Study on a series of pentanuclear planar ‘open’ clusters: synthesis, characterization, strong third-order optical nonlinearities and superior optical limiting properties

Abstract

A series of new candidates as nonlinear optical materials, heterothiometallic clusters [MS4Cu4X2(py)6] (X = I, M = W, 1, Mo, 2; X = Br, M = W, 3, Mo, 4) have been synthesized via two comparative synthetic routes for third-order NLO studies. Single-crystal X-ray diffraction shows that the neutral clusters assume pentanuclear planar ‘open’ structures. Their optical responses to incident pulsed laser exhibit very large optical limiting effects, which are simulated by using excited-state absorption theory, with limiting thresholds of 0.07–0.10 J cm−2 for clusters 1–4, and these effects should make this series of clusters among the most promising optical limiters. The strong third-order optical nonlinear absorption and optical self-focusing effects of clusters 1–4 in DMF solution were observed in the Z-scan measurements of an 8-ns pulsed laser at 532 nm. The corresponding effective third-order nonlinear optical susceptibilities of this series of clusters were found to be the largest among all kinds of heterothiometallic clusters, and their third-order nonlinear hyperpolarizabilities are also reported. The nonlinear effects for these four clusters in DMF solution have also been investigated by time-resolved pump–probe experiments to provide direct evidence on the physical origin of the observed OL effects. The nonlinear optical properties of this series of clusters 1–4 were compared with other NLO cluster compounds studied previously and revealed a qualitative structure/NLO property correlation.