Third-order nonlinear optical (NLO) materials possess unique conjugated π-electron systems in their molecular structure, enabling nonlinear optical conversion processes like optical frequency doubling and frequency tripling. Organic conjugated pigment molecules, representing this category, offer significant advantages such as large nonlinear optical coefficient, low direct-current permittivity, substantial mechanical strength, good chemical stability, and processability. The π-electron conjugated structure of these molecules enhances the third-order nonlinear polarizability χ³. Azo aromatic organic compounds containing NN double bonds demonstrate excellent charge transport channels, thus exhibiting promising nonlinear optical properties. The molecule benzo [b] naphtho [2', 3':5,6][1, 4] dithiino [2, 3-i] thianthrene-5, 7, 9, 14, 16, 18-hexanone (PA) features a rigid conjugated plane with significant delocalization, it is connected to the electron-donating group (phenyl) through the conjugate bridge chain NN, and the large π-conjugated parent structure is connected to the electron-donating azo group. This enhances the delocalization of electrons, allowing for the design of organic molecular transport materials with D-π-A-π-D structural molecules. PA exhibits remarkable rigidity in its conjugated plane and delocalization properties, especially due to its extensive π-conjugated core structure linked with electron-donating azobenzene groups, resulting in significant third-order nonlinear polarization responses under intense laser conditions. Therefore, the molecular structure, transition dipole moment, molecular charge distribution, electrostatic potential (ESP) distribution, electronic spectra, and third-order nonlinear optical properties of PA and its 36 derivatives containing azobenzene are theoretically calculated to investigate the effects of introducing azobenzene into the 1, 10-position, 1, 11-position, 1, 12-position, 1, 13-position, 2, 11-position, 2, 12-position, and 2, 13-position of the PA molecules, respectively. Furthermore, the effects of introducing different electron-donating groups such as —NH₂, —NHCH₃, —N(CH₃)₂, —NPh₃, and —KZ (N-phenylcarbazole) in the para-position of the azobenzene ring on the third-order nonlinear optical properties are further explored. Based on the trend of these changes, this investigation provides theoretical bases for the design and synthesis of diazobenzene PA-containing third-order nonlinear optical materials with excellent properties.

Density functional theory (DFT) with B3LYP and CAM-B3LYP methods, coupled with the 6-311++g(d,p) basis set, is employed to conduct comprehensive structural and vibrational calculations of PA and its 36 derivatives containing azobenzene moieties. In addition, a natural orbital charge distribution analysis is conducted. Transition dipole moments, electrostatic potential (ESP) distributions, frontier molecular orbitals, and electron absorption spectra of molecules f1-f6 are computed based on TD-B3LYP/6-311++g(d,p) theory. Subsequently, Multiwfn 3.8 software is utilized for results of in-depth processing of CAM-B3LYP and finite field method to investigate the third-order nonlinear optical properties of all 37 molecules.

Results reveal that the six molecules f1-f6 adopt D-π-A-π-D structures (Fig. 5 and Table 2) with energy gap values ranging from 1.33 to 2.03 eV (Fig. 6), characteristic of organic semiconductor materials. Transition dipole moments of azobenzene-containing groups [—N(CH₃)₂, —NPh₃, and —KZ (N-phenylcarbazole)] are calculated at different positions (1, 10; 1, 11; 1, 12; 1, 13; 2, 11; 2, 12) under B3LYP/6-311++g(d, p) and CAM-B3LYP/6-311++g(d, p) levels. While numerical differences exist between the two methods, the trends remain consistent (Fig. 5). Notably, the introduction of these groups results in maximum transition dipole moment values for f-series molecules. Analysis indicates that the transition dipole moment μ₀₁ primarily influences the third-order NLO coefficient γ, with larger μ₀₁ values correlating with improved NLO performance. It is noteworthy that the significant enhancement in the third-order NLO performance observed when these groups are introduced at 2, 12 sites (i.e., f-series). The electrostatic potential (ESP) distributions for molecules f1-f6 reveal negative charges predominantly congregating near the six ketone groups (Fig. 5), while positive charges mainly distribute along the molecular chains. The strongest absorption peaks and lowest energy absorption peaks occur in the order f1→f2→f3→f4→f6→f5 (Table 3 and Fig. 7), with charge transfer spectra (CTS) analysis (Table 4) revealing corresponding increases in the third-order NLO coefficient γ with increasing charge transfer amounts. Introducing azobenzene-containing groups at different positions (1, 10; 1, 11; 1, 12; 1, 13; 2, 11; and 2, 12) of PA resulted in varying third-order NLO coefficient γ, with relatively smaller values observed for introductions at 1, 10 and 1, 13 positions, ranging from 4.508×10⁵ to 51.565×10⁵ a. u., and larger coefficients observed for introductions at 2, 11 and 2, 12 positions, particularly with the latter yielding maximum third-order NLO coefficient γ ranging from 10.443×10⁵ to 73.815×10⁵ a.u., nearly an order of magnitude larger than reported diazo derivatives (3.10×10⁵ to 7.50×10⁵ a.u.) (Table 3 and Fig. 5). Across the six series, introducing —NH₂, —NHCH₃, —N(CH₃)₂, —KZ (N-phenylcarbazole), and —NPh₃ groups sequentially increases the third-order NLO coefficient γ, with significant enhancements observed particularly with —N(CH₃)₂, —KZ (n-phenylcarbazole), and —NPh₃ groups.

The electronic absorption spectra and third-order nonlinear optical properties of PA derivative molecules have been studied based on the density-functional theory B3LYP and long-range effect positive CAM-B3LYP methods. The results show that 36 molecules of the six series a-f exhibit a D-π-A-π-D structure. Particularly, molecules in the f series, where azobenzene-containing groups are introduced at positions 2,12 of the parent PA molecule, exhibit notably enhanced transition dipole moments. The third-order nonlinear optical coefficient γ of these molecules reaches 10⁷ orders of magnitude atomic units (10^-33 esu), indicating good third-order nonlinear optical properties and optimal substitution positions. In comparison to the PA molecule, molecules f4-f6, featuring azobenzene terminated with strong electron-donating groups such as —N(CH₃)₂, —KZ (N-phenylcarbazole), and —NPh₃ at the 2, 12 position, exhibited significantly heightened absorption peak wavelengths. Moreover, the third-order nonlinear optical coefficient γ of these molecules increased by 2.6 to 6.3 times, indicating the efficacy of introducing azobenzene with strong electron-donating groups at the 2, 12 position of the PA molecule in enhancing system’s third-order optical properties. This suggests that introducing azobenzene containing strong electron donor groups at the 2, 12 position of the PA molecule enhances the third-order nonlinear optical properties of the system, thus facilitating the development of superior third-order nonlinear optical materials.