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

A three orders of magnitude increase in nonlinear optical response by external electric field on Cryptand[2.2.2] (C222) based alkaline earthides

A new series of alkaline earthides based on Cryptand [2.2.2] (C222) containing nine complexes is designed by carefully placing alkali metals and alkaline earth metals inside and outside the C222 complexant, respectively i.e., M1(C222)M2 (M1 = Li, Na, K; M2 = Be, Mg, Ca). The designed complexes are reasonably stable both electronically and thermodynamically, as revealed through their vertical ionization potentials (VIPs) and interaction energies, respectively. Moreover, the true alkaline earthide nature of the complexes is confirmed through NBO and FMO analyses showing the negative charges and HOMOs over the alkaline earth metals, respectively. The further validity of true earthide characteristic is represented graphically by the spectra of partial density of states (PDOS). HOMO-LUMO gaps of the compounds are also very small (from 2.23 to 2.83 eV) when compared with pure cage's (C222) H-L gap i.e., 5.63 eV. All these features award these complexes with very small values of transition energies (ΔE) ranging from 0.68 to 2.06 eV ultimately resulting in remarkably high hyperpolarizability values up to 2.7 × 10⁵ au (for Na⁺(C222)Mg^-). Furthermore, applying external electric field (EEF) on the complexes enhances hyperpolarizability further. A remarkable increase of 1000 folds has been seen when hyperpolarizability of K⁺(C222)Ca^- is calculated after EEF application i.e., from 8.79 × 10⁴ au to 2.48 × 10⁷ au; when subjected to 0.001 au external electric field.

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.

Superalkali-doped borazine and lithiated borazine complexes: diffuse excess electron and large first-hyperpolarizability

A number of superalkali (M₃O / M₃S; M = Li, Na, K)-doped borazine and hexalithio borazine complexes are considered for the theoretical study of their electronic structure and quadratic polarizability. Electron-rich O/S atom of superalkali species remains very close to one boron atom of the ring through non-covalent interaction. The first-hyperpolarizability increases rather significantly upon superalkali doping. The chosen complexes possess diffuse excess electron which is located on the superpalkali moiety of borazine complexes and at the ring site of lithiated borazines. First-hyperpolarizability of M₃O(S)@B₃N₃Li₆ complexes are significantly larger than that of the corresponding M₃O(S)@B₃N₃H₆ complexes. The magnitude of first-hyperpolarizability of Li₃S@B₃N₃Li₆ is larger than that of Li₃S@B₃N₃H₆ by about three orders of magnitude.

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.

Designing a new class of excess electron compounds with unique electronic structures and extremely large non-linear optical responses

New Ca⁺-1-M′⁻ (M′ = Li, Na, and K) compounds with typical alkalide features and electride-like characteristics have been obtained.

A nonlinear optical switch induced by an external electric field: inorganic alkaline–earth alkalide

Exploring a new type of nonlinear optical switch molecule with excess electron character is extremely important for promoting the application of excess electron compounds in the nonlinear optical (NLO) field. Here, we report external electric field (EEF) induced second-order NLO switch molecules of inorganic alkaline–earth alkalides, M(NH₃)₆Na₂ (M = Mg or Ca). The centrosymmetric structure of M(NH₃)₆Na₂ is destroyed in the presence of an EEF, and then a long-range charge transfer process occurs. It has been found that excess electrons are gradually transferred from one Na atom to the other Na atom through the inorganic metal cluster M(NH₃)₆. Finally, the excess electrons are completely located on one of the two Na atoms. In particular, the electronic contribution of the static first hyperpolarizability (β^e₀) for M(NH₃)₆Na₂ exhibits a large significant difference when the EEF is switched on. The β^e₀ value of M(NH₃)₆Na₂ is 0 when EEF = 0, while the peak β^e₀ values are 5.95 × 10⁶ (a.u.) for Mg(NH₃)₆Na₂ (EEF = 58 × 10⁻⁴ (a.u.)) and 1.83 × 10⁷ (a.u.) for Ca(NH₃)₆Na₂ (EEF = 53 × 10⁻⁴ (a.u.)). This work demonstrates that the compounds M(NH₃)₆Na₂ can serve as potential candidates for NLO switches.

The inorganic alkaline–earth alkalide M(NH₃)₆Na₂ (M = Mg or Ca) can serve as a potential candidate for a nonlinear optical switch.

Electronic and Vibrational Hyperpolarizabilities of Lithium Substituted (Aza)benzenes and (Aza)naphthalenes

In this article we report results for the electronic and vibrational hyperpolarizabilities of ten molecules: Li@benzene, Li@pyridine, Li@pyrimidine, and Li@pyrazine; Li@naphthalene, Li@quinoline, Li@isoquinoline, Li@cinnolin, Li@quinazoline, and Li@quinoxaline. An electron correlation study shows that second-order many-body perturbation theory and density functional theory with CAM-B3LYP and M05-2X functionals give results for the electronic hyperpolarizabilities in good agreement with coupled cluster with single and doubles reference values. Static and dynamic vibrational corrections were computed through the perturbation theoretical method of Bishop and Kirtman and using a variational approach. In general, we obtained notable discrepancies between the results obtained by the two methods for the pure vibrational corrections because of the deficiency of the perturbation method to properly treat low-frequency normal modes present in the investigated systems. However, both methods give results similar to the zero-point vibrational average corrections.

Electronic structure and large second-order non-linear optical property of COT derivatives - a theoretical exploration

A new strategy to design new molecules based on a fused hydrocarbon ring system comprising a COT ring and two 5-membered rings has been proposed for the study of second order NLO properties. The four charge transferring groups -NR2 (R = H, Li, Na and K) in conjunction with a sufficient number of electron withdrawing groups lead to significant variation of structural parameters and polarity. The charge transfer characteristics can be strongly modulated by introducing calcium metal atoms at suitable sites. Ca metal atoms end-capping the nitrogen ends of two adjacent -CN groups lead to electride character while a Ca metal atom bonded directly to the COT ring leads to greater charge transfer. The size of the alkali metal atom has been found to have a dramatic effect on the enhancement of first hyperpolarizability. The most significant electronic asymmetry induced by the larger potassium metal atom strongly enhances the magnitude of first hyperpolarizability. The variation of first hyperpolarizability has been satisfactorily explained in terms of TD-CAMB3LYP calculated spectroscopic parameters in light of the two-state model.

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.

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.

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.

All-metal electride molecules CuAg@Ca7M (M = Be, Mg, and Ca) with multi-excess electrons and all-metal polyanions: molecular structures and bonding modes as well as large infrared nonlinear optical responses

All-metal electride molecules, CuAg@Ca₇M (M = Be, Mg and Ca), have been designed and researched in theory for the first time.

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.

Theoretical study on alkali-metal doped N3H3 complexes: an in-depth understanding of the origin of electride and alkalide and their large nonlinear optical properties

By doping the model complexant N3H3 with one or two lithium atoms, the geometrical and electronic structures as well as static electric properties of the resulting Li(N3H3), (N3H3)Li' and Li(N3H3)Li' complexes can be explored using the B3LYP, BHandHLYP, CAM-B3LYP and MP2 methods. All three complexes, especially Li(N3H3), were found to have large first hyperpolarizabilities (β 0). Meanwhile, Li(N3H3) and Li(N3H3)Li' exhibited electride and alkalide characteristics, respectively. The dependance of electric properties of alkalide Li(N3H3)Li' on the alkali atoms involved and the complexant layer number were revealed by investigating the related M(N3H3)Li' and Li(N3H3)M' (M = Na and K), and Li(N3H3) n Li' (n = 2, 3) systems. Note that the β 0 value of alkalide M(N3H3)M' increased not only with the increasing atomic number of the M'(-) anion but also with that of the M(+) cation, which differs from previously reported cases. In addition, the electric properties of the Li(N3H3)Li' alkalide were enhanced by increasing the complexant layers. However, it was found that both the complexant-complexant and the complexant-Li' interactions reduced with the addition of N3H3 layers, so no stable structures were found for larger Li(N3H3) n Li' complexes. Graphical Abstract Geometrical and electronic structures as well as static electric properties of Li(N3H3), (N3H3)Li' and Li(N3H3)Li' complexes were explored using B3LYP, BHandHLYP, CAM-B3LYP and MP2 methods.

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.