Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion-Induced and Aggregation-Enhanced Light Emission

Coherent Modulation of the Aggregation Behavior and Intramolecular Charge Transfer in Small Molecule Probes for Sensitive and Long-term Nerve Agent Monitoring

Aggregation-induced emission (AIE) shows promising performance in chemical sensing relying on the change of the emission behavior of the probe molecule monomers to the aggregated product. However, whether the response contrast could be further boosted by utilizing the emission property of the aggregated probe and the aggregated product remains a big challenge. Here, an exciting AIE probe regulation strategy was proposed by coherently modulating the aggregation behavior and the intramolecular charge transfer (ICT) property of the probes and thus an aggregated-to-aggregated colorimetric-fluorescent dual-mode detection was achieved. The blue emissive film obtained with the optimal AIE probe has been proven to be effective to recognize the vapor of nerve agent analog DCP in air by emitting a sharp green fluorescence. In addition, a porous polymer-based wet sensing chip loaded with the probe enables the immediate response to DCP vapor with a limit of detection (LOD) of 1.7 ppb, and it was further integrated into a wearable watch device for long-term monitoring of DCP vapor up to two weeks. We expect the present probe design strategy would greatly deepen the AIE-based science and provide new insights for long-term monitoring sensors toward trace hazardous substances.

Dynamic Light Scattering and Its Application to Control Nanoparticle Aggregation in Colloidal Systems: A Review

Colloidal systems and their control play an essential role in daily human activities, but several drawbacks lead to an avoidance of their extensive application in some more productive areas. Some roadblocks are a lack of knowledge regarding how to influence and address colloidal forces, as well as a lack of practical devices to understand these systems. This review focuses on applying dynamic light scattering (DLS) as a powerful tool for monitoring and characterizing nanoparticle aggregation dynamics. We started by outlining the core ideas behind DLS and how it may be used to examine colloidal particle size distribution and aggregation dynamics; then, in the last section, we included the options to control aggregation in the chemically processed toner. In addition, we pinpointed knowledge gaps and difficulties that obstruct the use of DLS in real-world situations. Although widely used, DLS has limits when dealing with complicated systems, including combinations of nanoparticles, high concentrations, and non-spherical particles. We discussed these issues and offered possible solutions and the incorporation of supplementary characterization approaches. Finally, we emphasized how critical it is to close the gap between fundamental studies of nanoparticle aggregation and their translation into real-world applications, recognizing challenges in colloidal science.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C_3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C₃ conformers of NH cryptands can be enlarged more remarkably than the C_3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C₃ conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F^- to I^- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy^-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn²⁺. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn²⁺ ions. The second, with ClO₄^- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn²⁺ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al³⁺ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.