Multi-State Second-Order Nonlinear Optical Switches Incorporating One to Three Benzazolo-Oxazolidine Units: A Quantum Chemistry Investigation.

This contribution employs quantum chemistry methods to describe the variations of the second nonlinear optical responses of molecular switches based on benzazolo-oxazolidine (BOX) units, connected by -linkers, along their successive opening/closing. Under the fully closed forms, all of them display negligible first hyperpolarizability () values. When one BOX is opened, which is sketched as →, a push-pull -conjugated segment is formed, having the potential to enhance and to set the depolarization ratio (DR) to its one-dimensional-like value (DR = 5). This is observed when only one BOX is open, either for the monoBOX species (→) or for the diBOX (→) and triBOX (→) compounds, i.e., when the remaining BOXs stay closed. The next BOX openings have much different effects. For the diBOXs, the second opening (→) is associated with a decrease of , and this decrease is tuned by controlling the conformation of the -linker, i.e., the centrosymmetry of the whole compound because vanishes in centrosymmetric compounds. For the triBOXs, the second opening gives rise to a -shape compound, with a negligible change of , but a decrease of the DR whereas, along the third opening, remains similar and the DR decreases to the typical value of octupolar systems (DR = 1.5).