Theoretical investigations on electronic spectra and the redox-switchable second-order nonlinear optical responses of rhodium(I)-9,10-phenanthrenediimine complexes

The redox-switchable second-order nonlinear optical (NLO) properties of a series of Rh(I) complexes have been studied based on density functional theory (DFT) calculations. The analysis of the electronic structure shows that the Rh(I) ion acts as the oxidation center in a one-electron-oxidized process, while both the Rh(I) ion and the 9,10-phenanthrenediimine (phdi) ligand act as reduction centers in a one-electron-reduced process. Different redox centers lead to different charge-transfer (CT) features, which alter the static first hyperpolarizabilities of the neutral complexes. Our DFT calculations indicated that these complexes show large second-order NLO responses and that the redox process can significantly enhance these NLO responses. For complexes 2 and 3, the βtot values of the one-electron-reduced species 2− and the one-electron-oxidized species 3+ are ∼10.0 and ∼8.5 times larger, respectively, than those of the corresponding neutral complexes. Therefore, complexes 2 and 3 are promising candidates for redox-switchable NLO molecular materials. The large NLO responses of the oxidized species are mainly related to ligand-to-ligand charge-transfer (LLCT) transitions when combined with intraligand charge-transfer (ILCT) transitions, while the results for the reduced species are strongly associated with metal-to-ligand charge-transfer (MLCT) transitions.

Graphical abstractThe static first hyperpolarizabilities of complexes 2, 2−, 3 and 3+.Figure optionsDownload full-size imageDownload high-quality image (114 K)Download as PowerPoint slideHighlights► The –NH2 substituted 3 displays desirable NLO properties. ► Oxidized or reduced center is different in redox process. ► Complexes 2 and 3 are promising candidates for redox-switchable NLO materials.