The Excess Electron in a Boron Nitride Nanotube: Pyramidal NBO Charge Distribution and Remarkable First Hyperpolarizability

Metalloborospherenes with a Stabilized Classical Fullerene-like Borospherene B40 Act as Nonlinear Optical Switches, Electron Reservoirs, Molecular Capacitors, and Molecular Reactors

Using a new method of η5-Li and η6-Mg atoms capping the faces of the classical fullerene-like borospherene Td B40, we theoretically predict an exohedral metalloborospherene Td Mg10Li12&B40 molecule. Remarkably, a newfangled endoexo cage isomerism is proposed. Further, embedding Mg atoms in the Td B40 cage forms endohedral derivatives. Due to the intramolecular pull-push electron transfer relay, these obtained molecules possess unequal multilayered and alternant spherical charge distribution. The outer is an excess electron layer, bringing a molecular nonlinear switch character and an electron reservoir behavior with strong electron-donating and -accepting abilities. The middle (Mg2+)10(Li+)12 and the outer layers together constitute an electric double layer, presenting the behavior of a molecular capacitor where the electronic charge-discharge process occurs in the outer excess electron layer. The inner part is an empty cage B4026- with a strong negative electric field. The valence electrons of the embedded Mg atoms are transformed into new excess electrons and added in the outer excess electron layer, also exhibiting the charging behavior of the molecular capacitor. Considering the chemical reaction in the inner cage, the embedded Mg atom is ionized, forming an Mg2+ cation and 2e under the strong negative electric field; meanwhile, 2e is powerfully pushed into the outer excess electron layer. This chemical process shows a generalized Coulomb explosion, and thus the exohedral metalloborospherene molecules with cage B4026- may act as molecular reactors. The new species mark the genesis of classical fullerene-like borospherene chemistry and stimulate their applications in molecular nonlinear optical and nanoelectronics.

Geometric, Electronic, and Optoelectronic Properties of Carbon-Based Polynuclear C3O[C(CN)2]2M3 (where M = Li, Na, and K) Clusters: A DFT Study

Carbon-based polynuclear clusters are designed and investigated for geometric, electronic, and nonlinear optical (NLO) properties at the CAM-B3LYP/6-311++G(d,p) level of theory. Significant binding energies per atom (ranging from -162.4 to -160.0 kcal mol^-1) indicate excellent thermodynamic stabilities of these polynuclear clusters. The frontier molecular orbital (FMOs) analysis indicates excess electron nature of the clusters with low ionization potential, suggesting that they are alkali-like. The decreased energy gaps (E_H-L) with increased alkali metals size revael the improved electrical conductivity (σ). The total density of state (TDOS) study reveals the alkali metals' size-dependent electronic and conductive properties. The significant first and second hyperpolarizabilities are observed up to 5.78 × 10³ and 5.55 × 10⁶ au, respectively. The β_o response shows dependence on the size of alkali metals. Furthermore, the absorption study shows transparency of these clusters in the deep-UV, and absorptions are observed at longer wavelengths (redshifted). The optical gaps from TD-DFT are considerably smaller than those of HOMO-LUMO gaps. The significant scattering hyperpolarizability (β_HRS) value (1.62 × 10⁴) is calculated for the C3 cluster, where octupolar contribution to β_HRS is 92%. The dynamic first hyperpolarizability β(ω) is more pronounced for the EOPE effect at 532 nm, whereas SHG has notable values for second hyperpolarizability γ(ω).

Theoretical Study of Alkaline-Earth Metal (Be, Mg, and Ca)-Substituted Aluminum Nitride Nanocages With High Stability and Large Nonlinear Optical Responses

By replacing one Al or N atom of aluminum nitride nanocage Al12N12 with an alkaline-earth metal atom, two series of compounds, namely, M@Al12N11 and M@Al11N12 (M = Be, Mg, and Ca), were constructed and investigated in theory. The substituted effect of alkaline-earth metal on the geometric structure and electronic properties of Al12N12 is studied in detail by density functional theory (DFT) methods. The calculated binding energies, HOMO–LUMO gaps, and VIE values of these compounds reveal that they possess high stability, though the NBO and HOMO analyses show that they are also excess electron compounds. Due to the existence of diffuse excess electrons, these alkaline-earth metal-substituted compounds exhibit larger first hyperpolarizabilities (β0) than pure Al12N12 nanocage. In particular, these considered compounds exhibit satisfactory infrared (IR) (>1800 nm) and ultraviolet (UV) (˂ 250 nm) transparency. Therefore, these proposed excess electron compounds with high stability may be regarded as potential candidates for new UV and IR NLO molecules.

Design a novel type of excess electron compounds with large nonlinear optical responses using group 12 elements (Zn, Cd and Hg)

Based on the interesting Janus-type all-cis1,2,3,4,5,6-hexafluorocyclohexane (1) molecule, a novel type of excess electron compounds MF-1-MH (MF = Li, Na and K, MH = Zn, Cd and Hg) were designed theoretically. The geometric structures, electronic structures and nonlinear optical properties of MF-1-MH compounds were studied by density functional theory. Our results show that in Li-1-MH, the obvious charge transfer between Li and MH can be observed while in Na/K-1-MH, the charge transfer between Na/K and MH is negligible. Particularly, the MF-1-MH exhibit remarkable nonlinear optical (NLO) response and the first hyperpolarizability of the K-1-Zn almost achieve 1.0 × 10⁶ au. We hope this work will further enrich the family of excess electron compounds, so that more experimental interests and efforts can be attracted to propose and synthesize new excellent NLO materials.

Oxacarbon superalkali C3X3Y3 (X = O, S and Y = Li, Na, K) clusters as excess electron compounds for remarkable static and dynamic NLO response

An intriguing class of excess electron oxacarbon superalkali clusters is explored for nonlinear optical response through density functional theory (DFT) methods at CAM-B3LYP/6-311++G(d,p). These superalkali clusters shows noticeable binding energies per atom (E_b) which reveals their thermodynamic stabilities (-86.45 ∼ -119.44 kcal mol^-1). The obtained significant VIPs values also suggest the electronic stability of these clusters. The VIP values range from 2.06 eV to 3.42 eV. These clusters show remarkable electronic properties and their HOMO-LUMO gaps (E_H-L) are significantly reduced. The lowest H-L gap of 0.96 eV is obtained for C₃O₃K₃ while the highest H-L gap of 2.07 eV is calculated for C₃S₃Li₃. The obtained PDOS spectra further provide evidence for the superior electronic properties of these clusters. The clusters show excellent nonlinear optical properties as revealed from remarkable values (1.6 × 10⁶ au) of static first hyperpolarizability. The controlling factors for hyperpolarizability are also explored by using conventional two-level model. The calculated values of β_o are correlated nicely with β_tl. The crucial excitation energy is the key factor in controlling the first hyperpolarizability. In these excess electron clusters, the second hyperpolarizability (γ_o) response increases up to 4.3 × 10⁹ au. Moreover, the calculated scattering hyperpolarizability (β_HRS) values are quite significant in these clusters and the highest value of 1.3 × 10⁶ au is calculated for C₃S₃K₃. Additionally, these clusters also possess larger dynamic nonlinearities. The dynamic second hyperpolarizability with dc-Kerr effect increases up to 1.0 × 10¹¹ au. The remarkable values for refractive index (n₂) also suggest the excellent nonlinearity of these superalkali clusters.

Remarkable static and dynamic NLO response of alkali and superalkali doped macrocyclic [hexa-]thiophene complexes; a DFT approach

In this study, the nonlinear optical (NLO) response of alkali metal atom (Li, Na and K) and their corresponding superalkali (Li₃O, Na₃O and K₃O) doped six membered cyclic thiophene (6CT) has been explored. The optimized geometries of complexes; Li@6CT, Na@6CT, K@6CT, Li₃O@6CT, Na₃O@6CT and K₃O@6CT depict that the superalkalis and alkali metals interact through the active cavity of 6CT. Interaction energies reveal that superalkalis have higher interaction with 6CT than alkali metals. The nonlinear optical (NLO) response of the reported complexes is estimated via both static and dynamic hyperpolarizabilities which are further rationalized by the HOMO–LUMO gap, natural bond orbital (NBO) charge transfer, dipole moment, polarizabilities and β_vec. A remarkably high NLO response is computed for Na₃O@6CT among all of the complexes. The static hyperpolarizability of the Na₃O@6CT complex is 5 × 10⁴ au along with the highest β_vec value (2.5 × 10⁴ au). High charge transfer (1.53e⁻) and small E_H–L gap (2.96 eV) is responsible for such a large NLO response. For dynamic NLO responses, electro-optic Pockel's effect (EOPE) and second-harmonic generation (SHG) are explored. A very large quadratic nonlinear optical response (3.8 × 10⁻¹² au) is observed for the Na₃O@6CT complex. Moreover, the absorption spectrum of the Na₃O@6CT complex shows ultra-high transparency in the ultraviolet and visible regions unlike any other of its counterparts.

In this study, the nonlinear optical (NLO) response of alkali metal atom (Li, Na and K) and their corresponding superalkali (Li₃O, Na₃O and K₃O) doped six membered cyclic thiophene (6CT) has been explored.

Coinage metalides: a new class of excess electron compounds with high stability and large nonlinear optical responses

The possibility of using coinage metal atoms as excess electron acceptors is examined for the first time by designing a new class of M⁺-1-M'^- (M = Li, Na, and K; M' = Cu, Ag, and Au) compounds termed "coinage metalides" on the basis of an intriguing Janus-type all-cis1,2,3,4,5,6-hexafluorocyclohexane (1) molecule. Under the large facial polarization of 1, the outermost ns¹ electrons of alkali metal atoms can be transferred to coinage metal atoms, forming diffuse excess electrons around them. Consequently, the resulting M⁺-1-Cu^- and M⁺-1-Ag^- compounds exhibit significantly large nonlinear optical (NLO) responses. In particular, these novel M⁺-1-M'^- compounds exhibit much higher stability (larger VIEs and E_c values) than that of the corresponding M⁺·1·M'^- (M, M' = Li, Na, and K) alkalides. We hope this work could open up new possibilities for NLO material design by using coinage metal atoms as excess electron acceptors and, on the other hand, attract more experimental interest and efforts to synthesize such stable compounds in the laboratory.

How Different Substitution Positions of F, Cl Atoms in Benzene Ring of 5-Methylpyrimidine Pyridine Derivatives Affect the Inhibition Ability of EGFRL858R/T790M/C797S Inhibitors: A Molecular Dynamics Simulation Study

Lung cancer is the most frequent cause of cancer-related deaths worldwide, and mutations in the kinase domain of the epidermal growth factor receptor (EGFR) are a common cause of non-small-cell lung cancers, which is a major subtype of lung cancers. Recently, a series of 5-methylpyrimidine-pyridinone derivatives have been designed and synthesized as novel selective inhibitors of EGFR and EGFR mutants. However, the binding-based inhibition mechanism has not yet been determined. In this study, we carried out molecular dynamic simulations and free-energy calculations for EGFR derivatives to fill this gap. Based on the investigation, the three factors that influence the inhibitory effect of inhibitors are as follows: (1) The substitution site of the Cl atom is the main factor influencing the activity through steric effect; (2) The secondary factors are repulsion between the F atom (present in the inhibitor) and Glu762, and the blocking effect of Lys745 on the phenyl ring of the inhibitor. (3) The two factors function synergistically to influence the inhibitory capacity of the inhibitor. The theoretical results of this study can provide further insights that will aid the design of oncogenic EGFR inhibitors with high selectivity.

Theoretical investigation of perfect fullerene-like borospherene Ih-B20 protected by alkaline earth metal: multi-layered spherical electride molecules as electric field manipulated second-order nonlinear optical switches

A perfect fullerene-like borospherene B₂₀ with 12 B₅ rings stabilized in the electride molecule (Mg²⁺)₁₂&B₂₀¹⁸⁻ + 6e⁻.

Remarkable second and third order nonlinear optical properties of organometallic C6Li6–M3O electrides

Electrides are excess electron compounds with excellent nonlinear optical properties.

Superhalogen doping: a new and effective approach to design materials with excellent static and dynamic NLO responses

The first-ever example where superhalogen doping alone is introduced as a new and effective approach to impart large NLO responses.

Nonlinear optical properties of aluminum nitride nanotubes doped by excess electron: a first principle study

Aluminum nitride nanotubes (AlNNTs) doped by the excess electron, e@AlNNT and M@N-AlNNT (M = Li, Na, K), have been designed and their geometrical, electronic, and nonlinear optical (NLO) properties have been explored theoretically. The results showed that the excess electron narrows the energy gap between HOMO and LUMO values (E_H-L) of the doped systems in the range of 3.42-5.37 eV, which is due to a new energy level HOMO formed for the doped excess electron, with higher energy than the original HOMO of AlNNT. Importantly, the doped excess electron considerably increases the first hyperpolarizability (β₀) from 130 a.u. of the undoped AlNNT to 646 a.u. for e@AlNNT, 2606 a.u. for Li@N-AlNNT, while 1.14 × 10⁵ a.u. for Na@N-AlNNT, and 1.37 × 10⁶ a.u. for K@N-AlNNT. The enormous β₀ values for Na@N-AlNNT and K@N-AlNNT are attributed to the low transition energy. These results demonstrate that AlNNTs are a promising material in high-performance NLO nanomaterials for electronic devices.

Successive lithiation of acetylene, ethylene and benzene: a comprehensive computational study of large static second hyperpolarizability

This work is a revisit of the study of the electron correlation effect of lithium substitution on the second hyperpolarizability (10⁶ a.u.) of acetylene, ethylene and benzene. The large quenching of mean second hyperpolarizability has been addressed by CCSD calculations. The inclusion of triple excitation in the MP4 method generally overestimates second hyperpolarizability in comparison to the MP4SDQ method. The present CCSD γ_av value of C₆Li₆: 405 × 10⁴ a.u. obtained with a relatively larger basis set established the earlier prediction of Sadlej et al. [Phys. Chem. Phys. Chem., 2000, 2, 3393-3399] where degenerate non-dipolar transitions in low lying excited states play the crucial role. The successive lithiation results in gradual red shifting of transition energy leading to significant enhancement of second hyperpolarizability. Most of the chosen DFT functionals predict the correct qualitative trend of second hyperpolarizability. The quantitatively different results may be attributed to the case when the ground state wave function cannot be approximated by a single SD.

Substituent effects on the electronic structures and nonlinear optical properties of Li-doped nano-carbon bowl

A series of Li-corannulene-(NH₂)_n and Li-corannulene-(NO₂)_n (n = 1, 2, 5) compounds have been theoretically designed and investigated using density functional theory. In this work, two models are systematically investigated to explore the important factors for enhancing the static first hyperpolarizability by introducing the substitution group. It is revealed that energy gaps (E_gap) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all compounds are in the range of 4.149-4.934 eV. Different DFT methods are adopted to calculate polarizabilities and the first hyperpolarizabilities of Li-corannulene-(NH₂)_n and Li-corannulene-(NO₂)_n (n = 1, 2, 5) compounds. It is revealed that polarizability values of the systems increase with increasing number of NH₂/NO₂ substitution group. Moreover, it is found that the first hyperpolarizabilities of Li-corannulene-(NO₂)_n are larger than those of Li-corannulene-(NH₂)_n, which can be attributed to the lower transition energies. In contrast to the NH₂ substitution group, NO₂ substitution group can be more powerful in increasing the first hyperpolarizability of Li-doped corannulene. We hope that this study can provide a new idea for designing nonlinear optical materials using the NH₂ and NO₂ groups.

A comparative theoretical study on the electrical and nonlinear optical properties of Li atom adsorbed on AlN and BN single-walled nanotubes

The geometrical structures, electrical properties, and nonlinear optical (NLO) properties of AlNNT-Li and BNNT-Li nanotube systems were investigated by means of the density functional theory (DFT) method. Frontier molecular orbitals and density of states analyses show that adsorption of the Li atom can significantly narrow the wide HOMO-LUMO gaps of pure AlNNT and BNNT. The results reveal that AlNNT-Li and BNNT-Li systems containing diffuse excess electrons can be regarded as inorganic electrides. The formation of diffuse excess electrons leads to a decrease in transition energies, thereby increasing the first hyperpolarizabilities (β ₀) of AlNNT-Li and BNNT-Li. This work may contribute to the development of potential high-performance NLO materials. Graphical abstract The structural characteristics and nonlinear optical properties of the AlNNT-Li and BNNT-Li systems were studied by means of density functional theory. Introduction of Li atoms greatly enhances the static first hyperpolarizabilities of AlNNT-Li and BNNT-Li.

A theoretical study on superalkali-doped nanocages: unique inorganic electrides with high stability, deep-ultraviolet transparency, and a considerable nonlinear optical response

By doping an Al12N12 nanocage with superalkali Li3O, a series of Li3O@Al12N12 compounds were theoretically designed and investigated for the first time. Computational results reveal that these species contain diffuse excess electrons, and thus can be regarded as inorganic electrides of a new type. As expected, these proposed electrides possess considerable first hyperpolarizabilities (β0) up to 1.86 × 10(7) au. In particular, the lowest-energy Li3O@Al12N12 exhibits high stability and excellent deep-ultraviolet transparency. Moreover, the effects of superalkali and nanocage subunits on the NLO responses of M3O@Al12N12 (M = Li, Na, K) and Li3O@X12Y12 (X = B, Al; Y = N, P) are systemically investigated. Results show that the respective substitution of Na3O and B12P12 for Li3O and Al12N12 can bring a larger β0 for such electrides. This study may be significant in terms of providing an effective strategy to design thermally stable inorganic electrides as potential high-performance NLO molecules.

Tuning the inter-molecular charge transfer, second-order nonlinear optical and absorption spectra properties of a π-dimer under an external electric field

Different strengths of external electric fields enhance the stability, control the inter-molecular charge transfer and strengthen the nonlinear optical responses of a π-dimer.

Effect of alkaline earth metal at the single wall CNT mouth on the electronic structure and second hyperpolarizability

In the present work, electronic structure and second hyperpolarizability of a number of alkaline earth metals (M [Formula: see text] Be, Mg and Ca) complexes with carbon nanotube (CNT) has been studied by using different DFT functional. The complexes have sufficient thermal stability. Significant amount of charge transfer from metal to CNT results in stronger ground state polarization. The second hyperpolarizability obtained at different DFT functional (BHHLYP, CAM-B3LYP, B2PLYP, [Formula: see text]B97XD) showed a consistent trend. The magnitude of second hyperpolarizability of M@CNT[3,0] complexes enhances rather appreciably when a second metal atom is introduced into other mouth position. The longitudinal component of second hyperpolarizability of M@CNT[3,0]@M complexes increases with increasing size of metal atom. The magnitude of second hyperpolarizability of Ca@CNT[3,0]@Ca complex is comparable with Fe([Formula: see text]-C[Formula: see text]B[Formula: see text]. However, widening/lengthening of CNT markedly reduces the cubic responses. The two state model can qualitatively explain the variation of second hyperpolarizability.

Connecting effect on the first hyperpolarizability of armchair carbon-boron-nitride heteronanotubes: pattern versus proportion

Carbon-boron-nitride heteronanotubes (BNCNT) have attracted a lot of attention because of their adjustable properties and potential applications in many fields. In this work, a series of CA, PA and HA armchair BNCNT models were designed to explore their nonlinear optical (NLO) properties and provide physical insight into the structure-property relationships; CA, PA and HA represent the models that are obtained by doping the carbon segment into pristine boron nitride nanotube (BNNT) fragments circularly around the tube axis, parallel to the tube axis and helically to the tube axis, respectively. Results show that the first hyperpolarizability (β0) of an armchair BNCNT model is dramatically dependent on the connecting patterns of carbon with the boron nitride fragment. Significantly, the β0 value of PA-6 is 2.00 × 10(4) au, which is almost two orders of magnitude larger than those (6.07 × 10(2) and 1.55 × 10(2) au) of HA-6 and CA-6. In addition, the β0 values of PA and CA models increase with the increase in carbon proportion, whereas those of HA models show a different tendency. Further investigations on transition properties show that the curved charge transfer from N-connecting carbon atoms to B-connecting carbon atoms of PA models is essentially the origin of the big difference among these models. This new knowledge about armchair BNCNTs may provide important information for the design and preparation of advanced NLO nano-materials.

Electronic structures and second hyperpolarizabilities of alkaline earth metal complexes end-capped with NA2 (A = H, Li, Na)

The ground state structures and NLO properties of a number of alkaline earth metal complexes end-capped with NA₂ groups (A = H, Li, Na) are calculated by employing the CAM-B3LYP, wB97XD and B2PLYP functionals along with MP2 and CCSD(T) for 6-311++G(d,p), 6-311++G(3df,3pd), aug-cc-pVTZ, aug-pc-2 and Hypol basis sets.

"Dancing inside the ball": the structures and nonlinear optical properties of three Sc2S@C3v(8)-C82 isomers

Recently, the crystal structures and electrochemical properties of the isomers (Sc2S "trapped" in C82) have been reported, in which the Sc2S is located inside the different positions of the C82 cage. In the present work, three isomers of endohedral metallofullerenes Sc2S@C3v(8)-C82 (A, B, and C) have been designed to explore the effect of the position of Sc2S on their interaction energies and nonlinear optical properties. Among three isomers, the Sc2S is located in different positions of the C82 cage: the angles of Sc-S-Sc in A, B, and C are 104.9, 114.8, and 115.7°, respectively. Furthermore, the analysis of natural bond orbital (NBO) charge indicates that the electron-transfer is from the Sc2S to the adjacent carbon atoms of the C82 cage. The interaction energy of B is the smallest among three isomers which is -226.2 kcal mol(-1). It was worth mentioning that their first hyperpolarizabilities (β tot) were studied, we found that their β tot values were related to the positions of Sc2S: C (2100) > B (1191) > A (947 au). We hope that the present work can provide a new strategy to promote the nonlinear optical properties of endohedral metallofullerenes by changing the positions of the encapsulated molecular. Graphical abstract Three isomers of endohedral metallofullerenes Sc2S@C3v(8)-C82 (A, B, and C) have been designed to explore the position effect of Sc2S on the interaction energies and nonlinear optical properties. Among three isomers, the Sc2S in B has the most stable position. Significantly, the first hyperpolarizability is related to the position of Sc2S inside the C82 cage, which provides a novel strategy to enhance the first hyperpolarizability by the Sc2S revolving inside the C82 cage.

Second hyperpolarizability of delta shaped disubstituted acetylene complexes of beryllium, magnesium, and calcium

Present theoretical study involves the delta shape complexes of beryllium, magnesium, and calcium where the metal atom interacts perpendicularly with disubstituted acetylene. Most of the complexes are found to be fairly stable. The dependence of second-hyperpolarizability on the basis set with increasing polarization and diffuse functions has been examined which showed the importance of 'f-type' type polarization function for heavy metal (Mg, Ca) and 'd-type' polarization function for beryllium. Larger second hyperpolarizability has been predicted for complexes having significant ground state polarization and low lying excited states favoring strong electronic coupling. Transition energy plays the most significant role in modulating the second hyperpolarizability.

Role of Excess Electrons in Nonlinear Optical Response

The excess electron is a kind of special anion with dispersivity, loosely bounding and with other fascinating features, which plays a pivotal role (promote to about 10(6) times in (H2O)3{e}) in the large first hyperpolarizabilities (β0) of dipole-bound electron clusters. This discovery opens a new perspective on the design of novel nonlinear optical (NLO) molecular materials for electro-optic device application. Significantly, doping alkali metal atoms in suitable complexants was proposed as an effective approach to obtain electride and alkalide molecules with excess electron and large NLO responses. The first hyperpolarizability is related to the characteristics of complexants and the excess electron binding states. Subsequently, a series of new strategies for enhancing NLO response and electronic stability of electride and alkalide molecules are exhibited by using various complexants. These strategies include not only the behaviors of pushed and pulled electron, size, shape, and number of coordination sites of complexants but also the number and spin state of excess electrons in these unusual NLO molecules.

A new strategy for simultaneously enhancing nonlinear optical response and electron stability in novel cup–saucer+–cage−-shaped sandwich electride molecules with an excess electron protected inside the cage

A new strategy of utilizing the cage size effect for simultaneously enhancing the β₀ and the VIP.

Hydrogen-bond-directed-linking solving transparence-efficiency tradeoff in nonlinear optical molecule

It is well known that settling transparency-efficiency tradeoff is important to design nonlinear optical (NLO) materials. In this work, we constructed one-dimensional polymeric cyanoacetylene (NCCCH)n by hydrogen-bond-directed-linking to understand this tradeoff from molecular level. Results show that the first hyperpolarizability of (NCCCH)n (n=2-8) gradually increased with the increase of n, and what is more important is that the red-shifts, associated with the increase of n, were very little. It is proposed that these polymeric structures possess double-degenerated charge transitions, which contribute to the hyperpolarizability in an additive fashion, and that the coupled oscillators are gradually improved, which lead to the increase of the first hyperpolarizability. Therefore, we propose the hydrogen-bond-directed-linking idea is helpful to develop the potential high-performance NLO materials.

The V-shaped polar molecules encapsulated into Cs (10528)-C72: stability and nonlinear optical response

Recently, a new sulfide cluster fullerene, Sc2S@Cs (10528)-C72 containing two pairs of fused pentagons has been isolated and characterized (Chen et al., J. Am. Chem. Soc., 2012, 134, 7851). Inspired by this investigation, we propose a question: what properties will be influenced by the interaction between the encapsulated V-shaped polar molecule and C72? To answer this question, four encapsulated metallic fullerenes (EMFs) M2N@C72 (M = Sc or Y, N = S or O) along with pristine Cs-C72 (10528) were investigated by quantum chemistry methods. The results show that the Egap (3.01-3.14 eV) of M2N@C72 are significantly greater than that of pristine Cs-C72 (10528) (2.34 eV). This indicates that the stabilities of these EMFs increase by encapsulating the V-shaped polar molecule into the fullerene. Furthermore, the natural bond orbital (NBO) charge analysis indicates electron transfer from M2N to C72 cage, which plays a crucial role in enhancing first hyperpolarizability (βtot). The βtot follows the order of 1174 au (Y2O@C72) ≈ 1179 au (Sc2O@C72) > 886 au (Y2S@C72) ≈ 864 au (Sc2S@C72) > 355 au (C72). This indicates that the βtot of M2N@C72 is more remarkable than that of pristine Cs-C72 (10528) due to the induction effect of the encapsulated molecule. Compared with sulfide cluster fullerenes (Y2S@C72 and Sc2S@C72), oxide cluster fullerenes (Sc2O@C72 and Y2O@C72) show much larger βtot due to the small ionic radius and the large electronegativity of oxygen. In contrast, the metal element (scandium and yttrium) has a slight influence on the βtot. Thus, oxide cluster fullerenes are candidates to become promising nonlinear optical materials with higher performance.

Doping the alkali atom: an effective strategy to improve the electronic and nonlinear optical properties of the inorganic Al12N12 nanocage

Under ab initio computations, several new inorganic electride compounds with high stability, M@x-Al12N12 (M = Li, Na, and K; x = b66, b64, and r6), were achieved for the first time by doping the alkali metal atom M on the fullerene-like Al12N12 nanocage, where the alkali atom is located over the Al-N bond (b66/b64 site) or six-membered ring (r6 site). It is revealed that independent of the doping position and atomic number, doping the alkali atom can significantly narrow the wide gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (EH-L = 6.12 eV) of the pure Al12N12 nanocage in the range of 0.49-0.71 eV, and these doped AlN nanocages can exhibit the intriguing n-type characteristic, where a high energy level containing the excess electron is introduced as the new HOMO orbital in the original gap of pure Al12N12. Further, the diffuse excess electron also brings these doped AlN nanostructures the considerable first hyperpolarizabilities (β0), which are 1.09 × 10(4) au for Li@b66-Al12N12, 1.10 × 10(4), 1.62 × 10(4), 7.58 × 10(4) au for M@b64-Al12N12 (M = Li, Na, and K), and 8.89 × 10(5), 1.36 × 10(5), 5.48 × 10(4) au for M@r6-Al12N12 (M = Li, Na, and K), respectively. Clearly, doping the heavier Na/K atom over the Al-N bond can get the larger β0 value, while the reverse trend can be observed for the series with the alkali atom over the six-membered ring, where doping the lighter Li atom can achieve the larger β0 value. These fascinating findings will be advantageous for promoting the potential applications of the inorganic AlN-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical (NLO) materials.

Beryllium-cyclobutadiene multidecker inverse sandwiches: electronic structure and second-hyperpolarizability

Beryllium forms stable sandwich and inverse sandwich complexes with the cyclobutadiene molecule. Two types of multidecker complexes are designed. Multidecker inverse sandwiches are found to be thermally more stable than the corresponding sandwich complexes. The average distance between two consecutive metals and the two consecutive cyclobutadiene rings increase gradually on increasing size of the chosen inverse sandwich complexes. The density functional theory functionals B3LYP, BHHLYP, BLYP, M06, CAM-B3LYP, and B2PLYP in conjunction with the 6-311++G (d, p) basis set have been employed for calculating the third-order electric response properties of the chosen beryllium-cyclobutadiene complexes and the results obtained for each functional are found to have a consistent trend. Compared to the normal sandwich compounds the second-hyperpolarizability of inverse sandwiches is predicated to be larger, which fairly correlates with the extent of ground-state polarization. The significant enhancement of cubic polarizability of higher-order multidecker inverse sandwiches arises from the strong coupling between the ground and the low lying charge transfer excited states. The rather strong enhancement of second-hyperpolarizability on increasing size of the beryllium-multidecker inverse sandwiches may provide a new route to design efficient nonlinear optical materials.