Design a novel type of excess electron compounds with large nonlinear optical responses using group 12 elements (Zn, Cd and Hg)
Graphical abstract
Table of contents synopsis: A novel type of NLO compounds which were designed by using transition metal atoms (Zn, Cd and Hg) and alkali metal atoms (Li, Na and K).
Introduction
The novel materials with large nonlinear optical (NLO) response hold important potential for the applications in optical switching [1], optical communication [2], optical computing [2], and other laser devices [[3], [4], [5], [6], [7]]. Therefore, over the past several decades, people have been designing and developing many different types of NLO materials [[8], [9], [10], [11], [12], [13], [14], [15], [16]]. In the field of inorganic NLO materials, materials with larger second and third-order nonlinearities are widely designed for second harmonic generation (SHG) and sum-frequency generator (SFG) [17]. For example, silicate mixed oxide materials containing trans-connected MO6 octahedral chains [18], rare-earth borate [19], and recent inorganic metal cyanurates (C3N3O3)3- groups [20] were already reported in the literature.
Moreover, organic nonlinear (NLO) materials also attract much attention due to their remarkable properties, such as variable molecular structure, large laser damage thresholds, greatly high and fast response, etc [21,22]. Therefore, many strategies are adopted to construct efficient organic NLO materials. Experimentally, Ivanova et al. investigated the self-assembly, crystal structures, optical properties and potential as NLO materials of eight novel organic crystals of derivatives of barbituric acid [23]. And Jiang et al. combined the advantages of organic and inorganic materials to construct novel semiorganic NLO crystals which have high physico-chemical stability and excellent NLO properties [11]. Theoretically, Nagai et al. investigated the NLO properties of hexagonal shaped finite graphene fragments with two types of edge shapes [24]. They found the NLO properties of the graphene fragment with zigzag edge are better than those with armchair edge, which is attributed to the intermediate diradical characters in zigzag edge.
Furthermore, metalloorganic compounds are composed of metal atoms (ions) and organic ligands that connected through chemical bonds, which possess the characteristics of both inorganic and organic compounds. Since the first paper about metallo-organic compound with nonlinear optical effects was reported in 1986 [25], more organometallic NLO materials have been proposed successively, such as Bis(thiourea) cadmium chloride (BTCC) single crystals [26], the octopolar alkynylruthenium complexes [27] and the octupolar 2D coordination network [Cd3 (μ3-OH) (pyridine)6 (L11a)3](ClO4)2 [8].
Till date, introduction of excess electrons in organic systems have become a mature strategy for constructing splendid NLO materials. Therefore, many various excess electrons compounds with greatly high NLO responses have been widely studied. For instance, using superatom clusters as building blocks to synthesize novel excess electron compounds with unusual nonlinear optical properties, such as BLi6-X (X = F, LiF2, BeF3, BF4) Motifs [28], the superhalogen (Al7) doped graphitic carbon nitride (g-C3N4) [29] and the oxacarbon superalkali C3X3Y3 (X = O, S and Y = Li, Na, K) clusters [30]. At present, alkalides and electrides are representative kinds of excess electron compounds. Alkalides is a novel kind of excess electron compounds that alkali metal anions (such as Li−, Na− and K−) occupy the anionic sites. Inspired by the electrides synthesized by Dye [31,32], Li's group proposed a series of new excess electron compounds with large NLO responses [[33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]], such as Li− [9]aneN3 [36], Li–H3C4N2···Na2 [40], Li@B10H14 [42], K+(e@C20F20)− and (K3O)+(e@C20F20)− [44]. Up to now, the general strategy used by Li's group is doping alkali metal atoms into some special organic complexing agents with electron-donating/accepting group, such as pyrrole [35], cyclic polyamine [36] and fluorocarbon [49]. Moreover, recently reported the alkali-metal-doped Zn12O12 nanocage [50] and the Ca12O12-based nano systems [51] also provide us with excellent ideas for constructing electrides.
Recently, an interesting organic molecule all-cis1,2,3,4,5,6-hexafluorocyclohexane (hereafter 1) was synthesized by O'Hagan group successfully [52]. In the 1, the axial fluorines present a negative face and the axial hydrogens present a positive face, leading to an abnormally large facial polarization of 1 (μ = 6.2 D) [52]. Due to its characteristic, the 1 was chosen as a complexant to design many excess electron compounds MF-1-MH [53,54] (metal atom on the side of the axial fluorines, denoted as MF and metal atom on the side of the axial hydrogens, denoted as MH, see Fig. 1). In most MF-1-MH, the metals with the unpaired ns electron, such as Li, Na, K, Cu, Ag and Au, act as MF and MH. And then, the ns electron of MF pushed by F atoms transfers to the MH, resulting in the MH with excess electron, that is, MH anion. Nevertheless, Hou et al. designed theoretically a new kind of excess electron compounds Li-1-MH (MH = Be, Mg and Ca) [55] in which the alkaline-earth atom with paired ns electrons acts as MH. These Li-1-MH are named as alkaline-earthide which exhibit extremely large nonlinear optical responses. The group 12 elements, Zn, Cd and Hg, also possess paired ns electrons which are similar to the alkaline-earth atoms. It is highly motivated to explore if the MF-1-MH with Zn, Cd and Hg serving as MH can have excellent NLO responses.
Here, we have theoretically investigated geometric characteristics, electronic structures and first hyperpolarizabilities (β0) of the MF-1-MH (MF = Li, Na and K, MH = Zn, Cd and Hg). In Li-1-MH, the obvious charge transfer between Li and MH can be observed while in Na/K-1-MH, the charge transfer between Na/K and MH is negligible. Moreover, the calculated β0 of most MF-1-MH achieve 105 au. which is about one order of magnitude higher than the calculated β0 of MF-1-MH (MF = Li, Na and K, MH = Cu, Ag and Au) [54]. We hope this study will not only provide a new idea for designing outstanding NLO compounds but also inspire experimental chemists to synthesize the novel NLO materials.
Section snippets
Computational details
The density functional theory was used in our calculation. The functional M06-2X [[56], [57], [58], [59], [60], [61]] was chosen hybrid meta exchange-correlation functional since the method has been proven to be accurate enough. Some computational tests had been performed so that appropriate basis set can be selected to optimize the molecular structure. Many basis sets were used to optimize the structure of Li-1-Zn (see Table S1 and Table S2). And in the process of optimization, the cc-pvtz
Structures, parameters and stabilities of MF-1-MH
The optimized structures and some parameters of MF-1-MH (MF = Li, Na and K; MH = Zn, Cd and Hg) with all real frequencies are presented in Fig. 1 and Table 1, respectively. As shown in Table 1, the MF-1-MH have C3v symmetry except for the Li-1-Hg that has C1 symmetry. In Na/K-1-MH compounds, the values of dMH-H and dMF-MH decrease with increasing atomic number of MH. In contrast, the values of dMF-F and dMF-MH of MF-1-Zn/Cd/Hg increase with increasing atomic number of MF. For example, it can be
Conclusion
In summary, we designed a new type of excess electron compounds, namely MF-1-MH (MF = Li, Na and K; MH = Zn, Cd and Hg). Our results show that in Li-1-MH, the obvious charge transfer between Li and MH can be observed while in Na/K-1-MH, the charge transfer between Na/K and MH is negligible. Moreover, these newly proposed species exhibit significantly large β0 values (up to 988734 au) as expected, which is a firm evidence that transition metal atoms Zn, Cd and Hg have great potential as the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is financially supported by Scientific and Technological Development Project of Jilin Province, China (Grant No. 20200201104JC).
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2022, PolyhedronCitation Excerpt :To the specific, for the Janus-type molecule 1, the positive and negative faces of 1 display an obvious tendency to bind both cations and anions, so that 1 can push the ns1 valence electron from the fluorine face (negative face) to the hydrogen face (positive face) and forming an excess-electron system [49]. Therefore, more and more research works have been reported based on the Janus-type molecule 1 from recent theoretical studies, such as MF-1-MH (MF = Li, Na and K; MH = Zn, Cd and Hg) [50], M − LCaL − M (M = Li or Na, L = C6F6H6) [51], M+−1−M′− (M = Li, Na, and K; M′ = Cu, Ag, and Au) [52], and M+·1·M′− (M, M′ = Li, Na, and K) [53] etc. Recently, Hou et al. constructed theoretically a series of alkali metal atom doped compounds, namely M-F6C6H6 (M = Li, Na and K) [54].
The electronic structures and nonlinear optical properties of Alkali and Alkali earth metal atoms doped C<inf>6</inf>H<inf>6</inf>Cl<inf>6</inf>: A density functional theoretical study
2022, Journal of Molecular Graphics and ModellingCitation Excerpt :Therefore, the Janus-type molecule 1 have potential to push the ns1 valence electron from the fluorine face (negative face) to the hydrogen face (positive face) and provide extraordinary stability for the formed complexes, thus forming an excess-electron system. In fact, a series of recent studies that focus on 1 molecule also indicate that 1 could be utilized to construct novel excellent alkalides, such as M+·1 M′− (M, M′ = Li, Na, and K) [47], MF-1-MH (MF = Li, Na and K; MH = Zn, Cd and Hg) [48], Li-1-M (M = Be, Mg and Ca) [49], M−LCaL−M (M = Li or Na, L = C6F6H6) [50], M+-1-M′− (M = Li, Na, and K; M′ = Cu, Ag, and Au) [51] and Ca+-1-M′− (M′ = Li, Na, and K) [52]. And the largest first hyperpolarizability (β0) of these alkalides reaches 3.51 × 106 au.