FIGURE 7 Plots of\(-x\rho_{\text{xx}}^{(2)}\left(r\right)\)and \(-y\rho_{\text{yy}}^{(2)}\left(r\right)\) for OLi3@(GDY/GTY)
It is interesting to compare the β 0 values of our superalkali salts of graphynes with the previously reported superatom doped complexes. Theoretical studies on the three series of AM3@GDY, M2X@GDY, and M3F@GDY complexes show that intramolecular charge-transfer mechanism in the D-A system can play a key role in enhancing nonlinear optical properties.32,35,36A comparison between OM3+@GDY(β 0 = 2.2×105–2.8×105 au) and AM3@GDY (AM = Li, Na, and K)32(β 0 = 9.2×103–1.6×105 au) indicates that doping superalkali OM3 may be more beneficial to enhance the NLO response than AM3. The computational results of Shehzadi et al.36suggested that the β 0 values of superalkalis doped graphdiyne [M2X@GDY (M = Li, Na, K and X = F, Cl, Br)] are in the range of 6.2×103–6.6×104 au, which are much lower than those (2.2×105–2.8×105au) of OM3+@GDY. Literatures reported19,62,63that ionization energy of OM3 is lower than the corresponding M2F (M = Li, Na, and K). Therefore, superalkali OM3 can induce greater charge transfer in the doped compounds than M2F does, and consequently, OM3, as a dopant, can increase theβ 0value of GDY more effectively. Besides, the excellent NLO response of OM3+@GDY is strongly superior to that of superalkaline-earth metal doped GDY (β 0=1.1×104 au), and even preferable to those of cationic M3F@GDY+ (M = Li, Na, and K) (1.1×105–1.6×105au).35 This means that using superalkali OM3 as a dopant may be a better choice. Noteworthily, the β 0 values of OM3+@(GDY/GTY) are much larger than those of previously reported superalkali (M3O+, M = Li, Na, and K) supported graphene nanoflakes (GR) (8.4×102–3.1×103au),64 which indicates that the largely π-conjugated graphyne is superior to graphene in producing complexes with large β 0 values.
Besides large NLO responses, excellent NLO molecules should have favored transparency spectral ranges (infrared or deep-ultraviolet bands) of electronic absorption spectra, which cannot be ignored in practice. The ultraviolet-visible (UV-vis) absorption spectra of the systems are obtained by using the TD-CAM-B3LYP method. As one can observe from Figure 8, both pristine GDY and GTY have maximum absorption in ultraviolet region (wavelength < 400 nm). However, we can find that, upon the introduction of superalkalis, the strongest absorption peak shifts from short wavelength to long wavelength. Besides, the main absorption region of OM3+@GDY is from 250 to 1000 nm. That is to say, there are obvious absorption peaks within the visible or near visible region. The OM3+@GTY complexes have visible and infrared absorption regions at wavelength > 300 nm. Obviously, both the OM3+@GDY and OM3+@GTY complexes have a deep-ultraviolet transparent region at wavelength ≤ 200 nm, demonstrating that they could be used as new candidates for deep-ultraviolet NLO materials.
Overall, all of these superalkali salts of graphynes with high structural stability not only have excellent NLO response but also have a satisfactory working waveband in the deep-UV region, which may be a new member of the high-performance NLO material family.