Oct 5, 2015

Singlet–Triplet Splittings in the Luminescent Excited States of Colloidal Cu+:CdSe, Cu+:InP, and CuInS2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons

Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b08547
Publication Date (Web): September 21, 2015
Copyright © 2015 American Chemical Society


Abstract Image
The electronic and magnetic properties of the luminescent excited states of colloidal Cu+:CdSe, Cu+:InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet–triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu+-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu+-doped semiconductor nanocrystals.
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Bidirectional Electron Transfer Capability in Phthalocyanine–Sc3N@Ih–C80 Complexes

Department of Organic Chemistry, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
IMDEA Nanociencia, Faraday 9, 28049 Madrid, Spain
§ Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b06454
Publication Date (Web): September 24, 2015
Copyright © 2015 American Chemical Society


Abstract Image
To activate oxidative and/or reductive electron transfer reactions, N-pyridyl-substituted Sc3N@Ih–C80 (4) and C60 (3) fulleropyrrolidines have been prepared and axially coordinated to electron-rich (1) or electron-deficient (2) Zn(II)phthalocyanines (Zn(II)Pcs) through zinc-pyridyl, metal–ligand coordination affording a full-fledged family of electron donor–acceptor ensembles. An arsenal of photophysical assays as they were carried out with, for example, 1/4 and 2/4 show unambiguously that a Zn(II)Pc-to-Sc3N@Ih–C80 photoinduced electron transfer takes place in the former ensemble, whereas a Sc3N@Ih–C80-to-Zn(II)Pc electron transfer occurs in the latter ensemble. To the best of our knowledge, this is the first time that a fullerene-based molecular building block shows an electron transfer dichotomy, namely acting both as electron-acceptor or electron-donor, and its outcome is simply governed by the electronic nature of its counterpart. In light of the latter, the present work, which involves the use of Sc3N@Ih–C80, one of the most abundant and easy-to-purify endohedral metallofullerenes, is, on one hand, a paradigmatic change and, on the other hand, an important milestone en-route toward the construction of easy-to-prepare molecular materials featuring switchable electron transfer reactivity.

Sep 22, 2015

2nd coordination sphere controlled electron transfer of iron hangman complexes on electrodes probed by surface enhanced vibrational spectroscopy

Chem. Sci., 2015, Advance Article

DOI: 10.1039/C5SC02560E

Iron hangman complexes exhibit improved catalytic properties regarding O2 and H2O2 reduction, which are attributed to the presence of a proton donating group in defined vicinity of the catalytic metal centre. Surface enhanced resonance Raman (SERR) and IR (SEIRA) spectro-electrochemistry has been applied concomitantly for the first time to analyse such iron hangman porphyrin complexes attached to electrodes in aqueous solution. While the SERR spectra yield information about the redox state of the central iron, the SEIRA spectra show protonation and deprotonation events of the 2nd coordination sphere. To investigate the influence of a proton active hanging group on the heterogeneous electron transfer between the iron porphyrin and the electrode, two hangman complexes with either an acid or ester functional group were compared. Using time resolved SERR spectroscopy the electron transfer rates of both complexes were determined. Complexes with an acid group showed a slow electron transfer rate at neutral pH that increased significantly at pH 4, while complexes with an ester group exhibited a much faster, but pH independent rate. SEIRA measurements were able to determine directly for the first time a pKavalue of 3.4 of a carboxylic hanging group in the immobilized state that shifted to 5.2 in D2O buffer solution. The kinetic data showed an increase of the heterogeneous electron transfer rate with the protonation degree of the acid groups. From these results, we propose a PCET which is strongly modulated by the protonation state of the acid hanging group via hydrogen bond interactions.

Graphical abstract: 2nd coordination sphere controlled electron transfer of iron hangman complexes on electrodes probed by surface enhanced vibrational spectroscopy

On the Mechanisms of Hydrogen-Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2Mg2O5].+: Remarkable Electronic Structure Effects

Angewandte Chemie International Edition
Volume 54Issue 40pages 11861–11864September 28, 2015
DOI: 10.1002/anie.201505336


Mechanistic insight into the homolytic cleavage of the O[BOND]H bond of water by the heteronuclear oxide cluster [Ga2Mg2O5].+ has been derived from state-of-the-art gas-phase experiments in conjunction with quantum chemical calculations. Three pathways have been identified computationally. In addition to the conventional hydrogen-atom transfer (HAT) to the radical center of a bridging oxygen atom, two mechanistically distinct proton-coupled electron-transfer (PCET) processes have been identified. The energetically most favored path involves initial coordination of the incoming water ligand to a magnesium atom followed by an intramolecular proton transfer to the lone-pair of the bridging oxygen atom. This step, which is accomplished by an electronic reorganization, generates two structurally equivalent OH groups either of which can be liberated, in agreement with labeling experiments.

Aug 31, 2015

Multimetallic catalysed cross-coupling of aryl bromides with aryl triflates

Link to Full Article

Multimetallic catalysed cross-coupling of aryl bromides with aryl triflates

  • Laura K. G. Ackerman,
  • Matthew M. Lovell
  • Daniel J. Weix
    • Nature
      • The advent of transition-metal catalysed strategies for forming new carbon-carbon bonds has revolutionized the field of organic chemistry, enabling the efficient synthesis of ligands, materials, and biologically active molecules123. In cases where a single metal fails to promote a selective or efficient transformation, the synergistic cooperation4 of two distinct catalysts—multimetallic catalysis—can be used instead. Many important reactions rely on multimetallic catalysis5678910, such as the Wacker oxidation of olefins678and the Sonogashira coupling of alkynes with aryl halides910, but this approach has largely been limited to the use of metals with distinct reactivities, with only one metal catalyst undergoing oxidative addition1112. Here, we demonstrate that cooperativity between two group 10 metal catalysts—(bipyridine)nickel and (1,3-bis(diphenylphosphino)propane)palladium—enables a general cross-Ullmann reaction (the cross-coupling of two different aryl electrophiles)131415. Our method couples aryl bromides with aryl triflates directly, eliminating the use of arylmetal reagents and avoiding the challenge of differentiating between multiple carbon–hydrogen bonds that is required for direct arylation methods1617. Selectivity can be achieved without an excess of either substrate and originates from the orthogonal reactivity of the two catalysts and the relative stability of the two arylmetal intermediates. While (1,3-bis(diphenylphosphino)propane)palladium reacts preferentially with aryl triflates to afford a persistent intermediate, (bipyridine)nickel reacts preferentially with aryl bromides to form a transient, reactive intermediate. Although each catalyst forms less than 5 per cent cross-coupled product in isolation, together they are able to achieve a yield of up to 94 per cent. Our results reveal a new method for the synthesis of biaryls, heteroaryls, and dienes, as well as a general mechanism for the selective transfer of ligands between two metal catalysts. We anticipate that this reaction will simplify the synthesis of pharmaceuticals, many of which are currently made with pre-formed organometallic reagents123, and lead to the discovery of new multimetallic reactions.
        • A general cross-Ullmann reaction catalysed by a combination of nickel and palladium.

Aug 28, 2015

Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation

Abstract Image

Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2(rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites, and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 10–12 Pa to 105 Pa results in (i) increase in the band gap from 2.42 to 2.91 eV (direct transitions) or 2.88 to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process.
J. Phys. Chem. A, Article ASAP
DOI: 10.1021/acs.jpca.5b05031

Aug 27, 2015

Saved: Size-Dependent Appearance of Intrinsic Oxq “Activated Oxygen” Molecules on Ceria Nanoparticles

Size-Dependent Appearance of Intrinsic Oxq “Activated Oxygen” Molecules on Ceria Nanoparticles by Xing Huang and Matthew J. Beck via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1MQ8zWI

Aug 25, 2015

Saved: High Stability of Immobilized Polyoxometalates on TiO2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation

High Stability of Immobilized Polyoxometalates on TiO2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation by Sarah M. Lauinger, Jordan M. Sumliner, Qiushi Yin, Zihao Xu, Guijie Liang, Elliot N. Glass, Tianquan Lian and Craig L. Hill via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1JtFHy4

Switching on elusive organometallic mechanisms with photoredox catalysis

  • Jack A. Terrett,
  • James D. Cuthbertson,
  • Valerie W. Shurtleff
  • David W. C. MacMillan
    • Link to Full Article
      • Nature

        Transition-metal-catalysed cross-coupling reactions have become one of the most used carboncarbon and carbonheteroatom bond-forming reactions in chemical synthesis. Recently, nickel catalysis has been shown to participate in a wide variety of C−C bond-forming reactions, most notably Negishi, SuzukiMiyaura, Stille, Kumada and Hiyama couplings12. Despite the tremendous advances in C−C fragment couplings, the ability to forge C−O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C–O bond-forming step (formally known as the reductive elimination step) to occur via a Ni(III) alkoxide intermediate. Here we demonstrate that visible-light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbonoxygen coupling reaction using abundant alcohols and aryl bromides. More notably, we have developed a general strategy to ‘switch on’ important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron-transfer catalysts.
        Modulating oxidation states of nickel enables challenging carbon-heteroatom coupling.

Aug 24, 2015

Saved: Toward Facet Engineering of CdS Nanocrystals and Their Shape-Dependent Photocatalytic Activities

Toward Facet Engineering of CdS Nanocrystals and Their Shape-Dependent Photocatalytic Activities by Xixi Wang, Maochang Liu, Zhaohui Zhou and Liejin Guo via The Journal of Physical Chemistry C: Latest Articles (ACS Publications) http://ift.tt/1h90mkv

Saved: Three-terminal energy harvester with coupled quantum dots

Three-terminal energy harvester with coupled quantum dots by Holger Thierschmann via Nature Nanotechnology - AOP - nature.com science feeds http://ift.tt/1h90m44