The Li-substituted effect on the geometries and second-order nonlinear optical properties of indeno[1,2-b]fluorene

The structures with all real frequencies and static first hyperpolarizabilities (βtot) of a series of Lin@indeno[1,2-b]fluorene (n = 1–6) salts have been calculated by the second-order Møller–Plesset (MP2) theory. The results show that the Li-substituted effect can significantly enhance the βtot values of Lin@indeno[1,2-b]fluorene compared with that of non-substituted molecule indeno[1,2-b]fluorene. Especially, the βtot value (18640 a.u.) for Li3@indeno[1,2-b]fluorene is the largest among all systems. According to the two-level model, the calculated fΔμ/ΔE3 values further explain the large second-order nonlinear optical (NLO) responses of Lin@indeno[1,2-b]fluorene. In addition, the investigations on frequency-dependent first hyperpolarizabilities (β(ω)) show that the dispersion has less influence on the β(ω) values at low frequency area of all studied systems. It is our expectation that this work will be beneficial for further theoretical and experimental studies on the NLO properties of the lithium salts.