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 106 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 sit...

Role of Excess Electrons in Nonlinear Optical Response.

Electrides are ionic substances in which the anionic species is stoichiometrically replaced with localized electrons that reside within crystal voids. Originally discovered in 1983, the past decade has seen a sharp rise in the number of known electride materials, most notably the isolation of the first air- and water-stable electride. As the presence of localized interstitial electrons cannot be directly detected experimentally, researchers have turned to density-functional theory (DFT) to discover new electrides. In this work, we survey eight common theoretical descriptors of electrides for their efficacy in identifying these materials. Illustrative examples are presented for all classes of electrides: organic, inorganic, 2D, elemental, and molecular electrides. In general, density-based descriptors such as the electron localization function (ELF) and localized-orbital locator (LOL) are shown to be the most consistently reliable. Limitations of DFT treatments of electrides are also discussed.

Theoretical Descriptors of Electrides.

Focusing on the interesting new concept of all-metal electride, centrosymmetric molecules e–+M2+(Ni@Pb12)2–M2++e– (M = Be, Mg, and Ca) with two anionic excess electrons located at the opposite ends of the molecule are obtained theoretically. These novel molecular all-metal electrides can act as infrared (IR) nonlinear optical (NLO) switches. Whereas the external electric field (F) hardly changes the molecular structure of the all-metal electrides, it seriously deforms their excess electron orbitals and average static first hyperpolarizabilities (β0e(F)). For e–+Ca2+(Ni@Pb12)2–Ca2++e–, a small external electric field F = 8 × 10–4 au (0.04 V/A) drives a long-range excess electron transfer from one end of the molecule through the middle all-metal anion cage (Ni@Pb12)2– to the other end. This long-range electron transfer is shown by a prominent change of excess electron orbital from double lobes to single lobe, which forms an excess electron lone pair and electronic structure Ca2+(Ni@Pb12)2–Ca2++2e–. Therefor...

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Efficient External Electric Field Manipulated Nonlinear Optical Switches of All-Metal Electride Molecules with Infrared Transparency: Nonbonding Electron Transfer Forms an Excess Electron Lone Pair

Alkaline-earthide: A new class of excess electron compounds Li-C6H6F6-M (M = Be, Mg and Ca) with extremely large nonlinear optical responses

Strategies to design alkalides with excellent NLO responses are well established in the literature. On the contrary, alkaline earthides are quite scarce mainly because of low electron affinities of alkaline earth elements. Herein, we report two series of alkaline earthides M+(26Adz)M- (M+ = Li and Na; M- = Be, Mg and Ca), based on 26Adz as a complexant and alkali metals as sources of electrons for alkaline earth metals. These complexes are characterized as alkaline earthides by negatives charges on alkaline earth metals (NBO analysis) and presence of polarized-p HOMO orbital on alkaline earth meals. Presence of HOMO on alkaline earth metals is also confirmed through PDOS spectra. HOMO-LUMO gaps of the compounds are reduced significantly and range from 2.15 to 2.85 eV as compared to 5.17 eV HOMO-LUMO gap of pure cage. Besides H-L gaps, these compounds possess remarkably large nonlinear optical responses with the first hyperpolarizabilities (βo) up to 1.0 × 106 au for Na +(26Adz)Mg-. The nonlinear optical response of alkaline earthides is superior than those of alkalides and it is attributed to the presence of excess electron on alkaline earth metals in polarized p-orbitals. The hyperpolarizabilities are rationalized through two level method. The projection of hyperpolarizability on the dipole moment, βvec for both series are comparable to βo (static first hyperpolarizability) which indicate that the charge transfer is parallel to the molecular dipole moments. The alkaline earthides possess very small transition energies (ΔE) ranging from 0.6 to 2.4 eV. The UV–Vis analysis reveals that these alkaline earthides are transparent in ultraviolet region of the electromagnetic spectrum. The designed alkaline earthides are potential candidates for UV-transparent high performance NLO materials for applications in optical communication, optical computing and other laser devices. Present results will certainly inspire experimentalists and theoreticians to design and synthesize other alkaline earthides-based NLO materials with even better performance.

Hui Li

Papers

The structure and large nonlinear optical properties of a novel octupolar electride Li@36Adz