Design of ratiometric monoaromatic fluorescence probe via modulating intramolecular hydrogen bonding: A case study of alkaline phosphatase sensing

Monoaromatic molecules are a category of molecules containing a single aromatic ring which generally emit light in the ultraviolet (UV) region. Despite their facile preparation, the UV emission greatly limits their application as organic probes. In this study, we developed a general method to red shift the emission of monoaromatic molecules. Significant fluorescence red-shift (∼100 nm per intramolecular hydrogen bonding) can be achieved by introducing intramolecular hydrogen bonding units to benzene, a typical monoaromatic molecule. Upon increasing the number of hydrogen bonding units on the benzene ring, UV, blue, and green emissions are screened, which are switchable by simply breaking/restoration the intramolecular hydrogen bonding. As a demonstration, with the breaking of one intramolecular H-bonding, the green emission (λ_em^max = 533 nm) of 2,5-dihydroxyterephthalic acid (DHTA) changed to cyan (λ_em^max = 463 nm) upon the formation of its phosphorylated form (denoted as PDHTA), which, in the presence of alkaline phosphatase (ALP), hydrolyzed and recovered the green emission. By taking advantage of the switchable emission colors, ratiometric in vitro and endogenous ALP sensing was achieved. This general approach offers a great promise to develop organic probes with tunable emissions for fluorescence analysis and imaging by different intramolecular hydrogen bonding.

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 of TPE-Nap was studied by luminescence and absorption spectra, and was attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N isomerization restriction and the excited-state intramolecular proton transfer (ESIPT) process. Polymorphs TPE-Nap-Y and TPE-Nap-O were prepared from TPE-Nap, and their emission color and intensity were compared. TPE-Nap-Y is a yellow block crystal with a very weak yellow emission, with its main peak at 565 nm, while TPE-Nap-O is 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 of TPE-Nap-Y between the benzene ring of the TPE unit and the Nap unit was 54.08°, while that of TPE-Nap-O was 14.19°. Interestingly, the TPE unit assumed propeller-like nonplanar conformations that likely led to different intermolecular interactions, such as C–H⋯O interactions (2.529 Å and 2.617 Å) in TPE-Nap-O and C–H⋯π interactions (3.224 Å and 3.791 Å) in TPE-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.

Organic polymorphs displaying different emission colors and intensities were obtained from aggregation-enhanced emission (AEE)-active tetraphenylethene derivatives, and their luminescent properties were affected mainly by molecular conformation.