Shiling Sun’s research while affiliated with Northeast Normal University and other places

It is well known that ammonia borane (BH3NH3) is one of the simplest donor-acceptor complex. The donor-acceptor bond (B-N bond) is formed by sharing lone pair electrons between BH3 and NH3 groups. In present work, different strength of external electric fields (Eext) are applied along Z-axis direction to investigate the electric field induced effect on BH3NH3 structure and properties. Interestingly, we have found that the lone pair electrons can be gradually moved in the “channel” between the BH3 and NH3 groups by modulating Eext. The donor-acceptor bond (B-N bond) is gradually elongated until it breaks when the Eext range from 0.0000 to 0.0321 au. The B-N bond is the shortest at Eext = -0.0519 au. Interestingly, the negative charge on BH3 groups sharply decreases from 0.161 to 0.005 (for NH3 groups decreases from 0.161 to 0.005) and the electron cloud of HOMO-2 exhibits an obvious transformation at “broking bonding” Eext ranging from 0.0320 to 0.0321 au, indicating the electron movement induced by electric-field is the main reason to change the structure and stability of BH3NH3. Further, atoms in molecules (AIM) analysis shows that B•••N interaction are similar to that of hydrogen bonds at Eext = -0.0767 au and the iconicity of B-N bond in BH3NH3 are confirmed by its low electron density for B-N ρ(B-N) (0.103~0.073) in the region of Eext (0～0.025 au).

In 2003, a novel compound 2 containing the benzo-15-crown-5 moiety has been synthesized and described. In present work, we have designed two compounds 1 (benzo-12-crown-4) and 3 (benzo-18-crown-6) on the basis of compound 2. Further, nine configurations N*M (N = 1, 2 and 3; M = Li+, Na+ and K+) are designed by the compounds 1, 2 and 3 complexing alkali metal cations. Density functional calculation is performed to investigate the effect of ring sizes and the nature of alkali metal cations on the interaction energy, charge transfer and nonlinear optical properties. The results indicate that the interaction energy of N*M depends on both the ring sizes and the nature of alkali metal cations. Moreover, the amount of net charge transfer is related to the diameters of the alkali metal cations. In addition, the calculated nonlinear optical properties reveal that the compound 2 has the largest first hyperpolarizability among three compounds 1, 2 and 3. However, the alkali metal cations give rise to different effects on the nonlinear optical properties. Significantly, the order of the first hyperpolarizability can be explained by the transition energy and the dipole moment variation within the two-state approximation.

In 2006, the Au–C22H14 with a covalent bond between an individual pentacene (C22H14) and a gold (Au) atom was synthesised and characterised, and its nonlinear optical (NLO) properties were explored. To further investigate the NLO properties from molecules to materials, three kinds of different dimers (Au–C22H14)2 (2, 3 and 4) were designed to probe the monomer accumulation modes on the structures and NLO properties. The results indicate that Au atoms doping breaks the conjugate structures of the two pentacenes to different extent. On the other hand, their NLO properties investigated by three density functional theory methods Becke-Half-and-Half-LYP (BHandHLYP), Coulomb-attenuating method Becke-3-Lee–Yang–Parr (CAM-B3LYP) and Minnesota 2005 double the amount of nonlocal exchange (M05-2X) show the same order, and 2 has the largest first hyperpolarisability (βtot) than the other molecules. At the same time, natural bond orbital analysis shows the Au atoms play a crucial role in pushing electron density. Meanwhile, the frontier molecular orbital analysis shows that charge transfer has occurred between the two pentacene molecules and Au atoms. As a result, the order of transition energy is opposite to the order of βtot values. Because the pentacene is taken as a simplified fragment of the graphene, our present work may be beneficial to the development of high-performance NLO materials.

The nonlinear optical (NLO) properties of Λ-shaped diarylethene (DAE) derivatives 1a(b)–4a(b) and their NLO switching effects were studied by using the density functional theory (DFT) methods. The results demonstrate that all of the open-ring molecules and their own closed-ring forms meet the model of NLO switching tuned by photoisomerization. The βtot values of 1b–3b are 16 times as small as that of their open-ring forms, and βtot value of 4b is 4 times as large as that of 4a. The spin interactions of open-shell closed-ring molecules are larger than that of their open-ring forms, and it could increase the NLO responses to some degree. Nature bond orbital (NBO) calculations indicate that large charge differences between electron-deficient and electron-rich centers are beneficial to charge transfer (CT), and the overlap between frontier molecular orbital (FMO) is also advantageous to the CT and NLO responses. Time-dependent density functional theory (TD-DFT) calculations show βtot values of all molecules meet the two-level model very well, and the smaller the ΔEge, the larger the βtot value.

Recently, C60Cl8 (C2v) has been experimentally synthesized (Nature materials, 2008, 7, 790) by addition of eight chlorine atoms to C60 (C2v), which is associated with a Stone-Wales transformation of C60 (Ih). In this work, the first hyperpolarizabilities (βtot) of C60 (C2v) and C60Cl8 (C2v) are investigated. After the Stone-Wales transformation and chlorine addition reaction, the βtot values slightly increase from 0 of C60 (Ih) to 60 of C60 (C2v) and 502 au of C60Cl8 (C2v), respectively. To further enhance the first hyperpolarizability, the endohedral fullerene derivate Li@C60Cl8 by encapsulating a lithium (Li) atom inside the C60Cl8 has been designed. Interestingly, the electron transfer between Li and C60Cl8 leads to an extremely large βtot value of 25569 au, which is greatly larger (51 times) than the 502 au of C60Cl8. It shows that the encapsulated Li effect plays an important role to enhance the first hyperpolarizability. So the Li@C60Cl8 can be considered as a candidate for high-performance nonlinear optical materials.

The structures and second-order nonlinear optical (NLO) properties of a series of chlorobenzyl-o-carboranes derivatives (1–12) containing different push-pull groups have been studied by density functional theory (DFT) calculation. Our theoretical calculations show that the static first hyperpolarizability (βtot) values gradually increase with increasing the π-conjugation length and the strength of electron donor group. Especially, compound 12 exhibits the largest βtot (62.404×10−30 esu) by introducing tetrathiafulvalene (TTF), which is about 76 times larger than that of compound 1 containing aryl. This means that the appropriate structural modification can substantially increase the first hyperpolarizabilities of the studied compounds. For the sake of understanding the origin of these large NLO responses, the frontier molecular orbitals (FMOs), electron density difference maps (EDDMs), orbital energy and electronic transition energy of the studied compounds are analyzed. According to the two-state model, the lower transition energy plays an important role in increasing the first hyperpolarizability values. This study may evoke possible ways to design preferable NLO materials.

The dynamic first hyperpolarizabilities of a series of 1,10-phenanthroline Ru(II) complexes were carried out using density functional theory (DFT). The results indicate that these complexes have large second-order nonlinear optical (NLO) responses. Specially, complex 6b has a maximal first hyperpolarizability βtot value. The first hyperpolarizabilities can be tuned by changing the ancillary ligand, introducing electron-acceptor group NO2 and/or increasing π-conjugation on phenanthroline. Calculations on absorption spectra demonstrate that the second-order NLO responses of complexes in series a are ascribed to the intraligand charge transfer (ILCT), while the complexes in series b exhibit metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) transition at relatively low-energy absorptions.

Recent studies show that non-dipolar molecular structures, having multidimensional (nD) nonlinear optical (NLO) characteristics, can be regarded as excellent candidates for the second-order NLO applications. Then, the second-order NLO response of a series of o-carborane (C2B10H12) derivatives was investigated by density functional theory (DFT) method. The results suggest that their second-order nonlinearities are dominated by two second-order polarizability tensor components (βzzz and βzyy). Electronic transitions to crucial excited states show that y-polarized transition, contributed to the off-diagonal second-order polarizability tensor (βzyy), possesses lower transition energy compared with z-polarized transition accounted for the diagonal second-order polarizability tensor (βzzz), which led to the large in-plane nonlinear anisotropy (μ = βzyy/βzzz) value, as well as good 2D second-order NLO properties. Moreover, the βtot values gradually increase from the monosubstituted (MS) to disubstituted (DS) o-carboranes. With increasing the strength of donor groups in DS molecules, the βtot values are also enhanced. The micromechanism of the second-order NLO properties has been analyzed from the standpoint of dipole moment, transition electronic dipole moment and transition energy. Our studies show that these complexes can be used as excellent 2D second-order NLO materials from the standpoint of large β, high transparency, and μ values.

In this work, density functional theory (DFT) combined with the finite field (FF) method has been adopted to analyze the second-order nonlinear optical (NLO) properties of the triarylborane (TAB) derivatives obtained by introducing different inductive electron groups into the phenylene ring of the TAB (RTAB, where R=2-C6H5-C2B10H10(1), R=F(2), R=Me(3), R=NO2(4), R=NH2(5)). The static first hyperpolarizabilities (β tot) of the RTAB molecules can be switched by binding one F− to the boron center (RTAB′) or one-electron reduction (RTAB″). The DFT-FF calculations show that the β tot values of 2′, 3′ and 5′ decrease while those of 1′ and 4′ increase compared with the values of their neutral molecules, which was attributed to the fact that the charge transfers of 3 and 5 become smaller and those of 1 and 4 become larger by binding one F− ion to the boron center, according to time-domain DFT (TD-DFT) analysis. However, the incorporation of one electron enhances the second-order NLO properties of the RTAB molecules remarkably, especially for system 1. It is notable that the β tot value of reduced form 1″ is 508.69×10−30 esu, i.e. abou 578 times larger than that of system 1. Frontier molecular orbital (FMO) and natural bond orbital (NBO) analyses suggest that the reversal of the charge distribution between the neutral molecules and their reduced forms leads to low HOMO-LUMO energy gaps (E 0) and thus large β tot values for the reduced forms.