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.

Controlling excited-state dynamics via protonation of naphthalene-based azo dyes.

Azo dyes are a class of photoactive dyes that constitute a major focus of chemical research due to their applications in numerous industrial functions. This work explores the impact of protonation on the photophysics of four naphthalene-based azo dyes. The pKa value of the dyes increases proportionally with decreasing Hammett parameter of p-phenyl substituents from 8.1 (R = -H, σ = 0) to 10.6 (R = -NMe2, σ = -0.83) in acetonitrile. Protonation of the dyes shuts down the steady-state photoisomerization observed in the unprotonated moieties. Fluorescence measurements reveal a lower quantum yield with more electron-donating p-phenyl substituents, with overall lower fluorescence quantum yields than the unprotonated dyes. Transient absorption spectroscopy reveals four excited-state lifetimes (<1 ps, ∼3 ps, ∼13 ps, and ∼200 ps) exhibiting faster excited-state dynamics than observed in the unprotonated forms (for 1-3: 0.7-1.5 ps, ∼3-4 ps, 20-40 ps, 20-300 min; for 4: 0.7 ps, 4.8 ps, 17.8 ps, 40 ps, 8 min). Time-dependent density functional theory (TDDFT) elucidates the reason for the loss of isomerization in the protonated dyes, revealing a significant change in the lowest excited state potential energy nature and landscape upon protonation. Protonation impedes relaxation along the typical rotational and inversion isomerization axes, locking the dyes into a trans-configuration that rapidly decays back to the ground state.

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.

Lessons Learned from Catalysis to Qubits: General Strategies to Build Accessible and Accurate First-Principles Models of Point Defects.

Defects and dopants play critical roles in defining the properties of a material. Achieving a mechanistic understanding of how such properties arise is challenging with current experimental methods, and computational approaches suffer from significant modeling limitations that frequently require a posteriori fitting. Consequently, the pace of dopant discovery as a means of tuning material properties for a particular application has been slow. However, recent advances in computation have enabled researchers to move away from semiempirical schemes to reposition density functional theory as a predictive tool and improve the accessibility of highly accurate first-principles methods to all researchers. This Perspective discusses some of these recent achievements that provide more accurate first-principles geometric, thermodynamic, optical, and electronic properties simultaneously. Advancements related to supercells, basis sets, functionals, and optimization protocols, as well as suggestions for evaluating the quality of a computational model through comparison to experimental data, are discussed. Moreover, recent computational results in the fields of energy materials, heterogeneous catalysis, and quantum informatics are reviewed along with an evaluation of current frontiers and opportunities in the field of computational materials chemistry.

Controlling product selectivity in oxidative desulfurization using an electrodeposited iron oxide film.

Sulfur-containing compounds must be removed from raw fuel oils before use and recently, there has been an effort to identify and optimize a more energy efficient method of oil processing. One promising route is electrochemical oxidative desulfurization (ODS), and in this work, we investigate an electrodeposited iron oxide film (FeOx(OH)y) as a working electrode to catalyze the oxidation of dibenzothiophene (DBT). The FeOx(OH)y film displays unexpected selectivity for the DBT sulfoxide (DBTO)-departing from the catalytic behavior of gold, which favors the dimerization of DBT. In addition, we observe a morphological change within our FeOx(OH)y film from γ-FeOOH to γ-Fe2O3. This change provides insight to the activity of each structure for ODS as the rate of oxidation increases after the incorporation of γ-Fe2O3. Our experimental observations are corroborated with DFT calculations, which suggest that the adsorption energy of DBT on Au is significantly greater than on the FeOx(OH)y, favoring the formation of dimeric and oligomeric products. Calculations also demonstrate that DBT binds preferably in a monodentate configuration but that oxidation occurs via DBT bound via a bidentate configuration. Monodentate binding on γ-FeOOH is significantly stronger than binding on γ-Fe2O, resulting in easier conversation to bidentate binding on γ-Fe2O3.

Photoredox Product Selectivity Controlled by Persistent Radical Stability.

The use of photoredox catalysis for the synthesis of small organic molecules relies on harnessing and converting the energy in visible light to drive reactions. Specifically, photon energy is used to generate radical ion species that can be harnessed through subsequent reaction steps to form a desired product. Cyanoarenes are widely used as arylating agents in photoredox catalysis because of their stability as persistent radical anions. However, there are marked, unexplained variations in product yields when using different cyanoarenes. In this study, the quantum yield and product yield of an α-aminoarylation photoredox reaction between five cyanoarene coupling partners and N-phenylpyrrolidine were characterized. Significant discrepancies in cyanoarene consumption and product yield suggested a chemically irreversible, unproductive pathway in the reaction. Analysis of the side products in the reaction demonstrated the formation of species consistent with radical anion fragmentation. Electrochemical and computational methods were used to study the fragmentation of the different cyanoarenes and revealed a correlation between product yield and cyanoarene radical anion stability. Kinetic modeling of the reaction demonstrates that cross-coupling selectivity between N-phenylpyrrolidine and the cyanoarene is controlled by the same phenomenon present in the persistent radical effect.

Ultra-fast excited-state dynamics of substituted trans-naphthalene azo moieties.

In this work we untangle the ultrafast deactivation of high-energy excited states in four naphthalene-based azo dyes. Through systematic photophysical and computational study, we observed a structure-property relationship in which increasing the electron donating strength of the substituent leads to longer lived excited states in these organic dyes and faster thermal reversion from the cis to trans configuration. In particular, azo dyes 1-3 containing less electron donating substituents show three distinct excited-state lifetimes of ∼0.7-1.5 ps, ∼3-4 ps, and 20-40 ps whereas the most electron donating dimethyl amino substituted azo 4 shows excited-state lifetimes of 0.7 ps, 4.8 ps, 17.8 ps and 40 ps. While bulk photoisomerization of all four moieties is rapid, the cis to trans reversion lifetimes vary by a factor of 30 with τreversion decreasing from 276 min to 8 min with increasing electron donating strength of the substituent. In order to rationalize this change in photophysical behavior, we explored the excited-state potential energy surfaces and spin-orbit coupling constants for azo 1-4 through density functional theory. The increase in excited-state lifetime for 4 can be attributed to geometric and electronic degrees of freedom of the lowest energy singlet excited-state potential energy surface.

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.

Enhanced basicity of an electron donor-acceptor complex.

An electron donor-acceptor (EDA) complex forms between 1,4-dicyanobenzene and N-phenylpyrrolidine, which are coupling partners for the α-aminoarylation photoredox reaction. Calculations and experiments demonstrate the EDA complex is a better base than N-phenylpyrroline. A re-analysis of the α-aminoarylation reaction suggests that the EDA complex is a proton acceptor in the reaction.

Elucidation of complex triplet excited state dynamics in Pd(II) biladiene tetrapyrroles.

Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks' sp3-hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectroscopy revealed that higher-energy excitation (λexc ∼ 350-500 nm) led to an additional lifetime (i.e., an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states (λexc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ∼20 µs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the 'extra' lifetime observed upon higher energy excitation was revealed using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin-orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

Untangling product selectivity on clean low index rutile TiO2 surfaces using first-principles calculations.

Computational modeling of metal oxide surfaces provides an important tool to help untangle complex spectroscopy and measured catalytic reactivity. There are many material properties that make rational catalytic design challenging, and computational methods provide a way to evaluate possible structural factors, like surface structure, individually. The mechanism of water oxidation or oxygen evolution is well studied on some anatase surfaces and the rutile TiO2 (110) surface but has not yet been mapped on other low-index Miller rutile surfaces that are present in most experimental nano-titania catalysts. Here first principles calculations provide new insights into water oxidation mechanisms and reactivity of the most common low-index Miller facets of rutile TiO2. The reactivity of three surfaces, (101), (010), and (001), are explored for the first time and the product selectivity of multistep electron transfer on each surface is compared to the well-studied (110) surface. Density functional theory shows that a peroxo, O(p), intermediate is more favorable for water oxidation on all facets. The ˙OH radical formation is favored on the (001) facet resulting in a high overpotential for oxygen evolution reaction (OER). The (101) and (110) facets have low overpotentials, ∼0.3 V, and favor two-electron proton-coupled electron transfer to produce H2O2. The only facet that prefers direct OER is (001), leading to O2 evolution in a four-electron process with an overpotential of 0.53 V. A volcano plot predicts the selectivity and activity of low-index Miller facets of rutile TiO2, revealing the high activity of the peroxo OER mechanism on the (010) facet.

Atomic-Layer-Deposited Aluminum Oxide Thin Films Probed with X-ray Scattering and Compared to Molecular Dynamics and Density Functional Theory Models.

A better understanding of amorphous aluminum oxide's structure and electronic properties is obtained through combined experimental and computational approaches. Grazing incidence X-ray scattering measurements were carried out on aluminum oxide thin films grown using thermal atomic layer deposition. The corresponding pair distribution functions (PDFs) showed structures similar to previously reported PDFs of solid-state amorphous alumina and molten alumina. Structural models based on crystalline alumina polymorphs (PDFgui) and amorphous alumina (molecular dynamics, MD) were examined for structural comparisons to the experimental PDF data. Smaller MD models were optimized and verified against larger models to allow for quantum chemical electronic structure calculations. The electronic structure of the amorphous alumina models yields additional insight into the band structure and electronic defects present in amorphous alumina that are not present in crystalline samples.

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.

Insights into the Mechanism of an Allylic Arylation Reaction via Photoredox-Coupled Hydrogen Atom Transfer.

Despite widespread use as a synthetic method, the precise mechanism and kinetics of photoredox coupled hydrogen atom transfer (HAT) reactions remain poorly understood. This results from a lack of detailed kinetic information as well as the identification of side reactions and products. In this report, a mechanistic study of a prototypical tandem photoredox/HAT reaction coupling cyclohexene and 1,4-dicyanobenzene (DCB) using an Ir(ppy)3 photocatalyst and thiol HAT catalyst is reported. Through a combination of electrochemical, photochemical, and spectroscopic measurements, key unproductive pathways and side products are identified and rate constants for the main chemical steps are extracted. The reaction quantum yield was found to decline rapidly over the course of the reaction. An unreported cyanohydrin side product was identified and thought to play a key role as a proton acceptor in the reaction. Transient absorption spectroscopy (TAS) and quantum chemical calculations suggested a reaction mechanism that involves radical addition of the nucleophilic DCB radical anion to cyclohexene, with cooperative HAT occurring as the final step to regenerate the alkene. Kinetic modeling of the reaction, using rate constants derived from TAS, demonstrates that the efficiency of the reaction is limited by parasitic absorption and unproductive quenching between excited Ir(ppy)3 and the cyanohydrin photoproduct.

Parameter space exploration reveals interesting Mn-doped SrTiO3 structures.

The rich chemistry of the SrTiO3 is often modified, intentionally or unintentionally, through the inclusion of defects and dopants. Much computational effort using periodic boundary DFT has been dedicated towards understanding how these observed properties arise from the disordered perovskite structure, but the range of possible defect chemistries arising from different computational modeling choices has not been thoroughly explored. In this study, we calculate the geometric and electronic properties for a systematic range of supercells, from approximately 40 atoms to approximately 320 atoms, with each atomic vacancy and doped with Mn ions to isolate the contribution of supercell size to predicted properties. Our thorough analysis of the electronic and geometric structure of each defected supercell shows high variability, illustrating the need to map the parameter space in order to achieve a comprehensive model of disordered perovskites. Our results additionally reveal fundamental insights into dopant chemistry in SrTiO3, and we report new potential geometric and electronic structures for Mn dopants that can be used to justify and guide additional experimental investigation into this complex material.

Proton-controlled non-exponential photoluminescence in a pyridylamidine-substituted Re(I) complex.

Chemical intuition and well-known design principles can typically be used to create ligand environments in transition metal complexes to deliberately tune reactivity for desired applications. However, intelligent ligand design does not always result in the expected outcomes. Herein we report the synthesis and characterization of a tricarbonyl rhenium (2,2'-bipyridine) 4-pyridylamidine, Re(4-Pam), complex with unexpected photophysical properties. Photoluminescence kinetics of Re(4-Pam) undergoes non-exponential decay, which can be deconvolved into two emission lifetimes. However, upon protonation of the amidine functionality of the 4-pyridylamidine to form Re(4-PamH), a single exponential decay is observed. To understand and rationalize these experimental observations, density functional theory (DFT) and time-dependent density functional theory (TDDFT) are employed. The symmetry or asymmetry of the protonated or deprotonated 4-pyridylamidine ligand, respectively, is the key factor in switching between one and two photoluminescence lifetimes. Specifically, rotation of the dihedral angle formed between the bipyridine and 4-Pam ligand leads to two rotamers of Re(4-Pam) with degenerate triplet- to ground-state transitions.

Site-Selective Orbital Interactions in an Ultrathin Iron-Carbene Photosensitizer Film.

We present the first experimental study of the frontier orbitals in an ultrathin film of the novel hexa-carbene photosensitizer [Fe(btz)3]3+, where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene). Resonant photoelectron spectroscopy (RPES) was used to probe the electronic structure of films where the molecular and oxidative integrities had been confirmed with optical and X-ray spectroscopies. In combination with density functional theory calculations, RPES measurements provided direct and site-selective information about localization and interactions of occupied and unoccupied molecular orbitals. Fe 2p, N 1s, and C 1s measurements selectively probed the metal, carbene, and side-group contributions revealing strong metal-ligand orbital mixing of the frontier orbitals. This helps explain the remarkable photophysical properties of iron-carbenes in terms of unconventional electronic structure properties and favorable metal-ligand bonding interactions-important for the continued development of these type of complexes toward light-harvesting and light-emitting applications.

Spin multiplicity effects in doublet versus singlet emission: the photophysical consequences of a single electron.

Ambient-stable fluorescent radicals have recently emerged as promising luminescent materials; however, tailoring their properties has been difficult due to the limited photophysical understanding of open-shell organic systems. Here we report the experimental and computational analysis of a redox pair of π-conjugated fluorescent molecules that differ by one electron. A π-dication (DC) and π-radical cation (RC) demonstrate different absorption spectra, but similar red emission (λ emiss,max = ∼630 nm), excitation maxima (λ exc,max = ∼530 nm), fluorescence lifetimes (1-10 ns), and even excited-state (non-emissive) lifetimes when measured by transient absorption spectroscopy. Despite their experimental similarities, time-dependent density functional theory (TDDFT) studies reveal that DC and RC emission mechanisms are distinct and rely on different electronic transitions. Excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly occupied molecular orbitals. This relationship between the excited-state dynamics of RC and its π-backbone geometry illustrates a potential strategy for developing π-conjugated radicals with new emission properties. Additionally, by comparing TDDFT and CIS (configuration interaction singles) excitations, we show that unrestricted TDDFT accurately reproduces experimental absorption spectra and provides an opportunity to examine the relaxed excited-state properties of large open-shell molecules like RC.

Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy.

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeII NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3 MLCT state, from the initially excited 1 MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3 MC state, in competition with vibrational relaxation and cooling to the relaxed 3 MLCT state. The relaxed 3 MLCT state then decays much more slowly (7.6 ps) to the 3 MC state. The 3 MC state is rapidly (2.2 ps) deactivated to the ground state. The 5 MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3 MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.

Excited State Dynamics of Bistridentate and Trisbidentate RuII Complexes of Quinoline-Pyrazole Ligands.

Three homoleptic ruthenium(II) complexes, [Ru(Q3PzH)3]2+, [Ru(Q1Pz)3]2+, and [Ru(DQPz)2]2+, based on the quinoline-pyrazole ligands, Q3PzH (8-(3-pyrazole)-quinoline), Q1Pz (8-(1-pyrazole)-quinoline), and DQPz (bis(quinolinyl)-1,3-pyrazole), have been spectroscopically and theoretically investigated. Spectral component analysis, transient absorption spectroscopy, density functional theory calculations, and ligand exchange reactions with different chlorination agents reveal that the excited state dynamics for Ru(II) complexes with these biheteroaromatic ligands differ significantly from that of traditional polypyridyl complexes. Despite the high energy and low reorganization energy of the excited state, nonradiative decay dominates even at liquid nitrogen temperatures, where triplet metal-to-ligand-charge-transfer emission quantum yields range from 0.7 to 3.8%, and microsecond excited state lifetimes are observed. In contrast to traditional polypyridyl complexes where ligand exchange is facilitated by expansion of the metal-ligand bonds to stabilize a metal-centered state, photoinduced ligand exchange occurs in the bidentate complexes despite no substantial MC state population, while the tridentate complex is extremely photostable despite an activated decay route, highlighting the versatile photochemistry of nonpolypyridine ligands.