Hsiao-An Pan

National Taiwan University, T’ai-pei, Taipei, Taiwan

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Publications (13)74.16 Total impact

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    ABSTRACT: Manganese oxide/carbon (MnOx/C) composite powders showing high performance as lithium-ion battery anode are synthesized by a facile co-precipitation process followed by thermal calcination between 400 °C and 700 °C in N2, where the as-deposited MnO2 is reduced progressively to Mn3O4 and then to MnO. The role of conductive additive is investigated by adopting two carbon (C) materials of different dimensionalities, including carbon black (CB) nanoparticles and micron-sized graphitic flakes (GFs). For MnOx/CB composite, the cycling stability is remarkably enhanced with increasing calcination temperature, and this is due to increasing content of MnO, which exhibits superior redox reversibility than the oxides having higher Mn valences. Attempt to achieve single-phase MnO at higher temperature (700 °C), nevertheless, leads to deteriorated cycle performance because of the formation of large oxide particles having poor contact with CB. The use of the two-dimensional GFs creates the “balls-on-plate” oxide-C configuration. This configuration facilitates MnO formation at lower temperature and simultaneously enables retention of good oxide-C contact, leading to significantly enhanced cycling stability and rate performance. The MnOx/GF composites obtained by calcination at 500–600 °C show specific capacities of 550–600 mAh g−1-(oxide + C) with no capacity fading after 150 cycles.
    Journal of Power Sources 05/2014; 253:373–380. · 5.26 Impact Factor
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    ABSTRACT: Scientists have made tremendous efforts to gain understanding of the water molecules in proteins via indirect measurements such as molecular dynamic simulation and/or probing the polarity of the local environment. Here we present a tryptophan analogue that exhibits remarkable water catalysed proton-transfer properties. The resulting multiple emissions provide unique fingerprints that can be exploited for direct sensing of a site-specific water environment in a protein without disrupting its native structure. Replacing tryptophan with the newly developed tryptophan analogue we sense different water environments surrounding the five tryptophans in human thromboxane A2 synthase. This development may lead to future research to probe how water molecules affect the folding, structures and activities of proteins.
    Nature Communications 11/2013; 4:2611. · 10.74 Impact Factor
  • 224th ECS Meeting; 10/2013
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    ABSTRACT: Using a bimetallic Au(I) complex bearing alkynyl-(phenylene)3-diphosphine ligand (A-3), we demonstrate that the fluorescence can be exquisitely harvested upon T1 → Tk (k > 1) excitation followed by Tk → Sj (j, k > 1) intersystem crossing (ISC) back to the S1 state. Upon S0 → S1 355 nm excitation, the S1 → T1 intersystem crossing rate has been determined to be 8.9 × 108 s–1. Subsequently, in a two-step laser pump–probe experiment, following a 355 nm laser excitation, the 532 nm T1 → Tk probing gives the prominent blue 375 nm fluorescence, and this time-dependent pump–probe signal correlates well with the lifetime of the T1 state. Careful examination reveals the efficiency of Tk → Sj (j, k > 1) reverse intersystem crossing to be 5.2%. The result is rationalized by a mechanism incorporating substantial involvement of metal-to-ligand charge transfer (MLCT) in the Tk (Sj) states, enhancing the rate of Tk → Sj ISC, which is competitive with the rate of Tk → T1 internal conversion. This mechanism is also proven to be operative in the A-3 solid film and should be universally applicable to the transition-metal complexes possessing a dominant ππ* configuration in the lowest-lying states. From an energy point of view, the UV fluorescence (375 nm) generated by green (532 nm) excitation can be recognized as a signal up-conversion process.
    The Journal of Physical Chemistry C 09/2013; 117(40):20494–20499. · 4.84 Impact Factor
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    ABSTRACT: Two pyrimidine chelates with the pyridin-2-yl group resided at either 5 or 4-positions are synthesized. These chelates are then utilized in synthesizing of a new class of heteroleptic Ir(III) metal complexes; namely: [Ir(b5ppm)2(fppz)] (1), [Ir(b5bpm)2(fppz)] (2), [Ir(b4bpm)2(fppz)] (3) and [Ir(b5bpm)(fppz)2] (4), for which the abbreviations b5ppm, b5bpm, b4bpm, and fppz represent chelates derived from 2-t-butyl-5-(pyridin-2-yl)pyrimidine, 2-t-butyl-5-(4-t-butylpyridin-2-yl)pyrimidine, 2-t-butyl-4-(4-t-butylpyridin-2-yl)pyrimidine and 3-trifluoromethyl-5-(pyridin-2-yl) pyrazole, respectively. The single crystal X-ray structural analyses were executed on 1 to reveal their coordination arrangement around the Ir(III) metal element. The 5-substituted pyrimidine complexes 1, 2 and 4 exhibited first emission peak wavelength (λmax) located in the range 452 ~ 457 nm with high quantum yields, while the emission of 3 with 4-substituted pyrimidine was red shifted substantially to longer wavelength with λmax = 535 nm. These photophysical properties were discussed under the basis of computational approaches, particularly the relationship between emission color and the relative position of nitrogen atoms of pyrimidine fragment. For application, organic light emitting diodes (OLEDs) were also fabricated using 2 and 4 as dopants, attaining the peak external quantum, luminance and power efficiencies of 17.9 % (38.0 cd/A and 35.8 lm/W) and 15.8 % (30.6 cd/A and 24.8 lm/W), respectively. Combining sky blue-emitting 2 and red-emitting [Os(bpftz)2(PPh2Me)2] (5), the phosphorescent white OLEDs were demonstrated with stable pure-white emission at CIE coordinate of (0.33, 0.34), and peak luminance efficiency of 35.3 cd/A, power efficiency of 30.4 lm/W, and external quantum efficiency up to 17.3 %.
    ACS Applied Materials & Interfaces 07/2013; · 5.90 Impact Factor
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    ABSTRACT: We have designed and synthesized a series of Au(I) complexes bearing either an alkynyl–(phenylene)n–diphosphine (A-0–A-3) or a (phenylene)n–diphosphine (B-1–B-5) bridge, among which the effective distance between Au(I) and the center of the emitting ππ* chromophore can be fine-tuned via the insertion of various numbers of phenylene spacers. We then demonstrated for the first time in a systematic manner the decrease of rate constant for S1 → T1 intersystem crossing (ISC) kisc as the increase of the effective distance. The results also unambiguously showed that the phosphorescence could be harvested via higher S0 → Sn (n > 1) electronic excitation, followed by fast Sn → Tm ISC and then the population at T1 state, bypassing the relatively slow S1 → T1 ISC. The results unify a recent report on higher excited-state relaxation dynamics for the late transition metal complexes (J. Am. Chem. Soc.2012, 134, 7715–7724). The dual, far separated fluorescence and phosphorescence of the titled complexes make feasible the white light generation in a single molecule unit, as successfully demonstrated using complex B-3 as a dopant to fabricate organic light emitting diodes.
    The Journal of Physical Chemistry C. 05/2013; 117(19):9623–9632.
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    ABSTRACT: Electron donor-acceptor types of multibranched triarylamine end-capped triazines have been systematically investigated by steady-state electronic spectroscopy, electrochemistry, femtosecond fluorescence anisotropy and solvent relaxation dynamics. The results, together with computational approach, have gained in-depth insight into their excited-state properties, especially the interactions between branches. Among different branched triarylamines of one, two and three arms, the interbranch interaction between each arm is weak, as evidenced by their nearly identical absorption spectral profile and frontier orbitals analyses. Upon S0 → S1 excitation, the electronic delocalization in the three-branched triarylamine end-capped triazine is resolved to be 680 ± 130 fs, followed by a slow (28 ± 3 ps) electronic localization into one branch and consequently a rotational depolarization of 2.0 ± 0.1 ns. Similar delocalization dynamics was resolved for the two-branched triarylamine end-capped triazine (electronic delocalization: 500 ± 90 fs, twisting localization: 21 ± 5 ps, rotational depolarization: 700 ± 30 ps). The comparable electron delocalization and solvent relaxation time scale may set up a new paradigm to investigate their specific correlation in the early time domain.
    The Journal of Physical Chemistry A 11/2012; · 2.77 Impact Factor
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    ABSTRACT: Cationic iridium complexes incorporated 4,5-diaza-9,9'-spirobifluorene as N(∧)N ancillary ligands, in which one (2) or two (3) phenyl groups were introduced onto 4,5-diazafluorene to afford intraligand π-π interactions. The X-ray crystal structures of complexes 2 and 3 show that the pendant phenyl ring forms strong intramolecular face-to-face π-stacking with the difluorophenyl ring of the cyclometalated ligand with distances of 3.38 Å for complex 2 and 3.40 and 3.46 Å for complex 3. This π-π stacking interaction minimizes the expansion of the metal-ligand bonds in the excited state, resulting in a longer device lifetime in the light-emitting electrochemical cell (LEC) devices.
    Inorganic Chemistry 10/2012; · 4.59 Impact Factor
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    ABSTRACT: Panchromatic Os(II) sensitizers for dye-sensitized solar cells (DSCs) were prepared. A DSC based on TF-52 showed promising performance characteristics: short-circuit photocurrent density J(SC) =23.3 mA cm(-2) , open-circuit photovoltage V(OC) =600 mV, fill factor (FF)=0.633 and power conversion efficiency η=8.85 % under AM 1.5G simulated one-sun irradiation.
    Angewandte Chemie International Edition 04/2012; 51(23):5642-6. · 11.34 Impact Factor
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    ABSTRACT: Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.
    Journal of the American Chemical Society 04/2012; 134(17):7488-96. · 10.68 Impact Factor
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    ABSTRACT: A series of 2-pyridyl pyrazoles 1a and 1-5 with various functional groups attached to either pyrazole or pyridyl moieties have been strategically designed and synthesized in an aim to probe the hydrogen bonding strength in the ground state versus dynamics of excited-state intramolecular proton transfer (ESIPT) reaction. The title compounds all possess a five-membered-ring (pyrazole)N-H···N(pyridine) intramolecular hydrogen bond, in which both the N-H bond and the electron density distribution of the pyridyl nitrogen lone-pair electrons are rather directional, so that the hydrogen bonding strength is relatively weak, which is sensitive to the perturbation of subtle chemical substitution and consequently reflected from the associated ESIPT dynamics. Various approaches such as (1)H NMR (N-H proton) to probe the hydrogen bonding strength and absorption titration to assess the acidity-basicity property were made for all the title analogues. The results, together with supplementary support provided by a computational approach, affirm that the increase of acidity (basicity) on the hydrogen bonding donor (acceptor) sites leads to an increase of hydrogen-bonding strength among the title 2-pyridyl pyrazoles. Luminescence results and the associated ESIPT dynamics further reveal an empirical correlation in that the increase of the hydrogen bonding strength leads to an increase of the rate of ESIPT for the title 2-pyridyl pyrazoles, demonstrating an interesting relationship among N-H acidity, hydrogen bonding strength, and the associated ESIPT rate.
    The Journal of Physical Chemistry A 04/2012; 116(18):4438-44. · 2.77 Impact Factor
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    ABSTRACT: With the motivation of assembling cyclometalated complexes without nitrogen-containing heterocycle, we report here the design and systematic synthesis of a class of Ir(III) metal complexes functionalized with facially coordinated phosphite (or phosphonite) dicyclometalate tripod, together with a variety of phosphine, chelating diphosphine, or even monocyclometalate phosphite ancillaries. Thus, treatment of [IrCl(3)(tht)(3)] with stoichiometric amount of triphenylphosphite (or diphenyl phenylphosphonite), two equiv of PPh(3), and in presence of NaOAc as cyclometalation promoter, gives formation of respective tripodal dicyclometalating complexes [Ir(tpit)(PPh(3))(2)Cl] (2a), [Ir(dppit)(PPh(3))(2)Cl] (2b), and [Ir(dppit)(PMe(2)Ph)(2)Cl] (2c) in high yields, where tpitH(2) = triphenylphosphite and dppitH(2) = diphenyl phenylphosphonite. The reaction sequence that afforded these complexes is established. Of particular interest is isolation of an intermediate [Ir(tpitH)(PPh(3))(2)Cl(2)] (1a) with monocyclometalated phosphite, together with the formation of [Ir(tpit)(tpitH)(PPh(3))] (3a) with all tripodal, bidentate, and monodentate phosphorus donors coexisting on the coordination sphere, upon treatment of 2a with a second equiv of triphenylphosphite. Spectroscopic studies were performed to explore the photophysical properties. For all titled Ir(III) complexes, virtually no emission can be observed in either solution at room temperature or 77 K CH(2)Cl(2) matrix. Time-dependent DFT calculation indicates that the lowest energy triplet manifold involves substantial amount of metal centered (3)MC dd contribution. Due to its repulsive potential energy surface (PES) that touches the PES of ground state, the (3)MC dd state executes predominant nonradiative deactivation process.
    Inorganic Chemistry 02/2012; 51(3):1785-95. · 4.59 Impact Factor
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    ABSTRACT: Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state intramolecular proton transfer (ESIPT), chemical modification has been performed at C(2)-C(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission (λ(max) ∼530 nm), whereas excited-state equilibrium is established for both II and III, as supported by the dual emission and the associated relaxation dynamics. The forward ESIPT (normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)(-1) and (22 ps)(-1), respectively, while the backward ESIPT rates are (11 ps)(-1) and (48 ps)(-1). The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III, respectively, giving a corresponding free energy change of 0.59 and -0.47 kcal/mol between normal and tautomer species. For III, normal and tautomer emissions in solid are maximized at 435 and 580 nm, respectively, achieving a white light generation with Commission Internationale de l'Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III is also successfully fabricated with maximum brightness of 665 cd m(-2) at 20 V (885 mA cm(-2)) and the CIE coordinates of (0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.
    Journal of the American Chemical Society 09/2011; 133(44):17738-45. · 10.68 Impact Factor

Publication Stats

54 Citations
74.16 Total Impact Points

Institutions

  • 2011–2014
    • National Taiwan University
      • • Department of Chemical Engineering
      • • Department of Chemistry
      T’ai-pei, Taipei, Taiwan
  • 2013
    • Industrial Technology Research Institute
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2012
    • National Tsing Hua University
      • Department of Chemistry
      Hsinchu, Taiwan, Taiwan