The Effect of Conjugation Length and NN Location on the Nonlinear Optical Switching Properties of Azobenzene Derivatives

Abstract

The trans-to-cis photoisomerization behavior of azobenzene endows them with excellent photo-switching potential, and the modulation of the conjugated system can effectively optimize their nonlinear optical (NLO) responses and switching efficiency. In this study, we systematically investigated, through density functional theory (DFT) calculations, the effects of conjugation length and the position of the NN bond on the electronic structures, excited-state properties, and NLO performances of azobenzene derivatives modified with –NH₂ and –NO₂ groups. The results indicated that extending the conjugated system significantly reduced the HOMO-LUMO energy gap. When the NN bond was near the –NO₂, the charge transfer efficiency was notably enhanced, and the static first hyperpolarizability of the trans configuration reached up to 17.54 × 10³ a.u. Additionally, the synergy between conjugation elongation and the position of the NN bond improved the photo-switching efficiency, which could be as high as 3.28 in that of the 2–2 system. Moreover, having the NN bond adjacent to the –NO₂ lowered the isomerization energy barrier, further optimizing the photo response properties. This work provides a theoretical basis for designing high efficient photo-switching materials.