The Structures And The Nonlinear Optical Properties Of The Novel Alkalides

Due to the potential applications in the optical computer, optical process ofinformation, optical communication, etc, the study of the non-linear optical (NLO)properties has attracted much attention. In previous work, it has been demonstratedthat the introduction of loosely bound electrons is a new effective strategy to enhancethe β0value of a system. Alkalide is a new kind of representative compound thatcontains loosely bound electrons. In this work, we have theoretically designed a seriesof new alkalides, and investigated the structures and nonlinear optical properties ofsuch species. The main contributions are as follows:(1) A series of M2+(H5Aza222)–M'–(M=Be, Mg, Ca; M'=Li, Na, K) alkalidesthat contain alkaline earth metal cations complexed by the H5Aza222–cage have beeninvestigated for the first time using the CAM-B3LYP method. These alkaline earth-based alkalides not only present unusual structural features but also exhibitextraordinarily large static first hyperpolarizabilities (β0) up to1.98×105au.Different from the previously reported Adz cage complexant akalides where the insidealkali metal located at the center of the cage, the inside alkaline metal M atom of theMH5Aza222M' alkalide is not centrally located in the Aza222complexant in presentwork. Due to the lossely bound excess electron, all the M2+(H5Aza222)–M'–compounds show fairly small ΔE values, which is the reason why these speciesexhibit considerably large β0values. In addition, the relationships between the β0values of M2+(H5Aza222)–M'–and the atomic number of the M'–anion, the atomicnumber of the M2+cation, and the M–M' distance are also explored.(2) New alkalides M+aza222M'-(M, M'=Li, Na, K) are designed by inserting thealkali atom into the azacryptand[2.2.2] complexants. Using the Density FunctionalTheory (DFT) with the B3LYP functional, the optimized structures of the alkalidesM⁺aza222M'-(M, M'=Li, Na, K) were obtained. The nonlinear optical (NLO)properties of these species were calculated by the BHandHLYP method. The results indicate that the M+aza222M'-alkalides exhibit very large firsthyperpolarizabilities (β0) up to1.0×106a.u.(for M=Li, M'=K). By comparing theβ0values among various organic alkalides, the aza222is found to be preferableto the previously investigated complexants in enhancing the firsthyperpolarizabilities of alkalides. This work reveals that the proper organiccomplexant can greatly enhance the NLO responses for the alkalides.(3) The nine optimized structures of the organic M⁺@H₆Aza222M'-·2MeNH2(M, M'=Li, Na, K) alkalides with real frequencies were obtainedusing the Density Functional Theory (DFT) with the B3LYP functional and the6-31G basis set. Studies of the structures reveal that the inside M atom shows atendency to move towards one side of the H6Aza222cage with decreasing atomicnumber. Also, we found that one MeNH2molecule that is close to alkali atom Mcan also be considered as a complexant, while the other MeNH2molecule did notinteract with the inside alkali atom M. The BHandHLYP method was employedto calculate the nonlinear optical (NLO) properties of these species, therelationships between the first hyperpolarizabilities (β0) and the structuralfeatures of M⁺@H₆Aza222M'-·2MeNH2are also investigated. The results showthat the M⁺@H₆Aza222M'-·2MeNH2alkalides exhibit dramatically large β0values, and the largest β0value is1280342au for the Na+@H6Aza222K-·2MeNH2species. It is also found that the first hyperpolarizabilities ofM⁺@H₆Aza222M'-·2MeNH2increased with the increasing atomic number ofalkali metal atom (M') which resides outside the H6Aza222complexant.(4) Novel alkalides with high NLO responses have been designed for thefirst time by introducing the superalkali Li3into the calix[4]pyrrole complexant.The structures of the novel superalkali-based alkalides Li³⁺(calix[4]pyrrole)M-(M=Li, Na and K) were obtained at the B3LYP/6-311+G(d) level. By using theCAM-B3LYP method, the NLO properties of the Li³⁺(calix[4]pyrrole)M-alkalides have been investigated systematically. Results show that theLi³⁺(calix[4]pyrrole)M-species have considerably large firsthyperpolarizabilities (β0). It is also found that the β0values ofLi³⁺(calix[4]pyrrole)M-not only correlate to the M atomic number but to therelative position between Li3plane and the N-N-N-N (N4) plane ofcalix[4]pyrrole. When M=K and the Li3plane is parallel to the N4plane P- [(Li³⁺(calix[4]pyrrole)K-], the largest β0up to30050au has been achieved. Ourinvestigation on such superalkali-based alkalides provides new candidates to thealkalide family.