Enhancement of second-order nonlinear optical response in boron nitride nanocone: Li-doped effect.

The unusual properties of Li-doped boron nitride nanomaterials have been paid further attention due to their wide applications in many promising fields. Here, density functional theory (DFT) calculations have been carried out to investigate the second-order nonlinear optical (NLO) properties of boron nitride nanocone (BNNC) and its Li-doped BNNC derivatives. The natural bond orbital charge, electron location function, localized orbital locator and frontier molecular orbital analysis offer further insights into the electron density of the Li-doped BNNC derivatives. The electron density is effectively bounded by the Li atom and its neighboring B atoms. The Li-doped BNNC molecules exhibit large static first hyperpolarizabilities (ß(tot)) up to 1.19×10³ a.u. for Li@2N-BNNC, 5.05×10³ a.u. for Li@2B-BNNC, and 1.08×10³ a.u. for Li@BN-BNNC, which are significantly larger than that of the non-doped BNNC (1.07×10² a.u.). The further investigations show that there are clearly dependencies of the first hyperpolarizabilities on the transition energies and oscillator strengths. Moreover, time-dependent DFT results show that the charge transfer from BNNC to Li atom becomes more pronounced as doping the Li atom to BNNC. It is also found that the frequency-dependent effect on the first hyperpolarizabilities is weak, which may be beneficial to experimentalists for designing Li-doped BNNC molecules with large NLO responses.

Strategy for enhancing second-order nonlinear optical properties of the Pt(II) dithienylethene complexes: substituent effect, π-conjugated influence, and photoisomerization switch.

The second-order nonlinear optical (NLO) properties of a series of Pt(II) dithienylethene (DTE) complexes possessing the reversible photochromic behavior have been investigated by density functional theory (DFT) combined with the analytic derivatives method. The results show that the calculated static first hyperpolarizabilities (ßtot) of the open-ring and closed-ring systems significantly increase in the range of 2.1-4.5 times through strengthening of the electron-withdrawing ability of the substituent R (R = H, CF3, NO2) and an increase of the number of thiophene rings. Moreover, there is a large enhancement of the ßtot values from the open-ring systems to the corresponding closed-ring systems. This efficient enhancement is attributed to the better delocalization of the π-electron system, the more obvious degree of charge transfer, and the larger f(os)/E(gm)(3) (f(os) is the oscillator strength, and E(gm) is the transition energy between the ground and the excited states) values in the closed forms according to the bond length alternation (BLA) and time-dependent density functional theory (TDDFT) calculations. In addition, the dispersion has less influence on the frequency-dependent first hyperpolarizabilities (ßtot(ω)) of the studied systems at the low-frequency area ω (0.000-0.040 au). Our present work would be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.

Computational investigation on redox-switchable nonlinear optical properties of a series of polycyclic p-quinodimethane molecules.

The polycyclic p-quinodimethanes are proposed to be the novel candidates of the high-performance nonlinear optical (NLO) materials because of their large third order polarizabilities (γ). We investigate the switchable NLO responses of a series of polycyclic p-quinodimethanes with redox properties by employing the density functional theory (DFT). The polycyclic p-quinodimethanes are forecasted to exhibit obvious pure diradical characters because of their large y 0 index (the y 0 index is a value between 0 [closed-shell state] and 1 [pure biradical state]). The γ values of these polycyclic p-quinodimethanes and their corresponding one-electron and two-electron reduced/oxidized species are calculated by the (U)BHandHLYP method. The γ values of polycyclic p-quinodimethanes and their corresponding one-electron reduced species are all positive and significantly different. The large differences of the γ values are due to a change in the transition energy and are related to the different delocalization of the spin density, which demonstrates that the NLO switching is more effective on one-electron reduction reactions. Therefore, the study on these polycyclic p-quinodimethanes provides a guideline for a molecular design of highly efficient NLO switching.

Modulation of the second-order nonlinear optical properties of the two-dimensional pincer Ru(II) complexes: substituent effect and proton abstraction switch.

The static second-order nonlinear optical (NLO) properties on a series of the two-dimensional (2D) pincer Ru(II) complexes with the substituted Tpy and H(2)SCS tridentate ligands (Tpy = 2,2':6',2″-terpyridyl and H(2)SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl) have been investigated by density functional theory (DFT). Introducing different donor/acceptor substituents to two ligands has an influence on the static first hyperpolarizabilities (ß(tot)) of the 2D systems. Compared to the reference system 1 [Ru(H(2)SCS)(Tpy)](+), introducing the branches with strong electron acceptor group (p-NO(2)-phenylethynyl) to the Tpy ligand or the branches with strong electron donor group (p-NH(2)-phenylethynyl) to the H(2)SCS ligand can effectively improve the ß(tot) values. Time-dependent DFT (TDDFT) calculations indicate that the enhanced ß(tot) values of the substituted systems are dominated by the intraligand charge transfer (ILCT), metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) transitions. Furthermore, the proton abstraction plays an important role in tuning the second-order NLO response. Particularly, for system 5 bearing the branches with NO(2) groups on H(2)SCS ligand, there is a dramatic enhancement in the ß(tot) values for its deprotonated forms. The ß(tot) values of the monodeprotonated system 5-H and the dideprotonated system 5-2H (58.712 × 10(-30) and 761.803 × 10(-30) esu) are about 7.58 times and 36.4 times larger than their diprotonated system 5, respectively. The second-order NLO responses based on substituent effect and proton abstraction switch are two-dimensional in characteristic with the large off-diagonal tensor values.