Black phosphorus, the result of white P under high pressure, has received much attention as a promising anode material for Li-ion batteries (LIBs). However, the final product of lithiation, P63/mmc Li3P, is not satisfactory due to its poor conductivity. In this article we explore the high-pressure phase diagram of the Li–P system through first-principles swarm-intelligence structural search and present two hitherto unknown stable Li-rich compounds, Fm-3m Li3P at 4.2 GPa and P6/mmm Li5P at 10.3 GPa. Metallic Li5P exhibits interesting structural features, including graphene-like Li layers and P-centered octadecahedrons, where P is 14-fold coordinated with Li. Interestingly, both compounds exhibit good dynamical and thermal stability properties at ambient pressure, and the theoretical capacity of P6/mmm Li5P reaches 4326 mAhg–1, the highest among the already known Li–P compounds. Additionally, their mechanical properties are also favorable for electrode materials. Our work represents a significant step toward the performance improvement of Li–P batteries and understanding Li–P compounds.

Boron carbides have become one of excellent candidates of superhard materials. However, the reported BC4 does not belong to the superhard material. Recently, first-principles structure searching calculations have become a powerful tool to discover ground or metastable state structures with intriguing properties. Here, three more stable structures of BC4 than the reported one are uncovered by using swarm structural searches. They all satisfy dynamical and mechanical stability. All atoms in I41/amd and R-3m structures are in sp3 bonding states, whereas Cm structure is a mixture of sp2 and sp3 ones. Interestingly, I41/amd or R-3m BC4 is metallic, while Cm BC4 is a semiconductor. The most stable I41/amd-structured BC4 is an excellent superhard material from the standpoint of high bulk modulus (370 GPa), high shear modulus (353 GPa), large Young’s modulus (804 GPa), low Poisson’s ratio (0.14), and acceptable Vickers hardness value (55.7 GPa). Our work is also important for fully understanding the structure and chemical bond of boron carbides.

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 (O22–) with an O–O distance of 1.40 Å at 100 GPa. Notably, O22– 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 BexOy (x = 1–3, y = 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.

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), and carbazole (Cz), diphenylamine (DPA), or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Because of the large, rigid π-conjugated structure and strong electron-withdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, that is, with or without the phenyl ring as π-bridge, are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical, and electroluminescent properties. Yellow, orange, red, and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A–1 (14.8%), 34.5 cd A–1 (16.9%), 12.8 cd A–1 (10.1%), and 13.2 cd A–1 (15.1%), with Commission Internationale de L’Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40), and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.Keywords: dicyanopyrazino phenanthrene derivatives; intramolecular charge transfer excited states; long-wavelength emitters; organic light-emitting diodes; thermally activated delayed fluorescence;

Recently, a chiral conjugated macrocyclic compound containing alternately arranged donor and acceptor motifs exhibited unique advantages (i.e. high absorption coefficient, broad absorption range, and high electron mobility) in organic photovoltaics (OPVs). Understanding the structure–property relationship at the microscopic level is a prerequisite for further performance optimization or improvement. Here, we employed time-dependent density functional theory (TDDFT) to investigate the electronic circular dichroism (CD), UV-vis absorption, charge transport, and second-order nonlinear optical (NLO) properties of the four chiral compounds. The experimental UV-vis/CD spectra of compound 1 were well reproduced by our calculations and could be used to assign the electron transition properties and absolute configuration (AC) with high confidence. The electronic absorption spectra, charge transport properties, and open circuit voltage of compound 1 in OPVs have been rationalized by comparing cyclic and acyclic structures. The designed compounds 2–3 are expected to exhibit excellent performances in OPVs in view of their small energy gaps, large oscillator strengths, and smaller electron reorganization energies. Moreover, the first hyperpolarizability (βtot) of compound 4 is 27 times larger than that of P-nitroaniline. Thus, our studied compounds are also excellent candidates for second-order NLO materials.

•Experimental UV–Vis/CD spectra were well reproduced by our simulations.•The designed complexes possess remarkable large β values.•The different stereoisomers (SS and RS) have great effect on the NLO properties.•The complex also could act as NLO switching material.Chiral transition metal complexes are of great interest in the second-order nonlinear optical (NLO) field due to their intrinsic non-centrosymmetric structure and the combination advantage of both inorganic and organic compounds. Very recently, the chiral macrocyclic imine Ni(II) coordination complex 1 with outstanding photophysical properties has been reported. The understanding of the structure-property relationship at the microscopic level is very important to further improve its performance. Here, time-dependent density functional theory (TDDFT) calculations have been used to investigate linear, chiroptical, and second-order nonlinear optical (NLO) properties of the eleven complexes with different stereoisomers (e.g. SS and RS) and substituent groups. The simulated UV–Vis/CD spectra of the complex 1 are in good agreement with the experimental ones, which can be used to assign the electron transition properties and absolute configuration (AC) with high confidence. It is found that stereoisomers and different substituent groups have great effect on the photophysical properties such as electronic absorption wavelengths, electron transition properties, and NLO responses. In particular, the designed complex 6 (SS stereoisomer) has the largest β value (32.6 × 10−30 esu), which is about 187 times as large as the organic urea molecule. The analysis of electronic transition indicates that ML'CT/IL'CT charge transfer is mainly responsible for its NLO response. More interestingly, complex 3 with SS stereoisomer could act as NLO switching material because it exhibits obvious different NLO response values from neutral state to the cationic states (3+ or 32+). The effects of different functionals and solvent effects on the UV–Vis/CD spectra were also considered.The structure-property relationships of second-order NLO properties of chiral nickel(II) complexes were established with the aid of the DFT calculations.Download high-res image (161KB)Download full-size image

Relative to advanced cathode materials, anode materials have become one of the key factors to hamper the performance improvement of lithium-ion batteries (LIBs). Recently, two-dimensional (2D) transition metal carbides (e.g. MXenes) have drawn great attention due to their high Li storage ability. However, metal-rich 2D transition metal carbides as anodes usually need surface functionalization, leading to a decrease in the rate performances. Here, we propose that increasing the carbon composition in 2D TaxCy is beneficial for not only eliminating surface functionalization but also greatly improving battery performance. First-principles swarm structural searches were used to explore structures and stabilities of 2D TaxCy (x = 1 and y = 1–4, or x = 2 and y = 1). Besides reproducing the reported 2D TaC, TaC2 and Ta2C are found to be stable, and have high thermal stabilities. Metallic TaC2 and Ta2C provide good electronic conductivity. Intriguingly, carbon-rich TaC2 contains carbon dimers exposed on the surface, enabling it to directly adsorb Li atoms. After adsorption of two-layer Li atoms, its structural integrity is well preserved. The resultant specific capacity, diffusion energy barrier, and open-circuit-voltage (OCV) of TaC2 are much better than those of commercial graphite, f-Ti3C2 or the Ti2C monolayer. Compared with TaC2, TaC, and Ta2C as anode materials, the overall performance of carbon-rich TaxCy is better. Our work provides a useful strategy for designing new-type 2D transition metal materials for LIBs.

Relative to advanced cathode materials, anode materials have become one of the key factors to hamper the performance improvement of lithium-ion batteries (LIBs). Recently, two-dimensional (2D) transition metal carbides (e.g. MXenes) have drawn great attention due to their high Li storage ability. However, metal-rich 2D transition metal carbides as anodes usually need surface functionalization, leading to a decrease in the rate performances. Here, we propose that increasing the carbon composition in 2D TaxCy is beneficial for not only eliminating surface functionalization but also greatly improving battery performance. First-principles swarm structural searches were used to explore structures and stabilities of 2D TaxCy (x = 1 and y = 1–4, or x = 2 and y = 1). Besides reproducing the reported 2D TaC, TaC2 and Ta2C are found to be stable, and have high thermal stabilities. Metallic TaC2 and Ta2C provide good electronic conductivity. Intriguingly, carbon-rich TaC2 contains carbon dimers exposed on the surface, enabling it to directly adsorb Li atoms. After adsorption of two-layer Li atoms, its structural integrity is well preserved. The resultant specific capacity, diffusion energy barrier, and open-circuit-voltage (OCV) of TaC2 are much better than those of commercial graphite, f-Ti3C2 or the Ti2C monolayer. Compared with TaC2, TaC, and Ta2C as anode materials, the overall performance of carbon-rich TaxCy is better. Our work provides a useful strategy for designing new-type 2D transition metal materials for LIBs.

•The electron transition properties of the studied compounds were assigned.•The relation between intermolecular interaction and charge transport was determined.•The effect of different substitutions on photophysical properties was probed.•The studied compounds exhibit large second-order nonlinear optical response.Recently, two phenanthro[9,10-d]imidazole derivatives exhibited excellent advantages in organic light-emitting devices (i.e. high luminous efficiency, high carrier mobility, and low turn-on voltage). However, the relationship between their photophysical properties and the structural characters or intermolecular interactions remain elusive, which is considerable importance to further performance improvement. Currently, density functional theory (DFT) and time-dependent DFT (TD-DFT) have become powerful tools to rationalize photophysical properties and to design new materials with improvement performance. The simulated electron absorption and emission wavelengths of compounds 1 and 2 are in good agreement with the experimental ones. For the studied compounds, the involvement of tert-butyl moiety has negligible effect on energy level and distribution of frontier molecular orbitals (FMOs), whereas greatly affects electron transition of deep energy level and charge transport property. Synergy of π-π and CH···π intermolecular interactions is responsible for the bipolar carrier transport, while CH···π for hole transport. The incorporation of NH2 on phenanthro[9,10-d]imidazole and NO2 on diphenylamino part is an effective way to tune FMOs energy level and intramolecular charge transfer, leading to the substantial enhancement of the second-order nonlinear optical (NLO) response. Our work is also important for understanding photophysical properties and designing photoelectric materials of phenanthro[9,10-d]imidazole derivatives.Download high-res image (258KB)Download full-size image

•Photophysical properties of chiral covalent organic cages were firstly studied.•The electron transition properties of the studied compounds were assigned.•The relation between substitutions and photophysical properties was established.•Chiral covalent organic cages are the potential second-order NLO materials.Chiral organic compounds are the excellent second-order nonlinear optical (NLO) materials due to their intrinsic non-symmetric structures and combined with the merits of organic compounds. Here, the ground electron structures, excited-state electron transition, and second-order NLO property of novel chiral covalent organic cages (COCs) (Solomek et al., 2017) consisting of naphthalene-1,4:5,8-bis(dicarboximide) (NDI) units, have been fully investigated by DFT/TDDFT. The simulated electron absorption wavelengths are in good agreement with experimental ones, allowing us to assign their electron transition characters with high confidence. Based on the experimental structures, we designed eight compounds to probe the effect of different substitutions on photophysical properties. It is found that the substitution of NH2 and NO2 groups at opposite side of NDI and replacement of cyclohex-1,2-diyl with benzene are the most effective way to not only tune energy gaps and electron transition properties but also enhance the NLO response. For instance, the second-order NLO value of compound 2-h is about 80 times as large as the organic urea molecule. Our work is also important for fully understanding photophysical properties and extending potential applications of chiral COCs.Download high-res image (58KB)Download full-size image

Fe–P binary compounds have attracted much attention, particularly under high pressure, since they are the constituents of the Earth's core. However, most studies focus on the single stoichiometry of Fe–P binary compounds at high pressure, and their whole phase diagram and relative stabilities have been unexplored thus far. Herein, first principles swarm structure predictions are performed to find stable structures of Fe–P compounds with various FexPy (x = 1–4 and y = 1, or x = 1 and y = 2) compositions. Then, their phase diagram and relative stabilities are reliably determined based on predicted structures. Specifically, the FeP, Fe2P and Fe4P compounds are found to be stable in the pressure range of 0–400 GPa. The Fe3P compound decomposes into Fe2P and Fe4P above 214 GPa. FeP2 becomes unstable above 82 GPa. Notably, two new phases (i.e. C2/c-structured Fe4P and Cmcm-structured Fe3P) are found to be more stable than the previously reported phases. In addition, the XRD pattern of the predicted Cmcm-structured Fe3P matches the experimental patterns, and we are awaiting future experimental confirmation. Electronic band calculations show that the Fe–P binary compounds are metallic, with a pronounced Fe 3d component crossing the Fermi level. Cmcm-structured Fe3P is ferromagnetic. Our study not only provides useful information for the further study of Fe–P binary compounds but also for the determination of the Earth's core components.

Heterohelicenes have attracted much attention because the involvement of heteroatom into helicene skeleton effectively modulates physical properties and greatly widens the application of helicenes. Recently, boron-fused double helicenes (compounds 1 and 2), exhibiting unique photophysical properties, were reported [J. Am. Chem. Soc., 2016, 138, 5210]. Fully understanding their microelectronic structures is rather important for further performance optimization or improvement. Here, we employed density functional theory to investigate the electronic transition properties, circular dichroism (CD), charge transport, and second-order nonlinear optical (NLO) response of the seven boron-fused double helicenes. Our simulated UV-vis/CD spectra of compound 2 are in good agreement with experimental ones. Different electron-donor or electron-acceptor substituents have considerable effect on frontier molecular orbital energy level, absorption wavelength, electron transition property, and second-order NLO response values. The designed compound 7 with two electron donors (TTF) and two electron acceptors (TCNQ) is expected to be excellent second-order NLO material in view of large first hyperpolarizability value and inherent asymmetric structure. The study of charge transport indicates that incorporation boron and oxygen atoms into intermolecular π–π packing units is an effective way to realize ambipolar charge transport.

Lithium sulfide (Li2S) as an electrode material not only has high capacity but also overcomes many problems caused by pure sulfur electrodes. In particular, the battery performance of nanoscale (Li2S)n clusters is much better than that of bulk sized Li2S. However, the structures, stability, and properties of (Li2S)n clusters, which are very important to improve the performance of Li–S batteries, are still unexplored. Herein, the most stable structures of (Li2S)n (n = 1–10) are reliably determined using the advanced swarm-intelligence structure prediction method. The (Li2S)n (n ≥ 4) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the Li2S monomer, each sulfur atom in the clusters is coordinated with more lithium atoms, thus lengthening the Li–S bond length and decreasing the Li–S bond activation energy. Notably, the adsorption energy gradually increases on the considered anchoring materials (AMs) as the cluster size increases. Moreover, B-doped graphene is a good AM in comparison with graphene or N-doped graphene. The predicted characteristic peaks of infrared, Raman, and electronic absorption spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (Li2S)n clusters and improving the performance of Li–S batteries.

•The P1¯-BeN4 can be synthesized by the reaction of Be3N2 and N2 at 25.4 GPa.•BeN4 with P1¯symmetry has the high energy density of 3.60 kJ g−1.•N∞ chains in P1¯-BeN4 transform to N10 rings network in P21/c phase at 115.1 GPa.•P1¯-BeN4 is metallic, whereas P21/c-BeN4 is an insulator.Polynitrogens are the ideal rocket fuels or propellants. Due to strong triple N≡N bond in N2, the direct polymerization of nitrogen is rather difficult (i.e. extreme high temperature and high pressure). However, the use of nitrides as precursors or the reaction of N2 with other elements has been proved to be an effective way to obtain polynitrogens. Here, with assistance of the advanced first-principles swarm-intelligence structure searches, we found that P1¯-BeN4, containing infinite zigzag-like polymeric nitrogen chains, can be synthesized by compressing the mixture of Be3N2 and N2 at 25.4 GPa, which is greatly lower than 110 GPa for synthesizing cubic gauche nitrogen and other polynitrogen compounds (e.g. bulk CNO at 52 GPa and SN4 at 49 GPa). Its structural stability can be attributed to the coexistence of ionic Be-N and covalent N-N bonds. Intriguingly, this phase has high kinetic stability and remains metastable at ambient pressure. The exceptional properties, including high energy density (3.60 kJ g−1), high nitrogen content (86.1%), high dynamical stability, and low polymerization pressure, make P1¯-structured BeN4 a promising high energy material. Infinite nitrogen chains in P1¯-BeN4 transform to N10 rings network in P21/c phase at 115.1 GPa. P1¯-BeN4 is metallic, while P21/c-BeN4 is an insulator.Download high-res image (259KB)Download full-size image

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 [Nat. Mater. 2015, 14, 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 Cmmm-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.

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 FexSyHz (x = 1–2; y = 1–2; z = 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 Cmc21 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 Cmc21-structured Fe2SH3. The different superconducting mechanisms between Fe2SH3 and H3S were compared and analyzed.

Chiral transition metal complexes not only have large nonlinear optical (NLO) response but also meet the non-centrosymmetric requirement of second-order NLO materials. Therefore, chiral transition metal complexes become very active in the NLO area. Recently, the second-order NLO response of chiral dinuclear Re(I) complex 2 has been found to be 1.5 times larger than that of KH2PO4 (KDP) based on experimental measurement. However, its NLO origin has not been determined and a structure–property relationship has not been established at the microscopic level, which are very important to further improve the performance. It is found that charge transfer from metal to ligand is mainly responsible for its NLO origin. Based on complex 2, the designed complexes have remarkably large second-order NLO activity. For instance, the designed complex 9 has a very large second-order NLO response value (115.81 × 10−30 esu), which is about 668 times larger than the organic molecule urea. Moreover, time-dependent density functional theory (TDDFT) calculations have been used to investigate their UV-Vis/CD spectra. The simulated circular dichroism (CD) spectra of the complex 2 are in good agreement with the experimental ones, which can be used to assign the absolute configurations (ACs) of chiral dinuclear Re(I) complexes with high confidence. The electronic absorption wavelengths, electron transition properties, and the second-order NLO responses strongly depend on the nature of substituent, different ligands (pyridine and isoquinoline) and their combinations. Based on NBO analysis, the interactions between [Re(CO)3Cl] fragments and ligands are of n → σ* character.

Sodium superoxide (NaO2) has attracted considerable attention as the main discharge product in Na–air batteries due to its specific energy, which exceeds that of the Li-ion battery. Pressure has become an irreplaceable tool to improve or alter the physical properties of a given material. By utilizing first-principles swarm structure-searching predictions, herein, we for the first time investigate the structures and electronic properties of NaO2 in the pressure range of 0–20 GPa. It is found that the orthorhombic Pnnm structure at ambient pressure transforms to another orthorhombic Immm structure at approximately 4.6 GPa, and subsequently to the tetragonal P4/mbm structure at 6.7 GPa. The pressure-induced structural transitions are mainly derived from the denser polyhedral packing and higher coordination number. It is interesting to find that the superoxide group (O2−) is maintained over the entire pressure range considered. Analysis of the electronic band structure and density of states shows that the structures found exhibit intriguing half-metallic magnetism. This study enables an opportunity to understand the structures and electronic properties of NaO2 at high pressures.

•The lithium oC24 phase is dynamically stable in the pressure range of 90–200 GPa.•The predicted Tc increases with pressure, and reaches 13.64 K at 200 GPa.•There appears the reconstruction of the Fermi surface under compression.Elements with low atomic numbers are expected to have high superconducting transition temperatures (Tc). Lithium, the lightest metallic element, has therefore been extensively investigated for superconductivity below 65 GPa. A new metallic phase of Li, oC24, has been recently identified above 95 GPa, but its superconductivity has remained unexplored. We report here an ab initio investigation of the phase׳s structural, electronic, dynamical, and superconducting properties at pressures of 100–200 GPa, and show that this phase is dynamically stable within the considered pressure region. The predicted Tc increases with pressure, and reaches 13.64 K at 200 GPa. This observation is a consequence of the reconstruction of the Fermi surface and the rapidly increased electronic density of states at the Fermi level under compression.

Helicene and its derivatives have received considerable attention as candidates for organic photoelectronic materials. Recently, novel quinoxaline-fused [7]carbohelicene derivatives have exhibited unique structural and photophysical properties, especially in the crystal state. However, their structure–property relationships have not been fully understood from their micromechanisms, which is also important to further improve their performance. Herein, the electronic transitions, electronic circular dichroism (CD), second-order nonlinear optical (NLO) responses and charge transport properties of five quinoxaline-fused [7]carbohelicene derivatives have been investigated based on density functional theory calculations. The experimental UV-Vis/CD spectra of the studied compounds were reproduced well by our calculations. Thus, we can assign their electron transition properties and absolution configurations (ACs) with high confidence. It is found that the CD bands of quinoxaline-fused [7]carbohelicene derivatives mainly originate from exciton coupling between quinoxaline, phenyl or 4-methoxyphenyl groups and [7]carbohelicene, which is in sharp contrast to [7]carbohelicene. More interestingly, these derivatives possess large first hyperpolarizability values. For example, the βHRS value of compound 6 is 32.96 × 10−30 esu, which is about 190 times larger than that of the organic urea molecule. The bandwidth of the valence band of compound 2 is comparable to that of the conduction band and slightly larger than that of tris(8-hydroxyquinolinato)aluminium. This means that compound 2 is a potential candidate as an ambipolar charge transport material.

Alkali metal peroxides have a wide range of industrial applications (e.g., energy storage and oxygen source). It is well known that pressure can cause profound structural and electronic changes, leading to the fundamental modification of the physical properties. Here, we reported the structural phase transition, lattice dynamics, and electronic properties of alkali metal (Li, Na, K, and Rb) peroxides by using the unbiased structure searching techniques and first-principles density functional calculations in the pressure range of 0–100 GPa. The predicted first-order phase transitions pressures in Li2O2, Na2O2, K2O2 and Rb2O2 occur at ∼84, 28, 7 and 6 GPa, respectively, which closely correlates with the electronegativity of alkali metals. Different phase transition mechanisms and complex phase transition structures have been observed for these alkali metal peroxide compounds. These predicated high-pressure phases are thermodynamically stable against decomposition into alkali metal oxides plus O2 or alkali metals plus O2. Interestingly, the character of the peroxide group (O22−) is maintained under the considered pressure range. Phonon calculations using the quasi harmonic approximation confirm that these structures are dynamically stable. The band gaps for the studied alkali metal peroxides increase with increasing pressure. This work provides an opportunity for understanding the structures and electron properties of alkali metal peroxides at high pressures.

•Relationship between intermolecular interactions and charge transport properties.•Intermolecular π-stacking interactions have much effect on the hole transport.•Hydrogen bonding interactions are mainly responsible for the electron transport.•A competitive relationship occurs between π-stacking and HB interactions.Polycyclic aromatic hydrocarbons (PAHs) with the electron-withdrawing groups such as halogen atom, cyanide, perfluoroalkyl (PFA), or perfluoroary, etc. exhibit good air stability and better solid-state charge carrier mobility. To obtain a better understanding of structure property relationships of this kind of compounds, a series PAH(CF3)n derivatives a1, a2, b1, b2, c1, and c2, which contain different numbers of trifluoromethyls and benzene rings, were chosen and studied by both band-like model and hopping model. Their crystals contain different intermolecular interactions. It turns out that intermolecular hydrogen bonding interactions are mainly responsible for electron transport, while π-stacking interactions dominate hole transport. When the π-stacking and intermolecular hydrogen bonding interactions coexist in the same direction, a competitive relationship occurs between hole and electron transport, which tend to cause enhancement of electron transport, and restrain hole transport.Graphical abstract

Although substitution with fluorine creates stability in organic electronic materials by altering the molecular crystal packing, the charge transport properties of the materials are significantly affected. Phenyl–perfluorophenyl (π–πF) interaction is a unique intermolecular interaction formed between electropositive perfluorophenyl and electronegative non-fluorinated phenyl, and may have a different charge transport as compared to the π–π interaction formed between ordinary phenyl rings. Three crystals with both π–πF interaction and intermolecular hydrogen bonding interaction were chosen to study the relationship between intermolecular interactions and their charge transport properties in both the band-like model and the hopping model. In contrast to ordinary π–π interaction, which has been reported to be mainly responsible for hole transport, the π–πF interaction is mainly responsible for electron transport. Thus, intermolecular π–πF interaction is an effective packing style to realize the n-type charge carrier. In summary, C–H⋯F interactions are mainly responsible for electron transport while the C–H⋯O interaction is responsible for hole transport.

•Relationship between intermolecular interactions and charge transport properties.•Intermolecular π–π interactions have much effect on the hole transport.•Hydrogen bonding interactions are mainly responsible for the electron transport.A fundamental understanding of the relationship between intermolecular interactions and transport properties in organic semiconducting materials is significant for their potential applications as electronic device element. Carrier transport properties of thiazole/thiophene-based oligomers with trifluoromethylphenyl groups 1, 2, and 3, in which the type and strength of the intermolecular interactions are different, were investigated within the framework of band model. The results show that π–π stacking interactions are mainly responsible for the hole transport, while hydrogen bonding interactions have a great influence on the electron transport. The specific transport mechanism could be explained by analyzing the density of states (DOS) and Γ point wave functions.

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 5 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.

Time-dependent density functional theory (TDDFT) calculations have been used to investigate chiroptical, linear, and second-order nonlinear optical (NLO) properties of the novel tetrathiafulvalenylallene in both neutral and two cationic states for the first time. The calculated UV-Vis/ECD spectra of the studied compound are in good agreement with the experimental ones, which can be used to assign its absolute configuration (AC) with high confidence. From neutral state to the two cationic states, the studied compound exhibits pronounced different chiroptical effects and second-order NLO response values. For example, the calculated β0 value of 12+ is 10.36 times as large as that of 1, while the β0 value of 14+ is 46.51 times as large as that of 1. These effects mainly result from the structural modifications of TTF units in the redox process. It is found that charge transfer between the tetrathiafulvalene (TTF) unit and the allene framework plays a key role in determining the chiroptical properties and electronic transition properties. It is interesting to find that the two benzene rings have vanishingly small effects on the chiroptical properties. The studied compound could act as both a chiroptical switch and NLO switch material from the standpoint of different chiroptical and NLO responses, reversible redox processes, and high stability. The effects of different functionals and basis sets, including solvent effects on the UV-Vis/ECD spectra were also considered.

The electronic circular dichroism (CD), UV-Vis absorption and emission spectra, charge transport, and nonlinear optical properties of the novel azaboradibenzo[6]helicene have been investigated by density functional theory (DFT) for the first time. The calculated absorption and emission energies are in good agreement with the experimental ones. The simulated CD spectra nicely reproduce the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling, which can be used to assign its absolute configuration (AC) with high confidence. The electronic transition properties have been assigned and analyzed. The observed CD bands mainly result from exciton-coupling of the ortho-fused aromatic rings. The adiabatic potential energy surface method was used to calculate reorganization energy of the studied compound. The hole reorganization energy is slightly smaller than that of the electron reorganization energy. The largest bond-length changes upon reduction and oxidation are mainly localized on the rings containing B–N bonds. It is found that the photophysical properties of azaboradibenzo[6]helicene can be effectively tuned upon substitution. In view of the second-order polarizability value and intrinsic non-centrosymmetric electronic structure, the studied compounds have the possibility to be excellent second-order nonlinear optical materials.

The chiroptical, linear, and second-order nonlinear optical (NLO) properties of chiral mononuclear and dinuclear zinc complexes have been investigated for the first time at density functional theory level. The calculated electron absorption energies and oscillator strengths are in reasonable agreement with the experimental ones. 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 zinc complexes with high confidence. Based on these calculated results, the electron transition property and chiroptical origin have been analyzed and assigned. The results show that the coordinated Zn atoms have certain effects on the chiroptical property. The larger nonlinear optical response mainly results from interligand and intraligand charge transfer. The studied complexes have a possibility to be excellent second-order nonlinear optical material from the standpoint of first hyperpolarizability value, transparency and intrinsic non-centrosymmetric electronic structure.

•A novel fluorescent photochromic switching unit has been synthesized.•Fluorescence was switched “on–off” reversibly by the photochromism of naphthopyran.•The quenching mechanism was investigated by experiments and DFT calculations.•The potential application of the molecule in non-destructive readout was studied.A naphthopyran-bridge-benzimidazole dyad which exhibits both fluorescence and photochromism was synthesized and its fluorescence photoswitching was investigated. Irradiation with UV light induces the isomerization of the naphthopyran component to the corresponding merocyanine. The fluorescence of the dyad was switched reversibly between on and off upon UV irradiation and thermal bleaching of the naphthopyran. Using ultraviolet illumination a pattern was created on a polymethylmethacrylate doped film with the dyad. Thus either a non-destructive photoswitch or an image recording system becomes available. The measurement of redox potentials by cyclic voltammetry combined with electronic spectra and a molecular energy diagram of the individual naphthopyran and benzimidazole demonstrated that the transformation of naphthopyran induced energy and electron transfer from the fluorescent benzimidazole to the photochromic naphthopyran, a feature which was also supported by our DFT calculations.Graphical abstract

•The MLCT transition is mainly responsible for the low-energy absorption band with relative smaller oscillator.•The first hyperpolarizability values of the v shaped complexes are larger than that of the linear shape complex.•These complexes exhibit two-dimensional second-order nonlinear optical (NLO) character.•The variation of first hyperpolarizabilities of the studied complexes can be explained by the two-level model.The photophysical properties of the linear and v shaped Pt(II) triarylborons with a 2,2′-bpy core derivatives have been investigated by density functional theory (DFT) method. The calculated electronic absorption wavelengths are in agreement with experimental ones, which can be described as a mixed transition of intra-ligand charge transfer (ILCT), ligand to ligand charge-transfer (LLCT), and metal-to-ligand charge transfer (MLCT). It is found that the MLCT transition is mainly responsible for the low-energy absorption band with relative smaller oscillator strength, while the high-energy absorption band mainly derives from ILCT and LLCT transition. Moreover, the electron absorption wavelengths are not only dependent on the position of the Ph-BMes2 but also on the electron-accepting ability of the acceptor groups. The first hyperpolarizability values of the v shaped complexes are larger than that of the linear shape complex, which indicates that larger intramolecular charge transfer for the v shaped complexes will come into being under the external electric field. Moreover, these complexes exhibit two-dimensional second-order nonlinear optical (NLO) character. Thus, the studied complexes have a possibility to be excellent second-order NLO materials. Based on the two-level model, the variation of first hyperpolarizabilities of the studied complexes can be explained by the combined effect of the difference between the ground state and excited state dipole moment, the oscillator strength, and the cube of the transition energy.The first hyperpolarizability values of the v shaped complexes are larger than that of the linear shape complex, which indicates that larger intramolecular charge transfer for the v shaped complexes might occur under the external electric field. And the changes in first hyperpolarizability values can be explained by the combined effect of the difference between the ground state and excited state dipole moment, the oscillator strength, and the cube of the transition energy.

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure–property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV–vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10−30 esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (62+2+2+ and 64+4+4+), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.

Calculations of the excitation energies, oscillator and rotational strengths, and the UV–Vis and ECD spectra of novel dinuclear zirconium complexes containing two homochiral N atoms have been performed for the first time. The effects of different functionals and basis sets including solvent effect on UV–Vis and ECD spectra were systemically investigated. The good agreement between the simulated UV–Vis and ECD spectra and the experimental ones allows us to assign the absolute configuration with high confidence. The electronic transition and chiroptical properties have been assigned and analyzed. Moreover, the studied complex has a possibility to be excellent second-order nonlinear optical material.Graphical abstractCalculations of the excitation energies, oscillator and rotational strengths, and the UV-vis and ECD spectra of novel dinuclear zirconium complexes containing two homochiral N atoms have been performed for the first time.Research highlights► The absolute configuration of dinuclear zirconium complex has been assigned for the first time. ► The maximum electronic transitions can be assigned as LLCT and LMCT. ► The positive and negative peaks of the ECD spectra are dominated by LLCT and intraligand CT. ► The studied complex has a possibility to be excellent second-order nonlinear optical material.

To get insights of the effect of multiple intermolecular interactions on the charge transport ability of polymorphs, we systematically investigated nine crystals in two kinds of polymorphs within the framework of band model, in which the type and strength of the weak interactions are different. The results show that: (i) the dispersions are relative to the π-stacking area. The bigger area the is, the larger dispersion the is. Thus, it can enhance the charge transport ability. (ii) Hydrogen bonding interactions have a great influence on the dispersion of valence band (VB) rather than conduction band (CB). (iii) When π-stacking and hydrogen bonding interactions coexist in the polymorphs, they enhance the dispersions of CB and restrain that of VB. (iv) When the type of weak interactions increases, the dispersions will be enlarged. Understanding the effect of multiple weak interactions on the polymorphs is a theoretical guide to design novel organic semiconductors with efficient charge transport ability.Graphical abstractHighlights► The effect of multiple intermolecular interactions on charge transfer was studied. ► The band structure shows that dispersion is proportional to the π-stacking area. ► The hydrogen bonding interactions have a great influence on the dispersion of VB. ► The dispersion of CB is enhanced and VB is restrained when the interactions coexist. ► When the type of weak interactions increases, the dispersions will be enlarged.

A theoretical study of polynuclear lithium compounds has shown that these species display large calculated nonlinear optical (NLO) responses. These compounds are based on aromatic subunits connected through polyhedral inorganic core (Li7O6 or Li8O6). These compounds show the calculated first hyperpolarizabilities (β) ranging from 262.55 to 16336.35 × 10−33 esu. The results show that subtle structural modification can substantially enhance the first hyperpolarizability. A basis for understanding the origin of these large NLO responses is proposed based on consideration of the molecular orbitals and electronic transition features of the compounds and the two-state model. Charge transfer from central core to the peripheral phenyl groups plays a key role in the nonlinear optical response. Moreover, the effects of different functionals and basis sets on first hyperpolarizability were systemically investigated.

The electronic absorption and emission spectra, second-order polarizability and reorganization energy of the twenty silafluorenes and spirobisilafluorenes derivatives have been studied at the density functional theory level. The results show that the second-order polarizability (β) increases with increase in the number of the branches due to cooperative enhancement of the charge transfer, whereas the reorganization energy (λ) follows the opposite trend for the studied compounds. The properties (β and λ) of the compounds at the 3, 6-positions substitution are much better than those of compounds at the 2, 7-positions substitution. The effects of donor/acceptor (D/A) substitution and different spiroatoms (silicon or carbon) on second-order polarizability and reorganization energy are also discussed. It is noted that the charge transport properties can be tuned by changing the donor/acceptor (D/A) substitution, and the acceptor substitution can greatly reduce the reorganization energy. The electronic absorption spectra show that all studied compounds can meet the requirement of nonlinear optical (NLO) transparency. Thus, increasing the number of branches and acceptor substitution can remarkably enhance performance of this kind of compounds. Based on larger β, smaller λ and excellent optical transparency, this kind of compounds have a possibility to be excellent second-order NLO or charge transport materials.

Due to the different molecular stacking conformations, two kinds of intermolecular interactions, arene–arene π-stacking interaction and Cu–Cu interaction coexist in the polymorphs of [C6F5Cu]2(4,4′-bipy) crystals, 3-α and 3-β. However, the relative magnitude of the two kinds of intermolecular interactions in 3-α and 3-β is different. With the help of first-principle band structure calculations, the relationship between the charge transport abilities and the intermolecular interactions in the two polymorphs was investigated for the first time. The analysis of band structures and Г point wave functions of the band-edge state in the valence band of crystal 3-α shows that the Cu–Cu interaction so-called cuprophilic interaction determines the hole transport ability, although this interaction is weaker than that in crystal 2 of C6F5Cu(py) discussed in our previous work, which is a promising hole transport material. For polymorph crystal 3-β, the wave functions of LUMO are mainly localized on the bipyridine (bpy) groups, which are result from the arene–arene π-stacking interaction between the bpy groups. Such a π–π stacking interaction dominates the electron transport ability in the conduction band of 3-β and makes the electron main carrier for transporting. The results are also supported by the analysis of effective masses and density of states (DOS). Thus, the charge transport properties are dominated by different intermolecular interactions due to the different molecule stacking in the two polymorphs.

Recently, a chiral conjugated macrocyclic compound containing alternately arranged donor and acceptor motifs exhibited unique advantages (i.e. high absorption coefficient, broad absorption range, and high electron mobility) in organic photovoltaics (OPVs). Understanding the structure–property relationship at the microscopic level is a prerequisite for further performance optimization or improvement. Here, we employed time-dependent density functional theory (TDDFT) to investigate the electronic circular dichroism (CD), UV-vis absorption, charge transport, and second-order nonlinear optical (NLO) properties of the four chiral compounds. The experimental UV-vis/CD spectra of compound 1 were well reproduced by our calculations and could be used to assign the electron transition properties and absolute configuration (AC) with high confidence. The electronic absorption spectra, charge transport properties, and open circuit voltage of compound 1 in OPVs have been rationalized by comparing cyclic and acyclic structures. The designed compounds 2–3 are expected to exhibit excellent performances in OPVs in view of their small energy gaps, large oscillator strengths, and smaller electron reorganization energies. Moreover, the first hyperpolarizability (βtot) of compound 4 is 27 times larger than that of P-nitroaniline. Thus, our studied compounds are also excellent candidates for second-order NLO materials.

The electronic circular dichroism (CD), UV-Vis absorption and emission spectra, charge transport, and nonlinear optical properties of the novel azaboradibenzo[6]helicene have been investigated by density functional theory (DFT) for the first time. The calculated absorption and emission energies are in good agreement with the experimental ones. The simulated CD spectra nicely reproduce the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling, which can be used to assign its absolute configuration (AC) with high confidence. The electronic transition properties have been assigned and analyzed. The observed CD bands mainly result from exciton-coupling of the ortho-fused aromatic rings. The adiabatic potential energy surface method was used to calculate reorganization energy of the studied compound. The hole reorganization energy is slightly smaller than that of the electron reorganization energy. The largest bond-length changes upon reduction and oxidation are mainly localized on the rings containing B–N bonds. It is found that the photophysical properties of azaboradibenzo[6]helicene can be effectively tuned upon substitution. In view of the second-order polarizability value and intrinsic non-centrosymmetric electronic structure, the studied compounds have the possibility to be excellent second-order nonlinear optical materials.

Time-dependent density functional theory (TDDFT) calculations have been used to investigate chiroptical, linear, and second-order nonlinear optical (NLO) properties of the novel tetrathiafulvalenylallene in both neutral and two cationic states for the first time. The calculated UV-Vis/ECD spectra of the studied compound are in good agreement with the experimental ones, which can be used to assign its absolute configuration (AC) with high confidence. From neutral state to the two cationic states, the studied compound exhibits pronounced different chiroptical effects and second-order NLO response values. For example, the calculated β0 value of 12+ is 10.36 times as large as that of 1, while the β0 value of 14+ is 46.51 times as large as that of 1. These effects mainly result from the structural modifications of TTF units in the redox process. It is found that charge transfer between the tetrathiafulvalene (TTF) unit and the allene framework plays a key role in determining the chiroptical properties and electronic transition properties. It is interesting to find that the two benzene rings have vanishingly small effects on the chiroptical properties. The studied compound could act as both a chiroptical switch and NLO switch material from the standpoint of different chiroptical and NLO responses, reversible redox processes, and high stability. The effects of different functionals and basis sets, including solvent effects on the UV-Vis/ECD spectra were also considered.

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure–property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV–vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10−30 esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (62+2+2+ and 64+4+4+), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.

Chiral transition metal complexes not only have large nonlinear optical (NLO) response but also meet the non-centrosymmetric requirement of second-order NLO materials. Therefore, chiral transition metal complexes become very active in the NLO area. Recently, the second-order NLO response of chiral dinuclear Re(I) complex 2 has been found to be 1.5 times larger than that of KH2PO4 (KDP) based on experimental measurement. However, its NLO origin has not been determined and a structure–property relationship has not been established at the microscopic level, which are very important to further improve the performance. It is found that charge transfer from metal to ligand is mainly responsible for its NLO origin. Based on complex 2, the designed complexes have remarkably large second-order NLO activity. For instance, the designed complex 9 has a very large second-order NLO response value (115.81 × 10−30 esu), which is about 668 times larger than the organic molecule urea. Moreover, time-dependent density functional theory (TDDFT) calculations have been used to investigate their UV-Vis/CD spectra. The simulated circular dichroism (CD) spectra of the complex 2 are in good agreement with the experimental ones, which can be used to assign the absolute configurations (ACs) of chiral dinuclear Re(I) complexes with high confidence. The electronic absorption wavelengths, electron transition properties, and the second-order NLO responses strongly depend on the nature of substituent, different ligands (pyridine and isoquinoline) and their combinations. Based on NBO analysis, the interactions between [Re(CO)3Cl] fragments and ligands are of n → σ* character.

Lithium sulfide (Li2S) as an electrode material not only has high capacity but also overcomes many problems caused by pure sulfur electrodes. In particular, the battery performance of nanoscale (Li2S)n clusters is much better than that of bulk sized Li2S. However, the structures, stability, and properties of (Li2S)n clusters, which are very important to improve the performance of Li–S batteries, are still unexplored. Herein, the most stable structures of (Li2S)n (n = 1–10) are reliably determined using the advanced swarm-intelligence structure prediction method. The (Li2S)n (n ≥ 4) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the Li2S monomer, each sulfur atom in the clusters is coordinated with more lithium atoms, thus lengthening the Li–S bond length and decreasing the Li–S bond activation energy. Notably, the adsorption energy gradually increases on the considered anchoring materials (AMs) as the cluster size increases. Moreover, B-doped graphene is a good AM in comparison with graphene or N-doped graphene. The predicted characteristic peaks of infrared, Raman, and electronic absorption spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (Li2S)n clusters and improving the performance of Li–S batteries.