Inverse Sodium Hydride: Density Functional Theory Study of the Large Nonlinear Optical Properties

Rational Design, Stabilities and Nonlinear Optical Properties of Non-Conventional Transition Metalides; New Entry into Nonlinear Optical Materials

Electronic and nonlinear optical properties of endohedral metallofullerenes are presented. The endohedral metallofullerenes contain transition metal encapsulated in inorganic fullerenes X₁₂Y₁₂ (X = B, Al & Y = N, P). The endohedral metallofullerenes (endo-TM@X₁₂Y₁₂) possess quite interesting geometric and electronic properties, which are the function of the nature of the atom and the size of fullerene. NBO charge and frontier molecular orbital analyses reveal that the transition metal encapsulated Al₁₂N₁₂ fullerenes (endo-TM@Al₁₂N₁₂) are true metalides when the transition metals are Ni, Cu and Zn. Endo-Cr@Al₁₂N₁₂ and endo-Co@Al₁₂N₁₂ are at the borderline between metalides and electrides with predominantly electride characteristics. The other members of the series are excess electron systems, which offer interesting electronic and nonlinear optical properties. The diversity of nature possessed by endo-TM@Al₁₂N₁₂ is not prevalent for other fullerenes. Endo-TM@Al₁₂P₁₂ are true metalides when the transition metals are (Cr-Zn). HOMO-LUMO gaps (E_H-L) are reduced significantly for these endohedral metallofullerenes, with a maximum percent decrease in E_H-L of up to 70%. Many complexes show odd-even oscillating behavior for E_H-L and dipole moments. Odd electron species contain large dipole moments and small E_H-L, whereas even electron systems have the opposite behavior. Despite the decrease in E_H-L, these systems show high kinetic and thermodynamic stabilities. The encapsulation of transition metals is a highly exergonic process. These endo-TM@X₁₂Y₁₂ possess remarkable nonlinear optical response in which the first hyperpolarizability reaches up to 2.79 × 10⁵ au for endo-V@Al₁₂N₁₂. This study helps in the comparative analysis of the potential nonlinear optical responses of electrides, metalides and other excess electron systems. In general, the potential nonlinear optical response of electrides is higher than metalides but lower than those of simple excess electron compounds. The higher non-linear optical response and interesting electronic characteristics of endo-TM@Al₁₂N₁₂ complexes may be promising contenders for potential NLO applications.

Inorganic electrides of alkali metal doped Zn12O12 nanocage with excellent nonlinear optical response

Finding new materials with exceptionally large nonlinear optical response is an interesting and challenging avenue for scientific research. Here, we report the alkali metal doped Zn₁₂O₁₂ nanocages as inorganic electrides with excellent nonlinear optical response. Density functional theory calculations have been performed for geometric, electronic and nonlinear optical response of exo- and endohedrally alkali metal doped Zn₁₂O₁₂ nanoclusters. For exohedral doping, all different possible doping sites are considered for decoration of alkali metal on the nanocage. The electride nature of the complexes is highly dependent on the position of alkali metal doping. All exohedral complexes except for alkali metal doping on six membered ring (r₆) are electride in nature, as revealed from frontier molecular orbital analysis. Interaction energies reveal that all doped nanoclusters except endo-K@Zn₁₂O₁₂ are thermodynamically stable. The exothermic encapsulation of alkali metals in Zn₁₂O₁₂ nanocages is in marked contradiction with other inorganic fullerenes where encapsulation is an endothermic process. The barriers for boundary crossing are also evaluated in order study the interconversion of exo- and endohedral complexes. Doping of alkali metal significantly influences the properties of nanocages. HOMO-LUMO (H-L) gap is reduced significantly whereas hyperpolarizability is increased several orders of magnitude. The NLO response of exohedrally doped complexes is higher than the corresponding endohedral complexes, which is in mark contradiction with the behavior of phosphide or nitride nanocages. The highest first hyperpolarizability of 1.0 × 10⁵ au is calculated for K@r₆-Zn₁₂O₁₂ complex. Third order NLO response of these complexes is calculated and compared with the best systems reported in the literature at the same level of theory.

DFT and QTAIM studies on the reduction of carbon monoxide by superalkalis

Carbon monoxide (CO) is a toxic gas molecule with no positive electron affinity, which makes it difficult to reduce it into CO¯. In this work, we perform density functional theory (DFT) and quantum theory of atoms in molecule (QTAIM) based studies on the interaction of CO molecule with superalkali (SA) clusters. Our findings suggest that this interaction results in SA(CO) complexes, which are stabilized by purely ionic as well as partially covalent bonds although their binding energy decreases with the increase in the size of SA clusters. In these ionic complexes, the electron is transferred from the SA cluster to the CO molecule. This suggests the single-electron reduction of the CO molecule by interacting with superalkalis. This work may offer some novel insights into the detection and reduction of stable CO molecule and related systems.

Design of the NnH3n+1+ series of “non-metallic” superalkali cations

A new series of non-metallic superalkali cations, N_nH_3n+1⁺ by using ammonium (NH₄⁺) cations, possessing vertical electron affinity (EA_v), 4.39 eV for n = 1 to 2.39 eV for n = 5 has been proposed. This series can be continued for obtaining new superalkali cations, for instance N₉H₂₈⁺ with an EA_v of 1.84 eV. The EA_v of N_nH_3n+1⁺ cations is governed by the electron localization on the central N-atom. The EA_v of N_nH_3n+1⁺ cations decays exponentially with an increase in n.

Transition metal doping: a new and effective approach for remarkably high nonlinear optical response in aluminum nitride nanocages

NLO response of early transition metal (Sc)-doped aluminum nitride nanocages is comparable to those of their alkali-metal-doped analogues.

Second-order NLO responses of two-cavity inorganic electrides Lin@B20H26 (n = 1, 2): evolutions with increasing excess electron number and various B-B connection sites of B20H26

Confining excess electrons in a specific space is an effective strategy to design nonlinear optical (NLO) molecules. The complexants with excess electrons are usually organic compounds, but these compounds are thermally unstable and thus hardly meet the processing requirements of NLO materials. To obtain better thermostability and NLO response molecules, in this work, inorganic compounds of B₂₀H₂₆ isomers containing two cavities were proposed. With the two included cavities, B₂₀H₂₆ can be doped by one or two Li atoms to form electrides of Li@B₂₀H₂₆ and Li₂@B₂₀H₂₆. These electrides show larger NLO responses, with respect to the corresponding undoped complexant of B₂₀H₂₆. Particularly, Li₂@B₂₀H₂₆ has the largest β₀ value of 108 846 a.u. (MP2/6-31+G(d) level) that is 850 times as large as that of corresponding B₂₀H₂₆. Moreover, the change of β₀ values with excess electron number is remarkable for two of the isomers, and differences between the β₀ values among those isomers are also significant owing to various B-B connection sites between the two cavities. Therefore, the present inorganic electrides have not only better performance due to the magnitude of their β₀ values but also better behavior on the molecular-level modulation of NLO.

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room-Temperature-Stable Organic Electride

Excess-electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀ ). A room-temperature-stable, excess-electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005, 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶  au, which revealed its potential as a high-performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶  au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro-group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro-substituted Na@(TriPip222) compounds.

Theoretical study of the non linear optical properties of alkali metal (Li, Na, K) doped aluminum nitride nanocages

The effect of alkali metal (Li, Na, and K) doping in aluminum nitride (Al₁₂N₁₂) nanocages is studied through density functional theory (DFT) methods.

A theoretical study on novel alkaline earth-based excess electron compounds: unique alkalides with considerable nonlinear optical responses

On the basis of stable alkaline earth metal ammines, a series of M(NH3)6NaCl and M(NH3)6Na2 (M = Mg and Ca) excess electron compounds were theoretically constructed and studied by using the density functional theory. The electride or alkalide characteristics of these compounds are verified by their electronic structures, HOMOs, and small VIE values. It is worth noting that the M(NH3)6Na2 alkalides have novel electronic structures that contain double alkali metal anions. As expected, all the noncentrosymmetric M(NH3)6NaCl and M(NH3)6Na2 compounds possess considerable first hyperpolarizabilities (β0) up to 123 050 au, which can be attributed to low excitation energies (ΔEs) and large oscillator strength (f0) of their crucial excited states. In addition, results reveal that the Ca(NH3)6-based species with lower ΔEs and larger transition moments (Δμ) show larger β0 values compared with the corresponding Mg(NH3)6-based ones with similar geometries. This study may be significant in terms of designing excess electron compounds of new-type, especially alkalides with multiple alkali metal anions.

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.

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.

Design of Lewis acid-base complex: enhancing the stability and first hyperpolarizability of large excess electron compound

In the present paper, a new type of Lewis acid-base complex BX3...Li@Calix[4]pyrrole (X = H and F) was designed and assembled based on electride molecule Li@calix[4]pyrrole (as a Lewis base) and the electron deficient molecule BX3 (as a Lewis acid) by employing quantum mechanical calculation. The new Lewis acid-base complex offers an interesting push-excess electron-pull (P-e-P) framework to enhance the stability and nonlinear optical (NLO) response. To measure the nonlinear optical response, static first hyperpolarizabilities (β 0) are exhibited. Significantly, point-face assembled Lewis acid-base complex BF3...Li@Calix[4]pyrrole (II) has considerable first hyperpolarizabilities (β 0) value (1.4  ×  106 a.u.), which is about 117 times larger than reported 11,721 a.u. of electride Li@Calix[4]pyrrole. Further investigations show that, in BX3...Li@Calix[4]pyrrole with P-e-P framework, a strong charge-transfer transition from the ground state to the excited state contributes to the enhancement of first hyperpolarizability. Theory calculation of enthalpies of reaction (ΔrH0) at 298 K demonstrates that it is feasible to synthetize the complexes BX3...Li@Calix[4]pyrrole. In addition, compared with Li@Calix[4]pyrrole, the vertical ionization potential (VIP) and HOMO-LUMO gap of BX3...Li@Calix[4]pyrrole have obviously increased, due to the introduction of the Lewis acid molecule BX3. The novel Lewis acid-base NLO complex possesses not only a large nonlinear optical response but also higher stability.

Structural Characteristics and Large Non-Linear Optical Responses of New Alkaline Earth-Based Alkalides

A series of M2+(H5Aza222)–M′– (M = Be, Mg, Ca; M′ = Li, Na, K) alkalides that contain alkaline earth metal cations complexed by the H5Aza222– cage have been investigated using the CAM-B3LYP method. These alkaline earth-based alkalides not only present unusual structural features but also exhibit extraordinarily large static first hyperpolarizabilities (β0) up to 1.98 × 105 au. By comparing the β0 values among alkalides with various complexants, the Aza222 cage is found to be preferable to the previously investigated calix[4]pyrrole and n6adamanzane (n = 2, 3) complexants in enhancing the first hyperpolarizabilities of alkalides. In addition, the relationships between the β0 values of M2+(H5Aza222)–M′– and the atomic number of the M′– anion, the atomic number of the M2+ cation, and the M–M′ distance are explored.

An effective method for accurate prediction of the first hyperpolarizability of alkalides

The proper theoretical calculation method for nonlinear optical (NLO) properties is a key factor to design the excellent NLO materials. Yet it is a difficult task to obatin the accurate NLO property of large scale molecule. In present work, an effective intelligent computing method, as called extreme learning machine-neural network (ELM-NN), is proposed to predict accurately the first hyperpolarizability (β(0)) of alkalides from low-accuracy first hyperpolarizability. Compared with neural network (NN) and genetic algorithm neural network (GANN), the root-mean-square deviations of the predicted values obtained by ELM-NN, GANN, and NN with their MP2 counterpart are 0.02, 0.08, and 0.17 a.u., respectively. It suggests that the predicted values obtained by ELM-NN are more accurate than those calculated by NN and GANN methods. Another excellent point of ELM-NN is the ability to obtain the high accuracy level calculated values with less computing cost. Experimental results show that the computing time of MP2 is 2.4-4 times of the computing time of ELM-NN. Thus, the proposed method is a potentially powerful tool in computational chemistry, and it may predict β(0) of the large scale molecules, which is difficult to obtain by high-accuracy theoretical method due to dramatic increasing computational cost.

Quantum mechanical design and structure of the Li@B10H14 basket with a remarkably enhanced electro-optical response

An innovative type of lithium decahydroborate (Li@B(10)H(14)) complex with a basketlike complexant of decaborane (B(10)H(14)) has been designed using quantum mechanical methods. As Li atom binds in a handle fashion to terminal electrophilic boron atoms of the decaborane basket, its NBO charge q (Li) is found to be 0.876, close to +1. This shows that the Li atom has been ionized to form a cation and an anion at the open end of B(10)H(14). The most fascinating feature of this Li doping is its loosely bound valence electron, which has been pulled into the cavity of the B(10)H(14) basket and become diffuse by the electron-deficient morphological features of the open end of the B(10)H(14) basket. Strikingly, the first hyperpolarizability (beta(0)) of Li@B(10)H(14) is about 340 times larger than that of B(10)H(14), computed to be 23,075 au (199 x 10(-30) esu) and 68 au, respectively. Besides this, the intercalation of the Li atom to the B(10)H(14) basket brings some distinctive changes in its Raman, (11)B NMR, and UV-vis spectra along with its other electronic properties that might be used by the experimentalists to identify this novel kind of Li@B(10)H(14) complex with a large electro-optical response. This study may evoke the possibility to explore a new thriving area, i.e., alkali metal-boranes for NLO application.

Theoretical study on nonlinear optical properties of the Li(+)[calix[4]pyrrole]Li(-)dimer, trimer and its polymer with diffuse excess electrons

The static (hyper)polarizabilities of the dimer and trimer with diffuse excess electrons, [Li(+)[calix[4]pyrrole]Li(-)](n), are firstly investigated by the DFT(B3LYP) method in detail. For the dimer and trimer, a Li atom inside each calix[4]pyrrole unit is ionized to form a diffuse excess electron. The results show that the dimer and trimer containing two and three excess electrons, respectively, have very large first hyperpolarizablities as 2.3 x 10(4) and 4.0 x 10(4) au, which are 30 and 40 times larger than that of the corresponding [calix[4]pyrrole](n) (n = 2, 3) without Li atom. Also, beta values of dimer and trimer are twice and four times as large as that of monomer containing one excess electron. Obviously, not only excess electron but also the number of excess electron plays an important role in increasing the first hyperpolarizability. Moreover, the (hyper)polarizabilities of the [Li(+)[calix[4]pyrrole]Li(-)](n) polymer are investigated at ab initio level by using the elongation finite-field (elongation FF) method. All the oligomers of the [Li(+)[calix[4]pyrrole]Li(-)](n) with many excess electrons exhibit very large first hyperpolarizability and large second hyperpolarizability. The present investigation shows that by introducing several and more excess electrons into the nonlinear optical (NLO) materials will be an important strategy for improving their NLO properties, which will be helpful for design of NLO materials.