A-Site Doping to Alter Oxygen Vacancy Diffusion in SrTiO3.

Doped SrTiO3 is a promising material for many applications where oxygen vacancy diffusion is either critical to device function or a source of failure. This work provides new insight into long-range oxygen vacancy diffusion in SrTiO3 doped with Mn2+, Cr3+, or Fe2+ on the A-site. Density functional theory and the nudged elastic band method are used to calculate oxygen vacancy diffusion barriers adjacent to the dopant and at remote sites. Relative to the pure SrTiO3 structure, doping was found to raise the diffusion barrier for VO •• vacancies and lower the diffusion barrier for VO x vacancies. Furthermore, a doping trapping radius of 6 Šwas found for both the VO x and VO •• vacancies. Counterintuitively, trapping was observed even in supercells where vacancies and dopants are both positively charged. These results provide new insight into how less common A-site doping can change the electronic structure of this important material.

Acid Violet 3: A Base-Activated Water-Soluble Photoswitch.

Acidic azo dyes are widely used for their vibrant colors. However, if their photophysics were better understood and controllable, they could be integrated into many more applications such as photosensing, photomedicine, and nonlinear optics. Here, the proton-controlled photophysics of a widely used acid, hydrazo dye, acid violet 3 (AV3) is explored. Density functional theory is used to predict the ground- and excited-state potential energy surfaces, and the proposed photoisomerization mechanism is confirmed with spectroscopic experiments. The ground-state and first two excited-state surfaces of the three readily accessible protonation states, AV3-H, AV3, and AV3+H, are investigated along both the dihedral rotation and inversion coordinates. The deprotonated AV3-H undergoes photoisomerization with blue light (λex = 453 nm) through a dihedral rotation mechanism. Upon the formation of the cis-isomer, the reversion of AV3-H is predicted to occur through a mixed rotational and inversion mechanism. In contrast, AV3 and its protonated form, AV3+H, do not undergo photoisomerization because there is no driving force for either the rotation or inversion of the azo bond in the excited state. In addition, when the azo bond is acidic, the ground-state dihedral rotation reversion mechanism barrier is lower. The mechanistic insights gained here through the combination of theory and experiment provide a roadmap to control the reactivity of AV3 across 11 orders of magnitude of proton concentration, making them interesting candidates for a range of pharmaceuticals.

Efficiently predicting directional carrier mobilities in organic materials with the Boltzmann transport equation.

Describing charge carrier anisotropy in crystalline organic semiconductors with ab initio methods is challenging because of the weak intermolecular interactions that lead to both localized and delocalized charge transport mechanisms. Small polaron hopping models (localized) are generally used to describe materials with small charge carrier mobilities, while periodic band models (delocalized) are used to describe materials with high charge carrier mobilities. Here, we prove the advantage of applying the constant relaxation time approximation of the Boltzmann transport equation (BTE) to efficiently predict the anisotropic hole mobilities of several unsubstituted (anthracene, tetracene, pentacene, and hexacene) and substituted (2,6-diphenylanthracene, rubrene, and TIPS-pentacene) high-mobility n-acene single crystals. Several density functionals are used to optimize the crystals, and the composite density functional PBEsol0-3c/sol-def2-mSVP predicts the most experimentally similar geometries, adequate indirect bandgaps, and the theoretically consistent n-acene charge transport mobility trend. Similarities between BTE and Marcus mobilities are presented for each crystal. BTE and Marcus charge carrier mobilities computed at the same geometry result in similar mobility trends, differing mostly in materials with more substitutions or structurally complex substituents. By using a reduced number of calculations, BTE is able to predict anisotropic carrier mobilities efficiently and effectively for a range of high-mobility organic semiconductors.

Real Temperature Model of Dynamic Disorder in Molecular Crystals.

Charge carrier mobilities in ordered organic semiconductors are limited by inherent vibrational phonons that scatter carriers. To improve a material's intrinsic mobility, restricting particularly detrimental modes with molecular substitutions may be a viable strategy. Here, we develop a probabilistic temperature-dependent displacement model that we couple with the density functional dimer projection protocol to predict effective electronic coupling fluctuations. The phonon-induced deviations from the equilibrium electronic couplings are used to infer the detriment of low-frequency phonons on charge carrier mobilities in a set of organic single crystals. We show that asymmetric sliding motions in pentacene and 2,6-diphenylanthracene induce large electronic coupling fluctuations, whereas seesawlike motions cause large fluctuations in rubrene, 9,10-diphenylanthracene, and, 2,6-diphenylanthracene. Vibrational analyses revealed that the asymmetric sliding phonon in rubrene persists only in the low-mobility direction of the crystal. Therefore, rubrene's intrinsic high mobility is likely due to the absence of this source of disorder in its high-mobility conduction channels. This model can be used to identify particularly harmful or helpful phonons in crystalline materials and may provide design rules for developing materials with intrinsically low disorder.

Purification and refolding protocol for cold-active recombinant esterase AaSGNH1 from Aphanizomenon flos-aquae expressed as insoluble inclusion bodies.

Microbial esterases are a highly desirable tool for numerous biosynthetic and biotechnological applications requiring ester bond cleavage. Once identified, microbial esterases are often produced recombinantly in Escherichia coli to enhance yield and ease of purification. In this study a polyhistidine-tagged SGNH esterase gene (AaSGNH1), originating from the cyanobacterium Aphanizomenon flos-aquae, was cloned into an over-expression plasmid and expressed in BL21(DE3) cells. The recombinant esterase enzyme was produced as inactive inclusion bodies which were insoluble in 8 M urea but readily solubilized by the detergent Empigen BB®. Crucially, the procurement of active enzyme required controlled removal of detergent during column chromatography and dialysis steps. The refolded esterase was characterized with respect to its ability to catalyze the cleavage of p-nitrophenol esters of different chain lengths (C2, C8, C16). In addition, the temperature and pH optima were determined and it was found that the enzyme was most active at low temperatures (5-15 °C) and under alkaline conditions (pH 8-10). It was found that the kinetic properties of AaSGNH1 were remarkably similar to other SGNH esterases described thereby validating that the protein was effectively refolded. Overall, this study provides a simple strategy for isolating cold-active recombinant esterase enzyme when expressed as inclusion bodies.