The Marcus dimension: identifying the nuclear coordinate for electron transfer from ab initio calculations.

The Marcus model forms the foundation for all modern discussion of electron transfer (ET). In this model, ET results in a change in diabatic potential energy surfaces, separated along an ET nuclear coordinate. This coordinate accounts for all nuclear motion that promotes electron transfer. It is usually assumed to be dominated by a collective asymmetric vibrational motion of the redox sites involved in the ET. However, this coordinate is rarely quantitatively specified. Instead, it remains a nebulous concept, rather than a tool for gaining true insight into the ET pathway. Herein, we describe an ab initio 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.

27Al Solid-State Magic-Angle Spinning NMR Studies of Aluminum Powder Particle Surfaces Treated with a Methyltriethoxysilane Coupling Agent under Acidic Conditions.

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 27Al 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.

Surface Chemistry Controls the Density of States in Metallic Nanoparticles.

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.

Asymmetries in the Electronic Properties of Spheroidal Metallic Nanoparticles, Revealed by Conduction Electron Spin Resonance and Surface Plasmon Resonance.

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.

Structural and solvent control over activation parameters for a pair of retro Diels-Alder reactions.

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.

Photothermal Effectiveness of Magnetite Nanoparticles: Dependence upon Particle Size Probed by Experiment and Simulation.

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.

On-demand curing of polydimethylsiloxane (PDMS) using the photothermal effect of gold nanoparticles.

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.

Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition.

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.

Probing ligand-induced modulation of metallic states in small gold nanoparticles using conduction electron spin resonance.

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).

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.

Effect of Protonation upon Electronic Coupling in the Mixed Valence and Mixed Protonated Complex, [Ni(2,3-pyrazinedithiol)2].

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.

Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles.

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.

Structural, Electronic, and Magnetic Characterization of a Dinuclear Zinc Complex Containing TCNQ(-) and a µ-[TCNQ-TCNQ](2-) Ligand.

A dinuclear zinc complex containing both a σ-dimerized 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligand ([TCNQ-TCNQ](2-)) and TCNQ(-) was synthesized for the first time. This is the first instance of a single molecular complex with a bridging [TCNQ-TCNQ](2-) 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](2-) 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.

Comparing the energetic and dynamic contributions of solvent to very low barrier isomerization using dynamic steady-state vibrational spectroscopy.

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.

We use the photothermal effect of gold nanoparticles (AuNPs) to provide billion-fold enhancement of on-demand bulk-scale curing of polyurethane. We follow the course of this polymerization using infrared spectroscopy, where we can observe the loss of both isocyanate and alcohol stretches, and the rise of the urethane modes. Application of 12.5 MW cm-2 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.

Fe3O4 nanoparticles as robust photothermal agents for driving high barrier reactions under ambient conditions.

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.

Electron-Transfer Reactions of Electronically Excited Zinc Tetraphenylporphyrin with Multinuclear Ruthenium Complexes.

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.

Concentration-dependent dynamics of hydrogen bonding between acetonitrile and methanol as determined by 1D vibrational spectroscopy.

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.

Solvent versus temperature control over the infrared band shape and position in Fe(CO)3(η(4)-ligand) complexes.

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.

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.