High-Generation Second-Order Nonlinear Optical (NLO) Dendrimers: Convenient Synthesis by Click Chemistry and the Increasing Trend of NLO Effects

摘要

Dendrimers are defect-free and perfect monodisperse macro-molecules with a regular and highly three-dimensional branched structure, which can bring out many special properties: nanometer size, globular shape, multivalent character, and modularity of the assembly. These characteristics make them competitive candidates for applications in a variety of fields including catalysis, biology, and materials science. Generally, highly branched dendrimers could be constructed through a divergent (a growing pattern from a multivalent core) or convergent (a dendron is grafted on to the core) synthetic strategy. However, the tedious multistep synthetic protocols often make the preparation of dendrimers relatively costly and involve some difficulties, such as the incomplete reaction of the end groups in the divergent approach (leading to the structural defects) and steric hindrance between the reactive segments and core molecule (hampering the formation of high generations) in the convergent strategy. Versatile methodologies to address these issues have now rendered the synthesis of dendrimers more precise and economical. As typical examples, through the combination of the divergent and convergent approaches (a "double-stage" method), proposed by Frechet and co-workers, the synthetic efficiency could be raised rapidly; also, by the utilization of the powerful Cu~I-catalyzed 1,3-dipolar cyclo-addition reactions between azides and alkynes (the Sharpless "click" reaction). many divergently built-up dendrimers were yielded after the pioneering work of Frechet and coworkers. However, there are currently very few reports of the use of "click chemistry" for both parts of a combined divergent and convergent approach.