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.