Janus second-order nonlinear optical dendrimers: their controllable molecular topology and corresponding largely enhanced performance†
A new type of Janus dendrimers, consisting of two different side dendrons with the dipole orientation of the second-order nonlinear optical (NLO) chromophore moieties partially in a non-centrosymmetric direction, was intelligently designed and synthesized in order to enhance the macroscopic NLO performance and break through the limitation of NLO efficiency in the current molecular topological structure of azo chromophore-based polymers. This kind of Janus dendritic structure was constructed by the combination of convergent and divergent methods, with the utilization of a powerful “click chemistry reaction”. The obtained three dendrimers, D-13N, D-17N and D-21N, show very high NLO performance, especially the dramatically enhanced NLO coefficient of 299 pm V−1 for D-13N, which is the highest value ever reported for polymers containing a simple azo chromophore. The new dendrimers provide a clear structure–properties relationship between high NLO efficiency and the controllable molecular topology with the non-centrosymmetrical alignment of dipole orientation, thus opening up a new avenue for the further development of NLO dendrimers with high performance and more importantly providing some clues for the rational design of functional dendrimers with controllable molecular topology.