Author
Masafumi Ueda, School of Science, Kitasato University
Keywords
Aggregation-induced emission, Coumarin, π-conjugated system, Distorted/twisted molecular structure, Room temperature phosphorescence, Thianthrene
Abstract
Oxygen-containing triskelion-shaped AIEgens composed of coumarins (1A-D) and benzocoumarins (2F, 2G, 2H) were successfully synthesized. X-ray crystallography revealed that their molecular structures are unique π-twisted helical geometries and Janus structures. These were racemic crystals consisting of (P,P,P)-and (M,M,M)-forms. These coumarin dyes showed slightly weak fluorescence emission (ΦF＝0.1～14%) in diluted common organic solvents due to molecular inversion caused by their isomerization between (P,P,P)-and (M,M,M)-forms. Conversely, these nano-aggregates in THF/H2O demonstrated remarkable aggregation-induced emission enhancement characteristics because of the restriction of molecular motions. Interestingly, compound 1A exhibited strong yellow phosphorescence emission at liquid nitrogen temperature. Sulfur-containing macrocycle with thianthrene, TC[4]TT was successfully obtained from macrocyclization and acid-induced condensation in 5% yield in 2 steps. TC[4]TT adopted an octagonal structure recessed inwards and possessed an internal cavity that could include small organic solvents, such as CHCl3 and CS2. The crystal structures of TC[4]TT(CHCl3)2 and TC[4]TT(CS2)2 constructed honeycomb structure and porous channel stemming from its polygonal structure. Furthermore, TC[4]TT in CH2Cl2 solution exhibited fluorescence (λFluo.＝447 nm at room temperature) and phosphorescence emission (λPhos.＝516 nm at 77 K) and showed redshifted spectra compared to those of thianthrene. This indicates that macrocyclization by a sulfur bridge of thianthrene might encourage more efficient emissions.