Role of Excess Electrons in Nonlinear Optical Response - The Journal

Jan 28, 2015 - Hong-Liang Xu received his B.S. degree from Inner Mongolia University in 2002 and then his Ph.D. in chemistry under the supervision of ...
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Role of Excess Electrons in Nonlinear Optical Response Rong-Lin Zhong,† Hong-Liang Xu,*,† Zhi-Ru Li,*,‡ and Zhong-Min Su*,† †

Institute of Functional Material Chemistry, Department of Chemistry, National & Local United Engineering Lab for Power Battery, Northeast Normal University, Changchun 130024, Jilin, People’s Republic of China ‡ State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ABSTRACT: 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 sites of complexants but also the number and spin state of excess electrons in these unusual NLO molecules.

T

he last few decades have witnessed progress in the design and synthesis of nonlinear optical (NLO) materials for their wide applications in optical communication, optical computing, dynamic image processing, and other laser devices.1−9 Primarily because of their role in various potential applications, the first or second hyperpolarizabilities (β or γ) of molecules have recently received a lot of attention especially for organic NLO molecules because they have exhibited many attractive features, such as very large and ultrafast response and ready processability.2,4 Up to now, great efforts have been devoted to enhance the hyperpolarizabilities of organic molecules, including use of molecules with abundant πelectrons,2 introduction of donor/acceptor groups10 and consideration of interesting molecules with diradical character,11 and so forth. For example, Nakano and colleagues have shown that switching on an external electric field along the electron correlation direction produces a giant enhancement of the γ in a polyaromatic diradicaloid with intermediate diradical character.11 Champagne et al. reported that the recognition of cations by molecular switches can give rise to large contrasts of β.12 It is worthy to note that the delocalization characteristic of π-electrons is the physical origin of the large hyperpolarizabilities of organic molecules.10 Significantly, we revealed that the loosely bound excess electron plays a crucial role in the large static first hyperpolarizability (β0) of solvated electrons system in 2004.13,14 Concretely, the β0 of the molecular cluster dipole-bound excess electron system (H2O)3{e} is dramatically larger (about 106 times) than that of corresponding molecular cluster (H2O)3. It was proposed that the compounds with dispersive excess electrons might be considered as candidates, which opens a new perspective on proposing strategies to design novel NLO molecular materials. © XXXX American Chemical Society

The dramatic effect of an excess electron on the extraordinary β0 of a cluster anion opens new perspectives on the design and preparation of novel NLO molecular materials. The investigations on compounds with excess electrons could be tracked to the work of Sir Humphry Davy.15−17 He reacted potassium with dry and gaseous ammonia, producing the potassium−ammonia compounds with beautiful colors about 200 years ago. After that, many scientists followed the investigations on the interactions between ammonia and alkali atoms, and especially liquid ammonia was obtained.15,18 In 1916, Gibson and Argo were aware that the free electron in liquid ammonia is the crucial factor leading to the blue color and other fascinating properties of an alkali ammonia solution.19 This is the first time that the term “solvated electrons” was invoked.15 Inspired by the solvated electrons, Dye’s group synthesized the first compounds with excess electrons, Cs+(18-crown-6)2e− in 1983, which were designated as an electride.20,21 Generally, electrides are composed of trapped electrons and alkali ions intercalated within organic complexant cages. In 2003, Matsuishi et al. reported the synthesis and properties of a single-crystal, nearly metallic inorganic electride 12CaO·7Al2O that is thermally stable and Received: December 7, 2014 Accepted: January 28, 2015

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can be exposed to air.22 The above introductions show that the compounds with excess electrons are a kind of fascinating chemical substance in which the excess electrons occupy a diffuse Rydberg orbital and are loosely bound.23,24 Owing to these fascinating features of excess electrons, corresponding compounds have shown broadly potential applications in catalysis25 and optical26,27 and conductive materials.28,29 It is worthy to note that the excess electron with dispersive features also brings the large NLO response. Naturally, electride and alikalide molecules with an excess electron are emphatically focused as a new kind of NLO molecules. In this Perspective, recent investigations regarding the electric (hyper)polarizability of solvated electrons, electrides, and alikalide molecules with excess electrons are highlighted. First of all, a remarkable first hyperpolarizability of the solvated electron system (molecular cluster anion) was induced by the loosely bound excess electrons, which reveals a new idea to design molecules with large NLO response. However, these solvent molecular clusters are weakly bonded by noncovalent interactions that limit their further applications. Therefore, our particular emphasis has been placed on recent quantum chemical investigations on electride and alikalide molecules with excess electrons for their potential applications in NLO fields. For the calculation of the first hyperpolarizability, selecting appropriate computational methods and basis sets is particularly important. The sophisticated quadratic configuration interaction including single and double substitutions (QCISD) method30 is suitable to consider the intermolecular interactions of dipole-bound electron clusters, but it is very costly for relatively larger systems. Therefore, the second-order Møller− Plesset perturbation (MP2) method31 was widely used for some electride molecules because it preferably solves the dynamical correlation problem with economic cost. Recently, a few new density functional theory (DFT) methods were recommended,32,33 which expand corresponding calculations to large molecules and nanosystems with excess electrons. We have tested these functionals, and results show that M06-2X34 and CAM-B3LYP35,36 are suitable to reproduce the tendency of post-SCF methods (such as MP2). For other properties of electride and alkalide molecules, concrete efforts should be devoted to study the electronic second hyperpolarizability and vibrational NLO properties along with Kirtman and coworkers.37 Maroulis pointed out that the studies of the basis set effect show the importance of embedding diffuse basis functions in accurately calculating the hyperpolarizabilities.38−40 On the electronic properties of electride crystals, the plane waves/pseudopotentials (PW/PS) approaches are used but still unsatisfactory.41 Up to now, despite a lot of progress made in applying quantum chemistry methods, developing new methods to simulate such special electronic structure remains a big challenge because of the intrinsic deficiency of many existing theoretical methods for wide applications. Therefore, it is worthy to develop new strategies to reveal the relationship between the NLO properties and structures of electrides, which might depict a more clear physical picture for us. Solvated electrons in molecular cluster anions have been intriguing topics.42−46 The solvated electron plays a prominent role in many important processes of physics, chemistry, and biochemistry.47,48 In radiobiological physics and chemistry, the essentiality of the solvated electron has been revealed.49 The researchers in the quantum chemistry and molecular dynamics fields have found that an excess electron bound on either the

cluster interior or the cluster surface exhibits different excess electron bound states including interior, surface, and networkpermeating states.50−52 The bound state of the excess electron influences the electronically excited-state relaxation dynamics, binding energies, as well as electronic and molecular spectra.53 In 2004, we found that the loosely bound excess electron is the prominent factor, leading to the large first hyperpolarizability of a hydrogen fluoride trimer cluster anion, (FH)2{e}(HF), with an excess electron of the partial interior state,13 and a noncentrosymmetrical water trimer cluster anion, (H2O)3{e}, with a surface state excess electron.14 In the two theoretical papers, the ab initio calculations indicate that the diffuse character of the orbital occupied by the excess electron leads to an extraordinary increase of the NLO response as compared with the corresponding neutral clusters. For example, the static first hyperpolarizabilities (β0) of the (H2O)3{e} anion is 1.72 × 107 au, which is dramatically larger than that (35 and 46 au) of (H2O)3 and (H2O)3F−. It is the first unambiguous demonstration of the dramatic effect (about 106 times) of an excess electron on the β0. Relatively, Fonseca et al. have studied the dipole polarizability of water with an interior state excess electron, (H2O)7−, by Monte Carlo simulation and quantum chemistry calculations.54 It is found that the bound excess electron contributes with 274 au to the total dipole polarizability of 345 au. The dramatic effect of the excess electron on the extraordinary β0 of the cluster anion opens new perspectives on the design and preparation of novel NLO molecular materials and sets up a new NLO material field for neutral electride and alkalide molecules with an excess electron.

Doping of alkali metal atoms is an effective approach to achieve compounds with excess electrons. On the basis of work on the solvated electron systems, further investigations of molecular materials were focused on the compounds with loosely bound excess electrons. Inspired by the Dye group’s work on electrides, we proposed that doping of alkali metal atoms is an effective approach to achieve compounds with excess electrons. In 2005, two categories of Lidoped (HCN)n (n = 1−3) cluster models with electride characteristic were designed as prototypes, in which the 2s valence electron of the Li atom became a loosely bound excess electron because of the interaction between the Li atom and the (HCN)n part.55 This structural characteristic (alkali cation and excess electron) is similar to that of the electrides in nature. It is worthy to note that the two clusters have very low lying excited states to bring a large β0 value, which is obviously dominated by the position of the Li atom. Interestingly, in Li···(HCN)n, the 2s electron is pulled toward the H atom, and the pulled excess electron is localized between the H atom and the Li atom, which results in a relatively stable electride molecule with Li+ and a Rydberg-like diffuse excess electron. However, in (HCN)n···Li, the 2s electron of the Li atom is pushed out by the N atom, and the pushed excess electron is localized near the Li atom at the opposite side of the (HCN)n part. Results show that the β0 (1.53 × 104 au) of HCN···Li with the pushed excess electron is larger than that (3.40 × 103 au) of Li···HCN with a pulled excess electron. Subsequently, the complexants with pushed electron characteristics were considered to form the excess electron 613

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Figure 1. Structures, first hyperpolarizabilities (β0), and excess electron bound states of calix[4]pyrrole without an excess electron, electride molecule Li+(calix[4]pyrrole)e− with a pushed excess electron, and alkalide molecule Li+(calix[4]pyrrole)K− with an alkali anion.

Table 1. Complexant shape, Bound State of an Excess Electron, First Hyperpolarizability (β0, au), and Vertical Detachment Energies (VDEs) of Anions/VIPs (eV) of Neutral Compounds with Excess Electrons molecule (FH)2{e}(HF) (H2O)3{e} Li+(calix[4]pyrrole)e− Li−[9]aneN3 Li@36adz Li+(calix[4]pyrrole)K− (K+@n6adz)K− Li−H3C4N2···Na2 Li@B10H14 Li@F4B10H10

shape spoon V cup petal cage cup cage hexagon plane basket basket

bound state

β0

partial interior partial interior pushed pushed pushed* K anion K anion pushed pulled pulled

× × × × × × × × × ×

9.20 1.72 7.33 5.23 2.07 3.60 3.20 1.41 2.31 3.23

6

10 107 103 104 105 104 105 106 104 104

VDE/VIPa

ref

0.45 0.15 4.12 2.88 1.84

13 14 56 57 60 64 68 69 70 71

6.12 6.78

a

VDE and VIP represent the detachment and ionization energies of the electron in chemical substances. According to references, the anions clusters are described by VDE, while the neutral compounds are usually described by VIP.

because the flexibility and space of the complexants increases with the increase of petal number. On the other hand, the alkalide molecules Li(NH3)nNa (n = 1−4) were comparably studied.65,67 A prominent coordination number dependence of the β0 value found that the β0 increases with n value. Interestingly, the β0 value (7.79 × 104) of this smaller inorganic molecule Li(NH3)4Na with high flexibility of the complexant is five times larger than that of the bigger organic molecule Li@ calix[4]pyrrole-Na (1.48 × 104 au). In this context, the β0 is strongly related to the flexibility of the complexants. This work suggests that two important factors should be taken into account to enhance the first hyperpolarizability of alkalide molecules, namely, the coordination number around the cation and the flexibility of the complexant. Subsequently, 18 structures of new organic alkalides (M+@n6adz)M− (M = Li, Na, K; n = 2, 3) with the alkali metal cation M+ lying near the center of the adz cage and the alkali metal anion M− located outside were studied.68 For alkalide molecules, the effect of cage size of the complexant on the β0 is that a majority of the smaller cage complexant corresponds to the larger β0 value. It is worth noting that the β0 values (1.6 × 105−3.2 × 105 au) of M+@n6adz)K− are considerably larger than others, which reveals that the dependence of the alkali atomic number on β0 is significant. In addition, our partners found that the β0 of lithium salt Li−H3C4N2/lithium salt electride H4C4N2···Na269 is 8.59 × 102/1.50 × 104 au. Interestingly, it is found that the β0 of Li−H3C4N2···Na2 is 93 times larger than that of the electride H4C4N2···Na2. Therefore, a combination of lithium salt and electride effects is a new strategy to enhance the first hyperpolarizability of organic molecules. The above results show that the β0 values of compounds with pushed excess electrons are remarkable, while compounds with pulled excess electrons were rarely discussed. It is worthy to

for obtaining potential compounds with relative stability and remarkable NLO response. Organic complexants including calix[4]pyrrole,56 cyclic polyamine,57 cyclacene,58 organic amines,59,60 fluorocarbon,61 and so forth have been designed to interact with alkali metal atoms. For example, in Li@ calix[4]pyrrole, the s valence electron of the Li atom is pushed to form an excess electron.56 In this context, it is better to be expressed as Li+(calix[4]pyrrole)e−, which exhibits an electride character according to Dye’s investigations.16,17 As shown in Figure 1, the β0 of Li+(calix[4]pyrrole)e− is almost 20 times larger than that of calix[4]pyrrole. Therefore, the pushed excess electron from the Li atom plays a pivotal role in the large first hyperpolarizability of Li+(calix[4]pyrrole)e−. By increasing alkali atoms, the electrides may form alkalides, which are chemical compounds with alkali anions.62 The alkali anion is an interesting excess electron bound state, in which the excess electron locates on the alkali atom and envelops the alkali atom (see Figure 1). The β0 values of a lot of designed alkalide molecules have also been reported.63−66 It is worth noting that the β0 of alkalide molecule Li+(calix[4]pyrrole)K− is almost 4 times larger than that of Li@calix[4]pyrrole. Therefore, on the basis of electride molecules, increasing an alkali atom to form the alkalide molecules with alkali anions is a new approach to obtaining compounds with large β0 values. To further enhance the β0 of compounds with pushed excess electrons, we considered strategies including the effect of shape, size, flexibility of the complexants, alkali atomic number, and combination of lithium salt/electride. First, three electride molecules referred to three-petal-shaped Li-[9]aneN3, fourpetal-shaped Li-[12]aneN4, and five-petal-shaped Li-[15]aneN5 were considered as prototype models.57 Results show that the β0 value increases with increasing petal number (n). It reveals an effect of complexant size on the β0 of electrides molecules 614

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fullerene cage and tubiform cyclacenes have been discussed.58,75,76 Correspondingly, our further plan was to propose suitable strategies to enhance the β0 of nanostructure-confined excess electrons.

note that the bounded states of excess electrons are significantly dependent on the features of the complexant, which is an important factor relating to the β0 of corresponding compounds (details are shown in Table 1). In 2009, we were inspired to probe into the research of novel inorganic NLO molecules.70 The pulled electron complexants were utilized to design a new type of electride molecule Li@B10H14 with a large β0.71 The most innovative and diverse feature of this electride molecule is the excess electron bound state, which is diffused and pulled into the cavity of the B10H14 basket with four electron-deficient terminal H atoms and two electron-deficient terminal boron atoms, as shown in Figure 2. Besides, further

Confining excess electrons in a specific space to reduce the dispersivity might be an effective strategy to obtain stable NLO molecules with excess electrons. In 2009, we proposed that perfluorinated fullerene cages confine the excess electrons due to sufficient attractive potentials of the cages.73,74,77 The SOMO and the spin density map of e2@C60F60 shows that the excess electrons are indeed encapsulated inside of the C60F60 cage, which exhibits a considerably large VIP (4.67−4.95 eV), showing their higher electron stability. The VIP of the sphere-shaped e2@C60F60 is greater than that of the capsule-shaped e2@C60F60 (D6h), indicating that the excess electron prefers to reside in the cage with the higher symmetry and the large C60F60 cage is an efficient container of excess electrons. Furthermore, compounds with large β0 for NLO application were designed based on perfluorinated fullerenes.78,79 For example, the alkali metal atoms and superalkali atoms were considered as the source of the electrons and C20F20 as the electron container to construct K+(e@C20F20)− and (K3O)+(e@C20F20)−. As shown in Figure 3, the electron from alkali metal atoms and superalkali atoms

Figure 2. Singly occupied molecular orbital (a) and the mechanism of generating the excess electrons (b) in Li@calix[4]pyrrole with a pushed excess electron and in Li@B10H14 with a pulled excess electron.

investigations indicated that a synergistic effect of conical push and inward pull in fluoro derivative Li@F4B10H10 induces a large β0 and higher vertical ionization potentials (VIPs). In addition, quantum chemical calculations of enthalpies of reaction at 298 K for B10H14 and its lithium/fluoro derivatives highlight the changes in their thermodynamical aspects, which make fluorinated decaborane a better choice for the design of compounds with an excess electron. On the other hand, it is found that the stability of compounds with excess electrons might be characterized by the VIP values. Table 1 shows that cluster anions and molecules with pushed excess electrons have relatively lower VIPs, but molecules with pulled excess electrons have higher VIPs. This shows a relationship between the excess electron stability and the feature of the complexant. As mentioned above, the key to the formation of excess electrons is that alkali atoms interact with cavity complexants. In these compounds, the excess electrons were pushed away from the organic complexants56 (crown ethers and cyclic polyamines) or pulled toward inorganic complexants70 (decahydroborate and its derivatives). These results indicate that confining excess electrons in a specific space to reduce the dispersivity might be an effective strategy to obtain stable NLO molecules with excess electrons. Recently, nanostructures with a cavity are interesting topics because of their unique structural and electronic properties.58,72−74 For example, endohedral fullerenes with alkali atoms trapped inside of the hollow

Figure 3. SOMOs of M+(e@C20F20)− and M3O+(e@C20F20)− (M = Na and K) show the excess electron mainly confined in the C20F20 cage. Reprinted with permission from ref 79. Copyright 2012, RSC.

are transferred to the inner cavity of the C20F20, forming confined excess electrons and large VIP values (6.53 eV). Interestingly, the β0 of (K3O)+(e@C20F20)− is 1.30 × 105 au is obviously larger than that (6.00 × 102 au) of K+(e@C20F20)−. It indicates that the interaction between superalkali atoms and perfluorinated fullerenes is an effective strategy to construct compounds with confined excess electrons and excellent NLO property. To further enhance the β0 of cage-confined excess electron systems, we design (CH2)4 as an unusual σ bridge to construct a new kind of electride molecular salt, e−@C20F19− (CH2)4−NH2···Na+ with an excess electron anion inside of the C20F19 cage. The β0 of e−@C20F19−(CH2)4−NH2···Na+ is 9.50 × 106 au, and the VIP is 4.76 eV, showing that the long σ chain bridge provides assistance for long-range charge transfer. 615

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Table 2. First Hyperpolarizability (β0, au) of Nanostructures with an Excess Electron Compared with Other Reported Nanostructures, Traditional D−π−A Molecules, and Organic Metal Compounds molecule −

e @C20F19−(CH2)4−NH2···Na (K3O)+(e@C20F20)− K+[e@3C8(O)] − 2 L3 with 3Na@9C3 Li@N−BNNT LiCN···Li LiCN···Li@BNNT push−pull polyenes Li−CNT−(NH2)2 pyrazine−Ru compound CNT−NH2

vital contribution

β0

e @cage and σ-bridge e−@cage e−@multicage multi-e− and multicup e−@BNNT Li salt and pushed e− e−@BNNT D−π−A Li salt and π-tube MLCT π-tube

× × × × × × × × × × ×

molecular type +

−

electride electride electride electride nanotube Li salt-electride nanotube organic polymer nanotube metal complex nanotube

9.50 1.30 7.10 2.20 3.40 3.10 1.06 1.70 6.80 3.77 1.32

ref 6

10 105 105 105 104 105 104 105 105 104 105

78 79 80 81 85 86 86 7 8 9 10

B cavity. However, the distribution of the electron in Li@N− BNNT is more diffuse and pyramidal from the B-rich edge to the N-rich edge, which is fascinating compared with Li@B− BNNT. Correspondingly, the β0 of Li@N−BNNT is 3.40 × 104 au, which is obviously larger than 1.35 × 103 au of Li@B− BNNT. Therefore, we found that the diffuse and pyramidal distribution of the excess electron is the key factor to determine the larger first hyperpolarizability, which reveals useful information for scientists to develop new electro-optic applications of BNNTs. On the other hand, BNNTs are widely applied as protective shields for many species because they are electronic insulators with a wide band gap independent of tube chirality and morphology.83 We proposed that BNNT might be a cavitycomplexant to confine the excess electron of a small Li salt electride molecule LiCN···Li expressed as Li+CN−Li+···e−, which is another potential strategy to design compounds with excess electrons at ambient temperature and environment (Figure 4).86 A systematical comparison on the encapsulation

To further enhance the NLO response, different multicage electride molecules with an excess electron were constructed. Interesting effects of the cage unit size, number of cage units, and bridge unit on β0 of novel multicage electrides are revealed.80 In detail, the small C8 cage with an O-bridge exhibits a larger β0. Besides, the β0 increases with increasing cage unit number. Assembling these effects, the constructed multicage electride structure with three small C8 cage units connected by the O-bridge (K+[3C8(O)]−) exhibits a considerable β0 (7.1 × 105 au), which is about 55 times larger than that (1.30 × 104 au) of the single-cage electride molecule Na3O+(e@C20F20)−. On the other hand, by utilizing multidoping alkali atoms on a multiunit complexant, unusual manipulative effects of spin multiplicity and excess electron number on the structure and static first hyperpolarizability (β0) were revealed based on new electride molecules with multiexcess electrons.81 First, our associates showed that β0 increases rapidly with increasing excess electron number accompanying molecular size enhancement. Besides, the β0 (2.27 × 105 au) of the low-spin structure 2 L3 is significantly larger than that (1.1 × 104 au) of the highspin structure 4L3 due to one excess electron spin reversal. The low-spin structure with doubly and singly occupied frontier orbitals is relevant to the low transition energy and complex distribution of electron density, which dramatically enhances β0. Notably, the β0 of modified nanostructures such as CNT− NH210 and Li−CNT−(NH2)28 are about 1.32−6.80 × 105 au. The β0’s of traditional D−π−A organic molecules7 and organic metal compounds9 are on the order of about 104−105 au. Significantly, the β0 (1.30 × 105−9.50 × 106 au) of nanostructure-confined excess electrons exhibits a new strategy to obtain molecules with large molecular hyperpolarizibilities. The fascinating effect on hyperpolarizabilities of the nanostructure-confined excess electrons has been shown though the above comparisons (more details are listed in Table 2). Boron nitride nanotubes (BNNTs) have attracted increasing interest since their discovery in 1995.82 Recently, many investigations show that noncovalent interactions can be utilized to functionalize BNNTs, which indicates that the electronic structure of BNNTs is also sensitive to external atoms and molecules.83,84 In 2012, we reported a quantum chemical investigation on the lithium-atom-doped minorcaliber BNNT fragments to understand their interactions.85 Corresponding structures, charge populations, and the first hyperpolarizabilities were studied. Results show that the 2s electron of the Li atom could be effectively bound by the cavities formed by the B atoms. Interestingly, the electron in e@BNNT and Li@B−BNNT is mainly bound by the endmost

Figure 4. First hyperpolarizability (β0, au) VIP (eV) of LiCN···Li, the BNNT fragment, and LiCN···Li@BNNT. Reprinted with permission from ref 86. Copyright 2012, RSC.

of the LiCN···Li electride molecule within the BNNT system showed that the VIP is increased. Therefore, by comparison with LiCN···Li, the encapsulated complexes are more difficult to oxidize. Significantly, the BNNT-encapsulated LiCN···Li exhibits a considerable β0 value (1.06 × 104 au), which is significantly (almost 380 times) larger than the 28 au of BNNT. Besides, it is easier to encapsulate LiCN···Li from the B-rich edge rather than the N-rich edge of BNNT due to the lower energy barrier. In this context, a new strategy of enhancing the VIP and obtaining a large β0 of the NLO molecule is proposed to encapsulate the electride molecules within nanotubes. Therefore, assembling of the molecules with excess electrons, capturing alkali metal atoms with the polymer or nanoscale 616

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Biographies

complexants, is a promising approach to move the corresponding investigation from molecules to condensed-phase materials.

Rong-Lin Zhong received his B.S. degree from Northeast Normal University (NENU) in 2010. He is currently a Ph.D. student at NENU under the supervision of Prof. Su. His research interest focuses on the theoretical investigations of nonlinear optical properties of molecules.

Capturing alkali metal atoms with polymer or nanoscale complexants is a promising approach to move corresponding investigation from molecules to condensed-phase materials.

Hong-Liang Xu received his B.S. degree from Inner Mongolia University in 2002 and then his Ph.D. in chemistry under the supervision of Prof. Li from Jilin University in 2008. He has worked as a post doctor and lecturer in the group of Prof. Su since 2008. He has been an associate professor since 2012 at NENU. His research interests includes theoretical design of nonlilnear optical molecules with an excess electron and the lithium effect on π-conjugate systems. http://chem.nenu.edu.cn/ teacher_show.php?teacher_id= 127&typeid=44

In this Perspective, we mainly summarized a series of quantum chemistry investigations on the structures and electronic first hyperpolarizability of electride and alkalide compounds. Results show that the excess electron plays an important role in the remarkable first hyperpolarizabilities of these compounds. A series of new strategies for enhancing the NLO response and electronic stability are exhibited based on electride and alkalide NLO molecules. These strategies include the behaviors of the pushed and pulled electron, size, shape, and number of coordination sites of electride and alkalide compouds, as well as the number and spin state of the excess electrons. Up to now, an increasing number of experiments related to NLO response of compounds with excess electrons have been reported. For example, Sagar and co-workers investigated hydrated electrons at the water/air interface via secondharmonic generation.87 Besides, the ceramic electride is demonstrated to provide surface-enhanced Raman scattering.29 Despite a lot of investigations about the NLO properties of compounds with excess electrons being reported in the literature, great challenges still remain to move these fascinating properties from academic studies to NLO applications. Further investigations on inorganic electride molecules including full metal electride molecules are necessary to separate more compounds at ambient temperature and environment. Considering the laser region, developments of ultraviolet and infrared NLO materials on the basis of systems with excess electrons are interesting topics and worth looking into. Besides, the NLO switch applications will receive continuous attention. In contrast to single-molecular magnets, novel molecular optics may be established on the basis of the development and application of molecular NLO materials. Therefore, further investigations on the potential NLO applications of these compounds are positive and worth looking at further. It is our expectation that new high-performance NLO materials with excess electrons will be applied in optic-electronic fields by the efforts of experimental scientists. Through multidisciplinary endeavors, fruitful results and more and more applicable materials based on compounds with excess electrons will appear soon.

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Zhi-Ru Li has been a full professor since 1994 at Jilin University. He worked as a visiting scholar at Boston College in 1994−1995, at California University in 1998−1999, and a collaborative researcher at Kyushu University, Japan in 2005. His current research interests include theoretical studies of intermolecular interactions, superatom systems, the special topology of molecules, and the electronic structures and nonlinear optical properties of nanomolecules and compounds with excess electrons. http://tcclab.jlu.edu.cn/html/ zhiruli.html Zhong-Min Su received his B.S. in 1983 and then his Ph.D. in inorganic chemistry under the supervision of Prof. Rong-Shun Wang and Prof. Chi-Ming Che from NENU in 1997. He has been a full professor at NENU since 1994. Later, he worked as a visiting scholar in the group of Prof. Chi-Ming Che and Prof. Guan-Hua Chen of The University of Hong Kong. His research interests focus on functional materials chemistry and quantum chemistry. http://subsite.nenu.edu. cn/professor/pro/show.php?id=54

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Science Foundation of China (NSFC) (21003019, 21173098, and 21473026), the Science and Technology Development Planning of Jilin Province (201201062 and 20140101046JC), the Computing Center of Jilin Province provided essential support, and H.-L.X. acknowledges support from the Hong Kong Scholars Program and Project funded by the China Postdoctoral Science Foundation (2014M560227).

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REFERENCES

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*E-mail: [email protected] (H.-L.X.). *E-mail: [email protected] (Z.-R.L.). *E-mail: [email protected] (Z.-M.S.). Notes

The authors declare no competing financial interest. 617

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