Organic polymorphs based on an AEE-active tetraphenylethene salicylaldehyde Schiff-base derivative: the effect of molecular conformation on luminescence properties

An aggregation-enhanced emission (AEE)-active tetraphenylethene salicylaldehyde Schiff-base derivative,TPE-Nap, was prepared using a facile synthesis. The AEE property ofTPE-Napwas studied by luminescence and absorption spectra, and was attributed to the C & xe001;N isomerization restriction and the excited-state intramolecular proton transfer (ESIPT) process. PolymorphsTPE-Nap-YandTPE-Nap-Owere prepared fromTPE-Nap, and their emission color and intensity were compared.TPE-Nap-Yis a yellow block crystal with a very weak yellow emission, with its main peak at 565 nm, whileTPE-Nap-Ois an orange plate crystal that gave a stronger orange emission, with its main peak at 583 nm. Single crystal diffraction data were used to demonstrate the structure-property relationship. The most unique feature was that the torsion angle ofTPE-Nap-Ybetween the benzene ring of the TPE unit and the Nap unit was 54.08 degrees, while that ofTPE-Nap-Owas 14.19 degrees. Interestingly, the TPE unit assumed propeller-like nonplanar conformations that likely led to different intermolecular interactions, such as C-HMIDLINE HORIZONTAL ELLIPSISO interactions (2.529 angstrom and 2.617 angstrom) inTPE-Nap-Oand C-HMIDLINE HORIZONTAL ELLIPSIS pi interactions (3.224 angstrom and 3.791 angstrom) inTPE-Nap-Y. These were influenced by the torsion angle, although the molecules in both crystals were arranged in a similar end-to-end slip-stacking mode. These results inferred that the molecular conformation was evidently affected by luminescent properties. Crystals possessing a slightly twisted molecular conformation exhibited stronger emission than those possessing a heavily twisted molecular conformation. These investigations will expand the research on the relationship between the molecular conformation and the emission properties of organic solids, and might provide a new development strategy for organic polymorphs.