Publications
Peer-reviewed discoveries spanning photothermal chemistry, nanoscale electronic structures, and spectroscopy.
2026
Mapping Proton‐Coupled Electron Transfer With Real Space Coordinates
Authors
Adam Šrut, Martin Diefenbach, Marvin Kronenberger, Benjamin J. Lear, Vera Krewald
Journal
Angewandte Chemie International Edition
Abstract
Both electron transfer (ET) and proton transfer (PT) are common steps in energy conversion across chemistry and biology. Coupling of these two transfer events reduces the energy demand relative to either individual process, but introduces a fundamentally new process-proton-coupled electron transfer (PCET)-with its own demands for a suitable theoretical description. Conceptualization of PCET usually involves a square scheme representing ET, PT, and PCET steps, each generating states with different energies. While intuitive, these square schemes do not offer structural insights such as the identities and contributions of nuclear motions to PCET. Herein, we present a computational approach that maps these square schemes onto real space coordinates (Å), from which ground and excited-state potential energy surfaces can be generated. This mapping involves the identification of PT and ET coordinates and reconstruction of the potential energy surfaces in orthogonalized coordinates. We find qualitative differences in key features of the surfaces for two distinct processes within PCET, namely concerted proton-electron transfer and hydrogen atom transfer, which may help to distinguish different PCET scenarios.
2025
Photothermal Curing of Polydimethyldisiloxane: Carbon Black Composites Results in Changes to Polymer Topography, Cross-Link Density, and Mass Density
Authors
Anthony Katona, Benjamin J. Lear
Journal
The Journal of Physical Chemistry C
Abstract
Photothermal heating using light-absorbing nanoparticles has recently emerged as a means to drive the rapid on-demand curing of thermally cured thermosets. However, the effects of extreme localized heat on the resulting polymers have rarely been considered. Herein, we examine the Shore hardness, gel fraction, cross-link density, and mass density of polydimethylsiloxane bearing 1% carbon black nanoparticles. We explore these properties as a function of curing under three conditions: 60 s exposure to 3.5 W/cm 2 of 808 nm continuous wave laser light, resting at room temperature for 48 h, and heating in a 150 °C oven for 60 min. We also explore 15 combinations of these conditions. We find that photothermal treatment has a significant impact on the Shore hardness, cross-link density, and mass density. However, we also find that the order in which these treatments are applied is important, where applying nonphotothermal curing to the polymer composite substantially ameliorates the effects of subsequent photothermal treatment.
Practical Guide to Automated TEM Image Analysis for Increased Accuracy and Precision in the Measurement of Particle Size and Morphology
Authors
Kristen M. Aviles, Benjamin J. Lear
Journal
Abstract
A common desire in nanoscience is to describe the size and morphology of nanoparticles as observed from TEM images. Many times, this analysis is done manually, a lengthy process that is prone to errors and ambiguity in the measurements. While several research groups have reported excellent advances in machine-learned approaches to automated TEM image processing, the tools that they have developed often require specialized software or significant knowledge of coding. This state of affairs means that a majority of researchers in the field of nanoscience are not well-equipped to incorporate these advances into their normal workflows. In this tutorial, we describe how to use Weka segmentation within the free and open source program FIJI to automatically identify and characterize nanoparticles from TEM images. The approach we outline is not meant to discount the excellent results of groups working at the forefront of machine learning image analysis; rather, it is meant to bring similar tools to a broader audience by demonstrating how such processing can be done within the GUI-based interface of FIJIa program already commonly used within nanoscience research. We also discuss the advantages that arise from automatic processing of TEM images, including repeatability, time savings, the ability to process low-contrast images, and the additional types of characterization that can be performed following identification of particles. The overall goal is to provide an accessible tool that enables a more robust and repeatable analysis and descriptions of nanoparticles.
Rapid photothermal curing of PDMS on paper
Authors
Alessandra P. Blasone, Anthony Katona, Benjamin J. Lear
Journal
Abstract
Photothermal heating of PDMS is used to cure the polymer on paper. Compared to conventional heating in an oven, photothermal heating provides greater acceleration of the curing and small changes to the substrate’s properties.
Structured Polymer-Derived Ceramic Composites via Near-Infrared Thermal Stereolithography
Authors
Evelyn Wang, Shruti Gupta, Charles Rafalko, Benjamin J. Lear, Michael A. Hickner
Journal
Abstract
High Resolution Image Download MS PowerPoint Slide We have developed near-infrared (NIR) thermal stereolithography (SLA) to print 2.5D-structured polymer-derived ceramic (PDC) composites with high SiC particle loadings in a PDC matrix. When combined with polymer infiltration and pyrolysis (PIP), this approach overcomes the challenges associated with traditional ultraviolet-based printing techniques when printing composite resins, namely, low light penetration, limited particle loadings, high shrinkage, and weak mechanical properties. Using an NIR laser to deliver spatially controlled thermal energy to the surface of a reactive resin pool induces localized thermally initiated free-radical polymerization in a top-down SLA configuration. After printing the green body, postprocessing methods, including debinding and PIP, are employed to densify and strengthen the printed samples. A Si–O–C x support network was formed in the debinded samples using a small amount of preceramic polymer in the printing resin to maintain the structural integrity of this porous preform. After 5 cycles of PIP, the PDC composites demonstrated a flexural strength of 74.3 ± 13.7 MPa with a density of 2.31 g/cm 3 . Different 2.5D lattice designs were fabricated by using this printing and materials processing method, and a compressive strength of 32.8 ± 11.2 MPa was obtained for lightweight honeycomb structures with an effective density of 1.07 g/cm 3 .
Thermal Stereolithography of SiC-Loaded Acrylate Resins with Polymer-Derived Ceramic Infiltration
Authors
Evelyn Wang, Shruti Gupta, Joseph Fortenbaugh, Caillin J. Ryan, Christopher M. DeSalle, Jeffrey R. Shallenberger, Douglas E. Wolfe, Benjamin J. Lear, Michael A. Hickner
Journal
ACS Applied Engineering Materials
Abstract
High Resolution Image Download MS PowerPoint Slide The implementation of stereolithography (SLA) for fabricating 3D-structured polymer-derived ceramics (PDCs) has greatly improved the resolution, manufacturing potential, and widespread capability to produce complicated component geometries in ceramic materials. However, different material systems impose challenges to the traditional UV SLA photo-cross-linking process due to a narrow window of material selection requirements─UV transparency, UV degradation resistance, the ability to support the photoinduced radical curing mechanism, and ambient shelf life stability. Herein, a near-infrared (NIR) thermal SLA printing technology is demonstrated on a composite thermally curable acrylate-based highly loaded resin to overcome current issues with UV light-driven SLA additive manufacturing of preceramic polymers (PCP). For this thermal SLA cross-linking method, a high-intensity NIR laser (λ = 808 nm) was used to generate localized thermal heating at the resin pool interface, which led to rapid, targeted thermal free-radical polymerization and solidification of the SiC particle-laden acrylate-based resin during laser scanning. Thermally cured printed parts were demonstrated using a gantry-based movement platform and a resin pool in a top-down laser scanning configuration. After printing, the green bodies were debinded, followed by polymer infiltration and pyrolysis (PIP) during postprocessing, which enhanced the mechanical strength of the pyrolyzed samples. This work demonstrated the fabrication of a reinforced PDC composite material with crystalline silicon carbide (SiC) fillers and an amorphous matrix made of silicon oxycarbide (SiOC) and silicon carbonitride (SiCN). The flexural strength of the NIR-printed samples reached 48 MPa with a fracture toughness of 4 MPa·m 1/2 .
Using Exam Preparation and Reflection to Introduce Artificial Intelligence Tools in Honors General Chemistry
Authors
Morgan A. Vincent, Benjamin J. Lear
Journal
Abstract
Abstract We report an intervention, performed during the Fall of 2024, in which a large language model artificial intelligence chatbot was introduced as a tool for pre-exam study and postexam reflection within an honors general chemistry course (CHEM 110H) at Penn State University. Through a combination of a structured 75 min instructional session and scaffolded pre- and postexam AI-based assignments, students engaged with AI tools such as ChatGPT, Microsoft Copilot, and Google Gemini intended to support content review and metacognitive development. Pre- and postcourse surveys revealed significant increases in students’ frequency of AI use, academic confidence in using AI, and perceived utility of AI, alongside reductions in anxiety regarding ethics of its use in academics. Survey data also showed a shift toward more positive and cohesive student perceptions of AI, suggesting that reflective, ethically framed AI integration can promote autonomous, meaningful engagement with new technologies without compromising academic performance. These findings highlight the value of intentional AI training in STEM education and underscore the need for continued study across diverse educational contexts.
Using Post Synthetic Treatment of AuNPs to Improve Uniformity of Their Thiol Ligand Coverage and Electronic Properties
Authors
Benjamin P. Kaercher, Benjamin J. Lear
Journal
Abstract
Gold nanoparticles are valued for their unique electronic properties and electronic structure as well as the tunability of their size and surface chemistry. However, there exist a number of open questions regarding the connection among size, morphology, surface chemistry, and electronic structure. Herein, we report a study that tracks changes in these aspects for small (∼3 nm diameter) hexanthiolate-protected gold nanoparticles aged in a dialysis bag that allowed free diffusion of ligand, but not metallic core. Using TEM, we find that statistically significant changes in size accompany aging. Using inductively coupled plasma atomic/optical emission spectroscopy, we find that statistically significant changes in the ligand-to-gold ratio accompany aging. Finally using a modified Evans NMR technique, we observe that statistically significant changes in the electronic structure accompany aging. Examining all these aspects, we conclude that the only meaningful connection between them is that both the ligand-to-gold ratio and the electronic structure properties experience a “focusing” in their values during aging, meaning that the standard deviation in these values decreases. We therefore suggest that during aging, the nanoparticles undergo a restructuring that leads to this focusing of the electronic structure. Additionally, we demonstrate that this dialysis treatment is an effective means to obtain particles with a more consistent electronic structure at the population level.
2024
Effective Photothermal Curing of PDMS Using Ultralow Loadings of Carbon Black
Authors
Anthony Katona, Benjamin J. Lear
Journal
Abstract
Photothermal curing of polymer nanocomposites inherently requires the addition of a light-absorbing photothermal agent to thermalize the incident light. For applications where the final product is desired to be clear and colorless, this can lead to an apparent contradiction in requirements. Herein, we demonstrate that carbon black is an excellent photothermal agent that leads to large photothermal enhancement for the curing of polydimethylsiloxane, even at extremely low loadings (1 × 10 –6 w/w) that result in composites that are scarcely more colored than the pure polymer. We demonstrate this photothermal enhancement using infrared spectroscopy to track the rate of reaction, demonstrate that photothermal curing produces dense cross-linked polymers, and develop a computational model for the heating that verifies that significant heating is expected even at these low loadings. The model also reveals that the penetration depth of light through a sample, thermal conduction through a sample, and convective cooling at the sample’s surface control the thermal profiles through the sample. In particular, we find that the peak temperature is always some distance below the surface and that this distance, and the final maximum temperature, can be controlled via the loading of the photothermal agent.
Analysis of synthetic parameters for coating aluminum powders with phenyltriethoxysilane coupling agent and their effects on powder flow behavior
Authors
Lillian M. Mawby, Bellamarie Ludwig, Benjamin J. Lear
Journal
Abstract
No abstract available in repository.
Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion
Authors
Adam Šrut, Benjamin J. Lear, Vera Krewald
Journal
Angewandte Chemie International Edition
Abstract
A long-standing question in electron transfer research concerns the number and identity of collective nuclear motions that drive electron transfer or localisation. It is well established that these nuclear motions are commonly gathered into a so-called electron transfer coordinate. In this theoretical study, we demonstrate that both anti-symmetric and symmetric vibrational motions are intrinsic to bridged systems, and that both are required to explain the characteristic shape of their intervalence charge transfer bands. Using the properties of a two-state Marcus-Hush model, we identify and quantify these two coordinates as linear combinations of normal modes from ab initio calculations. This quantification gives access to the potential coupling, reorganization energy and curvature of the potential energy surfaces involved in electron transfer, independent of any prior assumptions about the system of interest. We showcase these claims with the Creutz-Taube ion, a prototypical Class III mixed valence complex. We find that the symmetric dimension is responsible for the asymmetric band shape, and trace this back to the offset of the ground and excited state potentials in this dimension. The significance of the symmetric dimension originates from geometry dependent coupling, which in turn is a natural consequence of the well-established superexchange mechanism. The conceptual connection between the symmetric and anti-symmetric motions and the superexchange mechanism appears as a general result for bridged systems.
Titelbild: Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion (Angew. Chem. 31/2024)
Authors
Adam Šrut, Benjamin J. Lear, Vera Krewald
Journal
Abstract
No abstract available in repository.
Using ChatGPT-4 to Teach the Design of Data Visualizations
Authors
Benjamin J. Lear
Journal
Abstract
Modern scientific communication revolves around data visualizations. While chemistry curricula include basic instruction on constructing data visualizations, the design of these visualizations is rarely taught. There are two main reasons for this lack of instruction: (i) most instructors were never taught design themselves and thus struggle to articulate design concepts, and (ii) refining data visualizations has traditionally required detailed knowledge of specialized software, which required excessive time to teach within existing curricula. This article reports my recent experience using the AI tool ChatGPT-4 to teach the design of data visualizations. ChatGPT-4 can process data, create, and display data visualizations─eliminating the need to teach specialized software. The article makes the case that this tool can help overcome the above barriers, by leveraging everyday language as the interface for creating data visualizations. The use of everyday language means that one need not possess a specialized design lexicon, but only understand the basics of design─which can be surprisingly easy to learn and teach. This article introduces basic graphic design principles, demonstrates using them with ChatGPT-4 to produce data visualizations, discusses the relative strengths and weaknesses of this approach, reports student perceptions of their experience with the tool, and touches on outcomes from teaching the design of data visualizations using this tool. The overall conclusion is that ChatGPT-4 offers an opportunity to provide meaningful instruction that moves beyond the mechanics of constructing data visualizations, focusing instead on designing effective visualizations. This approach helps prepare students to communicate their science more effectively.
2023
Dependence of Photothermal Rate Enhancements of Urethane Formation upon the Nature of Light Exposure and Solution Viscosity
Authors
Nathaniel C. Ginder, Benjamin J. Lear
Journal
The Journal of Physical Chemistry C
Abstract
The photothermal effect of nanoparticles naturally results in nanometer scale heat sources. Despite their highly localized nature, these heat sources can be used to drive bulk scale chemical transformations. However, due to the localization in both time and space, it is reasonable to expect that the time scale of heating, as well as transport of reactive species into and out of the heated volumes during this time, might play a role in determining the efficacy of photothermal heating for driving chemical reactions. Herein, we report an investigation into these effects for the reaction between hexamethylene diisocyanate and a series of alcohols to form urethane bonds. The length of photothermal heating is controlled via the duration of light exposure, using either a modulated continuous wave (2 min duty cycle) or a nanosecond pulse (8 ns pulses) laser that deliver nearly the same total energy to the system. Mass transport is controlled by changing the alcohol from butanol to butanediol to a polyester diol, resulting in reaction mixtures that change their viscosity from 1.66 to 206 cSt. We use infrared spectroscopy to follow the urethane production and associated isocyanate consumption. We then fit the course of the reaction to a kinetic model from which we extract rate constants used to quantify the degree of photothermal rate enhancement. For the chemical systems used, we find no significant dependence on viscosity. We also find that, for the light sources used, the average rate enhancement is not significantly affected by the length of light exposure, but the rate of the reaction during the time of exposure increases with larger instantaneous power.
The Marcus dimension: identifying the nuclear coordinate for electron transfer from ab initio calculations
Authors
Adam Šrut, Benjamin J. Lear, Vera Krewald
Journal
Abstract
approach for quantifying the ET coordinate and demonstrate it for a series of dinitroradical anions. Using sampling methods at finite temperature combined with density functional theory calculations, we find that the electron transfer can be followed using the energy separation between potential energy surfaces and the extent of electron localization. The precise nuclear motion that leads to electron transfer is then obtained as a linear combination of normal modes. Once the coordinate is identified, we find that evolution along it results in a change in diabatic state and optical excitation energy, as predicted by the Marcus model. Thus, we conclude that a single dimension of the electron transfer described in Marcus-Hush theory can be described as a well-defined nuclear motion. Importantly, our approach allows the separation of the intrinsic electron transfer coordinate from other structural relaxations and environmental influences. Furthermore, the barrier separating the adiabatic minima was found to be sufficiently thin to enable heavy-atom tunneling in the ET process.
2022
27Al Solid-State Magic-Angle Spinning NMR Studies of Aluminum Powder Particle Surfaces Treated with a Methyltriethoxysilane Coupling Agent under Acidic Conditions
Authors
Lillian M. Mawby, Bellamarie Ludwig, Benjamin J. Lear
Journal
Abstract
We report on the reaction between methyltriethoxysilane (MTES) and micrometer-sized aluminum particles, facilitated by HCl. This reaction ultimately produces silane-coated aluminum particles. Using 27 Al magic-angle spinning solid-state nuclear magnetic resonance, we find that aluminum powder starts with a mixture of tetrahedrally, pentahedrally, and octahedrally coordinated aluminum, with the pentahedral species dominating. In the presence of HCl, however, the aluminum undergoes a restructuring, so that octahedrally coordinated aluminum is the dominant species. Using diffuse reflectance infrared spectroscopy to confirm the deposition of silane, we find that this restructuring of the aluminum in the presence of HCl is both a sufficient and necessary condition for the deposition of the silane.
Rapid Photothermal Synthesis of Polyurethane from Blocked Isocyanates
Authors
Sarah Phillips, Nathaniel C. Ginder, Benjamin J. Lear
Journal
Abstract
Though it has been shown that the photothermal heating by pulsed lasers can provide localized heat to cure polymers at an enhanced rate without bulk temperature changes, such an approach has been unsuccessful at driving chemical transformations that require large increases in bulk temperature─such as the curing of blocked isocyanates. We show that photothermal heating using 1 W of continuous wave laser power directed toward a mixture of 6 wt % carbon black in a blocked isocyanate is sufficient to reach temperatures near 142 °C. Additionally, using both infrared and nuclear magnetic resonance spectroscopies, we demonstrate that this heat is sufficient to drive the deblocking of a trimer of hexamethylene diisocyanate blocked by methyl ethyl ketoxime. We also show that 1 s of such heating produces the same degree of deblocking as 8 h in an oven held at 160 °C. Finally, we demonstrate that photothermal heating can also drive the formation of a urethane bond that is spectroscopically identical to that produced after oven heating at 160 °C for 1 h. This work shows that photothermal heating with carbon black and a CW laser can provide bulk heat necessary for high temperature reactions while maintaining the photothermally induced kinetic advantage of localized heat.
Surface Chemistry Controls the Density of States in Metallic Nanoparticles
Authors
Nicholas P. Litak, Lillian M. Mawby, Benjamin J. Lear
Journal
Abstract
Ligand-stabilized colloidal metallic nanoparticles are prized in science and technology for their electronic properties and tunable surface chemistry. However, little is known about the interplay between these two aspects of the particles. A particularly glaring absence concerns the density of electronic states, which is fundamental in explaining the electronic properties of solid-state materials. In part, this absence owes to the difficulty in the experimental determination of the parameter for colloidal systems. Herein, we demonstrate the density of electronic states for metallic colloidal particles can be determined from their magnetic susceptibility, measured using nuclear magnetic resonance spectroscopy. For this study, we use small alkanethiolate protected gold nanoparticles and demonstrate that changes in the surface chemistry, as subtle as changes in alkane chain length, can result inasmuch as a 3-fold change in the density of states at the Fermi level for these particles. This suggests that surface chemistry can be a powerful tool for controlling the electronic behavior of the materials to which they are attached, and suggests a paradigm that could be applied to other metallic systems, such as other metal nanoparticles, doped semiconductor systems, and even 2D metals. For all of these metallic systems, the Evans method can serve as a simple means to probe the density of states near the Fermi level.
2021
Asymmetries in the Electronic Properties of Spheroidal Metallic Nanoparticles, Revealed by Conduction Electron Spin Resonance and Surface Plasmon Resonance
Authors
Santina S. Cruz, Vadim Tanygin, Benjamin J. Lear
Journal
Abstract
Using electron spin resonance spectroscopy, we demonstrate that the morphological asymmetries present in small spheroidal metallic nanoparticles give rise to asymmetries in the behavior of electrons held in states near the metal’s Fermi energy. We find that the effects of morphological asymmetries for these spheroidal systems are more important than the effects of size distributions when explaining the asymmetry in electronic behavior. This is found to be true for all the particles examined, which were made from Cu, Ag, Pd, Ir, Pt, and Au, bearing dodecanethiolate ligands. In the case of the Ag particles, we also demonstrate that the same model used to account for morphological effects in the electron spin resonance spectra can be used to account for small asymmetries present in the plasmon spectrum. This result demonstrates that the electronic properties of even small particles are tunable via morphological changes.
2020
Dependence of Core Electronics of Gold Nanoparticles on Ligand, Solvent, and Sample Preparation
Authors
Jonathan W. Fagan, Benjamin J. Lear
Journal
The Journal of Physical Chemistry C
Abstract
Electron spin resonance (ESR) spectroscopy is used to probe the electronic properties of the metallic cores of small (∼2 nm) gold nanoparticles protected by 1-hexanethiol and 1-dodecanethiol, suspended in either n -hexane or THF. Analysis of the ESR spectra allows extraction of the principal components of the g -tensor for the metallic electrons in the core. We find that the values associated with the g -tensor are sensitive to the identity of both the ligand and the solvent. We also find that the handling of the samples can affect the measured g -values, with common manipulations such as freezing and thawing the sample or precipitating and resuspending the nanoparticles increasing the measurement-to-measurement distributions of the measured g -values. The degree of these perturbations also depends on the identity of the ligand and solvent. These results stress the importance of the design and handling of colloidal systems when seeking to use their electronic behaviors.
Preparation and Oxygen Sensitivity of a Range of Noble-Metal Nanoparticles (Ir, Pt, and Au) Protected by a Series of Chalcogen–Dodecane Ligands (S, Se, and Te)
Authors
Vadim Tanygin, Benjamin J. Lear
Journal
Abstract
Iridium, platinum, and gold nanoparticles, protected with sulfur, selenium, or tellurium dodecane ligands, were synthesized under ambient laboratory conditions. These nine nanoparticles were characterized by thermogravimetric analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). XPS was used to determine the degree of oxidation present at the metal–chalcogen interface at the time of synthesis and after 1 week of aging under ambient laboratory conditions. Upon synthesis, interfaces involving sulfur atoms were found to have no degree of oxidation and to retain this lack of oxidation over the course of 1 week. In contrast, all interfaces involving tellurium were found to have some degree of oxidation (28, 77, and 76% for Ir, Pt, and Au particles, respectively) at the time of synthesis, and this degree of oxidation increased over the course of 1 week to 38, 83, and 92% for Ir, Pt, and Au, respectively. For interfaces involving selenium, all interfaces initially lacked any oxidation and the iridium and platinum interface was found to be stable over 1 week. On the other hand, the gold interface oxidized over time, reaching 60% oxidation after 1 week under ambient laboratory conditions. Thus, our work shows that IrS, IrSe, PtS, PtSe, and AuS provide metal–ligand interfaces that are stable, with respect to oxidation under ambient conditions.
2019
Controlled Rapid Formation of Polyurethane at 700 K: Thermodynamic and Kinetic Consequences of Extreme Photothermal Heating
Authors
Emma N. Van Burns, Benjamin J. Lear
Journal
The Journal of Physical Chemistry C
Abstract
The photothermal effect of nanoparticles has proven to be an effective means of substantially increasing the rate of chemical transformations, by factors of up to 10 9 . For thermally activated processes, such a large increase in rate implies a corresponding increase in temperature large enough that it would be expected to affect the steady-state concentrations of products and reactants. We test this hypothesis by following the exothermic reaction between hexamethylene diisocyanate and poly-bis(triethylol) heptanedioate to produce a cross-linked polyurethane under both ambient and photothermal conditions. We demonstrate that the photothermal effect increases the reaction rate by a factor of 7.4 × 10 6 and decreases the effective equilibrium constant by a factor of at least 3 × 10 4 . These two changes provide kinetic and thermodynamic temperature estimates of 732 ± 21 and 683 ± 28 K, respectively. Remarkably, though both estimates of temperature are extreme, the chemical species produced under photothermal heating are the same as produced under ambient conditions.
Nanoscale heat for organic transformations: a photothermally driven retro Diels–Alder reaction
Authors
Andrea L. Widstrom, Benjamin J. Lear
Journal
Abstract
No abstract available in repository.
Photothermal Control over the Mechanical and Physical Properties of Polydimethylsiloxane
Authors
R. Joseph Fortenbaugh, Sabrina Carrozzi, Benjamin J. Lear
Journal
Abstract
Though it is known that the photothermal effect of nanoparticles can be used to greatly increase the rate of polymer curing, at present, little is known about how the parameters of photothermal curing affect the desirable chemical and physical properties of the cured polymer. We report the swelling, gel fraction, and Young’s modulus for the thermoset polydimethylsiloxane cured under a variety of photothermal and traditional conditions. We find that all of these properties can be tuned via the intensity of light and propose that the crosslink density within the thermoset decreases with increasing intensity of light during curing.
Structural and solvent control over activation parameters for a pair of retro Diels-Alder reactions
Authors
Andrea L. Widstrom, Benjamin J. Lear
Journal
Abstract
We report the temperature dependent NMR of two Diels-Alder adducts of furan: one formed with maleic anhydride and the other with N-methylmaleimide. These adducts are the products of so-called ‘click’ reactions, widely valued for providing simple, reliable, and robust reactivity. Under our experimental conditions, these adducts undergo a retro Diels-Alder reaction and we use our temperature dependent NMR to determine the rates of these reactions at multiple temperatures-ultimately providing estimates of the activation parameters for the reversion. We repeat these measurements in three solvents. We find that, in all solvents, the barrier to reversion is larger for the adduct formed with N-methylmaleimide. The barrier to reversion for this adduct is relatively insensitive to changes in solvent while the adduct formed with maleic anhydride responds more strongly to changes in solvent polarity. The differences in reaction barrier and solvent dependence arises because the adduct formed with N-methylmalemide is more stable-leading to a larger barrier to reversion-while the adduct formed with maleic anhydride experiences a larger change in dipole during the reaction-leading to a larger solvent dependence.
2018
Photothermal Effectiveness of Magnetite Nanoparticles: Dependence upon Particle Size Probed by Experiment and Simulation
Authors
Robert J. Johnson, Jonathan D. Schultz, Benjamin J. Lear
Journal
Abstract
The photothermal effect of nanoparticles has proven efficient for driving diverse physical and chemical processes; however, we know of no study addressing the dependence of efficacy on nanoparticle size. Herein, we report on the photothermal effect of three different sizes (5.5 nm, 10 nm and 15 nm in diameter) of magnetite nanoparticles (MNP) driving the decomposition of poly(propylene carbonate) (PPC). We find that the chemical effectiveness of the photothermal effect is positively correlated with particle volume. Numerical simulations of the photothermal heating of PPC supports this observation, showing that larger particles are able to heat larger volumes of PPC for longer periods of time. The increased heating duration is likely due to increased heat capacity, which is why the volume of the particle functions as a ready guide for the photothermal efficacy.
2017
On-demand curing of polydimethylsiloxane (PDMS) using the photothermal effect of gold nanoparticles
Authors
R. Joseph Fortenbaugh, Benjamin J. Lear
Journal
Abstract
The photothermal effect of gold nanoparticles (AuNPs) produces extremely localized heat that can be harnessed to drive large scale chemical reactions by simultaneously generating many individual reactions on the nanoscale. We use the photothermal effect to enhance the curing rate of polydimethylsiloxane (PDMS) by a factor of 4.9 × 109. Photothermal curing occurs via crosslinking reactions between vinyl and Si–H groups of the pre-polymer, and the course of the reaction was followed by monitoring the disappearance of infrared bands associated with these functional groups. Using mass spectroscopy, we verify that the major polymer m/z peaks are identical for both traditionally and photothermally cured polymers, indicating that the photothermal effect of AuNPs is an effective way in which to supply on-demand curing of PDMS.
2016
Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition
Authors
Anthony Cirri, Alexey Silakov, Lasse Jensen, Benjamin J. Lear
Journal
Journal of the American Chemical Society
Abstract
Alkanethiolate protected gold nanoparticles are one of the most widely used systems in modern science and technology, where the emergent electronic properties of the gold core are valued for use in applications such as plasmonic solar cells, photocatalysis, and photothermal heating. Though choice in alkane chain length is not often discussed as a way in which to control the electronic properties of these nanoparticles, we show that the chain length of the alkyl tail exerts clear control over the electronic properties of the gold core, as determined by conduction electron spin resonance spectroscopy. The control exerted by chain length is reported on by changes to the g-factor of the metallic electrons, which we can relate to the average surface potential on the gold core. We propose that the surface potential is modulated by direct charge donation from the ligand to the metal, resulting from the formation of a chemical bond. Furthermore, the degree of charge transfer is controlled by differences between the dielectric constant of the medium and the ligand shell. Together, these observations are used to construct a simple electrostatic model that provides a framework for understanding how surface chemistry can be used to modulate the electronic properties of gold nanoparticles.
Effect of Protonation upon Electronic Coupling in the Mixed Valence and Mixed Protonated Complex, [Ni(2,3-pyrazinedithiol)2]
Authors
Steven R. Kennedy, Puja Goyal, Morgan N. Kozar, Hemant P. Yennawar, Sharon Hammes‐Schiffer, Benjamin J. Lear
Journal
Abstract
We demonstrate that protonation of a mixed valence molecule, generating a mixed valence mixed protonated (MVMP) state, results in a severe reduction in the electronic coupling intimately connected with electron transfer kinetics. This phenomenon is illustrated by synthesizing a mixed valence molecule, [Ni(2,3-pyrazinedithiol)2], that can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.
Probing ligand-induced modulation of metallic states in small gold nanoparticles using conduction electron spin resonance
Authors
Anthony Cirri, Alexey Silakov, Lasse Jensen, Benjamin J. Lear
Journal
Physical Chemistry Chemical Physics
Abstract
Thiolate-protected gold nanoparticles have a rich history as model systems for understanding the physical and chemical properties of metallic nanoscale materials that, in turn, form the basis for applications in areas such as molecular electronics, photocatalytic systems, and plasmonic solar cells. It is well known that the electronic properties of gold nanoparticles can be tuned by modifying the geometry, size and dielectric surrounding of the particle. However, much less is known of how modifications to the surface chemistry modulates the electronic properties of gold nanoparticles. In part, this stems from the fact that there are few good tools for measuring the electronic properties with the sensitivity required for following the response to subtle changes in surface chemistry. In this work, we demonstrate conduction spin electron resonance (CESR) to be a sensitive and selective probe to determine how changes in surface chemistry of gold nanoparticles affect the metallic states near the Fermi energy. Using a series of para-substituted aromatic thiolate ligands, we find that the g-factor, as measured using CESR, correlates well with experimental and computational parameters often used to understand ligand effects in classical inorganic complexes. This suggests classical inorganic reasoning can function as a framework for understanding how to control the electronic properties of gold nanoparticles using their surface chemistry.
Steady-State Spectroscopic Analysis of Proton-Dependent Electron Transfer on Pyrazine-Appended Metal Dithiolenes [Ni(pdt)2], [Pd(pdt)2], and [Pt(pdt)2] (pdt = 2,3-Pyrazinedithiol)
Authors
Steven R. Kennedy, Morgan N. Kozar, Hemant P. Yennawar, Benjamin J. Lear
Journal
Abstract
We report the structural, electronic, and acid/base properties of a series of ML2 metal dithiolene complexes, where M = Ni, Pd, Pt and L = 2,3-pyrazinedithiol. These complexes are non-innocent and possess strong electronic coupling between ligands across the metal center. The electronic coupling can be readily quantified in the monoanionic mixed valence state using Marcus-Hush theory. Analysis of the intervalence charge transfer (IVCT) band reveals that that electronic coupling in the mixed valence state is 5800, 4500, and 5700 cm(-1) for the Ni, Pd, and Pt complexes, respectively. We then focus on their response to acid titration in the mixed valence state, which generates the asymmetrically protonated mixed valence mixed protonated state. For all three complexes, protonation results in severe attenuation of the electronic coupling, as measured by the IVCT band. We find nearly 5-fold decreases in electronic coupling for both Ni and Pt, while, for the Pd complex, the electronic coupling is reduced to the point that the IVCT band is no longer observable. We ascribe the reduction in electronic coupling to charge pinning induced by asymmetric protonation. The more severe reduction in coupling for the Pd complex is a result of greater energetic mismatch between ligand and metal orbitals, reflected in the smaller electronic coupling for the pure mixed valence state. This work demonstrates that the bridging metal center can be used to tune the electronic coupling in both the mixed valence and mixed valence mixed protonated states, as well as the magnitude of change of the electronic coupling that accompanies changes in protonation state.
2015
Comparing the Energetic and Dynamic Contributions of Solvent to Very Low Barrier Isomerization Using Dynamic Steady-State Vibrational Spectroscopy
Authors
Andrea N. Giordano, Benjamin J. Lear
Journal
The Journal of Physical Chemistry A
Abstract
We report the solvent-dependent dynamics of carbonyl site exchange for Fe(CO)3(η(4)-norbornadiene) (FeNBD) in a series of linear and nonlinear alkanes. The barrier to exchange is very low (∼1.5 kcal/mol), and the resulting carbonyl dynamics are rapid enough to lead to a change in the vibrational spectra, which we use to extract the ultrafast rates of exchange from linear Raman spectra of FeNBD. The dynamics of the carbonyl exchange has a weak dependence upon the solvent, and we analyze this dependence in terms of energetic (reaction field) and dynamic (Kramers theory) models of solvent effects. We find that both models can reproduce the observed solvent dependence but that the dynamic model provides a more physically satisfying picture for the solvent effects than does the energetic model. Finally, we find that cyclohexane is more strongly coupled to the dynamics of FeNBD than are the noncyclic alkanes.
Billion-fold rate enhancement of urethane polymerization via the photothermal effect of plasmonic gold nanoparticles
Authors
Kaitlin M. Haas, Benjamin J. Lear
Journal
Abstract
of 532 nm light to a solution of isocyanate and alcohol with 0.08% w/v of 2 nm AuNPs results in the billion-fold enhancement of the rate of curing. This result is intriguing, as it demonstrates the ability of nanoscale heat to drive bulk transformations. In addition, the reaction is strongly exothermic and results in a relatively weak bond, both of which would preclude the use of bulk-scale heat, highlighting the unique utility of the photothermal effect for driving thermal reactions.
Isolation and Chemical Transformations Involving a Reactive Intermediate of MOF-5
Authors
Juyeong Kim, Michelle Dolgos, Benjamin J. Lear
Journal
Abstract
We report the isolation of a nonporous plate-like intermediate species (MOF- i ) obtained during the synthesis of MOF-5 and the testing of this intermediate’s reactivity toward three metal ions (Zn II, Cu II, and Mn II ) in N, N -dimethylformamide at 120 °C. We obtained interpenetrated MOF-5 crystals from the reaction between MOF- i and Zn(NO 3 ) 2 ·6H 2 O, accompanied by a change in morphology from a plate to a cube. Reaction with CuCl 2 ·2H 2 O did not disrupt the plate-like morphology of MOF- i, but it did result in the replacement of Zn II by Cu II and formation of a novel porous copper MOF. MOF- i showed no reactivity toward MnCl 2 . Our results demonstrate that MOF- i imparts a selective reactivity that is different from the individual metal ions employed in conventional synthesis of MOFs and suggests that reactive intermediates may be useful in extending the diversity of metal–organic frameworks.
Structural, Electronic, and Magnetic Characterization of a Dinuclear Zinc Complex Containing TCNQ– and a μ-[TCNQ–TCNQ]2– Ligand
Authors
Juyeong Kim, Alexey Silakov, Hemant P. Yennawar, Benjamin J. Lear
Journal
Abstract
A dinuclear zinc complex containing both a σ-dimerized 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligand (TCNQ-TCNQ) and TCNQ(-) was synthesized for the first time. This is the first instance of a single molecular complex with a bridging TCNQ-TCNQ ligand. Each zinc center is coordinated with two 2,2’-bipyrimidines and one TCNQ(-), and the remaining coordination site is occupied by a TCNQ-TCNQ ligand, which bridges the two zinc centers. The complex facilitates π-stacking of TCNQ(-) ligands when crystallized, which gives rise to a near-IR charge-transfer transition and strong antiferromagnetic coupling.
Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles
Authors
Anthony Cirri, Alexey Silakov, Benjamin J. Lear
Journal
Angewandte Chemie International Edition
Abstract
The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chemistry of the AuNPs. Specifically, the conduction electron spin resonance (CESR) spectra of AuNPs of diameter 1.8-1.9 nm are sensitive to ligand exchange of hexanethiol for 4-bromothiophenol on the surface of the nanoparticle. Chemisorption of the aromatic ligand leads to a shift in the metallic electron’s g-factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Analysis by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chemical nature of the ligand controls the valence band structure of AuNPs.
Synthesis and characterization of the gold dithiolene monoanion, (Bu4N)[Au(pdt = 2,3-pyrazinedithiol)2]
Authors
Steven R. Kennedy, Morgan N. Kozar, Hemant P. Yennawar, Benjamin J. Lear
Journal
Abstract
No abstract available in repository.
2014
Concentration-Dependent Dynamics of Hydrogen Bonding between Acetonitrile and Methanol As Determined by 1D Vibrational Spectroscopy
Authors
Brian G. Alberding, Benjamin J. Lear
Journal
The Journal of Physical Chemistry A
Abstract
Solutions of acetonitrile (MeCN) in methanol (MeOH) at various concentrations have been investigated by variable temperature Raman spectroscopy. In the ν(CN) region of the spectrum, the variable temperature spectra at each concentration show two overlapping bands from hydrogen bound and free MeCN. These two species undergo dynamic exchange that gives rise to increasing coalescence of the two bands with increasing temperature. By simulation of the band shape, the rate of exchange was determined at each temperature. Arrhenius plots yielded values for the activation energy, Ea, and the natural log of the pre-exponential factor, ln[A/s(-1)], for the hydrogen bond formation/cleavage. Both of these dynamic parameters were found to depend on the relative amounts of MeCN and MeOH in the solutions. In particular, two different concentration regimes of dynamic hydrogen bonding were observed. First, at low MeCN concentration, the dynamics are largely independent of changes in MeCN concentration. Second, at higher MeCN concentration (above ∼0.2 MeCN mole fraction) the dynamics are strongly dependent on further increases of MeCN content. Over the range of MeCN mole fractions that we studied (0.03-0.5), the ln[A/s(-1)] changes from 32.5 ± 0.1 to 30.1 ± 0.2 and Ea changes from 3.73 ± 0.08 to 2.7 ± 0.1 kcal/mol. We suggest the observed changes in dynamics arise from changes in the local solvent microstructure that occur above a critical mole fraction of MeCN.
Electron-Transfer Reactions of Electronically Excited Zinc Tetraphenylporphyrin with Multinuclear Ruthenium Complexes
Authors
Jane Henderson, Starla D. Glover, Benjamin J. Lear, D. G. Walker, Jay R. Winkler, Harry B. Gray, Clifford P. Kubiak
Journal
The Journal of Physical Chemistry B
Abstract
Transient absorption decay rate constants (kobs) for reactions of electronically excited zinc tetraphenylporphyrin ((3)ZnTPP*) with triruthenium oxo-centered acetate-bridged clusters [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), or 4-dimethylaminopyridine (dmap) (3), were obtained from nanosecond flash-quench spectroscopic data (quenching constants, kq, for (3)ZnTPP*/1-3 are 3.0 × 10(9), 1.5 × 10 (9), and 1.1 × 10(9) M(-1) s(-1), respectively). Values of kq for reactions of (3)ZnTPP* with 1-3 and Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)2 [L = cpy (4), py (5), dmap (6)] monomeric analogues suggest that photoinduced electron transfer is the main pathway of excited-state decay; this mechanistic proposal is consistent with results from a photolysis control experiment, where growth of characteristic near-IR absorption bands attributable to reduced (mixed-valence) Ru3O-cluster products were observed.
Fe 3 O 4 nanoparticles as robust photothermal agents for driving high barrier reactions under ambient conditions
Authors
Robert J. Johnson, Kaitlin M. Haas, Benjamin J. Lear
Journal
Abstract
Magnetite nanoparticles (MNPs) show remarkable stability during extreme photothermal heating (≥770 K), displaying no change in size, crystallinity, or surfactants. The heat produced is also shown as chemically useful, driving the high-barrier thermal decomposition of polypropylene carbonate. This suggests MNPs are better photothermal agents (compared to gold nanoparticles), for photothermally driving high-barrier chemical transformations.
Quantitative Assessment of the Connection between Steric Hindrance and Electronic Coupling in 2,5-Bis(alkoxy)benzene-Based Mixed-Valence Dimers
Authors
Angela M. Bischof, Shaopeng Zhang, Tara Y. Meyer, Benjamin J. Lear
Journal
The Journal of Physical Chemistry C
Abstract
The effect of the bridging ligand on electronic delocalization was examined in a series p -bis(alkoxy)benzene dimers relevant to conducting polymers used for organic devices. Using spectroscopic methods, the degree of delocalization for an ethylene-bridged p -bis(alkoxy)benzene dimer was determined and compared to the electronic coupling for directly coupled and phenylene-bridged p -bis(alkoxy)benzene dimers reported previously. Despite a significant increase in distance (53%) between the redox-active sites, the ethylene-bridged compound exhibited a higher electronic coupling than either of the others previously reported. The increased coupling can be attributed to the lower rotational barrier to planarization for the ethylene-bridged dimer. This result highlights the need to minimize both sterics and distance between redox active sites in molecular systems designed for promoting electron mobility and provides quantitative evidence that an optimal balance between these parameters can be achieved.
2013
Degradation of polypropylene carbonate through plasmonic heating
Authors
Kaitlin M. Haas, Benjamin J. Lear
Journal
Abstract
We report the thermal degradation of a solid film of polypropylene carbonate, driven by the photothermal effect of gold nanoparticles. We provide characterization of the products of this chemical reaction and use the known activation barrier for this chemical reaction to discuss the temperatures obtained in the film. In addition, we report the efficiency of the reaction as a function of nanoparticle concentration and find nanoparticles to be significantly more effective than an organic dye at driving this reaction.
Silica Nanoparticles for Enhanced Carrier Transport in Polymer-Based Short Channel Transistors
Authors
Ali Veysel Tunç, Andrea N. Giordano, Bernhard Ecker, Enrico Da Como, Benjamin J. Lear, Elizabeth von Hauff
Journal
The Journal of Physical Chemistry C
Abstract
Electronic disorder in conducting polymers represents a fundamental limit for developing high performance polymer-based transistors (TFTs). Nanoscaled manipulation of polymer morphology with electrically inert nanostructures is an interesting and flexible strategy to enhance ordering in polymer films. We show that blending poly[2-methoxy,5-(3′,7′-dimethyloctyloxy)]-1,4-phenylene vinylene (MDMO-PPV) with silica nanoparticles leads to an increase in TFT performance, including an increase in hole mobility by over 10 times. By means of Raman spectroscopy we correlate variations in polymer structure induced by the silica to improvements in the electrical properties. We compare these results to MDMO-PPV blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Although PCBM leads to similar results in longer channel TFTs, no improvement in short channel behavior is observed. These results demonstrate a simple way to realize short channel polymer TFTs with enhanced performance.
Direct Test of the Equivalency of Dynamic IR and Dynamic Raman Spectroscopies As Techniques for Observing Ultrafast Molecular Dynamics
Authors
Andrea N. Giordano, Seth M. Morton, Lasse Jensen, Benjamin J. Lear
Journal
The Journal of Physical Chemistry A
Abstract
We report the temperature-dependent infrared (IR) and Raman spectra of Fe(CO)3(η(4)-norbornadiene). This molecule undergoes carbonyl ligand site exchange on the vibrational time scale, and the effect of this exchange is observable as coalescence of the carbonyl bands in both the IR and Raman spectra. We outline a theory that we used to account for these effects and report simulations of the experimental spectra. We used these simulations to extract the carbonyl ligand exchange rates at various temperatures from the IR and Raman data. This data was used to calculate the activation energy for carbonyl exchange, yielding activation energies of 1.2 ± 0.2 and 1.4 ± 0.1 kcal/mol from the IR and Raman data, respectively. These activation energies are statistically identical and are consistent with previously reported values. This constitutes the first direct comparison between dynamic IR and Raman spectroscopies, and we find them to give identical results.
Solvent versus Temperature Control over the Infrared Band Shape and Position in Fe(CO)3(η4-Ligand) Complexes
Authors
Andrea N. Giordano, Benjamin J. Lear
Journal
The Journal of Physical Chemistry A
Abstract
The solute-solvent interactions between Fe(CO)3(η(4)-cyclooctatetraene) (FeCOT) and 27 solvents were examined by infrared (IR) spectroscopy. The observed change in band shape and position of the carbonyl bands as a function of solvent was found to be very similar to that previously observed in temperature-dependent IR experiments of Fe(CO)3(η(4)-norborndiene) (FeNBD). While for FeNBD the change in band shape results from dynamic exchange of carbonyl ligands, temperature-dependent IR experiments in ethyl acetate show that the observed changes are not a result of carbonyl ligand site exchange for FeCOT. We therefore concluded that the solvent dependence of the IR spectra must be a consequence of a static solute-solvent interaction. We find that the linear solvation energy model (J. Am. Chem. Soc. 1977, 99, 6027-6038; Chem. Soc. Rev. 1993, 22, 409-416) provides a satisfactory account for the spectral changes due to the solvent. From this model, we are able to conclude that the solute-solvent interactions of this system are influenced by the solvent’s polarizability and hydrogen bonding acidity. We also observed interdependence between the change in fwhm and band positions for all three carbonyl bands, which brings us to the conclusion that the observed changes in the IR carbonyl band shape of FeCOT are a consequence of the solute-solvent interactions, rather than any solvent friction effects. This implies that care must be taken to separate the effects of chemical dynamics and solvatochromism when examining IR spectra of molecules suspected of exhibiting dynamically broadened vibrational spectra.
Synthesis and characterization of ruthenium polypyridyl complexes with hydroxypyridine derivatives: effect of protonation and ethylation at the pyridyl nitrogen
Authors
Juyeong Kim, Hemant P. Yennawar, Benjamin J. Lear
Journal
Abstract
A new series of ruthenium polypyridyl complexes with a hydroxypyridine ligand were prepared, and their properties were investigated spectroscopically and electrochemically. Particular focus is paid to the effects of protonation-deprotonation and ethylation of the hydroxypyridine ligand, which affects the NMR, electronic spectroscopy, and electrochemistry of the complex. The changes to the UV-vis spectrum were used to determine a pka of 10.5 for the hydroxypyridine nitrogen. In the NMR, protonation of the hydroxypyridine ligand of the complex causes changes in the chemical shifts of the protons on both the hydroxypyridine and bipyridine rings, indicating some degree of electronic communication between these ligands. In addition, it is found that deprotonation of the hydroxypyridine ligand strongly affects the redox potential of the ruthenium metal center, shifting it more negative by 0.4 V. While the electrochemistry of the protonated complex contains irreversible electrochemical events, both deprotonation and subsequent ethylation of the hydroxypyridine ligand result in reversible electrochemistry for all events within the solvent window. For the ethylated complex, we search for a ligand to ligand charge transfer band, corresponding to electron transfer between bipyridine ligands in the mixed valence state. Despite the potential for electronic coupling between ligands through the metal center, we were unable to find any spectroscopic evidence of such electronic coupling.
2011
Extent of M2δ to Ligand π-Conjugation in Neutral and Mixed Valence States of Bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) Dimetal Complexes (MM), Where M = Mo or W, and Their Adducts with Tris(pentafluorophenyl)boron
Authors
Philip C. Bunting, Malcolm H. Chisholm, Judith C. Gallucci, Benjamin J. Lear
Journal
Journal of the American Chemical Society
Abstract
The reaction between W(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W(2)(T(i)PB)(2)(nic)(2), 2, and T(i)PBH. Compound 2 is related to the previously reported molybdenum analog, Mo(2)(T(i)PB)(2)(nic)(2), 1. Compounds 1 and 2 react with 2 equiv of B(C(6)F(5))(3) in THF to form the adducts M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2), 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2)·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M(2) δ to π* MLCT transitions, and upon complexation with B(C(5)F(5))(3) to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE(1/2) = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE(1/2) values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M(2) δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C(5)Me(5))(2)Co leads to the anions 1B(-) and 2B(-), which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV-vis-NIR absorption. The EPR spectra of 1B(-) and 2B(-) are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B(-)) and 4500 cm(-1) (2B(-)), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations (Bu(t)CO(2))(3)M(2)-μ(2)-(O(2)C-CO(2))(+), to which they are remarkably similar, as well as with other organic-based mixed valence systems.
M2δ to ligand π-conjugation: testbeds for current theories of mixed valence in ground and photoexcited states of molecular systems
Authors
Malcolm H. Chisholm, Benjamin J. Lear
Journal
Abstract
Redox active quadruply bonded units, M(2), can be combined so that they either (i) are bridged by an organic linker or (ii) function as a bridge between two identical organic ligands. When two M(2) units are linked together by an organic group that affords M(2)δ-bridge π-conjugation the electronic structure of each M(2) unit is perturbed by the other in the ground state, the photoexcited states, and the mixed valence oxidized form. Similarly when a M(2) center links two organic π systems represented by L, the two organic units are coupled by Lπ*-M(2)δ-Lπ* interactions in their ground state, their photoexcited states, and the mixed valence reduced state. The photoexcited states of the neutral complexes (both case i and ii) provide examples of excited state mixed valence. In case (i), the positive charge may be localized on one dinuclear center or may be delocalized over both M(2) units. Similarly in (ii), the electron may be localized on one ligand or delocalized over both. In this tutorial review, spectroscopic studies (UV-vis-NIR absorption, steady state emission, EPR, and time resolved infrared) of these mixed valence systems employing carboxylate tethers are described and the data are discussed in terms of contemporary theories of mixed valence ions.
Synthesis and characterization of trans-M2(TiPB)2(O2C-CHCH-2-C4H3S)2 (M = Mo or W) and comments on the metal-to-ligand charge transfer bands in MM quadruply bonded complexes of the type trans-M2(TiPB)2L2, where TiPB = 2,4,6-triisopropylbenzoate and L = π-accepting carboxylate ligand
Authors
Brian G. Alberding, Malcolm H. Chisholm, Benjamin J. Lear, V. Naseri, Carly R. Reed
Journal
Abstract
The preparation and characterization of the compounds trans-M(2)(T(i)PB)(2)(O(2)C-CH=CH-2-C(4)H(3)S)(2) where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate are reported. The optical spectra of the new compounds are compared with those of related trans-M(2)(T(i)PB)(2)L(2) compounds where L = O(2)C-C(6)H(4)-4-CN, O(2)C-α,α’-terthienyl (TTh), and O(2)C-4-C(6)H(4)N-B(C(6)F(5))(3), that show strong metal-to-ligand charge transfer bands because of M(2)δ to Lπ conjugation, and are notably temperature dependant due to the various conformations of the two trans-L groups. Upon cooling the spectral features sharpen as the planar geometry that optimizes M(2)δ-Lπ conjugation is favored. As the electronic coupling of the two trans-Lπ systems increases the (0,0) electronic transition gains intensity indicating a greater nesting of the ground state (S(0)) and excited state (S(1)) potential energy surfaces. These features are discussed in terms of the related electronic coupling of [M(2)]-[M(2)] complexes.
2010
Electroabsorption of Dimers Containing MM (M = Mo, W) Quadruply Bonded Units: Insights into the Electronic Structure of Neutral Coupled Redox Centers and Their Relationship with Mixed Valence Ions
Authors
Malcolm H. Chisholm, Benjamin J. Lear, Alberto Moscatelli, Linda A. Peteanu
Journal
Abstract
The electroabsorption spectra for the metal-to-ligand charge transfer transition in complexes containing oxalate and terephthalate bridged MM quadruply bonded units, (MM)(pivalate)(3)-mu(2)-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (|Deltamu|) and the change in polarizability (Deltaalpha) that accompany this electronic transition are found to be small and not to follow the behavior expected on the basis of the two-state model. In addition, the trend in the value of Deltaalpha for the neutral states is mirrored by the trend in the degree of electronic coupling (H(AB)) for the strongly coupled mixed valence states formed by the same complexes in their singly oxidized states.
USING ELECTROABSORPTION SPECTROSCOPY TO GAIN INSIGHT INTO THE GROUND- AND EXCITED-STATE MIXED VALENCE PROPERTIES OF A SERIES OF DIMERS FORMED FROM METAL-METAL QUADRUPLY BONDED UNITS
Authors
Benjamin J. Lear, Phil C. Bunting, Malcolm H. Chisholm
Journal
Abstract
No abstract available in repository.
2009
EVOLUTION OF THE MLCT BAND FOLLOWING CHANGES IN OXIDATION STATE FOR HIGHLY COUPLED MIXED VALENCE COMPLEXES
Authors
Benjamin J. Lear, Malcolm H. Chisholm
Journal
The Knowledge Bank (The Ohio State University)
Abstract
Author Institution: The Ohio State University, Department of Chemistry, Columbus, Ohio 43210
Inter- or intramolecular electron transfer between triruthenium clusters: we’ll cross that bridge when we come to it
Authors
Starla D. Glover, John C. Goeltz, Benjamin J. Lear, Clifford P. Kubiak
Journal
Coordination Chemistry Reviews
Abstract
No abstract available in repository.
Mixed Valency at the Nearly Delocalized Limit: Fundamentals and Forecast (Eur. J. Inorg. Chem. 5/2009)
Authors
Starla D. Glover, John C. Goeltz, Benjamin J. Lear, Clifford P. Kubiak
Journal
European Journal of Inorganic Chemistry
Abstract
Abstract The cover picture shows a Class II/III mixed‐valence system, {[Ru3O(OAc)6(CO)(pyridine)]2–pyrazine}–1, which undergoes picosecond intramolecular electron transfer (right). In nearly delocalized mixed‐valence complexes, rates of ET depend highly on solvent dynamics. Freezing of the solution causes a localized‐to‐delocalized transition, and rates of ET increase. We find that, for Class II/III mixed‐valence complexes, solvent dynamical parameters control rates of ET and tend to localize otherwise delocalized electronic states. Details are presented in the Microreview by C. P. Kubiak et al. on p. 585 ff.
Oxalate Bridged MM (MM = Mo2, MoW, and W2) Quadruply Bonded Complexes As Test Beds for Current Mixed Valence Theory: Looking beyond the Intervalence Charge Transfer Transition
Authors
Benjamin J. Lear, Malcolm H. Chisholm
Journal
Abstract
The spectroscopic features of a series of oxalate bridged complexes ((t)BuCO(2))(3)MM-mu(2)-O(2)CCO(2) (where MM = Mo(2), MoW, and W(2)) in their neutral and singly oxidized (mixed valence) states are examined as a function of temperature and solvent. A large degree of electronic coupling between the two MM centers is evident, principally involving the MM delta orbitals mediated by the oxalate bridge pi* orbital. In the oxidized states these mixed valence ions show solvent independent intervalence charge transfer (alternatively termed charge resonance) bands, consistent with assignment to Class III (or electronically delocalized) within the Robin-Day classification scheme. In both the neutral and oxidized states these complexes also show an intense metal-to-ligand charge-transfer (MLCT) transition, involving the lowest unoccupied molecular orbital (LUMO) of the bridge. The solvent and temperature dependence of this transition is also reported along with an inspection and simulation of the vibronic features, which are notably altered when switching between the neutral and the mixed valence states as well as with variation of the nature of the MM unit. Collectively, these observations allow us to comment on the validity and limitations of current theories dealing with mixed valence ions that have hitherto ignored the information that can be gained from MLCT transitions.
2008
Mixed Valency at the Nearly Delocalized Limit: Fundamentals and Forecast
Authors
Starla D. Glover, John C. Goeltz, Benjamin J. Lear, Clifford P. Kubiak
Journal
European Journal of Inorganic Chemistry
Abstract
Abstract Mixed valency is important in many areas of chemistry, from synthetic to biological systems, from the simplest systems to the most highly complex. The purpose of our research is to understand the principles of mixed valency in localized, nearly delocalized, and delocalized systems, and to apply our knowledge to the rational design of molecular devices. This microreview discusses our group’s research over the last ten years. It reviews methods of estimating picosecond electron‐transfer lifetimes from infrared spectral lineshapes, studies of the localized‐to‐delocalized transition and the effects of frozen solvents, charge gating by non‐covalent interactions, electron‐transfer‐gated electron transfer, and the distribution of charge within a molecule probed and controlled by an applied electric field. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
2007
Electron transfer at the class II/III borderline of mixed valency: dependence of rates on solvent dynamics and observation of a localized-to-delocalized transition in freezing solvents
Authors
Starla D. Glover, Benjamin J. Lear, J. Catherine Salsman, Casey H. Londergan, Clifford P. Kubiak
Journal
Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences
Abstract
The dependence of the rates of intramolecular electron transfer (ET) of mixed-valence complexes of the type {[Ru3O(OAc)6(CO)(L)]2-BL}-1, where L is the pyridyl ligand and BL is the pyrazine on solvent type and temperature is described. Complexes were reduced chemically to obtain the mixed-valence anions in acetonitrile (CH3CN) and methylene chloride (CH2Cl2). Rate constants for intramolecular ET were estimated by simulating the observed degree of nu(CO) infrared (IR) bandshape coalescence in the mixed-valence state. In the strongly coupled mixed-valence states of these complexes, the electronic coupling, HAB, approaches lambda/2, where lambda is the total reorganization energy. The activation energy is thus nearly zero, and rate constants are in the ‘ultrafast’ regime where they depend on the pre-exponential terms within the frequency factor, nuN. The frequency factor contains both external (solvent dynamics) and internal (molecular vibrations) contributions. In general, external solvent motions are slower than internal vibrations, and therefore control ET rates in fluid solution. A profound increase in the degree of nu(CO) IR bandshape coalescence is observed as the temperature approaches the freezing points of the solvents methylene chloride (f.p. -92 degrees C) and acetonitrile (f.p. -44 degrees C). Decoupling the slower solvent motions involved in the frequency factor nuN for ET by freezing the solvent causes a transition from solvent dynamics to internal vibration-limited rates. The solvent phase transition causes a localized-to-delocalized transition in the mixed-valence ions that accelerates the rate of ET.
Origins of Cooperative Noncovalent Host−Guest Chemistry in Mixed Valence Complexes
Authors
Benjamin J. Lear, Clifford P. Kubiak
Journal
The Journal of Physical Chemistry B
Abstract
The electronic effects resulting from noncovalent host-guest interactions between calix[6]arene and a ruthenium dimer, [Ru3O(OAc)6(CO)(ppy)]2-mu-pz (ppy=4-phenyl pyridine, pz=pyrazine), are presented. The noncovalent interaction is between the calix[6]arene and the ppy ligands of the dimer. The dimer can bind 2 equiv of calix[6]arene. The complex [Ru3O(OAc)6(CO)(ppy)]2-mu-pz forms a highly stable mixed valence ion with strong electronic coupling between the two Ru3 clusters. The strength of the electronic interaction is found to be moderated by calix[6]arene binding. Addition of calix[6]arene to the mixed valence ion causes the electronic coupling to decrease. The binding of calix[6]arene is found to be cooperative. The origins of cooperative binding are developed in terms of the potential energy surfaces associated with the symmetric and asymmetric mixed valence ion. In particular, it is found that symmetry breaking (through the binding of a single calix[6]arene) destabilizes the mixed valence state. Restoration of symmetry (through the binding of a second calix[6]arene) increases the stability of the mixed valence ion and provides an additional driving force for the binding of the second calix[6]arene.
Solvent Dynamical Control of Ultrafast Ground State Electron Transfer: Implications for Class II−III Mixed Valency
Authors
Benjamin J. Lear, Starla D. Glover, J. Catherine Salsman, Casey H. Londergan, Clifford P. Kubiak
Journal
Journal of the American Chemical Society
Abstract
We relate the solvent and temperature dependence of the rates of intramolecular electron transfer (ET) of mixed valence complexes of the type {[Ru3O(OAc)6(CO)(L)]2-BL}-1, where L = pyridyl ligand and BL = pyrazine. Complexes were reduced chemically or electrochemically to obtain the mixed valence anions in seven solvents: acetonitrile, methylene chloride, dimethylformamide, tetrahydrofuran, dimethylsulfoxide, chloroform, and hexamethylphosphoramide. Rate constants for intramolecular ET were estimated by simulating the observed degree of nu(CO) IR band shape coalescence in the mixed valence state. Correlations between rate constants for ET and solvent properties including static dielectric constant, optical dielectric constant, the quantity 1/epsilonop - 1/epsilonS, microscopic solvent polarity, viscosity, cardinal rotational moments of inertia, and solvent relaxation times were examined. In the temperature study, the complexes displayed a sharp increase in the ket as the freezing points of the solvents methylene chloride and acetonitrile were approached. The solvent phase transition causes a localized-to-delocalized transition in the mixed valence ions and an acceleration in the rate of ET. This is explained in terms of decoupling the slower solvent motions involved in the frequency factor nuN which increases the value of nuN. The observed solvent and temperature dependence of the ket for these complexes is used in order to formulate a new definition for Robin-Day class II-III mixed valence compounds. Specifically, it is proposed that class II-III compounds are those for which thermodynamic properties of the solvent exert no control over ket, but the dynamic properties of the solvent still influence ket.
2006
Charge Gating and Electronic Delocalization over a Denderimeric Assembly of Trinuclear Ruthenium Clusters
Authors
Benjamin J. Lear, Clifford P. Kubiak
Journal
Abstract
A zeroth-order dendrimer was formed using a tridentate bridging ligand, 2,4,6-tri-4-pyridyl-S-triazine, and the redox-active trinuclear ruthenium cluster Ru3O(OAc)6(CO)(py)(H2O). The electronic properties of this dendrimer were probed using cyclic voltammetry. IR spectroelectrochemistry was performed at both low (-30 degrees C) and room temperature. The IR spectroelectrochemical response at -30 degrees C was straightforward, but at room temperature, the dendrimer exhibits an unusual and complex series of electronic behaviors, including intramolecular cluster-to-bridging-ligand charge transfer, gated electron transfer, and dynamic exchange on the IR time scale.
2005
Observation and dynamics of “mixed-valence isomers” and a thermodynamic estimate of electronic coupling parameters
Authors
Casey H. Londergan, J. Catherine Salsman, Benjamin J. Lear, Clifford P. Kubiak
Journal
Abstract
No abstract available in repository.
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