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Many interesting multifunctional materials are obtained under the application of an external electric field, which arouses extensive interest of experimental and theoretical researchers. In the present work, the structure–property relationships of a cross-linked single-walled carbon nanotube (CL-SWCNT) with bisaromatic amine, upper CNT(3, 3), and lower CNT(3, 3) are investigated under a longitudinal external electric field (F). The results show that the effect of the external electric field induces an intramolecular charge transfer (CT) from the lower CNT(3, 3) to the upper CNT(3, 3), which can bring extraordinary first hyperpolarizabilities (βtot). Upon increasing F, the βtotvalues are increased obviously. Significantly, the largest βtotvalue of 4.55 × 104au is present at F= 50 × 10–4au. Moreover, the βzcomponents for the CL-SWCNT are dominant that contribute greatly to the βtotvalues compared to other components, which is mainly attributed to the intramolecular CT transition. The current work shows that the external electric field appears as a promising strategy for enhancing the nonlinear optical properties and motivates further efforts in the development of carbon nanotubes.

The introduction of the Li3O and BeF3can exhibit an interlayer charge-transfer transition character for pure phenalenyl (PLY) dimer. A considerable question emerged: Can we modulate the interlayer charge-transfer transition character with different superalkalis and superhalogens? In the present work, nine complexes were designed with three superalkalis (Li3O, Li3S, and Na3O) and three superhalogens (BeF3, BeCl3, and MgF3) introduced into the PLY dimers, respectively. Results showed that the geometric structure, interlayer charge transfer, and electron transition character of PLY dimer were alternated by the superalkalis and superhalogens, especially for superalkali, with the increasing atomic number for alkali metals, interlayer distance and interaction energy between PLY monomers increased, while first hyperpolarizability (βtot) decreased. The results of TD-DFT calculation indicated that the variation of βtotwas mainly caused by the alteration of the difference of the dipole moments between the ground state and the crucial excited state Δμ. Otherwise, due to the introduction of superatoms, the distribution of HOMO and LUMO was changed greatly as compared to that of pure PLY dimer. In addition, aromaticity was also analyzed, and it was intriguing to find that introduction of superatoms caused asymmetric distribution of aromaticity along the central-carbon axis. All of the above consequences suggested that the introduction of superatoms was really an effective strategy to alter the structure and the nonlinear optical property of the system.

Based on s-indaceno[1,2,3-cd;5,6,7-c′d′]diphenalene (1) consisting of two phenalenyl moieties, the monomer 2and its dimer 22are designed by boron and nitrogen atoms substituting the central carbon atoms of phenalenyl moieties. Calculated energy decompose analysis (EDA) shows that the orbital interaction for 22possesses a large attractive contribution of −18.31 kcal mol–1, which is dominated by the π–π stacking interaction between the upper and the lower π-conjugated units. Interestingly, the natural population analysis (NPA) charge and the transition density matrix (TDM) show that both intramolecular charge transfer and intermolecular charge transfer (CT) exist in 22. Further, the first hyperpolarizability (βtot= 4.56 × 104au) of 2with intramolecular CT is greatly larger than that of reported molecule 3(5.45 × 103au) with intermolecular CT. Significantly, 22exhibits the largest βtotvalue to be 1.42 × 105au, which is caused by combining the intra- and intermolecular CT transitions (βx= 1.40 × 105au and βz= 2.27 × 104au). Correspondingly, highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) (intramolecular CT) in the low-energy electronic transition of 22is 68%, while HOMO → LUMO + 1 (intermolecular CT) is 18%, which demonstrates that the intramolecular CT effect on the βtotvalue is stronger than the case of the intermolecular CT effect. The present work might provide rich insight into designing and developing potential second-order optical nonlinearity materials with inter- and intramolecular CT characters.

Phenalenyl π-dimer (PLY2) has recently attracted intensive research interest due to its unique structure and binding characteristics (two-electron/12-center bonding). The directional transfer of electron or electron pair under the external electric field can produce a new structure with interesting properties. In the present work, we investigate for the first time the effect of the external electric field along the main molecule axis on PLY2. Two unpaired electrons between two layers are gradually shifted to the upper layer with increasing of the external electric field strength (Fext): the weaker the two-electron/12-center bonding, the stronger the electrostatic interaction between two layers. Significantly, a small increment of Fextmakes a big difference: the interlayer distance in the PLY2is sharply elongated from 3.241 Å (Fext= 203 × 10–4au) to 3.485 Å (Fext= 204 × 10–4au), which leads to the two-electron/12-center bonding breaking at 204 × 10–4au. Therefore, the Fext= 204 × 10–4au is regarded as the critical electric field. In this case, the interaction between two layers in PLY2is exclusively governed by the electrostatic interaction. Besides this, the effect of the external electric field brings some distinctive changes in its diradical character (y0), the Wiberg bond index (WBI), the interaction energy (Eint), and the frontier molecular orbital (FMO) that can be used to explore the conversion between bonding and electrostatic interactions. This study can deepen the understanding for the effect of the external electric field on structures and electric properties for molecule and be an open a door for the discovery and development of new switching devices.

Recently, the N-connecting pattern of the BN-segment has been shown as a suitable strategy to enhance the static first hyperpolarizability (β0) of carbon–boron–nitride heterojunction nanotubes (J. Phys. Chem. C2013, 117, 10039–10044). In this work, we report a quantum chemical investigation on the lithiation effect to further reveal the mechanism of modification. Interestingly, the lithiation effect is significantly dependent on the activating segment of the heterojunction nanotubes. For lithiation on the BN-segment, the β0(3.22 × 104au) of Li5–BN-1a is larger than that (1.42 × 104au) of Li5–BN-2a, which shows that the N-connecting pattern of the BN-segment linking to the C-segment is an efficient way to enhance the β0of heterojunction nanotubes. However, for lithiation on the C-segment, the β0(6.03 × 104au) of Li5–BN-1b is even slightly smaller than that (6.97 × 104au) of Li5–BN-2b. Besides, results show that activating the C-segment is a more effective strategy than activating the BN-segment for enhancing the β0of carbon–boron–nitride heterojunction nanotubes by lithiation. The new knowledge about heterojunction nanotubes might provide important information for designing nonlinear optical molecules by rationally introducing lithium atoms on carbon–boron–nitride heterojunction nanotubes.

Adamantane (Ad) is the parent molecule of diamondoids, which has been the subject of much chemical interest because of its highly symmetric cage structure. Many valuable papers demonstrated that the H atom of the methine position in Ad is highly reactive. Compared with highly symmetric carbon structures such as graphene and single-walled carbon nanotubes (SWCNTs), Ad only possesses σ bonds. In this work, we report a quantum chemical calculation on three complexes (Ad-Li, Ad-Na, and Ad-K), which were obtained by substituting the H atom of methine position in Ad with alkali metal (Li, Na, and K). Interestingly, the substitution by alkali metals leads to absorption within the visible region. The maximum absorption wavelengths of the complexes show a red shift trend from the Li to the K complex. This trend indicates that the crucial transition energy becomes smaller, which might lead to a larger nonlinear optical response. Among the three structures, the largest first static hyperpolarizability (βtot) of the K complex was 76626 au, which is about 45 times larger than a prototypical second-order nonlinear optical (NLO) molecule of p-nitroaniline (βtot= 1679 au). Therefore, our results show that alkali metal substituted Ads may be novel potential candidates for high-performance NLO materials.

An increasing number of scientists have focused on carbon–boron-nitride heteronanotubes because of their particularly adjustable properties, as shown in many fields. In this work, four isoelectronic models (BN-n, n= 1–4) were systematically investigated to explore the crucial factor for enhancing the static first hyperpolarizibility by doping the BN segment into the carbon nanotube (CNT) with differently connecting patterns. Theoretical results show that the N-connecting pattern might increase the contribution of the BN segment to the crucial transition states, which obviously increases the occupied orbital energy while the unoccupied orbital energy is slightly influenced. Correspondingly, the transition energy of BN-1 is smaller than that of BN-2. As a result, the static first hyperpolarizability of BN-1 is 1.05 × 104au, which is remarkably larger than the 4.37 × 102au of BN-2. The results indicate that, compared to the B-connecting pattern, the N-connecting pattern of the BN segment linking to the conjugated CNT segment is a more efficient way to enhance the first hyperpolarizability of heteronanotubes. It is our expectation that the new knowledge about the carbon–boron-nitride heteronanotubes could provide valuable information for scientists to develop the potential nonlinear optical nanomaterials by introducing BN segments into suitable positions of CNTs.

Trumpet-shaped carbon nanocone (CNC) is used as a π-conjugated bridge to design high-performance nonlinear optical (NLO) material. Owing to the attractive trumpet-shaped structure, O2N–CNC–NH2exhibits nonlinear optical behavior that is both surprisingly and qualitatively distinct from conventional π-conjugated organic species. It has been shown that the electron-acceptor nitryl (−NO2) at the apex of the trumpet and the electron-donor amino (−NH2) at the bottom edge of the trumpet to form O2N–CNC–NH2is beneficial to improve the NLO response of CNC. Significantly, the first hyperpolarizabilities (βtot) of O2N–CNC–NH2show an arc-shaped change as −NH2substituting different H atoms along the arc-shaped bottom edge of CNC. Interestingly, the βtotvalues of H–CNC–NH2also show an arc-shaped change. By comparison of the βtotvalues of O2N–CNC–NH2and H–CNC–NH2, it has been found that no matter which bottom H of H–CNC–H is substituted by NH2, the βtotvalue of H–CNC–NH2will be increased about 1.6 times after substituting the top H with −NO2into the corresponding molecule. This study may stimulate the search for new types of NLO materials based on CNC for application.

Very recently, tubiform multilithium salts have been investigated as new candidates for high-performance nonlinear optical (NLO) materials because of their large static first hyperpolarizability (β0) (J. Phys. Chem. C2009, 113, 4984−4986). The interesting question of how to further enhance the β0value of the multilithium salt, by widening or lengthening the tubiform cyclacene, is studied in this work. On one hand, the effect of widening on the β0value of tubiform multilithium salts is investigated using Lin-[n]cyclacene (n= 5–8). We found that the β0value (15497 au) of Li8-[8]cyclacene is 3 times larger than the 5028 au value of the Li5-[5]cyclacene, with the total number of carbon atoms increasing from 20 to 32. On the other hand, lengthening Li5-[5]cyclacene to form Li5–CNT (5,0) results in the dramatically enhanced β0value of 146642 au, which is 29 times larger than the value for the Li5-[5]cyclacene, even though the total number of carbon atoms is increased only from 20 to 30. Further, the second harmonic generation (SHG) β(−2ω;ω,ω) and the electro-optical Pockels effect (EOPE) β(−ω;ω,0) increase with an increase in frequency (ω) from 0.0000 to 0.0200 au. Because cyclacenes have been proposed to be super-short single-walled carbon nanotubes, it is our expectation that this work could provide more useful information for the development of nonlinear optical nanomaterials.

How do the number and location of lithium atoms affect the first hyperpolarizability (βtot) of graphene? In this paper, based on pentacene, a series of graphene (multi)lithium salts Lin@pentacene (n= 1, 2, 3, 4, 5, and 6) have been designed to answer this question. βtotobviously increases stepwise with an increase in the number of lithium atoms: 1369−1843 for Li@pentacence < 3510−4081 for Li2@pentacence < 6933−7934 for Li3@pentacence < 11 188−12 145 for Li4@pentacence < 14 904 au for Li5@pentacence, which are much larger than pentacence. This pattern suggests that the lithium salt effect on the first hyperpolarizability is very large. Unexpectedly, when an additional lithium atom is doped into the graphene multilithium salt Li5@pentacence, which leads to Li6@pentacence, the βtotvalue dramatically increases to a value of 4 501 764 au with a remarkable increase of 302-fold in contrast to Li5@pentacence. On the other hand, when the number of lithium atoms is equal, the location of lithium atoms also affects the βtotvalue: the closer the lithium atoms are clustered, the larger the βtotvalue: for Li3@pentacence, 6933 au of system 10 < 7401 au of system 9 < 7934 au of system 8. Furthermore, their transition energies (ΔE) are also obtained. The results show that ΔEdecreases stepwise with an increase in the number of the lithium atoms, and ΔEof Li6@pentacence sharply decreases to 0.299 eV, which may explain the huge βtotvalue. This study may stimulate the search for new types of graphene NLO materials based on alkali metals for NLO application.

Four knot-isomers of Möbius cyclacene are composed of 15 nitrogen-substituted benzo rings. They are non-Möbius cyclacenes without a knot (0), Möbius cyclacenes with a knot (1), non-Möbius cyclacenes with two knots (2), and Möbius cyclacenes with three knots (3). Their structures and nonlinear optical properties are systematically studied. The order of first hyperpolarizability (β0) is 4693 (0) < 10 484 (2) < 25 419 (3) < 60 846 au (1). The β0values of the knot-isomers with knot(s) are larger than that of the knot-isomer without a knot. It shows that the β0value can be dramatically increased (13 times) by twisting the knot(s) to the cyclacene. Two noticeable relationships between the number of knots and the first hyperpolarizability have been observed: (i) the β0values of one surface Möbius cyclacene (1and 3) with an odd number of knots are larger than that of two-surface non-Möbius cyclacenes (0and 2) with an even number of knots. (ii) For the one-surface Möbius cyclacenes, the β0value for 1with one knot is larger than that for 3with three knots. On the other hand, the largest component of β0is alternated for the four knot-isomers. The largest components are βzfor the 0and βyfor the 1and 2. The largest component turns back to the βzfor the 3.

Due to the novel π-conjugated structure, carbon nanocone (CNC) has aroused the interest of the chemists. In the present work, we found that the replacement of two terminal carbon atoms by two gold atoms led to the obvious red shift of the absorption peak of Au2-CNC. Significantly, the latitudinal (y-axis) external electric field (EEF) can regulate the absorption peak, which is related to the second-order nonlinear optical response (βtot). The corresponding results are as follows: i. The intensity of the shoulder peak at about 300 nm increased mildly when EEFchanged from 0 to −50 × 10–4a.u., which caused the βtotvalues to decrease gradually. ii. A new broad absorption peak appeared at about 550 nm when EEFchanged from 0 to 50 × 10–4a.u., which generated increased βtotvalues. We hope that the present work can provide a fresh strategy to regulate and control the nonlinear optical property.

On the basis of tubiform [5]cyclacene, a new multilithium salt Li5-[5]cyclacene and a new electride Li−Li5-[5]cyclacene are designed in theory. As five Li atoms substituting the five edge H atoms of the [5]cyclacene form the noncentrosymmetric Li5-[5]cyclacene, the natural bond orbital (NBO) charges of these five Li atoms (Li+= 0.840) show that Li5-[5]cyclacene is a new multilithium salt. The β0value is greatly increased about 1200 times from 7 ([5]cyclacene) to 7938 au (Li5-[5]cyclacene) by the lithiation effect. Specially, the Li-doped effect on the static first hyperpolarizability in the new electride Li−Li5-[5]cyclacene has been explored for the first time. An additional Li atom doping into this new tubiform multilithium salt Li5-[5]cyclacene leads to a new tubiform electride Li−Li5-[5]cyclacene, and the β0value increases about 5 times from 7938 (Li5-[5]cyclacene) to 35 384 au (Li−Li5-[5]cyclacene). This shows that the Li-doped effect greatly increases the first hyperpolarizability.