Photoisomerization-switchable second-order nonlinear optical responses of dithienylethene-containing boron derivatives: a theoretical study

The switchable second-order nonlinear optical properties of a series of dithienylethene-containing boron compounds possessing reversible photochromic behavior have been systemically investigated based on density functional theory. The significant differences on the static first hyperpolarizabilities caused by the photoisomerization reaction confirm the switchable nonlinear optical behavior. Results show that the closed-ring 1c-4c possess large static first hyperpolarizabilitieswith respect to their corresponding open-ring 1o-4o, which is due to the better pi-conjugation and a more obvious degree of charge transfer. Specifically, the static first hyperpolarizability values increase in the range of 2.4-5.2 times with the strengthening electron-withdrawing ability of the substituent R (R = H, CF3, NO2) on the pyridine and elongating the conjugated chains. It indicates that introducing boron-based functional groups to the dithienylethene monomer and modifying the pyridine ring can effectively improve the switching nonlinear optical responses.