Benzimidazole-Piperazine-Coumarin/Cucurbit[7]uril Supramolecular Photoinduced Electron Transfer Fluorochromes for Detection of Carnosol by Stimuli-Responsive Dye Displacement and pK a Tuning

A new fluorescent dye (4PBZC) comprising coumarin (C), piperazine (P), and benzimidazole (BZ) was designed, prepared, and complexed to cucurbit[7]uril (CB7) to detect carnosol (CAR), an anti-breast cancer drug, in sub-nanomolar concentrations utilizing the supramolecular indicator displacement assay strategy, the CB7-assisted pK _a shift, and the CB7-retarded photoinduced electron transfer process. The host-guest complexation was confirmed by UV-visible absorption, fluorescence, and ¹H NMR spectroscopy, which established the binding of 4PBZC to CB7. CB7 preferentially binds the indicator dye (4PBZC) via the protonated BZ residue compared to the neutral BZ one, demonstrated by a higher binding constant of the complex in its di-protonated form, which led to an increase in the pK _a of the BZ moiety by ca. 3.0 units after the addition of CB7. In aqueous solution (pH 6), switching the emission signals between 4PBZH⁺C/CB7 (ON state) and 4PBZC (OFF state) was achieved by displacement of the protonated dye from the cavity of CB7 by the CAR analyte. An efficient sensor was obtained for the sensitive detection of CAR in aqueous solution with a low detection limit of 0.148 ng/mL (0.45 nM) and a linear range from 20 to 627 ng/mL.

An overview from simple host-guest systems to progressively complex supramolecular assemblies

Supramolecular chemistry involving macrocyclic hosts is a highly interdisciplinary and fast-growing research field in chemistry, biochemistry, and materials science. Host-guest based supramolecular assemblies, as constructed through non-covalent interactions, are highly dynamic in nature, and can be tuned easily using their responses to various external stimuli, providing a convenient approach to achieve excellent functional materials. Macrocyclic hosts, particularly cyclodextrins, cucurbit[n]urils, and calix[n]arenes, which have unique features like possessing hydrophobic cavities of different sizes, along with hydrophilic external surfaces, which are also amenable towards easy derivatizations, are versatile cavitands or host molecules to encapsulate diverse guest molecules to form stable host-guest complexes with many unique structures and properties. Interestingly, host-guest complexes possessing amphiphilic properties can easily lead to the formation of various advanced supramolecular assemblies, like pseudorotaxanes, rotaxanes, polyrotaxanes, supramolecular polymers, micelles, vesicles, supramolecular nanostructures, and so on. Moreover, these supramolecular assemblies, with varied morphologies and responsiveness towards external stimuli, have immense potential for applications in nanotechnology, materials science, biosensors, drug delivery, analytical chemistry and biomedical sciences. In this perspective, we present a stimulating overview, discussing simple host-guest systems to complex supramolecular assemblies in a systematic manner, aiming to encourage future researchers in this fascinating area of supramolecular chemistry to develop advanced supramolecular materials with superior functionalities, for their deployment in diverse applied areas.

Circularly Polarized Luminescence of Achiral Metal-Organic Colloids and Guest Molecules in a Vortex Field

Recently, scientists have reported a range of chiral fluorescence materials or chiral composites that can emit circularly polarized luminescence. Herein, two achiral metal-organic colloidal solutions were studied, showing active circularly polarized luminescence, which is observed in vortex stirring. The absolute values for g_lum are 0.05 and 0.03 and the plus or minus sign of g_lum depends on the colloidal structure and stirring direction, which make the property easy to manipulate. Further, the host-guest interaction study suggests both electrostatic interactions and coordination bonding may influence the chiroptical property from the colloidal solution to the guest molecule. Rhodamine 6G and its carboxylic acid derivative exhibit good quantum yields and acceptable g_lum values in the colloidal solution.

Heparin sensing based on multisite-binding induced highly ordered perylene nanoaggregates

Heparin sensing based on highly ordered perylene nanoaggregates with ultra-low fluorescence and the use of host–guest complexes to improve the sensitivity.