Daniel A. Poulsen

CSU Mentor, Long Beach, California, United States

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Publications (17)119.16 Total impact

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    ABSTRACT: Self-assembly and light-induced mechanical switching of azobenzene derivatives deposited on GaAs(110) were explored at the single molecule level using scanning tunneling microscopy (STM). 3,3′,5,5′-Tetra-tert-butylazobenzene (TTB-AB) molecules in the trans isomer configuration were found to form well-ordered islands on GaAs(110). After exposure to ultraviolet (UV) light, the TTB-AB molecules exhibited conformational changes attributed to trans to cis photoisomerization. Photoisomerization of TTB-AB/GaAs is observed to occur preferentially in one-dimensional (1D) stripes. This 1D cascade behavior differs significantly from optically induced switching behavior observed when TTB-AB molecules are placed on a gold surface.
    The Journal of Physical Chemistry C. 12/2011; 116(1):1052–1055.
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    ABSTRACT: We have used scanning tunneling microscopy to investigate the structure and photoswitching behavior of azobenzene molecules functionalized with bulky spacer groups and adsorbed onto Au(111). We find that positioning tert-butyl "legs" in a canted arrangement on the azobenzene phenyl rings quenches photoisomerizability of the molecule on Au(111). Addition of cyano groups at the para positions changes the molecular self-assembly significantly, but does not alter the quenched photoisomerizability. This behavior likely arises from a combination of molecule-surface interactions, molecule-molecule interactions, and alteration of azobenzene electronic structure resulting from the position-specific addition of tert-butyl groups.
    The Journal of Chemical Physics 12/2010; 133(23):234707. · 3.12 Impact Factor
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    ABSTRACT: We demonstrate a simple and versatile approach to produce hierarchical assemblies of nanoparticles by combining block copolymers and small molecules. Directed nanoparticle assemblies can be achieved with a variety of nanoparticles and small molecules without modification of the nanoparticle ligands. This novel approach opens up new routes toward the fabrication of nanoparticle-based functional devices using a “Bottom-Up” approach.
    Nanotechnology (IEEE-NANO), 2010 10th IEEE Conference on; 09/2010
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    ABSTRACT: New soluble quinacridone-based molecules have been developed as electron donor materials for solution-processed organic solar cells. By functionalizing the pristine pigment core of quinacridone with solubilizing alkyl chains and light absorbing/charge transporting thiophene units, i.e., bithiophene (BT) and thienylbenzo[c][1,2,5]thiadiazolethienyl (BTD), we prepared a series of multifunctional quinacridone-based molecules. These molecular donors show intense absorption in the visible spectral region, and the absorption range and intensity are well-tuned by the interaction between the quinacridone core and the incorporated thiophene units. The thin film absorption edge extends with the expansion of molecular conjugation, i.e., 552 nm for N,N'-di(2-ethylhexyl)quinacridone (QA), 592 nm for 2,9-Bis(5'-hexyl-2,2'-bithiophene)-N,N'-di(2-ethylhexyl)quinacridone (QA-BT), and 637 nm for 4-(5-hexylthiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (QA-BTD). The change of molecular structure also influences the electrochemical properties. Observed from cyclic voltammetry measurements, the oxidation and reduction potentials (vs ferrocene) are 0.7 and -1.83 V for QA, 0.54 and -1.76 V for QA-BT, and 0.45 and -1.68 V for QA-BTD. Uniform thin films can be generated from both single component molecular solutions and blend solutions of these molecules with [6,6]-phenyl C70-butyric acid methyl ester (PC70BM). The blend films exhibit space-charge limited current (SCLC) hole mobilities on the order of 1×10(-4) cm(2) V(-1) S(-1). Bulk heterojunction (BHJ) solar cells using these soluble molecules as donors and PC70BM as the acceptor were fabricated. Power conversion efficiencies (PCEs) of up to 2.22% under AM 1.5 G simulated 1 sun solar illumination have been achieved and external quantum efficiencies (EQEs) reach as high as ∼45%.
    ACS Applied Materials & Interfaces 09/2010; 2(9):2679-86. · 5.90 Impact Factor
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    ABSTRACT: We report the synthesis and characterization of boron(subphthalocyanine) derivatives with bithiophene and quaterthiophene as axial ligands, i.e., thiophene-subphthalocyanine dyads (nT-SubPcs), and their application in organic photovoltaic cells (OPVs). Thin films of nT-SubPcs prepared via solution processing can act as the electron donor in bilayer OPVs with evaporated C(60) as the electron acceptor. The photophyscial and morphological properties of the nT-SubPcs are studied to rationalize OPV device parameters. The single-crystal X-ray structure is solved for two dyads to show the molecular structures in the solid state, and UV-vis spectroscopy and fluorescence spectroscopy are used to characterize the effect of conjugated thiophene ligands on the photophysical properties, i.e., absorption and photoluminescence quantum yield. Cyclic voltammetry, density functional theory (DFT) calculations, and low-temperature photoluminescence spectra show that photoluminescence yields depend on the overall flexibility of the SubPc derivatives and not on the oxidation potential or electronic relationship of the ligand and macrocycle molecular orbitals. We show with grazing-incidence X-ray scattering and atomic force microscopy (AFM) that careful choice of ligand structure can improve the crystallinity of thin films that leads to a relative increase in short-circuit current in OPV device. Our work clearly demonstrates that SubPcs can be used as light-harvesting chromophores in a matrix of a crystalline organic semiconductor for OPVs.
    ACS Applied Materials & Interfaces 09/2010; · 5.90 Impact Factor
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    ABSTRACT: Single-molecule-resolved scanning tunneling microscopy of tetra-tert-butyl azobenzene (TTB-AB) molecules adsorbed onto Au(111) reveals chirality selection rules in their photoswitching behavior. This observation is enabled by the fact that trans-TTB-AB molecules self-assemble into homochiral domains. Cis-TTB-AB molecules produced via photoisomerization are found in two distinct conformations with final state chirality determined by the initial trans isomer chirality. Based on these observations and ab initio calculations, we propose a new inversion-based dynamical photoswitching mechanism for azobenzene molecules at a surface.
    Physical Review Letters 04/2010; 104(17):178301. · 7.73 Impact Factor
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    ABSTRACT: Surface-bound photoactive organic molecules reveal substantially different photomechanical switching properties compared to when they are in solution-based environments. Metal surfaces, for example, often reduce photomechanical activity due to molecule-substrate interactions. Semiconductor surfaces are expected to induce different molecular switching behavior due to the presence of a band gap, potentially resulting in longer excited-state lifetimes and enhanced control of photomechanical properties. Here we report our exploration of single-molecule-resolved self-assembly and photomechanical switching capability of azobenzene derivatives on semiconducting GaAs(110) using variable temperature scanning tunneling microscopy.
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    ABSTRACT: The use of cross-linked polymer nanoparticles was explored to achieve site isolation of different emitters within a single emissive layer in an electroluminescent device. Encapsulation of the iridium emitters within the polymer nanoparticles led to the desired effect with minimal energy transfer from high bandgap chromophores to lower bandgap ones. The nanoparticles were easily dispersed in organic solvent for film casting while preserving particle shape. They behave as light emitting "inks", enabling the tuning of electroluminescence through simple changes in the ratios of nanoparticles in the emissive film.
    Nano Letters 03/2010; 10(4):1440-4. · 13.03 Impact Factor
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    ABSTRACT: A series of solution-processable and crystalline platinum−acetylide oligomers containing a thienyl−benzothiadiazole−thienyl core and oligothiophene alkynyl ligands are synthesized and characterized. X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH−π interactions in single crystals. These oligomers show two intense and broad absorption bands in the visible spectral region, with the short-wavelength absorption band being strongly dependent on the oligothiophene length. In neat films, all the oligomers form large crystalline domains of several hundred nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities on the order of 10−5−10−4 cm2 V−1 s−1. The photovoltaic properties of these oligomers were evaluated by fabricating bulk heterojunction devices with fullerene derivatives (PC61BM and PC71BM) and some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly properties have potential for solution-processed organic photovoltaics.
    Chemistry of Materials - CHEM MATER. 02/2010; 22(7).
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    Advanced Materials 01/2010; 22(1):77-82. · 14.83 Impact Factor
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    ABSTRACT: Precise control of the spatial organization of nanoscopic building blocks, such as nanoparticles, over multiple length scales is a bottleneck in the 'bottom-up' generation of technologically important materials. Only a few approaches have been shown to achieve nanoparticle assemblies without surface modification. We demonstrate a simple yet versatile approach to produce stimuli-responsive hierarchical assemblies of readily available nanoparticles by combining small molecules and block copolymers. Organization of nanoparticles into one-, two- and three-dimensional arrays with controlled inter-particle separation and ordering is achieved without chemical modification of either the nanoparticles or block copolymers. Nanocomposites responsive to heat and light are demonstrated, where the spatial distribution of the nanoparticles can be varied by exposure to heat or light or changing the local environment. The approach described is applicable to a wide range of nanoparticles and compatible with existing fabrication processes, thereby enabling a non-disruptive approach for the generation of functional devices.
    Nature Material 10/2009; 8(12):979-85. · 35.75 Impact Factor
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    ABSTRACT: Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule-molecule interactions and molecule-surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having approximately 10 nm pitch. These findings can be summarized into photoswitching selection rules that highlight the important role played by a molecule's nanoscale environment in determining its switching properties.
    Nano Letters 03/2009; 9(3):935-9. · 13.03 Impact Factor
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    ABSTRACT: Here, a new series of crosslinkable heteroleptic iridium (III) complexes for use in solution processed phosphorescent organic light emitting diodes (OLEDs) is reported. These iridium compounds have the general formula of (PPZ-VB)2Ir(CˆN), where PPZ-VB is phenylpyrazole (PPZ) vinyl benzyl (VB) ether; and the CˆN ligands represent a family of four different cyclometallating ligands including 1-phenylpyrazolyl (PPZ) (1), 2-(4,6-difluorophenyl)pyridyl (DFPPY) (2), 2-(p-tolyl)pyridyl (TPY) (3), and 2-phenylquinolyl (PQ) (4). With the incorporation of two crosslinkable VB ether groups, these compounds can be fully crosslinked after heating at 180 °C for 30 min. The crosslinked films exhibit excellent solvent resistance and film smoothness which enables fabrication of high-performance multilayer OLEDs by sequential solution processing of multiple layers. Furthermore, the photophysical properties of these compounds can be easily controlled by simply changing the cyclometallating CˆN ligand in order to tune the triplet energy within the range of 3.0–2.2 eV. This diversity makes these materials not only suitable for use in hole transporting and electron blocking but also as emissive layers of several colors. Therefore, these compounds are applied as effective materials for all-solution processed OLEDs with (PPZ-VB)2IrPPZ (1) acting as hole transporting and electron blocking layer and host material, as well as three other compounds, (PPZ-VB)2IrDFPPY (2), (PPZ-VB)2IrTPY(3), and (PPZ-VB)2IrPQ(4), used as crosslinkable phosphorescent emitters.
    Advanced Functional Materials 02/2009; 19(7):1024 - 1031. · 10.44 Impact Factor
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    Nick H Werstiuk, Daniel A Poulsen
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    ABSTRACT: QTAIM-DI-VISAB analyses at the CCSD and B3PW91 levels were used to characterize the bonding of the cyclopropylcarbinyl (1) and the so-called 'nonclassical' bicyclobutonium and 1-methylbicyclobutonium cations, 2 and 3 as well as the transition state for rearrangement of 1 to 2. These analyses involved obtaining QTAIM molecular graphs and delocalization indexes (DIs) for pairs of atoms that were correlated with the proximities of atomic basins (VISAB). This study showed that the supposed nonclassical bicyclobutonium and 1-methylbicyclobutonium cations do not exhibit penta-coordinate carbons at their equilibrium geometries as has been claimed. Both species are best described as distorted cyclobutyl cations that exhibit a single ring critical point in the topology of the charge density.
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    ABSTRACT: We have used single-molecule-resolved scanning tunneling microscopy to measure the photomechanical switching rates of azobenzene-derived molecules at a gold surface during exposure to UV and visible light. This enables the direct determination of both the forward and reverse photoswitching cross sections for surface-mounted molecules at different wavelengths. In a dramatic departure from molecular behavior in solution-based environments, visible light does not efficiently reverse the reaction for azobenzene-derived molecules at a gold surface.
    Applied Physics Letters 03/2008; 92(12):123107-123107-3. · 3.52 Impact Factor
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    ABSTRACT: Bipolar transport polymers have been developed as host materials for electroluminescent devices by incorporating both electron-transporting and hole-transporting functionalities into copolymers. Two different copolymers having the same molecular weight (Mn 30 kg/mol) and the same fraction of electron-transporting monomers (fOXA = 0.50) have been synthesized in the form of random and diblock copolymers, respectively. The effect of molecular structure and film morphology of these bipolar polymers on device performance has been studied. For the diblock copolymers, pronounced phase segregation forming different nanomorphologies has been observed by modern microscopic techniques, which is not observed for the random counterparts under the same thin film preparation conditions. The results of single-layer polymer light emitting diodes (PLEDs) show that the nanophase separation morphology of diblock copolymers has a significant effect on device performance:  lowering charge transport and facilitating the hole−electron recombination leads to a much higher quantum efficiency. Applying this high triplet block copolymer as host, a high external quantum efficiency of 5.4% at the brightness of 900 cd/m2 was achieved for single-layer PLEDs with a green-emitting complex dopant.
    Macromolecules 10/2007; 40(23). · 5.93 Impact Factor
  • Daniel A. Poulsen, Nick H. Werstiuk
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    ABSTRACT: While studies on the experimental photolytic and thermolytic extrusion of nitrogen from tert-butyldiazomethane and tert-butyldiazirine and the decomposition of other precursors have shown a mixture of C−H and C−C insertion products depending on conditions, the analogous trimethylsilyldiazomethane undergoes solely Si−C insertion. Description of the singlet tert-butylmethylene intermediates potentially involved in the C−H and C−C insertion reactions and were addressed through computational means by Armstrong et al. (J. Am. Chem. Soc. 1995, 117, 3685−3689). In addition to re-examining singlet tert-butylmethylene at a higher level of theory [CCSD/6-311+G(d,p)], we have studied the silicon and germanium analogues trimethylsilylmethylene and trimethylgermylmethylene. A computational atoms-in-molecules and atomic-basin-delocalization-indices analysis established that the singlet carbenes, while exhibiting varying degrees of delocalization, are not bridged species based on the fact that none possess a pentacoordinate methyl group. In addition, from the results, we are able to make a prediction of solely a Ge−C insertion product for the extrusion of nitrogen from trimethylgermyldiazomethane. Most importantly, we demonstrated that a combination of quantum theory of atoms in molecules (QTAIM) molecular graphs, the evaluation of delocalization indices, and a visualization of the closeness of atomic basinsa QTAIM-DI-VISAB analysisshould be considered as the method of choice for unambiguously characterizing the bonding between pairs of atoms not only of carbenes but of other reaction intermediates such as carbocations, carbanions, and radicals.

Publication Stats

197 Citations
119.16 Total Impact Points


  • 2010
    • CSU Mentor
      Long Beach, California, United States
  • 2009–2010
    • University of California, Berkeley
      • • Department of Materials Science and Engineering
      • • Department of Physics
      Berkeley, MO, United States
  • 2008–2009
    • Lawrence Berkeley National Laboratory
      • • Geochemistry Department
      • • Materials Sciences Division
      Berkeley, CA, United States