of potassium and copper loadings. The results showed that impregnation of different loadings of Cu and K promoters to precipitated iron-based catalyst was found to have significant influences on the crystallographic structure, morphological and physical properties of iron-based catalysts, as well as catalytic activity, stability and selectivity performances during FT synthesis. 100Fe7Cu3K sample exhibited the highest catalytic activity and stability in TOS tests. The catalyst is capable of working under a variety of temperatures and space velocities at a same activity level with no significant activity loss. Proper control of Cu and K contents is essential to obtain maximum C-5+ yield since the changes in the relative loadings of both Cu and K promoters affect hydrocarbon product distribution. 100Fe7Cu3K sample has high secondary alpha-olefin hydrogenation activity as olefin/paraffin ratio decreases with increasing carbon number while C-19+ selectivity is relatively low compared to other samples. The effects of space velocity and temperature on alpha-olefin selectivity are strongly dependent on the amounts of Cu and K promoters and chain length. The maximum alpha-olefin/n-paraffin ratio has been obtained over 100Fe3Cu1K sample at 543 K with a space velocity of 3 NL/h/g-cat. (C) 2011 Elsevier B.V. All rights reserved

An effective electrocatalyst based on platinum nanoparticles supported with graphene nanoplatelets and carbon black hybrid for PEM fuel cells

A facile synthesis at room temperature and at solid-state directly on the support yielded small, homogeneous and well-dispersed Pt nanoparticles (NPs) on CB-carbon black, GNP-graphene nanoplatelets, and CB-GNP-50:50 hybrid support. Synthesized Pt/CB, Pt/GNP and Pt/CB:GNP NPs were used as electrocatalysts for polymer electrolyte membrane fuel cell (PEMFC) reactions. HRTEM results displayed very small, homogeneous and well-dispersed NPs with 1.7, 2.0 and 4.2 nm mean-diameters for the Pt/CB-GNP, Pt/GNP and Pt/CB electrocatalysts, respectively. Electrocatalysts were also characterized by RAMAN, XRD, BET and CV techniques. ECSA values indicated better activity for graphene-based supports with 19 m2 g−1Pt for Pt/GNP and 55 m2 g−1Pt for Pt/CB-GNP compared to 10 m2 g−1Pt for Pt/CB. Oxygen reduction reaction (ORR) studies and fuel cell tests were in parallel with these results where highest maximum power density of 377 mW cm−2 was achieved with Pt/CB-GNP hybrid electrocatalyst. Both fuel cell and ORR studies for Pt/CB-GNP indicated better dispersion of NPs on the support and efficient fuel cell performance that is believed to be due to the prevention of restacking of GNP by CB. To the best of our knowledge, Pt/GNP and Pt/CB-GNP electrocatalysts are the first in literature to be synthesized with the organometallic mild synthesis method using Pt(dba)3 precursor for the PEMFC applications

Deoxygenation of oleic acid: Influence of the synthesis route of Pd/mesoporous carbon nanocatalysts onto their activity and selectivity

International audienceSupported Pd nanocatalysts were prepared by deposition of Pd nanoparticles (NPs) onto spherical mesoporous carbon beads (MB) functionalized by thermal or acidic treatement The Pd NPs were synthesized by decomposition of [Pd-2(dba)(3)] (dba: dibenzylideneacetone) under dihydrogen either directly on the carbon supports without stabilizer leading to naked Pd NPs (Pd/MB series) or in solution in the presence of a stabilizer (polymer (PVP series) or triphenylphosphine (TPP series)) to obtain stable colloidal solutions that were further used to impregnate the carbon materials to have carbon-deposited Pd NPs. The NPs deposited on carbon displayed a Pd loading from 0.5 to 14.8 wt.% and were characterized by different techniques (nitrogen physisorption at 77 K, H-2-chemissorption and TPD, XRD, XPS and HRTEM). Their catalytic performance in deoxygenation of oleic acid was evaluated in batch and flow reaction conditions. Flow conditions led to superior results compared to batch. No aromatic compounds were detected as side products, but in the case of the Pd/MB series, octadecanol and octadecane were significantly formed suggesting the involvement of a deoxygenation mechanism in which the hydrocarbons were produced via both decarbonylation/decarboxylation and dehydration steps. Further experiments carried out in H-2/N-2 mixture or in pure N-2 highlighted the key role of hydrogen. For a N-2/H-2 of 2.5:1 the dehydration route was crossing out and even no traces of octadecanol nor octadecane were detected. Then, complete removal of H-2 produced heptadecene in a high excess compared to heptadecane (almost 7-1) thus suggesting the decarbonylation/decarboxylation steps as the main route. ICP-OES measurements indicated no leaching of palladium and simple washing of catalysts with mesitylene allowed recycling without any change in conversion or product distribution