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Search Results: 1 - 10 of 494506 matches for " K. Müllen "
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First-principles modeling of the polycyclic aromatic hydrocarbons reduction
D. W. Boukhvalov,X. Feng,K. Müllen
Physics , 2011, DOI: 10.1021/jp2024928
Abstract: Density functional theory modelling of the reduction of realistic nanographene molecules (C42H18, C48H18 and C60H24) by molecular hydrogen evidences for the presence of limits in the hydrogenation process. These limits caused the contentions between three-fold symmetry of polycyclic aromatic hydrocarbon molecules and two-fold symmetry of adsorbed hydrogen pairs. Increase of the binding energy between nanographenes during reduction is also discussed as possible cause of the experimentally observed limited hydrogenation of studied nanographenes.
En ben d r
Rikke von Müllen
Dansk Universitetspaedagogisk Tidsskrift , 2010,
Abstract:
Sally Browns syv effektive kritikmetoder
Rikke von Müllen
Dansk Universitetspaedagogisk Tidsskrift , 2007,
Abstract: Skulle du v re s uheldig, at du ikke kom med til DUNkonferencen i maj, gik din v rkt jskasse glip af Sally Browns syv l ringsfremmende, tidsbesparende m der at give studerende l bende, fremadrettet kritik p . Metoderne pr senteres her sammen med et kig ind i klassev relset hos en ekstern underviser med erfaringer i at benytte lignende v rkt jer.
Undervisningsevaluering og udvikling af underviserkompetencer
Rikke von Müllen
Dansk Universitetspaedagogisk Tidsskrift , 2006,
Abstract: Konklusionen p denne artikel er, at kriteriebaseret og deltagerbaseret evaluering er interessante alternativer til de forskellige former for tilfredshedsevaluering, n r fokus er p udvikling af underviserkompetencer. Formerne har hver deres styrker og svagheder, men kan begge v re med til at sikre, at kvaliteten af data fra studenterevalueringer bliver h jere, og at underviserne ikke f ler sig d mt af ukvalificerede.
At forberede forberedelsen. Fra den p dagogisk-didaktiske v rkt jskasse
Rikke von Müllen
Dansk Universitetspaedagogisk Tidsskrift , 2011,
Abstract: Denne korte artikel pr senterer en simpel model, som illustrerer, hvordan man med fordel kan systematisere den praksis at s tte tid af i undervisningen til at forberede og kvalificere de studerendes selvst ndige studiearbejde mellem undervisningsgangene.
Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane - a NEXAFS Study
K. Medjanik,S. A. Nepijko,H. J. Elmers,G. Sch?nhense,P. Nagel,M. Merz,S. Schuppler,D. Chercka,M. Baumgarten,K. Müllen
Physics , 2010,
Abstract: It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds. Microcrystals of tetra- and hexamethoxypyrene as donors with the strong acceptor tetracyanoquinodimethane (TMPx/HMPx - TCNQy) were grown from solution via vapour diffusion in different stoichiometries x:y = 1:1, 1:2 and 2:1. Owing to the element specificity of NEXAFS, the oxygen and nitrogen K-edge spectra are direct spectroscopic fingerprints of the donating and accepting moieties. The orbital selectivity of the NEXAFS resonances allows to precisely elucidate the participation of specific orbitals in the charge-transfer process. In the present case charge is transferred from methoxy-orbitals 2e (PI*) and 6a1 (SIGMA*) to the cyano-orbitals b3g and au (PI*) and - to a weaker extent - to b1g and b2u (SIGMA*). The occupation of 2e reflects the anionic character of the methoxy groups. Surprisingly, the charge transfer increases with increasing HMP content of the complex. As additional indirect signature, all spectral features of the donor and acceptor are shifted to higher and lower photon energies, respectively. Providing quantitative access to the relative occupation of specific orbitals, the approach constitutes the most direct probe of the charge-transfer mechanism in organic salts found so far. Although demonstrated for the specific example of pyrene-derived donors with the classical acceptor TCNQ, the method is very versatile and can serve as routine probe for novel CT-complexes on the basis of functionalized polycyclic aromatic hydrocarbons.
Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s
G. Heimel,M. Daghofer,J. Gierschner,E. J. W. List,A. C. Grimsdale,K. Müllen,D. Beljonne,J. -L. Brédas,E. Zojer
Physics , 2005, DOI: 10.1063/1.1839574
Abstract: The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab-initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.
A new charge-transfer complex in UHV co-deposited tetramethoxypyrene and tetracyanoquinodimethane
K. Medjanik,S. Perkert,S. Naghavi,M. Rudloff,V. Solovyeva,D. Chercka,M. Huth,S. A. Nepijko,T. Methfessel,C. Felser,M. Baumgarten,K. Müllen,H. J. Elmers,G. Sch?nhense
Physics , 2010, DOI: 10.1103/PhysRevB.82.245419
Abstract: UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane (TMP1-TCNQ1) on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared (IR) spectroscopy and scanning tunnelling spectroscopy (STS). The formation of an intermolecular charge-transfer (CT) compound is evident from the appearance of new reflexes in XRD (d1= 0.894 nm, d2= 0.677 nm). A softening of the CN stretching vibration (red-shift by 7 cm-1) of TCNQ is visible in the IR spectra, being indicative of a CT of the order of 0.3e from TMP to TCNQ in the complex. Characteristic shifts of the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of from DFT calculations (Gaussian03 with hybrid functional B3LYP). STS reveals a HOMO-LUMO gap of the CT complex of about 1.25 eV being much smaller than the gaps (>3.0 eV) of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.
Charge-Transfer Controlled Crystallization of a Model Oligomer for Donor-Acceptor-Polythiophenes
Ralph Rieger,Volker Enkelmann,Klaus Müllen
Materials , 2010, DOI: 10.3390/ma3031904
Abstract: A model donor-acceptor oligomer consisting of benzodithiophene-diketone and thiophene has been investigated with regard to its molecular packing and opto-electronic properties. The crystal structure suggests that the packing is dominated by charge-transfer interactions between the electron-rich part of the molecule and the electron-poor part. A series of observations corroborate this assumption, among them are a charge-transfer band in the film absorption spectra and exceptionally low π-π distances. A detailed analysis of the energetic levels of the present system reveals that only the HOMO level of the acceptor is shifted by conjugation to the donor. The results can contribute to the development of improved materials for organic electronics.
WINROP Identifies Severe Retinopathy of Prematurity at an Early Stage in a Nation-Based Cohort of Extremely Preterm Infants
Pia Lundgren, Elisabeth Stoltz Sj?str?m, Magnus Domell?f, Karin Kllen, Gerd Holmstr?m, Anna-Lena H?rd, Lois E. Smith, Chatarina L?fqvist, Ann Hellstr?m
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0073256
Abstract: Objective To evaluate the ability of a postnatal weight-gain algorithm (WINROP) to identify sight-threatening retinopathy of prematurity (ROP type 1) in a nation-based extremely preterm infant cohort. Methods This study enrolled all 707 live-born extremely preterm (gestational age [GA] <27 weeks) infants, born 2004–2007 in Sweden; the Extremely preterm Infants in Sweden Study (EXPRESS). WINROP analysis was performed retrospectively in 407 of the infants using weekly weight gain to assess the preterm infant’s risk of developing ROP type 1 requiring treatment. GA, birthweight (BW), and weekly postnatal weight measurements were entered into WINROP. WINROP signals with an alarm to indicate if the preterm infant is at risk for ROP type 1. Results In this extremely preterm population, WINROP correctly identified 96% (45/47) of the infants who required treatment for ROP type 1. The median time from alarm to treatment was 9 weeks (range, 4–20 weeks). Conclusions WINROP, an online surveillance system using weekly weight gain, identified extremely preterm infants at risk for ROP type 1 requiring treatment at an early stage and with high sensitivity in a Swedish nation-based cohort.
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