Nonplanar Donor–Acceptor Chiral Molecules with Large Second-Order Optical Nonlinearities: 1,1,4,4-Tetracyanobuta-1,3-diene Derivatives

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of five 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives and elucidated structure–property relationships from the micromechanism. The experimental UV–vis absorption and circular dichroism (CD) spectra were well reproduced by our calculations at TDB3LYP/6-31+G* level of theory. The electron transition property and chiroptical origin have been assigned and analyzed. The results show that the studied compounds possess large molecular first hyperpolarizabilities, especially for compound <b>5</b> which has a value of 35 × 10^–30 esu, which is comparable with the measured value for highly π-delocalized phenyliminomethyl ferrocene complex and about 200 times larger than the average first hyperpolarizability of the organic urea molecule. Despite the nonplanarity of these compounds, efficient intramolecular charge transfer (CT) from electron donor to electron acceptor moieties was observed, which plays the key role in determining the NLO response. The intramolecular charge transfer cooperativity was also probed. In view of the first hyperpolarizability values, intrinsic noncentrosymmetric electronic structure, and high stability, the studied compounds have the possibility to be excellent second-order NLO materials

A Stable, Magnetic, and Metallic Li₃O₄ Compound as a Discharge Product in a Li–Air Battery

The Li–air battery with the specific energy exceeding that of a Li ion battery has been aimed as the next-generation battery. The improvement of the performance of the Li–air battery needs a full resolution of the actual discharge products. Li₂O₂ has been long recognized as the main discharge product, with which, however, there are obvious failures on the understanding of various experimental observations (e.g., magnetism, oxygen K-edge spectrum, etc.) on discharge products. There is a possibility of the existence of other Li–O compounds unknown thus far. Here, a hitherto unknown Li₃O₄ compound as a discharge product of the Li–air battery was predicted through first-principles swarm structure searching calculations. The new compound has a unique structure featuring the mixture of superoxide O₂^– and peroxide O₂^2–, the first such example in the Li–O system. The existence of superoxide O₂^– creates magnetism and hole-doped metallicity. Findings of Li₃O₄ gave rise to direct explanations of the unresolved experimental magnetism, triple peaks of oxygen K-edge spectra, and the Raman peak at 1125 cm^–1 of the discharge products. Our work enables an opportunity for the performance of capacity, charge overpotential, and round-trip efficiency of the Li–air battery

Theoretical Study on the Chiroptical Optical Properties of Chiral Fullerene C₆₀ Derivative

Time-dependent density functional theory (TDDFT) calculations have been used to investigate UV/CD spectra and nonlinear optical (NLO) property of the C₆₀-fullerene bisadduct (<i>R</i>,<i>R</i>,^f,s<i>A</i>)-[CD(+)280] for the first time. The electron transition natures of the four main measured bands are analyzed, and their results are used to designate the excited states involved in an electron-transfer process of the studied compound. On a comparative scale, the predicted excitation energies and oscillator strengths are in reasonable agreement with the observed values, demonstrating the efficiency of TDDFT in predicting the localized and charge transfer transitions. The good agreement between the experimental and the simulated CD spectra shows that TDDFT calculations can be used to assign the absolute configurations (ACs) of chiral fullerene C₆₀ derivatives with high confidence. The observed large dissymmetry ratio <i>g</i> (<i>g</i> = Δε/ε) at about 700 nm results from the orbital characters of the local fullerene excited state, which leads to large transition magnetic dipole moment and small transition electronic dipole moment. The different functionals and solvent effects on UV/CD spectra were also considered. The studied compound has a possibility to be an excellent second-order NLO material from the standpoint of transparency and large second-order polarizability value

Photophysical Properties of Chiral Tetraphenylethylene Derivatives with the Fixed Propeller-Like Conformation

The recent synthesized helical tetraphenylethylene (TPE) exhibits broad application prospects such as display, catalysis, and medical imaging. A full understanding of the intricate relation between structure and property is rather important to structural design and performance improvement. Here, we employed density functional theory (DFT) and time-dependent DFT to calculate their ground- and excited-state structures, electron transition properties, optical rotation (OR), and second-order nonlinear optical (NLO) properties. For compound <b>1</b>, the simulated UV–vis/CD spectra and calculated OR value are in reasonable agreement with the experimental ones, allowing us to reliably assign the electron transition and determine the absolute configuration. Intriguingly, TPE derivatives are excellent candidates for the second-order NLO materials in view of the large first hyperpolarizability values and intrinsic asymmetric structures. The intramolecular charge transfer cooperativity for this kind of compound was achieved through involvement of the donor and acceptor substituent groups or their combinations. The charge transfer within TPE plays a key role in determining the chiral origin and electron transition properties, whereas the contribution of peripheral phenyl rings is fairly small. Moreover, the designed compounds <b>5</b> and <b>7</b> are potential materials for the fluorescent probe

Silicon Framework-Based Lithium Silicides at High Pressures

The bandgap and optical properties of diamond silicon (Si) are not suitable for many advanced applications such as thin-film photovoltaic devices and light-emitting diodes. Thus, finding new Si allotropes with better bandgap and optical properties is desirable. Recently, a Si allotrope with a desirable bandgap of ∼1.3 eV was obtained by leaching Na from NaSi₆ that was synthesized under high pressure [<i>Nat. Mater.</i> <b>2015</b>, <i>14</i>, 169], paving the way to finding new Si allotropes. Li is isoelectronic with Na, with a smaller atomic core and comparable electronegativity. It is unknown whether Li silicides share similar properties, but it is of considerable interest. Here, a swarm intelligence-based structural prediction is used in combination with first-principles calculations to investigate the chemical reactions between Si and Li at high pressures, where seven new compositions (LiSi₄, LiSi₃, LiSi₂, Li₂Si₃, Li₂Si, Li₃Si, and Li₄Si) become stable above 8.4 GPa. The SiSi bonding patterns in these compounds evolve with increasing Li content sequentially from frameworks to layers, linear chains, and eventually isolated Si ions. Nearest-neighbor Si atoms, in <i>Cmmm</i>-structured LiSi₄, form covalent open channels hosting one-dimensional Li atom chains, which have similar structural features to NaSi₆. The analysis of integrated crystal orbital Hamilton populations reveals that the SiSi interactions are mainly responsible for the structural stability. Moreover, this structure is dynamically stable even at ambient pressure. Our results are also important for understanding the structures and electronic properties of LiSi binary compounds at high pressures

Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides

The oxidation state of an element influences its chemical behavior of reactivity and bonding. Finding unusual oxidation state of elements is a theme of eternal pursuit. As labeled by an alkali-earth metal, barium (Ba) invariably exhibits an oxidation state of +2 by a loss of two 6s valence electrons while its inner 5p closed shell is known to remain intact. Here, we show through the reaction with fluorine (F) at high pressure that Ba exhibits a hitherto unexpected high oxidation state greater than +2 in three pressure-stabilized F-rich compounds BaF₃, BaF₄, and BaF₅, where Ba takes on the role of a 5p element by opening up its inert 5p shell. Interestingly enough, these pressure-stabilized Ba fluorides share common structural features of Ba-centered polyhedrons but exhibit a diverse variety of electronic properties showing semiconducting, metallic, and even magnetic behaviors. Our work modifies the traditional knowledge on the chemistry of alkali-earth Ba element established at ambient pressure and highlights the major role of pressure played in tuning the oxidation state of elements

Pressure-Induced Stable Beryllium Peroxide

Beryllium oxides, at ambient pressure, have been extensively studied due to their unique chemical bonds and applications. However, the long-desirable target beryllium peroxide (BeO₂) has not been reported, thus far. Currently, the application of pressure has become a powerful tool in finding unusual stoichiometric compounds with exotic properties. Here, swarm structural searches in combination with first-principles calculations disclosed that the reaction of BeO and oxygen, at pressures above 89.6 GPa, yields BeO₂. Interestingly, this reaction pressure is lower than the phase transition pressure (106 GPa) of pure BeO. BeO₂ crystallizes in FeS₂-type structure, whose remarkable feature is that it contains peroxide group (O₂^2–) with an O–O distance of 1.40 Å at 100 GPa. Notably, O₂^2– is maintained in the pressure range of 89.6–300 GPa. The chemical bonding analysis shows that the uniformly distributed ionic Be–O and covalent O–O bonding network plays a key role in determining its structural stability. BeO₂ is a direct band gap nonmetal, and its band gap becomes larger with increase of pressure, which is in sharp contrast with BaO₂. Moreover, phase diagram of Be–O binary compounds with various Be_<i>x</i>O_<i>y</i> (<i>x</i> = 1–3, <i>y</i> = 1–6) compositions at pressures of up to 300 GPa was reliably built. Our results are also important for enriching the understanding of beryllium oxides

Structure and Electronic Properties of Fe₂SH₃ Compound under High Pressure

Inspired by the diverse properties of hydrogen sulfide, iron sulfide, and iron hydrides, we combine first-principles calculations with structure prediction to find stable structures of Fe–S–H ternary compounds with various Fe_<i>x</i>S_<i>y</i>H_<i>z</i> (<i>x</i> = 1–2; <i>y</i> = 1–2; <i>z</i> = 1–6) compositions under high pressure with the aim of finding novel functional materials. It is found that Fe₂SH₃ composition stabilizes into an orthorhombic structure with <i>Cmc</i>2₁ symmetry, whose remarkable feature is that it contains dumbbell-type Fe with an Fe–Fe distance of 2.435 Å at 100 GPa, and S and H atoms directly bond with the Fe atoms exhibiting ionic bonding. The high density of states at the Fermi level, mainly coming from the contribution of Fe-3d, indicates that it satisfies the Stoner ferromagnetic condition. Notably, its ferromagnetic ordering gradually decreases with increasing pressure, and eventually collapses at a pressure of 173 GPa. As a consequence, magnetic and nonmagnetic transition can be achieved by controlling the pressure. In addition, there is a very weak electron–phonon coupling in <i>Cmc</i>2₁-structured Fe₂SH₃. The different superconducting mechanisms between Fe₂SH₃ and H₃S were compared and analyzed

Gold as a 6p-Element in Dense Lithium Aurides

The negative oxidation state of gold (Au) has drawn a great attention due to its unusual valence state that induces exotic properties in its compounds, including ferroelectricity and electronic polarization. Although monatomic anionic gold (Au^–) has been reported, a higher negative oxidation state of Au has not been observed yet. Here we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. First-principles calculations in combination with swarm structural searches disclosed chemical reactions between Au and Li at high pressure, where stable Li-rich aurides with unexpected stoichiometries (e.g., Li₄Au and Li₅Au) emerge. These compounds exhibit intriguing structural features like Au-centered polyhedrons and a graphene-like Li sublattice, where each Au gains more than one electron donated by Li and acts as a 6p-element. The high negative oxidation state of Au has also been achieved through its reactions with other alkali metals (e.g., Cs) under pressures. Our work provides a useful strategy for achieving diverse Au anions

Gold with +4 and +6 Oxidation States in AuF₄ and AuF₆

An important goal in chemistry is to prepare compounds with unusual oxidation states showing exciting properties. For gold (Au), the relativistic expansion of its 5d orbitals makes it form high oxidation state compounds. Thus far, the highest oxidation state of Au known is +5. Here, we propose high pressure as a controllable method for preparing +4 and +6 oxidation states in Au via its reaction with fluorine. First-principles swarm-intelligence structure search identifies two hitherto unknown stoichiometric compounds, AuF₄ and AuF₆, exhibiting typical molecular crystal character. The high-pressure phase diagram of Au fluorides is rather different from Cu or Ag fluorides, which is indicated by stable chemical compositions and the pressures needed for the synthesis of these compounds. This difference can be associated with the stronger relativistic effects in Au relative to Cu or Ag. Our work represents a significant step forward in a more complete understanding of the oxidation states of Au