Assembly of Sandwich-Like Supermolecules Li Salts CpLi-C60: Structures, Stabilities, and Nonlinear Optical Properties

Theoretical study on exohedral complexes C6H6TMB40 (TM = Sc-Ni)

Exohedral borospherene complexes comprised of 3d transition metal (TM) atoms with borospherene B₄₀ and benzene (C₆H₆) molecules, C₆H₆TMB₄₀, were systematically studied by using density functional theory (DFT). Results show that in the ground states, the bonding type between TM and B₄₀ changes from η⁷ (TM = Sc-V) to η⁶ (TM = Cr-Fe) and then to η⁷ (TM = Co, Ni) with the increasing number of d electrons. Except for C₆H₆TiB₄₀ and C₆H₆FeB₄₀ being triplets, all C₆H₆TMB₄₀ clusters have the lowest spin. Namely, the ground spin state with an even number of electrons is a singlet state, and the ground spin state with an odd number of electrons is a doublet state. The investigated C₆H₆TMB₄₀ clusters (TM = Sc-Ni) possess higher stabilities through the ionic-covalent interactions between transition metals, benzene and the B₄₀ cage.

The encapsulated lithium effect on the first hyperpolarizability of C60Cl2 and C60F2

In this paper, we report a study on the structure and first hyperpolarizability of C60Cl2 and C60F2. The calculation results show that the first hyperpolarizabilities of C60Cl2 and C60F2 were 172 au and 249 au, respectively. Compared with the fullerenes, the first hyperpolarizability of C60Cl2 increased from 0 au to 172 au, while the first hyperpolarizability of C60F2 increased from 0 au to 249 au. In order to further increase the first hyperpolarizability of C60Cl2 and C60F2, Li@C60Cl2 and Li@C60F2 were obtained by introducing a lithium atom to C60Cl2 and C60F2. The first hyperpolarizabilities of Li@C60Cl2 and Li@C60F2 were 2589 au and 985 au, representing a 15-fold and 3.9-fold increase, respectively, over those of C60Cl2 and C60F2. The transition energies of four molecules (C60Cl2, Li@C60Cl2, C60F2, Li@C60F2) were calculated, and were found to be 0.17866 au, 0.05229 au, 0.18385 au, and 0.05212 au, respectively. A two-level model explains why the first hyperpolarizability increases for Li@C60Cl2 and Li@C60F2.

"Dancing inside the ball": the structures and nonlinear optical properties of three Sc2S@C3v(8)-C82 isomers

Recently, the crystal structures and electrochemical properties of the isomers (Sc2S "trapped" in C82) have been reported, in which the Sc2S is located inside the different positions of the C82 cage. In the present work, three isomers of endohedral metallofullerenes Sc2S@C3v(8)-C82 (A, B, and C) have been designed to explore the effect of the position of Sc2S on their interaction energies and nonlinear optical properties. Among three isomers, the Sc2S is located in different positions of the C82 cage: the angles of Sc-S-Sc in A, B, and C are 104.9, 114.8, and 115.7°, respectively. Furthermore, the analysis of natural bond orbital (NBO) charge indicates that the electron-transfer is from the Sc2S to the adjacent carbon atoms of the C82 cage. The interaction energy of B is the smallest among three isomers which is -226.2 kcal mol(-1). It was worth mentioning that their first hyperpolarizabilities (β tot) were studied, we found that their β tot values were related to the positions of Sc2S: C (2100) > B (1191) > A (947 au). We hope that the present work can provide a new strategy to promote the nonlinear optical properties of endohedral metallofullerenes by changing the positions of the encapsulated molecular. Graphical abstract Three isomers of endohedral metallofullerenes Sc2S@C3v(8)-C82 (A, B, and C) have been designed to explore the position effect of Sc2S on the interaction energies and nonlinear optical properties. Among three isomers, the Sc2S in B has the most stable position. Significantly, the first hyperpolarizability is related to the position of Sc2S inside the C82 cage, which provides a novel strategy to enhance the first hyperpolarizability by the Sc2S revolving inside the C82 cage.

Second hyperpolarizability of multimetallocenes [Cp–Mn–Cp] of Be, Mg and Ca

Multimetallocene complexes ( Cp – M n– Cp ) of Be , Mg and Ca have been considered for the theoretical study of static second hyperpolarizability using a number of DFT functionals. Owing to the cooperative effect in bonding, beryllium forms multiberyllocene complexes ( Cp – Be n– Cp ) which have sufficient thermal stability with respect to dissociation into neutral fragments up to n = 10. On the other hand, multimetallocene complexes of Mg and Ca are found to be stable for n ≤ 5 which may be due to the weaker covalent bonding interaction between the larger metal atoms. The rather small variation of linear and cubic polarizabilities of Cp – Be n– Cp complexes beyond n = 5 arises from the rather weaker charge transfer transitions. The difference in NLO property among the investigated metal complexes arises from the extent of charge transfer from the terminal metal atoms and the distance between them. The charge transfer at longer distances in the ground state of Mg and Ca complexes leads to more intense electronic transition — the spectroscopic parameters of which strongly favors the enhancement of second hyperpolarizability.

One lithium atom binding with P-nitroaniline: lithium salts or lithium electrides?

Recently, both lithium (Li) salts and Li electrides formed by one Li atom interacting with ligand complexes, have been widely investigated. An interesting question emerges: is the configuration of one Li atom interacting with ligand complexes a Li salt or electride? In the present work, four configurations n-Li-PNA (n = 1-4) were obtained by binding one Li atom with the p-nitroaniline (PNA) at different positions to explore this question. The results show that 1-Li-PNA and 2-Li-PNA are typical Li salts, and 4-Li-PNA is a typical Li electride. Significantly, 3-Li-PNA possesses both characteristics of Li salt and electride. At the same time, 3-Li-PNA has the largest first hyperpolarizability (2.9 × 10(6) au) by ROMP2 method compared with the other three configurations. Furthermore, the first hyperpolarizability of 3-Li-PNA is about 2600 times larger than that of PNA. Further, the vertical ionization potential (VIP) and interaction energy (E int) indicate that 3-Li-PNA is less stable than 1-Li-PNA and 2-Li-PNA (Li salts), but is more stable than 4-Li-PNA (Li electrides).

The V-shaped polar molecules encapsulated into Cs (10528)-C72: stability and nonlinear optical response

Recently, a new sulfide cluster fullerene, Sc2S@Cs (10528)-C72 containing two pairs of fused pentagons has been isolated and characterized (Chen et al., J. Am. Chem. Soc., 2012, 134, 7851). Inspired by this investigation, we propose a question: what properties will be influenced by the interaction between the encapsulated V-shaped polar molecule and C72? To answer this question, four encapsulated metallic fullerenes (EMFs) M2N@C72 (M = Sc or Y, N = S or O) along with pristine Cs-C72 (10528) were investigated by quantum chemistry methods. The results show that the Egap (3.01-3.14 eV) of M2N@C72 are significantly greater than that of pristine Cs-C72 (10528) (2.34 eV). This indicates that the stabilities of these EMFs increase by encapsulating the V-shaped polar molecule into the fullerene. Furthermore, the natural bond orbital (NBO) charge analysis indicates electron transfer from M2N to C72 cage, which plays a crucial role in enhancing first hyperpolarizability (βtot). The βtot follows the order of 1174 au (Y2O@C72) ≈ 1179 au (Sc2O@C72) > 886 au (Y2S@C72) ≈ 864 au (Sc2S@C72) > 355 au (C72). This indicates that the βtot of M2N@C72 is more remarkable than that of pristine Cs-C72 (10528) due to the induction effect of the encapsulated molecule. Compared with sulfide cluster fullerenes (Y2S@C72 and Sc2S@C72), oxide cluster fullerenes (Sc2O@C72 and Y2O@C72) show much larger βtot due to the small ionic radius and the large electronegativity of oxygen. In contrast, the metal element (scandium and yttrium) has a slight influence on the βtot. Thus, oxide cluster fullerenes are candidates to become promising nonlinear optical materials with higher performance.

Third-order NLO property of beryllium-pyridyne complexes

Six pyridyne isomers and their complexes with beryllium have been considered for the theoretical study of the third-order polarizability. The NLO properties are calculated by employing the DFT functionals BLYP, B3LYP, BHHLYP, B3PW91, BP86 and B2PLYP for the 6-311++G(d,p) basis set. The C - Be bond length in the complexes varies within 1.644 Å–1.771 Å indicating covalent interactions between the metal and pyridynes. The present investigation reveals that the magnitude of second-hyperpolarizability of pyridynes strongly enhances upon complex formation with beryllium. The maximum hyperpolarizability has been predicted for the 2,5-diberyllium pyridine complex. The lowest value of hyperpolarizability is obtained for the 2,3- and 3,4-diberyllium pyridine complexes. The chosen DFT methods predict almost identical pattern of variation of NLO property. The variation of second-hyperpolarizability has been satisfactorily explained by the excitation energy and transition dipole moment associated with the most dominant excited state.

Doping the alkali atom: an effective strategy to improve the electronic and nonlinear optical properties of the inorganic Al12N12 nanocage

Under ab initio computations, several new inorganic electride compounds with high stability, M@x-Al12N12 (M = Li, Na, and K; x = b66, b64, and r6), were achieved for the first time by doping the alkali metal atom M on the fullerene-like Al12N12 nanocage, where the alkali atom is located over the Al-N bond (b66/b64 site) or six-membered ring (r6 site). It is revealed that independent of the doping position and atomic number, doping the alkali atom can significantly narrow the wide gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (EH-L = 6.12 eV) of the pure Al12N12 nanocage in the range of 0.49-0.71 eV, and these doped AlN nanocages can exhibit the intriguing n-type characteristic, where a high energy level containing the excess electron is introduced as the new HOMO orbital in the original gap of pure Al12N12. Further, the diffuse excess electron also brings these doped AlN nanostructures the considerable first hyperpolarizabilities (β0), which are 1.09 × 10(4) au for Li@b66-Al12N12, 1.10 × 10(4), 1.62 × 10(4), 7.58 × 10(4) au for M@b64-Al12N12 (M = Li, Na, and K), and 8.89 × 10(5), 1.36 × 10(5), 5.48 × 10(4) au for M@r6-Al12N12 (M = Li, Na, and K), respectively. Clearly, doping the heavier Na/K atom over the Al-N bond can get the larger β0 value, while the reverse trend can be observed for the series with the alkali atom over the six-membered ring, where doping the lighter Li atom can achieve the larger β0 value. These fascinating findings will be advantageous for promoting the potential applications of the inorganic AlN-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical (NLO) materials.

Beryllium-cyclobutadiene multidecker inverse sandwiches: electronic structure and second-hyperpolarizability

Beryllium forms stable sandwich and inverse sandwich complexes with the cyclobutadiene molecule. Two types of multidecker complexes are designed. Multidecker inverse sandwiches are found to be thermally more stable than the corresponding sandwich complexes. The average distance between two consecutive metals and the two consecutive cyclobutadiene rings increase gradually on increasing size of the chosen inverse sandwich complexes. The density functional theory functionals B3LYP, BHHLYP, BLYP, M06, CAM-B3LYP, and B2PLYP in conjunction with the 6-311++G (d, p) basis set have been employed for calculating the third-order electric response properties of the chosen beryllium-cyclobutadiene complexes and the results obtained for each functional are found to have a consistent trend. Compared to the normal sandwich compounds the second-hyperpolarizability of inverse sandwiches is predicated to be larger, which fairly correlates with the extent of ground-state polarization. The significant enhancement of cubic polarizability of higher-order multidecker inverse sandwiches arises from the strong coupling between the ground and the low lying charge transfer excited states. The rather strong enhancement of second-hyperpolarizability on increasing size of the beryllium-multidecker inverse sandwiches may provide a new route to design efficient nonlinear optical materials.

INTERACTION OF BERYLLIUM WITH ACCEPTOR HYDROCARBONS: ELECTRONIC STRUCTURE AND SECOND HYPERPOLARIZABILITY

Some selected acceptor-Be hydrocarbon complexes have been considered for evaluation of second hyperpolarizability at different DFT functional. All the complexes have been found thermally stable and there is a significant increase of second hyperpolarizability compared to the ligands. Second hyperpolarizability has been explained in terms of charge transfer interaction. Co-operative interaction is required for maximization of second hyperpolarizability. Localized charge transfer in the vicinity of metal alone cannot increase second hyperpolarizability while charge delocalization over entire molecule is mandatory. Substitution by more electropositive metals e.g. Mg , Ca have greater enhancement in longitudinal component of γ. Basis set 6-311++G** is reasonable with respect to computational cost at aug-cc-pVnZ's (n = D, T, Q) for evaluation of second hyperpolarizability for the present complexes.

Enhanced reverse saturable absorption in graphene/Ag2S organic glasses

G/Ag2S composites were synthesized for the first time by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis demonstrated that Ag2S nanoparticles with a diameter of about 130 nm uniformly covered the graphene surfaces. G/Ag2S composites were dispersed in methyl methacrylate (MMA), polymerized at 75 °C for 30-35 min, and finally dried at 45 °C for 10 h, to afford G/Ag2S/PMMA organic glasses. The nonlinear absorption (NLA) properties of the G/Ag2S/PMMA organic glasses with different amounts of G/Ag2S were investigated by an open-aperture Z-scan technique. The experimental results showed that the G/Ag2S/PMMA organic glass with an appropriate amount of G/Ag2S exhibited enhanced reverse saturable absorption (RSA) properties compared to G/PMMA and Ag2S/PMMA organic glasses, which was attributed to the notable synergistic effects between graphene and Ag2S. Both one-photon absorption (OPA) in Ag2S and two-photon absorption (TPA) in graphene played important roles in RSA processes of the G/Ag2S/PMMA organic glasses. The effective NLA coefficient βeff of the G/Ag2S/PMMA organic glasses was in the order of 10(3) cm GW(-1). Thus this kind of organic glasses have great promise in optical limiter and optical shutter applications.