Synthesis of two novel indolo[3,2-b]carbazole derivatives with aggregation-enhanced emission property

Abstract

Luminogenic materials with AIE (aggregation-induced emission) and AIEE (aggregation-induced enhanced emission) characteristics have attracted substantial research interest due to their potential applications in optics, electronics environmental and the biological sciences. Here, we successfully synthesized two novel indolo[3,2-b]carbazole derivatives (compounds 3 and 4) with strong symmetrical groups, which were demonstrated to exhibit superior AIEE properties. Both 3 and 4 contain the same π-conjugated core structure, but slightly differ in the cyano-substituted stilbene group, giving rise to a high charge mobility, similar optical properties and energy band-gaps. The bulky cyano group attached to a vinylene moiety generates intramolecular co-planarity and prevents π–π stacking, whereby the emission in the aggregates can be enhanced. In addition, it is found that the cyano group also plays an important role in the formation of J-type aggregation, which is advantageous to the appearance of AIE and/or AIEE properties. A slight modification of the terminal substituent group renders compounds 3 and 4 AIEE properties entirely. As revealed by crystallographic data, the multiple intermolecular hydrogen bonding in 4 restricts intramolecular vibration and rotation, which enables 4 to emit intensely in the solid state. Moreover, cyano groups endow 3 with an improved self-assembly ability under the proper conditions. The molecules of 3 self-organize into one-dimensional ordered microfibers and/or nanorods that show a significant optical waveguide with different water fractions, indicating that the aqueous suspension of microfibers favors fluorescence emission. Our results demonstrate that the AIEE properties of the novel indolo[3,2-b]carbazole derivatives are highly related to the molecular structure and aggregate packing.