Multinuclear Staircase Oligomers Based on the (Et2C2B4H4)Fe(η6-C6H6) Sandwich Unit: Quantitative Tailorable and Redox Switchable Nonlinear Optics

To provide a new approach for enhancing second-order nonlinear optical (NLO) response, a series of multinuclear staircase oligomers composed of Fe(Et2C2B4H4) ferracarborane clusters and benzene rings have been systematically designed and investigated. The calculations of geometric and electronic structures, electronic absorption spectra, first hyperpolarizabilities, and first hyperpolarizability densities were carried out by density functional theory and time-dependent density functional theory. It is found that the NLO properties have obvious multiple relationships with the numbers of the sandwich unit. Moreover, all of the possibilities for one-, two-, and three-electron oxide complexes have also been considered. The results first revealed some remarkable changes upon charge transfer pattern which is from the unexpected interrupted to desired continuous forms in the two-electron oxidation reactions, accompanied by significant differences in the relevant second-order NLO properties. The considerable ON/OFF switching ratios for the hyperpolarizability values declare that these multinuclear sandwich complexes can be deemed as high-efficiency redox-triggered NLO switches. Hence, we hoped that the conception and performance in our work will strongly provide a fundamental guideline and reference for further research for novel NLO materials.