Electronic Absorption Spectra and Third-Order Nonlinear Optical Property of Dinaphtho[2,3-b:2’,3’-d]Thiophene-5,7,12,13-Tetraone (DNTTRA) and Its Phenyldiazenyl Derivatives: DFT Calculations

Third-order nonlinear optical (NLO) materials have broad application prospects in high-density data storage, optical computer, modern laser technology, and other high-tech industries. The structures and frequencies of Di-naphtho[2,3-b:2’,3’-d]thiophene-5,7,12,13-tetraone (DNTTRA) and its 36 derivatives containing azobenzene were calculated by using density functional theory B3LYP and M06-2X methods at 6-311++g(d, p) level, respectively. Be-sides, the atomic charges of natural bond orbitals (NBO) were analyzed. The frontier orbitals and electron absorption spectra of A-G5 molecule were calculated by TD-DFT (TD-B3LYP/6-311++g(d, p) and TD-M06-2X/6-311++g(d, p)). The NLO properties were calculated by effective finite field FF method and self-compiled program. The results show that 36 molecules of these six series are D-π-A-π-D structures. The third-order NLO coefficients γ (second-order hyperpolarizability) of the D series molecules are the largest among the six series, reaching 10 7 atomic units (10 −33 esu) of order of magnitude, showing good third-order NLO properties. Last, the third-order NLO properties of the azobenzene ring can be improved by introducing strong electron donor groups (e.g. -N(CH 3 ) 2 or -NHCH 3 ) in the azobenzene ring, so that the third-order NLO materials with good performance can be obtained. The third-order NLO coefficient (γ) of azobenzene is the largest when azobenzene is introduced at 2.10 sites. The third-order NLO coefficients (γ) of azobenzene at 3.9 sites, 2.9 sites and 2.10 sites are much larger than those of azobenzene at 1.8 sites, 1.11 sites and 4.8 sites. The results show that azobenzene at 1.8 sites, 1.11 sites and 4.8 sites is not as good as azobenzene at 3.9 sites, 2.9 sites and 2.10 sites, which is consistent with the structural copla-narity analysis of the preceding molecules. The best substitution of azobenzene for A molecule is at 2.10 sites. Among the six series of substituted derivatives such as B, C, D, E, F and G, the third-order NLO coefficient (γ) of the three series derivatives such as B, C and D is obviously larger than that of the three series derivatives such as E, F and G. The third-order NLO coefficients of D series derivatives are the largest in the six series of substituted derivatives, reaching 10 7 order of magnitude atomic units (10 −33 esu), showing good third order NLO properties. The third-order NLO coefficients (γ) of each series of derivatives have the same change rule, that is, they increase in turn according to the substitution of OH, -OCH 3 , -NH 2 , -NHCH 3 , -N(CH 3 ) 2 . This is mainly due to the induction effect and conjugation effect of the introduced donor electrons. -N(CH 3 ) 2 and-NHCH 3 have strong electron-donating induction and conjugation effects, while -OH and -OCH 3 have relatively weak induction and conjugation effects.


Introduction
Third-order nonlinear optical (NLO) materials have the characteristics of fast nonlinear optical response, large nonlinear polarizability, wide response band, high optical damage domain value, excellent flexibility, good chemical and thermal stability, and are easy to modify molecular structure. This kind of material has broad application prospects in high-density data storage, optical computer, modern laser technology, and other high-tech industries [1] [2] [3] [4].
Therefore, many chemists are attracted to design and synthesize new compounds with strong nonlinear optical effects. A large number of research results show that the third-order nonlinearity of materials is closely related to the type of molecular structure [5] [6]. For example, the linear and nonlinear polarizabilities of polyene and polymethine dyes have been shown to be highly correlated with the bond length alternation between adjacent C-C bonds in the conjugated chain. At present, the research focus of nonlinear optical materials is mainly focused on the construction of organic molecules containing electron donor (D), electron acceptor (A) and π-conjugation [7]. For example, D-π-A, A-π-D-π-A, D-π-A-π-D and other organic molecules [8] [9], focus on increasing the third-order NLO coefficients (second-order hyperpolarizability), so as to improve the optical properties of organic materials [10] [11] [12].
Quinone heterocyclic compounds are a kind of conjugated organic functional materials [13]. The intramolecular charge transfer compound synthesized by using it as the electron donor has become the preferred material for organic superconductors [14] [15] [16]. It is widely used in organic semiconductors, airport devices, organic light-emitting diodes, photovoltaic cells and other fields. At present, anthraquinone and naphthoquinone derivatives have been widely studied in the third-order nonlinear optical materials. The nonlinear optical effect of this kind of molecule mainly depends on its molecular configuration. The third-order hyperpolarizability of this kind of molecule increases obviously with the growth of its conjugated chain. At the same time, the sulfur-containing heterocycle in the conjugated heterocycle structure contributes significantly to its third-order hyperpolarizability due to its high hole mobility values and air stability, so it has attracted the attention of chemical researchers [17] [18] [19] [20] [21]. For example, Dibenzo [b,i] thiazene-5,7,12,14-tetraketone was synthesized from 2,3-dichloro-1,4-naphthoquinone and sodium sulfide by Gao's research group [13].
Azo aromatic organic compounds are a kind of nonlinear optical materials with good properties. Because of its photosensitivity and photoisomerism, it has a great potential application in nonlinear fields such as optical information sto-  [27]. Many researchers have studied its third-order nonlinear optical properties. For example, Guo et al. [28] study the third-order nonlinearity of materials by increasing π electron delocalization; Kang et al. [29] study the influence of different substituents on the nonlinear optical properties of azobenzene derivatives; Wang et al. [30] report that the introduction of other organic molecules on azobenzene molecules will enhance the nonlinear optical properties of azobenzene.
In recent years, there have been experimental and theoretical reports on the structure and NLO properties of quinone heterocycles containing diazobenzene [31] [32] [33] [34] [35]. DNTTRA has good rigid conjugate plane and good delocalization. The large π conjugated parent structure would interact with the azo group after conjugated bridging the electron donor of phenyl, which can further enhance the delocalization of the electron, and thus can be designed as an organic electron transport material. These structural molecules belong to D-π-A-π-D type organic molecules, and their chemical modification can improve the nonlinear optical properties of the molecules. Therefore, in this work, we calculated the molecular structure and third-order NLO of DNTTRA and 36 derivatives as shown in Figure 1 and Figure S1. The effects of azobenzene on the third-order NLO properties of DNTTRA molecules at 3.9 sites, 2.9 sites, 2.10 sites, 1.8 sites, 1.11 sites and 4.8 sites were also discussed. Secondly, we further studied the introduction of -OH, -OCH 3 , -NH 2 , -NHCH 3 , -N(CH 3 ) 2 into the para position of azobenzene ring. It provides a theoretical reference for the further design and synthesis of the third-order nonlinear optical materials containing DNTTRA with excellent properties.
The center of thiophene ring is chosen as the coordinate origin when calculating the third-order NLO properties. The XY plane is a conjugate plane, the Z axis is perpendicular to the molecular plane, and the dipole moment of the molecule is roughly the same as the X axis.
Among the 37 molecules, A is C2 point group and the rest are C1 point group.
Among the 37 molecules calculated, the length of C-C single bond ranges from 0.143 to 0.150 nm; whereas the single bond length of C-N ranges from 0.137 to 0.145 nm [9]. These structure parameters are similar with our previous works [37].
Among the 37 molecular structures, the dihedral angle of naphthalene and thiophene is between 2.5˚ and 4.8˚, and the dihedral angle of naphthalene and azobenzene is between 1.4˚ and 1.8˚, shown that the molecule is a quasi-planar molecule. Compared with B, C and D, the molecular planarity is poor. The degree of conjugation in conjugated system is consistent with the degree of overlap sites, 3.9 sites, and 2.10 sites respectively.

Charge Analysis
In order to explore the correlation between charge transfer and NLO property of organic -NHCH 3 , -N(CH 3 ) 2 and other groups respectively, the positive charge increases, but does not change its positive charge characteristics. Therefore, these molecules can be regarded as D-π-A-π-D structures. With the enhancement of electron-donating ability of substituents introduced by para-position of benzene ring, the electron delocalization increases, which predicts that the third-order NLO properties of molecules will also be enhanced.

Frontier Orbitals and Electron Absorption Spectra
The molecular gap value is closely related to the intramolecular charge transfer [37]. In order to better understand the nature of molecular charge transfer, the B3LYP and M06-2X methods were used to calculate the energy gap of A-G5 molecules. The calculation results are shown in Figure 2 and Table S1.   Figure 4, Figure 5 and Table S2. It can be seen from Figure 4   the electron transition energy is relatively reduced, the lowest energy absorption peak is significantly red shifted, and the electron is easy to be excited. Therefore, it can be predicted that the azobenzene has strong third-order NLO properties.

Third-Order NLO Properties
Using the finite field FF method, an electric field ranging from 0.001 to 0.  Table 2. Table 2 shows that the γ xxxx of the third-order polarizability of 37 molecules is the largest, indicating that the third-order NLO properties of DNTTRA derivatives are mainly due to charge transfer in the X-axis direction. Compared with molecule A, the third-order NLO coefficients (γ) increased significantly when azobenzene was introduced at 3.9 sites, 2.9 sites, 2.10 sites, 1.8 sites, 1.11 sites,  Therefore, the third-order NLO coefficient (γ) of N(CH 3 ) 2 and -NHCH 3 mainly depends on the electron-donating ability of azobenzene para-substituents. Azobenzene was introduced at the 2.10 sites of DNTTRA, and strong donor electron groups -N(CH 3 ) 2 and -NHCH 3 were introduced at azobenzene para-position.
These molecules can be designed as third-order NLO materials with good properties.

Conclusion
The in the azobenzene ring, so that the third-order NLO materials with good properties can be obtained. Computational Chemistry

Supporting Information
A B C D E F G Figure S1. The optimized structures of A-G molecules.