The photo-controllable third-order nonlinear optical (NLO) switches have drawn ever-increasing attention due to considerable research potential in the emerging field of nonlinear optics. A class of materials, which contain photosensitive groups but cannot express directly switching properties under light conditions, can also exhibit the characteristics of excellent photo-controllable NLO switches after external regulation. Azobenzene is a kind of classic photo-isomerized molecule and has good π coplanar property and excellent electron channel, which can engender third-order NLO response under push and pull electron action. It is a feasible strategy to design photo-controlled NLO switch materials by introducing azo groups. Nevertheless, the trans-cis isomerization behaviors of some azobenzene derivatives are interfered by other groups or external factors, further inhibiting the conversion of photo-controllable third-order NLO properties. Once these external interference factors are found and removed, the photo-controllable NLO behaviors of such azobenzene derivatives will be opened. In our work, a special azobenzene derivative was synthesized and reported, which was unable to produce cis-trans isomerization due to the H⁺ effect of self-dissociation, and the H⁺ effect could be shield by introducing organic groups or bases. The adjusted materials can easily undergo reversible cis-trans isomerization reaction, and the Z-scan test shows the complete inversions of third-order NLO properties before and after UV irradiation. The adjusted materials in trans configuration show the reverse saturation absorption (RSA) and self-defocusing properties. After UV irradiation, the materials convert into cis configuration and exhibit saturation absorption (SA) and strong self-focusing behaviors. To gain a deeper understanding of the light-adjusted third-order NLO switch behaviors, density functional theory (DFT) calculations of (CH₃)₂L were carried out. For trans-(CH₃)₂L, HOMO and LUMO are mainly localized on the azobenzene unit, where π-π^* transition between the two orbitals is displayed. The azobenzene unit in the trans-(CH₃)₂L is considered to have considerable contribution to the generation of third-order nonlinear-ity. For cis-(CH₃)₂L, the electron cloud density of HOMO is mainly populated on the azobenzene unit, whereas the electron cloud density distribution of LUMO appears on the entire molecule, suggesting significant intramolecular charge transfer (ICT) from azobenzene to the entire molecule. The effect of ICT in the cis structure dominants the generation of third-order nonlinearity. The remarkable third-order NLO transformation result from the rearrangement of the electronic structures, which makes them generate different response mechanisms under the laser stimulation.

Key words: cis-trans isomerization, photo-controllable, third-order NLO switch, protons effect, performance transformation