Redox-switching second-order nonlinear optical responses of N^N^N ruthenium complexes

► One-electron reduction of ruthenium complexes can enhance the NLO responses. ► Complex 1 acts as the multi-state redox NLO switchable materials. ► Reduction processes decrease the low energy absorption of complex 1. ► Complex 3e1reduced has the largest β value among all the complexes studied. ► Complex 1 and 1e1reduced have different donors and acceptors.