Martin M Nielsen

Technical University of Denmark, København, Capital Region, Denmark

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Publications (27)221.85 Total impact

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    ABSTRACT: We report on the synthesis and characterization of fluorescent halogen substituted anthracene-bridge-aniline (ABA) supermolecules that undergo structural reorganization on photoexcitation to form transient complexes. The syntheses were achieved in high yields on a large scale and the molecular structures were confirmed by single crystal X-ray diffraction. The photophysics of the ABA supermolecules were investigated using steady state and time resolved optical spectroscopy. Despite the presence of heavy atoms the series of ABA molecules have high quantum yields of fluorescence from both a locally excited anthracene state (LE) and an excited state complex (exciplex, EP) in non-polar solvents. The kinetics of the excited state processes were established in decalin from the time-resolved emission, and was shown to be strongly influenced by an electron-transfer state (ET). For quantitative studies of the excited state dynamics, the presence of this state required the development of a numerical three-excited-state kinetic model to replace the commonly used two-excited-state model. The experimental results shows that the reaction rates are strongly influenced both by substituents and solvent, illustrating the importance of including all relevant states in the kinetic modeling. Ultimately it is established that the excited state dynamics can conveniently be followed by optical methods, and the applicability of the system as a model system in time-resolved X-ray scattering experiments is discussed.
    06/2014;
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    ABSTRACT: Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.
    Nature 05/2014; · 38.60 Impact Factor
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    ABSTRACT: In time-resolved laser pump, X-ray probe wide-angle X-ray scattering experiments on systems in solution the structural response of the system is accompanied by a solvent response. The solvent response is caused by reorganization of the bulk solvent following the laser pump event, and in order to extract the structural information of the solute, the solvent response has to be treated. Methodologies capable of doing so include both theoretical modelling and experimental determination of the solvent response. In the work presented here, we have investigated how to obtain a reproducible solvent response-the solvent term-experimentally when applying laser pump, X-ray probe time-resolved wide-angle X-ray scattering. The solvent term describes difference scattering arising from the structural response of the solvent to changes in the hydrodynamic parameters: pressure, temperature and density. We present results based on NIR and dye mediated solvent heating, and demonstrate that the solvent response is independent of the heating method. The NIR heating is shown to be rendered unusable by higher order effects under certain experimental conditions, while the dye mediated solvent heating is demonstrated to exhibit first order behaviour with respect to the amount of energy deposited in the solution. We introduce a standardized method for recording solvent responses in laser pump, X-ray probe time-resolved X-ray wide-angle scattering experiments by using dye mediated solvent heating. Furthermore, we have generated a library of solvent terms, which can be used to describe the solvent term in any TRWAXS experiment, and made it available online.
    Physical Chemistry Chemical Physics 08/2013; · 3.83 Impact Factor
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    ABSTRACT: Building a detailed understanding of the structure function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 +/- 0.03 angstrom. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.
    Journal of Physical Chemistry Letters 01/2013; 4(11):1972-1976. · 6.59 Impact Factor
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    ABSTRACT: A joint experimental and theoretical study of singlet exciton diffusion in spin‐coated poly(3‐hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co‐facial π–π aggregates of polymer chromophores and about 100 times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions. Singlet exciton diffusion in poly(3‐hexylthiophene) (P3HT) is studied using a combination of experimental and theoretical methods. It is shown that exciton diffusion is faster along the co‐facial π–π aggregates than between them. The fastest exciton diffusion is observed for intermediate molecular weight polymers and it correlates with the degree of chromophore aggregation and with exciton delocalization along the polymer chain.
    Laser Physics Review 01/2013; 3(11). · 10.04 Impact Factor
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    ABSTRACT: We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) FeII complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS–LS difference spectra measured at thermal spin crossover, and reference HS–LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe 1s pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales.
    Journal of Electron Spectroscopy and Related Phenomena 09/2012; 188:166-171. · 1.71 Impact Factor
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    ABSTRACT: Time-resolved X-ray diffraction and optical reflectivity are used to directly measure three different acoustic oscillations of InAs nanowires. The oscillations are excited by a femtosecond laser pulse and evolve at three different time scales. We measure the absolute scale of the initial radial expansion of the fundamental breathing eigenmode and determine the frequency by transient optical reflectivity. For the extensional eigenmode we measure the oscillations of the average radial and axial lattice constants and determine the amplitude of oscillations and the average extension. Finally we observe a bending motion of the nanowires. The frequencies of the eigenmodes are in good agreements with predictions made by continuum elasticity theory and we find no difference in the speed of sound between the wurtzite nanowires and cubic bulk crystals, but the measured strain is influenced by the interaction between different modes. The wurtzite crystal structure of the nanowires however has an anisotropic thermal expansion.
    Nano Letters 07/2010; 10(7):2461-5. · 13.03 Impact Factor
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    ABSTRACT: The present communication describes the identification and structural characterization of a photo-induced transient trinuclear Ag-Pt-Pt complex, in which a pronounced internal structural change of the excited-state PtPOP moiety is observed upon complexation with the Ag ion.
    Physical Chemistry Chemical Physics 07/2010; 12(26):6921-3. · 3.83 Impact Factor
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    ABSTRACT: Molecular movies: Time-resolved X-ray scattering provides direct structural information on an electronically excited complex while it is formed in the bimolecular reaction between excited octahydrogen[tetrakis-mu-diphosphito-1kappaP:2kappaP'-diplatinate](4-) (PtPOP*) and thallium ions. In the exciplex one thallium(I) and two platinum(II) ions are found to be collinear.
    Angewandte Chemie International Edition 05/2009; 48(23):4180-4. · 11.34 Impact Factor
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    ABSTRACT: By use of specular X-ray reflectivity (XR) the structure of a metal-covered organic thin film device is measured with angstrom resolution. The model system is a Langmuir-Blodgett (LB) film, sandwiched between a silicon substrate and a top electrode consisting of 25 A titanium and 100 A aluminum. By comparison of XR data for the five-layer Pb2+ arachidate LB film before and after vapor deposition of the Ti/Al top electrode, a detailed account of the structural damage to the organic film at the buried metal-molecule interface is obtained. We find that the organized structure of the two topmost LB layers (approximately 5 nm) is completely destroyed due to the metal deposition.
    Nano Letters 03/2009; 9(3):1052-7. · 13.03 Impact Factor
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    ABSTRACT: The structures of the ground and excimer states of perylene pairs are calculated [using density functional theory (DFT) and time-dependent DFT techniques] in a free as well as a crystal environment, and their spectroscopic properties are studied for the most stable configurations. The vertical transition energies for the absorption and emission bands are obtained, and they are in good agreement with experimental data. In these calculations, up to six excited states are considered. With the calculated structures of the ground and excimer states, the scattering factors are analyzed as a function of the concentration of excimers in a crystal. The intensity of the 110, 005, and 0 10 0 reflections are found to be fairly sensitive to the presence of excimers in the crystal. The finite (nanosecond) lifetime of the excimer may make it possible to observe this state using time-resolved X-ray diffraction techniques.
    The Journal of Physical Chemistry A 10/2008; 112(35):8179-87. · 2.77 Impact Factor
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    ABSTRACT: Computationally efficient simulations of grazing-incidence X-ray diffraction (GIXD) are discussed, with particular attention given to textured thin polycrystalline films on supporting substrates. A computer program has been developed for simulating scattering from thin films exhibiting varying degrees of preferred orientation. One emphasized common case is that of a `fibre' symmetry axis perpendicular to the sample plane, resulting from crystallites having one well defined crystal facet towards the substrate, but no preferred in-plane orientation. Peak splitting caused by additional scattering from the totally substrate-reflected beam (two-beam approximation) and refraction effects are also included in the program, together with the geometrical intensity corrections associated with GIXD measurements. To achieve `user friendliness' for scientists less familiar with diffraction, the mathematically simplest possible descriptions are sought whenever feasible. The practical use of the program is demonstrated for a selected thin-film example, perylene, which is of relevance for organic electronics.
    Journal of Applied Crystallography 04/2008; 41(2). · 3.34 Impact Factor
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    ABSTRACT: Blends and other multicomponent systems are used in various polymer applications to meet multiple requirements that cannot be fulfilled by a single material. In polymer optoelectronic devices it is often desirable to combine the semiconducting properties of the conjugated species with the excellent mechanical properties of certain commodity polymers. Here we investigate bicomponent blends comprising semicrystalline regioregular poly(3-hexylthiophene) and selected semicrystalline commodity polymers, and show that, owing to a highly favourable, crystallization-induced phase segregation of the two components, during which the semiconductor is predominantly expelled to the surfaces of cast films, we can obtain vertically stratified structures in a one-step process. Incorporating these as active layers in polymer field-effect transistors, we find that the concentration of the semiconductor can be reduced to values as low as 3 wt% without any degradation in device performance. This is in stark contrast to blends containing an amorphous insulating polymer, for which significant reduction in electrical performance was reported. Crystalline-crystalline/semiconducting-insulating multicomponent systems offer expanded flexibility for realizing high-performance semiconducting architectures at drastically reduced materials cost with improved mechanical properties and environmental stability, without the need to design all performance requirements into the active semiconducting polymer itself.
    Nature Material 01/2007; 5(12):950-6. · 35.75 Impact Factor
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    ABSTRACT: Organic electronics technology, in which at least the semiconducting component of the integrated circuit is an organic material, offers the potential for fabrication of electronic products by low-cost printing technologies, such as ink jet, gravure offset lithography and flexography. The products will typically be of lower performance than those using the present state of the art single crystal or polysilicon transistors, but comparable to amorphous silicon. A range of prototypes are under development, including rollable electrophoretic displays, active matrix liquid crystal (LC) displays, flexible organic light emitting diode displays, low frequency radio frequency identification tag and other low performance electronics. Organic semiconductors that offer both electrical performance and stability with respect to storage and operation under ambient conditions are required. This work describes the development of reactive mesogen semiconductors, which form large crosslinked LC domains on polymerization within mesophases. These crosslinked domains offer mechanical stability and are inert to solvent exposure in further processing steps. Reactive mesogens containing conjugated aromatic cores, designed to facilitate charge transport and provide good oxidative stability, were prepared and their liquid crystalline properties evaluated. The organization and alignment of the mesogens, both before and after crosslinking, were probed by grazing incidence wide-angle X-ray scattering of thin films. Both time-of-flight and field effect transistor devices were prepared and their electrical characterization reported.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 11/2006; 364(1847):2779-87. · 2.89 Impact Factor
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    ABSTRACT: We show that charge-transporting polymer chains in the brush conformation can be synthesized from a variety of substrates of interest, displaying a high degree of stretching and showing up to a 3 orders of magnitude increase in current density normal to the substrate as compared with a spin-coated film. These nanostructured polymeric films may prove to be suitable for electronic devices based on molecular semiconductors as current fabrication techniques often provide little control over film structure.
    Nano Letters 04/2006; 6(3):573-8. · 13.03 Impact Factor
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    ABSTRACT: The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.
    The Journal of Physical Chemistry B 01/2006; 109(47):22319-25. · 3.61 Impact Factor
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    ABSTRACT: The dehydrogenation kinetics of pure and nickel (Ni)-doped (2w/w%) magnesium hydride (MgH2) have been investigated by in situ time-resolved powder X-ray diffraction (PXD). Deactivated samples, i.e. air exposed, are investigated in order to focus on the effect of magnesium oxide (MgO) surface layers, which might be unavoidable for magnesium (Mg)-based storage media for mobile applications. A curved position-sensitive detector covering 120∘ in 2θ and a rotating anode X-ray source provide a time resolution of 45s and up to 90 powder patterns collected during an experiment under isothermal conditions. A quartz capillary cell allowed the in situ study of gas/solid reactions. Three phases were identified: Mg, MgH2 and MgO and their phase fractions were extracted by Rietveld refinement or integration of selected reflections from each phase. Dehydrogenation curves were constructed and analysed by the Johnson–Mehl–Avrami formalism in order to derive rate constants at different temperatures. The apparent activation energies for dehydrogenation of pure and Ni-doped magnesium hydride were EA≈300 and 250kJ/mol, respectively. Differential scanning calorimetry gave, EA=270kJ/mol for dehydrogenation of the Ni-doped sample. The relatively high activation energies are due to MgO surface layers, retarding the diffusion of hydrogen (H2) out of MgH2/Mg. The observed difference in EA of ca. 50kJ/mol is likely due to the catalytic effect of Ni on the recombination of H atoms to H2 molecules verified by theoretical considerations.
    International Journal of Hydrogen Energy 01/2006; 31(14):2052-2062. · 3.55 Impact Factor
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    ABSTRACT: Charge transport properties in organic semiconductors depend strongly on molecular order. Here we demonstrate field-effect transistors where drain current flows through a precisely defined array of nanostripes made of crystalline and highly ordered molecules. The molecular stripes are fabricated across the channel of the transistor by a stamp-assisted deposition of the molecular semiconductors from a solution. As the solvent evaporates, the capillary forces drive the solution to form menisci under the stamp protrusions. The solute precipitates only in the regions where the solution is confined by the menisci once the critical concentration is reached and self-organizes into molecularly ordered stripes 100-200 nm wide and a few monolayers high. The charge mobility measured along the stripes is 2 orders of magnitude larger than the values measured for spin-coated thin films.
    Nano Letters 01/2006; 5(12):2422-5. · 13.03 Impact Factor
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    ABSTRACT: Interchain interactions have a profound effect on the optical as well as charge transport properties of conjugated polymer thin films. In contrast to oligomeric model systems in solution-deposited polymer thin films the study of such effects is complicated by the complex microstructure. We present here a detailed study of interchain interaction effects on both charged polarons as well as neutral excitons in highly crystalline, high-mobility poly-3-hexylthiophene (P3HT) as a function of molecular weight. We find experimental evidence for reduced exciton bandwidth and increased polaron delocalization with increasing conjugation length and crystalline quality. From comparative studies of field-effect transistor characteristics, film morphology, and optical properties our study provides a microscopic understanding of the factors which limit the charge transport in P3HT to field-effect mobilities around 0.1 cm2∕V s, and which will need to be addressed to improve mobility further.
    Physical Review B 01/2006; 74(11). · 3.66 Impact Factor
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    ABSTRACT: Spin-coated poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films of different molecular weights (Mn= 9-255 kg/mol), both in the pristine and annealed state, were studied in an effort to elucidate changes in the polymer packing structure and the effects this structure has on the optoelectronic and charge transport properties of these films. A model based on quantum chemical calculations, wide-angle X-ray scattering, atomic force microscopy, Raman spectroscopy, photoluminescence, and electron mobility measurements was developed to describe the restructuring of the polymer film as a function of polymer chain length and annealing. In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the F8 and BT units, and the BT units in neighboring chains are close to one another. Annealing films to sufficiently high transition temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain ("alternating structure"), and the torsion angle between F8 and BT units is reduced. This restructuring, dictated by the strong dipole on the BT unit, subsequently affects the efficiencies of interchain electron transfer and exciton migration. Films exhibiting the alternating structure show significantly lower electron mobilities than those of the pristine high molecular weight films, due to a decrease in the efficiency of interchain electron transport in this structure. In addition, interchain exciton migration to low energy weakly emissive states is also reduced for these alternating structure films, as observed in their photoluminescence spectra and efficiencies.
    Journal of the American Chemical Society 10/2005; 127(37):12890-9. · 10.68 Impact Factor

Publication Stats

481 Citations
221.85 Total Impact Points

Institutions

  • 2013
    • Technical University of Denmark
      • Department of Physics
      København, Capital Region, Denmark
  • 2010–2013
    • University of Copenhagen
      • • Centre for Molecular Movies
      • • Niels Bohr Institute
      København, Capital Region, Denmark
  • 2009
    • IT University of Copenhagen
      København, Capital Region, Denmark