Redox-switchable second-order nonlinear optical responses of TEMPO-dithiolate ligand and (tempodt)M complexes (M = Pt, Pd)

Density functional theory (DFT) combined with the finite field (FF) method has been carried out to investigate the switching nonlinear optical (NLO) action of the TEMPO-bound dithiolate ligands and metal (Pt, Pd) complexes. The TEMPO unit is a redox-active radical and can stably exist in different redox states (TEMPOH, TEMPO+, TEMPOradical dot, TEMPO−). The DFT–FF calculations show that a substantial enhancement in second-order polarizability has been obtained in TEMPO+-bound dithiolate ligand, with a value of 461 × 10−30 esu. It is because that the TEMPO moiety as donor strength has been lowered by one-electron-reduced, and then the TEMPO moiety becomes the acceptor character in one-electron-oxidized species 1L+. On the whole, the βtot values of metal complexes within their four different states exhibit excellent NLO switching characters. Thus, this kind of metal complexes has a possibility to be excellent switching second-order NLO materials. Among four states of the metal complexes, the radical PtLradical dot and hydroxylamine PtLH species are nonlinearity “on”, while the one-electron-oxidized 3PtL+ and one-electron-reduced 3PtL− species are nonlinearity “off”. It can be seen from the following the real process. Specifically, the electron-acceptor ability of the diimine ligand decreases within its reduction process. However, the electronic character of dithiolate ligand will be changed from electron-donor to electron-acceptor when it is oxidated.