Optical and nonlinear optical properties of rippled C84 derivatives comprising cyclically fused nonbenzenoid rings

Due to the widespread application of nonlinear optical materials, designing and synthesizing high-performance nonlinear optical materials has always been a hot research direction. Based on the reported C₈₄ molecular carbon containing cyclic defects, eight new derivatives are designed by introducing donor/acceptor units or their combinations. Optical and nonlinear optical properties of the reported molecule and eight new designed derivatives are investigated in detail by using DFT and TD-DFT. The research manifests that they are all narrow bandgap derivatives, and all have small reorganization energy and large static first hyperpolarizability. The simultaneous introduction of stronger electron donor and electron acceptor is an effective method to increase optical nonlinearity. The introduction of donor/acceptor units causes a red shift in the strong absorption wavelength, and the stronger the electron-donating/deficient ability of the donor/acceptor unit, the more pronounced the red shift. But when both donor and acceptor units are introduced simultaneously, the situation changes. That is, when the donor NH₂ unit is introduced simultaneously with acceptor units of different intensities, the strong absorption band undergoes a significant blue shift, and the greater the intensity of the acceptor unit, the more obvious the blue shift of the strong absorption band. However, when the donor TTF unit with stronger electron-donating ability is introduced simultaneously with acceptor units of different intensities, the strong absorption peak still shows a red shift. Considering their large static first hyperpolarizability and small electron/hole reorganization energy, it is expected that they will become candidates for nonlinear optical materials and bipolar charge transport materials. Moreover, the origin of nonlinearity was studied utilizing DR analysis, hyperpolarizability density analysis and hyperpolarizability contribution decompositions analysis. This work will provide guidance for the rational design and synthesis of molecules with excellent optoelectronic properties in the future.