Designing Alkalides with Considerable Nonlinear Optical Responses

Article pubs.acs.org/Organometallics

Designing Alkalides with Considerable Nonlinear Optical Responses and High Stability Based on the Facially Polarized Janus all-cis1,2,3,4,5,6-Hexafluorocyclohexane Wei-Ming Sun,*,† Bi-Lian Ni,† Di Wu,‡ Jian-Ming Lan,† Chun-Yan Li,*,† Ying Li,‡ and Zhi-Ru Li‡ †

The Department of Basic Chemistry, The School of Pharmacy, Fujian Medical University, Fuzhou 350108, People’s Republic of China ‡ Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: The first synthesis of facially polarized all-cis-1,2,3,4,5,6-hexafluorocyclohexane (1) was a tour de force of organic chemistry and opened up new possibilities for molecular design. In view of its large facial polarization, 1 was first utilized as a complexant to design a series of organic alkalides, namely M+·1·M′− (M, M′ = Li, Na, K), in this work. The alkalide identities of these proposed species were guaranteed by their HOMOs and VIE values, as well as NBO analysis. Our computational results show that these novel alkalides possess large complexation energies and thus exhibit high stability due to the strong electrostatic interaction between the alkali-metal ions and the axial fluorine or hydrogen atoms of 1. In particular, it is revealed that these novel alkalides possess remarkably large nonlinear optical responses with the first hyperpolarizabilities (β0) up to 1.45 × 106 au. Moreover, the feasibility of using 1 to design superalkali-based alkalide and superalkalide was also examined. We hope that this work will promote further application of 1 and, on the other hand, attract more research interest and efforts in designing and synthesizing new alkalides. materials with high performance.16,17,19 Alkalides are special compounds in which the anionic sites are occupied by alkalimetal anions (e.g., Li−, Na−, K−, Rb−, and Cs−). The existence of such special species has been known for more than 40 years since the first alkalide was successfully synthesized and characterized by Dye et al.24,25 In 1999, Dye and his coworkers26 successfully synthesized two room-temperaturestable alkalides, i.e. K+(Me6Aza222)K− and K+(Me6Aza222)Na−, which improved the prospect of applications of alkalides. To further extend the application of alkalides in the NLO field, there is a requirement to design new alkalides with large NLO response. Conventionally, an alkalide is generated by doping a complexant molecule with two alkali-metal atoms, in which one acts as a cation and the other carries the negative charge. Hence, choosing a proper complexant to simultaneously stabilize the alkali-metal cation and anion is expected to be a new approach to yield novel stable alkalides. Recently, the O’Hagan group27 reported the seminal synthesis of the Janustype molecule all-cis-1,2,3,4,5,6-hexafluorocyclohexane (hereafter 1), a molecule with one hydrocarbon face and one fluorocarbon face (see Figure 1). In particular, this special

1. INTRODUCTION The design and synthesis of novel materials with excellent nonlinear optical (NLO) properties has aroused great interest in view of their extensive applications in optical communication, optical computing, optical switching, optical logic, dynamic image processing, and other laser devices.1−9 In particular, organic NLO molecules with large first or second hyperpolarizabilities have recently received a great deal of attention because they exhibit many attractive features, such as very large and ultrafast response, ready processability, high laser damage thresholds, smaller dielectric constants, etc.6,7 Hitherto great efforts have been devoted to enhancing the nonlinear optical response of organic molecules, and many strategies have been proposed, including utilization of molecules with abundant π electrons,6,8 introduction of donor/acceptor groups,7 application of bond length alternation (BLA) theory,9 doping alkali-metal atoms,10−12 synthesis of octupolar molecules,13 consideration of interesting molecules with diradical character,14 etc. Especially, it has been reported that the introduction of excess electrons into organic molecules can effectively reduce the transition energies of their crucial excited states and thus result in the significant enhancement of NLO responses.15−23 As a representative of excess electron compounds, alkalides exhibit significant potential to serve as molecular NLO © XXXX American Chemical Society

Received: June 29, 2017

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DOI: 10.1021/acs.organomet.7b00491 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Structure and two-faced Janus polarity of 1 as well as the resulting geometric structure of M+·1·M′− (M, M′ = Li, Na, K).

Figure 2. Geometric structure and HOMO of M·1·M′ (M, M′ = Li, Na, and K).

molecule has an unusually large facial polarization and exhibits the largest reported molecular dipole moment (6.2 D) of any aliphatic compound reported so far, which opens up new possibilities for molecular design.28 In view of the large facial polarization, 1 exhibits an extraordinary propensity to bind with both cations and anions in the gas phase via its negative and positive faces, respectively, resulting in extraordinarily stable complexes.29 Recently, the interactions between compound 1 and benzene,30 B12F122−,31 and small planar/nonplanar molecules and materials32 have also been investigated. Considering the separate positive and negative facial polarities, it is highly expected that 1 could be utilized to design novel alkalides by the simultaneous binding of alkalimetal cations and anions with its negative and positive faces, respectively. Thus, this hypothesis was verified by doping two alkali-metal atoms on the two different polarized faces of 1 in this work. Under the action of the lone pairs of the axial F atoms, the s electron of the lower alkali-metal atom is ejected to the upper alkali-metal atom (see Figure 1), which yields a novel series of alkalides, i.e., M+·1·M′− (M, M′ = Li, Na, K). Our results show that all of the designed alkalides not only possess strong electrostatic interaction between 1 and alkali-metal ions but also exhibit remarkably large first hyperpolarizabilities (β0) up to 1.45 × 106 au. Furthermore, the feasibility of using 1 as a complexant to construct superalkali-based alkalide and superalkalide was also investigated in the present work. It is our hope that this work will not only enrich the variety of alkalides but also provide inspiration for experimental chemists to synthesize such potential NLO materials in the laboratory to promote the potential application of 1.

As shown in Table 1, for the compounds with the same M atom but different M′ atoms, the distance between the lower M atom and axial F atom (dM−F) hardly varies upon changing the upper alkali metal M′, indicating that the M′ species have little influence on the position of the M atom. Instead, the dependence of dM′−H and dM−M′ on the atomic number of M′ is very obvious: that is, the larger the atomic number of M′, the much longer the M′−H and M−M′ distances. For example, with Li serving as the lower M, dM′−H increases in the order 2.874 Å (M′ = Li) < 3.027 Å (M′ = Na) < 3.517 Å (M′ = K) while dM−M′ increases in the order 6.507 Å (M′ = Li) < 6.672 Å (M′ = Na) < 7.207 Å (M′ = K). This obvious increasing tendency can be attributed to the gradually increasing atomic radii of M′.33 Similarly, when M′ is fixed, it is found that the M−F and M−M′ distances also strongly correlate with the atomic number of M, while the dependence of dM′−H on M is relatively unobvious. It is also interesting to compare the geometric parameters of M·1·M′ with those of related compounds. For instance, the Na−F distances (2.180−2.192 Å) of Na·1·M′ (M′= Li, Na, K) are shorter than that (2.28 Å) of the reported 1·Na + compound,29 indicating that the Na+ cations are bound more tightly in these Na·1·M′ compounds. Moreover, the distance between the alkali-metal cation and anion (dM−M′) has been proved to be an important geometric parameter that can affect the NLO properties of alkalides, namely, the longer dM−M′ may lead to the larger NLO response.34 Thus, the M−M′ distances of these M·1·M′ compounds were also compared with those of previously reported alkalides. It is noted that the M−M′ distances (6.507−8.192 Å) of M·1·M′ are much longer than those of 5.244−6.344 Å for (M+@36adz)M′− (M, M′ = Li, Na, K)19 and 5.778−6.533 Å for (M+@36adz)M′− (M = Cu, Ag, Au; M′ = Li, Na, K).35 Especially, the Li−M′ distances of the Li·1·M′ compounds are even longer than the corresponding Li+−M′− distances in Li+(calix[4]pyrrole)M′− (M′ = Li, Na, K)17 by 2.848, 2.724, and 2.575 Å, respectively. In addition, it is of great significance to understand the strength of the interaction between 1 and the introduced alkalimetal atoms. Thus, the complexation energies (Ec) of the M·1· M′ complexes were also evaluated and are given in Table 1. It is shown that the M·1·M′ compounds possess very large negative Ec values (−95.84 to −139.01 kcal/mol), indicating that the three segments of M·1·M′ are tightly bound and thus produce remarkably stable species. In particular, no matter which alkalimetal atom (Li, Na, or K) serves as M′, the Ec values decrease in the order Li·1·M′ (−131.85 to −139.01 kcal/mol) > Na·1· M′ (−106.56 to −112.62 kcal/mol) > K·1·M′ (−95.84 to −102.14 kcal/mol), indicating the apparent dependence of Ec

2. RESULTS AND DISCUSSION 2.1. Structural Characteristics. Nine equilibrium structures of M·1·M′ (M, M′ = Li, Na, K) were obtained and are shown in Figure 2. The results show that all of the obtained compounds possess a similar geometric structure with C3v symmetry. Selected geometrical parameters of these resulting compounds are summarized in Table 1. From Figure 2 and Figure S1 in the Supporting Information, it can be seen that the integrity of 1 is well preserved in the resulting compounds. In comparison with the isolated 1, a small contraction of 0.016−0.022 Å occurs in the covalent C−C bonds, whereas the axial C−F bonds and C−H bonds are slightly elongated by 0.045−0.077 and 0.016−0.030 Å, respectively. The minor changes in covalent bond lengths in 1 after complexation indicates that the interaction between the alkali-metal ions and the axial fluorine or hydrogen atoms of 1 is largely electrostatic, involving ion−dipole and ion-induced− dipole potentials as proposed by Ziegler et al.29 B

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Table 1. Distances between the Lower M and Axial F Atoms (dM−F, in Å), Distances between the Upper M′ and Axial H Atoms (dM′−H, in Å), Distances between the M and M′ Atoms (dM−M′, in Å), Complexation Energies (Ec, in kcal/mol), NBO Charges on M (QM, in |e|) and M′ (QM′, in |e|), and VIE Values (in eV) of the M·1·M′ (M, M′ = Li, Na, K) Compounds M·1·M′

symmetry

dM−F

dM′−H

dM−M′

Ec

QM

QM′

VIE

Li·1·Li Li·1·Na Li·1·K Na·1·Li Na·1·Na Na·1·K K·1·Li K·1·Na K·1·K

C3v C3v C3v C3v C3v C3v C3v C3v C3v

1.840 1.847 1.850 2.180 2.182 2.192 2.536 2.542 2.545

2.874 3.027 3.517 2.930 3.097 3.567 2.967 3.131 3.582

6.507 6.672 7.207 7.002 7.185 7.721 7.507 7.691 8.192

−139.01 −135.66 −131.85 −112.62 −110.09 −106.56 −102.14 −99.63 −95.84

0.833 0.833 0.799 0.860 0.855 0.788 0.909 0.907 0.870

−0.509 −0.559 −0.498 −0.536 −0.576 −0.478 −0.606 −0.643 −0.584

3.830 3.701 3.359 3.525 3.413 3.114 3.358 3.250 2.964

Table 2. Calculated Mean Dipole Moments (μ0, in au), Mean Polarizabilities (α0, in au), Mean First Hyperpolarizabilities (β0, in au), Transition Energies (ΔE, in eV), Oscillator Strengths ( f 0), Differences in Dipole Moment (Δμ, in D) between the Ground and Excited States of the Crucial Excited State, and Estimated β0 Values under the Two-Level State (Δμ·f 0/ΔE3, in au) for the M+·1·M′− (M, M′ = Li, Na, K) Alkalides complex −

Li ·1·Li Li+·1·Na− Li+·1·K− Na+·1·Li− Na+·1·Na− Na+·1·K− K+·1·Li− K+·1·Na− K+·1·K− +

μ0 3.515 3.612 2.900 5.309 5.309 3.795 6.413 6.526 5.780

α0

β0

362 421 663 474 611 1290 433 538 1164

× × × × × × × × ×

3.49 5.97 2.08 2.52 4.57 1.17 1.34 2.84 1.45

4

10 104 105 105 105 106 105 105 106

ΔE

f0

Δμ

1.999 2.012 1.811 1.875 1.854 1.709 2.126 2.155 1.747

0.329 0.290 0.407 0.226 0.157 0.271 0.216 0.239 0.258

7.030 8.170 9.843 6.198 10.896 11.862 7.859 12.363 12.518

Δμ·f 0/ΔE3 2.29 2.30 5.35 1.68 2.12 5.09 1.40 2.34 4.80

× × × × × × × × ×

103 103 103 103 103 103 103 103 103

and the axial negative fluorine (positive hydrogen) atoms of 1 leads to the large negative Ec values, as mentioned above. The alkalide identities of these M+·1·M′− compounds are supported by their highest occupied molecular orbitals (HOMOs). Just as shown in Figure 1, the diffuse electron cloud enwraps the M′ atom and creates an M′− anion in each M+·1·M′− compound. From Table 1, it is noted that Na− possesses the largest negative charges for the M+·1·M′− alkalides with the same M but different M′. For instance, the QM′ values of Li+·1·M′− vary in the order −0.559 |e| (M′ = Na) > −0.509 |e| (M′ = Li) > 0.498 |e| (M′ = K). The same case has been also reported for Li+(calix[4]pyrrole)M− (M = Li, Na, K)17 as well as Li3+(calix[4]pyrrole)M− and Li3O+(calix[4]pyrrole)M− (M = Li, Na, K).36 Moreover, it should be mentioned that the QM′ values (−0.478 to −0.643 |e|) of these proposed M+·1·M′− alkalides are much larger than those (−0.253 to −0.361 |e|) of Li+(calix[4]pyrrole)M′− (M′ = Li, Na, K),17 and those (−0.260 to −0.485 |e|) of Li3+(calix[4]pyrrole)M− and Li3O+(calix[4]pyrrole)M− (M = Li, Na, K),36 demonstrating that the better performance of 1 in comparison to calix[4]pyrrole in driving the charge transfer from M to M′ is because of its large facial polarity. In addition, the vertical ionization energies (VIEs) of M+·1· M′− are in the range of 2.964−3.830 eV (see Table 1), which are even smaller than the ionization energy (3.89 eV)37 of the Cs atom. Such low VIE values further confirm that these studied compounds contain loosely bound diffuse electrons. From Table 1, it is observed that, for the M+·1·M′− alkalides with the same M but different M′, the VIE values decrease with the increasing M′ atomic number, such as 3.830 eV (M′ = Li) > 3.701 eV (M′ = Na) > 3.359 eV (M′ = K) for Li+·1·M′−, implying that the smaller the electron affinity of M′, the more

values on the atomic number of M. This may be related to the fact that the electrostatic interaction between the M and F atoms of 1 is reduced with the increasing M atomic number. According to Coulomb’s law, the electrostatic interaction between two point charges is inversely proportional to the square of the distance between them. Hence, the electrostatic interaction between M and F decreases when dM−F increases with the increasing M atomic radii, leading to the above decreasing trend of Ec. Similarly, with the same M, the Ec values are also slightly reduced along with the increasing M′ atomic number, such as Li·1·Li (−139.01 kcal/mol) > Li·1·Na (−135.66 kcal/mol) > Li·1·K (−131.85 kcal/mol) for Li·1· M′. This can also be understood by the fact that the electrostatic interaction between M′ and axial H of 1 decreases when the dM′−H increases along with the increasing M′ atomic number. Consequently, Li·1·Li is considered to be the most stable compound among the M·1·M′ (M, M′ = Li, Na, K) series with an Ec value of −139.01 kcal/mol, which is even larger that that of −132.40 kcal/mol for Na+·1·Cl− at the same computational level in each series. 2.2. Alkalide Characteristics. To explore the electronic features of M·1·M′ (M, M′ = Li, Na, K), NBO calculations were performed at the M06-2X/6-31+G(d, p) level and the related results are given in Table 1. It is noted that the NBO charges on the upper M′ atoms (QM′) are negative (−0.478 to −0.643 |e|), demonstrating the alkalide characteristics of these compounds. Meanwhile, the charges on the M atoms (QM) are 0.788−0.909 |e|, which indicate that the separate positive and negative facial polarities of 1 lead to charge transfer from the lower M to the upper M′. Thus, the studied species can be written as M+·1·M′− (M, M′ = Li, Na, K), in which the strong electrostatic interaction between the alkali-metal cation (anion) C

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Figure 3. Dependences of (a) the polarizability α0 and (b) the first hyperpolarizability β0 values on the atomic number of M′− for M+·1·M′−.

diffuse the electron cloud. For the M+·1·M′− series with the same M′ but different M, the VIE values also decrease as the M atomic number increases. For example, the varying order of VIE for M+·1·Li− is 3.830 eV (M = Li) > 3.525 eV (M = Na) > 3.358 eV (M = K), which is reasonable considering the decreasing IE order of M, i.e., 5.39 eV (Li) > 5.14 eV (Na) > 4.34 eV (K).37 2.3. Nonlinear Optical Properties. As pointed out above, the M+·1·M′− (M, M′ = Li, Na, K) complexes exhibit typical alkalide characteristics with loosely bound excess electrons around M′−. Hence, it is highly expected that they will exhibit considerable NLO responses. Thus, the static electric properties of these M+·1·M′− alkalides were calculated and are given in Table 2. From Table 2, it is observed that these novel M+·1·M′− alkalides possess remarkably large β0 values of 3.49 × 104 to 1.45 × 106 au, which are significantly larger than that of 127 au for uncomplexed 1, indicating that doping two alkali-metal atoms on the different faces of 1 greatly enhances its NLO response. It is noted that the β0 value (1.45 × 106 au) of K+·1· K− is between 1 and 2 orders of magnitude greater than those of previously reported potassides, including Li+(calix[4]pyrrole)K− (35934 au at the MP2 level),17 (K+@26adz)K− (318354 au at the BHandHLYP level),19 and Li3O+(calix[4]pyrrole)K− (33252 au at the CAM-B3LYP level).36 Consequently, these proposed M+·1·M′− alkalides with high stability can be considered as excellent NLO molecules with high performance. To better visualize the results, the dependences of polarizability (α0) and the first hyperpolarizability (β0) values on the M′− atomic number of M+·1·M′− are exhibited in Figure 3. From Figure 3a, an obvious dependence of α0 on the atomic number of M′− anion is observed for the M+·1·M′− species. The same holds true for the β0 values (see Figure 3b). Take the Li+·1·M′− series as examples. Their α0 values increase in the order 362 au (M′ = Li) < 421 au (M′ = Na) < 663 au (M′ = K), and β0 values show the same sequence of 3.49 × 104 au (M′ = Li) < 5.97 × 104 au (M′ = Na) < 2.08 × 105 au (M′ = K), which are in accordance with the increasing M′− atomic number. This is the same case for Li+(calix[4]pyrrole)M− (M = Li, Na, K)17 as well as Li3+(calix[4]pyrrole)M− and Li3O+(calix[4]pyrrole)M− (M = Li, Na, K).36

In addition, it is observed that, with the same M+, the lithide and sodide possess approximately equal μ0 values while the potasside exhibits a much smaller μ0. However, the case is opposite for the α0 and β0 values. It can be seen from Figure 3 and Table 2 that the α0 and β0 values of potassides are much larger than those of lithides and sodides with the same M+. Taking the K+·1·M′− series as examples, the α0 and β0 values (1164 and 1.45 × 106 au) of K+·1·K− are much larger than those of K+·1·Na− (538 and 2.84 × 105 au) and K+·1·Li− (433 and 1.34 × 105 au), whereas the μ0 value (5.780 au) of K+·1·K− is smaller than those of K+·1·Na− (6.526 au) and K+·1·Li− (6.413 au). How can we understand the dependence of β0 values on the M′− atomic number of these M+·1·M′− alkalides? We may find some clues from the two-level model proposed by Oudar and Chemla.38,39 For the static case, the expression of the two-level model in literature6 is employed as

β0 ∝

Δμ·f0 ΔE3

where ΔE, f 0, and Δμ are the transition energy, oscillator strength, and the difference in the dipole moment between the ground state and crucial excited state, respectively. According to the two-level expression, β0 is proportional to f 0 and Δμ, whereas is inversely proportional to the third power of ΔE. Thus, the small ΔE value is considered to be the decisive factor in the large β0 of alkalides, which has been confirmed by many previous reports.17,35,36 Herein, the crucial excited states of M+· 1·M′− and corresponding ΔE, f 0, and Δμ values are presented in Figure 4 and Table 2, respectively. From Figure 4, it can be seen that all of the crucial excitations for M+·1·M′− originate from their HOMO orbitals, which are mainly composed of the diffused excess electrons around M′−. Due to the fact that diffuse excess electrons can be easily excited, all of the studied alkalides exhibit small transition energies (ΔE) of 1.709−2.155 eV (see Table 2), which justifies the remarkably large β0 values of these proposed alkalides. Especially, it is noted that the potassides possess smaller ΔE values of 1.709−1.811 eV in comparison to those of 1.854− 2.155 eV for lithides and sodides. Meanwhile, the f 0 and Δμ values of crucial excited states for the potassides are larger than those of corresponding lithides and sodides in each series. Accordingly, the combination of smaller ΔE and larger f 0 and D

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is confirmed by the estimated β 0 values of M + ·1·M′ − simultaneously considering the three factors ΔE, Δμ, and f 0 under the two-level state model. From Table 2, it is observed that the potassides always show much larger estimated β0 values (4.80 × 103 to 5.35 × 103 au) than those (1.40 × 103 to 2.34 × 103 au) of lithides and sodides. 2.4. Possibility of Using 1 To Design SuperalkaliBased Alkalide and Superalkalide. More recently, superalkali clusters40 with chemical characteristics similar to those of alkali-metal atoms have been successfully used as the source of excess electrons in designing alkalides with large NLO responses.34,36,41 Hence, the feasibility of using 1 as a complexant to design superalkali-based alkalides was also investigated in the present work. Among various superalkalis, M3O species (M = Li, Na, K)42,43 have been extensively studied by theoretical and experimental researchers. The Li3O+ and Na3O+ cations have been successfully detected in the crystalline salts Li3O+NO2− and Na3O+NO2−,44−46 while the Na3O and K3O molecules have been utilized to assemble unusual superatom compounds.47 Consequently, a series of M3O·1·K (M = Li, Na, and K) compounds were also constructed and are shown in the Figure 5. As shown in Figure 5, the M3O molecules preserve their structural integrity after complexing with 1 but show a tendency from a planar structure toward a triangular pyramid. In these structures, the face formed by three M ligands of superalkali is parallel to the face formed by three axial F atoms of 1 along with M facing toward F. However, the Li3O subunit turns on the principal axis of 1 by a small angle in Li3O·1·K, resulting in the lower C3 symmetry of Li3O·1·K in comparison with the C3vsymmetric Na3O·1·K and K3O·1·K. In addition, the distance between K and the axial H atom of M3O·1·K increases in the order 3.535 Å (M = Li) < 3.587 Å (M = Na) < 3.602 Å (M = K), indicating that the position of the upper K atoms can be

Figure 4. Crucial transitions of the M+·1·M′− (M, M′ = Li, Na, K) alkalides.

Δμ values is responsible for the larger β0 values of the potassides in comparison to those of lithides and sodides. This

Figure 5. Optimized geometric structures and HOMOs of M3O·1·K (M = Li, Na, K) and Li3O·1·Li3O. E

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Table 3. NBO Charges on the Lower Superalkali Subunit (QSA) and the Upper K or Li3O (QM′), VIE Values (in eV), Complexation Energies (Ec, in kcal/mol), Mean Dipole Moments (μ0, in au), Mean Polarizabilities (α0, in au), and Mean First Hyperpolarizabilities (β0, in au) of the M3O·1·K (M = Li, Na, K) and Li3O·1·Li3O Compounds compound

symmetry

QSA

QM ′

VIE

Ec

μ0

α0

Li3O·1·K Na3O·1·K K3O·1·K Li3O·1·Li3O

C3 C3v C3v C3

0.842 0.876 0.913 0.877

−0.557 −0.632 −0.688 −0.866

3.275 3.263 3.088 3.467

−104.44 −93.42 −85.38 −116.35

3.043 2.457 3.551 4.118

631 640 679 616

β0 7.60 5.46 4.78 2.65

× × × ×

104 104 104 104

acceptor to design a new kind of superalkalide (see Figure 5). As shown in Table 3, the NBO charge on the upper Li3O unit is −0.866 |e|, demonstrating the superalkalide identity of this Li3O+·1·Li3O− compound. Obviously, the Li3O− carries more negative charge than the K− anions in the M3O+·1·K− alkalides. However, the VIE value (3.467 eV) of Li3O+·1·Li3O− is larger than those of 3.088−3.275 eV for M3O+·1·K−, indicating that the excess electron is more stable in Li3O+·1·Li3O− in comparison with M3O+·1·K−. This may be attributed to two facts: on the one hand, the excess electron in the HOMO of Li3O+·1·Li3O− can distribute over the whole Li3O framework; on the other hand, the hydrogen bonds between the O atom of Li3O and axial H atoms of 1 are able to strengthen the bound effect to the excess electron.48 As a result, the Li3O+·1·Li3O− exhibits relatively smaller α0 and β0 values (616 and 2.65 × 104 au, respectively) in comparison to those of M3O+·1·K−. Even so, the β0 value of this novel superalkalide is still ca. 4 times as large as that (6366 au) of the state of the art organic 4-(N,Ndimethylamino)-4′-nitrostilbene (DANS),49 which continues to be a useful reference. In particular, the Ec value (−116.35 kcal/ mol) of Li3O+·1·Li3O− is larger than those of M3O+·1·K−, indicating its higher stability. Consequently, this proposed Li3O+·1·Li3O− superalkalide is also a potential NLO molecule with high performance.

affected by the lower superalkali species. The strong interaction between 1 and the (super)alkali-metal unit are guaranteed by the large complexation energies (Ec) ranging from −85.38 to −104.44 kcal/mol. In addition, the Ec values decrease in the order −104.44 kcal/mol (M = Li) > −93.42 kcal/mol (M = Na) > −85.38 kcal/mol (M = K) for M3O·1·K, indicating the dependence of Ec values on the M atomic number of the M3O units. The NBO charges and VIE values of M3O·1·K (M = Li, Na, K) were also calculated and are given in Table 3. The NBO analyses reveal that these designed M3O-based compounds can also be written as M3O+·1·K− (M = Li, Na, K). From Table 3, the charges on the upper K− anions vary from −0.557 to −0.688 |e|, indicating the alkalide characteristics of these species. This is also supported by their HOMOs (see Figure 5). The charges on the M3O units (QSA) are in the range of 0.842− 0.913 |e|, implying that the M3O cluster behaves like an alkalimetal atom and donates an electron to the upper K atom in these compounds. In addition, it is observed from Table 3 that the charges on the superalkali cations and K− anions of these M3O+·1·K− alkalides show increasing tendencies of 0.842 |e| (M = Li) < 0.876 |e| (M = Na) < 0.913 |e| (M = K) and −0.557 |e| (M= Li) < −0.632 |e| (M = Na) < −0.688 |e| (M = K), respectively. This is reasonable considering the decreasing VIE order of 3.485 eV (Li3O) > 3.201 eV (Na3O) > 2.673 eV (K3O) for the superalkali units because the smaller the VIE of the superalkali unit, the more easily the superalkali unit loses its valence electrons. In addition, the decreasing VIE order of 3.275 eV (M = Li) > 3.263 eV (M = Na) > 3.088 eV (M = K) for M3O+·1·K− is also in line with the decreasing VIE order of M3O. Focusing on the NLO responses of M3O+·1·K−, it is noted that these potassides also exhibit considerable β0 values of 4.78 × 104 to 7.64 × 104 au, which are larger than those of 2.93 × 104 to 3.47 × 104 au for M3O+(calix[4]pyrrole)K− (M = Li, Na, K).36 This demonstrates the better performance of 1 in comparison to calix[4]pyrrole in synthesizing unconventional superalkali-based alkalides with a large NLO response. Moreover, as shown in Table 3, it is found that the α0 values increase with the increasing atomic number of M in the M3O units, whereas the β0 values gradually decrease as the M atomic number of M3O increases. To be specific, the α0 values increase in the order 631 au (M = Li) < 640 au (M = Na) < 679 au (M = K), and, in contrast, the β0 values reduces in the sequence of 7.60 × 104 au (M = Li) > 5.46 × 104 au (M = Na) > 4.78 × 104 au (M = K) for M3O+·1·K−. This phenomenon may be utilized to regulate the linear and nonlinear optical properties of such M3O+·1·K− alkalides. In addition, the superalkalis have also been used as excess electron acceptors to design a series of compounds termed as superalkalides by Mai et al.48 Hence, 1 was simultaneously combined with two Li3O molecules, where one serves as an excess electron donor and the other acts as an excess electron

3. CONCLUSIONS A new type of organic alkalide, namely M+·1·M′− (M, M′ = Li, Na, K), has been designed by utilizing the recently synthesized all-cis-1,2,3,4,5,6-hexafluorocyclohexane (1) as a complexant. The alkalide characteristics of these compounds are confirmed by an NBO analysis as well as their VIE values and HOMOs. Because of the large facial polarization of 1, the formed alkalimetal cation and anion in these complexes are strongly bound to the fluorine face and hydrogen face of 1 by electrostatic interactions, respectively, resulting in large complexation energies. In particular, all of the proposed alkalides exhibit remarkably large NLO responses, especially for the potassides. Moreover, 1 has also been successfully employed to construct a series of superalkali-based alkalides and a new kind of superalkalide with considerable NLO response and high stability. Hence, this study demonstrates that, with two different polarized fluorine and hydrogen faces, 1 emerges as an attractive complexant for the design of novel alkalides with large NLO responses and high stability. 4. COMPUTATIONAL DETAILS The choice of suitable methods and basis sets for molecular structure and property calculations is crucial in modern computational quantum chemistry. To choose a proper method and basis set for structural optimization, a computational test was performed to investigate the effects of different methods and basis sets on the optimized geometrical structures by sampling the Li+·1·Li− compound. It is found that the geometrical parameters obtained by the M06-2X method are close to the corresponding values from the second-order F

DOI: 10.1021/acs.organomet.7b00491 Organometallics XXXX, XXX, XXX−XXX

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Møller−Plesset perturbation (MP2) method (see Table S1 in the Supporting Information). Hence, this novel hybrid meta exchange− correlation functional M06-2X proposed by Zhao and co-workers50,51 has been chosen to obtain the geometric structures of the studied compounds in conjunction with the 6-31+G(d, p) basis set, which can show molecular structure close to the that obtained by the larger basis set (see Table S2 in the Supporting Information). With regard to the calculation of the static electric properties, the MP2 method, accompanied by the 6-31+G(d,p) basis set, is chosen to calculate the dipole moments (μ0), polarizabilities (α0), and first hyperpolarizabilities (β0) of the studied systems by the finite-field approach, considering that the computed results from the MP2 method can be close to those obtained from the more sophisticated correlation methods, such as the quadratic configuration interaction with single and double excitations (QCISD).52 Furthermore, the employed 6-31+G(d,p) basis set is also appropriate to compute the static electric properties of the studied systems because it can provide results close to those obtained by the larger basis set (see Table S3 in the Supporting Information). The magnitude of the applied electric field is chosen as 0.001 au for the calculation of the hyperpolarizability, which is proven to be the most adequate value for the numerical differentiation.16−19,48 In the finite-field approach, it is known that the total energy of a molecule in a weak static electric field (F) can be expressed as E = E 0 − μα Fα −

1 1 ααβFαFβ − βαβγ FαFβFγ + ... 2 6

1 (αxx + αyy + αzz) 3

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00491. Computational test on the methods and basis sets used for geometry optimization and the effect of basis sets on the computed electric properties and optimized structures of 1, Na+·1·Cl−, and M3O (M = Li, Na, K) (PDF) Cartesian coordinates for all calculated structures (XYZ)

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*E-mail for W.-M.S.: [email protected]. *E-mail for C.-Y.L.: [email protected]. ORCID

Wei-Ming Sun: 0000-0002-9882-0511 Di Wu: 0000-0002-7000-0597 Zhi-Ru Li: 0000-0002-1384-0725 Notes

(1)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21173095, 21573089, 21603032), State Key Development Program for Basic Research of China (Grant No. 2013CB834801), the Natural Science Foundation of Fujian Province (Grant Nos. 2016J05032, 2016J01771, 2016J01685), Youth Scientific Research Project of Fujian provincial Health and Family Planning Commission (Grant No. 2016-1-37), and Academic Foundation for Professor of Fujian Medical University (Grant No. JS14009). The authors also acknowledge the National Supercomputing Center in Shenzhen for providing computational resources.

(3)

where βi =

3 (β + βijj + βikk ) 5 iii

i, j, k = x, y, z

The mean dipole moment (μ0) is noted as μ0 = (μx2 + μy2 + μz2 )1/2

AUTHOR INFORMATION

Corresponding Authors

(2)

β0 = (βx2 + βy2 + βz2)1/2

ASSOCIATED CONTENT

S Supporting Information *

in which E0 is the molecular total energy without the electric field, Fα is the electric field component along the α direction, and μα, ααβ, and βαβγ are the dipole moment, the polarizability, and the first hyperpolarizability, respectively. Herein, the static mean polarizability (α0) and mean first hyperpolarizability (β0) are defined as α0 =

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(4)

In addition, the TD-M06-2X calculations were performed to obtain the transition energies and oscillator strengths of the crucial excited states (the excited state with the largest oscillator strength) for the studied systems, which has been widely used in the relevant works,10,53−55 while the difference of dipole moments between the ground state and crucial excited state was evaluated by employing the configuration interaction singles (CIS) method with the identical 631+G(d,p) basis set.10,56−58 The vertical ionization energies (VIE) and of the studied systems were calculated as the total energy difference between the cationic and neutral compound with the same geometry as the neutral compound at the MP2/6-31+G(d,p) level. The complexation energies (Ec) of M+·1·M′− (M, M′ = alkali metal or superalkali), defined as Ec = E[M+·1·M′−] − E(1) − E(M+) − E(M′−), were calculated at the MP2/6-31+G(d, p) level. We used the counterpoise procedure59 to eliminate the basis set superposition error (BSSE) effect given as60

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