121 results on '"John M. Papanikolas"'
Search Results
2. A Semiconductor‐Mediator‐Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation
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Fujun Niu, Degao Wang, Lenzi J. Williams, Animesh Nayak, Fei Li, Xiangyan Chen, Ludovic Troian‐Gautier, Qing Huang, Yanming Liu, M. Kyle Brennaman, John M. Papanikolas, Liejin Guo, Shaohua Shen, Thomas J. Meyer, and UCL - SST/IMCN/MOST - Molecular Chemistry, Materials and Catalysis more...
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Semiconductors ,Organic Chemistry ,Water ,General Chemistry ,Photosynthesis ,Oxidation-Reduction ,Catalysis - Abstract
A hybrid semiconductor-mediator-catalyst photoelectrochemical (PEC) cell mimicking the PSII is proposed for efficient water oxidation. Addition of carbazole mediator results in a significant enhancement in PEC water oxidation performance as it promotes interfacial hole transfer from the Fe2O3 light absorber to the Ru(carbene) catalyst, a function similar to the tyrosine residue in photosystem II. Such a semiconductor-mediator-catalyst assembly greatly inspires the rational design of high-performance artificial photosynthesis systems. more...
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- 2022
Catalog
3. A Quantitative Model of Charge Injection by Ruthenium Chromophores Connecting Femtosecond to Continuous Irradiance Conditions
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Thomas P. Cheshire, Jéa Boodry, Erin A. Kober, M. Kyle Brennaman, Paul G. Giokas, David F. Zigler, Andrew M. Moran, John M. Papanikolas, Gerald J. Meyer, Thomas J. Meyer, and Frances A. Houle
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Chemical Physics (physics.chem-ph) ,Physics - Chemical Physics ,General Physics and Astronomy ,FOS: Physical sciences ,Physical and Theoretical Chemistry - Abstract
A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion and adsorption kinetics during sample preparation, and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles, decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of an approximately 500 nm isosbestic point, validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging 1011 s-1 to 1012 s-1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation, and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature., Comment: 81 pp - main paper and supplementary material. 34 figures more...
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- 2022
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4. Observation of Phonon Propagation in Germanium Nanowires Using Femtosecond Pump–Probe Microscopy
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Christopher W. Pinion, Emma E. M. Cating, James F. Cahoon, Erika M. Van Goethem, and John M. Papanikolas
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Materials science ,business.industry ,Resolution (electron density) ,Nanowire ,chemistry.chemical_element ,Germanium ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,010309 optics ,Photoexcitation ,Thermal conductivity ,chemistry ,0103 physical sciences ,Femtosecond ,Microscopy ,Optoelectronics ,Electrical and Electronic Engineering ,0210 nano-technology ,business ,Ultrashort pulse ,Biotechnology - Abstract
The excited-state dynamics in individual Ge nanowires (NWs) are imaged using ultrafast pump–probe microscopy with high spatial (∼600 nm) and temporal (∼500 fs) resolution. Photoexcitation of the NW... more...
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- 2019
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5. It Is Good to Be Flexible: Energy Transport Facilitated by Conformational Fluctuations in Light-Harvesting Polymers
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Leah M. Rader Bowers, Zachary A. Morseth, Egle Puodziukynaite, John R. Reynolds, Li Wang, Kirk S. Schanze, and John M. Papanikolas
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Materials science ,010304 chemical physics ,Polymers ,Chromophore ,Molecular Dynamics Simulation ,010402 general chemistry ,Kinetic energy ,Osmium ,01 natural sciences ,Ruthenium ,0104 chemical sciences ,Surfaces, Coatings and Films ,Photoexcitation ,Polyfluorene ,chemistry.chemical_compound ,Molecular dynamics ,chemistry ,Chemical physics ,Excited state ,0103 physical sciences ,Materials Chemistry ,Organometallic Compounds ,Emission spectrum ,Physical and Theoretical Chemistry ,Excitation - Abstract
We investigate the mechanism of energy transfer between ruthenium(II) (Ru) and osmium(II) (Os) polypyridyl complexes affixed to a polyfluorene backbone (PF-RuOs) using a combination of time-resolved emission spectroscopy and coarse-grained molecular dynamics (CG MD). Photoexcitation of a Ru chromophore initiates Dexter-style energy hopping along isoenergetic complexes followed by sensitization of a lower-energy Os trap. While we can determine the total energy transfer rate within an ensemble of solvated PF-RuOs from time-dependent Os* emission spectra, heterogeneity of the system and inherent polymer flexibility give rise to highly multiexponential kinetics. We developed a three-part computational kinetic model to supplement our spectroscopic results: (1) CG MD model of PF-RuOs that simulates molecular motions out to 700 ns, (2) energy transfer kinetic simulations in CG MD PF-RuOs that produce time-resolved Ru and Os excited-state populations, and (3) computational experiments that interrogate the mechanisms by which motion aids energy transfer. Good agreement between simulated and experimental emission transients reveals that our kinetic model accurately simulates the molecular motion of PF-RuOs during energy transfer. Simulated results indicate that pendant flexibility allows 81% of the excited state to sensitize an Os trap compared to a 48% occupation when we treat pendants statically. Our computational experiments show how static pendants are only able to engage in local energy transfer. The excited state equilibrates across a domain of complexes proximal to the initial excitation and becomes trapped within that unique, frozen locality. Side-chain flexibility enables pendants to swing in and out of the original domain spreading the excited state out to ±30 pendant complexes away from the initial excitation. more...
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- 2021
6. Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations
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Frances A. Houle, Gerald J. Meyer, M. Kyle Brennaman, David F. Zigler, Andrew M. Moran, Thomas P. Cheshire, Thomas J. Meyer, John M. Papanikolas, and Paul G. Giokas
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Materials science ,Photoluminescence ,010304 chemical physics ,Relaxation (NMR) ,Chromophore ,010402 general chemistry ,01 natural sciences ,Molecular physics ,0104 chemical sciences ,Surfaces, Coatings and Films ,Intersystem crossing ,Engineering ,Affordable and Clean Energy ,Excited state ,0103 physical sciences ,Ultrafast laser spectroscopy ,Physical Sciences ,Chemical Sciences ,Materials Chemistry ,Physical and Theoretical Chemistry ,Ground state ,Absorption (electromagnetic radiation) - Abstract
Maximizing the efficiency of solar energy conversion using dye assemblies rests on understanding where the energy goes following absorption. Transient spectroscopies in solution are useful for this purpose, and the time-resolved data are usually analyzed with a sum of exponentials. This treatment assumes that dynamic events are well separated in time, and that the resulting exponential prefactors and phenomenological lifetimes are related directly to primary physical values. Such assumptions break down for coincident absorption, emission, and excited state relaxation that occur in transient absorption and photoluminescence of tris(2,2'-bipyridine)ruthenium(2+) derivatives, confounding the physical meaning of the reported lifetimes. In this work, we use inductive modeling and stochastic chemical kinetics to develop a detailed description of the primary ultrafast photophysics in transient spectroscopies of a series of Ru dyes, as an alternative to sums of exponential analysis. Commonly invoked three-level schemes involving absorption, intersystem crossing (ISC), and slow nonradiative relaxation and incoherent emission to the ground state cannot reproduce the experimentally measured spectra. The kinetics simulations reveal that ultrafast decay from the singlet excited state manifold to the ground state competes with ISC to the triplet excited state, whose efficiency was determined to be less than unity. The populations predicted by the simulations are used to estimate the magnitudes of transition dipoles for excited state excitations and evaluate the influence of specific ligands. The mechanistic framework and methodology presented here are entirely general, applicable to other dye classes, and can be extended to include charge injection by molecules bound to semiconductor surfaces. more...
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- 2020
7. Interfacial electron transfer yields in dye-sensitized NiO photocathodes correlated to excited-state dipole orientation of ruthenium chromophores
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Cory J. Flynn, James F. Cahoon, John M. Papanikolas, Yejee Han, Jillian L. Dempsey, Eric S. Rountree, Robert J. Dillon, and Leila Alibabaei
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Organic Chemistry ,Non-blocking I/O ,chemistry.chemical_element ,02 engineering and technology ,General Chemistry ,Chromophore ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Catalysis ,Nanocrystalline material ,0104 chemical sciences ,Ruthenium ,Electron transfer ,Dipole ,chemistry ,Excited state ,Thin film ,0210 nano-technology - Abstract
Interface dynamics of nanocrystalline NiO thin films sensitized with two ruthenium polypyridyl chromophores have been investigated to examine the influence that excited-state dipole orientation and the position of the bipyridine radical formed in the charge-separated state have on interfacial electron transfer yields. In ultrafast transient absorption experiments, the charge separated state is observed on the nanosecond timescale for the trifluoromethyl-substituted chromophore, [Ru(flpy)2(dcb)]2+ (flpy = 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, dcb = 4,4′-dicarboxy-2,2′-bipyridine), but not for [Ru(bpy)2(dcb)]2+ (bpy = 2,2′-bipyridine). Differences are attributed to the positioning of the bipyridine radical formed in the charge separated state; for [Ru(flpy)2(dcb)]2+, the electron is localized on the flpy ligand distal to the surface, whereas for [Ru(bpy)2(dcb)]2+, the electron is localized on the dcb ligand, proximal to the NiO surface. Enhanced photovoltaic performance is observed for dye-sensitized solar cell devices prepared with [Ru(flpy)2(dcb)]2+, demonstrating that enhanced charge separation can be correlated with device efficiency. more...
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- 2018
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8. Role of Structure in Ultrafast Charge Separation and Recombination in Naphthalene Diimide End-Capped Thiophene Oligomers
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Melissa K. Gish, Kirk S. Schanze, Austin L. Jones, and John M. Papanikolas
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Materials science ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Oligomer ,Acceptor ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Photoexcitation ,chemistry.chemical_compound ,General Energy ,Intersystem crossing ,chemistry ,Picosecond ,Ultrafast laser spectroscopy ,Femtosecond ,Thiophene ,Physical and Theoretical Chemistry ,0210 nano-technology - Abstract
The photophysics of a series of thiophene oligomers (Tn) with and without naphthalene diimide (NDI) acceptor end groups were investigated using femtosecond transient absorption spectroscopy. Photoexcited thiophene oligomers (n = 4, 6, 8, 10, and 12) exhibit complex length-dependent excited-state dynamics on the picosecond time scale due to rapid structural relaxation and intersystem crossing. The incorporation of NDI end groups leads to ultrafast charge separation after selective excitation of the thiophene donor. Initial location of photoexcitation dictates the time scale of charge separation and, therefore, recombination. Photoexcitations near the NDI acceptor result in fast charge separation in all Tn-NDI2 oligomers, whereas excitations near the center of the oligomer must undergo a length-dependent long-range electron-transfer or energy-transfer/electron-transfer step to create a charge-separated state. Oligomer structure plays a role in the charge separation and recombination processes, where T4-NDI2... more...
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- 2018
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9. Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO2
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Melissa K. Gish, Robert A. Binstead, Michael R. Norris, Alexander M. Lapides, Joseph L. Templeton, Javier J. Concepcion, Thomas J. Meyer, M. Kyle Brennaman, Leila Alibabaei, Wenjing Song, John M. Papanikolas, Robert J. S. Brown, and Animesh Nayak more...
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Materials science ,chemistry.chemical_element ,02 engineering and technology ,Chromophore ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Electrochemistry ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Catalysis ,Ruthenium ,Photoexcitation ,Microsecond ,Electron transfer ,General Energy ,chemistry ,Ultrafast laser spectroscopy ,Physical and Theoretical Chemistry ,0210 nano-technology - Abstract
Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]4+, −[RuaII–RubII–OH2]4+, on nanocrystalline TiO2 electrodes, starting from either −[RuaII–RubII–OH2]4+ or −[RuaII–RubIII–OH2]5+, have been investigated. Transient absorption measurements for TiO2–[RuaII–RubII–OH2]4+ in 0.1 M HPF6 or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO2–[RuaII–RubIII–OH2]5+, in a variety of media, generates −[RuaIII–RubIII–OH2]6+. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[RuaII–RubIII–OH2]5+ assembly to −[RuaII–RubII–OH2]4+. In a parallel reaction, −[RuaIII–RubIII–OH2]6+ formed by electron injectio... more...
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- 2018
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10. Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition
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M. Kyle Brennaman, Seth L. Marquard, Kyung Ryang Wee, Leila Alibabaei, Thomas J. Meyer, John M. Papanikolas, Caroline E. Reilly, and Robert J. Dillon
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Photocurrent ,Aqueous solution ,Materials science ,Inorganic chemistry ,02 engineering and technology ,Chromophore ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Catalysis ,Atomic layer deposition ,Catalytic oxidation ,Water splitting ,General Materials Science ,0210 nano-technology ,Layer (electronics) - Abstract
Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor−π–acceptor (D−π–A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer ( more...
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- 2017
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11. Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications
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Subhadip Goswami, John M. Papanikolas, Kirk S. Schanze, Melissa K. Gish, Jiliang Wang, Suchismita Guha, John R. Reynolds, Bethy Kim, Amrit Laudari, and Jeff L. Hernandez
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chemistry.chemical_classification ,Materials science ,Band gap ,General Chemical Engineering ,Auxochrome ,chemistry.chemical_element ,02 engineering and technology ,General Chemistry ,Polymer ,Chromophore ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,chemistry ,Picosecond ,Ultrafast laser spectroscopy ,Materials Chemistry ,0210 nano-technology ,Platinum ,Spectroscopy - Abstract
A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore DPP-C8-Th-Py. The polymers Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer Poly-DPP-Th-Pt has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as ... more...
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- 2017
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12. Intrinsic gain and gain degradation modulated by excitation pulse width in a semiconducting conjugated polymer
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Simon E. Lappi, John M. Papanikolas, Zach E. Lampert, and C. Lewis Reynolds
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Amplified spontaneous emission ,Photoluminescence ,Active laser medium ,Materials science ,business.industry ,Heterojunction ,02 engineering and technology ,Laser pumping ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Dissociation (chemistry) ,Electronic, Optical and Magnetic Materials ,0103 physical sciences ,Thermal ,Optoelectronics ,Electrical and Electronic Engineering ,010306 general physics ,0210 nano-technology ,business ,Excitation - Abstract
We have previously reported that substantially higher optical gain values can be achieved in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) through use of transient excitation conditions. In the present paper, we report on a systematic investigation of this behavior to elucidate the physical mechanisms involved, which enables us to distinguish between the fundamental intrinsic gain and an excitation induced degraded gain. Using pump laser pulses having temporal widths longer and shorter than the photoluminescence (PL) decay time of MEH-PPV, both quasi-steady-state (QSS) and transient excitation regimes are explored in our encapsulated waveguide heterostructures [Si(1 0 0)/SiO2/MEH-PPV/poly(methyl methacrylate)]. Under transient excitation (25 ps pump pulses), extremely large optical gain is observed, reaching a value of 700 cm−1 at a maximum pump energy density of 85 µJ/cm2. However, under QSS conditions (8 ns pulses), considerably lower gain coefficients are achieved with a maximum of ∼130 cm−1 at an energy density of 2,000 µJ/cm2; this factor of 5 decrease in optical gain performance is observed at the same excitation density as that for transient excitation using ps pulses. We have also employed unencapsulated waveguide structures [Si(1 0 0)/SiO2/MEH-PPV/air], which allows us to achieve additional insight on gain degradation under QSS conditions. It is clear that the gain measured under transient conditions is more representative of the intrinsic gain whereas that determined in the QSS regime is degraded by defect-mediated dissociation of emissive states due to localized thermal and oxidative damage to the films. It is in the QSS regime in which most optical gain measurements to date have been performed. These results suggest that further optimization of MEH-PPV – and most likely other conjugated polymers – as a robust optical gain medium can be achieved by consideration of the excitation pulse width. more...
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- 2017
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13. Ruthenium Dyes, Charge Transfer, and the Sun
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Thomas P Cheshire, Jeb Boodry, Erin A Kober, Bruno M. Aramburu-Trošelj, M. Kyle Brennaman, Paul M. Giokas, David Zigler, Andrew M. Moran, John M. Papanikolas, Gerald Meyer, Thomas Meyer, and Frances Houle more...
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The performance of dye-sensitized solar cells (DSSC) and dye-sensitized photoelectrosynthesis cells (DSPEC)—and their constituent chromophores, catalysts, and substrates—is commonly evaluated by photocurrents, product fluxes, and time-resolved spectroscopies. Kinetic models of solar harvesting systems are often built from the data using phenomenological methods (i.e. sum-of-exponential or global fit analyses). Such models cannot be predictive, and therefore offer limited fundamental insights to the efficiencies of charge injection and photocatalysis. We describe an approach to modeling the molecular photophysics, interfacial electron transfer, and charge separation that yields a comprehensive kinetic framework for these processes. Simulations of femtosecond to steady-state dynamics using this framework provide a detailed picture of dye cycling under both pulsed-monochromatic and continuous broadband illumination at 1 sun intensity. The competition between molecular transitions and charge injection will be discussed, including the potential implications for the design of chromophore-catalyst assemblies. more...
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- 2021
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14. Ultrafast kinetics of supramolecules with a Ru(II)- or Os(II)-polypyridyl light absorber, cis-Rh(III)Cl2-polypyridyl electron collector, and 2,3-bis(2-pyridyl)pyrazine bridge
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David F. Zigler, John M. Papanikolas, Theodore R. Canterbury, Karen J. Brewer, Hannah J. Sayre, Travis A. White, José Á. Rodríguez-Corrales, M. Kyle Brennaman, and Zachary A. Morseth
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education.field_of_study ,Pyrazine ,010405 organic chemistry ,Chemistry ,Population ,Kinetics ,Bridging ligand ,010402 general chemistry ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Inorganic Chemistry ,chemistry.chemical_compound ,Excited state ,Ultrafast laser spectroscopy ,Materials Chemistry ,Physical and Theoretical Chemistry ,education ,Ground state ,Bimetallic strip - Abstract
The femtosecond transient absorption spectra (fsTA) and excited state kinetics for a series of six structurally related mixed-metal polypyridyl supramolecules are reported. Each complex consists of one or two light absorbers (LA) with Ru(II) or Os(II) centers attached to a Rh(III)-centered electron collector (EC) by an aromatic bridging ligand (BL). The resulting bimetallic and trimetallic complexes have LA-BL-EC and LA-BL-EC-BL-LA architectures, respectively. Excitation at 470 nm light populates metal-to-bridging ligand charge transfer states (MLCT), showing a transient absorption band near 380 nm due to π → π∗ transitions of a bridging ligand-localized radical anion and a transient bleach around 525 nm resulting from formal oxidation of the LA metal in the excited state. Loss of the ligand localized radical signal during the first 10 ps reflects conversion of the excited state population from an MLCT state into metal-to-metal (i.e. M(dπ)-to-Rh(dσ∗)) charge transfer states (MMCT). Each complex shares a similar ultrafast component, indicating that the kinetics governing MLCT → MMCT population transfer do not depend on the nature of the LA. Return to the ground state, however, is strongly LA dependent and controlled by the free-energy difference between the MMCT state and ground state, as well as an associated large reorganization energy. more...
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- 2017
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15. Ultrafast Recombination Dynamics in Dye-Sensitized SnO2/TiO2 Core/Shell Films
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John M. Papanikolas, Thomas J. Meyer, Melissa K. Gish, Alexander M. Lapides, Joseph L. Templeton, and M. Kyle Brennaman
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Chemistry ,Shell (structure) ,02 engineering and technology ,Electron ,Orders of magnitude (numbers) ,Chromophore ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Photoexcitation ,Picosecond ,Ultrafast laser spectroscopy ,General Materials Science ,Physical and Theoretical Chemistry ,Atomic physics ,0210 nano-technology ,Recombination - Abstract
Interfacial dynamics are investigated in SnO2/TiO2 core/shell films derivatized with a Ru(II)-polypyridyl chromophore ([RuII(bpy)2(4,4′-(PO3H2)2bpy)]2+, RuP) using transient absorption methods. Electron injection from the chromophore into the TiO2 shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO2 core and must tunnel through the TiO2 shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for t... more...
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- 2016
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16. Direct observation of light-driven, concerted electron–proton transfer
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John M. Papanikolas, M. Kyle Brennaman, David W. Thompson, Li Wang, Christopher J. Gagliardi, Prateek Dongare, and Thomas J. Meyer
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Multidisciplinary ,Aqueous solution ,Chemistry ,Direct observation ,02 engineering and technology ,Electron ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Molecular physics ,Spectral line ,0104 chemical sciences ,Adduct ,Physical Sciences ,Ultrafast laser spectroscopy ,Light driven ,0210 nano-technology ,Ultrashort pulse - Abstract
Significance Concerted proton-coupled electron transfer (EPT) reactions in which both electrons and protons transfer in tandem are at the heart of many chemical and biological conversions including photosystem II. We report here the direct observation of absorption bands arising from photoEPT transitions, in this case, in H-bonded complexes between N -methyl-4,4′-bipyridinium cation and biologically relevant donors including tyrosine. The importance of these observations follows from the earlier experimental observations by Taube and coworkers on intervalence transfer in mixed-valence complexes. The observation of these photoEPT transitions and the appearance of reactive radical products also points to a possible, if inefficient, role in DNA photodamage and, possibly, in the formation of reactive oxygen intermediates. more...
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- 2016
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17. Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore–Catalyst Assemblies
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Benjamin D. Sherman, Kirk S. Schanze, Thomas J. Meyer, Zachary A. Morseth, Alex J. Burnett, Gyu Leem, Kyung Ryang Wee, and John M. Papanikolas
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Materials science ,Renewable Energy, Sustainability and the Environment ,Layer by layer ,Inorganic chemistry ,Energy Engineering and Power Technology ,02 engineering and technology ,Substrate (electronics) ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Tin oxide ,01 natural sciences ,Polyelectrolyte ,0104 chemical sciences ,Indium tin oxide ,chemistry.chemical_compound ,Fuel Technology ,chemistry ,Chemistry (miscellaneous) ,Materials Chemistry ,Polystyrene ,Cyclic voltammetry ,0210 nano-technology ,Mesoporous material - Abstract
Layer-by-Layer (LbL) polyelectrolyte self-assembly occurs by the alternate exposure of a substrate to solutions of oppositely charged polyelectrolytes or polyions. Here, we report the application of LbL to construct chromophore–catalyst assemblies consisting of a cationic polystyrene-based Ru polychromophore (PS-Ru) and a [Ru(tpy)(2-pyridyl-N-methylbenzimidazole) (OH2)]2+ water oxidation catalyst (RuC), codeposited with poly(acrylic acid) (PAA) as an inert polyanion. These assemblies are deposited onto planar indium tin oxide (ITO, Sn:In2O3) substrates for electrochemical characterization and onto mesoporous substrates consisting of a SnO2/TiO2 core/shell structure atop fluorine doped tin oxide (FTO) for application to light-driven water oxidation in a dye-sensitized photoelectrosynthesis cell. Cyclic voltammetry and ultraviolet–visible absorption spectroscopy reveal that multilayer deposition progressively increases the film thickness on ITO glass substrates. Under an applied bias, photocurrent measureme... more...
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- 2016
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18. Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO2
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Thomas J. Meyer, Zachary A. Morseth, Toan V. Pho, Benjamin D. Sherman, John M. Papanikolas, John R. Reynolds, Kirk S. Schanze, and Matthew V. Sheridan
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Hydroquinone ,010405 organic chemistry ,Chemistry ,Ligand ,010402 general chemistry ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Catalysis ,chemistry.chemical_compound ,Terthiophene ,Catalytic oxidation ,Benzyl alcohol ,Alcohol oxidation ,Polymer chemistry ,General Materials Science ,Trifluoromethanesulfonate - Abstract
The ligand 5-PO3H2-2,2':5',2″-terthiophene-5-trpy, T3 (trpy = 2,2':6',2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru(T3)(bpy)(OH2)](2+) (T3-Ru-OH2, bpy = 2,2'-bipyridine). T3 contains a phosphonic acid group for anchoring to a TiO2 photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO2, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO2 photoanode, T3 displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm(2)) arising from hydroquinone oxidation. Addition of [Ru(bpy)(OTf)][OTf]2 (bpy = 2,2'-bipyridine, OTf(-) = triflate) to T3 at the free trpy ligand forms the molecular assembly, T3-Ru-OH2, with the oxidative catalyst fragment: [Ru(trpy)(bpy)(OH2)](2+). The new assembly, T3-Ru-OH2, was used to perform efficient light-driven oxidation of phenol (230 μA/cm(2)) and benzyl alcohol (25 μA/cm(2)) in a dye-sensitized photoelectrosynthesis cell. more...
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- 2016
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19. Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO2
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Erinn C. Brigham, Leila Alibabaei, Erik M. Grumstrup, M. Kyle Brennaman, David F. Zigler, Li Wang, Melissa K. Gish, Thomas J. Meyer, Robert J. Dillon, Dennis L. Ashford, Zachary A. Morseth, Gerald J. Meyer, and John M. Papanikolas more...
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Ligand ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Biochemistry ,Catalysis ,0104 chemical sciences ,chemistry.chemical_compound ,Electron transfer ,Bipyridine ,Colloid and Surface Chemistry ,chemistry ,Excited state ,Titanium dioxide ,Surface modification ,Perchloric acid ,Absorption (chemistry) ,0210 nano-technology - Abstract
Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru(III/)* couple, E(+/)*, in 0.1 M perchloric acid (HClO4(aq)) to be tuned from -0.69 to -1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350-800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3-30 and 30-500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to E(+/)*. The correlation between the exponentially diminishing density of TiO2 sub-band acceptor levels and injection rate is well described using Marcus-Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the (3)MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the (3)MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials. more...
- Published
- 2016
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- View/download PDF
20. Completing a Charge Transport Chain for Artificial Photosynthesis
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Leah M. Rader Bowers, Thomas J. Meyer, John M. Papanikolas, Michael S. Eberhart, Ludovic Troian-Gautier, Bing Shan, and M. Kyle Brennaman
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010405 organic chemistry ,chemistry.chemical_element ,General Chemistry ,Chromophore ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Biochemistry ,Electron transport chain ,Catalysis ,0104 chemical sciences ,Ruthenium ,Artificial photosynthesis ,Electron transfer ,Colloid and Surface Chemistry ,Reaction rate constant ,chemistry ,Radical ion ,Excited state - Abstract
A ruthenium polypyridyl chromophore with electronically isolated triarylamine substituents has been synthesized that models the role of tyrosine in the electron transport chain in photosystem II. When bound to the surface of a TiO2 electrode, electron injection from a Ru(II) Metal-to-Ligand Charge Transfer (MLCT) excited state occurs from the complex to the electrode to give Ru(III). Subsequent rapid electron transfer from the pendant triarylamine to Ru(III) occurs with an observed rate constant of ∼1010 s-1, which is limited by the rate of electron injection into the semiconductor. Transfer of the oxidative equivalent away from the semiconductor surface results in dramatically reduced rates of back electron transfer, and a long-lived (τ = ∼165 μs) triarylamine radical cation that has been used to oxidize hydroquinone to quinone in solution. more...
- Published
- 2018
21. π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells
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Subhadip Goswami, John M. Papanikolas, Kirk S. Schanze, Russell W. Winkel, Melissa K. Gish, and Jiliang Wang
- Subjects
Intersystem crossing ,Materials science ,Excited state ,General Materials Science ,Singlet state ,Chromophore ,Triplet state ,Photochemistry ,Acceptor ,HOMO/LUMO ,Polymer solar cell - Abstract
An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low. more...
- Published
- 2015
- Full Text
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22. Ultrafast, Light-Induced Electron Transfer in a Perylene Diimide Chromophore-Donor Assembly on TiO2
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Leila Alibabaei, M. Kyle Brennaman, Erik M. Grumstrup, Melissa K. Gish, Alexander M. Lapides, Dennis L. Ashford, Joseph L. Templeton, Thomas J. Meyer, John M. Papanikolas, and Michael R. Norris
- Subjects
Kinetics ,Chromophore ,Photochemistry ,chemistry.chemical_compound ,Electron transfer ,Aniline ,Succinimide ,chemistry ,Diimide ,Ultrafast laser spectroscopy ,Organic chemistry ,General Materials Science ,Physical and Theoretical Chemistry ,Perylene - Abstract
Surface-bound, perylenediimide (PDI)-based molecular assemblies have been synthesized on nanocrystalline TiO2 by reaction of a dianhydride with a surface-bound aniline and succinimide bonding. In a second step, the Fe(II) polypyridyl complex [Fe(II)(tpy-PhNH2)2](2+) was added to the outside of the film, also by succinimide bonding. Ultrafast transient absorption measurements in 0.1 M HClO4 reveal that electron injection into TiO2 by (1)PDI* does not occur, but rather leads to the ultrafast formation of the redox-separated pair PDI(•+),PDI(•-), which decays with complex kinetics (τ1 = 0.8 ps, τ2 = 15 ps, and τ3 = 1500 ps). With the added Fe(II) polypyridyl complex, rapid (25 ps) oxidation of Fe(II) by the PDI(•+),PDI(•-) redox pair occurs to give Fe(III),PDI(•-) persisting for400 μs in the film environment. more...
- Published
- 2015
- Full Text
- View/download PDF
23. Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications
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Thomas J. Meyer, John M. Papanikolas, Aaron K. Vannucci, Joseph L. Templeton, M. Kyle Brennaman, Melissa K. Gish, and Dennis L. Ashford
- Subjects
South carolina ,Chemistry ,General Chemistry ,Archaeology - Abstract
Applications Dennis L. Ashford,† Melissa K. Gish,† Aaron K. Vannucci,‡ M. Kyle Brennaman,† Joseph L. Templeton,† John M. Papanikolas,† and Thomas J. Meyer*,† †Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599, United States ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States more...
- Published
- 2015
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24. Pump–probe microscopy: Visualization and spectroscopy of ultrafast dynamics at the nanoscale
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Michelle M. Gabriel, John M. Papanikolas, Erik M. Grumstrup, Erika M. Van Goethem, and Emma E. M. Cating
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Nanostructure ,Chemistry ,Chemical physics ,Temporal resolution ,Exciton ,Excited state ,Microscopy ,General Physics and Astronomy ,Nanotechnology ,Physical and Theoretical Chemistry ,Transport phenomena ,Spectroscopy ,Plasmon - Abstract
Excited state dynamics at the nanoscale provide important insight into the influence of structural features such as interfaces, defects, and surfaces on material properties. Pump–probe microscopy combines the spatial resolution of far-field optical microscopy with the temporal resolution of ultrafast spectroscopy, and has emerged as a powerful technique for characterizing spatial variation in dynamical phenomena across nanometer length scales. It has helped correlate dynamical phenomena with specific structural features in a variety of materials, shedding light on how excited state behaviors can dramatically differ from one member of the ensemble to the next, and even at different points within a single structure. It has also enabled direct imaging of transport phenomena such as free carrier diffusion, exciton migration and plasmon propagation in nanostructures. This ability to observe individual objects provides unique insight into complex materials where heterogeneous behavior makes it difficult, if not impossible, to reach clear and quantitative conclusions. more...
- Published
- 2015
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- View/download PDF
25. The University of North Carolina Energy Frontier Research Center: Center for Solar Fuels
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Thomas J. Meyer, Catherine M. Heyer, Gerald J. Meyer, Ralph L. House, and John M. Papanikolas
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Materials science ,Meteorology ,Renewable Energy, Sustainability and the Environment ,Energy Engineering and Power Technology ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Engineering physics ,0104 chemical sciences ,Frontier ,Fuel Technology ,Chemistry (miscellaneous) ,Materials Chemistry ,Center (algebra and category theory) ,0210 nano-technology ,Research center - Published
- 2016
- Full Text
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26. Ru(bpy)32+ derivatized polystyrenes constructed by nitroxide-mediated radical polymerization. Relationship between polymer chain length, structure and photophysical properties
- Author
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Zhuo Chen, Toan Pho, John M. Papanikolas, Kirk S. Schanze, Zachary A. Morseth, John R. Reynolds, Shahar Keinan, Zhen Fang, Zhenya Hu, Egle Puodziukynaite, Junlin Jiang, and Gyu Leem
- Subjects
chemistry.chemical_classification ,Nitroxide mediated radical polymerization ,Polymers and Plastics ,Chemistry ,Organic Chemistry ,Radical polymerization ,chemistry.chemical_element ,Bioengineering ,Polymer ,Chromophore ,Photochemistry ,Biochemistry ,Ruthenium ,chemistry.chemical_compound ,Polymer chemistry ,Click chemistry ,Molar mass distribution ,Polystyrene - Abstract
A series of polystyrene-based light harvesting polymers featuring pendant polypyridyl ruthenium complexes has been synthesized. The polymer backbones were prepared by nitroxide-mediated radical polymerization with a variable average molecular weight (Mn) ranging from ∼5500 to ∼24 000 g mol−1. Pendant Ru(II) polypyridyl complexes were grafted to the polymer backbone by azide–alkyne click chemistry to afford chromophore loaded polymers. The resulting polystyrene-based polychromophores with pendant Ru(II) polypyridyl complexes (PS-Ru) were characterized by nuclear magnetic resonance and infrared spectroscopy, confirming the high efficiency of the click grafting. The photophysical and electrochemical properties of the series of PS-Ru polymers were characterized in solution and investigated as a function of polymer chain length and solvent. The electrochemical properties of PS-Ru maintained the characteristics of the individual Ru(II) polypyridyl units. Emission quantum yield and lifetime studies reveal that the metal-to-ligand charge transfer (MLCT) excited states are quenched to a variable extent depending on the molecular weight of the polymers, consistent with intramolecular energy transfer and self-quenching in polymers with longer chain lengths. To support the synthetic effort, molecular dynamics simulations of the polypyridyl ruthenium derivatized polystyrenes in different solvents were conducted. more...
- Published
- 2015
- Full Text
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27. Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers
- Author
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Charles J. Zeman, John M. Papanikolas, Kirk S. Schanze, Melissa K. Gish, and Austin L. Jones
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Singlet oxygen ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Oligomer ,Fluorescence ,Photoinduced electron transfer ,0104 chemical sciences ,chemistry.chemical_compound ,Intersystem crossing ,chemistry ,Ultrafast laser spectroscopy ,Thiophene ,Physical and Theoretical Chemistry ,0210 nano-technology ,Spectroscopy - Abstract
A series of linear thiophene oligomers containing 4, 6, 8, 10, and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of nonacceptor-substituted thiophene oligomers, Tn, and corresponding NDI end-capped series, TnNDI2 (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the Tn and TnNDI2 series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond–microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the TnNDI2 oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the Tn oligomers gives rise to efficient fluorescence and intersystem crossing to a tr... more...
- Published
- 2017
28. Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites
- Author
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Seokhyoung Kim, James F. Cahoon, John M. Papanikolas, Amar Kumbhar, David Hill, James R. McBride, Lenzi J. Williams, Jonathan K. Meyers, and Emma E. M. Cating
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chemistry.chemical_classification ,Materials science ,Mechanical Engineering ,Iodide ,Inorganic chemistry ,Nanowire ,Halide ,Bioengineering ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0104 chemical sciences ,Crystallinity ,chemistry ,Chemical engineering ,Phase (matter) ,Halogen ,General Materials Science ,Crystallite ,0210 nano-technology ,Stoichiometry - Abstract
Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor–liquid–solid (VLS) growth of lead halide (PbX2; X = Cl, Br, or I) NWs and conversion to LHP. We present a kinetic model of the PbX2 NW growth process in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI2, we show that the NWs are single-crystalline, oriented in the ⟨1210⟩ direction, and composed of a stoichiometric PbI2 shaft with a spherical Pb tip. Low-temperature vapor-phase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI3) perovski... more...
- Published
- 2017
29. Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump-Probe Microscopy
- Author
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Caleb A. Christie, Emma E. M. Cating, John M. Papanikolas, James F. Cahoon, Joseph D. Christesen, Christopher W. Pinion, and Erik M. Grumstrup
- Subjects
Materials science ,Silicon ,Analytical chemistry ,chemistry.chemical_element ,Bioengineering ,02 engineering and technology ,01 natural sciences ,Molecular physics ,Quality (physics) ,0103 physical sciences ,Microscopy ,General Materials Science ,Silicon nanowires ,010302 applied physics ,business.industry ,Mechanical Engineering ,General Chemistry ,Orders of magnitude (numbers) ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Semiconductor ,chemistry ,Surface trap ,0210 nano-technology ,business ,Recombination - Abstract
Surface trap density in silicon nanowires (NWs) plays a key role in the performance of many semiconductor NW-based devices. We use pump-probe microscopy to characterize the surface recombination dynamics on a point-by-point basis in 301 silicon NWs grown using the vapor-liquid-solid (VLS) method. The surface recombination velocity (S), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship S = d/4τ from measurements of the recombination lifetime (τ) and NW diameter (d) at distinct spatial locations in individual NWs. We find that S varies by as much as 2 orders of magnitude between NWs grown at the same time but varies only by a factor of 2 or three within an individual NW. Although we find that, as expected, smaller-diameter NWs exhibit shorter τ, we also find that smaller wires exhibit higher values of S; this indicates that τ is shorter both because of the geometrical effect of smaller d and because of a poorer quality surface. These results highlight the need to consider interwire heterogeneity as well as diameter-dependent surface effects when fabricating NW-based devices. more...
- Published
- 2017
30. Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes
- Author
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Thomas J. Meyer, Leila Alibabaei, John M. Papanikolas, and Robert J. Dillon
- Subjects
education.field_of_study ,Quenching (fluorescence) ,Materials science ,Non-blocking I/O ,Population ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,Electrochemistry ,01 natural sciences ,Photocathode ,0104 chemical sciences ,Excited state ,Ultrafast laser spectroscopy ,General Materials Science ,0210 nano-technology ,education ,Recombination - Abstract
The hole-injection and recombination photophysics for NiO sensitized with RuP ([RuII(bpy)2(4,4′-(PO3H2)2-bpy)]2+) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron–hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway. more...
- Published
- 2017
31. Light Harvesting and Charge Separation in a π-Conjugated Antenna Polymer Bound to TiO2
- Author
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Egle Puodziukynaite, Zhen Fang, John R. Reynolds, Junlin Jiang, Alexander T. Gilligan, John M. Papanikolas, Kirk S. Schanze, Zachary A. Morseth, and Gyu Leem
- Subjects
chemistry.chemical_classification ,Materials science ,Scanning electron microscope ,Polymer ,Conjugated system ,Chromophore ,Photochemistry ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Photoexcitation ,Polyfluorene ,chemistry.chemical_compound ,General Energy ,chemistry ,Transmission electron microscopy ,Excited state ,Physical and Theoretical Chemistry - Abstract
This paper describes the photophysical and photoelectrochemical characterization of a light harvesting polychromophore array featuring a polyfluorene backbone with covalently attached Ru(II) polypyridyl complexes (PF-Ru-A), adsorbed on the surface of mesostructured TiO2 (PF-Ru-A//TiO2). The surface adsorbed polymer is characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, providing evidence for the morphology of the surface adsorbed polymer and the mode of binding. Photoexcitation of the Ru(II) complexes bound to the metal oxide surface (proximal) results in electron injection into the conduction band of TiO2, which is then followed by ultrafast hole transfer to the polymer to form oxidized polyfluorene (PF+). More interestingly, chromophores that are not directly bound to the TiO2 interface (distal) that are excited participate in site-to-site energy transfer processes that transport... more...
- Published
- 2014
- Full Text
- View/download PDF
32. Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces
- Author
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Thomas J. Meyer, Zachary A. Morseth, Byron H. Farnum, M. Kyle Brennaman, and John M. Papanikolas
- Subjects
Chemistry ,Fermi level ,Doping ,Analytical chemistry ,Nanoparticle ,General Chemistry ,Electrolyte ,Biochemistry ,Molecular physics ,Catalysis ,Photoinduced electron transfer ,symbols.namesake ,Electron transfer ,Colloid and Surface Chemistry ,Ultrafast laser spectroscopy ,symbols ,Thin film - Abstract
Photoinduced, interfacial electron injection and back electron transfer between surface-bound [Ru(II)(bpy)2(4,4'-(PO3H2)2-bpy)](2+) and degenerately doped In2O3:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (ΔG(o)'). By controlling the external bias, ΔG(o)' was varied from 0 to -1.8 eV for electron injection and from -0.3 to -1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using Marcus-Gerischer theory gave an experimental estimate of λ = 0.56 eV for the reorganization energy of the surface-bound Ru(III/II) couple in acetonitrile with 0.1 M LiClO4 electrolyte. more...
- Published
- 2014
- Full Text
- View/download PDF
33. Ultrafast Carrier Dynamics in Individual Silicon Nanowires: Characterization of Diameter-Dependent Carrier Lifetime and Surface Recombination with Pump–Probe Microscopy
- Author
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Michelle M. Gabriel, James F. Cahoon, John M. Papanikolas, Erik M. Grumstrup, Ernest L. Vallorz, Joseph D. Christesen, J. Kirschbrown, Emma E. M. Cating, and Christopher W. Pinion
- Subjects
education.field_of_study ,Materials science ,business.industry ,Population ,Physics::Optics ,Carrier lifetime ,Electron ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,General Energy ,Optics ,Femtosecond ,Microscopy ,Optoelectronics ,Charge carrier ,Physical and Theoretical Chemistry ,education ,business ,Ultrashort pulse ,Excitation - Abstract
Ultrafast charge carrier dynamics in silicon nanowires (NWs) grown by a vapor–liquid–solid mechanism were interrogated with optical pump–probe microscopy. The high time and spatial resolutions achieved by the experiments provide insight into the charge carrier dynamics of single nanostructures. Individual NWs were excited by a femtosecond pump pulse focused to a diffraction-limited spot, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Photoexcited carriers undergo both electron–hole recombination and diffusional migration away from the excitation spot on similar time scales. The evolution of the carrier population is monitored by a delayed probe pulse that is also focused to a diffraction-limited spot. When the pump and probe are spatially overlapped, the transient signal reflects both recombination and carrier migration. Diffusional motion is directly observed by spatially separating the pump and probe beams, enabling carriers to be generated in one locatio... more...
- Published
- 2014
- Full Text
- View/download PDF
34. Ultrafast Carrier Dynamics of Silicon Nanowire Ensembles: The Impact of Geometrical Heterogeneity on Charge Carrier Lifetime
- Author
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Joseph D. Christesen, John M. Papanikolas, Christopher W. Pinion, James F. Cahoon, Emma E. M. Cating, Ernest L. Vallorz, Michelle M. Gabriel, Erik M. Grumstrup, and J. Kirschbrown
- Subjects
Materials science ,Silicon ,Absorption spectroscopy ,chemistry.chemical_element ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Photoexcitation ,General Energy ,chemistry ,Picosecond ,Ultrafast laser spectroscopy ,Direct and indirect band gaps ,Charge carrier ,Physical and Theoretical Chemistry ,Atomic physics ,Spectroscopy - Abstract
Ultrafast carrier dynamics in silicon nanowires with average diameters of 40, 50, 60, and 100 nm were studied with transient absorption spectroscopy. After 388 nm photoexcitation near the direct band gap of silicon, broadband spectra from 400 to 800 nm were collected between 200 fs and 1.3 ns. The transient spectra exhibited both absorptive and bleach features that evolved on multiple time scales, reflecting contributions from carrier thermalization and recombination as well as transient shifts of the ground-state absorption spectrum. The initially formed “hot” carriers relaxed to the band edge within the first ∼300 fs, followed by recombination over several hundreds of picoseconds. The charge carrier lifetime progressively decreased with decreasing diameter, a result consistent with a surface-mediated recombination process. Recombination dynamics were quantitatively modeled using the diameter distribution measured from each sample, and this analysis yielded a consistent surface recombination velocity of ... more...
- Published
- 2014
- Full Text
- View/download PDF
35. Photophysical Characterization of a Helical Peptide Chromophore–Water Oxidation Catalyst Assembly on a Semiconductor Surface Using Ultrafast Spectroscopy
- Author
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Melissa K. Gish, Thomas J. Meyer, John M. Papanikolas, Derek M. Ryan, Stephanie E. Bettis, Leila Alibabaei, and Marcey L. Waters
- Subjects
Materials science ,Kinetics ,Chromophore ,Photochemistry ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Catalysis ,chemistry.chemical_compound ,Electron transfer ,General Energy ,Catalytic oxidation ,chemistry ,Pyridine ,Ultrafast laser spectroscopy ,Physical and Theoretical Chemistry ,Spectroscopy - Abstract
We report a detailed kinetic analysis of ultrafast interfacial and intra-assembly electron transfer following excitation of an oligoproline scaffold functionalized by chemically linked light-harvesting chromophore [Ru(pbpy)2(bpy)]2+ (pbpy = 4,4′-(PO3H2)2-2,2′-bipyridine, bpy = 2,2′-bipyridine) and water oxidation catalyst [Ru(Mebimpy)(bpy)OH2]2+ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine). The oligoproline scaffold approach is appealing due to its modular nature and helical tertiary structure. They allow for the control of electron transfer distances in chromophore–catalyst assemblies for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The proline chromophore–catalyst assembly was loaded onto nanocrystalline TiO2 with the helical structure of the oligoproline scaffold maintaining the controlled relative positions of the chromophore and catalyst. Ultrafast transient absorption spectroscopy was used to analyze the kinetics of the first photoactivation step for oxidation of wa... more...
- Published
- 2014
- Full Text
- View/download PDF
36. Light-Harvesting Polymers: Ultrafast Energy Transfer in Polystyrene-Based Arrays of π-Conjugated Chromophores
- Author
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Zhuo Chen, John M. Papanikolas, Kirk S. Schanze, Erik M. Grumstrup, and Alexander T. Gilligan
- Subjects
chemistry.chemical_classification ,Quenching (fluorescence) ,Materials science ,Polymer ,Chromophore ,Photochemistry ,Surfaces, Coatings and Films ,chemistry.chemical_compound ,Polymerization ,chemistry ,Excited state ,Ultrafast laser spectroscopy ,Materials Chemistry ,Copolymer ,Polystyrene ,Physical and Theoretical Chemistry - Abstract
Energy transfer along a nonconjugated polymer chain is studied with a polystyrene-based copolymer of oligo(phenylene-ethynylene) (OPE) donor and thiophene-benzothiadiazole (TBT) acceptor pendants. The graft copolymers are prepared from reversible addition-fragmentation transfer polymerization (RAFT) and copper(I)-catalyzed azide-alkyne "click" reaction. The singlet energy transfer from donor to accept is studied via fluorescence emission and ultrafast transient absorption spectroscopy. Near unity quenching of the OPE excited state by the TBT moiety occurs on multiple time scales (2-50 ps) dependent on where the initial exciton is formed on the polymer. more...
- Published
- 2013
- Full Text
- View/download PDF
37. Watching Photoactivation in a Ru(II) Chromophore–Catalyst Assembly on TiO2 by Ultrafast Spectroscopy
- Author
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Li Wang, Thomas J. Meyer, John M. Papanikolas, David W. Thompson, and Dennis L. Ashford
- Subjects
Chemistry ,Chromophore ,Photochemistry ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Ion ,Catalysis ,Photoexcitation ,Microsecond ,General Energy ,Excited state ,Physical and Theoretical Chemistry ,Absorption (chemistry) ,Spectroscopy - Abstract
This paper examines the ultrafast dynamics of the initial photoactivation step in a molecular assembly consisting of a chromophore (denoted [RuaII]2+) and a water-splitting catalyst (denoted [RubII]2+) anchored to TiO2. Photoexcitation of the chromophore is followed by rapid electron injection from the Ru(II) metal-to-ligand charge-transfer (MLCT) excited state. The injection process was followed via the decay of the bpy radical anion absorption at 375 nm. Injection is ∼95% efficient and exhibits multiple kinetic components with decay times ranging from more...
- Published
- 2013
- Full Text
- View/download PDF
38. Ultrafast Formation of a Long-Lived Charge-Separated State in a Ru-Loaded Poly(3-hexylthiophene) Light-Harvesting Polymer
- Author
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Shahar Keinan, John M. Papanikolas, Li Wang, Kirk S. Schanze, Aaron C. Brown, Egle Puodziukynaite, Erik M. Grumstrup, and John R. Reynolds
- Subjects
chemistry.chemical_classification ,Materials science ,Polymer ,Photochemistry ,Polaron ,Photoinduced electron transfer ,Photoexcitation ,Electron transfer ,chemistry ,Excited state ,Ultrafast laser spectroscopy ,General Materials Science ,Physical and Theoretical Chemistry ,Spectroscopy - Abstract
A light-harvesting macromolecular assembly (PT-Ru) consisting of a poly(3-hexylthiophene) (P3HT) scaffold and pendant Ru(II) polypyridyl complexes that exhibits rapid and efficient formation of a long-lived charge-separated state is described here. Photoinduced electron transfer from the polymer backbone to Ru(II) was investigated by femtosecond transient absorption spectroscopy. Photoexcitation at 388 nm results in the excitation of both the polymer backbone and Ru(II) complexes, with relative excitation probabilities of 60 and 40%, respectively. The dominant pathway (∼85%) for decay of the polymer excited state is direct electron transfer from the polymer scaffold to Ru(II), forming a positive polaron and a reduced complex [RuII(L)2(L–)]+, denoted Ru(I). The charge-separated state PT+•-Ru(I) is long-lived, persisting for 20–60 μs, and is attributed to the high mobility of holes on the polymer backbone, which facilitates spatial separation of the electron and hole, delaying recombination. The remaining 1... more...
- Published
- 2013
- Full Text
- View/download PDF
39. Hybrid Standing Wave and Whispering Gallery Modes in Needle-Shaped ZnO Rods: Simulation of Emission Microscopy Images Using Finite Difference Frequency Domain Methods with a Focused Gaussian Source
- Author
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John M. Papanikolas, Brian P. Mehl, James K Parker, Ralph L. House, and J. Kirschbrown
- Subjects
Materials science ,business.industry ,Plane wave ,Physics::Optics ,Rod ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,Standing wave ,Cross section (physics) ,Resonator ,General Energy ,Optics ,law ,Frequency domain ,Optical cavity ,Physical and Theoretical Chemistry ,Whispering-gallery wave ,business - Abstract
Two-photon emission microscopy is used to investigate the photoluminescence properties of individual ZnO rods. The rods are 10–20 μm in length with a tapered cross section that varies from 1 to 2 μm at the midpoint to several hundred nanometers at the ends. The tapered shape and hexagonal cross section result in complex optical resonator modes that lead to periodic patterns in the two-photon emission image. Finite-difference frequency domain methods using a series of excitation sources, including focused Gaussian, point dipole, and plane wave, suggest that resonator modes have both standing wave (Fabry-Perot) and whispering gallery mode character, whose relative contributions vary along the rod axis. more...
- Published
- 2013
- Full Text
- View/download PDF
40. Direct Imaging of Free Carrier and Trap Carrier Motion in Silicon Nanowires by Spatially-Separated Femtosecond Pump–Probe Microscopy
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James F. Cahoon, Brian P. Mehl, John M. Papanikolas, Christopher W. Pinion, Michelle M. Gabriel, David F. Zigler, Joseph D. Christesen, Emma E. M. Cating, J. Kirschbrown, and Erik M. Grumstrup
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Materials science ,Microscope ,business.industry ,Ambipolar diffusion ,Mechanical Engineering ,Dynamics (mechanics) ,Nanowire ,Bioengineering ,General Chemistry ,Condensed Matter Physics ,law.invention ,Photoexcitation ,Optics ,law ,Temporal resolution ,Femtosecond ,Optoelectronics ,General Materials Science ,business ,Recombination - Abstract
We have developed a pump-probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼10(4) cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales. more...
- Published
- 2013
- Full Text
- View/download PDF
41. Ultrafast Recombination Dynamics in Dye-Sensitized SnO
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Melissa K, Gish, Alexander M, Lapides, M Kyle, Brennaman, Joseph L, Templeton, Thomas J, Meyer, and John M, Papanikolas
- Abstract
Interfacial dynamics are investigated in SnO
- Published
- 2016
42. ChemInform Abstract: Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells
- Author
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Christopher J. Dares, Leila Alibabaei, Robert J. Dillon, Ralph L. House, Gerald J. Meyer, John M. Papanikolas, Thomas J. Meyer, M. Kyle Brennaman, Melissa K. Gish, and Dennis L. Ashford
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Hydrogen ,chemistry.chemical_element ,Nanoparticle ,General Medicine ,Oxygen ,Cathode ,Photocathode ,law.invention ,Catalysis ,chemistry ,Chemical engineering ,law ,Water splitting ,Carbon - Abstract
The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore–catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future. more...
- Published
- 2016
- Full Text
- View/download PDF
43. Ultrafast photoinduced charge transfer in pi-conjugated electron systems: Effects of structure, delocalization, and energetics (Conference Presentation)
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Melissa K. Gish, Omar F. Mohammed, Amani A. Alsam, John M. Papanikolas, Charles J. Zeman, Shawkat M. Aly, Kirk S. Schanze, and Austin L. Jones
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chemistry.chemical_classification ,Delocalized electron ,chemistry.chemical_compound ,Materials science ,chemistry ,Organic solar cell ,Diimide ,Phenylene ,Thiophene ,Electron acceptor ,Photochemistry ,Oligomer ,Polymer solar cell - Abstract
Photoinduced charge transfer is a key step in the mechanism of charge generation in organic solar cells. Charge transfer typically occurs from a photoexcited conjugated polymer donor to an electron acceptor. In an effort to better understand the primary events in solar cells, we have investigated photoinduced charge transfer in model donor-acceptor systems consisting of pi-conjugated oligomer donors that are covalently linked to diimide electron acceptors. These studies utilized oligo(thiophene), oligo(phenylene ethynylene) and oligo(fluorene) pi-conjugated systems with lengths varying from 4 to 12 repeat units linked to naphthalene diimide electron acceptors. Excitation with 100 femtosecond pulses at wavelengths correspoinding to the conjugated oligomer absorption band(s) leads to rapid photoinduced charge transfer to produce a charge separated state, (oligomer+)-(NDI-), which subsequently decays on timescales ranging from 100 ps to 5 ns. The dynamics of the forward and reverse electron transfer reactions depend strongly on the structure and length of the pi-conjugated oligomers, with the fastest rates occurring for oligo(thiophene)s, and considerably slower rates for oligo(phenylene ethynylene)s. The talk will discuss the structure-property relationships and energetic correlations that control the dynamics of charge separation and recombination. more...
- Published
- 2016
- Full Text
- View/download PDF
44. Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells
- Author
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John M. Papanikolas, M. Kyle Brennaman, Thomas J. Meyer, Melissa K. Gish, Gerald J. Meyer, Ralph L. House, Christopher J. Dares, Robert J. Dillon, Dennis L. Ashford, and Leila Alibabaei
- Subjects
Hydrogen ,Chemistry ,Nanoparticle ,chemistry.chemical_element ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Biochemistry ,Oxygen ,Catalysis ,Photocathode ,Cathode ,0104 chemical sciences ,law.invention ,Colloid and Surface Chemistry ,law ,Water splitting ,0210 nano-technology ,Carbon - Abstract
The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore–catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future. more...
- Published
- 2016
45. Role of Macromolecular Structure in the Ultrafast Energy and Electron Transfer Dynamics of a Light-Harvesting Polymer
- Author
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Kirk S. Schanze, John M. Papanikolas, Robert J. Dillon, Zachary A. Morseth, Alexander T. Gilligan, John R. Reynolds, and Toan V. Pho
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Materials science ,Quenching (fluorescence) ,010405 organic chemistry ,Chromophore ,010402 general chemistry ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Photoexcitation ,Electron transfer ,Molecular dynamics ,Excited state ,Ultrafast laser spectroscopy ,Femtosecond ,Materials Chemistry ,Physical and Theoretical Chemistry - Abstract
Ultrafast energy and electron transfer (EnT and ET, respectively) are characterized in a light-harvesting assembly based on a π-conjugated polymer (poly(fluorene)) functionalized with broadly absorbing pendant organic isoindigo (iI) chromophores using a combination of femtosecond transient absorption spectroscopy and large-scale computer simulation. Photoexcitation of the π-conjugated polymer leads to near-unity quenching of the excitation through a combination of EnT and ET to the iI pendants. The excited pendants formed by EnT rapidly relax within 30 ps, whereas recombination of the charge-separated state formed following ET occurs within 1200 ps. A computer model of the excited-state processes is developed by combining all-atom molecular dynamics simulations, which provides a molecular-level view of the assembly structure, with a kinetic model that accounts for the multiple excited-state quenching pathways. Direct comparison of the simulations with experimental data reveals that the underlying structure has a dramatic effect on the partitioning between EnT and ET in the polymer assembly, where the distance and orientation of the pendants in relation to the backbone serve to direct the dominant quenching pathway. more...
- Published
- 2016
46. Growth and Post-Deposition Treatments of SrTiO3 Films for Dye-Sensitized Photoelectrosynthesis Cell Applications
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Rene Lopez, John M. Papanikolas, Robin R. Knauf, Animesh Nayak, Robert W. Call, Jillian L. Dempsey, Robert J. Dillon, and Leila Alibabaei
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Photocurrent ,Auxiliary electrode ,Materials science ,Standard hydrogen electrode ,Oxide ,Analytical chemistry ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Tin oxide ,01 natural sciences ,Reference electrode ,0104 chemical sciences ,Pulsed laser deposition ,chemistry.chemical_compound ,chemistry ,Electrode ,General Materials Science ,0210 nano-technology - Abstract
Sensitized SrTiO3 films were evaluated as potential photoanodes for dye-sensitized photoelectrosynthesis cells (DSPECs). The SrTiO3 films were grown via pulsed laser deposition (PLD) on a transparent conducting oxide (fluorine-doped tin oxide, FTO) substrate, annealed, and then loaded with zinc(II) 5,10,15-tris(mesityl)-20-[(dihydroxyphosphoryl)phenyl] porphyrin (MPZnP). When paired with a platinum wire counter electrode and an Ag/AgCl reference electrode these sensitized films exhibited photocurrent densities on the order of 350 nA/cm(2) under 0 V applied bias conditions versus a normal hydrogen electrode (NHE) and 75 mW/cm(2) illumination at a wavelength of 445 nm. The conditions of the post-deposition annealing step-namely, a high-temperature reducing atmosphere-proved to be the most important growth parameters for increasing photocurrent in these electrodes. more...
- Published
- 2016
47. Controlling Morphology and Chain Aggregation in Semiconducting Conjugated Polymers: The Role of Solvent on Optical Gain in MEH-PPV
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Zach E. Lampert, John M. Papanikolas, C. Lewis Reynolds, and M. Osama Aboelfotoh
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Polymer morphology ,chemistry.chemical_classification ,Vinyl Compounds ,Morphology (linguistics) ,Materials science ,Optical Phenomena ,Polymers ,Polymer ,Conjugated system ,Chlorobenzenes ,Surfaces, Coatings and Films ,Solvent ,Semiconductors ,chemistry ,Chemical engineering ,Chain (algebraic topology) ,Polymer chemistry ,Solvents ,Materials Chemistry ,Physical and Theoretical Chemistry ,Furans - Abstract
We report the results of a detailed investigation that addresses the influence of polymer morphology and chain aggregation, as controlled by the chemical nature of the solvent, on the optical gain properties of the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Using the variable stripe length technique in the picosecond regime, we have extensively studied the optical gain performance of asymmetric planar waveguides formed with thin MEH-PPV films spin-cast from concentrated chlorobenzene (CB) and tetrahydrofuran (THF) solutions onto thermally oxidized silicon substrates. CB and THF solvents were chosen based on their known ability to promote and effectively limit aggregate formation, respectively. Very large net gain coefficients are demonstrated, reaching values of 330 and 365 cm(-1), respectively, when optically pumping the waveguides with a maximum energy density of 85 μJ/cm(2). Our results clearly demonstrate that polymer morphology, and hence, the chain conformation dependence of the degree of aggregation in the films as controlled by the solvent, has minimal impact on the net gain. Moreover, the waveguides exhibit low loss coefficients of 10-20 cm(-1) at the ASE wavelength. These results question the importance of polymer morphology and aggregate formation in polymer-based optical devices operating at high excitation densities in the stimulated emission regime as would be characteristic of lasers and optical amplifiers. more...
- Published
- 2012
- Full Text
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48. Light Harvesting in Microscale Metal–Organic Frameworks by Energy Migration and Interfacial Electron Transfer Quenching
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Wenbin Lin, John M. Papanikolas, Liqing Ma, Caleb A. Kent, Demin Liu, and Thomas J. Meyer
- Subjects
Quenching (fluorescence) ,Chemistry ,General Chemistry ,Photochemistry ,Biochemistry ,Catalysis ,Electron transfer ,Colloid and Surface Chemistry ,Excited state ,Metal-organic framework ,Luminescence ,Absorption (electromagnetic radiation) ,Microscale chemistry ,Visible spectrum - Abstract
Microscale metal-organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers. Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N',N'-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching. more...
- Published
- 2011
- Full Text
- View/download PDF
49. Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone
- Author
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Li Wang, Robert A. Binstead, John M. Papanikolas, David J. Stewart, Stephanie E. Bettis, Thomas J. Meyer, and M. Kyle Brennaman
- Subjects
Electron transfer ,Proton ,Chemistry ,Excited state ,Intramolecular force ,General Materials Science ,Protonation ,Physical and Theoretical Chemistry ,Proton-coupled electron transfer ,Ground state ,Photochemistry ,Adduct - Abstract
The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ (1) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [ReII(bpy–•)(CO)3(4,4′-bpy)]+*, is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give ReI(bpy–•)(CO)3(4,4′-bpy)]0 with k = (1.8 ± 0.2) × 109 M–1 s–1, followed by proton transfer from H2Q to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+. Protonation triggers intramolecular bpy•– → 4,4′-bpyH+ electron transfer. In the second pathway, preassociation occurs between the ground state and H2Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H2Q in concert with intramolecular bpy•– → 4,4′-bpyH+ electron transfer to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+ with k = (1.0 ± 0.4) × 109 s–1 in 3:1 CH3CN/H2O. more...
- Published
- 2011
- Full Text
- View/download PDF
50. The End Is Different than The Middle: Spatially Dependent Dynamics in ZnO Rods Observed by Femtosecond Pump–Probe Microscopy
- Author
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Ralph L. House, John M. Papanikolas, J. Kirschbrown, and Brian P. Mehl
- Subjects
Materials science ,genetic structures ,business.industry ,Dynamics (mechanics) ,Pump probe ,Molecular physics ,Rod ,Pump probe microscopy ,Optics ,Ultrafast laser spectroscopy ,Femtosecond ,Microscopy ,General Materials Science ,Physical and Theoretical Chemistry ,business ,Recombination - Abstract
Pump–probe transient absorption microscopy was used to follow the electron–hole (e–h) recombination dynamics at different points within individual needle-shaped ZnO rods to characterize spatial dif... more...
- Published
- 2011
- Full Text
- View/download PDF
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