Switching of molecular second-order polarisability in solution

Second-order nonlinear optical ( NLO) polarisabilities or first hyperpolarisabilities, beta, of molecules may be manipulated by reversibly modifying the properties of specific parts of active molecules, usually by on/off switching. This may involve reducing the donor capacity of the electron-rich fragment of a typical donor - acceptor species, D - A, by oxidation or protonation. Conversely, the acceptor behaviour of A may be altered by reduction, or by deprotonation. Alteration of the first hyperpolarisability may also involve structural or chemical modification of the bridging group, thereby interfering with the communication between D and A. Much of the article focuses on redox behaviour of NLO-active metal-containing compounds, mainly describing dipolar species in which the donor fragment containing a metal is oxidised, but giving examples of switching behaviour in octopolar species and of protonation/ deprotonation behaviour.