Tunable Aggregation-Induced Emission of Imidazole Hydrazones by pH and Anions

Tunable Emission of Low-Dimensional Organic Metal Halides by Stoichiometric Ratio and Metal Center

Low-dimensional (LD) organic metal halides (OMHs) have a bright future due to their excellent photoelectric characteristics and unique structure. However, the synthesis and emission control of LD-OMHs are still unclear. Herein, the different dimensional (zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D)) of OMHs were obtained by the reaction of 1,4-diazabicyclo (2.2.2) octane with PbBr2 in different stoichiometric ratios. This discovery shows that the structure and properties of OMHs can be regulated while maintaining the functional organic cations of OMHs, which broadens the path for the development of functional LD-OMHs. Among them, 0D-OMH 1 and 1D-OMH 3 have narrow-band (full width at half-maximum (fwhm) = 74 nm) and broad-band (fwhm = 201 nm) emission, respectively. We found that when organic cations have no contribution to the formation of conduction band minimum and valence band maximum, and the distances between polyhedrons are larger than the van der Waals diameter of the halogen atom, the effect of phonons on exciton transitions can be reduced to achieve a narrow-band emission. Further, Cu(I)- and Mn (II)-based 0D-OMHs were synthesized, which have high photoluminescence quantum yield (PLQY) (33.97 and 47.33%, respectively). When the emitting of 0D-OMHs produced by the interaction of the metal-center and halogens, the asymmetric planar metal-halogen structure will result in a higher PLQY.

Tunable Aggregation-induced Emission and Emission Colors of Imidazolium and Pyridinium Based Hydrazones

Aggregation-induced emission (AIE) materials have drawn great attention for their wide applications as optical materials. The applications of AIE materials, however, are restricted by the complicated syntheses, hydrophobic properties and short emission wavelengths. Herein, an imidazolium based hydrazone (E)-1-(4-methoxyphenyl)-2-((1-methyl-1H-imidazol-2-yl)methylene)hydrazine hydrochloride (1) and a pyridinium based hydrazone (E)-1-(4-methoxyphenyl)-2-(pyridin-4-ylmethylene)hydrazine hydrochloride (2) have been synthesized. Notably, 1 and 2 in crystals show distinct green and near-infrared (NIR) fluorescence, with emission peaks at 530 and 688 nm, and Stokes shifts of 176 and 308 nm, respectively. After grinding the crystals to powder, the absolute fluorescence quantum yield (ΦF) of 1 is increased from 4.2% to 10.6%, and the ΦF of 2 is increased from 0.2% to 0.7%. X-ray crystallography studies together with theoretical calculations indicate that the enhanced emission of 1 arises from hydrogen bonding induced rigid network, and the fluorescence in the NIR region and large Stokes shift of 2 are attributed to its twisted molecular structure and strong push-pull effect.

Helical Self-Assembly and Fe3+ Detection of V-Shaped AIE-Active Chiral Tetraphenylbutadiene-Based Polyamides

Chiral aggregation-induced emission (AIE) molecules have drawn attention for their helical self-assembly and special optical properties. The helical self-assembly of AIE-active chiral non-linear main-chain polymers can produce some desired optical features. In this work, a series of V-shaped chiral AIE-active polyamides P1-C3, P1-C6, P1-C12 and linear P2-C3, P2-C6, bearing n-propyl/hexyl/dodecyl side-chains, based on tetraphenylbutadiene (TPB), were prepared. All target main-chain polymers exhibit distinct AIE characteristics. The polymer P1-C6 with moderate length alkyl chains shows better AIE properties. The V-shaped main-chains and the chiral induction of (1R,2R)-(+)-1,2-cyclohexanediamine in each repeating unit promote the polymer chains display helical conformation, and multiple helical polymer chains induce nano-fibers helicity when the polymer chains aggregate and self-assemble in THF/H2 O mixtures. Simultaneously, the helical conformation polymer chains and helical nano-fibers cause P1-C6 produce strong circular dichroism (CD) signals with positive Cotton effect. Moreover, P1-C6 could also occur fluorescence quenching response to Fe3+ selectively with a low detection limit of 3.48 μmol/L.

Multi-Stimuli-Induced Mechanical Bending and Reversible Fluorescence Switching in a Single Organic Crystal

Fluorescent single crystals that respond to multiple external stimuli are of great interest in molecular machines, sensors, and displays. The integration of photo- or acid-induced fluorescence enhancement and bending in one organic crystal, however, has not been reported yet. Herein, we report the interesting plastic photomechanical bending and switching on of the fluorescence of an azine crystal in a single-crystal transformation, due to extended π-conjugation and molecular slippage. Moreover, the fluorescent plastic bending driven by multiple volatile acid vapors was firstly observed, and attributed to the synergistic effect of push-pull electronic structure and hydrogen bonding. The single crystal also shows high elasticity under external force. In addition, reversible fluorescence switching can be triggered by grinding and solvent fuming, as well as by the adsorption and desorption of HCl vapor. The integration of plastic, elastic bending and switch-on fluorescence into one single crystal provides a new strategy for next-generation smart materials.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.