Charles B. Musgrave

Singlet fission in pentacene through multi-exciton quantum states

Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here.

Organocatalyzed atom transfer radical polymerization driven by visible light

Precise control from a metal-free catalyst Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace. One of modern chemistrys great accomplishments has been the development of methods to assemble polymers in steady, orderly steps. However, order comes at a price, and often its the need for metal catalysts that are hard to remove from the plastic product. Theriot et al. used theory to guide the design of a metal-free light-activated catalyst that offers precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the Perspective by Shanmugam and Boyer). Science, this issue p. 1082; see also p. 1053 A metal-free catalyst offers comparable control to more commonly used metals without the drawback of product contamination. Atom transfer radical polymerization (ATRP) has become one of the most implemented methods for polymer synthesis, owing to impressive control over polymer composition and associated properties. However, contamination of the polymer by the metal catalyst remains a major limitation. Organic ATRP photoredox catalysts have been sought to address this difficult challenge but have not achieved the precision performance of metal catalysts. Here, we introduce diaryl dihydrophenazines, identified through computationally directed discovery, as a class of strongly reducing photoredox catalysts. These catalysts achieve high initiator efficiencies through activation by visible light to synthesize polymers with tunable molecular weights and low dispersities.

Efficient generation of H2 by splitting water with an isothermal redox cycle.

Isothermal Water Splitting Solar concentrators can create extremely high temperatures that can drive chemical reactions, including the thermal splitting of water to provide hydrogen. A metal oxide catalyst is needed that is usually cycled between hotter conditions where it is reduced and cooler conditions where it is reoxidized by water. This cycling can limit catalyst lifetime, which can be costly. Muhich et al. (p. 540; see the Perspective by Roeb and Sattler) developed an approach that allowed the redox cycle to be driven isothermally, using pressure swings. A thermal process for generating H2 from water uses pressure changes to recycle between catalyst redox states. [Also see Perspective by Roeb and Sattler] Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS’s overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the “hercynite cycle” exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

Quantum chemical study of the mechanism of aluminum oxide atomic layer deposition

The atomic layer deposition of alumina using water and trimethylaluminum is investigated using the density functional theory. The atomistic mechanisms of the two deposition half-cycles on Al–CH3* and Al–OH* surface sites are investigated. Both half-cycle reactions proceed through the formation of an Al–O Lewis acid-base complex followed by CH4 formation. The Al–O complexes are relatively stable, with formation energies between 0.57 and 0.74 eV. The CH4 formation activation energies range from 0.52 to 0.91 eV and both half-cycle reactions are exothermic with overall enthalpies of reaction between 1.30 and 1.70 eV.

Prediction of transition state barriers and enthalpies of reaction by a new hybrid density-functional approximation

We present a new hybrid density-functional method which predicts transition state barriers with the same accuracy as CBS-APNO, and transition state barriers and enthalpies of reaction with smaller errors than B3LYP, BHandHLYP, and G2. The accuracy of the new method is demonstrated on 132 energies, including 74 transition state barriers and 58 enthalpies of reaction. For 40 reactions with reliable experimental barriers, the absolute mean deviations of the transition state barriers are 0.9, 1.0, 3.1, 3.5, and 3.6 kcal/mol for the new method and the CBS-APNO, G2, B3LYP, and BHandHLYP methods, respectively. The absolute mean deviations of the enthalpies of reaction for 38 reactions with reliable experimental enthalpies are 1.2, 1.4, 3.0, and 5.9 kcal/mol for the new method and the G2, B3LYP, and BHandHLYP methods, respectively. For the new method the maximum absolute deviations for the barriers and enthalpies of reaction are 2.6 and 5.6 kcal/mol, respectively. In addition, we present a simple scheme for a hig...

Mechanism of homogeneous reduction of CO2 by pyridine: proton relay in aqueous solvent and aromatic stabilization.

We employ quantum chemical calculations to investigate the mechanism of homogeneous CO(2) reduction by pyridine (Py) in the Py/p-GaP system. We find that CO(2) reduction by Py commences with PyCOOH(0) formation where: (a) protonated Py (PyH(+)) is reduced to PyH(0), (b) PyH(0) then reduces CO(2) by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO(2), and finally (c) proton transfer (PT) from PyH(0) to CO(2) produces PyCOOH(0). The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH(0) to CO(2). However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6-16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pK(a) for PyH(0) of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH(0) formation, confirming PyH(0)s pK(a) as irrelevant for predicting PT from PyH(0) to CO(2). Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO(2), Py, and Py·CO(2) of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO(-) stabilizes the anionic radicals CO(2)(-) and Py(-) to facilitate low barrier ET. As the reduction of CO(2) proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO(2) reduction by stabilizing the PCET TS and the PyCOOH(0) product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH(3)OH through a series of one-electron reductions analogous to the PCET reduction of CO(2) examined here, where the electrode only acts to reduce PyH(+) to PyH(0).

Predicting ionic conductivity of solid oxide fuel cell electrolyte from first principles

First-principles quantum simulations complemented with kinetic Monte Carlo calculations were performed to gain insight into the oxygen vacancy diffusion mechanism and to explain the effect of dopant composition on ionic conductivity in yttria-stabilized zirconia (YSZ). Density-functional theory (DFT) within the local-density approximation with gradient correction was used to calculate a set of energy barriers that oxygen ions encounter during migration in YSZ by a vacancy mechanism. Kinetic Monte Carlo simulations were then performed using Boltzmann probabilities based on the calculated DFT barriers to determine the dopant concentration dependence of the oxygen self-diffusion coefficient in (Y2O3)x(ZrO2)(1−2x) with x increasing from 6% to 15%. The results from the simulations suggest that the maximum conductivity occurs at 7–9mol% Y2O3 at 600–1500K and that the effective activation energy increases at higher Y doping concentrations in good agreement with previously reported literature data. The increase i...

Reactions of methylamines at the Si(100)-2×1 surface

We have investigated the room temperature adsorption of methylamine, dimethylamine and trimethylamine using density functional theory (DFT) and multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. It was found that the reaction pathways of the amines resemble the precursor-mediated dissociative chemisorption of ammonia. Our calculations showed that although dissociation involving N–C bond cleavage is thermodynamically more favorable than the N–H dissociation pathway, the activation barrier for N–CH3 dissociation is significantly higher than that for N–H dissociation. This leads to selective cleavage of N–H bonds in the surface reactions of methylamine and dimethylamine, while trapping trimethylamine in its molecularly chemisorbed state through the formation of a Si–N dative bond. We also identified the products of the reactions of the amines on the Si(100)-2×1 surface by surface IR studies, confirming the theoretical predictions. The selectivity observed in the surface chemistr...

Atomic layer deposition of hafnium oxide: A detailed reaction mechanism from first principles

Atomic layer deposition (ALD) of hafnium oxide (HfO2) using HfCl4 and H2O as precursors is studied using density functional theory. The mechanism consists of two deposition half-reactions: (1) HfCl4 with Hf-OH sites, and (2) H2O with Hf-Cl sites. Both half-reactions exhibit stable intermediates with energies lower than those of the final products. We show that increasing the temperature reduces the stability of the complex. However, increasing temperature also increases the dissociation free-energy barrier, which in turn results in increased desorption of adsorbed precursors. Both half-reactions are qualitatively similar to the corresponding reactions of ZrO2 ALD using ZrCl4 and H2O.

A Correlated Electron View of Singlet Fission

Singlet fission occurs when a single exciton splits into multiple electron-hole pairs, and could dramatically increase the efficiency of organic solar cells by converting high energy photons into multiple charge carriers. Scientists might exploit singlet fission to its full potential by first understanding the underlying mechanism of this quantum mechanical process. The pursuit of this fundamental mechanism has recently benefited from the development and application of new correlated wave function methods. These methods-called restricted active space spin flip-can capture the most important electron interactions in molecular materials, such as acene crystals, at low computational cost. It is unrealistic to use previous wave function methods due to the excessive computational cost involved in simulating realistic molecular structures at a meaningful level of electron correlation. In this Account, we describe how we use these techniques to compute single exciton and multiple exciton excited states in tetracene and pentacene crystals in order to understand how a single exciton generated from photon absorption undergoes fission to generate two triplets. Our studies indicate that an adiabatic charge transfer intermediate is unlikely to contribute significantly to the fission process because it lies too high in energy. Instead, we propose a new mechanism that involves the direct coupling of an optically allowed single exciton to an optically dark multiexciton. This coupling is facilitated by intermolecular motion of two acene monomers that drives nonadiabatic population transfer between the two states. This transfer occurs in the limit of near degeneracies between adiabatic states where the Born-Oppenheimer approximation of fixed nuclei is no longer valid. Existing theories for singlet fission have not considered this type of coupling between states and, therefore, cannot describe this mechanism. The direct mechanism through intermolecular motion describes many experimentally observed characteristics of these materials, such as the ultrafast time scale of photobleaching and triplet generation during singlet fission in pentacene. We believe this newly discovered mechanism provides fundamental insight to guide the creation of new solar materials that exhibit high efficiencies through multiple charge generation.