We suggest here a family of non-linear optical chromophores that have weak response in the ground state and a significantly enhanced response in the triplet state - hence a non-linear optical 'switch'. Several promising molecules are proposed and their response mechanism, in which computation shows a significant enhancement of the first hyperpolarizability upon photoexcitation from the singlet to metastable triplet state, are investigated. (c) 2005 Elsevier B.V. All rights reserved.