Porous organic cages as a novel platform for second harmonic generation

Abstract

Second harmonic generation (SHG) is one of the most extensively applied nonlinear optical phenomena. Compared to traditional inorganic SHG materials, organic SHG materials, including non-centrosymmetric small molecular crystals and organic polymers, have been extensively explored for SHG optics due to their advantageous properties, including high light-induced damage threshold, ultrafast response speed, high polarization ratio, wide transparency range, and elevated dielectric constant. However, in organic SHG materials, most small molecular crystals often show poor thermal stability (<200 °C) due to their low molecular weight, which is an important obstacle for practical application under high temperatures. On the other hand, the difficulty in the growth of large-size crystals as well as the disorderly array of chromospheres on polymer chains associated with organic polymers have hindered their widespread application in SHG due to the requirements of processability and homogeneity. Therefore, it is significantly necessary to develop novel organic SHG material with high thermal stability, large-size crystals, excellent processability, high homogeneity, and a well SHG effect. In the contribution, we developed herein porous organic cages (POCs) as a novel platform for SHG optics. Thanks to its inherent macromolecular composition and zero-dimensional discrete structures, POCs can not only solve the instability problem of small molecule crystals but also overcome the drawbacks in growing large-size crystals for organic polymer. The obtained two POCs with chiral asymmetric centers (CPOC-R-5 and CPOC-S-7) display advantageous features for SHG optics including well SHG performance, large-size crystal (5 mm×3 mm×1 mm), high thermal stability (⩾250 °C), wide transparency window (750–2,000 nm) and high polarization ratio (up to 94.5%). This work thus develops POCs as a new platform toward the construction of organic NLO materials with the potential application in sorts of SHG optics.