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 π -linkers, along their successive opening/closing. Under the fully closed forms, all of them display negligible first hyperpolarizability ( β ) values. When one BOX is opened, which is sketched as C → O , a push-pull π -conjugated segment is formed, having the potential to enhance β 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 ( C → O ) or for the diBOX ( CC → CO ) and triBOX ( CCC → CCO ) compounds, i.e., when the remaining BOXs stay closed. The next BOX openings have much different effects. For the diBOXs, the second opening ( CO → OO ) is associated with a decrease of β , and this decrease is tuned by controlling the conformation of the π -linker, i.e., the centrosymmetry of the whole compound because β vanishes in centrosymmetric compounds. For the triBOXs, the second opening gives rise to a Λ -shape compound, with a negligible change of β , but a decrease of the DR whereas, along the third opening, β remains similar and the DR decreases to the typical value of octupolar systems (DR = 1.5).